United Kingdom: June 2018
The United Kingdom of Great Britain and Northern Ireland, often shortened to the United Kingdom (UK), or just Britain, is a sovereign country in Western Europe. It is a constitutional monarchy of four countries which were once separate: England, Wales, Scotland and Northern Ireland. The UK borders Ireland.
It is part of the United Nations, the Commonwealth of Nations, NATO, the G7, and formerly the European Union. It had the sixth largest economy in the world by nominal GDP in 2019.
About 95 percent of the UK's population are English speakers. 5.5 per cent of the population speak languages brought to the UK as a result of relatively recent immigration.
The UK has many cities. London is the largest city in the UK and is the capital city. There are also other large cities in England such as Birmingham, Manchester, Liverpool, Leeds, Bristol and Newcastle upon Tyne. Scotland has the large cities of Edinburgh and Glasgow. Cardiff and Swansea are in Wales and Derry and Belfast are in Northern Ireland.
Between the 17th and mid 20th-centuries, Britain became a world power. It became a colonial empire that controlled large areas of Africa, Asia, North America and Oceania. 
At its height in 1922, more than 458 million people lived in the British Empire, one-fifth of the Earth's population. Its area was 13,012,000 square miles: almost a quarter of the Earth's land area. The Empire was sometimes called 'the Empire on which the Sun never sets', meaning it is always daytime someplace in the Empire. Many countries left and became independent from the Empire in the 20th century, although Britain keeps links with most countries of its former empire and also still controls fourteen colonies.
History
 Prehistory 
Humans have lived in Britain for almost a million years. They did not live there all the time, probably because the climate was too extreme at times.
Archaeological remains show that the first group of modern people to live in the British Isles were hunter-gatherers after the last ice age ended. The date is not known: perhaps as early as 8000BC but certainly by 5000BC. They built mesolithic wood and stone monuments. Stonehenge was built between 3000 and 1600BC. Celtic tribes arrived from mainland Europe. Britain was a changing collection of tribal areas, with no overall leader. Julius Caesar tried to invade (take over) the island in 55BC but was not able to do so. The Romans successfully invaded in 43AD.
Written history began in Britain when writing was brought to Britain by the Romans. Rome ruled in Britain from 44AD to 410AD. They ruled the southern two-thirds of Great Britain. The Romans never took over Ireland and never fully controlled Scotland, the land north of the valleys of the River Forth and River Clyde. Their northern border varied from time to time and was marked sometimes at Hadrian's Wall (in modern England), sometimes at the Antonine Wall (in modern Scotland).
After the Romans, waves of immigrants came to Britain. Some were German tribes: the Angles, Saxons and Jutes. Others were Celts, like the Scoti, who came to Great Britain from Ireland. English and Scots are Germanic languages. They developed from Old English. This was spoken by the Anglo-Saxons in an area from the River Forth to the River Tamar.
 Middle Ages 
A later wave of immigration was that of the Vikings, during the Early Middle Ages or Viking Age. During the Viking invasion of Britain, they set up their own kingdom in north-western England, which the Anglo-Saxons named the "Danelaw", after the Danes who lived there and controlled the land. Vikings from Scandinavia also controlled most of the islands which are now part of Scotland, including the Outer Hebrides, the Inner Hebrides, and the Northern Isles (the Shetland Islands and the Orkney Islands). 
After a long period when Anglo-Saxon England was split into various kingdoms, it was made into one kingdom by Æthelstan (Athelstan) in 945 AD. In the 13th century, the lands of Wales were unified by force with England by the wars of Edward I of England ("Edward Longshanks").
 Early modern history 
 Union of the Crowns 
There were hundreds of years of fighting between both kingdoms of Great Britain. In 1603, when Queen Elizabeth I of England died, her closest relative was King James VI of Scotland. He became king of England and Ireland as well as the king of Scotland. The kingdoms of England, Ireland, and Scotland had the same monarch ever since. James VI and I was the first to be named "King of Great Britain", and he ordered the design of the Union Jack. The Union Jack has been the British national flag ever since.
 Union of 1707 
In 1707, the Parliaments of England and Scotland agreed on the Treaty of Union, which joined the two countries into one country called the United Kingdom of Great Britain under Queen Anne with the Acts of Union 1707. This union merged Scotland and England into one kingdom. England and Scotland kept their own laws, with English law in England and Wales and Scots law in Scotland. The division between the Church of Scotland and the Church of England continued. Ireland and Great Britain continued to have the same king, but Ireland did not become part of the new kingdom in 1707.
 Modern history 
 Union of 1801 
Scotland and England had already independently had much influence over Ireland since 1200. In 1800 laws were passed in the parliaments of Great Britain and Ireland to merge the two kingdoms and their two parliaments. The country was then called the United Kingdom of Great Britain and Ireland. The Union Jack was changed so that the flag of Saint Patrick (a red saltire) shows Ireland to be a part of the country.
In 1922 much of Ireland became independent from the United Kingdom as the Irish Free State (now called Ireland). However, six northern counties (called Northern Ireland) are part of the United Kingdom. The country was renamed the United Kingdom of Great Britain and Northern Ireland in 1927.
The new Parliament of Northern Ireland set up in the 1920s stopped working in the 1970s, because of The Troubles. However, devolution started again with the Northern Ireland Assembly after the Belfast Agreement (the "Good Friday Agreement") in 1998. Devolution in Scotland and Wales started the Scottish Parliament and the Welsh Parliament the same year.
The United Kingdom was a member state of the European Union (EU) and an older organization, the European Economic Community (EEC), from 1973 until Brexit in 2020.
In September 2024, the United Kingdom became the first G7 country to phase out coal power for electric generation, after 142 years of using the energy source.
Geography
The UK is northwest off the coast of mainland Europe. Around the UK are the North Sea, the English Channel and the Atlantic Ocean. The UK also rules, usually indirectly, a number of smaller places (mostly islands) around the world, which are known as British Overseas Territories. They were once part of the British Empire. Examples are Gibraltar (on the Iberian Peninsula next to the Strait of Gibraltar) and the Falkland Islands (in the south Atlantic Ocean).
In the British Isles, the UK is made up of four different countries: Wales, England and Scotland and Northern Ireland. The capital city of Wales is Cardiff. The capital city of England is London. The capital city of Scotland is Edinburgh and the capital city of Northern Ireland is Belfast. Other large cities in the UK are Birmingham, Glasgow, Leeds, Manchester, Liverpool, Sheffield, Bristol, Leicester, Coventry, Nottingham, Bradford, Newcastle Upon Tyne and Southampton.
The physical geography of the UK varies greatly. England consists of mostly lowland terrain, with upland or mountainous terrain only found north-west of the River Tees-River Exe line. The upland areas include the Lake District, the Pennines, the North York Moors, Exmoor, and Dartmoor. The lowland areas are typically traversed by ranges of low hills, frequently composed of chalk, and flat plains. Scotland is the most mountainous country in the UK and its physical geography is distinguished by the Highland Boundary Fault which goes across the Scottish mainland from Helensburgh to Stonehaven. The Royal Observatory, Greenwich is the defining point of the Prime Meridian.The weather of the United Kingdom is changeable and unpredictable. Summers are moderately warm, winters are cool to cold. Rain falls throughout the year, and more on the west than the east because of its northerly latitude and the warm water from the Atlantic Ocean's Gulf Stream. The usually moderate prevailing winds from the Atlantic may be interrupted by Arctic air from the northeast or hot air from the Sahara.
The United Kingdom is reducing greenhouse gas emissions. It has met the Kyoto Protocol targets. It has signed the Paris Agreement. The British government want the UK to be carbon neutral by the year 2050.
Climate
The United Kingdom has an oceanic climate.
The highest temperature ever recorded in the United Kingdom was 40.3 C F, on 19 July 2022 in Coningsby. The lowest temperature ever recorded was -27.2 C F, on February 11, 1895 & January 10, 1982 in Braemar, and December 30, 1995 in Altnaharra.
Following resumed negotiations a treaty was signed on 22 May 2025 that will formally transfer the sovereignty of the territory to Mauritius once it comes into effect, while the Diego Garcia military base remains under British control during a 99-year lease.
Politics
The United Kingdom is a parliamentary democracy based on a constitutional monarchy. The people of the United Kingdom vote for a members of Parliament to speak for them and to make laws for them. King Charles III is the King of the United Kingdom of Great Britain and Northern Ireland and is the head of state. The government, led by the Prime Minister, governs the country and appoints cabinet ministers. Today, the Prime Minister is Keir Starmer, who is the leader of the centre-left Labour Party.
Parliament is where laws are made. It has three parts: the House of Commons, the House of Lords, and the King. The House of Commons is the most powerful part. It is where Members of Parliament sit.
Scotland has its own devolved Parliament with the power to make laws on things like education, health and Scottish law. Northern Ireland and Wales have their own devolved legislatures which have some powers but less than the Scottish parliament. The Parliament of the United Kingdom is sovereign and it could end the devolved administrations at any time. The UK is a unitary state and not a federation of states.
Parliament
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  Elizabeth II greets NASA GSFC employees, May 8, 2007 edit.jpg
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  Elizabeth II reigned between 1952 until her death in 2022, becoming the longest reigining monarch in UK history. The current monarch is Charles III.
The Parliament of the United Kingdom is the legislature, the political assembly that makes laws and decides tax. The British people are represented by members of parliament (MPs) in the House of Commons of the United Kingdom. MPs are chosen in elections. The MPs in the House of Commons decide who will be the Prime Minister of the United Kingdom. The prime minister decides who will be in the British Government (His Majesty's Government). The government is not controlled by the king or queen, but by Parliament. In Britain, Parliament is made up of the House of Commons and the House of Lords.
Unlike the House of Commons, the people in the House of Lords are not elected. The people who sit in the House of Lords are called peers. Most peers are appointed by the government. There are some who are hereditary peers (who inherit their peerages from ancestors or other family members). Certain bishops in the established Church of England also attend the House of Lords. (The Church of England is the national church in England. The Church of Scotland does not have bishops, and neither Wales nor Northern Ireland has an established national church.) Together, the two houses make a bicameral legislature, in which the House of Commons has more power. In the past, the House of Lords had more power. Before the 20th century, the prime minister was often a member of the House of Lords. As the House of Lords lost its powers, as political reforms tried to improve democracy, the House of Commons became more powerful and the prime minister is now always a member of the House of Commons.
After the English Civil War during the Wars of the Three Kingdoms, Oliver Cromwell became Lord Protector, and the monarchy ended for a time. The British Isles were a republic, which Cromwell named the "Commonwealth of England, Scotland, and Ireland". Although the monarchy was restored after his death, the Crown slowly became the secondary power, and Parliament the first. Until the early twentieth century, only men who owned property could vote to choose MPs. In the nineteenth century, more people were given suffrage. In 1928, all men and women got the vote: this is called universal suffrage.
Almost all members of Parliament belong to political parties. The biggest parties are the Conservative Party, Labour Party, the Scottish National Party and the Liberal Democrats. Members of the same party agree to work together. A party (often with more than half the seats: a majority) forms the government. The leader of the party becomes the prime minister, who then chooses the other ministers. Because the government has a majority in Parliament, it can normally control what laws are passed.
The British Parliament is in Westminster, in London. It has power over the whole of the United Kingdom. Wales, Scotland and Northern Ireland each have their own parliaments as well, and these have more limited powers. England does not have a separate parliament. 
Scotland has the Scottish Parliament at Holyrood in Edinburgh. Wales has the Welsh Parliament in Cardiff. Northern Ireland has the Northern Ireland Assembly at Stormont in Belfast. There are also parliaments in the Isle of Man and in Jersey and Guernsey (the Channel Islands), which are all island states for which the UK has responsibility in international law. Man, Jersey, and Guernsey are "crown dependencies". Some British Overseas Territories have their own legislatures.
 Parts of the UK 
 Countries (nations) 
About 68 million people live in the UK (2022). They can be divided into four big nationalities based on the countries where they live (or where they were born or their ancestry). Each country has a demonym for its people (for example; England's people are English), but no matter which country someone is from, they have a British nationality.
 England is the biggest country, where most people in the UK live. People who live in England are called English. Their native language is called English, which is spoken by most people in England. England's land is split between regions. 
 Scotland, north of England, is the second biggest country. People who live there are called Scottish, and a Scottish person may be called a Scot. Some speak a language other than English: Scottish Gaelic, a Gaelic language. Scottish English, on the other hand, is a version of English. 
 Wales is to the west of England. Its people are called Welsh and they have their own Celtic language which is also called Welsh. Not everyone in Wales can speak Welsh, but almost everyone can speak English.
 Northern Ireland is the smallest country. Unlike the other three countries, it is not on the island of Great Britain: it is part of the island called Ireland. Northern Ireland takes up about a sixth of Ireland (with the Republic of Ireland taking up the remainder). People who live in Northern Ireland are either Irish, British, or Northern Irish. The people who live here speak English.
 Crown dependencies 
The crown dependencies are three nations which are not part of any of the four countries in the UK. They are: the Isle of Man, Jersey and Guernsey. Unlike the four countries, the governments of the crown dependencies have almost full power over the dependencies, with the exception of military and international relations. Everybody from a crown dependency has a British nationality.
 Overseas Territories 
The British Overseas Territories are former colonies of the British Empire which have not become independent from the UK. There are fourteen. Some have civilisations on them while others are military bases. Most of them have their own governments. The UK is responsible for their defence and international relationships. Everybody from an overseas territory has a British nationality.
Military
The United Kingdom has one of the most advanced militaries in the world, alongside such countries such as the USA and France, and operates a large navy (Royal Navy), a sizable army, (British Army) and an air force (Royal Air Force).
From the 18th century to the early 20th century, the United Kingdom was one of the most powerful nations in the world, with a large and powerful navy (due to the fact it was surrounded by sea, so a large navy was the most practical option). This status has faded in recent times, but it remains a member of various military groups such as the UN Security Council and NATO. It is also still seen as a great military power.
Economy
The United Kingdom is a developed country with the sixth-largest economy in the world. It was a superpower during the 18th, 19th and early 20th century and was considered since the early 1800s to be the most powerful and influential nation in the world, in politics, economics and in military strength.
Britain continued to be the biggest manufacturing economy in the world until 1908 and the largest economy until the 1920s. The economic cost of two world wars and the decline of the British Empire in the 1950s and 1960s reduced its leading role in global affairs. The United Kingdom still has strong economic, cultural, military and political influence and is a nuclear power. The United Kingdom holds a permanent seat on the United Nations Security Council, and is a member of the G8, NATO, World Trade Organization and the Commonwealth of Nations. The City of London, in the capital, is famous for being the largest centre of finance in the world.
Literature
William Shakespeare was an English playwright. He did write plays in the late 16th century. Some of his plays were Romeo and Juliet and Macbeth. In the 19th century, Jane Austen and Charles Dickens were novelists. Twentieth-century writers include the science-fiction novelist H. G. Wells and J. R. R. Tolkien. The children's fantasy Harry Potter series was written by J. K. Rowling. Aldous Huxley was also from the United Kingdom.
English language literature is written by authors from many countries. Eight people from the United Kingdom have won the Nobel Prize in Literature. Seamus Heaney is a writer who was born in Northern Ireland.
Arthur Conan Doyle from Scotland wrote the Sherlock Holmes detective novels. He was from Edinburgh. The poet Dylan Thomas brought Welsh culture to international attention.
Education
The nature of education is a devolved matter in Scotland, Wales and Northern Ireland. They have separate, but similar, systems of education with laws that a broad education is required from ages five to eighteen, except for in Scotland where school departure is allowed from the age of sixteen. Pupils attend state funded schools (academy schools, faith schools, grammar schools, sixth form colleges, further education, city technology colleges, studio schools) and other children attend independent fee-paying schools (known as public schools).
There have been universities in Britain since the Middle Ages. The "ancient universities" started at this time and in the Renaissance. They are: the University of Oxford, the University of Cambridge, the University of St Andrews, the University of Glasgow, the University of Aberdeen, and the University of Edinburgh. These are the oldest universities in the English-speaking world.
The University of Cambridge, the University of Oxford, and London universities (University College London, the London School of Economics, King's College London and Imperial College London) collectively form the Golden Triangle of universities in the south-east of England. A broader group of twenty universities form the Russell Group of research universities.
Media
The BBC is an organisation in the United Kingdom. It broadcasts in the United Kingdom and other countries on television, radio and the Internet. The BBC also sells its programs to other broadcasting companies around the world. The organisation is run by a group of twelve governors who have been given the job by the King on the advice of government ministers.
Transport
Road traffic in the United Kingdom drives on the left-hand side of the road (unlike the Americas and most of Europe), and the driver steers from the right-hand side of the vehicle. The road network on the island of Great Britain is extensive, with most local and rural roads having evolved from Roman and Medieval times. Major routes developed in the mid 20th Century were made to the needs of the motor car. The multi-lane high speed motorway (freeway) network was mostly built in the 1960s and 1970s. It links major towns and cities.
The system of rail transport was invented in England and Wales, so the United Kingdom has the oldest railway network in the world. It was built mostly during the Victorian era. At the heart of the network are five long-distance main lines which radiate from London to the major cities and secondary population centres with dense commuter networks and highs-speed lines in the regions. The newest part of the network connects London to the Channel Tunnel from St Pancras station. The system of underground railways in London, known as the Tube, has been copied by many other cities.
Most domestic air travel in the United Kingdom is between London and the major cities in Scotland and the North of England and Belfast. London-Heathrow is the nation’s largest airport and is one of the most important international hubs in the world. Other major airports with principal international service include London-Gatwick, Birmingham, Manchester and Glasgow. An extensive system of ferry networks operates. The Isle of Man and the Channel Islands also have domestic passenger and freight routs.
Languages
Major languages spoken in the United Kingdom other than English include Polish (500,000 approximate number of speakers in the United Kingdom), Eastern Panjabi or Punjabi (471,000), Bengali (400,000), Urdu (400,000), Cantonese (300,000), Greek (200,000), Southwestern Caribbean Creole English (170,000).
Native languages include:
Celtic languages
Brittonic or Bythonic languages
 Welsh
 Cornish
Goidelic or Gaelic languages
 Irish Gaelic
 Scottish Gaelic
 Manx language
Germanic languages
 English
 Scots
 Ulster Scots
Relations with countries and other areas
The UK has foreign relations with many countries.
EU, and the United Kingdom–European Union relations; In 2025, there was a new EU–UK deal, including: a defence and security pact, a fisheries agreement, an agrifood agreement, carbon border taxes, and border security. 
Belarus. The British ambassador was (recalled or) removed by British authorities in 2020; The reason was that (British) authorities were not satisfied with the government of Belarus.
Syria. The British ambassador was (recalled or) removed by British authorities in 2012; The embassy was also closed; The (British) authorities were not satisfied with the government of Syria.
Hong Kong. The UK has foreign relations with some places that are not countries. Hong Kong has an office in London; The trade office is linked (2024) to a case in the justice system in the UK, according to media. The link is supposed to be thru one of the employees of the trade office.


Physics: Physics is a branch of science. It is one of the most fundamental scientific disciplines. The main goal of physics is to explain how things move in space and time and understand how the universe behaves. It studies matter, forces and their effects.
The word physics comes from the Greek word ἡ φύσις, meaning "nature". Physics can also be defined as "that department of knowledge which relates to the order of nature, or, in other words, to the regular succession of events".
Physics is very important in engineering and developing new technologies, such as aviation, electronics and weapons. One reason for starting the mathematical field of calculus was to help develop mechanics, a branch of physics.
Modern physics connects ideas about the four laws of symmetry and conservation of energy, momentum, charge, and parity. 
Astronomy, now a part of physics, is the oldest natural science. In the past it was a part of 'natural philosophy' with other fields of science, such as chemistry and biology. During the scientific revolution, these fields became separate, and physics became a distinct field of knowledge.
 History 
 Ancient astronomy 
Astronomy is one of the oldest natural science. The Sumerians, and Ancient Egyptians studied the stars, mostly with a view to prediction and religion. The first Babylonian star maps date from about 1200 BC. That astronomical events are periodic also dates back to the Babylonians. Their understanding was not scientific, but their observations influenced later astronomy. Much astronomy came from Mesopotamia, Babylonia, Ancient Egypt, and Ancient Greece. Astronomers from Egypt built monuments that showed how objects in the sky moved, and most of the names for the constellations in the Northern hemisphere came from Greek astronomers.
 Natural philosophy 
Natural philosophy started in Greece around 650 BC when a movement of philosophers replaced superstition with naturalism, which refuted the spiritual. Leucippus and his student Democritus suggested the idea of the atom around this period.
 Physics in the medieval Islamic world 
Islamic scholars continued to study Aristotelian physics during the Islamic Golden Age. One main contribution was to observational astronomy. Some, like Ibn Sahl, Al-Kindi, Ibn al-Haytham, Al-Farisi and Avicenna, worked on optics and vision. In The Book of Optics, Ibn al-Haytham rejected previous Greek ideas concerning vision and proposed a new theory. He studied how light enters the eye, and developed the camera obscura. European scientists later built eyeglasses, magnifying glasses, telescopes, and cameras from this book.
 Classical physics 
Physics became a separate field of study after the scientific revolution. Galileo's experiments helped to create classical physics. Although he did not invent the telescope, he used it when he looked into the night sky. He supported Copernicus' idea that the Earth moved around the Sun (heliocentrism). He also investigated gravity. Isaac Newton used Galileo's ideas to create his three laws of motion and his law of universal gravitation. Together these laws explained the motion of falling bodies near the earth and the motion of earth and planets around the sun. 
In a couple centuries, the Industrial Revolution was in full swing and many more discoveries were made in many fields of science. The laws of classical physics are good enough to study objects that move much slower than the speed of light, and are not microscopic. When scientists first studied quantum mechanics, they had to create a new set of laws, which was the start of modern physics.
 Modern physics 
As scientists researched particles, they discovered what classical mechanics could not explain. Classical mechanics predicted that the speed of light varied, but experiments showed the speed of light stayed the same. This was predicted by Albert Einstein's theory of special relativity. Einstein predicted that the speed of electromagnetic radiation through empty space would always be the same. His view of space-time replaced the ancient idea that space and time were quite separate things.
Max Planck came up with quantum mechanics to explain why metal releases electrons when you shine a light at it, and why matter emits radiation. Quantum mechanics applies for very small things like the electrons, protons, and neutrons that make up an atom. People like Werner Heisenberg, Erwin Schrödinger, and Paul Dirac continued to work on quantum mechanics and eventually we got the Standard Model.
 Definition 
Physics is the study of energy and matter in space and time and how they are related to each other. Physicists assume the existence of mass, length, time and electric current and then define (give the meaning of) all other physical quantities in terms of these basic units. Mass, length, time, and electric current are never defined but the standard units used to measure them are always defined. In the International System of Units (abbreviated SI from the French Système International), the kilogram is the basic unit of mass, the metre is the basic unit of length, the second is the basic unit of time, and the ampere is the basic unit of electric current. In addition to these four units, there are three other ones: the mole, which is the unit of the quantity of matter, the candela which measures the luminous intensity (the power of lighting) and the kelvin, the unit of temperature.
Physics studies how things move, and the forces that make them move. For example, velocity and acceleration are used by physics to show how things move. Also, physicists study the forces of gravity, electricity, magnetism and the forces that hold things together.
Physics studies very large things, and very small things. For instance, physicists can study stars, planets and galaxies but could also study small pieces of matter, such as atoms and electrons.They may also study sound, light and other waves. As well as that, they could examine energy, heat and radioactivity, and even space and time. 
Physics not only helps people understand how objects move, but how they change form, how they make noise, how hot or cold they will be, and what they are made of at the smallest level. In short, physics is the branch of science that deals with properties of matter and energy along with the interaction between them.
 Physics and mathematics
Physics is a quantitative science because it is based on measuring with numbers. Mathematics is used in physics to make models that try to predict what will happen in nature. These predictions are compared to the way the real world works. Physicists are always working to make their models of the world better.
 Branches 
Classical mechanics contains major topics such as Newton's laws of motion, Lagrangian mechanics, Hamiltonian mechanics, kinematics, statics, dynamics, chaos theory, acoustics, fluid dynamics, continuum mechanics. Classical mechanics is all about forces acting on a body in nature, balancing forces, maintaining equilibrium state, etc. 
Electromagnetism is study of charges on a particular body. It contains subtopics such as Electrostatics, electrodynamics, electricity, magnetism, magnetostatics, Maxwell's equations, optics. 
Thermodynamics and statistical mechanics are related with temperature. It includes main topics such as Heat engine, kinetic theory. It uses terms such as heat(Q), work(W), and internal energy (U). First law of thermodynamics gives us the relation them by the following equation (ΔU = Q − W)
Quantum mechanics is the study of particle at the atomic level taking into consideration the atomic model. It includes subtopics Path integral formulation, scattering theory, Schrödinger equation, quantum field theory, quantum statistical mechanics.
 Relativity 
 Advanced knowledge 
 General description 
Physics is the science of matter and how matter interacts. Matter is any physical material in the universe. Everything is made of matter. Physics is used to describe the physical universe around us, and to predict how it will behave. 
Physics is the science concerned with the discovery and characterization of the universal laws which govern matter, movement and forces, and space and time, and other features of the natural world.
 Breadth and goals of physics 
The sweep of physics is broad, from the smallest components of matter and the forces that hold it together, to galaxies and even larger things. There are only four forces that appear to operate over this whole range. However, even these four forces (gravity, electromagnetism, the weak force associated with radioactivity, and the strong force which holds protons and neutrons in an atom together) are believed to be different parts of a single force.
Physics is mainly focused on the goal of making ever simpler, more general, and more accurate rules that define the character and behavior of matter and space itself. 
One of the major goals of physics is making theories that apply to everything in the universe. In other words, physics can be viewed as the study of those universal laws which define, at the most basic level possible, the behavior of the physical universe.
 Physics uses the scientific method 
Physics uses the scientific method. That is, data from experiments and observations are collected. Theories which attempt to explain these data are produced. Physics uses these theories to not only describe physical phenomena, but to model physical systems and predict how these physical systems will behave. Physicists then compare these predictions to observations or experimental evidence to show whether the theory is right or wrong.
The theories that are well supported by data and are especially simple and general are sometimes called scientific laws. Of course, all theories, including those known as laws, can be replaced by more accurate and more general laws, when a disagreement with data is found.
 Physics is quantitative 
Physics is more quantitative than most other sciences. That is, many of the observations in physics may be represented in the form of numerical measurements. Most of the theories in physics use mathematics to express their principles. Most of the predictions from these theories are numerical. This is because of the areas which physics has addressed work better with quantitative approaches than other areas. Sciences also tend to become more quantitative with time as they become more highly developed, and physics is one of the oldest sciences.
 Fields of physics 
Classical physics normally includes the fields of mechanics, optics, electricity, magnetism, acoustics and thermodynamics. Modern physics is a term normally used to cover fields which rely on quantum theory, including quantum mechanics, atomic physics, nuclear physics, particle physics and condensed matter physics, as well as the more modern fields of general and special relativity, but these last two are often considered fields of classical physics as they do not rely on quantum theory. Although this difference can be found in older writings, it is of little new interest as quantum effects are now understood to be of importance even in fields that before were called classical.
 Approaches in physics 
There are many ways to study physics, and many different kinds of activities in physics. The two main types of activities are the collection of data, and the development of theories.
Some subfields of physics can be studied by experiment. For example, Galileo Galilei invented kinematics by making experiments and studying the data. Experimental physics focuses mainly on an empirical approach. Some experiments are done to explore nature, and other experiments are performed to produce data to compare with the predictions of theories.
Some other fields in physics like astrophysics and geophysics are mostly observational sciences because most of their data has to be collected passively instead of through experimentation. Galileo, for example, could only look at Jupiter and discover that it has moons. However, observational programs in these fields use many of the same tools and technology that are used in the experimental subfields of physics. 
Theoretical physics often uses quantitative approaches to develop the theories that attempt to explain the data. In this way, theoretical physicists often use tools from mathematics. Theoretical physics often can involve creating quantitative predictions of physical theories, and comparing these predictions quantitatively with data. Theoretical physics sometimes creates models of physical systems before data is available to test and support these models.
These two main activities in physics, data collection, theory production and testing, use many different skills. This has led to a lot of specialization in physics, and the introduction, development and use of tools from other fields. For example, theoretical physicists use mathematics and numerical analysis and statistics and probability and computer software in their work. Experimental physicists develop instruments and techniques for collecting data, using engineering and computer technology and many other fields of technology. Often the tools from these other areas are not quite appropriate for the needs of physics, and need to be changed or more advanced versions have to be made.
It is frequent for new physics to be discovered if experimental physicists do an experiment that current theories cannot explain, or for theoretical physicists to generate theories which can then be put to the test by experimental physicists.
Experimental physics, engineering and technology are related. Experiments often need specialized tools such as particle accelerators, lasers, and important industrial applications such as transistors and magnetic resonance imaging have come from applied research.
 Physicists 
Prominent physicists
yes 20em
Galileo Galilei (1564–1642)
Christiaan Huygens (1629–1695)
Isaac Newton (1643–1727)
Leonhard Euler (1707–1783)
Joseph Louis Lagrange (1736–1813)
Pierre-Simon Laplace (1749–1827)
Joseph Fourier (1768–1830)
Nicolas Léonard Sadi Carnot (1796–1842)
William Rowan Hamilton (1805–1865)
Rudolf Clausius (1822–1888)
James Clerk Maxwell (1831–1879)
J. Willard Gibbs (1839–1903)
Ludwig Boltzmann (1844–1906)
Hendrik A. Lorentz (1853–1928)
Henri Poincaré (1854–1912)
Nikola Tesla (1856–1943)
Max Planck (1858–1947)
Albert Einstein (1879–1955)
Milutin Milanković (1879–1958)
Emmy Noether (1882–1935)
Max Born (1882–1970)
Niels Bohr (1885–1962)
Erwin Schrödinger (1887–1961)
Louis de Broglie (1892–1987)
Satyendra Nath Bose (1894–1974)
Wolfgang Pauli (1900–1958)
Enrico Fermi (1901–1954)
Werner Heisenberg (1901–1976)
Paul Dirac (1902–1984)
Eugene Wigner (1902–1995)
Robert Oppenheimer (1904–1967)
Sin-Itiro Tomonaga (1906–1979)
Hideki Yukawa (1907–1981)
John Bardeen (1908–1991)
Lev Landau (1908–1967)
Anatoly Vlasov (1908–1975)
Nikolay Bogolyubov (1909–1992)
Subrahmanyan Chandrasekhar (1910–1995)
John Archibald Wheeler (1911–2008)
Richard Feynman (1918–1988)
Julian Schwinger (1918–1994)
Feza Gürsey (1921–1992)
Chen Ning Yang (1922– )
Freeman Dyson (1923–2020 )
Gunnar Källén (1926–1968)
Abdus Salam (1926–1996)
Murray Gell-Mann (1929– )
Riazuddin (1930– )
Roger Penrose (1931– )
George Sudarshan (1931– )
Sheldon Glashow (1932– )
Tom W. B. Kibble (1932– )
Steven Weinberg (1933– )
Gerald Guralnik (1936–)
Sidney Coleman (1937–2007)
C. R. Hagen (1937–)
Ratko Janev (1939– )
Leonard Susskind (1940– )
Michael Berry (1941– )
Bertrand Halperin (1941–)
Stephen Hawking (1942–2018 )
Alexander Polyakov (1945–)
Gerardus 't Hooft (1946– )
Jacob Bekenstein (1947–)
Robert Laughlin (1950–)


Chemistry: Chemistry is the science that studies what everything is made of and how it changes. It looks at matter, which is anything that takes up space. Chemistry tries to understand how matter is built, how it behaves, and how it can change. Chemistry explores how tiny particles called atoms and molecules come together, break apart, or rearrange to form new substances. People often call chemistry the “central science” because it connects other sciences like physics (which studies energy and forces) and biology (which studies living things). Chemistry helps explain things all around us, like why iron rusts, how food gives us energy, and how soap cleans our hands.
The history of chemistry is very long. Thousands of years ago, people in places like Mesopotamia, Egypt, India, and China used early forms of chemistry in their daily lives. They practiced metalworking, made dyes for clothes, and learned to ferment food and drinks. These skills were based on watching and experimenting, even though people did not yet understand the science behind them. Later, a practice called alchemy became popular. Alchemists hoped to turn ordinary metals into gold and find a magic liquid that would give eternal life. Even though their ideas were not always scientific, alchemists invented tools and methods that helped future scientists. In the 1600s, chemistry began to change. Scientists like Robert Boyle said that experiments, not just ideas, should be used to understand the world. In the 1700s, Antoine Lavoisier carefully measured chemicals and discovered that matter cannot be created or destroyed, only changed. This became a key idea in modern chemistry. As time went on, scientists developed the atomic theory, created the periodic table, and learned how atoms bond together.
Chemistry is a type of physical science, which means it focuses on understanding matter and how it changes using careful measurements and natural laws. It is closely related to physics, especially in areas like heat and energy (thermodynamics), the behavior of tiny particles (quantum mechanics), and how fast reactions happen (kinetics). These help scientists understand how and why chemical reactions take place. While physics looks for broad, universal rules, chemistry often focuses on how specific molecules and materials behave. Chemists study how atoms come together to form molecules, how energy moves during reactions, and how things like temperature, pressure, and the amount of each substance can affect the speed of a reaction. Using tools and models, chemistry helps us explore both the tiny world of atoms and the larger world of solids, liquids, gases, and plasma. It plays a big role in helping us understand the materials that make up everything around us.
Chemistry is very important in both science and everyday life. It plays a big role in many areas, like medicine, farming, energy, and making new materials. You can find chemistry in everything from the food you eat to the soap you use and the medicine you take when you are sick. Thanks to chemistry, we have clean water, solar panels, plastic, and life-saving drugs. It helps explain everyday things, like why bread rises when you bake it, how soap gets rid of grease, and what happens when your body digests food. Chemists also study pollution, look for better ways to protect the environment, and create new materials that are safer and better for the planet. As the world faces problems like climate change, diseases, and limited resources, chemistry helps us find smart and useful solutions.
Chemistry is not a subject that stands alone. It is closely connected to many other sciences. Chemistry and physics are closely linked because the movement of atoms, energy, and tiny particles like electrons all follow the laws of physics. In biology, chemistry helps us understand how living things work. It explains how enzymes speed up reactions in the body, how DNA carries genetic information, and how cells turn food into energy. This area of science is called biochemistry. In geology, chemistry helps us learn what rocks and minerals are made of, how volcanoes work, and how elements move through Earth’s surface, oceans, and air. Chemistry is also important in environmental science, astronomy, engineering, and making new materials. Because of these connections, chemistry is one of the most important and wide-reaching sciences.
History
Prehistoric Chemistry
In prehistoric times, chemistry was not a real science like it is today. Instead, it was more like a mix of practical skills and simple observations that helped people survive. Early humans learned how to use the materials around them by trying things out and watching what happened. Over time, they began to understand how some things changed. This hands-on experience helped them slowly build a basic understanding of chemical changes, even though they did not know the science behind it. One of the most important achievements of early humans was learning how to control fire. Cooking was one of the most important uses of fire. It made food taste better and easier to digest. Cooking also helped kill germs, which made food safer to eat, especially meat. Since early humans did not have refrigerators, cooking helped food last longer and reduced the chance of getting sick from spoiled or raw meat. Fire was also used for other important tasks. For example, early people discovered that heating the tips of wooden tools made them harder and stronger. They also learned to shape wet clay into pots and bowls, then bake them in fire to make them tough and long-lasting.
By around 5000 BCE, people began to discover a new use for fire, making metal from rocks. This early process was called smelting. It marked the beginning of metallurgy, the science of working with metals. Before this, people mostly used native metals, which are found in nature in pure form, like small lumps of gold or copper. But now, they learned how to get copper out of rocks called ores, such as malachite or azurite. To smelt copper, they had to heat the ore in a very hot fire, hot enough to melt the metal and separate it from the rock. This was not easy. It required skill in building and controlling fire. Sometimes they used tools like bellows to blow air and make the fire hotter. They also had to choose the right rocks and other materials to help the process work better. The result was soft copper, which could be shaped into tools, weapons, and jewelry.
One of the earliest examples of prehistoric chemistry was using natural pigments for cave paintings. People ground up minerals like ochre (which is made of iron oxide), charcoal (which is made of carbon), and manganese dioxide into powders. They mixed these powders with animal fat or water to make paint. People used these paints for rock art and decorating their bodies. Another important skill was tanning hides. This meant soaking animal skins in plants with tannins, like tree bark. This process made the leather last longer, stay flexible, and be useful for making clothes or tools. Fermentation also started during this time. At first, it probably happened by accident when fruit or grains went sour. As early as 7000 BCE, people in China were mixing rice, honey, and fruit to make a kind of early alcoholic drink. At the same time, people in Mesopotamia and Egypt were learning how to brew beer and make bread rise by fermenting dough. These processes used wild yeasts and bacteria to turn sugars into alcohol or carbon dioxide, which changed the taste and texture of the food. Even though they did not know about microorganisms, early humans figured out how to make fermentation work.
Metals like gold, silver, and copper became very special. Gold was rare, shiny and beautiful, and also did not get dull or rust, so it was used for ceremonies and to show status. Silver was shiny and easy to shape, so it was used for jewelry and everyday items. Copper was valuable because it could be made into tools and weapons. People started mining these metals on purpose and trading them. One of the biggest discoveries was mixing copper with tin to make bronze. Bronze was first made about 3300 BCE, during the time between the late Stone Age and the early Bronze Age. The oldest proof of making bronze comes from Mesopotamia, an area that is now Iraq and parts of Syria and Turkey. People there started mixing copper with tin to make a metal that was stronger and lasted longer. This new way of making metal eventually spread to other places like the ancient Near East, Europe, and the Indus Valley. Bronze was stronger and harder than tin or copper alone. This was one of the first times people made a new material on purpose by combining substances. Bronze tools and weapons helped societies grow, build cities, and create professional armies.
Salt was one of the most important chemical substances in early civilization. It was not just used to add flavor to food. It helped people keep meat and fish from spoiling, so they could store food for a long time. This made it possible to travel farther, plan for different seasons, and build towns away from places with fresh food. Because salt was so useful, it became very valuable and was traded over long distances. Some trade routes and even early cities grew around places where salt was found, and in some cultures, salt was even used as money.
Ancient Chemistry
In ancient civilizations, chemistry started to become more organized and intentional. Unlike the earlier times when people learned mostly by guessing and trying things out, ancient cultures began to keep records and follow steps more carefully. Even though they did not know about atoms or molecules, they still learned how to change and use natural materials in useful ways. Civilizations like ancient Egypt, Mesopotamia, India, China, and Greece all made important contributions. They used chemistry for everyday needs like making metal tools, preserving food, and creating medicines. They also used it for spiritual or religious reasons, such as preparing bodies for burial (embalming), making perfumes, or using special materials in ceremonies. These cultures wrote down their ideas and methods on things like clay tablets and scrolls. This helped them improve their techniques and pass knowledge down to future generations.
In ancient Egypt, chemistry was closely connected to religion, medicine, and daily life. The Egyptians were skilled at using natural materials like minerals, dyes, and plant resins, especially when it came to preserving bodies for the afterlife. They used a special salt mixture called natron, made of natural chemicals like baking soda and salt, to dry out dead bodies and prevent decay. They also added oils, herbs, and resins to stop bad smells and protect the body. Egyptians were also talented metalworkers. They knew how to melt down copper ores like malachite in hot furnaces using charcoal to get pure metal. They used metals like gold, copper, and lead to make tools, jewelry, and statues. They also made green eye paint from malachite and black eyeliner (kohl) from a mineral called galena (lead sulfide). These were not just for beauty. They were believed to help protect their eyes from infections and bright sunlight. Egyptians also made perfumes and scented oils by mixing plant parts, resins, and fats. They used early versions of extraction methods, like soaking flowers in oils or using heat. Some even think they may have used very simple distillation to collect scents. One of their greatest chemical achievements was glass-making. As early as 1500 BCE, they were making colorful glass beads and jars using a mix of sand (silica), soda (a type of salt), and lime, all melted together in hot ovens. By adding different metals, they could change the color. Copper made blue, iron made green, and manganese made purple. Chemistry also played a role in Egyptian medicine. Ancient texts like the Ebers Papyrus included hundreds of recipes for medicines made from minerals, herbs, and animal parts. Some treatments were based on beliefs and rituals, but many were surprisingly accurate and showed real knowledge of how the human body reacted to certain substances.
In Mesopotamia, especially in places like Sumer and Babylon, people made important early discoveries in chemistry, mostly through practical work. They wrote down their knowledge on clay tablets using a writing system called cuneiform. These records show in detail how they used different materials. One of their biggest contributions was in metalworking. The Sumerians and Babylonians learned how to get metals like copper, tin, lead, silver, and gold from rocks called ores. They built furnaces hot enough to melt these ores. By mixing copper and tin, they created bronze, a strong metal used for making tools, weapons, and art. This marked the start of the Bronze Age. Pottery and ceramics were also important. They used kilns (hot ovens) to bake clay and to make glazes, which gave pottery shiny, colorful finishes. These glazes were made by mixing minerals and metal powders like copper or iron, which changed color when heated. Pottery was used in homes, temples, and for special objects. Mesopotamians also made progress in perfume-making. Ancient clay tablets from cities like Mari and Babylon describe how they got scents from plants, spices, and tree resins. They may have used early versions of distillation, heating ingredients and collecting the vapor to concentrate smells. These perfumes were used in religious ceremonies, for personal grooming, and sometimes in medicine. In medicine, Mesopotamian healers called asu (practical doctors) and ashipu (spiritual healers) made remedies by mixing minerals, herbs, and animal parts. Some ingredients, like sulfur, salts, and alkalis, had real chemical effects. Their treatments, written on Assyrian tablets, included directions for curing fevers, wounds, and stomach problems, often with both practical steps and magical prayers. Another interesting substance they used was bitumen, a sticky, black natural material similar to modern asphalt. They used it for building, waterproofing, and even to preserve bodies like in mummification.
In ancient India, people had a deep and detailed knowledge of chemistry, especially in medicine and spiritual practices. This knowledge was recorded in early texts like the Vedas and Ayurvedic writings. One major area of chemical knowledge was Ayurveda, the traditional Indian system of medicine. Ayurvedic healers used herbs, minerals, and even animal products to make medicines that matched each person’s body type, known as a dosha. These healers used careful steps like heating, grinding, and washing to prepare them. For example, they used mercury (called rasa) in special medicines called rasaushadhis, often mixing it with sulfur. Ancient Indian alchemy, called Rasayana Shastra, also played a big role. While it had mystical goals like living longer or turning base metals into gold, it led to real scientific progress. They used early chemical techniques like calcination (marana), distillation (patan), sublimation (sublimna), and fermentation (sandhana). They built and used tools such as yantras (distillers), musha (crucibles), and retorts (curved tubes), often made from clay or metal. Metallurgy, the science of working with metals, was another area where ancient India stood out. Indian blacksmiths were some of the first to smelt iron, and by the first millennium BCE, they had invented a special kind of strong steel called Wootz steel. This steel was famous for its strength and was traded all over the world. A great example of their skill is the Iron pillar of Delhi, built around the 4th century CE, which has not rusted much even after 1,600 years. India was also among the first cultures to extract and purify zinc. They worked with metals like gold, silver, copper, and lead. In daily life, Indian chemistry showed up in dyeing, cosmetics, and perfumery. Artisans made colorful dyes from plants and minerals for clothes and art. They made perfumes and incense by collecting scented oils from flowers and herbs using early forms of distillation. These recipes and tools were written down in books like the Brihat Samhita, which explained not just what to use, but how to prepare each substance step by step.
In ancient China, chemistry was closely connected to Taoist beliefs and natural ideas, especially the concepts of yin and yang and the Five Elements: wood, fire, earth, metal, and water. These ideas helped guide how people thought substances changed and interacted with each other. One of the most important parts of Chinese chemistry was alchemy, “the way of the elixir”. Alchemists tried to create special elixirs, magical mixtures that they believed could bring immortality or spiritual powers. They used materials like mercury, sulfur, arsenic, and other minerals and metals. While some of these elixirs were dangerous or poisonous, the work led to better knowledge of how different substances behaved and reacted. For example, Chinese alchemists learned how to heat cinnabar (a mercury mineral) to get liquid mercury. These experiments accidentally led to one of the greatest discoveries in chemistry: gunpowder. Around the time of the Tang Dynasty (7th–10th century CE), Taoist alchemists trying to make an elixir of life mixed sulfur, saltpeter, and charcoal, and instead discovered an explosive powder, gunpowder. This changed warfare forever and eventually spread around the world. The recipe was written down in military books like the Wujing Zongyao (written in 1044 CE). Besides alchemy and gunpowder, the Chinese were also masters of metallurgy. Long before Europe, they had created blast furnaces to make cast iron. They developed strong metal mixtures like bronze (copper and tin) and even steel. These metal tools and weapons helped China grow and succeed. Ancient China also developed medicine. Books like the Shennong Bencao Jing (written around the 1st century CE) listed hundreds of plants, minerals, and animal parts used to treat illness. Medicines were made using chemical techniques like grinding, boiling, fermenting, and roasting. Doctors noticed how different amounts, preparation styles, and combinations of ingredients affected the body, even though they explained it using traditional ideas like qi (life energy) and balance between organs. Finally, ancient Chinese people were experts in ceramics, dyeing, and glass-making. The art of making porcelain used special clay (kaolinite) and very high firing temperatures in kilns. They carefully controlled the heat and mixed minerals to make beautiful glazes. Chinese artists also used metal powders to make colorful finishes, like cobalt for blue, copper for green, and iron for brown or black.
Classical Theories of Matter
In the history of chemistry, classical theories of matter were the early ideas people had about what everything is made of and how it changes. These ideas came from ancient civilizations long before scientists understood atoms like we do today. Even though these early theories did not use experiments like modern chemistry, they helped people start thinking about the nature and structure of matter. These ideas set the stage for many important discoveries later in chemistry.
In ancient Greece, people had different ideas about what matter was made of. One early idea came from Empedocles in the 5th century BCE. He said all matter was made of four basic elements: earth, water, air, and fire. These elements could mix and change because of two forces: love (which brings things together) and strife (which pulls things apart). Later, Aristotle added a fifth element called aether, which he believed made up the sky and stars. He also linked the four elements to qualities like hot, cold, wet, and dry, which helped explain how matter could change. Another important idea came from Democritus and his teacher Leucippus. They said that everything was made of tiny, unbreakable particles called atoms. These atoms were different in shape and size, and how they combined created all the different materials around us. Even though they did not have experiments to prove it, this idea was very similar to what scientists believe today and was very different from Aristotle’s ideas.
In ancient India, there was an idea similar to atoms in a school of thought called Vaisheshika, started by a thinker named Kanada around the 2nd century BCE. They believed that everything is made of tiny, unbreakable particles called “anu” or “paramanu.” These particles combine in different ways to make all kinds of materials. They also created a way to group things based on qualities like taste, color, and how they feel. Even though their ideas were mixed with philosophy and spirituality, they were trying to explain how things change and why there are so many different materials. At the same time, in ancient China, people explained matter using the ideas of yin and yang and the Five Elements (called Wu Xing): wood, fire, earth, metal, and water. These were not elements like we think of them today but were seen as phases or processes that energy goes through. Each element had connections to different qualities, directions, body organs, and natural events. This system helped explain things like nature’s cycles, health, and balance in society, and it influenced Chinese medicine, alchemy, and ideas about the universe for many centuries.
Medieval Chemistry
During the medieval period (about the 5th to 15th century AD), chemistry was not yet a formal science like it is today. Instead, it was mixed with hands-on crafts, spiritual beliefs, and ideas passed down from ancient times. This period was like a bridge between ancient alchemy and the modern science of chemistry that would come later. People during this time used chemical knowledge in many parts of daily life. For example, medicine used herbal mixtures, metal-based compounds, and special liquids called elixirs to treat sickness. These were made through experiments and trial-and-error methods. Metalworking, or metallurgy, was also important. Craftsmen learned how to extract metals like iron, copper, tin, and lead from ores. They used fire and special tools to melt and combine metals, which involved real chemical changes like smelting and making alloys. Other industries like glassmaking and dyeing cloth also depended on chemistry. Workers learned how heat, minerals, and plants could change color or texture. Even though they did not fully understand why it worked, they gained skills and passed them on through guilds and workshops. This time in history helped keep old knowledge alive and slowly improved it, setting the stage for the science of chemistry to grow in the future.
During the medieval period, the Islamic world became one of the most advanced places for chemical knowledge and learning. After the fall of the Western Roman Empire, the Islamic Golden Age (from the 8th to the 14th century) saw a great effort to collect and build upon the science of earlier civilizations. Scholars in cities like Baghdad, Cairo, and Cordoba translated important books from Greek, Persian, and Indian languages into Arabic. But they did not just copy old ideas, they improved them. One famous scientist, Jabir ibn Hayyan (known in Europe as Geber), is often called the “father of Arab chemistry.” He wrote hundreds of books describing different substances and how they reacted with each other. He also created new ways to study materials using experiments. Jabir worked with processes like distillation, crystallization, and calcination. He even helped design tools like alembics and retorts, which were used in early chemistry labs. Jabir tried to understand matter by observing its properties, like how easily it burns or changes. Even though his ideas were influenced by alchemy and religious beliefs, he took important steps toward using experiments to test ideas. His work also introduced many Arabic words for chemicals and tools that later became part of European chemistry. Other Islamic scholars, like Abu Bakr al-Razi (Rhazes) and Al-Tughra'i, continued to build on Jabir’s work. Al-Razi was especially important in medicine. He made antiseptics, acids, and cures for different illnesses. He was also one of the first people to tell the difference between trying to turn metals into gold (a common alchemical goal) and doing useful science to understand how materials behave. These Islamic chemists helped create important tools and methods that were later shared with European scientists. Their books were translated into Latin in places like Sicily and Spain, allowing European scholars to learn from their experiments. This knowledge helped Europe begin its own journey toward modern chemistry.
In medieval Europe, alchemy was shaped by a mix of ancient ideas, Christian beliefs, and mystical thinking. European alchemists believed that changing metals was not just about science, it also had a spiritual meaning. For them, turning ordinary metals into gold was a symbol of purifying the soul and reaching a higher, more perfect state. One of their main goals was to create the Philosopher’s stone, a magical substance they thought could change metals into gold and even give eternal life. Even though alchemy was full of spiritual ideas, it also led to real scientific progress. Important European alchemists like Roger Bacon, Albertus Magnus, and Raymond Lull helped connect mystical ideas with careful observation and early experiments. Roger Bacon, a monk, believed that learning through experiments was important and studied things like light and materials. Albertus Magnus wrote a lot about minerals and metals and helped organize what people knew about them. Raymond Lull used symbols to explain complex ideas, and some of his systems were later used to describe chemical reactions.
During the Middle Ages, the translation of books played a big role in helping chemistry grow in Europe. As Europeans came into more contact with the Islamic world, through trade, the Crusades, and the reconquest of Spain, they discovered many scientific texts written in Arabic. These included both translations of ancient Greek writings and original works by Muslim scientists. Cities like Toledo in Spain and Palermo in Italy became important places where scholars translated Arabic texts into Latin. Thanks to this, the ideas of famous Islamic chemists like Jabir ibn Hayyan and Abu Bakr al-Razi became known across Europe. Their work helped inspire European scientists and planted the seeds for future experiments. At the same time, monasteries in Europe were important for keeping and sharing knowledge. Even though monks mainly focused on religion, they also worked on tasks that used chemical skills. For example, making fancy books called illuminated manuscripts required knowledge about how to make colorful inks from minerals and plants. Monks also brewed beer, made wine, preserved food, and prepared herbal medicines. These activities were based on chemical knowledge, even if the monks did not call it “chemistry.” Craft workers in cities also helped chemistry grow through hands-on work. People like blacksmiths, potters, glassmakers, tanners, dyers, and brewers learned how to change raw materials into useful things. They melted metals, made shiny glazes for pottery, created dyes from plants and bugs, and controlled how things fermented. These workers learned by doing, often as apprentices in guilds. Even though they were not scientists, they used chemistry in their everyday jobs and passed their knowledge down through generations.
By the late Middle Ages, as Europe moved into the early modern period, alchemy started to change. People were becoming less interested in trying to make gold or find ways to live forever, because those goals rarely worked. Instead, more people began to focus on real results. Things they could see, test, and use in everyday life. One important thinker during this time was Paracelsus, a doctor in the 1500s. He believed that alchemy should be used to help people, especially in medicine. He used minerals like mercury, arsenic, and antimony in treatments, substances that were usually feared. Paracelsus is famous for saying, "The dose makes the poison," which means that even dangerous substances can be helpful if used in the right amount. This idea became an important rule in medicine and toxicology. Paracelsus also believed that people should learn about nature by observing and experimenting, not just by reading old books. He studied how chemicals reacted and how they affected the human body. This helped move chemistry away from mystical ideas and closer to real science. Another key figure was Andreas Libavius, a German doctor and chemist. He believed that knowledge should be shared clearly and openly. In 1597, he wrote Alchemia, one of the first chemistry textbooks. In it, he explained tools, techniques, and substances in a simple and organized way. This was very different from the secret codes and symbols used by older alchemists.
The Birth of Chemistry
The birth of classical chemistry in the 1500s and 1600s was a big turning point. During this time, chemistry began to move away from the old mystical ideas of alchemy and become a more careful, scientific way of understanding the world. This change happened during the Scientific Revolution, a time when people started to focus more on observing nature, doing experiments, and thinking for themselves instead of just trusting old books and beliefs. Classical chemistry did not appear overnight. It grew slowly as scientists developed better ways to test ideas, improved their lab tools, and began to think differently about matter and how it changes. One important change was that scientists started to question the old idea that everything was made of four or five elements, like earth, water, fire, and air. Instead, they looked more closely at how substances were made and how they reacted in real-life experiments. This new way of thinking helped turn chemistry into a true science, based on facts, testing, and clear thinking.
Robert Boyle was an important figure in the history of chemistry. His book The Sceptical Chymist, published in 1661, is often seen as the moment when chemistry started to move away from old alchemical ideas and become a true science. He challenged long-held beliefs about what matter is made of and how it changes. At the time, many people still believed in the old idea that everything was made of four elements: earth, air, fire, and water. Boyle disagreed. Instead, he suggested that all matter is made of tiny particles, which he called "corpuscles." These particles could combine in different ways to form all the materials in the world. This idea was an early step toward what we now call atomic theory. One of Boyle’s biggest contributions was his new definition of a chemical element. He said an element is a substance that cannot be broken down into anything simpler by chemical methods. Even though he did not know about all the elements we recognize today, his definition helped future scientists, like Antoine Lavoisier, build a better system for understanding them. Boyle was also a strong supporter of careful, repeatable experiments. He believed scientists should write down exactly what they did and what happened, so others could try the same experiments and check the results. This idea of clear, honest reporting is a key part of how science works today. Unlike many alchemists who kept their findings secret, Boyle wanted to share knowledge. He helped start the Royal Society of London, a group that encouraged scientists to share ideas and discoveries openly.
During the 1500s and 1600s, as chemistry was changing, the tools and techniques used in the laboratory improved a lot. Old alchemical tools like retorts, alembics, crucibles, and furnaces were still used, but they were made better. A retort was often made of glass and metal. It was used to heat liquids and collect their vapors, and helped scientists separate parts of a mixture by heating it. An alembic was another tool that helped separate and cool down gases, turning them back into liquids. Over time, alembics were made with better seals and more consistent shapes so they worked better and gave more reliable results. Crucibles were small containers used to heat materials to very high temperatures. They were often made from strong ceramics that could handle the heat and be used many times, which was especially important when working with metals and minerals. Many chemical processes were also becoming clearer and more organized. For example, distillation was used to purify substances like alcohol or perfumes. Filtration was used to separate solid bits from liquids using cloth, paper, or fine mesh. Precipitation meant making a solid appear out of a liquid by mixing certain chemicals, useful for finding out how different substances react. Crystallization helped scientists purify salts and study the shapes of crystals. Glassmaking became very important too. In places like Venice and other parts of Europe, people learned to make clear, heat-resistant glass. This allowed scientists to see what was happening inside their experiments. Tools like flasks, beakers, funnels, test tubes, and condensers became more uniform, meaning they looked and worked the same in different labs. This made it easier for scientists to copy each other’s work and learn together. New measuring tools also made a big difference. Scientists started using balances (scales) that were more sensitive, which helped them measure materials more accurately. This led to the early idea that the total amount of matter stays the same during a reaction, a key idea in modern chemistry. Thermometers, improved by people like Galileo, let scientists track temperature changes during experiments. With better tools and more exact methods, chemistry became less about guessing and more about testing, recording, and sharing results.
In the world of medicine, a new idea called iatrochemistry began to change how people thought about health and healing. Iatrochemistry combined chemistry with medicine and was based on the bold ideas of a scientist named Paracelsus. At the time, most doctors believed in humoral theory, which said sickness came from imbalances in body fluids. But Paracelsus disagreed. He believed that diseases came from outside the body or from chemical problems inside it, and that they should be treated with chemical medicines, not just herbs or bloodletting. Paracelsus introduced the idea of using minerals and man-made substances to treat illnesses. This opened the door to pharmacology, the science of making and testing medicines. By the 1600s, doctors and chemists started working together more closely. They tested materials like antimony, mercury, arsenic, and different salts and tinctures to see if they could cure diseases. Some of these early treatments were harmful or did not work well, but the process of testing, observing, and improving them helped medicine become more scientific. Apothecaries and early pharmacy labs became important places where these treatments were studied and made. At the same time, growing industries in Europe helped chemistry move forward in other ways. As people needed more metals, dyes, ceramics, textiles, and glass, workers had to learn more about how materials changed and reacted. For example, mining and metalwork required people to understand how to find and melt metal ores, how to mix metals into alloys, and how to deal with waste. These jobs needed a good understanding of heat and chemical reactions, so chemists began studying materials more carefully. In the textile and dye industries, creating bright, long-lasting colors for fabrics took a lot of experimenting. Workers used substances like alum, indigo, madder, and cochineal (a red dye made from insects) and had to figure out how to make the colors stick to the cloth. Glassmakers and potters also used chemistry to improve their crafts. They learned to mix silica, soda, and lime in just the right amounts to make clear or colorful glass. By adding small amounts of metals like cobalt for blue, copper for green, or gold for red, they could make beautiful glassware. In trying to copy Chinese porcelain, European makers studied how clays and glazes behaved when heated in kilns, leading to more discoveries in chemistry.
One of the most important changes in science during the 1600s was the creation of scientific societies. These were groups where scientists could meet, share their work, and learn from each other. One of the first and most famous of these groups was the Royal Society of London, founded in 1660. It brought together people like doctors, inventors, and scientists who were all interested in studying nature in a careful, organized way. The Royal Society had a special motto: "Nullius in verba," which means "take nobody’s word for it." This meant they believed in evidence and experiments, not just trusting old books or famous people. Scientists had to prove their ideas through testing and clear results. This approach was a big step forward in turning chemistry into a serious science. Similar groups started in other countries too. In France, the Académie des Sciences was created in 1666. These societies gave chemistry a new home in public places where experiments could be watched, repeated, and discussed by others. Scientists now had places to show their work, get feedback, and improve their ideas based on what others had done. Another big change was the rise of scientific journals. These were like magazines for scientists to publish their discoveries. The Royal Society started one of the first, called Philosophical Transactions of the Royal Society, in 1665. It helped chemists share their methods, tools, and results with other scientists across Europe. This helped science move faster because others could repeat experiments, test new ideas, or improve old ones. Books like Physica Subterranea by Johann Joachim Becher and writings by Georg Ernst Stahl (who supported the phlogiston theory) showed how print helped scientists share and debate their ideas, even when they did not agree. In addition to books and journals, letters were very important. Scientists like Robert Boyle, Antoine Lavoisier, Isaac Newton, and Robert Hooke often wrote to each other, sharing drawings, experiments, and new ideas. These letters helped build an international community of scientists, even before the internet or telephones existed. At first, chemistry was not seen as important as math or physics. Some people thought it was too close to the strange ideas of alchemy. But chemistry began to earn respect because it was useful and connected to the Age of Enlightenment, a time when people believed in reason, progress, and discovery. Slowly, universities began teaching chemistry, and students got to learn in real labs, not just from books. This helped turn chemistry into a respected professional science, just like it is today.
The Chemical Revolution
At the start of the 18th century, chemistry still relied on old ideas from alchemy and natural philosophy. One of the most famous ideas was the phlogiston theory, first created by Johann Joachim Becher and later improved by Georg Ernst Stahl. This theory said that all things that could burn had a special substance called phlogiston inside them. Phlogiston was thought to be invisible and weightless, and was released when something burned. According to the phlogiston theory, when a material burned or when metal rusted, it was losing phlogiston into the air. For example, when wood turned into ash or iron became rust, people believed the phlogiston left the material, and what was left behind was called “dephlogisticated.” This idea also tried to explain other things, like how animals breathe, thinking that breathing was the slow release of phlogiston from the body. The phlogiston theory seemed to explain many observations about fire and burning, so it became very popular in European chemistry for many years. However, no one could ever find or measure phlogiston directly. Scientists could not isolate it or prove it was real. Sometimes they had to change the idea to fit what they saw. For example, when metals actually gained weight after burning, they said phlogiston had "negative weight," which did not make sense.
One of the biggest changes in 18th-century chemistry was the rise of pneumatic chemistry, which is the study of gases and their properties. Chemists like Stephen Hales helped start pneumatic chemistry by inventing special tools to catch and measure gases. He used a method to collect gases over water, which let scientists isolate different kinds of “airs” (what people called gases back then). This was a big deal because it gave chemists a way to study gases carefully and learn how they acted. Building on this work, Joseph Black made an important discovery with a gas called carbon dioxide, which he called “fixed air.” Black showed that fixed air was different from normal air. He found that this gas could put out flames and that it reacted with limewater to make a solid. These tests proved that fixed air had its own special chemical properties. Black’s discovery showed that air was not just one thing but a mix of different gases.
The study of gases led to many important discoveries. Henry Cavendish found hydrogen, which he called “inflammable air” because it burned easily with a pale blue flame. Cavendish also showed that when hydrogen burns, it combines with oxygen to make water. This was a big surprise because people used to think water was a basic element. Cavendish proved that water is actually made of two gases, which changed how scientists understood chemistry. At the same time, two chemists, Joseph Priestley and Carl Wilhelm Scheele, both discovered oxygen. They found this gas independently, but Priestley was the first to publish his results. He called it “dephlogisticated air” because he believed in the phlogiston theory, thinking oxygen was air that had lost something called phlogiston. Scheele probably found oxygen first but published later. He described oxygen’s strong ability to help things burn and support breathing, though he also still believed in the old phlogiston ideas. Even though their ideas about phlogiston were wrong, these discoveries of oxygen, hydrogen, and carbon dioxide changed chemistry forever. These findings helped lead to the end of the phlogiston theory and the start of modern chemistry.
Even though scientists had made big discoveries about gases and started to question the phlogiston theory, it was a French chemist named Antoine-Laurent de Lavoisier who finally proved the theory wrong and helped create modern chemistry. Lavoisier showed that burning and breathing were not about releasing an invisible substance called phlogiston. Instead, these were chemical reactions where things combined with oxygen, which is a part of air. Lavoisier did careful experiments where he weighed things before and after chemical reactions. He found out that the total weight stayed the same, even though the substances changed. This idea is called the conservation of mass, meaning matter cannot be made or destroyed, only changed into something new. This was a brand-new way of thinking that went against the old phlogiston ideas.
Besides his important experiments, Lavoisier knew it was necessary to have a clear and simple way to name chemicals. Before his work, chemical names were confusing because the same substance could have different names in different places. In 1787, Lavoisier and his helpers, like Berthollet and Guyton de Morveau, created a system to name chemicals based on what they were made of and their properties. This made it easier for scientists to talk and work together. In 1789, Lavoisier published a famous book called Traité Élémentaire de Chimie. This book is considered the first modern chemistry textbook. In it, he clearly explained what chemical elements are, substances that cannot be broken down into simpler parts by chemical means. He also listed many elements known at the time, such as oxygen, hydrogen, nitrogen, and some metals. This helped create the idea of elements that chemists still use today. Lavoisier’s work also led to a new way of studying chemistry called stoichiometry. This means understanding the exact amounts of substances used and produced in chemical reactions. He introduced balanced chemical equations that showed these reactions in symbols and numbers. Because of this, chemistry became a science based on measurements and math, making it easier to predict and repeat experiments.
The Rise of Modern Chemistry
In the 19th century, a big change in chemistry was the development of atomic theory by John Dalton, an English teacher and chemist. In 1803, Dalton said that everything is made of tiny, invisible particles called atoms. Each element is made of its own kind of atom, and each kind has a specific weight. Dalton’s idea was based on careful observations and experiments. He also explained that atoms combine in simple, whole-number amounts to form compounds. This helped explain why compounds always have the same kinds of atoms in the same amounts. For example, carbon and oxygen make two different compounds: carbon monoxide (CO) and carbon dioxide (CO₂). In these compounds, the amount of oxygen compared to carbon is always in simple ratios like 1 to 2. Dalton’s work supported the idea that matter is made of small, separate particles. He even made one of the first lists of atomic weights, comparing everything to the weight of hydrogen atoms. Dalton also used special symbols to stand for atoms and molecules, which later became the chemical symbols we use today.
In the 19th century, there was a big change in how scientists understood organic chemistry. For a long time, people believed in something called vitalism. This was the idea that organic compounds, chemicals found in living things, could only be made by living organisms because of a special "vital force." They thought these compounds could not be made in a lab. But in 1828, a German chemist named Friedrich Wöhler did an important experiment that changed everything. He heated a simple chemical called ammonium cyanate, which is inorganic (not made by living things), and it turned into urea, a compound found in the urine of living organisms. This showed that organic compounds could be made in the lab without any living organism involved. Wöhler’s discovery opened the door for scientists to study and create many more organic compounds. Chemists started working with a wide range of carbon-based substances like alcohols, acids, dyes, and medicines. One important result of Wöhler’s work was a new focus on chemical structure. Scientists discovered that certain groups of atoms, called functional groups, determine how molecules behave. Chemists like Justus von Liebig and August Kekulé helped develop ideas about how atoms bond together in molecules. Kekulé, for example, suggested the famous ring shape of the benzene molecule in 1865.
In the 1800s, one of the biggest breakthroughs in chemistry came from a Russian scientist named Dmitri Mendeleev. In 1869, Mendeleev created the periodic table which completely changed how scientists understood the building blocks of matter. At that time, about 63 elements were known, but they seemed like a random list with no clear pattern. Mendeleev had a smart idea. He arranged the elements in rows and columns based on their atomic weights and how they reacted chemically. When he did this, he noticed a repeating pattern in their properties. He called this the “periodic law”. What made his table special was that he left blank spaces where he thought unknown elements would one day fit. He even predicted what these missing elements would be like, what their atomic weights would be and how they would behave. Later, scientists discovered elements like germanium, scandium, and gallium, and they turned out to be almost exactly as Mendeleev had predicted.
In the 1800s, chemistry made big leaps forward, especially in the areas of analysis and energy. One of the most exciting new tools was spectroscopy, which was developed in the 1850s by two scientists named Robert Bunsen and Gustav Kirchhoff. They discovered that when elements are heated or excited, they give off light in unique patterns, like fingerprints. These patterns, called spectral lines, helped scientists identify which elements were present in a substance just by looking at the light it gave off. Using this new technique, Bunsen and Kirchhoff found two new elements, cesium in 1860 and rubidium in 1861, by their special spectral lines. Spectroscopy did not just help on Earth; it also helped scientists study stars. By looking at the light from stars, they could figure out what elements the stars were made of, even from millions of miles away. Spectroscopy started helping astronomers in the early 1800s. Scientists noticed that when sunlight passed through a special glass called a prism, it created a rainbow of colors, but with some dark lines missing. In 1802, W. H. Wollaston saw these dark lines, and in 1815, Joseph von Fraunhofer studied them more closely. These dark lines were called absorption lines, and they were very important. They showed that certain colors of light were being absorbed by gases in the Sun’s outer layers. This gave scientists their first clues about what the Sun was made of. Later, the same method helped astronomers figure out the chemical makeup of other stars, too. At the same time, chemists were also learning more about how energy works in chemical reactions. A scientist named Germain Hess came up with Hess’s Law, which says that the total energy change in a chemical reaction is the same, no matter how many steps it takes. Later, James Prescott Joule made another big discovery. He showed that heat and mechanical energy (like moving parts) are really just different forms of the same thing. His famous experiment with a paddle wheel helped prove this. These ideas became part of the first law of thermodynamics, which says that energy cannot be created or destroyed, it can only change form.
The Industrial Revolution in the 1800s brought big changes to the world, and chemistry played a major role in making that happen. One of the biggest breakthroughs was in steelmaking. In the 1850s, Henry Bessemer invented a process to turn melted iron into strong steel by blowing air through it to remove unwanted materials. This Bessemer process made steel much cheaper and easier to produce. As a result, steel was used to build railroads, ships, bridges, and buildings all over the world. At the same time, chemistry helped create synthetic dyes. In 1856, a young chemist named William Henry Perkin was trying to make medicine but accidentally created a bright purple dye called mauveine. This dye was the first of its kind and made colorful clothing affordable for many people. Perkin’s discovery started a new industry, and soon many companies were making different kinds of synthetic dyes for clothes and fabrics. Explosives were another area where chemistry made a big impact. Chemists like Ascanio Sobrero and Alfred Nobel developed powerful materials such as nitroglycerin and dynamite. These explosives were used in mining, building tunnels, and even in war. Nobel’s inventions made blasting safer and more effective. He later created the Nobel Prizes to honor great achievements in science and other fields. Chemistry also helped farmers by improving fertilizers. Scientists like Justus von Liebig discovered that plants need certain minerals to grow well. This led to the creation of artificial fertilizers like superphosphates and ammonium salts, which helped crops grow faster and better. These fertilizers supported a growing population and helped feed more people. Finally, chemistry changed medicine. Scientists learned how to take useful chemicals from plants and make them in labs. Important medicines like aspirin, morphine, and quinine were produced in large amounts, making them more available to people around the world. This helped save lives and treat diseases more effectively.
During the 1800s, chemistry became a more organized and respected science. One of the biggest changes was that chemistry started to be taught seriously at universities. Before this time, most science was done by individuals working on their own. But now, schools in Europe and North America began to understand that experimental science, like chemistry, could help improve society and make countries stronger. Chemistry got its own professors (called university chairs) and special laboratories where students could learn both ideas and hands-on experiments. One of the most famous schools for this was the University of Giessen in Germany, where Justus von Liebig started a chemistry lab in the 1820s. His lab became a model for others around the world. Liebig believed students should learn by doing research and experiments, not just by listening to lectures. Many important chemists were trained there, and the idea of the "chemical laboratory" became common in schools. Outside of schools, scientific societies also helped chemistry grow. In 1841, the Chemical Society of London was founded. It gave chemists a place to share their work, honor great discoveries, and influence government decisions. Similar groups formed in other countries, like the Deutsche Chemische Gesellschaft in Germany. These groups helped chemists stay connected and work together. Another important part of this time was the rise of science journals. Chemists needed a way to share their discoveries with others. In 1832, Liebig started a journal called Liebigs Annalen der Chemie, where chemists could publish their findings. It was carefully reviewed by other scientists, which helped make the information trustworthy. As more journals were created, new discoveries spread faster and helped science move forward. As chemistry became more organized, it also became more of a professional career. Chemists were no longer just curious hobbyists or workers in factories, they were seen as trained professionals with special skills. Companies started building their own industrial labs and hiring chemists to help make products like medicine, fabric dyes, and metals. This helped turn chemistry into an important part of both science and industry.
20th Century Chemistry
In the 20th century, chemistry changed a lot because scientists made important discoveries about atoms and even smaller parts inside them. Before, people thought atoms were the smallest pieces of matter and could not be broken down. But in 1897, J.J. Thomson discovered the electron, a tiny negatively charged particle inside the atom. This showed that atoms were made of smaller parts. Thomson created the “plum pudding” model, imagining electrons like little raisins stuck inside a positively charged pudding. Not long after, in 1911, Ernest Rutherford did the famous gold foil experiment. He found out that the atom is not like a pudding but has a tiny, dense, positively charged center called the nucleus, with electrons moving around it mostly through empty space. In 1913, Niels Bohr improved this idea. He said electrons move in specific paths or shells around the nucleus. They can jump between these shells by gaining or losing energy. This idea helped explain why atoms give off certain colors of light, called spectral lines. However, Bohr’s model did not work well for bigger atoms, so scientists kept exploring. In the 1920s and 1930s, a new field called quantum mechanics was created. Erwin Schrödinger suggested that electrons behave like waves, and their positions can only be described by probabilities, not exact locations. Werner Heisenberg added that we cannot know both an electron’s exact position and speed at the same time. This is called the Uncertainty Principle. These ideas came together into the modern quantum mechanical model of the atom, which is the foundation of chemistry today. It helps explain not only how atoms look and behave but also why elements are arranged in the periodic table, how atoms stick together in chemical bonds, and the shapes of molecules.
In 1913, Henry Moseley used X-rays to study atoms and discovered that an element’s place in the periodic table should be based on its atomic number (the number of protons in its nucleus), not its atomic weight. This helped fix problems in the old version of the table and made it more accurate. Moseley’s work also helped scientists find missing elements by spotting gaps in the sequence of atomic numbers. At the same time, scientists were learning about radioactivity. In 1896, Henri Becquerel discovered that uranium could give off invisible rays without any outside energy. Later, Marie and Pierre Curie studied this further and discovered new radioactive elements like polonium and radium. These elements gave off energy because their atoms were unstable and could break down on their own. This showed that atoms were not unchangeable after all. These discoveries led to a new field called nuclear chemistry, which focuses on the reactions that happen in an atom’s nucleus. In the 1930s, scientists like Otto Hahn, Fritz Strassmann, Lise Meitner, and Otto Frisch discovered nuclear fission, a process where atoms like uranium split into smaller parts and release a huge amount of energy. This discovery had a huge impact. During World War II, the U.S. used nuclear fission to build the atomic bomb in a secret project called the Manhattan Project. These bombs caused massive destruction and changed the world forever. After the war, people also began using nuclear fission to make electricity in nuclear power plants. Later, scientists studied nuclear fusion, the reaction that powers the sun. Fusion could give us cleaner and safer energy, but we are still working on how to control it on Earth.
Another important breakthrough in 20th-century chemistry was the rise of physical chemistry. Physical chemistry used the principles of physics to understand matter. In the United States, an early supporter of this field was Ira Remsen, who helped improve science education by promoting hands-on experiments in chemistry classes. This helped train a new generation of scientists who were both skilled and curious. One of the most important figures in physical chemistry was Gilbert N. Lewis. He came up with the idea that atoms bond by sharing pairs of electrons. He also created the Lewis dot structure, a simple way to show how atoms are connected. His work helped people understand things like covalent bonds (where atoms share electrons), acids and bases, and chemical energy. Later, in the 1930s, Linus Pauling took these ideas even further by using quantum mechanics to explain chemical bonds. He introduced the concept of hybridization, which helped explain the shapes of molecules, like the four-cornered tetrahedral shape of methane. Pauling also created the electronegativity scale, which shows how strongly atoms attract electrons when they bond. Pauling’s impact went beyond theory. He was also a leader in using tools like X-ray crystallography and spectroscopy to figure out the exact 3D shapes of molecules. These tools became especially important in biochemistry. Pauling used them to study proteins and discovered their basic structures, such as the alpha helix and beta sheet. His work helped start the field of structural biology, which looks at how the shape of a molecule affects what it does in the body.
In the 20th century, organic chemistry, grew very quickly and made a huge impact on the world. One of the biggest changes was that chemists invented new ways to build complicated molecules with great care and precision. These new methods, like the Grignard reaction or Wittig reaction, gave scientists powerful tools to create all kinds of useful chemicals, including medicines and materials. One important part of this progress was the rise of polymer chemistry, which led to the invention of new materials called plastics. In 1935, a chemist named Wallace Carothers at DuPont created nylon, the first completely man-made fiber. This showed that large, chain-like molecules (called polymers) could be designed to have special properties, like strength or flexibility. After nylon, scientists quickly made other important materials, such as polyethylene, polystyrene, PVC (polyvinyl chloride), Teflon, and polyester. These plastics changed everyday life. They were used in packaging, clothing, cars, buildings, and much more. At the same time, pharmaceutical chemistry, the science of making medicines, grew rapidly. In the early 1900s, chemists had already made useful drugs like aspirin and barbiturates, but the discovery of penicillin in 1928 by Alexander Fleming changed medicine forever. Penicillin could fight deadly bacterial infections, and its mass production helped save countless lives, especially during World War II. After that came sulfa drugs, the first widely used antibiotics, which helped treat many infections even before penicillin was common. Later in the century, scientists made even more important medicines, including antiviral drugs for diseases like HIV/AIDS, chemotherapy drugs to treat cancer, and medications for mental health and brain disorders. New advancements in stereochemistry and chiral synthesis made medicines safer and more effective. In the final decades of the century, techniques like combinatorial chemistry, high-speed drug testing, and computer-aided drug design helped scientists discover new medicines much faster.
In the middle of the 20th century, chemistry began to focus more on understanding the molecules that make up living things. One of the most important discoveries during this time was the structure of DNA, made by James Watson and Francis Crick in 1953. They used X-ray images taken by Rosalind Franklin and base-pairing rules discovered by Erwin Chargaff to figure out that DNA is shaped like a double helix, two strands twisted around each other. These strands are held together by base pairs. Adenine pairs with thymine, and guanine pairs with cytosine. This discovery helped connect chemistry with biology and gave rise to a new field called biochemistry, which studies how chemicals work inside living things. Scientists began to study important molecules like proteins, carbohydrates, lipids, and nucleic acids in more detail. They used special tools like X-ray crystallography, NMR (nuclear magnetic resonance), and mass spectrometry to see the shapes and structures of these molecules. Researchers also started to understand how enzymes speed up chemical reactions in the body, how cells communicate, and how metabolism works to give us energy. This new knowledge helped create another field called molecular biology, which focuses on how genes work and how cells use DNA to make proteins. In the 1970s, scientists developed ways to cut and paste DNA, creating a powerful technique called recombinant DNA technology. This let them move genes from one organism to another, helping launch the field of genetic engineering. Later, in the 21st century, scientists created an even more precise gene-editing tool called CRISPR-Cas9, which allows for direct changes to DNA. These discoveries changed many areas of life. In medicine, they led to new drugs, gene therapies, and personalized treatments based on a person’s DNA. In farming, they helped create genetically modified organisms (GMOs) that grow better and resist pests. In science, they gave researchers powerful tools to explore how life works, including how we inherit traits and how species evolve.
In the 20th century, analytical chemistry changed a lot because of new and powerful tools that helped scientists study chemicals more carefully and accurately. Before this time, scientists mostly used older methods called "wet chemistry," which involved mixing chemicals, making them change color, or forming solids to find out what was in a sample. These methods worked okay but were not precise enough for the harder problems chemists faced in fields like organic chemistry, biochemistry, and materials science. One big step forward was the invention of the mass spectrometer. This tool measures the mass and charge of tiny particles called ions. Scientists like Francis Aston and Arthur Jeffrey Dempster helped improve mass spectrometry, which became super useful for finding out the weights of molecules, discovering different forms of atoms (isotopes), and identifying unknown chemicals. Mass spectrometry is so sensitive it can detect even tiny amounts of substances, down to billionths or trillionths of a gram. Another important tool was nuclear magnetic resonance (NMR) spectroscopy, first shown in the 1940s and widely used by the 1950s and 1960s. NMR lets chemists study the magnetic properties of atoms like hydrogen and carbon in molecules. This helps them understand the exact structure of molecules, how atoms bond, and how molecules move around in liquids. Later, more advanced versions of NMR allowed even deeper study of complex molecules, especially in drug research and organic chemistry. X-ray crystallography also became very important during the 20th century. This technique uses X-rays to find the three-dimensional shapes of molecules with amazing detail. At first, it was used on simple crystals, but later scientists could use it to study big molecules like vitamins, hormones, and proteins. For example, Rosalind Franklin’s X-ray images helped explain the famous double-helix structure of DNA. Over time, X-ray crystallography helped scientists learn about many complicated biological molecules, which is very important for understanding how our bodies work and for making new medicines. Other helpful tools include infrared spectroscopy, which helps identify parts of molecules by measuring vibrations, and ultraviolet-visible (UV-Vis) spectrophotometry, which helps chemists watch chemical reactions and study certain types of molecules. Together, these instruments made it possible for chemists to study chemicals in ways they never could before, speeding up discoveries and helping create new materials, medicines, and technologies.
By the late 20th century, the chemical industry got much bigger, including not only old areas like medicine and dyes but also new ones like petrochemicals (chemicals made from oil), agrochemicals (chemicals for farming), and advanced materials. One big change came from petrochemistry. Scientists learned how to turn crude oil into many useful chemicals. These chemicals were then used to make plastics, synthetic rubber, and solvents. These new materials changed everyday things like packaging, buildings, cars, and many other products. Another very important invention was the Haber process (also called the Haber-Bosch process). It was developed in the early 1900s by Fritz Haber and later made into a big industrial method by Carl Bosch. This process uses high pressure, high temperature, and a special iron catalyst to turn nitrogen gas from the air and hydrogen gas into ammonia (NH₃). Ammonia is a key ingredient in nitrogen fertilizers. The Haber process had a huge impact on farming. It allowed factories to make nitrogen fertilizers in large amounts, which helped farmers grow much more food. This helped feed the rapidly growing world population and played a big role in the Green Revolution, a time when farming became much more productive. Today, more than half of the people on Earth depend on food grown with fertilizers made using the Haber process. Without it, natural soil would not have enough nitrogen to grow enough food for everyone.
In the field of energy, chemists made big progress in creating new ways to produce cleaner and better power, so people would not have to rely so much on fossil fuels like oil and coal. One important invention was the fuel cell, which changes chemical energy directly into electricity using reactions between hydrogen and oxygen. Although the idea of fuel cells started way back in 1839 with Sir William Grove, it was not until the mid-1900s that fuel cells became practical. In the 1960s, companies like General Electric built hydrogen-oxygen fuel cells to power NASA’s spacecraft because they were reliable and clean. Later, in the 1990s and 2000s, fuel cells became important for clean energy cars and power stations. Chemists worked on making better catalysts, materials like platinum, that helped fuel cells work faster and last longer. At the same time, batteries got much better thanks to advances in chemistry and materials science. Early batteries, such as the lead-acid battery invented in 1859, were heavy and did not store much energy. Nickel-cadmium batteries, made in the early 1900s, could be recharged but had problems like toxicity and losing capacity over time. A big breakthrough happened in the 1980s when John B. Goodenough and his team created the lithium cobalt oxide cathode, which led to the lithium-ion battery. Sony started selling these batteries in 1991. Lithium-ion batteries are lightweight and hold a lot of energy, which changed portable devices like laptops and smartphones. They also helped make electric cars better, allowing them to travel longer distances and pushing more people toward cleaner transportation. Chemists also played a big role in solar energy by improving solar panels that turn sunlight directly into electricity. The first practical silicon solar cell was made in 1954 by Bell Labs scientists Daryl Chapin, Calvin Fuller, and Gerald Pearson. This showed that solar energy could work well, although it was expensive at first. Over the years, chemists and materials scientists worked on making solar cells cheaper and more efficient by improving the materials inside them.
Beyond energy, materials science helped create many important inventions in fields like electronics, space travel, and computers. One big discovery was semiconductors, like silicon and germanium. These materials made it possible to invent the transistor in 1947 at Bell Labs by scientists John Bardeen, Walter Brattain, and William Shockley. Transistors replaced bulky vacuum tubes and made electronic devices smaller, faster, and more reliable. This invention became the building block for modern computers, phones, and other technology. In the 1980s, scientists Georg Bednorz and K. Alex Müller discovered high-temperature superconductors. These materials are used in powerful magnets for MRI machines and particle accelerators and could someday help build more efficient power systems. Chemists also worked on composite materials, which are made by combining two or more substances to make something stronger or better. For example, carbon fiber reinforced plastics and advanced ceramics are light but very strong and can resist heat and corrosion. These materials are used to build airplanes that use less fuel, safer and lighter cars, and even parts for rockets and spacecraft.
Even though the 20th century brought amazing discoveries in chemistry, it also caused serious problems for people and the planet. One major wake-up call came in 1962, when scientist and writer Rachel Carson published a book called Silent Spring. In it, she warned about the dangers of using pesticides like DDT without understanding the harm they could do. These chemicals were killing birds, harming wildlife, and even making people sick. Her book helped spark the environmental movement, leading to the banning of DDT in many countries and the creation of environmental protection laws and agencies. But that was not the only concern. Chemistry was also used in ways that hurt people, especially during wars. In World War I, mustard gas was used as a weapon. In the Vietnam War, chemicals like Agent Orange were sprayed to destroy forests, but they also harmed soldiers and civilians. These tragedies led to international agreements like the Chemical Weapons Convention in 1993, which aimed to stop countries from using chemistry to make weapons. Factories also caused pollution by releasing toxic chemicals into the air, water, and soil. This included things like heavy metals, greenhouse gases, and chlorinated hydrocarbons. Problems like acid rain, holes in the ozone layer from CFCs, and huge amounts of plastic waste showed that chemical innovation could have long-term effects on the Earth. These issues taught scientists and leaders that chemistry should be used more responsibly to protect both people and the environment.
21st Century Chemistry
Chemistry in the 21st century has continued to grow and improve in exciting ways. One of the biggest changes is the use of computers to model and predict chemical reactions. This area is called computational chemistry. It lets scientists create virtual experiments and watch how atoms and molecules behave without needing to do everything in a real lab. This saves time, money, and materials. A special method called Density Functional Theory (DFT) helps chemists understand how molecules are built, how they react, and how much energy they need. In the past, these tools could only handle small, simple molecules, but now they can work with much larger and more realistic ones. This is very useful in designing new medicines, materials, and catalysts. In 2013, three scientists, Martin Karplus, Michael Levitt, and Arieh Warshel, won the Nobel Prize in Chemistry for their work in combining different types of modeling. They found a way to blend quantum mechanics with classical physics to study big, complex systems like enzymes. Another exciting development is called ultrafast laser spectroscopy. This technique uses femtosecond laser pulses (that is one quadrillionth of a second) to take snapshots of chemical reactions as they happen. It is like watching atoms and molecules moving, bonding, and changing in real time. This helps scientists better understand what is happening during reactions, including how bonds break, electrons move, and energy flows.
Scientists now use amazing tools like atomic force microscopes (AFM), scanning tunneling microscopes (STM), and X-ray photoelectron spectroscopy (XPS) to look at atoms and molecules on surfaces. These tools are very important for creating things like better catalysts, tiny sensors, and nanomaterials that help clean up pollution. Scientists can also use special tools like optical tweezers to measure the forces on just one molecule at a time. Another field is nanochemistry. Chemists work with materials that are only a few nanometers wide. These materials, like quantum dots and nanoparticles, have special properties that can be used in solar panels, tiny computer chips, and targeted drug delivery that helps treat diseases more precisely. Scientists are designing better batteries, fuel cells, and machines that split water into hydrogen and oxygen, which could help us use clean energy. In biophysical chemistry, scientists combine chemistry with biology and computer science. They study how proteins fold, how enzymes work, and how cell membranes behave. Tools like cryogenic electron microscopes (cryo-EM) and NMR machines help them see the shapes of molecules, which is very helpful in fighting diseases and creating new medicines.
In the 21st century, analytical chemistry has become much more advanced and important in many areas of science and everyday life. One big improvement has been in mass spectrometry (MS), a tool that helps scientists figure out what different chemicals are and how much of each is present. In the past, MS was mostly used for small molecules. Today, it can study large molecules, complicated mixtures, and even single cells. Modern versions, like tandem mass spectrometry (MS/MS) and high-resolution machines such as time-of-flight (TOF) and orbitrap analyzers, are incredibly sensitive. They can detect tiny amounts of a substance, as little as one part in a trillion. These tools are used in areas like medicine, drug testing, studying proteins and cells, and checking for pollution in the environment. Another big part of analytical chemistry is chromatography, which helps separate mixtures. The most common types are gas chromatography (GC), high-performance liquid chromatography (HPLC), and the newer ultra-performance liquid chromatography (UPLC). UPLC, developed in the early 2000s, works faster and gives clearer results than older methods. Often, chromatography is combined with mass spectrometry to make hyphenated techniques like LC-MS/MS or GC-MS. These powerful tools can separate and identify thousands of substances in just one experiment, which is very useful in things like medical testing, making new medicines, and checking food safety. A big trend in the 21st century is making scientific tools smaller and more portable. This is called miniaturization. One exciting result of this is the creation of lab-on-a-chip and microfluidic devices. These tiny tools can do complex chemical tests on a small chip, using only tiny drops of liquid. Because they are small and easy to carry, these devices can be used outside of a regular lab, like at a patient’s bedside, in the field, or in places with few resources. They can give fast results, which is very helpful in emergencies or remote areas. These tools are used for many things, such as checking blood sugar for people with diabetes, testing for diseases, like COVID-19 or the flu, finding toxins in water or the environment, and analyzing drugs in crime investigations.
Today, analytical chemistry works closely with data science. Modern science tools create huge amounts of data, and chemists need better computer programs to help understand it all. They use things like statistics, machine learning, and pattern recognition to find useful information in the data. This helps chemists in detecting pollution in the air, water, or soil, even at tiny amounts (less than one part per billion). It also helps them find illegal drugs or chemicals in crime scenes. It also helps in tracking new pollutants like microplastics and PFAS, which used to be hard to measure. Using special tools like ICP-MS, chemists can find heavy metals (like lead or mercury) in people or the environment In medicine, analytical chemistry helps create and test new medicines. Chemists make sure drugs are pure, safe, and work the way they should. They also check that medicines stay good over time. Groups like the FDA (Food and Drug Administration) and EMA (European Medicines Agency) use these tests to decide if a medicine can be sold.
Biochemistry in the 21st century has grown really fast. One of the biggest achievements was the Human Genome Project, finished in 2003. Scientists were able to read the entire DNA sequence of humans for the first time. This helped researchers learn more about how genes work. How they are turned on and off, and how they make proteins. Scientists have also sequenced the DNA of many other living things, like tiny bacteria, helpful plants like Arabidopsis, and fruit flies. This helped them compare genes across species and better understand how life evolved and functions. One of the most powerful tools in today’s biochemistry is called CRISPR-Cas9. Discovered in 2012 by Emmanuelle Charpentier and Jennifer Doudna, CRISPR lets scientists edit DNA quickly and accurately. It has changed the way we do genetic engineering. With CRISPR, we could fix genetic diseases, improve crops, and study what different genes do in living things Another important area is proteomics, the study of proteins. Using mass spectrometry, scientists can now identify thousands of proteins in a sample and study how they are changed or interact with each other. This helps us understand how cells work on a very detailed level. Tools like cryogenic electron microscopes (cryo-EM) allow scientists to actually see large molecules and protein machines in 3D, almost down to the atom. This technology was so important that it won the Nobel Prize in Chemistry in 2017.
One important area is called metabolomics, which is the study of tiny molecules (called metabolites) in cells and tissues. These molecules help us see what's really happening inside living things. Scientists now combine information from genomics (DNA), transcriptomics (RNA), proteomics (proteins), and metabolomics to get a complete picture of how cells work. This big-picture approach is called systems biology. It helps scientists understand complex diseases like cancer, diabetes, and Alzheimer’s by looking at how everything in a cell connects and works together. Biochemistry also helps in drug discovery and personalized medicine. Using computer modeling and new tools, scientists can design medicines that match a person’s specific needs. A field called pharmacogenomics studies how people’s genes affect the way they respond to medicine. This helps doctors choose the right drug and the right dose for each person. Another interesting area is synthetic biology. This is when scientists engineer living cells to do new jobs, like making biofuels, biodegradable plastics, or even new medicines. They do this by reprogramming cells with carefully chosen DNA instructions. In agriculture, biochemists are helping to create GMO (genetically modified organisms) and gene-edited crops that are healthier, more nutritious, and more resistant to pests, diseases, and climate change. By studying plant biochemistry, scientists are finding ways to help plants grow better and use water and nutrients more efficiently. Biochemistry is also key in solving global health problems. One example was the creation of mRNA vaccines for COVID-19. These vaccines use molecules that tell our cells to make a harmless part of the virus, helping the immune system learn how to fight it. Scientists are now working on similar vaccines for other diseases like flu, HIV, and even cancer.
In the 21st century, chemistry has also become very important in helping us create better and cleaner ways to store and use energy. One big area of progress is rechargeable batteries. Lithium-ion batteries, which first came out in the 1990s, have gotten much better over the years. Today, they are used in many things like smartphones, laptops, and electric cars. Chemists have made these batteries safer and more powerful by creating better materials for the battery parts. Some of these materials include lithium iron phosphate (LiFePO₄) and nickel manganese cobalt (NMC). Scientists are also working on new types of batteries, like lithium-sulfur, solid-state, and sodium-ion batteries. These may be cheaper, safer, and last longer, which could help even more people use clean energy. Another exciting technology is the fuel cell. Fuel cells make electricity by combining hydrogen and oxygen, and the only thing they give off is water, so they do not pollute the air. This makes them a great option for clean cars. One type, called a PEM fuel cell, is especially useful. Chemists are working on better and cheaper catalysts, which are materials that help the fuel cell work faster. One challenge with fuel cells is how to store hydrogen safely and easily. Scientists are exploring special materials, like metal–organic frameworks (MOFs) and solid–state hydrides, that can hold hydrogen in a compact and safe way.
Chemists are also leading the way in improving solar energy. The most common solar panels today are made from silicon, but scientists are working on new materials that could make solar power cheaper, lighter, and more flexible. One interesting material is called a perovskite. In 2009, perovskite solar cells could only turn about 4% of sunlight into electricity. By the early 2020s, that number had jumped to over 25%. Perovskites are cheap to make and can be used on bendable surfaces, but they have some problems. They do not last very long, and they contain lead, which can be harmful. Chemists are working to fix these issues. Other types of solar panels include dye-sensitized solar cells (DSSCs) and organic solar cells, which can be see-through or flexible. These new types of solar cells could be used in windows or on clothing. Another interesting area is called photocatalysis. This means using sunlight to make chemical reactions happen. Scientists are developing materials like titanium dioxide mixed with tiny amounts of certain metals, that can use sunlight to split water into hydrogen and oxygen or turn carbon dioxide into useful fuels like methanol or methane. This is similar to how plants do photosynthesis, and scientists call their version artificial photosynthesis. Chemists are also helping to make green hydrogen, a clean fuel, using a method called electrolysis. This process splits water using electricity, and it works even better when chemists add catalysts that help speed up the reaction. Chemists are also working on ways to capture carbon dioxide (CO₂) from the air or from factories. They are designing new materials like amine-based liquids, metal-organic frameworks (MOFs), and porous carbons that can trap CO₂. Once captured, the CO₂ can be turned into fuels, plastics, or even medicines, helping to reduce pollution and fight climate change.
Chemistry is also helping to create new materials. One example is nanomaterials. These are materials made at an incredibly small scale. Chemists can now make nanoparticles, nanotubes, and nanowires with exact shapes and sizes. This has led to discoveries like carbon nanotubes and graphene, which are super strong and great at conducting electricity and heat. These materials are now being used in batteries, tiny sensors, and advanced electronics. Another new type of material are called smart materials. These materials can change when something around them changes like temperature, light, or electric signals. For example, some materials can “remember” a shape and return to it after being bent. Others can heal themselves after getting damaged. Smart materials are used in things like robotics, medical devices, and spacecraft. In medicine, special smart gels can deliver medicine exactly when and where it is needed or help new tissue grow. Polymers, which are long chains of molecules, have also improved. Chemists can now make biodegradable polymers that break down safely or design polymers for 3D printing, medicine, and even to act like proteins or DNA. Chemists can now use machine learning and computer simulations to test and find the best materials faster than ever before. Programs like the Materials Genome Initiative help scientists go from new idea to real product more quickly. Chemists are also designing electronic and photonic materials for things like flexible phone screens, glowing displays (OLEDs), and even quantum computers. Tiny particles called quantum dots are used in colorful TVs and medical imaging. And in the race to build quantum computers, special materials are helping create more stable and powerful qubits.
In the 21st century, chemistry faces many big and complicated challenges. One of the most serious is climate change. To help stop global warming, chemists are working to create cleaner energy sources. These include better solar panels, fuel cells, and batteries that do not rely on fossil fuels. But making these technologies affordable, safe, and easy to use on a large scale is still a tough problem. Another goal is to find ways to capture carbon dioxide, a major greenhouse gas, and turn it into useful fuels or chemicals. Right now, that process uses a lot of energy and is not very efficient. Green chemistry is another important focus. Many chemical industries today use toxic substances and create a lot of waste. Chemists are now trying to design cleaner ways to make products using renewable resources, safer materials, and less energy. This shift is difficult, especially when it comes to changing how big factories operate, but it is necessary for a more sustainable future. Plastic pollution is also a major concern. Plastics are everywhere, from packaging to clothes, but they do not break down easily and often end up in oceans and soil. Chemists are working to invent biodegradable and recyclable plastics that still work well but do not harm the planet. They're also trying to find better ways to remove microplastics and other plastic waste from the environment. In the world of medicine, chemists are helping to fight antibiotic resistance and new diseases. Bacteria are becoming harder to kill. There have not been many new antibiotics in recent years. Chemists must design new drugs and ways to deliver them more effectively. There is also growing interest in personalized medicine, where treatments are made to match a person’s genes. Water shortages and polluted water are big problems in many parts of the world. Chemists are developing new ways to clean water using less energy. They are also making better sensors to detect pollutants, like pesticides and heavy metals, in the water, soil, and air. Many high-tech devices, like smartphones and electric cars, need special materials such as lithium, cobalt, and rare earth elements. But mining these materials can harm the environment, and supplies are limited. Chemists are trying to find safer ways to extract these elements, reuse them, or even replace them with better alternatives.
Types of chemistry
There are several types of chemistry. Analytical chemistry looks at which chemicals are in things. For example, looking at how much arsenic is in food. Organic chemistry looks at things that have carbon in them. For example, making acetylene. Inorganic chemistry looks at things that do not have carbon in them. One example is making an integrated circuit. Theoretical chemistry tries to explain chemical data with mathematics and computers.
A large area of chemistry is polymer chemistry. This looks at plastics. One example is making nylon. Because plastics are made of carbon, polymer chemistry is part of organic chemistry. Another area is biochemistry. This looks at the chemistry of living things. An example would be seeing how arsenic poisons people. Biochemistry is also part of organic chemistry. There are many other small branches of chemistry.
Concepts of chemistry
Matter
Matter is anything that has mass and takes up space. This means it fills up some amount of room, even if we cannot always see it. Everything around us, from tiny grains of sand to huge planets and stars, is made of matter. Even things like air or steam, which seem invisible or light, are still matter because they are made of tiny particles that have mass and take up space. We can notice matter in many ways. Some things, like a rock or a cup of water, are easy to see and touch. Other things, like the air in a room or the gas in a balloon, are harder to notice, but they are still there. You can feel them when the wind blows or when a balloon gets bigger. Matter is really important in science, especially in chemistry and physics, because it helps us understand what everything is made of and how it behaves. All matter is made up of atoms and molecules, which are tiny building blocks.
Matter can exist in different states, or forms, depending on how its tiny particles are arranged and how much energy they have. The three most common states are solid, liquid, and gas. In a solid, the particles are packed closely together in a fixed pattern. They do not move around much, which is why solids have a definite shape and volume. For example, a rock or a pencil keeps its shape unless something forces it to change. In a liquid, the particles are still close, but they are not in a set pattern. They can slide past each other, which lets liquids flow and take the shape of their container. However, liquids still keep the same volume. A cup of water stays the same amount, whether it is in a glass or a bowl. In a gas, the particles are spread far apart and move very fast. Gases do not have a fixed shape or volume. They will spread out to fill any space they are in, like air in a balloon. Gases can also be compressed or squished into a smaller space. Besides these three, there are also other, more unusual states of matter. One is plasma, which is made of super-energized particles with electric charges. Plasma is found in things like stars and lightning. Another rare state is called a Bose–Einstein condensate (BEC). It happens only at very, very cold temperatures close to absolute zero. In this state, particles move so slowly they start to act like one single particle instead of many.
Matter can be grouped into two main types based on what it's made of: pure substances and mixtures. Pure substances are made of only one kind of material and have the same properties throughout. They always have the same makeup, no matter where you find them. Pure substances can be either elements or compounds. Elements are the simplest type of pure substance. They cannot be broken down into anything simpler using normal chemical methods. Each element is made of just one kind of atom. Examples include oxygen (O₂), hydrogen (H₂), and iron (Fe). There are over 100 known elements, and they are all listed in the periodic table. Compounds are also pure substances, but they are made of two or more different elements that are chemically bonded together in a fixed ratio. This means the elements combine in a specific way to form a new substance. Some examples are water (H₂O), carbon dioxide (CO₂), and salt (NaCl). In a compound, the elements lose their individual properties and form something completely new.
Mixtures are made when two or more substances are physically combined, not chemically joined. This means each part of the mixture keeps its own properties, and you can often separate them using simple methods like filtering, boiling, or picking them apart. Mixtures come in two main types: homogeneous and heterogeneous. A homogeneous mixture (also called a solution) looks the same all the way through. The different parts are evenly mixed, and you cannot see or easily separate them. For example, when salt dissolves in water, it becomes a solution. You cannot see the salt anymore, but it is still there. Another example is air, which is a mixture of gases. A heterogeneous mixture looks different in different parts. The substances are not evenly mixed, and you can often see and separate the different parts. Examples include a salad, sand and iron filings, or oil and water. In these mixtures, the parts stay separate and can be picked out or separated by hand or simple tools.
The properties of matter help us understand how different substances act, react, and change in various situations. These properties are usually divided into two main types: physical properties and chemical properties. Physical properties are things we can see, measure, or feel without changing what the substance is made of. These include things like color, smell, taste, melting point, boiling point, density, hardness, electrical conductivity, and solubility (how well something dissolves). For example, water boils at 100°C and freezes at 0°C. One special thing about physical properties is that they do not change the substance. You can measure them many times, and the substance stays the same. For instance, if you melt ice, it turns into liquid water, but it is still H₂O. That means it’s a physical change, not a chemical one, and it can be reversed by freezing the water again. Some physical properties depend on how much of the substance you have. These are called extensive properties, like mass and volume. Others stay the same no matter how much you have. These are called intensive properties, like density and boiling point.
Chemical properties describe how a substance can change into something completely new. Unlike physical properties, which can be seen or measured without changing the substance, chemical properties can only be observed when a chemical reaction happens. These reactions change the substance's molecular or atomic structure, meaning it becomes a different substance. Some common chemical properties include flammability (how easily something burns), reactivity with acids or bases, rusting of iron, tarnishing of silver, and the ability to decompose or oxidize. For example, when wood burns, it reacts with oxygen in the air and turns into ash, carbon dioxide, water vapor, and heat. This is a chemical change, and the fact that wood can burn is a chemical property. Another example is iron rusting. When iron is exposed to air and moisture, it reacts to form rust, which is a new substance with different properties.
Atoms
The idea of atoms goes all the way back to around 400 BCE, when ancient Greek thinkers like Democritus and Leucippus came up with the idea that all matter is made of tiny, invisible particles. They called these particles "atomos," which means "uncuttable" or "indivisible." Democritus believed that atoms had different shapes and sizes and moved through empty space, combining in different ways to make everything we see. However, this idea was just a guess. It was not based on experiments or evidence. Another famous philosopher, Aristotle, had a different idea. He believed that everything was made of just four elements: earth, water, air, and fire. Because Aristotle was very popular and influential, people accepted his ideas for almost 2,000 years, and the atom theory was mostly forgotten. That changed in the early 1800s, when an English scientist named John Dalton brought the idea of atoms back. In 1803, Dalton proposed the first modern atomic theory. He said that all matter is made of atoms, and that atoms cannot be created or destroyed. He also said that all atoms of the same element are exactly the same, and that chemical reactions happen when atoms are rearranged. Dalton’s ideas were based on experiments, especially from studying how elements combine in fixed amounts. But, Dalton thought atoms were just solid spheres like a ball with no parts inside.
As scientists built better tools and did more experiments, they began to learn much more about what was inside an atom. In 1897, a scientist named J.J. Thomson discovered the electron by using a special tube called a cathode ray tube. This showed that atoms are not solid and indivisible after all. Thomson came up with the "plum pudding model," where he imagined the atom as a ball of positive charge with negative electrons scattered inside it, like raisins in a pudding. But in 1911, Ernest Rutherford did a famous experiment with gold foil that changed this idea. He found that most of the atom is empty space, and that the positive charge is packed into a tiny, dense center called the nucleus. This led to the nuclear model of the atom, where electrons orbit around a central nucleus. However, this model still did not explain why the electrons did not just crash into the nucleus. Then in 1913, Niels Bohr improved Rutherford’s model by using ideas from quantum theory. Bohr said that electrons move in fixed energy levels or shells around the nucleus. He also said that electrons can jump from one level to another by gaining or losing energy. Bohr’s model worked well for explaining the behavior of hydrogen, the simplest atom, but it did not work as well for bigger atoms. In the 1920s, scientists created an even better model using quantum mechanics. Scientists like Erwin Schrödinger, Werner Heisenberg, and Max Born showed that electrons do not move in perfect orbits. Instead, they exist in areas called electron clouds or orbitals, which are regions where electrons are likely to be found. This modern model of the atom is the most accurate one we have today and helps explain how atoms behave in chemistry and physics.
An atom is the smallest unit of matter. It is the basic building block that makes up everything around us. At the center of every atom is the nucleus, a tiny, dense part that contains two types of particles: protons and neutrons. Protons have a positive charge, while neutrons have no charge, they are neutral. The number of protons in an atom’s nucleus is called the atomic number, and it tells us what element the atom is. For example, all hydrogen atoms have one proton, and all oxygen atoms have eight protons. The mass number of an atom is the total number of protons and neutrons in the nucleus. Because protons and neutrons are both heavy (compared to electrons), almost all of the atom’s mass is in the nucleus, even though it is very small. Outside the nucleus is where the electrons are found. Electrons are much smaller and have a negative charge. In older models, scientists thought electrons moved in set paths around the nucleus, like planets orbiting the sun. But today, we know from quantum mechanics that electrons move in regions called orbitals or electron clouds. These are areas where electrons are most likely to be found, but their exact position cannot be known for sure. Electrons are also arranged in energy levels or shells around the nucleus. Each level can hold only a certain number of electrons. Atoms are electrically neutral when they have the same number of protons and electrons. But sometimes, atoms can gain or lose electrons, which changes their overall charge. When an atom loses electrons, it becomes a positively charged ion, called a cation. When an atom gains electrons, it becomes a negatively charged ion, called an anion. Also, atoms of the same element can have different numbers of neutrons. These are called isotopes. Even though isotopes have the same number of protons and act the same in chemical reactions, their different numbers of neutrons make them have slightly different masses. Some isotopes are stable, while others can break down over time, releasing energy.
The electron configuration of an atom is the way its electrons are arranged in different energy levels and areas around the nucleus. This arrangement is very important because it helps determine how the atom will react with other atoms and form chemical bonds. In today’s model of the atom, called the quantum mechanical model, electrons do not travel in neat circles around the nucleus like planets around the sun. Instead, they move around in areas called orbitals, which are regions where there is a high chance of finding an electron. These orbitals are grouped into energy levels, also called shells, which are labeled with numbers like 1, 2, 3, and so on. Each energy level has one or more subshells, which are labeled as s, p, d, and f. The first energy level (1) has only an s subshell. The second level (2) has s and p subshells. The third level (3) has s, p, and d, and so on. Each orbital can hold up to two electrons. The Aufbau principle helps scientists understand how electrons fill the space around an atom’s nucleus. According to this rule, electrons always fill the lowest energy orbitals first before moving to higher ones. The order in which electrons fill these orbitals goes like this: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, and so on. This order is based on the energy levels of the orbitals, and sometimes it can seem a bit out of order. For example, 4s fills before 3d because 4s has a lower energy level than 3d. There are also other important rules to remember when talking about electron configuration. Pauli exclusion principle says that only two electrons can fit into the same orbital, and they must spin in opposite directions. Hund’s rules says that when electrons go into orbitals that have the same energy (like three 2p orbitals), they will first go in one at a time with the same spin. Only after each orbital has one electron will they start to pair up. This helps reduce repulsion between electrons. We use a special way to write electron configurations. For example, the configuration for a carbon atom, which has 6 electrons, is: 1s² 2s² 2p². This means carbon has 2 electrons in the 1s orbital, 2 in the 2s, and 2 in the 2p. For bigger elements, we often use noble gas shorthand to save time. This means we start with the symbol of the nearest noble gas (like neon or argon) in brackets, and then continue from there. For example, sodium has 11 electrons. Instead of writing out everything, we can write: [Ne] 3s¹. Here, [Ne] stands for the first 10 electrons (1s² 2s² 2p⁶), and the 11th electron is in the 3s orbital. The valence electrons, or the electrons in the outermost shell, are very important. They decide how an atom behaves in chemical reactions, how it bonds with other atoms and what kind of compounds it can form.
The Periodic Table
A long time ago, people only knew about a few natural elements like gold, silver, copper, and iron. As science progressed, especially in the 1600s and 1700s, scientists started discovering more elements and learning about chemical reactions. In the late 1700s, a scientist named Antoine Lavoisier made one of the first lists of elements. He also suggested that elements were basic substances that could not be broken down into anything simpler. By the early 1800s, more elements had been discovered, and scientists began to see patterns in their properties. The big breakthrough came in 1869 when a Russian chemist named Dmitri Mendeleev created a version of the periodic table. He arranged the 63 known elements by increasing atomic mass and grouped elements with similar properties into columns. What was really amazing about Mendeleev’s table was that he left spaces for elements that had not been discovered yet and predicted what their properties would be. When scientists later discovered elements like gallium, scandium, and germanium, their properties matched Mendeleev’s predictions, which made his table even more accepted. Although there were some problems with his system, like elements that did not fit perfectly by mass, Mendeleev’s table helped scientists understand the relationships between different elements. In the early 1900s, Henry Moseley improved the table by arranging elements by atomic number, which solved many of the problems. He showed that elements’ properties depend on the number of protons in their nucleus. This led to the modern periodic law, which says that elements’ properties change in a regular pattern based on their atomic number. Since then, the periodic table has grown to include over 100 elements, including many that are made in labs. It has also become more complex with the addition of new groups of elements like noble gases, transition metals, and lanthanides and actinides (the f-block elements).
The periodic table is a special chart that shows all the known chemical elements in a very organized way. It is arranged based on each element’s atomic number (how many protons it has), how its electrons are set up, and how it behaves in chemical reactions. The table is built to match how atoms work, especially how electrons move around the nucleus. This setup helps scientists understand how each element will act during chemical changes. The table has rows called periods and columns called groups or families. Periods go from left to right and are numbered from 1 to 7. Each period shows how many energy levels, or electron shells, the atoms have. As you move across a period, each element gets one more proton and one more electron, and they go into the same energy level. This changes things like the size of the atom and how strongly it attracts electrons. Groups go up and down and are numbered from 1 to 18. Elements in the same group have the same number of valence electrons. These are the electrons in the outer shell of the atom. Because of this, elements in the same group act in similar ways. For example, Group 1 is called the alkali metals (like lithium and sodium). They all have one valence electron and are very reactive, especially with water. Group 17 is the halogens (like fluorine and chlorine). They have seven valence electrons and are also very reactive. Group 18 is the noble gases (like helium and neon). They have full outer shells of electrons and do not react much with other elements.
The periodic table is also split into four big sections called blocks. The s, p, d, and f blocks. These blocks are based on how electrons fill up different parts of an atom, called orbitals. The s-block includes Groups 1 and 2, plus hydrogen and helium. In these elements, the s orbitals are being filled with electrons. The p-block includes Groups 13 to 18. These elements are filling up their p orbitals. The s-block and p-blocks together make up most of the main group elements, which include both metals and nonmetals. The d-block is in the middle of the table and includes Groups 3 to 12. These are the transition metals. They are special because they can form different types of charged ions and complex compounds. The f-block is at the bottom of the table and includes the lanthanides and actinides. These elements are filling their f orbitals. Even though they are placed separately to keep the table neat, they actually belong in the 6th and 7th rows (periods). The periodic table also shows trends, or patterns, in the properties of elements. Atomic radius (the size of an atom) gets smaller across a period from left to right because the nucleus pulls electrons in tighter. But it gets larger going down a group because more electron shells are added. Electronegativity (how strongly an atom attracts electrons) usually increases across a period and decreases down a group. Ionization energy (the energy needed to remove an electron) follows a similar pattern. It increases across a period and decreases down a group.
Basic concepts
The basic unit of an element is called an atom. An atom is the smallest building block that you can cut an element into without the element breaking down (turning into a lighter element, for example through nuclear fission or radioactive decay). A chemical compound is a substance made up of two or more elements. In a compound, two or more atoms are joined to form a molecule. The tiniest speck of dust or drop of liquid, that one can see is made up of many millions or billions of these molecules. Mixtures are substances where chemicals are mixed but not reacted. An example would be mixing sand and salt. This can be undone again to produce salt and sand separately. Chemical compounds are changed by a chemical reaction. An example would be heating sodium bicarbonate, common baking soda. It will make water, carbon dioxide, and sodium carbonate. This reaction cannot be undone.
One very important concept in chemistry is that different atoms interact with one another in very specific proportions. For example, two hydrogen atoms interacting with one oxygen atom lead to the water molecule, H2O. This relationship is known as the "Law of constant proportions" and leads to the idea of "stoichiometry", a term that refers to the ratios of different atoms in chemical compounds. For example, in water, there are always exactly 2 hydrogen atoms to 1 oxygen atom. In carbon dioxide, there are exactly 2 oxygen atoms for 1 carbon atom. These relationships are described using chemical formulas such as H2O (two hydrogen atoms and one oxygen atom) and CO2 (one carbon atom and two oxygen atoms).
Mole
Because atoms of different elements react with one another in very specific proportions but atoms of different elements have different weights, chemists often describe the number of different elements and compounds in terms of the number of "moles". A "mole" of any element contains the same number of atoms: 602,214,150,000,000,000,000,000 atoms. The atomic mass of an element can be used to see how much of the element makes a mole. For example, the atomic mass of copper is about 63.55. That means about 63.55 grams of copper metal has a mole of atoms. The atomic mass of chlorine is about 35.45. That means 35.45 grams of chlorine has a mole of atoms in it.
Moles can be used to see how many molecules are in chemical compounds, too. Copper(II) chloride is an example. CuCl2 is its chemical formula. There is one copper atom (63.55) and two chlorine atoms (35.45 · 2 = 70.90). Add all the molar masses of the elements together to get the molar mass of the chemical compound (63.55 + 70.90 = 134.45). That means in 134.45 grams of copper(II) chloride, there is one mole of copper(II) chloride molecules. This concept is used to calculate how much chemicals are needed in a chemical reaction if no reactants (chemicals that are reacted) should be left. If too much reactant is used, there will be some reactants left in the chemical reaction.
Acids and bases
Acids and bases are a common type of chemical. Using the simplest definitions, acids add H3O+ hydronium ions when in water, and bases add OH- hydroxide ions when in water. Acids can react with bases: the OH- takes the extra hydrogen from H3O+ to make an extra water molecule, H2O. The other parts of the acid and base make a salt.
An example would be reacting hydrochloric acid (HCl) and sodium hydroxide (NaOH). Hydrochloric acid releases H+ and Cl- ions in water. The base releases Na+ and OH- ions. The H+ and the OH- react to make water. There is a solution of sodium chloride (NaCl) left. Sodium chloride is a salt.
This definition of acids and bases, called the Arrhenius acid-base theory, is not used by modern chemists. It is too oversimplified for what really happens in water, and cannot describe anything dissolved in another solvent like ammonia. Instead, chemists use the Brønsted–Lowry acid–base theory and Lewis acid-base theory, which are more complicated but more useful in chemistry. Instead of looking at reactions in water, these theories focus on hydrogen ions and pairs of electrons.
Usefulness
Chemistry is very useful in everyday life and makes up the foundation of many branches of science. Most objects are made by chemists (people who do chemistry). Chemists are constantly working to find new and useful substances. Chemists make new drugs and materials like paints that we use every day.
Safety
Many chemicals are harmless, but there are some chemicals that are dangerous. For example, mercury(II) chloride is very toxic. Chromates can cause cancer. Tin(II) chloride pollutes water easily. Hydrochloric acid can cause bad burns. Some chemicals like hydrogen can explode or catch fire. To stay safe, chemists experiment with chemicals in a chemical lab. They use special equipment and clothing to do reactions and keep the chemicals contained. The chemicals used in drugs and in things like bleach have been tested to make sure they are safe if used correctly.


United States: "America" redirects here. For the continent, see Americas. For other uses, see United States (disambiguation) and America (disambiguation)
May 2019
The United States of America, also known as the United States (U.S.) or simply America, is a sovereign country mostly in North America. It is divided into 50 states. 48 of these states and the District of Columbia border each other between the Pacific and Atlantic Oceans. They are bordered by Canada to the north and Mexico to the south. The state of Alaska is in the northwestern area of the continent and is separated from the other 48 states by Canada making it an exclave. Alaska is bordered by Canada to its east. The state of Hawaii is a set of islands in the Pacific located within Polynesia and is about 2,200 miles (3,500 kilometers) from the mainland. The country also possesses territories, and insular areas, in the Caribbean and Pacific. The capital city is Washington, D.C and the largest city by population is New York City with a population of 8.8 million people. With a population of 340.1 million people and an area of 3.79 million square miles (9.83 million km2), the United States is the third most populated country in the world and the fourth-largest country in the world by total area.
The land that would one day become the United States was first settled by migrating tribes from Siberia that walked across a land bridge about 20,000 years ago during the Ice Age. These people were the ancestors of the Native Americans. European colonization began in the 16th and 17th century with several European nations setting up colonies in North America. 
The thirteen colonies of Great Britain along the Atlantic coast declared independence as a nation on July 4, 1776. They issued the Declaration of Independence, which announced their independence from Great Britain and their creation of a cooperative union. The colonies defeated Great Britain in the American Revolutionary War, making it the first successful colonial war of independence in history. The Philadelphia Convention adopted the current United States Constitution on September 17, 1787; its approval the following year made the states part of a single republic with a strong central government. The Bill of Rights, making up ten constitutional amendments guaranteeing many basic civil rights and freedoms, was approved in 1791.
In the 19th century, the United States had a legal policy of manifest destiny where they believed that it was their god given right to expand across the North American continent. They conquered and took over native lands and bought territory from France, the United Kingdom, Mexico, and Russia, and took over the Republic of Texas and later the Republic of Hawaii. 
Political arguments between the farming-based South and industrial North over the growth of the institution of slavery and states' rights began the American Civil War. In 1861, the southern states separated from the union and founded their own country called the Confederate States of America after anti-slavery candidate, Abraham Lincoln won the Presidential election. The Union's victory over the Confederacy prevented a permanent split of the country and led to the end of legal slavery in the United States. By the 1870s, the national wealth was the world's largest. The Spanish–American War and World War I confirmed the country's status as a military power. In 1945, the United States came out of World War II as a superpower and was the first country with nuclear weapons. After World War II the United States along with its allies founded the United Nations. Today the United States is a permanent member of the United Nations Security Council. The United States also became engaged in an arms race called the Cold War with the Soviet Union. The United States participated in the Space Race against the Soviet Union that produced rapid advancements in rocket technology. They created the Apollo 11 rocket and it was the first to send people to the moon. It was done by NASA, an American space agency. It went up to space on July 16, 1969, carrying three astronauts: Neil Armstrong, Buzz Aldrin and Michael Collins. On July 20, 1969, Armstrong and Aldrin became the first humans to land on the moon, while Collins stayed in orbit around the Moon. In 1991, the Soviet Union broke up, ending the Cold War and leaving the United States as the only superpower. 
Today, the United States is one of the world's most ethnically mixed and multicultural nations. It is the product of large-scale immigration from many countries. The U.S. economy is the largest national economy in the world, with an estimated 2020 gross domestic product (GDP) of US$ 20.9 trillion (about a quarter of worldwide GDP).
 Geography and environment 
The United States is a federal republic of fifty states, a federal district, and several territories.
The land area of the contiguous United States is 2959064 sqmi km2 0. Alaska, separated from the contiguous United States by Canada, is the largest state at 663268 sqmi km2 0. Hawaii, occupying an archipelago in the central Pacific, southwest of North America, is 10931 sqmi km2 0 in area.
The United States is the world's fourth largest nation by total area (land and water), ranking behind Russia, Canada, and China or behind Russia, Canada, and ahead of China. The ranking varies depending on how two territories disputed by China and India are counted and how the total size of the United States is measured: calculations range from 3676486 sqmi km2 0 to 3717813 sqmi km2 0 to 3794101 sqmi km2 0. Measured by only land area, the United States is third in size behind Russia and China, just ahead of Canada.
The coastal plain of the Atlantic seaboard gives way further inland to deciduous forests and the rolling hills of the Piedmont. The Appalachian Mountains divide the eastern seaboard from the Great Lakes and the grasslands of the Midwest. The Mississippi–Missouri River, the world's fourth longest river system, runs mainly north-south through the heart of the country. The flat, fertile prairie of the Great Plains stretches to the west, interrupted by a highland region in the southeast.
The Rocky Mountains, at the western edge of the Great Plains, extend north to south across the country, reaching altitudes higher than 14,000 feet (4,300 m) in Colorado. Further west is the rocky Great Basin and deserts such as the Chihuahua and Mojave. The Sierra Nevada and Cascade mountain ranges run close to the Pacific coast, both ranges reaching altitudes higher than 14,000 feet. The Alaskan Range passes through much of Alaska and is home to Denali which stands at a height of 20,310 ft (6,190 m) above sea level and is the tallest mountain in the United States and in North America. Denali is in fact the tallest mountain in the World on land from base to summit standing at about 18,000 ft (5,490 m), which is about 4,000 ft (1,220 m) higher than Mount Everest which sits at a height of about 14,000 ft (4,270 m) from base to summit. 
The United States, with its large size and geographical variety, includes most climate types. To the east of the 100th meridian, the climate ranges from humid continental in the north to humid subtropical in the south. The southern tip of Florida is tropical, as is Hawaii. The Great Plains west of the 100th meridian are semi-dry. Much of the western mountains are alpine. The climate is dry in the Great Basin, desert in the Southwest, Mediterranean in coastal California, and oceanic in coastal Oregon and Washington and southern Alaska. Most of Alaska is subarctic or polar. Extreme weather is not unusual—the states bordering the Gulf of Mexico are prone to hurricanes, and most of the world's tornadoes happen within the country, mainly in the Midwest's Tornado Alley.
The U.S. ecology is considered "megadiverse"; about 17,000 species of vascular plants occur in the contiguous United States and Alaska, and over 1,800 species of flowering plants are found in Hawaii, few of which occur on the mainland. The United States is home to more than 400 mammal, 750 bird, and 500 reptile and amphibian species. About 91,000 insect species have been described.
The Endangered Species Act of 1973 protects threatened and endangered species and their habitats. They are watched by the United States Fish and Wildlife Service. There are 63 national parks, and hundreds of other federally managed parks, forests, and wilderness areas. The United States government owns 28.8% of the country's land area. Most of this is protected, but some is leased for oil and gas drilling, mining, logging, or cattle ranching. 2.4% is used for military purposes.
An example of animals that are native to the United States such as opossums, raccoons, pumas, and bears. Endangered animals in the United States include the jaguar, the California condor, and the Florida panther.
There are many types of small trees and shrubs in the United States. Some of these are hackberries, hawthorn, serviceberry, blackberry, wild cherry, dogwood, and snowberry. Wildflowers grow all around the United States. They even grow in deserts and places with mountains. Wildflowers include forget-me-not, fringed and closed gentians, jack-in-the-pulpit, black-eyed Susan, columbine, and common dandelion, as well as many types of aster, orchid, lady's slipper, and wild rose.
 History 
 Native Americans 
It is believed that the indigenous peoples of the continental United States, including the natives of Alaska, moved in from Asia. They began arriving more than twelve thousand years ago, if not earlier. Some, such as the pre-Columbian Mississippian culture in the southeast, developed advanced farming, grand construction, and state-level communities. The native population of America decreased after Europeans arrived, and for different reasons, mostly sicknesses such as smallpox and measles.
 European settlers 
In 1492, Italian explorer Christopher Columbus from Genoa, under contract to the Spanish crown, reached some Caribbean islands, making the first Contact with the native people. On April 2, 1513, Spanish conquistador Juan Ponce de León landed on what he called "La Florida" — the first recorded European coming on what would become the U.S. mainland. Spanish settlements in the area were followed by ones in the present-day southwestern United States that drew thousands through Mexico. French fur traders established outposts of New France around the Great Lakes; France eventually claimed much of the North American interior, down to the Gulf of Mexico. The first successful English settlements were the Colony of Virginia in Jamestown in 1607 and the Pilgrims' Plymouth Colony in 1620. The 1628 chartering of the Massachusetts Bay Colony resulted in a wave of relocation; by 1634, New England had been settled by some 10,000 Puritans. Between the late 1610s and the American Revolution, about 50,000 convicts were shipped to Britain's American colonies. Beginning in 1614, the Dutch settled along the lower Hudson River, including New Amsterdam on Manhattan Island.
 Independence and expansion 
Tensions between American colonials and the British during the rebel period of the 1760s and early 1770s led to the American Revolutionary War, fought from 1775 through 1781. On June 14, 1775, the Continental Congress, a meeting in Philadelphia, established a Continental Army led by George Washington.The Congress said that "all men are created equal" and are born with "certain natural rights," and adopted the Declaration of Independence, written mostly by Thomas Jefferson, on July 4, 1776. That date is now celebrated every year as America's Independence Day. In 1777, the Articles of Confederation established a weak federal government that operated until 1789. Morocco was the first country in the world to recognize America’s independence.
After the British defeat by American forces helped by the French, Great Britain recognized the independence of the United States and the states' sovereignty over American land west to the Mississippi River. A constitutional convention was organized in 1787 by people who wanted to establish a stronger national government, with powers of taxation. The United States Constitution was approved in 1788, and the new republic's first Senate, House of Representatives, and President (George Washington) took office in 1789. The Bill of Rights, forbidding federal restriction of personal freedoms and certifying a range of legal protections, was adopted in 1791.
Attitudes toward slavery were shifting; a clause in the Constitution protected the African slave trade only until 1808. The northern states permanently stopped slavery between 1780 and 1804, leaving the slave states of the south as defenders of the "peculiar institution." The Second Great Awakening, beginning about 1800, made evangelicalism a force behind different social reform movements, including abolitionism.
Americans' eagerness to expand westward caused a long series of Indian Wars and an Indian removal policy that removed native peoples from their land. The Louisiana Purchase of French-claimed land under President Thomas Jefferson in 1803 almost doubled the nation's size. The War of 1812, declared against Britain over different complaints and fought to a draw, strengthened U.S. nationalism. A series of U.S. military invasions into Florida led Spain to give up it and other Gulf Coast territory in 1819. The United States took over the Republic of Texas in 1845. The idea of Manifest destiny became popular during this time. The 1846 Oregon Treaty with Britain led to U.S. control of the present-day American Northwest. The U.S. victory in the Mexican–American War resulted in the 1848 cession of California and much of the present-day American Southwest. The California Gold Rush of 1848–49 further encouraged western relocation. New railways made relocation easier for settlers and increased conflicts with Native Americans. Over a half-century, up to 40 million American bison, or buffalo, were killed for skins and meat and to ease the railways' spread. The loss of the buffalo, which were valuable to the plains Indians, caused many native cultures to become gone forever.
 Civil War and industrialization 
American Civil War Reconstruction era Spanish–American War
Tensions between slave and free states mounted with arguments over the relationship between the state and federal governments, as well as violent conflicts over the spread of slavery into new states. Abraham Lincoln, a candidate of the antislavery Republican Party, was elected president in 1860. Before he took office, seven slave states declared their secession and formed the Confederate States of America. With the Confederate attack upon Fort Sumter, the American Civil War began and four more slave states joined the Confederacy. Lincoln's Emancipation Proclamation committed the Union to end slavery. Following the Union victory in 1865, three changes to the U.S. Constitution secured freedom for the nearly four million African Americans who had been slaves, made them citizens, and gave them voting rights. The war and its resolution led to a big increase in federal power.
After the war, the assassination of Abraham Lincoln was followed by the Reconstruction era, where policies were put together directed at getting back and rebuilding the Southern states while securing the rights of the newly freed slaves. The resolution of the disputed 1876 presidential election by the Compromise of 1877 ended this era, and the Jim Crow laws soon began to negatively affect many African Americans. In the North, urbanization and a never-before-seen inflow of immigrants from Southern and Eastern Europe made the country's industrialization grow rapidly. The wave of immigration, lasting until 1929, provided labor and changed American culture. High tax protections, national infrastructure building, and new banking laws also encouraged growth. The 1867 Alaska Purchase from Russia completed the country's mainland expansion. The Wounded Knee Massacre in 1890 was the last major armed conflict of the Indian Wars. In 1893, the native monarchy of the Pacific Kingdom of Hawaii was ended in a secret and successful plan led by American residents. The United States took over the archipelago in 1898. Victory in the Spanish–American War the same year proved that the United States was a world power and led to the addition of Puerto Rico, Guam, and the Philippines. The Philippines gained independence fifty years later. Puerto Rico and Guam are still U.S. territories.
 World War I, Great Depression, and World War II 
The First World War started in Europe in 1914. The United States said it was neutral and did not join the war at first. Later, the Americans helped the British and French, even though many citizens, especially those from Ireland and Germany, were against it. In 1917, the United States joined the Allies. They helped defeat the Central Powers. At the end of the war, many Americans did not want to stay involved in Europe. The Senate did not approve the Treaty of Versailles (1919). The United States stayed out of the League of Nations. The country became more isolationist. 
In 1920, the women's rights movement gained the approval of a constitutional amendment to grant women the right to vote. For most of the 1920s, the country enjoyed a period of success, decreasing the inequality in the balance of payments while profiting from industrial farms. This period, known as the Roaring Twenties, ended with the Wall Street Crash of 1929 that triggered the Great Depression. 
After his election as president in 1932, Franklin D. Roosevelt responded with the New Deal, a series of policies that increased government interference in the economy. From 1920 to 1933 a prohibition banning alcohol was in place. The Dust Bowl of the 1930s left many poor farmer communities and encouraged a new wave of emigration to the West Coast.
The United States, officially neutral during the early stages of World War II, began supplying supplies to the Allies in March 1941, through the Lend-Lease program. On December 7, 1941, the country joined the Allies' fight against the Axis Powers, after the Japanese attack on Pearl Harbor. World War II boosted the economy by providing investment capital and jobs, making many women enter the labor market. Of the significant fighters, the United States was the only nation to be enriched by war. The discussions at Bretton Woods and Yalta created a new system of an international organization that placed the country and the Soviet Union at the heart of world affairs. In 1945, when the end of the Second World War in Europe came, and an international gathering held in San Francisco drafted the Charter of the United Nations, which came into force after the war. Having developed the first nuclear weapon, the government decided to use it in the Japanese cities of Hiroshima and Nagasaki in August of that same year. Japan gave up on September 2, ending the war.
 Cold War and civil rights era 
Cold War McCarthyism Civil Rights Movement Vietnam War Watergate scandal
After World War II, the United States and the Soviet Union started the Cold War. This was because both of them thought their own type of government was the best. The United States was supported by NATO, and the Soviet Union was supported by the Warsaw Pact. They never fought each other directly, but did many proxy wars. In the Korean War, the United States sent soldiers to help South Korea. They fought Chinese soldiers that were helping communist North Korea. In the 1960s, the Cuban Missile Crisis almost led to a real war between the two countries. However, the Soviets agreed to take their missiles out of Cuba if the United States did not attack them. Because of the Cold War, many Americans were scared of communist spies taking over the United States. This started the Second Red Scare, and hundreds of people were arrested because they were communist. Later, many cases in the Supreme Court made arresting communists illegal, because it is free speech.
In 1957, the Soviet Union launched a satellite named Sputnik 1 into outer space. Later, they were the first to put people in outer space. Because of this, President John F. Kennedy wanted to send a man to the Moon. This started the Space Race, and the United States began the Apollo program, a project used for outer space. In 1969, the United States launched Apollo 11, and was the first country to send people to the Moon.
The economy of the United States grew a lot during the 1950s and 1960s, with Americans having the highest average income in the world. Because of this, many people moved to large cities and suburbs, and had more children. The country built the Interstate Highway System, which made it easier to travel and move things across the country. In 1959, Alaska and Hawaii became U.S. states.
There were many social movements in the United States during this time. The Civil Rights Movement wanted to give African Americans more rights. It was led by people such as Rosa Parks, Martin Luther King, Jr. and Malcolm X. It was very successful, and helped make many laws that protected African Americans, such as the Civil Rights Act of 1964 and 1968. John F. Kennedy was murdered in 1963, and Lyndon B. Johnson became the President. Johnson sent United States soldiers to the Vietnam War, and many people did not like this. This started the counterculture movement. Counterculture included many things, such as people who wanted black nationalism, peace, and more rights for women. In 1969, the Stonewall riots in New York marked the start of the gay rights movement.
In 1974, Richard Nixon became the first President to resign. This was because of the Watergate scandal, where people broke into the Democratic headquarters and took a lot of information to help Nixon. During the Yom Kippur War, the United States helped Israel. Because of this, countries in OPEC did not let the United States buy their oil. This started an oil crisis. In the 1970s, the country's economy had a problem with stagflation, something that happens when prices go up but production does not. When Ronald Reagan was elected in 1980, he tried to stop this with lower taxes and less government control over the economy. These changes were called Reaganomics. The Cold War ended in 1991. This was because the Soviet Union dissolved. With no Soviet Union, the United States became the largest superpower in the world.
 Modern history 
September 11 attacks War in Afghanistan (2001–present) Iraq War Great Recession
Under President George H. W. Bush, the country took on a global dominant role worldwide, as in the Gulf War (1991). The longest economic expansion in modern American history, from March 1991 to March 2001, spanned the presidency of Bill Clinton and the dot-com bubble. A civil lawsuit and a sex scandal led to his impeachment in 1998, although he managed to finish his period. The 2000 presidential election was one of the most competitive in American history, it was settled by the Supreme Court: George W. Bush, son of George H. W. Bush, became president, even though he gained fewer votes than his opponent Al Gore.
On September 11, 2001, the terrorists of the Al-Qaeda group attacked the twin towers of the World Trade Center in New York City (which were destroyed) and the Pentagon near Washington, D.C., in a series of attacks that killed nearly three thousand people. In response, the Bush administration launched the "War on Terror." At the end of 2001, U.S. forces invaded Afghanistan, removed the Taliban government and destroyed Al-Qaeda's training camps. Taliban insurgents continued to fight a guerrilla war. In 2002, Bush began to discuss a regime change in Iraq. With a lack of support from NATO and without a clear UN order for military intervention, Bush organized a coalition of governments to begin an invasion. The coalition forces quickly invaded Iraq in 2003 and ended the government of Saddam Hussein. The following year, Bush was re-elected as the most voted president in an election.
In 2005, Hurricane Katrina, which would end up being the deadliest natural disaster in national history, caused severe destruction along the Gulf Coast: the city of New Orleans was mostly destroyed, with 1833 dead.
On November 4, 2008, during a global economic downturn, Barack Obama was elected president, having been the first African American to take office. In May 2011, American Special forces managed to kill Osama bin Laden, hiding in Pakistan. The following year, Barack Obama was re-elected. Under his second term, he led the war against the Islamic State and restored diplomatic relations with Cuba.
On November 8, 2016, the Republican Party leader Donald Trump defeated former First Lady Hillary Clinton for presidency.
The massacres in Orlando of June 12, 2016 at the gay disco Pulse (51 dead) and in Las Vegas on October 1, 2017 (60) are listed as the largest massacres in the country since 9/11. The murder of George Floyd in late May 2020 sparked protests all over the world that demanded racial justice. On January 6, 2021, the United States Capital in Washington, D.C. was stormed during a riot and violent attack against the U.S. Congress.
 Government 
The United States is the world's oldest surviving federation. It is a constitutional republic and representative democracy, "in which majority rule is tempered by minority rights protected by law." The government is controlled by a system of checks and balances from the United States Constitution. The constitution is the country's main legal document. There are three branches. They are the executive branch, the legislative branch, and the judicial branch. State governments and the federal government work in very similar ways. Each state has its own executive, legislative, and judicial branches. The executive branch of a state government is led by a governor, instead of a president.
 Executive branch 
The executive branch is the part of the government that enforces the law. Members of the U.S. Electoral College elect a president who is the leader of the executive branch, as well as the leader of the armed forces. The president may veto a bill that Congress has passed, so it does not become a law. The President may also make "executive orders" to ensure that people follow the law.
The president is in charge of many departments that control much of the day-to-day actions of the government. For example, the Department of Commerce makes rules about trade. The president chooses the heads of these departments and also nominates federal judges. However, the Senate, part of the legislative branch, must agree with all of the people the president chooses. The president may serve two 4-year terms. 
 Legislative branch 
The legislative branch makes laws. The legislative branch is called the United States Congress. Congress is divided into two "houses".
One house is the House of Representatives. The Representatives are each elected by voters from set areas within the states. The number of Representatives a state has is based on how many people live there. Representatives serve two-year terms. The total number of representatives today is 435. The leader of the House of Representatives is the Speaker of the House.
The other house is the Senate. In the Senate, each state is represented equally, by two senators. Because there are 50 states, there are 100 senators. The President's treaties or appointments of officials need the Senate's approval. Senators serve six-year terms. The Vice President of the United States serves as president of the Senate. In practice, the vice president is usually absent from the Senate, and a senator serves as president pro tempore, or temporary president, of the Senate.
Representatives and senators propose laws, called "bills", in their respective houses. A bill may be voted upon by the entire house right away or may first go to a small group, known as a committee, which may recommend a bill for a vote by the whole house. If one house votes to pass a bill, the bill then gets sent to the other house; if both houses vote for it, it is then sent to the president, who may sign the bill into law or veto it. If the president vetoes the bill, it is sent back to Congress. If Congress votes again and passes the bill with at least a two-thirds majority, the bill becomes law and cannot be vetoed by the president.
Under the American system of federalism, Congress may not make laws that directly control the states; instead, Congress may use the promise of federal funds, or special circumstances such as national emergencies, to encourage the states to follow federal law. This system is both complex and unique.
 Judicial branch 
The judicial branch is the part of government that interprets what the law means. The Judicial Branch is made up of the Supreme Court and many lower courts. If the Supreme Court decides that a law is not allowed by the Constitution, the law is said to be "struck down" and is no longer a valid law.
The Supreme Court is made up of nine judges, called justices, who are nominated by the President and confirmed by the Senate. One of these justices, called the chief justice, heads the court. A Supreme Court justice serves until he or she dies or resigns. When that happens, the president nominates someone new to replace the justice who left. If the Senate agrees with that choice, the person becomes a justice. If the Senate does not agree with the president's choice, then the president must nominate someone else.
Famous court cases such as Marbury v Madison (which was decided in 1803) have firmly established that the Supreme Court is the ultimate interpreter of the United States Constitution and has the power to strike down any law that conflicts with it.
 Politics 
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   TrumpPortrait.jpg
  Donald Trump, President of the United States since 2025
  VancePortrait.jpg
  JD Vance, Vice President of the United States since 2025
The United States of America consists of 50 states, 5 territories, and 1 district (Washington D.C.). States can make laws about things inside the state, but only federal laws can apply to more than one state or dealing with other countries. In some areas, if the federal government makes laws that say different things from the state laws, people must follow the federal law. Each state has a constitution of its own, different from the federal (national) constitution. Each of these is like the federal constitution because they say how each state's government is set up, but some also talk about specific laws.
The federal and most state governments are dominated by two political parties: the Republicans and the Democrats. There are many smaller parties; the largest of these are the Libertarian Party and the Green Party. People help in political campaigns that they like. They try to persuade politicians to help them; this is called lobbying. All Americans are allowed to do these things, but some have and spend more money than others, or in other ways do more in politics. Some people think this is a problem, and lobby for rules to be made to change it.
The USA's large cultural, economic, and military influence has made the foreign policy of the United States, or relations with other countries, a topic in American politics, and the politics of many other countries.
 Political divisions 
 States 
The United States conquered and bought new lands over time, and grew from the original 13 colonies in the east to the current 50 states, of which 48 of them are joined together to make up the contiguous United States. These states, called the "lower 48", can all be reached by road without crossing a border into another country. They go from the Atlantic to the east to the Pacific in the west. There are two other states which are not joined to the lower 48 states. Alaska can be reached by passing through British Columbia and the Yukon, both of which are part of Canada. Hawaii is in the middle of the Pacific Ocean.
Washington, D.C., the national capital, is a federal district that was split from the states of Maryland and Virginia in 1791. Not part of any US state, it used to be in the shape of a square, with the land west of the Potomac River coming from Virginia and the land east of the river coming from Maryland. In 1846, Virginia took back its part of the land. Some people living in DC want it to become a state, or for Maryland to take back its land, so that they can have the right to vote in Congress.
 Territories and possessions 
The United States territories and possessions consist of sixteen lands that do not state, many of which are colonial territories. None of them have any land borders with the rest of the US. People live in five of these places, which are de facto American:
Puerto Rico
American Samoa
Guam
U.S. Virgin Islands
Northern Mariana Islands
The Philippines was a possession of the United States. Palau, the Federated States of Micronesia, and other Pacific island nations were governed by the United States as a United Nations "Trust Territory". All of these places have become independent: the Philippines in 1946, Palau in 1947, and Micronesia in 1986.
The U.S. armed forces has bases in many countries, and the U.S. Navy's base at Guantanamo Bay was rented from Cuba after that country had a Communist revolution.
 Counties and cities 
All the states are divided into administrative subdivisions. Most of them are called counties, but Louisiana uses the word "parish," and Alaska uses the word "borough."
There are many cities in the United States. One city in each state is the state capital, where the government of the state meets and the governor works. This city is not always the largest in its state. For example, the city with the most people living in is New York City in New York State, but the state capital is Albany, New York. Some other big cities are Los Angeles, California; Chicago, Illinois; Seattle, Washington; Miami, Florida; Indianapolis, Indiana; Las Vegas, Nevada; Houston and Dallas, Texas; Philadelphia and Pittsburgh, Pennsylvania; Boston, Massachusetts; Denver, Colorado; St. Louis, Missouri, Memphis, Tennessee, Atlanta, Georgia, San Francisco, California, San Diego and Detroit, Michigan.
 Foreign relations and military 
The United States is very influential in global economics, politics, and the military. It is a permanent member of the United Nations Security Council and the headquarters of the United Nations is in New York City. It is a member of the G7, G20, and Organization for Economic Co-operation and Development. Almost all countries have embassies in Washington, D.C., and many have consulates around the country. Likewise, nearly all nations host American diplomatic missions. However, Iran, North Korea, Bhutan, and Taiwan do not have formal diplomatic relations with the United States. The United States has a "special relationship" with the United Kingdom and strong ties with Canada, Australia, New Zealand, Japan, South Korea, and Israel.
The president is the commander-in-chief of the country's armed forces and appoints its leaders, the secretary of defense and the Joint Chiefs of Staff. The United States Department of Defense administers the armed forces, including the Army, Marine Corps, Navy, and Air Force. The Coast Guard is run by the Department of Homeland Security in peacetime and by the Department of the Navy during times of war. In 2008, the armed forces had 1.4 million personnel on active duty, along with several hundred thousand each in the Reserves and National Guard for a total of 2.3 million troops. The Department of Defense also employed about 700,000 civilians, not including contractors.
The military budget of the United States in 2011 was more than $700 billion, 41% of global military spending and equal to the next 14 largest national military expenditures combined. At 4.7% of GDP, the rate was the second-highest among the top 15 military spenders, after Saudi Arabia. U.S. defense spending as a percentage of GDP ranked 23rd globally in 2012 according to the CIA. The proposed base Department of Defense budget for 2012, $553 billion, was a 4.2% increase over 2011; an additional $118 billion was proposed for the military campaigns in Iraq and Afghanistan. The last American troops serving in Iraq departed in December 2011; 4,484 service members were killed during the Iraq War. Approximately 90,000 U.S. troops were serving in Afghanistan in April 2012; by November 8, 2013 2,285 had been killed during the War in Afghanistan.
 Economy 
The United States has a capitalist economy. The country has rich mineral resources, with many gold, coal, and uranium deposits. Farming makes the country among the top producers of, among others, corn (maize), wheat, sugar, and tobacco. Housing contributes about 15% to the gross domestic product (GDP) of the United States. America produces cars, airplanes, and electronics. About 3/4 of Americans work in the service industry.
 Science and technology 
The United States is the world's foremost scientific power. It has the most Nobel Prize winners of any country. The U.S. spends more on research and development than any other country. It is the leading country in artificial intelligence development.
Notable American inventions are: the telephone, the electric telegraph, the electric light, the cinema, the gramophone, the airplane, the internet, and the video game console. Although the U.S. did not invent the automobile, the world's first mass-produced car was American.
 Demographics 
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The United States of America has people of many different race and ethnic backgrounds. 80% of the people in the United States descend from European immigrants. Many people are descended from Germany, England, Scotland, Ireland, Africa, and Italy. 13% of the people in the United States are African-American. Most of them descend from the African slaves that were brought to America. African Americans are concentrated in the Southern United States. Asian-Americans make up only 5% of the population in America but make up a bigger portion in the west coast. For example, in California, Asian-Americans make up 13% of the population of that state. Hispanic-Americans or people of Latin origins make up 15% of the nation. Mexicans are the largest Hispanic national group, followed by Puerto Ricans, Cubans, Salvadorans and Dominicans. The original peoples, called Native American, American Indians, or Amerindians and Inuit (Eskimos) are a very small group. There are 574 federally recognized Native American tribes in the United States.
11% of the people in the United States are foreign-born. 18% speak a language other than English at home. For people 25 and older, 80% are high school graduates while 25% have a bachelor's degree or higher.
The 2000 Census counted self-reported ancestry. It identified 43 million German-Americans, 30.5 million Irish-Americans, 24.9 million African-Americans, 24.5 million English-Americans, and 18.4 million Mexican Americans.
The United States has the largest number of immigrants of any country in the world. The United States attracts immigrants from other countries due to the American Dream. Most immigrants in the United States come from Mexico, China, India, the Philippines and El Salvador. 
 Money 
The social structure of the United States has a big range. This means that some Americans are much, much richer than others. The average (median) income for an American was $37,000 a year in 2002. However, the richest 1% of Americans have as much money as the poorest 90%. 51% of all households have access to a computer and 41% had access to the Internet in 2000, a figure which had grown to 75% in 2004. Also, 67.9% of American families owned their homes in 2002. There are 200 million cars in the United States, two for every three Americans. The debt has grown to over $21,000,000,000,000.
 Religion 
There are many different religions in the U.S. Statistically, the largest religion is Christianity, including groups such as Catholicism and Protestantism. Other religions include Hinduism, Islam, Judaism, Unitarian Universalism, Wicca, Druidry, Baha'i, Raelism, Zoroastrianism, Taoism, and Jainism. Religions which were founded within the United States include Eckankar, Satanism, and Scientology. Native American religions have various animistic beliefs.
The United States is one of the most religious countries in the Western World, and most Americans believe in God. The number of Christians in the U.S. has gone down. 86.2% called themselves Christian in 1990 and 78.4% said this in 2007. The others include Judaism (2.3%), Islam (0.8%), Buddhism (0.7%), Hinduism (0.4%), and Unitarian Universalism (0.3%). Those who have no religion are at 16.1%. There is a large difference between those who say that they belong to a religion and those who are members of a religious body of that religion.
Doubts about the existence of a God, gods or goddesses are higher among young people. Among the non-religious population of the U.S., there are deists, humanists, ignostics, atheists, and agnostics.
 Language 
English (American English) is the de facto national language. Although there is no official language at the federal level, some laws (such as U.S. naturalization requirements) standardize English. In 2010, about 230 million, or 80% of the population aged five years and older, spoke only English at home. Spanish, spoken by 12% of the population at home, is the second most common language and the most widely taught second language. Some Americans advocate making English the country's official language, as it is in at least twenty-eight states. Both Hawaiian and English are official languages in Hawaii by state law.
While neither has an official language, New Mexico has laws providing for the use of both English and Spanish, as Louisiana does for English and French. Other states, such as California, order the publication of Spanish versions of certain government documents including court forms. Many jurisdictions with large numbers of non-English speakers produce government materials, especially voting information, in the most commonly spoken languages in those jurisdictions.
Several insular territories grant official recognition to their native languages, along with English: Samoan and Chamorro are recognized by American Samoa and Guam, respectively; Carolinian and Chamorro are recognized by the Northern Mariana Islands; Spanish is an official language of Puerto Rico and is more widely spoken than English there.
Many Native American languages in the United States are endangered.
 Education 
In most states, children are required to attend school from the age of six or seven (generally, kindergarten or first grade) until they turn eighteen (generally bringing them through twelfth grade, the end of high school); some states allow students to leave school at sixteen or seventeen. About 12% of children are enrolled in parochial or nonsectarian private schools. Just over 2% of children are homeschooled.
 Culture 
American popular culture goes out to many places in the world. It has a large influence on most places, especially the Western world. American music is everywhere, and American movies and television shows can be seen in most countries.
 Federal holidays 
Flag
The American flag is made up of 50 stars on a blue background and has 13 stripes, seven red and six white. It is one of many symbols of the United States like the Bald Eagle. The 50 stars represent the 50 states. The red stands for courage, the blue stands for justice, and the white represents peace and cleanliness. The 13 stripes represent the 13 original colonies.
Cuisine
A hamburger is one of the popular foods of the United States. Fast food in the United States is home to many regional cuisines such as the Cuisine of the Southern United States, also known as Southern food. There are Americanized versions of Chinese, Greek, Japanese, Italian and Mexican cuisine. Native American cuisine is the cuisine of the indigenous Native Americans. A lot of American dishes are influenced by many countries around the world. American cuisine has Native American, British, French, German, and Spanish influences. Soul food is traditional southern African American food.
Music
The most popular genres in the United States are rock and roll, pop, country, R&B, and hip hop. Native American music is the indigenous music of the United States. American singers that became global superstars include Whitney Houston, Michael Jackson, and Madonna. 
Sports
Native Americans played lacrosse before Europeans arrived. Baseball is the country sport for the United States, and American football is the most popular sport. Basketball is also very popular in the USA, which the USA has its own league called the NBA.
Video games
The video game industry of the United States is one of the largest of any country. It is the second largest market for video games after China. Many of the world's largest video game developers are based in the USA, like Take-Two Interactive, Electronic Arts, Activision Blizzard, and Xbox Game Studios.


Science: Science is what we do to find out about the natural world. There are different kinds of science. Natural sciences study nature and the physical world. They include chemistry, biology, geology, astronomy, and physics. Social sciences study people and how societies work. They include, psychology, sociology, and economics. Applied sciences use the things we learn from science to solve problems. They include, engineering and medicine. Science uses mathematics, computer science and logic, which are sometimes called "formal sciences". Science makes observations and experiments. Science produces accurate facts, scientific laws, and theories. 'Science' also refers to the large amount of knowledge that has been found using this process.
Research uses the scientific method. Scientific research uses hypotheses based on ideas or earlier knowledge, which can be categorized through different topics. Then those hypotheses are tested by experiments. People who study and research science and try to find out everything about it are called scientists. Scientists study things by looking at them very carefully, by measuring them, and by doing experiments and tests. Scientists try to explain why things act the way they do, and predict what will happen.
The history of science is thousands of years old. The beginnings of modern science can be found in ancient Egypt and Mesopotamia (around 3000–1200 BCE). They developed early ideas in math, astronomy, and medicine. They influenced the Greeks, who tried to explain what happened in our world using natural causes. Later, during India’s Golden Age, more progress was made. This included the creation of the Hindu–Arabic number system.
After the Western Roman Empire fell during the Early Middle Ages (400–1000 CE), scientific progress in Europe slowed down. But progress started up again during certain periods. In the Islamic world, scholars kept, studied, and made Greek science better during the Islamic Golden Age. Later, when the Byzantine Empire got weaker, its scholars took Greek books to Western Europe, helping start the Renaissance.
From the 10th to 13th centuries, Europe re-learned Greek and Islamic science. This helped cause the scientific revolution in the 16th century. Science began to change a lot. New discoveries removed old ideas. The scientific method became more important. By the 19th century, science became more professional and organized. Instead of calling it “natural philosophy”, it was called “natural science”.
Etymology and Terminology
The history of the word “science” and the terms used in science tell us a lot about how the subject has changed over time and what it means in different cultures and philosophies. The word “science” comes from the Latin word “scientia,” which means “knowledge.” That word came from the verb “scire,” which means “to know.” In ancient Rome, scientia just meant any kind of knowledge you could get through learning or experience. It included many subjects like philosophy, speaking (rhetoric), and math. But back then, it did not mean what we now think of as science. In ancient Greece, a similar idea was called “epistēmē” (ἐπιστήμη). Philosophers like Aristotle and Plato used this word to mean knowledge that could be proven through reason. They saw it as different from “doxa,” which meant opinion, something that might not be true or could change.
The meaning of “science” started to change during the Scientific Revolution in the 1500s and 1600s. During this time, famous thinkers like Galileo Galilei, Johannes Kepler, Francis Bacon, and Isaac Newton began to create a new way of learning about the world. They used observation, experiments, and math to find answers. This method helped form the basis of what we now call modern science. Back then, people did not call it “science”, they called it “natural philosophy”, especially when studying the physical world like motion, planets, or light. The word “scientist” was not even invented until 1834. A man named William Whewell, who studied the history and philosophy of science, came up with it. He suggested “scientist” in a book review to describe someone who studies the natural world in a careful, organized way, just like the word “artist” describes someone who creates art.
Science words (or scientific terms) have changed and grown over time, just like science itself. In the past, many science terms came from Greek and Latin, because those were the main languages used by educated people in Europe for many centuries. For example, the word “biology” comes from the Greek words bios (which means “life”) and logos (which means “study”), so biology means “the study of life.” The word “physics” comes from the Greek word physis, meaning “nature.” “Chemistry” likely comes from the Arabic word al-kīmiyāʾ, which has roots in ancient Egyptian and Greek (Hellenistic) alchemy, a practice that tried to change substances and discover how materials work.
Scientific terms are very important because they help scientists be precise (use exact meanings), communicate clearly, and organize ideas and information. Each area of science has its own set of special words to describe things it studies. For example, in physics, words like “momentum,” “entropy,” and “quantum” have very specific meanings that are different from how they are used in everyday language. In biology, scientists name living things using a system called Linnaean binomial nomenclature. This system gives each organism a Latin name made of two parts: the genus and species. For example, humans are called Homo sapiens. Having standardized scientific words is important for making sure experiments can be repeated correctly (replicability), letting scientists check each other’s work (peer review), and helping scientists from all over the world work together, even if they speak different languages.
Many words used in science today come from the history and culture of the time when they were first used. Some scientific terms actually started out as jokes, creative ideas, or simple descriptions, but they became official because people started using them so much. For example, the term “Big Bang” was first used by astronomer Fred Hoyle in a radio broadcast in 1949. He did not believe in the Big Bang theory and actually used the name to make fun of it. But, the name caught on and is now the main word we use for how the universe began. Another example is the word “quark,” which is used for tiny particles that make up protons and neutrons. Physicist Murray Gell-Mann chose this word from a line in a book called Finnegans Wake by James Joyce. This shows how creativity and language play a role in science.
Scientific words often come from metaphors or comparisons to help people understand complex ideas. For example, the word “cell” in biology was first used by Robert Hooke in 1665. When he looked at cork under a microscope, he saw tiny box-like spaces that reminded him of the small rooms (cells) where monks lived. That is how the name “cell” was born. In computer science, the word “virus” is used to describe a program that can copy itself and infect other programs, just like real viruses do in the human body. Scientists used this name because it helped people understand how these harmful programs behave. These examples show that scientific words often come from real-world ideas, and they help us understand new or difficult concepts by connecting them to things we already know.
The language used in science is always changing to become more fair, clear, and inclusive. In the past, many scientific terms were based on ideas that focused too much on humans, Europe, or one gender. Over time, scientists have worked to replace these words with ones that are more neutral and accurate. For example, in astronomy, the word “planet” was officially redefined in 2006 by the International Astronomical Union (IAU). Because of this change, Pluto was no longer called a full planet, but instead a dwarf planet, which caused a lot of debate. In everyday science language, people now often say “human” instead of “man”, and “humankind” instead of “mankind”. These changes help make science feel more welcoming to everyone, no matter their background or identity.
As science keeps moving forward, new inventions and discoveries also bring new words. For example, scientists discovered CRISPR, a tool that can edit genes. The word CRISPR is an acronym (a short word made from initials) for “Clustered Regularly Interspaced Short Palindromic Repeats.” It shows how scientists create short terms to explain big, complex ideas. New fields like machine learning, artificial intelligence (AI), and quantum computing are also creating new words. These words need to be defined very clearly, so scientists from different areas can understand each other and work together without confusion.
History
Prehistoric science
The period of prehistoric science includes the Palaeolithic, Mesolithic, and Neolithic ages, which lasted from about 2.5 million years ago to around 3000 BCE. Even though these early efforts were not "science" the way we know it today, they showed careful observation, experiments, and the start of organized thinking. One of the first and most important forms of early science was making stone tools. It started with a human ancestor called Homo habilis and continued through different prehistoric cultures like the Acheulean, Mousterian, and Upper Paleolithic. People made tools like hand axes, scrapers, awls (used for poking holes), and spear tips. To do this, they had to understand how stones break, what kinds of rocks work best, and how to hit them with the right amount of force. Archaeologists (scientists who study ancient human history) have found evidence that these early humans learned by trial and error, and they shared their knowledge with others. Over time, they improved their tools to make them stronger and more useful. This kind of hands-on learning and passing down of skills is one of the earliest examples of using science to solve problems, especially in understanding materials and how to use them, what we now call materials science.
Early humans also showed basic knowledge of astronomy, the study of the sky. This can be seen in archaeological discoveries like the Blanchard bone (around 30,000 BCE) and the Lascaux Cave paintings (around 17,000 BCE). These objects and drawings may show that early people were watching the Moon’s phases or recognizing star groups like the Pleiades constellation. One of the earliest sky-watching structures is the Nabta Playa stone circle in southern Egypt, built around 6000 BCE. It is one of the oldest known examples of people using megaliths (large stones) to line up with the summer solstice sunrise, the longest day of the year. It may also have helped track seasonal rains, which were very important for survival in that dry area. These structures show that early people spent years carefully watching the sky, noticing patterns, and trying to predict natural cycles, like when rain or seasons would come. This was an early step toward the science of astronomy.
Another major scientific step happened during the Neolithic Revolution, around 10,000 BCE, when humans started farming. This time period was extremely important because people learned to grow plants and raise animals. They had to understand soil, water, and weather patterns to grow food successfully. Early farmers in places like the Fertile Crescent (in the Middle East), China’s Yellow River valley, and Mesoamerica (Central America) grew crops like wheat, barley, millet, rice, and maize (corn). They watched how these plants grew over many generations and chose the ones with the best traits, like more food, better taste, or the ability to survive without much water. This careful choosing is the beginning of genetics (even though they did not know that word), and their work helped people settle down, build villages, and eventually start civilizations. These farming skills are the beginning of what we now call agricultural science.
Even though prehistoric people did not write things down, they still had surprisingly advanced medical knowledge. One of the most amazing examples is trepanation, a kind of surgery where people removed parts of the skull. Scientists have found skulls from as far back as 7000 BCE in Europe, Africa, and South America that show signs of this surgery, and some of the skulls even show healing, which means people survived the operation. This suggests that early humans had basic knowledge of the body, special tools, and may have used natural medicines, like herbs, to reduce pain or stop infection. Another example is Ötzi the Iceman, a frozen body found in the Alps from around 3300 BCE. His body had tattoos that lined up with places used in acupuncture, a method of healing still used today. He was also carrying a medicinal fungus called Fomitopsis betulina, which may have been used to fight infection.
People in prehistoric times also began using math and measurement. They made tools like tally sticks to help them count. One of the oldest is the Lebombo bone from about 35,000 BCE, and another is the Ishango bone from around 20,000 BCE. These bones have carved marks that might show number patterns, multiples, or even prime numbers. Some scientists think they may also have tracked lunar cycles (the phases of the moon). Later, in places like Mesopotamia and the Indus Valley, early people started using standard measurements. They needed these for storing grain, trading goods, and building things. They created systems to measure volume and weight, which helped them with farming, business, and early forms of engineering.
In prehistoric times, early people showed amazing skills in building large stone structures, which we now call megalithic architecture. One of the oldest examples is Göbekli Tepe, built around 9600 BCE in what is now Turkey. It has huge stone pillars carved with animals, arranged in circles. These stones are very heavy, and moving and placing them would have taken many people working together, using simple tools and their understanding of leverage (how to move heavy things using force and angles). They probably also used basic measuring tools to plan where to put the stones. Other examples, like Stonehenge in England (about 3000 BCE) and the stone lines at Carnac in France, show that people in different places had similar building knowledge. These projects needed not just strength, but also careful planning, measuring, and even watching the sky to line up stones with seasons or sunrise points. This shows early knowledge of engineering and astronomy.
Early humans were also smart about taking care of the environment. They used fire in helpful ways. For example, Australian Aboriginals practiced something called fire-stick farming. They burned certain areas on purpose to help new plants grow, bring animals to the area, and make the land easier to manage. In the Andes Mountains (in South America), ancient farming communities built terraces (step-like farms on hills) and irrigation systems to control water. This helped them farm on steep land and stop soil from washing away. These people understood how to manage natural resources, like water and soil, in ways that are similar to today’s ideas of sustainability (protecting the environment while meeting human needs).
People also started to create ways to record and share information. Early forms of writing included cave art, petroglyphs (pictures carved in rock), and tokens (small clay objects used for counting). In Mesopotamia, around 8000 BCE, people used clay tokens to keep track of food and goods. Over time, this system turned into writing, known as cuneiform. Writing allowed people to store knowledge, like farm records and star charts, and pass it on to future generations. This step, moving from oral traditions (speaking stories and knowledge) to writing things down, helped humans build on knowledge over time. It was the beginning of being able to record science and discoveries so they would not be forgotten.
Ancient science
Ancient science was the early scientific knowledge and inventions created by some of the first civilizations in places like Mesopotamia, Egypt, the Indus Valley, ancient China, and Mesoamerica (Central America). This period lasted from around 3000 BCE to 600 BCE. In Mesopotamia, one of the first places where civilization began, science was used in areas like farming, government, and astronomy. The Sumerians and Babylonians created number systems, including one based on 60 (sexagesimal). This is why we still use 60 minutes in an hour and 360 degrees in a circle today. They were also great at studying the sky. On clay tablets written in cuneiform writing, they recorded things like star charts, calendars, and math problems. Babylonian astronomers carefully watched the Moon and planets, and used math to predict eclipses (when the Moon or Sun is covered) and planet alignments. One of their most important discoveries was the Metonic cycle, which is a 19-year cycle where the Moon’s phases happen on the same days of the calendar again.
Ancient Egyptian science was closely tied to farming, timekeeping, and building large monuments. The Egyptians created a 365-day calendar by watching the star Sirius (which they called Sopdet). Every year, Sirius rose in the sky just before the Nile River flooded. This was a very important event that made the soil good for growing crops. To prepare for this, the Egyptians had to be good at measuring the sky and water levels. They also used science to build huge structures like the pyramids. These amazing buildings needed careful planning, geometry (math involving shapes), and knowledge of materials and surveying (measuring land). Egyptian engineers were skilled at all of these things. A famous document called the Rhind Mathematical Papyrus, written around 1650 BCE, shows that Egyptian students learned math such as fractions, areas, volumes, and ratios. 
The Indus Valley Civilization (around 2600 to 1900 BCE) also had amazing science and technology, even though we still do not fully understand it because their writing system has not been decoded yet. Cities like Mohenjo-Daro and Harappa were built in neat grid patterns, had advanced drainage systems, and used standard sizes for things like weights and bricks. At a site called Lothal, scientists found a dockyard, which shows that people there understood how tides worked and how to build for the sea. The use of uniform bricks and measuring tools also shows they had organized production and possibly government systems to manage it. All of this points to high-level technical knowledge and careful planning.
In ancient China, science grew together with philosophy and government. People studied nature because they wanted to understand it better and use that knowledge to rule wisely. One early book called the I Ching (also called the Book of Changes) was written by at least the late 2nd millennium BCE. It used patterns and binary logic (ideas based on two choices, like yes/no or dark/light) to explain natural cycles and how the universe worked. By the time of the Zhou Dynasty (around 1046–256 BCE), Chinese scientists had invented many useful tools and ideas. They created early earthquake detectors (called seismographs), studied the stars (astronomy), used plants as medicine (herbal medicine), and learned how to work with metals and chemicals (metallurgy). They also made a complex calendar based on both the Moon’s phases and solar seasons. This calendar used a 60-part cycle (called the sexagenary cycle) and showed that they had a very advanced way of measuring time using math. The Chinese were also experts in bronze casting, making tools and objects out of melted metal. They made alloys (mixtures of metals) that had the same recipe every time, using careful control of heat and basic understanding of chemistry.
In Mesoamerica, the Olmec (around 1500–400 BCE) and later the Maya made amazing discoveries in astronomy, math, and building. The Maya used a base-20 number system (instead of base-10 like we use). They invented long count calendars that could track time for thousands of years. They watched the sky carefully and recorded things like the movements of Venus, eclipses, solstices, and planet alignments. Some of this information was written in books called codices, like the Dresden Codex. The Maya used these observations to plan religious festivals, decide when to plant crops, and even schedule political events. They built pyramids and temples that lined up perfectly with events in the sky. They were constantly watching the sky and collecting data, just like modern scientists.
In all of these ancient civilizations, medicine was based on practical experience and observation, even though it was often connected to religion or spiritual beliefs. In Ancient Egypt, a medical text called the Edwin Smith Papyrus (written around 1600 BCE) showed that Egyptian doctors had a logical and scientific approach to treating injuries. It includes detailed instructions for doing surgery and describes how to treat injuries to the head, spine, and arms or legs. It even gave predictions (called prognosis) about how serious the injuries were and what might happen to the patient, depending on where the injury was. In Mesopotamia, doctors used a book called the Diagnostic Handbook by a man named Esagil-kin-apli (around 1069 BCE). This book helped doctors figure out what was wrong with a patient by looking at their symptoms, then choosing treatments. These treatments might include herbal medicine, chants or prayers, and physical treatments like bandages or surgeries. In Ancient China, a book called the Shennong Bencaojing (written in the first millennium BCE) listed hundreds of medicinal herbs and their effects on the body. This book became the base of Traditional Chinese Medicine. It showed how Chinese doctors tested herbs, classified their properties, and wrote down their results, which helped build knowledge over many generations.
These early sciences also depended on good recordkeeping, which allowed people to share and pass on knowledge over time. Each civilization created its own writing system, including cuneiform in Mesopotamia, hieroglyphs in Egypt, oracle bone script in China, and Maya script in Mesoamerica. These writing systems helped people write down important information, like star charts and astronomy, math tables, medical recipes, and instructions for building things Because this knowledge was written down, it could be passed from one generation to the next, even if science back then was usually done by priests, scribes, or royal advisors, not scientists like we think of today.
Classical antiquity
Classical science started in ancient Greece, especially in a city called Miletus. There, early thinkers like Thales (around 624–546 BCE) tried to explain how the world works without using stories about gods or magic. Thales believed that water was the basic substance that everything came from. This idea started a new way of thinking called natural philosophy, which focused on finding natural causes for things we observe in the world. After Thales, other thinkers from Miletus also had big ideas. Anaximander said everything came from something called the apeiron (which means “the boundless” or unlimited). Anaximenes thought air was the most important element.
Another important thinker was Pythagoras (around 570–495 BCE). He believed that numbers and math were the true key to understanding the universe. His school studied geometry, music, and numbers. They discovered that when strings of certain lengths were plucked, they made sounds that matched simple number ratios. This showed a connection between music and math, and supported the idea that the whole universe had a kind of mathematical harmony, a concept called the “music of the spheres.” In medicine, a famous Greek named Hippocrates (around 460–370 BCE) made big changes. He said that illness was not caused by angry gods or magic, but by natural causes like diet, environment, and the balance of fluids in the body. He taught doctors to carefully observe patients, keep records, and study the effects of things like climate and food on health. Hippocrates also created the four-humor theory, which said the body had four fluids: blood, phlegm, black bile, and yellow bile. People believed for many centuries that staying healthy meant keeping these in balance. He also introduced ethical rules for doctors, such as those in the Hippocratic Oath, which is still remembered today. Because of Hippocrates, medicine became more scientific and professional.
Aristotle (384–322 BCE) was one of the most important thinkers in ancient science. He was a student of Plato and later became the teacher of Alexander the Great. Aristotle wrote about many subjects, including biology, physics, astronomy, psychology, and logic. In biology, Aristotle studied sea animals and did some of the first dissections (cutting open bodies to learn how they work). He grouped animals by their physical traits, which helped create early classification systems. In physics and astronomy, he believed the Earth was at the center of the universe (geocentric model), and that everything in the sky moved in perfect circles made of a special substance called aether. He thought the heavens were unchanging, unlike Earth, which was made of earth, water, air, and fire, things that could decay and change. Aristotle also created a famous way of thinking about why things happen, called the four causes. The material cause was what something is made of, the formal cause was the shape or structure of the thing, the efficient cause was what caused it to happen, and the final cause was its purpose or goal.
One of Aristotle’s students, Theophrastus (around 371–287 BCE), took these ideas further, especially in plant science. He wrote books like Enquiry into Plants and On the Causes of Plants, where he tried to classify and describe plants in a clear and organized way. He noticed details like where plants grow, how they reproduce, and how people use them. He divided plants into groups like trees, shrubs, and herbs, and because of his work, he is known as the “father of botany”. His ideas about plants were so good that scientists still read them over a thousand years later. After Alexander the Great died, Greek science continued to grow during the Hellenistic period (323–31 BCE). One of the most famous centers of learning was Alexandria, in Egypt. It had a huge library, scholars from many cultures, and support from powerful leaders. One of the most important scientists there was Euclid (around 300 BCE). He wrote a book called The Elements, which organized everything known about geometry into 13 books. He proved shapes and math rules using logic and step-by-step thinking. This method, called the axiomatic method, became the model for how scientists and mathematicians would work for thousands of years.
Archimedes of Syracuse (about 287–212 BCE) was a brilliant ancient scientist and inventor. He made important discoveries in physics, engineering, and math. One of his most famous ideas is called Archimedes' principle. It says that when you put something in water (or another liquid), the liquid pushes up on it with a force equal to the weight of the liquid the object pushes out of the way. This explains why some things float and others sink. Archimedes also studied levers, pulleys, and other machines, helping to create the basic rules of mechanical engineering. He figured out how to calculate the area and volume of different shapes and came up with a very good estimate for the number pi (π).
Eratosthenes (about 276–194 BCE) was the head librarian at the Library of Alexandria, one of the greatest learning centers of the ancient world. He was good at math, astronomy, and geography, and used these skills to measure the size of the Earth very accurately. He did this by comparing shadows in two cities at the same time during the summer solstice (the longest day of the year). He also invented a system of latitude and longitude to map locations, just like the grid system used in maps today. Eratosthenes tried to create a map of the entire known world. Aristarchus of Samos (about 310–230 BCE) was an early astronomer who had a bold idea: the Sun is at the center of the solar system, not the Earth. This idea is called the heliocentric model. At the time, most people believed the Earth was at the center, so his theory was not accepted, partly because they did not have strong telescopes or enough evidence. Even though his idea was ignored back then, his work later inspired scientists like Copernicus, who helped start the modern understanding of the solar system.
Claudius Ptolemy (about 100–170 CE) lived in Roman Egypt and was a famous ancient astronomer and scientist. He believed that the Earth was at the center of the universe, and he wrote a big book called the Almagest, where he explained this idea using a model called the geocentric model. To explain how the Sun, Moon, and planets move in the sky, he used epicycles, deferents, and equants, complicated ways of showing orbits that looked like circles within circles. Even though this model was wrong (we now know the Sun is at the center), Ptolemy’s system worked well for making predictions and was used for over 1,400 years. Ptolemy did not just study space, he also worked on optics (how light and vision work), maps, and music. He even wrote about astrology.
The Romans were more focused on practical science than on theories. They were great engineers and used science to build things like aqueducts (which carried water), roads, bridges, and concrete buildings. They understood how water flows, how to balance weight, and how to use strong building materials. A Roman engineer named Vitruvius wrote a book called De Architectura in the 1st century BCE, where he talked about architecture, human body proportions, and machines. He combined art and science to show how buildings should look beautiful but also work well. In medicine, a Roman doctor named Galen (around 129–200 CE) became very important. He studied how the body works by examining animals, learning about organs, bones, and how diseases spread. He believed in the four humors (like Hippocrates) and thought that keeping these in balance kept people healthy. Galen’s medical ideas were used in Europe and the Islamic world for many hundreds of years.
Islamic Golden Age
Science during the Islamic Golden Age (from about the 8th to the 14th century CE) was one of the most creative and important times in history. During this period, Muslim scholars made huge advances in astronomy, math, medicine, physics, chemistry, geography, and engineering. This progress happened because of several things which included leaders supported science, especially the Abbasid caliphs who helped fund research and learning. Books from Greece, India, Persia, and China were translated into Arabic, allowing people to learn from many different cultures, and Islamic culture encouraged learning (called ‘ilm in Arabic) and logical thinking. One of the most important places during this time was the House of Wisdom (Bayt al-Hikma) in Baghdad, created by Caliph Harun al-Rashid and expanded by his son Al-Ma'mun. It was a major center where scholars gathered to translate, study, and improve scientific knowledge. They translated famous works from Greek, Persian, and Indian scientists, like Aristotle, Galen, Euclid, and Ptolemy, into Arabic. One key figure was Hunayn ibn Ishaq, who translated over 100 medical books.
In astronomy, Muslim scientists made many improvements to older models from Greece and India. Al-Battani (also known as Albatenius, around 858–929 CE) carefully studied the Sun and calculated the length of the year as 365.2422 days, almost exactly what is known today. Al-Zarqali (also called Arzachel) made the Toledan Tables, which helped European scientists and even influenced Copernicus hundreds of years later. Al-Tusi (1201–1274 CE), working at the Maragheh observatory, invented a math model called the Tusi couple, which helped improve future ideas about how planets move. Muslim astronomers also built observatories (places to study the stars) in cities like Baghdad, Maragheh, Samarkand, and Shiraz. They invented tools like the astrolabe, sextant, and spherical instruments to measure stars and planets very precisely. These tools were used to find the correct times for prayer, determine the direction of Mecca (Qibla), and create Islamic calendars.
In the field of mathematics, Islamic scholars made major breakthroughs, especially in algebra, arithmetic, geometry, and trigonometry. One important mathematician, Muhammad ibn Musa al-Khwarizmi (around 780–850 CE), wrote a book called Al-Kitab al-Mukhtasar fi Hisab al-Jabr wal-Muqabala. This book laid the foundation for algebra. The word "algebra" actually comes from al-jabr, meaning "completion." Al-Khwarizmi also created step-by-step ways to solve quadratic equations and methods for doing calculations. In fact, the word “algorithm” comes from his name. Another scholar, Omar Khayyam (1048–1131), figured out how to solve cubic equations using geometry. Al-Kashi (died 1429) calculated the number pi to 16 decimal places and improved the way decimal numbers were used, something very important for modern math. In trigonometry, scientists like Al-Biruni and Nasir al-Din al-Tusi made accurate tables for sine, cosine, and tangent, helping to turn trigonometry into its own branch of mathematics.
In medicine, Islamic scientists made great progress by using ideas from ancient Greek thinkers and combining them with careful observation and testing. A famous doctor named Al-Razi (also known as Rhazes, 854–925) wrote a huge 25-volume medical book called Kitab al-Hawi. He was the first to clearly tell the difference between smallpox and measles and believed in learning through watching and testing patients. Another well-known doctor, Ibn Sina (also called Avicenna, 980–1037), combined medical knowledge from Greece, Persia, and India in his book The Canon of Medicine. This book was used as a guide in both Europe and the Islamic world for more than 600 years. He introduced important medical ideas like testing medicines, using quarantine to stop diseases from spreading, checking a patient’s pulse, and understanding how infections work. Muslim doctors also invented surgical tools, used antiseptics to stop infections, and developed special treatments for eye problems. One doctor, Ammar ibn Ali al-Mawsili, even invented a syringe to remove cataracts from the eye.
In the study of light and physics, Ibn al-Haytham (also called Alhazen, 965–1040) made discoveries that helped shape modern science. In his book Book of Optics, he showed that the Greek idea of vision, where light was thought to come out of the eyes, was wrong. He explained that light bounces off objects and goes into the eye, and he correctly described how the retina and lens work. Ibn al-Haytham tested his ideas with experiments using things like mirrors, lenses, and a camera obscura (an early version of a camera). He believed in forming hypotheses, testing them, and repeating experiments. These are ideas that are important to the modern scientific method. He also studied motion, mechanics, and inertia, and his work influenced later scientists in Europe, like Roger Bacon and Kepler.
In early Islamic times, chemistry was called alchemy, and Muslim scientists made big discoveries while trying to understand how materials change. One of the most important alchemists was Jabir ibn Hayyan (also called Geber), who lived in the 8th century. He wrote hundreds of books about how to heat, purify, and mix substances using methods like distillation and crystallization. He even described how to make aqua regia, a powerful acid that can dissolve gold. Jabir believed in testing ideas through careful experiments and recording results, which helped move alchemy closer to real, practical chemistry. He also used special lab tools like alembics (for distilling liquids), crucibles (for heating substances), and furnaces, helping to shape the way modern chemistry labs would work.
Muslim scientists also made great progress in geography, which is the study of the Earth and how it is measured. They traveled, observed nature, and made better maps. A brilliant scholar named Al-Biruni (973–1048) used math and trigonometry to measure the size of the Earth from the top of a mountain, and his answer was very close to what we know today. He also collected the exact locations (latitude and longitude) of over 600 cities and even suggested that the Earth spins, hundreds of years before scientists in Europe did. Another scholar, Al-Khwarizmi, corrected mistakes in older maps from the Greek scientist Ptolemy. Later, Al-Idrisi made a famous world map in 1154 for a European king, using knowledge from Arab traders, sailors, and explorers.
Islamic scientists also made amazing progress in engineering and building machines. They designed clever devices like early robots (called automata), water clocks, and machines that used water and air to move. In the 9th century, three brothers known as the Banu Musa wrote The Book of Ingenious Devices, which described over 100 inventions using gears, valves, and tubes called siphons. Later, a brilliant inventor named Al-Jazari (1136–1206) built fancy water clocks, mechanical toys that could be programmed to move, and one of the first crankshafts, a tool still used in engines today. These machines were not just for show; they were used in palaces, public fountains, and farming systems to help people in everyday life.
To spread knowledge, the Islamic world created many places for learning. These included schools called madrasas, hospitals known as bimaristans, libraries, and observatories where people could study the stars. Famous schools like the Nezamiyeh in Baghdad, Al-Qarawiyyin in Fez (Morocco), and Al-Azhar in Cairo became major centers for science and education. Scholars traveled from place to place, shared ideas, and copied books so they could be studied in different parts of the world. From Spain and North Africa all the way to Central Asia and India, science and learning were an important part of Islamic culture.
Medieval and Renaissance Europe
During the early Middle Ages (around the 5th to 10th centuries), science in Western Europe slowed down after the fall of the Western Roman Empire. Many ancient books and ideas were at risk of being lost. Luckily, monks in monasteries helped save this knowledge by copying old texts written in Latin. These included the works of famous thinkers like Aristotle, Pliny the Elder, Galen, and Boethius. One important scholar from this time was Isidore of Seville (around 560–636), who tried to collect and organize all human knowledge into one big book called Etymologiae. People at the time believed that studying nature helped them better understand God’s creation. Even though science was limited during this period, learning started to improve under Charlemagne (who ruled from 768–814). He supported education and helped start cathedral schools, which taught subjects like math, geometry, music, and astronomy (called the quadrivium), along with grammar, logic, and writing (the trivium).
Between the 11th and 13th centuries, science in Europe came back to life thanks to a big effort to translate ancient books. In places like Spain, Sicily, and parts of the Middle East, European scholars came in contact with the Islamic world and the Byzantine Empire. From them, they got access to many old Greek and Roman ideas, now improved by Arabic scholars. Translators like Gerard of Cremona worked hard to turn important Arabic and Greek books by Aristotle, Avicenna, Alhazen, and Galen into Latin so more people in Europe could understand them. These books taught about astronomy, medicine, math, and optics. This sparked the rise of universities in cities like Bologna, Paris, Oxford, and Salamanca. A new way of thinking called scholasticism became popular. Scholastic thinkers like Thomas Aquinas mixed Greek philosophy with Christian beliefs, and others like Albertus Magnus and Roger Bacon encouraged people to observe nature and do experiments to learn how the world works.
By the 14th century, some smart thinkers in Europe were starting to question old ideas about how things move. Two of these thinkers, Nicole Oresme and Jean Buridan, worked at the University of Paris and had new ideas about physics. Jean Buridan came up with a theory called impetus, which was an early idea similar to what we now call inertia. He said that once something is moving, it can keep moving on its own. It does not need to be pushed forever. This was a big change from the old belief (by Aristotle) that something had to keep being pushed to keep moving. Nicole Oresme did not agree with the old idea that Earth was the center of the universe (geocentric model). He also used graphs to show how things move, which was an early step toward kinematics, the study of motion.
The Renaissance (from the 1300s to the 1600s) was a time when science began to change a lot. People started to focus more on observing nature, doing experiments, and re-reading old science texts from ancient Greece. Scholars like Leonardo Bruni and Marsilio Ficino helped translate these important works so others could study them in more depth. One of the most famous Renaissance thinkers was Leonardo da Vinci (1452–1519). He is known for being a "universal man", or someone who could do many things well. He was an amazing artist, but he also studied anatomy (the human body), how water flows, and how machines work. Leonardo dissected human bodies and made some of the most accurate drawings of muscles, bones, and the heart for his time. He figured out many things about how the vascular system (blood vessels) and body parts worked, even though he never published his findings while he was alive.
A big moment in astronomy happened with a scientist named Nicolaus Copernicus (1473–1543). He suggested a heliocentric model of the universe, which means that the Sun is at the center, not the Earth. This idea was published in his book On the Revolutions of the Heavenly Spheres. In his model, Earth spins every day and goes around the Sun once a year. At the time, most people believed the Ptolemaic model, which said Earth was the center of everything. Even though Copernicus kept some old ideas like epicycles (small loops in orbits), his model was a huge change and led to a new way of thinking about the universe. Another important astronomer was Tycho Brahe (1546–1601). He did not use a telescope, but he made very careful observations of the stars and planets using special instruments. His data was extremely accurate and helped later scientists understand how the planets move.
Johannes Kepler (1571–1630), who used Tycho’s data, discovered three laws of planetary motion. He showed that planets do not move in perfect circles, but in elliptical orbits (oval-shaped). He also found that planets move faster when they are closer to the Sun. Galileo Galilei (1564–1642) combined experiments and astronomy. He used one of the first telescopes to look at the sky and made amazing discoveries. He saw craters on the Moon, sunspots, the phases of Venus, and moons orbiting Jupiter. All these things disproved the old idea that everything revolved around Earth and helped support Copernicus’s heliocentric model. Galileo also studied how objects fall, how pendulums swing, and how things move through the air. He showed that the laws of nature follow math rules, and we can understand them by doing experiments. In 1632, he wrote a book called Dialogue Concerning the Two Chief World Systems that supported heliocentrism. Because of this, he got in trouble with the Catholic Church, which still believed Earth was the center of the universe. Galileo was put on trial and spent the rest of his life under house arrest.
During the Renaissance, there were huge changes in medicine and anatomy (the study of the human body). A scientist named Andreas Vesalius (1514–1564) made a big breakthrough by dissecting real human bodies and studying them carefully. Before this, people mostly followed the ideas of an ancient doctor named Galen, but Vesalius found that Galen had made many mistakes because he studied animals, not humans. Vesalius wrote a book called De humani corporis fabrica (which means On the Fabric of the Human Body), where he corrected hundreds of errors and showed that anatomy should be based on direct observation. This helped make anatomy a real science based on what doctors could see and study for themselves. Later, another important doctor named William Harvey (1578–1657) discovered how blood moves through the body. He showed that the heart pumps blood in a circle, a system called circulation, which was very different from the old idea that blood came from the liver and just moved back and forth. Harvey proved this through experiments and live animal dissection (called vivisection). His work helped change medicine from just guessing and theory to something based on evidence and real body functions.
In math and physics, the Renaissance also saw big advances. A man named Simon Stevin (1548–1620) came up with decimal fractions, which made it easier to do math with numbers. For example, instead of writing 1/10 or 1/100, people could now write 0.1 or 0.01, something used all the time today. René Descartes (1596–1650), a French thinker, invented analytic geometry, which connected algebra with shapes and space. This helped scientists describe the world more clearly using equations. Another important figure was Francis Bacon (1561–1626). He believed science should be based on observation and experiments, not just guessing or using old books. In his book Novum Organum, he explained how scientists should use inductive reasoning, starting with observations and then finding general rules. His ideas became the philosophy of science and influenced many future scientists. Finally, Isaac Newton (1642–1727) brought together the work of earlier scientists like Kepler, Galileo, and Descartes. In the early 1700s, he created a complete system of physics and astronomy that explained how planets move and how objects fall on Earth. His work marked the end of the Renaissance and the start of the Scientific Revolution, a time when modern science truly began.
Age of Enlightenment
The Enlightenment was greatly influenced by the ideas of the Scientific Revolution, especially the work of Isaac Newton (1642–1727). In 1687, Newton wrote a book called Philosophiæ Naturalis Principia Mathematica, where he explained the laws of motion and the law of universal gravitation. He showed that the same force that makes an apple fall also keeps the Moon in orbit around the Earth. Newton also explained that planets move in elliptical (oval-shaped) paths because of gravity. He invented calculus (at the same time as another scientist named Gottfried Leibniz) to help describe things that change, like speed and movement. His experiments with light proved that white light is made up of many colors.
Because of Newton, many Enlightenment thinkers started to use math and science to understand more than just space or motion. In chemistry, Robert Boyle (1627–1691) was an important figure. In his book The Sceptical Chymist (1661), he rejected old ideas from Aristotle and alchemy and introduced new scientific ideas about matter. He discovered Boyle’s Law, which says that when you make a gas container smaller, the pressure goes up (as long as the temperature stays the same). This showed that gases behave in ways that can be measured and predicted. Boyle also believed that science should be based on careful experiments that could be repeated. The move from alchemy (an early, unscientific form of chemistry) to modern chemistry was led by Antoine Lavoisier (1743–1794), who is often called the “father of modern chemistry.” Lavoisier discovered and named the elements oxygen and hydrogen, and he proved that the old phlogiston theory (which said things burned by releasing a strange substance) was wrong. He showed that in a chemical reaction, the total mass stays the same, which is now known as the law of conservation of mass. Lavoisier also created a clear system for naming chemicals, which helped scientists better organize and classify the elements.
In biology and anatomy, scientists during the Enlightenment started to focus more on how living things are organized. Carl Linnaeus (1707–1778) invented the system we still use today for naming living things, called binomial nomenclature. He gave each plant or animal two names, a genus and a species. This helped scientists all over the world talk about the same organisms clearly and consistently. His book, Systema Naturae, first published in 1735, listed and organized thousands of plants and animals and became the foundation of modern taxonomy, the science of classification. At the same time, scientists were learning more about how the human body works. Albrecht von Haller studied things like how nerves work, how muscles move, and how the heart beats. Through experiments, he helped people understand how parts of the body function automatically, without us thinking about them. This was a big step forward in the science of physiology.
During the Enlightenment, astronomy made big advances thanks to better telescopes and the growth of observatories. Edmond Halley (1656–1742) used Newton’s laws to predict when a certain comet would come back. He was right, and that comet is now named Halley's Comet. This showed that gravity could explain how objects move in space. Later, William Herschel (1738–1822) discovered a new planet, Uranus, in 1781. He also found over 2,500 objects in the night sky, like nebulae (clouds of gas and dust) and star clusters. He built some of the biggest telescopes of his time. His work helped scientists realize that the universe is much bigger than just the Milky Way galaxy. Astronomy became more professional, with major observatories in cities like Greenwich, Paris, and Berlin. These places also helped improve navigation, which was important for travel and trade.
Scientific institutions were very important during the Enlightenment. The Royal Society of London (founded in 1660) and the French Academy of Sciences (founded in 1666) brought scientists together to share ideas. They published journals, like Philosophical Transactions, that explained experiments, new discoveries, and theories. These groups helped make science more organized, trustworthy, and easy to understand by others. They also started the idea of peer review, where other scientists check your work. As more people learned to read and write, science started to spread outside of just schools and universities. Salons, coffeehouses, and public lectures became places where people talked about science. Scientists were no longer just private thinkers, they became public figures who shared their knowledge with everyone.
During the Enlightenment, medicine improved because doctors started focusing more on observing patients and learning through hospital training. One important doctor, Giovanni Battista Morgagni (1682–1771), helped create modern diagnosis by showing that symptoms were linked to specific organs, not just to old ideas like imbalances of body fluids. This helped doctors understand that diseases could be traced to problems in certain parts of the body. Another key figure was Edward Jenner (1749–1823), who made the first successful vaccine. He noticed that people who got cowpox did not catch smallpox, a deadly disease. He tested this idea and proved that cowpox could protect people. In 1798, he published his results, starting the science of immunology and vaccination.
During this time, science became more separate from religion and was seen as a way to improve society. Famous thinkers like Denis Diderot, Voltaire, and Jean le Rond d’Alembert believed that science and reason could free people’s minds and make the world better. They worked on a big project called the Encyclopédie (1751–1772), which was like an early encyclopedia. It collected all known scientific and technical knowledge into one place and made it easier for people to learn about how things work, from optics and anatomy to mining and tools. Engineering and technology also made huge progress. James Watt made important changes to the steam engine in the 1760s and 1770s by adding a separate condenser, which made the engine much more efficient. His improvements helped power factories and machines, leading to the Industrial Revolution.
19th Century Science
During the 1800s, science made big changes, especially in how we understand heat, energy, electricity, and atoms. In the study of heat and energy, thermodynamics, scientists created the laws of thermodynamics. These laws helped people understand how steam engines work and how to make them more efficient. In 1824, a scientist named Sadi Carnot came up with the idea of the Carnot cycle, which showed that an engine’s efficiency depends on the temperature difference between where the heat comes from and where it goes. This idea helped lead to the second law of thermodynamics, which explains how energy spreads out. Later, Rudolf Clausius introduced the idea of entropy, which measures how energy becomes more disordered over time. Scientists William Thomson (Lord Kelvin) and James Prescott Joule created the first law of thermodynamics, which says that energy cannot be created or destroyed, only changed from one form to another.
In electricity and magnetism, scientists discovered that these two forces are connected. Michael Faraday, in the 1830s and 1840s, did experiments that showed how moving magnets can create electricity, a process called electromagnetic induction. This discovery is what makes electric generators and transformers work. Faraday also came up with the idea of fields, which are invisible areas where electric or magnetic forces can act. Later, in the 1860s, James Clerk Maxwell built on Faraday’s work and wrote four equations, called Maxwell's equations, that explain how electric and magnetic fields move and interact. These equations also predicted the existence of electromagnetic waves, which led to the invention of things like radios, TVs, and wireless communication.
At the beginning of the century, John Dalton brought back the idea of atomic theory. He said that every element (like oxygen or iron) is made up of its own kind of atom, and these atoms combine in fixed amounts to make compounds (like water or carbon dioxide). His ideas helped explain important chemistry rules, like the law of multiple proportions, which says that elements always combine in certain number patterns. Later, in 1869, Dmitri Mendeleev created the periodic table. He arranged the elements in order by atomic mass and grouped them by chemical properties. Mendeleev’s table was so smart that it even predicted new elements, like gallium and germanium, before they were discovered. When scientists later found these elements and they matched his predictions, it showed that the periodic law was a powerful way to understand and organize chemistry.
Meanwhile, biology was also changing in a major way. In 1859, Charles Darwin published a book called On the Origin of Species. In it, he explained his theory of evolution by natural selection. Darwin said that living things change over time. These changes happen because of small differences that are passed from parents to offspring. If a trait helps an organism survive and have more babies, that trait becomes more common over generations. This is how new species can form. Darwin got many of his ideas by observing animals and plants during his long trip on the ship HMS Beagle. He was also influenced by Thomas Malthus, who wrote about how populations grow, and Charles Lyell, who studied how the Earth changes slowly over time. Darwin’s theory replaced older ideas that species never change. It gave scientists a new way to understand life, one that was based on evidence, change, and history. It also started many important discussions about science, religion, and philosophy.
At the same time as other big discoveries in the 1800s, scientists also developed the cell theory, which says that all living things are made of cells, and that the cell is the basic unit of life. This idea was first introduced in the 1830s by Matthias Schleiden and Theodor Schwann. Later, Rudolf Virchow added that all cells come from other cells, which he summed up in the Latin phrase "omnis cellula e cellula". The invention and improvement of the light microscope helped scientists look more closely at cells. They were able to discover important parts inside cells, like the nucleus, mitochondria, and other tiny structures called organelles. These discoveries helped build the foundation for molecular biology and genetics, which study how cells work and how traits are passed down.
In medicine, the 1800s were a turning point as it began to become more scientific. This progress was made thanks to new knowledge in disease research, germs, surgery, and public health. One of the most important scientists was Louis Pasteur. He showed that tiny organisms (microbes) can cause disease and spoil food. This idea became known as the germ theory of disease. Pasteur proved that germs do not just appear out of nowhere (a belief called spontaneous generation). He also invented vaccines for rabies and anthrax, and created pasteurization, a method to heat food and drinks to kill harmful microbes and keep them from spoiling. Another important scientist was Robert Koch. He discovered the specific bacteria that cause tuberculosis, cholera, and anthrax. Koch also created new lab methods, like using agar plates to grow bacteria and staining techniques to make microbes easier to see under a microscope. He developed Koch’s postulates, a set of rules that scientists use to prove that a certain germ causes a specific disease.
In the 1840s, doctors began using anesthesia, like ether and chloroform, to perform pain-free surgeries. This was a huge breakthrough because it allowed surgeons to do more complex operations without hurting the patient. Around the same time, Joseph Lister started using carbolic acid (also called phenol) to clean wounds and tools, which greatly reduced the number of people who died from infections after surgery. His work helped start the use of antiseptic techniques in hospitals and led to the modern idea of clean, germ-free medical care. Because of these changes, hospitals became places of real healing and science, not just places where the sick were kept. At the same time, big cities were facing many diseases due to crowding and pollution from the Industrial Revolution. This led to early public health reforms. One important figure was Florence Nightingale, who worked during the Crimean War. She pushed for clean hospitals, better nursing, and proper training, which helped improve medical care all over the world.
In the study of geology, scientists began to understand that Earth is very old, much older than people once believed. Charles Lyell wrote a famous book called Principles of Geology (1830–1833), where he explained that natural processes like erosion (wearing down of land) and sedimentation (building up of layers) happen slowly and steadily over a long time. This idea is called uniformitarianism. Lyell’s work was important because it challenged the biblical view that Earth was only a few thousand years old. It also gave Charles Darwin the long time span needed for his theory of evolution to work. Other areas of geology, like stratigraphy (studying rock layers) and fossil correlation (matching fossils to different layers), helped scientists piece together Earth’s history. Studies in mineralogy (minerals) and paleontology (fossils) also helped explain what the Earth is made of and what life used to be like.
During the 19th century, people began to study human behavior and society in a more scientific way. This was the beginning of the social sciences. A thinker named Auguste Comte came up with the idea of positivism, which said that we should study society using facts and observations, just like scientists study nature. This led to the creation of sociology, a field that tries to understand how things like cities, factories, and social classes affect people’s lives. In economics, early ideas from Adam Smith were built upon by other thinkers like David Ricardo, Thomas Malthus, and John Stuart Mill. They used math and statistics to study how money, jobs, and resources are shared in society. 
At the same time, science became more organized in every area. Countries created national science academies and professional journals where scientists could publish and share their discoveries. Universities and research institutes trained new scientists, and governments or private groups started to fund science projects. Science also became something the public could enjoy and learn about. Museums and big events like the Great Exhibition of 1851 in London showed off new inventions and discoveries, proving that science was important for progress. Some amazing inventions of the time included the telegraph (for long-distance communication), the steamship, the electric light, and photography. These changes showed how science could improve daily life and move society forward.
20th and 21st Century Science
At the start of the 20th century, scientists realized that the old ideas of physics (called classical mechanics) could not fully explain how very small things (like atoms) or very large things (like galaxies) work. This led to the birth of modern physics. In 1905, a young scientist named Albert Einstein wrote four important papers that changed science forever. One of these introduced the special theory of relativity, which showed that time and space are not fixed. Instead, time can slow down and lengths can shrink when objects move very fast, close to the speed of light. He also came up with the famous equation E = mc², showing that mass and energy are two forms of the same thing. Later, in 1915, Einstein created the general theory of relativity, which explained gravity in a new way. Instead of thinking of gravity as a pulling force, he said that mass bends spacetime, and this bending is what is felt as gravity. This idea helped scientists understand things like black holes, gravitational lensing (how light bends around stars), and how the universe is expanding.
At the same time, a different kind of physics called quantum mechanics was being developed to understand how things work at the level of atoms and tiny particles. In 1900, Max Planck said that energy comes in small amounts, called quanta. Niels Bohr created a model of the atom where electrons move in specific orbits, helping to explain how atoms give off light. Werner Heisenberg and Bohr developed the Copenhagen interpretation, which included the uncertainty principle. This says we cannot know exactly where a particle is and how fast it is moving at the same time. Erwin Schrödinger came up with wave mechanics, which showed how particles can also act like waves. Paul Dirac combined quantum theory with Einstein’s ideas and predicted the existence of antimatter. These discoveries helped create many modern technologies, like semiconductors (used in computers), lasers, nuclear power, and even the new field of quantum computing.
In the 20th century, a new area of science called nuclear physics became very important and had a big impact on the world. It started when Henri Becquerel discovered radioactivity, and Marie and Pierre Curie studied it further. They learned how atoms can break apart in a process called atomic decay. In 1938, scientists Otto Hahn and Fritz Strassmann discovered something called nuclear fission, which is when the nucleus of an atom splits, releasing a huge amount of energy. Lise Meitner and Otto Frisch explained how it worked. This discovery led to the creation of the Manhattan Project, a secret program where scientists like Enrico Fermi and J. Robert Oppenheimer developed the first atomic bomb, which was used in World War II in 1945. But nuclear physics was not just used for weapons. It also led to the development of nuclear power plants, which generate electricity, and medical technologies like PET scans (used to see inside the body) and radiation therapy (used to treat cancer).
Meanwhile, in biology, scientists began to understand how traits are passed from parents to children. This started with the rediscovery of Gregor Mendel’s laws of inheritance, which led to the science of genetics. In 1953, James Watson and Francis Crick, using X-ray images from Rosalind Franklin and Maurice Wilkins, discovered the double helix structure of DNA. This showed how genetic information is stored in the sequence of nucleotides (the building blocks of DNA). This discovery led to molecular biology, which helped scientists understand how genes make proteins and how certain diseases are caused by genetic problems. Later, a huge international project called the Human Genome Project finished in 2003. Scientists mapped out all the DNA in a human, which includes over 3 billion base pairs and more than 20,000 genes. This started the field of genomics, and helped create new ways to study and treat diseases through personalized medicine and bioinformatics (the use of computers in biology).
Biotechnology in the late 1900s and early 2000s gave scientists amazing new tools to work with DNA and genes. These included Recombinant DNA technology which lets scientists combine DNA from different sources. PCR (polymerase chain reaction) is a method to quickly make millions of copies of a piece of DNA. CRISPR-Cas9, on the other hand, is a tool that lets scientists edit genes very precisely, like using scissors to cut and change DNA. These tools are now used in many ways. To create genetically modified crops, make medicines like insulin, perform gene therapy to treat diseases, and do quick and accurate disease testing. In medicine, a huge breakthrough came in 1928, when Alexander Fleming discovered penicillin, the first antibiotic. This completely changed how bacterial infections were treated and saved millions of lives. Later, vaccines were developed for deadly diseases like polio, measles, and HPV. More recently, mRNA vaccines (like some COVID-19 vaccines) have helped fight new diseases and protect public health around the world.
At the same time, information technology and computer science became a big part of science and everyday life. During World War II, the first electronic computers, ENIAC and Colossus, were built. Alan Turing created early ideas about how computers work and also helped crack secret codes during the war. In 1947, three scientists at Bell Labs, Bardeen, Brattain, and Shockley, invented the transistor, a tiny part that made it possible to build smaller and faster electronic devices. This led to the microprocessor and started the digital revolution. Later, scientists created the internet, which began as a government project called ARPANET. The internet grew into a huge, worldwide system that allows people to communicate, learn, shop, and share information.
Space science made huge leaps forward in the 20th century as humans began to explore beyond Earth for the first time. In 1957, the Soviet Union launched Sputnik 1, the first artificial satellite. Then, in 1961, Yuri Gagarin became the first human in space. One of the biggest milestones came in 1969, when NASA’s Apollo Program sent Neil Armstrong and Buzz Aldrin to the Moon, making them the first people to walk on the Moon. Robots also helped us explore space. The Voyager probes, launched in 1977, sent back amazing information about the outer planets and are still traveling through interstellar space (the space between stars). In 1990, the Hubble Space Telescope was launched into orbit. It took detailed pictures of galaxies, nebulae, and supernovae, helping scientists learn more about the expanding universe. In the 21st century, space exploration has kept growing. NASA’s Mars rovers explore the Red Planet. Space telescopes like Kepler and TESS search for exoplanets (planets around other stars). Private companies like SpaceX and Blue Origin are working on things like Mars missions and even space tourism.
At the same time, climate and environmental science became very important. Back in 1896, scientist Svante Arrhenius said that carbon dioxide (CO₂) in the air could raise Earth’s temperature. This is called the greenhouse effect. By the late 1900s, scientists had strong evidence that human activities, like burning fossil fuels, were changing the climate. They got this data from things like ice cores, satellites, and ocean studies. Today, scientists around the world work together through groups like the Intergovernmental Panel on Climate Change (IPCC) to study climate change and warn about its dangers. They say it is urgent to reduce CO₂ emissions to protect the planet. To help with this, we now use tools like climate models, remote sensing (using satellites to study Earth), and renewable energy (like solar and wind power) to fight climate change and find better ways to live sustainably.
Neuroscience and cognitive science are the sciences that study the brain and how we think, feel, and make decisions. In recent years, scientists have made big discoveries thanks to new brain scanning tools like fMRI and PET scans, which show real-time images of brain activity. These tools helped researchers find out that the prefrontal cortex (the front part of the brain) is important for decision-making, and the amygdala (a small part deep in the brain) helps process emotions like fear and anger. Today, scientists are trying to map all the connections in the brain. This is called connectomics. Also, brain-computer interfaces have been developed. These allow people who are paralyzed to move robotic arms or prosthetic limbs using only their brain signals. At the same time, fields like artificial intelligence (AI) and machine learning, which used to be mostly theory, are now real tools. They help with things like voice assistants, face recognition, medical diagnoses, and self-driving cars.
In the 21st century, scientists from different fields began working together more closely, creating powerful new areas of research. These include nanotechnology (the science of building tiny machines and materials at the scale of atoms and molecules), synthetic biology (designing and creating new living systems, such as bacteria that produce medicine), quantum information science (using the strange laws of quantum physics to make ultra-powerful computers), and systems biology (studying how all the parts of a living system work together). One of the biggest science projects of this century was at CERN, where scientists used the Large Hadron Collider (LHC), a huge machine that smashes particles together, to discover the Higgs boson in 2012. This particle helps explain how other particles get mass and is a key part of the Standard Model of physics. Scientists are also still trying to solve big mysteries like dark matter, dark energy, and how to unite quantum physics with gravity, which could lead to a unified theory of everything in the universe.
Branches
Natural sciences
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  The natural sciences seek to understand how the world and universe around us works. There are five major branches, indicated by the photos above taken clockwise from top left: astronomy, chemistry, Earth science, biology, and physics.
Natural sciences are a major area of science that focuses on understanding the natural world. Scientists in this field study how things in nature work by watching carefully, doing experiments, and creating models or theories. These models help explain why things happen and can even predict what might happen next. Natural sciences depend on evidence that can be seen, measured, and repeated. Scientists use the scientific method, which means they make guesses (called hypotheses), test them through experiments, and change their ideas if the results do not match. This process helps make sure that scientific conclusions are as accurate as possible.
There are several main areas of natural science, including physics, chemistry, biology, earth sciences, and astronomy. Each area focuses on different parts of nature, but they often work together in today’s research. Physics is the study of matter, energy, and the forces that affect them. Physicists explore everything from the tiniest particles, like electrons and quarks, to giant objects like stars and galaxies. They use different theories depending on the scale. Quantum mechanics for tiny particles and general relativity for huge cosmic objects. Physics has led to important laws and ideas that help explain the universe, such as Newton's laws of motion, the laws of thermodynamics, Maxwell's equations for electricity and magnetism, and the Standard Model, which organizes the basic building blocks of matter.
Chemistry is often called the central science because it connects physics and biology. It focuses on what matter is made of, how it is structured, how it behaves, and how it changes during reactions. Chemists study atoms and molecules to understand how they bond together, break apart, and move energy during chemical changes. Chemistry is divided into different areas. Organic chemistry studies compounds that contain carbon (like living things do). Inorganic chemistry looks at compounds without carbon. Physical chemistry explores how chemistry and physics work together. Analytical chemistry focuses on measuring and identifying substances. Biochemistry applies chemistry to biological systems like cells and DNA. Because of chemistry, there are drugs, plastics, batteries, fertilizers, and many other useful materials and tools.
Biology is the science of living things. It explores how organisms are built, how they work, how they grow, where they come from, and how they change over time. Biology includes many branches, such as cell biology (which studies the basic units of life, cells), genetics (which looks at heredity and DNA), ecology (which studies how organisms interact with each other and the environment), evolutionary biology (which looks at how life has changed over millions of years), and molecular biology (which focuses on life at the smallest scales, like proteins and genes). Biologists use tools like microscopes, DNA sequencing, and gene editing to explore life. Biology is important in health, farming, nature protection, and biotechnology, helping us make vaccines, improve crops, and protect endangered species.
Earth sciences, also called geosciences, are the sciences that study the Earth and everything that affects it. This includes the solid ground, the atmosphere (air and weather), the hydrosphere (water and oceans), and the biosphere (all living things). Earth scientists use ideas from physics, chemistry, and biology to understand how Earth’s systems work together. Earth sciences include geology, which looks at the layers of the Earth, rocks, and the history of the planet; meteorology, which studies weather and the atmosphere; oceanography, which focuses on oceans and sea life; and climatology, which examines long-term climate patterns and how they change. These studies are important for learning about natural disasters like earthquakes, volcanoes, and hurricanes, and for helping us manage natural resources and protect the environment.
Astronomy is the science that studies space, planets, stars, galaxies, and the universe. Astronomers observe the sky using telescopes and use theories and math to understand what they see. Modern astronomy is closely connected to physics, especially in a branch called astrophysics, which uses physical laws to explain how space objects behave. Astronomy has helped us discover just how huge the universe is and that there are other solar systems beyond our own. Scientists study amazing objects in space like black holes, neutron stars, and dark matter, a mysterious substance that we cannot see directly. Astronomy also helps us explore big questions, like where the universe came from, how it has changed over time, and what might happen to it in the future.
The natural sciences all use similar methods to learn about the world. First, scientists use empirical observation, which means they carefully collect data using tools or by studying things in nature. They also do experiments, where they test ideas in controlled settings to see what happens. Another method is mathematical modeling, where scientists use math and computers to describe and predict how things work. To make sure results are trustworthy, scientists follow rules like peer review (where other experts check their work), reproducibility (so others can repeat the experiment), and falsifiability (the idea that scientific claims must be testable and could be proven wrong). Scientists use numbers and statistics to support their conclusions with solid, unbiased evidence.
Today, the natural sciences often combine different subjects to solve big problems. These mixed areas are called interdisciplinary fields, like biophysics (biology + physics), astrochemistry (astronomy + chemistry), geobiology (geology + biology), and climate science (which combines many fields). These new fields help scientists understand complex challenges, such as climate change, disease outbreaks, clean energy, and space exploration. For example, studying the climate means using knowledge from physics, chemistry, biology, and computer modeling. Scientists in astrobiology use ideas from space science, chemistry, and biology to search for life on other planets.
The natural sciences have led to amazing technology and discoveries that have changed our lives. Physics helped create nuclear energy, computers, and electronics. Chemistry made new medicines, better farming products, and useful materials like plastics. Biology and medicine led to vaccines, treatments for genetic diseases, and ways to grow or repair body parts. Earth science helps us predict natural disasters and take care of the environment. Astronomy has led to better telescopes, cameras, and even new ways to process big data. Finally, the natural sciences are very important for education, government, and solving world problems. Understanding science helps people make smart choices about things like global warming, health, and new technology. Governments use science to make rules, spend money wisely, and protect people and nature. Many big science projects, like CERN (which studies tiny particles), the IPCC (which studies climate change), or the Human Genome Project (which mapped all human genes), show how countries and scientists work together around the world to learn and make progress.
Core Principles and Characteristics
Empiricism
Empiricism is a basic idea in science. It says that we learn things by using our senses, like seeing, hearing, or measuring, not just by guessing or following tradition. Instead of relying only on what people believe or think is true, science depends on what we can actually observe, test, and measure. This is why scientists use special tools like telescopes, microscopes, and sensors, to help them see things that are too small, too far, or otherwise invisible to the human eye. For example, when scientists look for planets outside our solar system, called exoplanets, they cannot see the planets directly. Instead, they look at tiny changes in the brightness of a star. If the star gets a little dimmer at regular times, it might mean a planet is passing in front of it. This method is based on careful observation, which is what empiricism is all about.
Empiricism also means that scientific results should be reproducible. This means that if someone does an experiment again, they should get the same results. A good example is Gregor Mendel’s pea plant experiments, where he discovered how traits are passed down from parents to offspring. Scientists were able to repeat his work and get the same patterns, showing that the results were reliable. The idea of empiricism was supported by philosophers like John Locke, who believed that people are not born with knowledge. Instead, he said the mind starts out like a blank slate, and everything we learn comes from experience. This idea is different from rationalism, which says we are born with certain ideas or that we can learn everything through logic alone. In science, empiricism helps make sure our knowledge is based on real-world evidence.
Empirical methods in science are not just about watching things happen, they also include doing careful experiments to test ideas. These experiments are done in a controlled way to help scientists figure out which things cause certain results. For example, when Alexander Fleming saw that a certain type of mold killed bacteria, it was not enough just to notice it. Scientists had to do more experiments to prove that the mold, later called Penicillium notatum, could really fight infections. This led to the discovery of penicillin, an important medicine. This kind of testing is how scientists confirm what works and what does not. Galileo also used experiments with ramps (inclined planes) to show that objects do not need a force to keep moving, which went against the old ideas from Aristotle. In more recent times, scientists used special detectors at LIGO to find gravitational waves, tiny ripples in space predicted by Einstein’s theory of general relativity 100 years earlier. This discovery was another example of how scientists use careful observation and testing to confirm or disprove scientific theories. Empirical methods are also used in big, complex areas like climate science. Scientists collect information from things like ice cores, satellites, and even tree rings to learn about Earth’s climate over time. These different sources of data help build models to understand and predict how the planet is changing.
Even though empiricism, the idea that knowledge comes from observation and experience, is a key part of science, it is not always simple. One challenge is deciding what counts as a valid observation and how to understand what we see. Sometimes, what scientists notice or pay attention to depends on the theories they already believe in. This idea is called “theory-laden observation.” Philosopher Thomas Kuhn explained that scientists often use models or ideas they already have to decide what to look for and how to explain it. For example, early astronomers had trouble understanding why planets sometimes seemed to move backward in the sky (called retrograde motion) until they used models like the Ptolemaic system or the later Copernican model, which helped them interpret what they were seeing. In some parts of science, such as quantum physics, things get even harder. Scientists cannot always observe tiny particles directly. Instead, they have to use mathematical models and experiments to figure out what is happening. Phenomena like quantum entanglement or wavefunction collapse are known through indirect evidence and complex data. In the social sciences (like psychology or sociology), empiricism works a bit differently. Scientists use tools like surveys, interviews, and experiments with people to gather data. But this kind of research can have its own problems. For example, people might give answers they think are expected (this is called social desirability bias), or they might act differently because they know they are being watched (called the Hawthorne effect or observer bias).
Rationality and Logic
Rationality and logic are very important in science. They help scientists make sure their ideas make sense, their theories fit together, and their conclusions are based on good reasoning. Rational thinking in science means using reason carefully. Scientists use it to come up with ideas (called hypotheses), to plan experiments, to understand results, and to compare different explanations. This helps make sure that conclusions come from real evidence, not just guesses. Logic gives rules for how to go from one idea to another. It helps scientists check if their arguments are valid. For example, in biology, if all mammals are warm-blooded and whales are mammals, then whales must be warm-blooded. This is a basic kind of logical thinking called deductive reasoning. Science also uses the rule of non-contradiction. This means something cannot be both true and not true at the same time. For example, a chemical cannot both contain and not contain the same element under the same conditions. This helps scientists make clear and reliable chemical models. There are also formal types of logic, like propositional logic, predicate logic, and modal logic. These are used in areas like computer science, artificial intelligence, and mathematical biology. They help scientists build computer programs, design experiments, and understand complex systems.
Logic and rational thinking are important in science not just to support good ideas, but also to find mistakes, hidden problems, or wrong assumptions in scientific theories. In physics, the discovery of quantum mechanics made scientists question old ideas. For example, classical physics said that objects behave in clear, predictable ways (called determinism) and that things far apart cannot affect each other instantly (called locality). But quantum mechanics showed that very small particles often behave in strange and random ways, so scientists had to use new kinds of logic to understand them. There is also a big problem between two important theories: general relativity and quantum field theory. Relativity explains how big things like planets and galaxies work, while quantum theory explains tiny particles. But the two theories do not fully agree with each other. This has led scientists to search for a new theory that combines both, such as string theory or loop quantum gravity. Science often uses a method called hypothetico-deductive reasoning. This means scientists start with a theory, then use logic to make predictions that can be tested. If experiments agree with the prediction, the theory is stronger. If not, the theory may need to change. For example, we know that antibiotics kill bacteria, not viruses. This logical idea helps doctors avoid giving antibiotics for viral infections, which helps prevent antibiotic resistance. In social sciences, logic is also important. Game theory is a tool that uses logic and reasoning to study how people make choices in situations where others are also making decisions. It helps explain things like voting, diplomacy between countries, and even how governments sell licenses for things like radio frequencies. Game theory helps people make better decisions by thinking logically about the actions of others.
Scientific thinking also includes the idea of parsimony, which is often called Occam's Razor. This idea says that if there are different explanations for something, the simplest one that still explains the facts should be chosen. This rule helps scientists choose the best theories in many fields, such as biology and climate science. For example, a long time ago, people believed that the Earth was at the center of the universe (the geocentric model). But this model needed many complicated parts, like epicycles, to explain how planets moved. Later, Copernicus suggested that the Sun was at the center instead (the heliocentric model). This simpler idea explained the same facts more easily, so it became the better choice. In brain science, a new idea called the Bayesian brain hypothesis says that the brain works by using probability to update what it believes. This means the brain uses logic and evidence, like a smart guesser. This idea is also used in computer fields like robotics and machine learning. Logic is also very important in doing science in a fair and careful way. For example, decision theory is a part of logic that helps scientists decide how to run medical studies. It helps them think clearly about risks, benefits, and what is unknown when testing new treatments.
 Scientific method 
Today, "science" usually refers to a way of pursuing knowledge, not just the knowledge itself. It is mainly about the phenomena of the material world. The Greek works into Western Europe from the 6th to 7th century B.C. revived "Philosophy". In the 17th and 18th centuries scientists increasingly sought to formulate knowledge in terms of laws of nature such as Newton's laws of motion. And during the 19th century, the word "science" became more and more associated with the scientific method itself. It was seen as a way to study the natural world, including physics, chemistry, geology and biology.
It was also in the 19th century that the term scientist was created by William Whewell. He meant it to tell the difference between those who looked for knowledge on nature from those who looked for other types of knowledge.
The scientific method is the name given to the methods used by scientists to find knowledge. The main features of the scientific method are:
 Scientists identify a question or a problem about nature. Some problems are simple, such as "how many legs do flies have?" and some are very deep, such as "why do objects fall to the ground?"
 Next, scientists investigate the problem. They work at it, and collect facts. Sometimes all it takes is to look carefully.
 Some questions cannot be answered directly. Then scientists suggest ideas, and test them out. They do experiments and collect data.
 Eventually, they find what they think is a good answer to the problem. Then they tell people about it.
 Later, other scientists may agree or not agree. They may suggest another answer. They may do more experiments. Anything in science might be revised if we find out the previous solution was not good enough.
 An example 
A famous example of science in action was the expedition led by Arthur Eddington to Principe Island in Africa in 1919. He went there to record where the stars were around the Sun during a solar eclipse. The observation of where the stars were shown that the apparent star positions close to the Sun were changed. In effect, the light passing the Sun was pulled towards the Sun by gravitation. This confirmed predictions of gravitational lensing made by Albert Einstein in the general theory of relativity, published in 1915. Eddington's observations were considered to be the first solid proof in favour of Einstein's theory.
 Practical impacts of scientific research 
Discoveries in fundamental science can be world-changing. For example:
{| class="wikitable" style="font-size:90%"
|-
! Research !! Impact
|-
| Static electricity and magnetism (1600)Electric current (18th century) || All electric appliances, dynamos, electric power stations, modern electronics, including electric lighting, television, electric heating, magnetic tape, loudspeaker, plus the compass and lightning rod.
|-
| Diffraction (1665) || Optics, hence fiber optic cable (1840s), cable TV and internet
|-
| Germ theory (1700) || Hygiene, leading to decreased transmission of infectious diseases; antibodies, leading to techniques for disease diagnosis and targeted anticancer therapies.
|-
| Vaccination (1798) || Leading to the elimination of most infectious diseases from developed countries and the worldwide eradication of smallpox.
|-
| Photovoltaics (1839) || Solar cells (1883), hence solar power, solar powered watches, calculators and other devices.
|-
| The strange orbit of Mercury (1859) and other researchleading to special (1905) and general relativity (1916) || Satellite-based technology such as GPS (1973), satnav and communications satellites.
|-
| Radio waves (1887) || Used in broadcast: radio (1906) and television (1927) entertainment. It is used in telephony, emergency services, radar (navigation and weather forecasting), medicine, astronomy, wireless communications, and networking. Radio research led to microwave cooking.
|-
| Radioactivity (1896) and antimatter (1932) || Cancer treatment (1896), Radiometric dating (1905), nuclear reactors (1942) and weapons (1945), PET scans (1961), and medical research (with isotopic labelling)
|-
|X-rays (1896)|| Medical imaging, including computer tomography
|-
| Crystallography and quantum mechanics (1900) || Semiconductor devices (1906), hence modern computing and telecommunications including the integration with wireless devices: the mobile phone
|-
|Plastics (1907)||Starting with bakelite, many types of artificial polymers for numerous applications in industry and daily life
|-
|Antibiotics (1880's, 1928) || Salvarsan, Penicillin, doxycycline. In 2018 Amoxicillin and amoxicillin/clavulanic acid were the most frequently used.
|-
|Nuclear magnetic resonance (1930's) || Nuclear magnetic resonance spectroscopy (1946), magnetic resonance imaging (1971), functional magnetic resonance imaging (1990's).
|-
|Genomics (1990s)||Genomics = genetics + medicine. It is the structure, function, evolution, mapping, and editing of genomes. A genome is an organism's complete set of DNA (or RNA). This makes up its genes. Vaccines for viruses are built by genomics.
|}
Philosophy of science
The philosophy of science is a subject that looks closely at how science works and what it really means. It asks big questions like: How do we know something is true in science? How do scientific ideas change over time? Can science truly be objective? People who study the philosophy of science try to understand the methods scientists use, the rules they follow, and how their discoveries affect the world. This field connects to other areas of philosophy, such as epistemology (the study of knowledge), metaphysics (the study of what exists), ethics, and logic. It also looks at how science is actually done in both natural sciences (like physics and biology) and social sciences (like psychology and sociology).
One of the biggest questions in the philosophy of science is called the demarcation problem. This means trying to figure out what counts as real science and what does not, like pseudoscience, which pretends to be scientific but doesn't follow the same rules. A famous philosopher named Karl Popper said that real science should be falsifiable. This means a scientific idea must make clear predictions that could be proven wrong with evidence. For example, Einstein’s theory of general relativity predicted that light would bend during a solar eclipse. Scientists tested this and confirmed it, showing the theory was falsifiable. On the other hand, Popper criticized things like Freud’s psychoanalysis, which tried to explain human behavior in ways that could not really be tested or proven wrong, because it could twist its explanations to fit any situation. Some later philosophers, like Imre Lakatos and Paul Feyerabend, said that science is not always so simple. They pointed out that many great scientific ideas started out as vague or hard to test. So, the question of what counts as real science is still debated, and there may not be one easy answer.
Scientific realism and anti-realism are two different ways of thinking about what science is really doing. People who support scientific realism, like Hilary Putnam and Richard Boyd, believe that scientific theories describe the world as it actually is, even the parts we cannot see, like electrons or black holes. They argue that science works so well, like making accurate predictions and helping us build amazing technology, because its ideas are mostly true, or at least close to the truth. On the other side, anti-realists, like Bas van Fraassen, believe something different. They say the job of science is not to find the absolute truth, but to come up with ideas and models that correctly predict what we can observe. This is called empirical adequacy. For example, in quantum mechanics (the science of very tiny particles), the math can predict things very well, but different scientists have totally different beliefs about what is really happening. Some realists believe in things like the Many-worlds Interpretation, while anti-realists prefer ideas like the Copenhagen Interpretation, which focus only on what we can measure and see.
Another big idea in the philosophy of science is how science explains things. One early idea was the deductive-nomological model, created by Carl Hempel. He said that to explain something scientifically, you had to show how it follows from general laws and starting conditions. For example, explaining why a planet moves in an ellipse (oval shape) using Newton's laws of gravity and motion. But not everyone agreed with this model. Some philosophers thought it did not explain things in everyday life or in complicated sciences like biology. So, newer thinkers, like Nancy Cartwright and James Woodward, introduced different ways to explain things, especially using causes and mechanisms. In this newer view, a good explanation tells us how something works step by step. For instance, if we want to explain how the heart pumps blood, we do not just give a law, we show how parts like pacemaker cells, ion channels, and muscle fibers all work together to make the heart beat. This kind of explanation is especially useful in subjects like neuroscience and medicine, where understanding the parts and how they interact is key.
One big question in the philosophy of science is called the problem of induction. This idea was first brought up by a philosopher named David Hume in the 1700s. He asked: How do we really know that what happened in the past will keep happening in the future? For example, we believe the sun will rise tomorrow because it has always done so. But Hume pointed out that we do not have a perfect reason to believe that, just past experience. Science often depends on these kinds of patterns, but there is no absolute proof that they will always hold true. Later on, a philosopher named Nelson Goodman added to this puzzle. He showed that it is not always easy to decide which patterns are meaningful. He created a strange example using the color “grue,” which means something is green before a certain time and blue after. This example showed that choosing which patterns to trust is harder than it seems. Today, some philosophers and scientists try to solve this problem using probability. This is called Bayesian reasoning, where people adjust their beliefs based on how likely something is and what they already know. But even this method has challenges, like being hard to calculate and including some personal judgment.
Another important thinker was Thomas Kuhn, who wrote a famous book in 1962 called The Structure of Scientific Revolutions. He said that science does not always grow in a straight line by slowly adding new facts. Instead, he said science works through paradigms, big ideas or frameworks that shape how scientists think, what they study, and what evidence they trust. Scientists work within a paradigm for a long time during “normal science,” but eventually they find anomalies, things that do not fit. When enough of these build up, a scientific revolution happens, and the old paradigm is replaced with a new one. For example, when scientists moved from Newton’s laws to Einstein’s theory of relativity, or from Ptolemy’s geocentric model (Earth at the center) to Copernicus’s heliocentric model (Sun at the center), these were paradigm shifts. Kuhn also said that different paradigms can be incommensurable, meaning they are so different that they cannot easily be compared. This idea was controversial. Philosophers like Karl Popper and Imre Lakatos disagreed with Kuhn. Karl Popper, for example, believed that science moves forward by falsification. This means that scientists should constantly test their ideas and try to prove them wrong. If a theory fails the test, it should be rejected. Popper thought that a good scientific theory must be one that could be proven false if the evidence does not support it. For example, saying “All swans are white” is a scientific idea because it can be tested, and even disproven by finding just one black swan. Another philosopher, Imre Lakatos, offered a different view. He did not think science was as dramatic as Kuhn’s idea of sudden "paradigm shifts," but he also did not fully agree with Popper. Lakatos said that science works through research programmes, groups of theories and ideas that develop over time. These programmes may face problems and make mistakes, but if they keep improving and helping us understand more, then they are still making progress. Lakatos believed science should still be rational and goal-directed, even if it changes slowly and builds on earlier ideas.
The philosophy of science does not just focus on how we know things or what reality is made of. It also asks important questions about ethics (what is right or wrong) and politics (how science affects society). These questions are especially important in fields like environmental science, artificial intelligence, biotechnology, and how we respond to pandemics. For example, when scientists or governments use computer models to make decisions about public health or climate change, philosophers ask: Are these models reliable? Are they being used fairly? Scientists also have to think about responsibility, how their discoveries might help or harm people. One idea that comes up is called the precautionary principle. This means that if a new technology might be dangerous and we do not fully understand the risks, we should be careful and not rush into using it. This idea is based on epistemic humility, which means recognizing the limits of our knowledge. A good example is CRISPR, a powerful gene-editing tool. While it can be used to treat diseases, it also raises big questions: Should we change human DNA? Should we allow “designer babies”? These are not just scientific questions, they are philosophical and ethical ones, too.
Philosophers also talk about whether there is only one correct way to do science. Some thinkers, like Paul Feyerabend, believed that there is no single method that all science must follow. He called this idea epistemological anarchism, which means that even unusual or untraditional approaches can lead to progress. This way of thinking is called pluralism, and it accepts that different fields of science may need different tools and ways of thinking. For instance, in climate science, scientists use a mix of physics, statistics, and information about human behavior to make predictions. In medicine, both controlled experiments and patient stories give useful, but very different, kinds of evidence. This shows that science does not always follow just one path. Instead, different methods can work together to give us a better understanding of the world.
Science and society
Scientific discoveries have played a big role in changing the world. They have helped improve how people live, work, and communicate. Some important examples include the invention of the printing press, the rise of factories during the Industrial Revolution, and modern tools like electricity, antibiotics, and the internet. These discoveries changed not only technology, but also how societies are organized. For example, in the 1700s, James Watt improved the steam engine. This invention made it easier to move goods and power machines. It helped build large cities and connect countries through trade. In 1928, Alexander Fleming discovered penicillin, the first antibiotic. This medicine saved many lives by treating infections and changed the way doctors and hospitals work all over the world.
Science also helps governments make better decisions. When leaders use evidence and facts, they can solve big problems like climate change, diseases, and food shortages. For example, the Intergovernmental Panel on Climate Change (IPCC) studies thousands of scientific papers to give advice about global warming. In 1987, the Montreal Protocol helped protect the ozone layer by banning harmful gases called CFCs. Scientists showed that CFCs were damaging the atmosphere, and the agreement helped the ozone layer begin to heal. During the COVID-19 pandemic, groups like the World Health Organization (WHO) and the Centers for Disease Control and Prevention (CDC) gave advice based on science. They told people how to stay safe using masks, vaccines, and social distancing. However, not everyone followed or trusted this advice equally. Science also helps countries work together. This is called science diplomacy. Projects like the International Space Station (ISS) or joint research in the Arctic bring scientists from different countries together. These efforts can build peace and cooperation between nations.
The connection between science and society goes both ways. Science changes the world, but society also shapes how science is done. People's values, culture, money, and politics all influence what research is supported and how results are used. For example, during the Cold War, the U.S. focused heavily on space research after the Soviet Union launched Sputnik 1 in 1957. This led to the creation of NASA and more investment in science and engineering education. Politics and fear of falling behind in space technology drove this big push. Social movements can also change science. In the 1970s, feminist groups pointed out that medical research mostly focused on men. Because of this, rules changed so women had to be included in clinical trials. This helped scientists understand how treatments can work differently in men and women. In farming, many people worry about genetically modified organisms (GMOs). This public concern has led to different rules in different places. For example, Europe uses the precautionary principle, which means being more careful about approving new GMO products than in countries like the U.S. Even the way scientific papers are shared is shaped by money and business models. Many journals charge people to read studies, which has caused debates about whether scientific knowledge should be free and open to everyone.
Science also helps grow economies and create new jobs. Many big industries, like biotechnology, renewable energy, artificial intelligence, and aerospace, are based on years of scientific research. For example, the semiconductor industry, which makes computer chips, came from early work in solid-state physics. Today, it is a global industry worth trillions of dollars. Governments often help science and technology grow by giving tax breaks, supporting patents, and creating partnerships between public and private groups. In countries like South Korea and Israel, spending more than 4% of the country’s income on research and development (R&D) has helped build strong technology industries. However, new technology can also cause problems. Sometimes, machines and computers take over jobs that people used to do. This is called technological unemployment. It raises hard questions about what to do when people lose their jobs because of automation. Ideas like universal basic income, job training programs, and better education are being discussed to help people adapt to these changes.
Understanding science is important for everyone. It helps people make smart choices, stay healthy, and take part in important debates. This is called scientific literacy. When people understand basic science, they can check if information is true and avoid being misled. If many people do not understand science, it can lead to misinformation. For example, some people still believe vaccines cause autism. This false idea came from a study that was later proven wrong and taken down, but the anti-vaccine movement still exists and causes real harm. Other examples include denying climate change or following pseudoscientific health ideas that are not supported by evidence. To help the public understand science better, many things are being done. Schools are improving science classes, scientists give public talks, and science journalists write easy-to-understand articles. Organizations like the American Association for the Advancement of Science (AAAS) and the British Science Association also organize science festivals and help scientists learn how to talk clearly to the public. People can also take part in citizen science. These are projects where regular people help collect data for scientific studies. Examples include watching birds through eBird or testing water through FreshWater Watch. These projects help people feel more connected to science and show that science is for everyone, not just experts.
As science becomes more powerful, it also raises ethical and philosophical questions. New technologies like gene editing, brain-computer links, and geoengineering can change human life and the planet in big ways. People must think carefully about what is right or wrong when using these tools. For example, a tool called CRISPR-Cas9 might one day cure genetic diseases. But it could also be used to create “designer babies,” which may increase unfairness in society or bring back dangerous ideas like eugenics. Artificial intelligence (AI) is another case. When AI is used to help police or choose job applicants, it can carry hidden biases from the data it was trained on. This has led to calls for fair and transparent AI systems. To deal with these big issues, there are special groups that work on science and ethics. These include The Hastings Center, the Nuffield Council on Bioethics, and UNESCO’s Bioethics Program. They help create rules and ideas that respect human rights, justice, and the limits of science. These discussions are not just for scientists. They also involve ethicists, lawyers, religious leaders, and the general public. This helps make sure that science is used in ways that are fair and good for everyone.
Many of today’s biggest problems, like diseases, climate change, and food shortages, affect the whole world. Solving them requires international cooperation and fair access to scientific knowledge. But there are still big differences between richer and poorer countries, often called the Global North and the Global South. Wealthy countries usually lead in science. They have more money for research, better equipment, and more universities. In contrast, many low- and middle-income countries struggle with problems like not enough funding, brain drain (when scientists leave to work in richer countries), and limited access to scientific tools and journals. Some groups are working to close this gap. For example, TWAS (The World Academy of Sciences) and INASP (International Network for the Availability of Scientific Publications) help support science in developing countries. They promote regional cooperation, open-access publishing, and training programs to build research skills. The idea of open science, supported by UNESCO, says that scientific knowledge should be freely available to everyone. It also encourages scientists to work together, share results, and be transparent about how they do their research. A good example of open science is what happened during the COVID-19 pandemic. Scientists around the world used platforms like GISAID to quickly share data about the virus’s genes. This helped track new versions of the virus and develop better responses faster. However, there are still challenges. Sometimes, patents and intellectual property rules make it hard for poorer countries to get important technologies, like vaccines or medical treatments. This shows that fairness in science is not just about sharing knowledge, it is also about making sure everyone can use it.
Criticisms and limitations
Science is the best tool humans have for understanding and predicting how nature works. But it is not perfect. Science has limits, and it is not always free from mistakes or bias. Science depends on observation, testing, and the idea that ideas should be falsifiable, which means they can be proven wrong if new evidence shows up. Because of this, science cannot answer questions that are metaphysical (about things beyond the physical world), emotional, or based on personal values. These kinds of questions cannot be tested or measured with tools and experiments. For example, science can study what happens in the brain during meditation, but it cannot fully explain what a person feels during that experience. This is part of a big question in philosophy and brain science called the problem of consciousness. Scientists also disagree about how best to explain complex systems. Some believe that everything can be broken down into smaller parts (reductionism), while others think that new properties appear when parts come together (emergentism). This is an important debate in areas like neuroscience and systems biology.
Some philosophers have said that science is not as objective as it may seem. For example, Thomas Kuhn argued that what scientists observe is affected by the theories they already believe. This idea is called theory-ladenness. In his famous book The Structure of Scientific Revolutions (1962), Kuhn said that science does not always move forward step by step. Instead, it goes through big changes called paradigm shifts, times when one way of thinking is replaced by another. A good example is how Einstein’s relativity replaced Newton’s mechanics. Kuhn also said that during these shifts, scientists working under different theories may not agree on what counts as proof or even what questions are important. This makes it hard to compare old and new ideas fairly. Another thinker, Paul Feyerabend, said that there is no single "scientific method" that always works. In his book Against Method (1975), he said that in the history of science, sometimes “anything goes.” He believed that too many strict rules in science can actually stop progress. One example is how Galileo’s telescope was at first seen as untrustworthy by many scientists of his time. People did not trust what it showed because it was new and did not fit their existing ideas. This shows that new tools and ideas are sometimes rejected, not because they are wrong, but because they challenge the way people already think.
One basic problem in science is called the problem of induction. It was explained by philosopher David Hume in the 1700s. The problem asks: how can we be sure that something will always happen just because it has happened many times before? For example, the sun has risen every day in history, but that does not guarantee it will rise tomorrow. Yet, science often assumes that nature follows regular patterns like this. Philosopher Karl Popper tried to solve this problem by saying that science should focus on proving ideas false, not just confirming them. This is called falsifiability. According to Popper, science moves forward by getting rid of wrong theories. But this approach also has problems. If an experiment gives unexpected results, scientists do not always throw out the theory. Sometimes they blame the tools, the way the data was read, or unknown factors. For example, if a telescope shows strange data about a planet, the issue might be with the telescope, not the theory. So it is not always clear when a theory has truly been disproven. This shows that scientific ideas are not final truths. They can change when new evidence appears or when old evidence is understood in a new way.
Social and cultural factors also affect science. They influence what gets studied, who gets to do the research, and how results are shared. Most science today is led by institutions in the Western world, and it is often written in English. This has raised concerns about epistemic colonialism, where knowledge from non-Western cultures, like Indigenous traditions or ancient medical systems, is ignored or only accepted when explained in Western scientific terms. For example, Ayurveda and Traditional Chinese medicine have been used for thousands of years. But in modern science, they are often not taken seriously unless specific parts of them are tested in lab conditions or clinical trials. There are also problems inside the scientific community. Many researchers are under pressure to publish lots of papers to keep their jobs. This is called "publish or perish." It can lead to rushed studies, weak results, and even bad practices like p-hacking (changing data to get the desired result). These issues are part of what's known as the replication crisis, when other scientists try to repeat a study but do not get the same results. One big study in 2015, called the Open Science Collaboration, tried to repeat 100 famous psychology experiments. Only 36% of them gave similar results. This showed that many scientific findings may not be as reliable as once thought.
Science works best when it is honest and independent. But sometimes, economic and political pressures can affect what scientists study, how results are shared, and whether the truth is told. For example, in the 1900s, tobacco companies paid for studies that made smoking look safer than it really was. They tried to hide the fact that smoking causes cancer. This delayed laws to protect people and led to millions of deaths. The fossil fuel industry has done something similar. By funding certain research groups and articles, they tried to make people doubt the strong scientific agreement that human actions are causing climate change. These actions are examples of corporate influence over science, which can reduce public trust and keep people from knowing the full truth. Governments can also interfere with science. During the early COVID-19 pandemic, some leaders delayed sharing important data or ignored the advice of health experts. This may have made the pandemic worse and hurt people’s trust in science and government decisions. To keep science honest, there must be transparency (open sharing of methods and results) and safeguards that protect scientists from outside pressure.
Science often uses controlled experiments to find clear answers. But some problems are too complex for this method to work perfectly. Fields like climate science, epidemiology (the study of diseases), and ecology deal with systems that have many parts working together. Because of this, scientists must use models and probabilities to make predictions. For example, climate models look at how things like greenhouse gases, ocean currents, and weather patterns interact. These models can show possible future trends, but they cannot predict exactly what will happen in a specific place at a specific time many years ahead. Sometimes people misunderstand this. They think that if science is uncertain, it must be wrong. But in complex systems, uncertainty is normal. It does not mean the science is bad. The same problem appears in nutritional science. It is hard to study diet because people eat many things at once, and other factors (like sleep or exercise) affect health too. This can lead to confusing or changing advice about what is healthy. Even in areas with experiments, like genetics, scientists have learned that things are more complicated than they once thought. Many diseases cannot be explained by a single gene. Instead, they are caused by interactions between many genes and environmental factors. This shows that reductionism, studying only small parts of a problem, can miss the bigger picture when dealing with complex systems.
Science is a powerful way to learn about the natural world, but it has limits. Some important questions cannot be answered by science alone. These include normative (what we should do), existential (why we exist), and metaphysical (beyond the physical world) questions. For example, science cannot fully answer “What is the meaning of life?”, “Is there a God?”, “What is morally right or wrong?”, or “What is consciousness?”. Science can study things we can see and test, like how the brain works. For instance, neuroscience can show how parts of the brain light up when we make choices. But it cannot tell us what we should do morally. This is known as the is–ought problem, explained by philosopher David Hume. In the same way, evolutionary biology can explain how helping others (altruism) may have developed in humans. But it cannot say whether we should be kind from a moral point of view. These types of questions are explored through philosophy, religion, ethics, and the humanities (like history and literature). Thinking that science is the only way to gain knowledge is called the scientism fallacy. It ignores the important insights from other areas of human thought, like art, culture, and spiritual traditions.
People’s trust in science is not the same everywhere. It depends on their background, politics, and past experiences. In some places, people see science as something only for experts or something that does not care about their real-life problems. This is especially true in communities that have been treated unfairly by scientific institutions in the past. One well-known case is that of Henrietta Lacks, an African-American woman whose cancer cells were taken without her permission in the 1950s. Her cells were used in many scientific breakthroughs, but her family was never asked or informed. This case is still discussed today in conversations about bioethics (the ethics of medical and scientific research) and trust in science. To build trust, it is not enough to just explain science better. Scientists also need to include different communities, listen to their concerns, and make research more open and fair. People also get confused when scientific advice changes. For example, during the COVID-19 pandemic, guidelines about wearing masks changed as scientists learned more. Some people saw this as a sign that science was unreliable. But in reality, this shows how science works, it updates when new evidence becomes available. That is a strength, not a weakness, but it must be clearly explained so the public understands why advice sometimes changes.


Isaac Newton: Sir Isaac Newton GBR FRS PRS (25 December 1643 – 31 March 1727) was an English physicist, mathematician and astronomer. He is well known for his work on the laws of motion, optics, gravity, and calculus also, he presents his theory of universal gravitation and three laws of motion.
Newton built the first practical reflecting telescope in 1668. He also developed a theory of light based on the observation that a prism decomposes white light into the colors of the rainbow. Newton also shares credit with Gottfried Leibniz for the development of calculus. 
Newton's ideas on light, motion, and gravity dominated physics for the next three centuries, until modified by Albert Einstein's theory of relativity.
After being knighted in 1705 because he was Master of the Royal Mint, he was "Sir" Isaac Newton.
 Life 
 Early life 
Isaac Newton was born (according to the Julian calendar, in use in England at the time) on Christmas Day, 25 December 1642 (N.S. 4 January 1643) "an hour or two after midnight", at Woolsthorpe Manor in Woolsthorpe-by-Colsterworth, a hamlet in the county of Lincolnshire, England. His father, also named Isaac Newton, died three months before his birth. When Newton was three, his mother, Hannah Ayscough, remarried with Reverend Barnabas Smith. Young Newton remained with his maternal grandmother, Margery Ayscough.
From 1655 to 1659, Newton was educated at The King's School, Grantham. When he was seventeen, he was removed from school. His mother tried to make him a farmer, but he did not like that.Westfall 1994 16–19 Henry Stokes, master at The King's School, requested his mother to send him back to school.White 1997 22
In June 1661, he was sent to the University of Cambridge to study. In 1666 the University closed temporarily due to the Great Plague of London and he went home to Woolsthorpe to study.
 Discoveries 
In 1666 Isaac Newton experimented with light, and found that different colors had different refractions. He began lecturing on this topic in 1670.
Newton explained the workings of the universe through mathematics. He described laws of motion and gravitation. These laws are math formulas that explain how objects move when a force acts on them. 
1687, Newton published his most famous book called the Philosophiæ Naturalis Principia Mathematica 1687 while he was a mathematics professor at Trinity College, Cambridge. In the Principia, Newton explained three basic laws that govern the way objects move. He then described his idea, or theory, about gravity. Gravity is the force that causes things to fall down. If a pencil fell off a desk, it will land on the floor, not the ceiling. In his book Newton also used his laws to show that the planets revolve around the suns in orbits that are oval, not round. Newton also discovered diffraction. This led him to enter the field of physics, where he prospered.
 Newton's Three Laws Of Motion 
Following are the three laws of motion.
 The first law (Law of Inertia)
 Newton's first law of motion states is that an object that is not being pushed or pulled by some force will stay still, or will keep moving in a straight line at a steady speed. It is easy to understand that a rocket will not move unless something pushes or pulls it. It is harder to understand that an object will continue to move without help. Think of the rocket again. If someone is flying a rocket and jumps off before the rocket is stopped, what happens? The rocket continues on until it goes into space. The tendency of an object to remain still, or keep moving in a straight line at a steady speed is called inertia.
 The second law (Law of Acceleration)
 The second law explains how a force acts on an object. An object accelerates in the direction the force is moving it. If someone gets on a bicycle and pushes the pedals forward the bicycle will begin to move. If someone gives the bicycle a push from behind, the bicycle will speed up. If the rider pushes back on the pedals the bicycle will slow down. If the rider turns the handlebars, the bicycle will change direction. The formula showing this law is F=m*a, or the force acting on an object is equal to mass times acceleration.
 The third law (Law of Reciprocal Actions)
 The third law states that if an object is pushed or pulled, the object will push or pull equally in the opposite direction. If someone lifts a heavy box, they use force to push it up. The box is heavy because it is producing an equal force downward on the lifter’s arms. The weight is transferred through the lifter’s legs to the floor. The floor presses upward with an equal force. If the floor pushed back with less force, the person lifting the box would fall through the floor. If it pushed back with more force the lifter would fly into the air.
One proposed "zeroth law" is the fact that at any instant, a body reacts to the forces applied to it at that instant. Likewise, the idea that forces add like vectors (or in other words obey the superposition principle ), and the idea that forces change the energy of a body, have each been proposed as a "fourth law".
 The discovery of the Law of Gravitation 
When people think of Isaac Newton, many think of him sitting under an apple tree watching an apple fall. Some people believe the apple fell onto his head. Newton understood that what makes things like apples fall to the ground is a specific kind of force — the force we call gravity. Newton thought that gravity was the force of attraction between two objects, such as an apple and the earth. He also thought that an object with more matter exerted the same force on smaller objects as they exerted on it. That meant that the large mass of the earth pulled objects toward it. That is why the apple fell down instead of up, and why people do not float in the air.
Isaac Newton went on thinking about gravity. Before Newton, people thought that only objects near to the earth would fall down. But Newton thought that gravity should not just be limited to the earth and the objects on it. What if gravity went to the moon and beyond?
Newton invented a formula for calculating the force of attraction between two bodies. He used it to calculate the force needed to keep the moon moving around the earth. Then he compared it with the force that made the apple fall downward. After allowing for the fact that the moon is much farther from the earth, and has a much greater mass, he discovered that the forces were the same. The moon is held in an orbit around the earth by the pull of earth’s gravity.
The formula invented by Newton is called the Law of gravitation.
 Impact 
Sir Isaac Newton’s calculations changed the way people understood the universe. No one had been able to explain why the planets stayed in their orbits. What held them up? Less than 50 years before Isaac Newton was born it was thought that the planets were held in place by an invisible shield. Isaac proved that they were held in place by the sun’s gravity. He also showed that the force of gravity was affected by distance and by mass. He was not the first to understand that the orbit of a planet was not circular, but more elongated, like an oval. What he did was to explain how this and other parts of celestial mechanics worked.
Isaac Newton was the first to discover the laws of gravitation and the laws of motion. He also established a new field in mathematics known as calculus, though the German Gottfried Leibniz had developed the ideas at the same time. His work has greatly contributed in the areas of science and mathematics making him one of the most influential scientists in human history and one of the greatest mathematician of all times.
The great physicist, Albert Einstein, thought that Newton's idea of gravity was not completely accurate. He corrected many of the things that Newton did.
 Death 
Isaac Newton died on 20 March 1728 in London, England.
He is buried in Westminster Abbey. He set the stage for many famous physicists to come. such as Albert Einstein, James Chadwick, and Stephen Hawking.


Leonardo da Vinci: Leonardo Vinci
Leonardo Da Vinci (15 April 1452 – 2 May 1519) was an Italian polymath who lived during the Renaissance. He is famous for his paintings. He was also a scientist, mathematician, engineer, inventor, anatomist, sculptor, architect, botanist, musician, and writer. Leonardo wanted to know everything about nature, and wanted to know how everything worked. He was very good at studying, as well as designing and making all sorts of inventions.
The art historian Helen Gardner said that no one has ever been quite like him because he was interested in so many things that he seems to have had the mind of a giant, and yet what he was like as a person is still a mystery.
Leonardo was born in Vinci, a small town near Florence, Italy. He was trained to be an artist by the sculptor and painter Verrocchio. He spent most of his life working for rich Italian noblemen. In his last years, he lived in an expensive home given to him by the King of France.
Two of his paintings are among the best-known in the world: the Mona Lisa and The Last Supper. He did many drawings. His best-known drawing is Vitruvian Man. Leonardo often thought of new inventions. He kept notebooks with notes and drawings of these ideas. Most of his inventions were never made. Some of his ideas were a helicopter, a tank, a calculator, a parachute, a robot, a telephone, evolution, and solar power.
 Life 
 Childhood, 1452–1466 
Leonardo was born on the 15th of April, 1452, in the Tuscan hill town of Vinci, in the valley of the Arno River. His grandfather, Antonio da Vinci, wrote down the details of the birth. Leonardo's parents were not married. His father was a Notary, Ser Piero da Vinci. His mother, Caterina, was a servant. She may have been a slave from the Middle East, or from China. His father later took custody of Leonardo, and his mother remarried and had 5 more children. Leonardo's full name was "Leonardo di ser Piero da Vinci", which means "Leonardo, the son of Messer (Mister) Piero da Vinci".
Leonardo spent his first five years living in a farmhouse with his mother. After that, he lived in Vinci with his father, his father's new wife Albiera, his grandparents, and uncle Francesco. When Leonardo grew up, he only wrote down two memories from his childhood. He remembered that when he was lying outside in his cradle, a large bird flew from the sky and hovered over him. Its tail feathers brushed his face. Leonardo's other important memory was when he found a cave while exploring in the mountains. He was terrified that some great monster might be hiding there, but he was also very excited and wanted to find out what was inside.
Leonardo started painting major works when he was about 29 or 30 years old. His first major painting was the Adoration of the Magi, and his last major painting was The Virgin and Child with Saint Anne, painted when he was about 66 or 67 years old. Giorgio Vasari wrote about Leonardo's life shortly after his death. He tells many interesting stories about how clever Leonardo was. He says that Leonardo painted a round wooden shield with a picture of snakes spitting fire. Messer Piero took his son's painting to Florence and sold it to an art dealer.
 Verrocchio's workshop, 1466–1476 
In 1466, when Leonardo was fourteen, his father took him to Florence, to be an apprentice to the artist Verrocchio.
Florence was an exciting place for a young person who wanted to be an artist. Many famous artists had lived in Florence, starting with Cimabue and Giotto in the 1200s. Everywhere a person looked, there were famous and beautiful artworks. The huge cathedral had an enormous new dome. The church of St John had doors that gleamed with gold and were said to be the most beautiful doors in the world. Another church had statues all around it by the most famous sculptors, including one by Leonardo's teacher Verrocchio.
If an artist was lucky, they would find a rich patron who would buy lots of their paintings. The richest family in Florence were the Medici. They had built themselves the finest palace in Florence, and liked buying paintings, statues and other beautiful things. They were also interested in the study of literature and philosophy. Many young artists hoped to get work from the Medici and their friends.
Verrocchio had a big workshop that was one of the busiest in Florence. Leonardo was learning to be an artist, so he had to learn drawing, painting, sculpting and model-making. While he was at the workshop, he learned many other useful skills: chemistry, metallurgy, metal working, plaster casting, leather working, mechanics and carpentry.
Leonardo was not the only young painter at Verrocchio's workshop. Many other painters trained there, or often visited. Some of them later became famous: Ghirlandaio, Perugino and Botticelli. These artists were all a few years older than Leonardo.
Giorgio Vasari tells an interesting story from this time in Leonardo's life. Verrocchio was painting a large picture of the Baptism of Christ. He gave Leonardo the job of painting one of the angels holding Jesus' robe on the left side of the picture. Vasari said that Leonardo painted the angel so beautifully that Verrocchio put down his brush and never painted again. When the painting is examined closely, it is possible to see that many other parts of the picture, such as the rocks, the brown stream and the background may have been painted by Leonardo as well. Verrocchio made a bronze statue of David at this time. It is believed that he used Leonardo as his model.
In about 1472, when he was twenty, Leonardo joined the Guild of St Luke, an organization of artists and doctors of medicine. Even after his father set him up in his own workshop, Leonardo still enjoyed working at Verrocchio's workshop. Leonardo's earliest known work is a drawing in pen and ink of the Arno River valley. It has the date 5 August, 1473. It is now in the Uffizi Gallery.
 Working life 1476–1499 
When Vasari writes about Leonardo, he uses words like "noble", "generous", "graceful," and "beautiful". Vasari tells us that as an adult, Leonardo was a tall handsome man. He was so strong that he could bend horseshoes with his bare hands. His voice was so beautiful that it charmed everyone that heard it. Almost everyone wanted to be his friend. He loved animals, was a vegetarian and would buy birds at the market to set free.
Very little is known about Leonardo's life and work between 1472 and 1481. People think he was busy in Florence. In 1478, he had an important commission to paint an altarpiece for the Monks of San Donato a Scopeto. The painting was to be the Adoration of the Magi (The Three Wise Men). The painting was never finished because Leonardo was sent away to Milan.
Leonardo was a very talented musician. In 1482, he made a silver lyre (a musical instrument) in the shape of a horse's head. At that time there was a new ruler in the city of Milan, in the north of Italy. Duke Ludovico il Moro was making other rulers nervous. Lorenzo Medici sent Leonardo to Milan as an ambassador. Lorenzo de' Medici wanted Leonardo to give Ludovico the lyre as a present from him. Leonardo wrote a letter to the Duke of Milan, telling him about all the clever and useful things that he could do, like making war machines. He wrote in the letter that he could "also paint". Leonardo did not know at the time that it was for painting that he would be mostly remembered. Leonardo stayed in Milan and worked for the Duke between 1482 and 1499. Part of his work was to design festivals and carnival processions. In Leonardo's notebooks are drawings of theatre costumes, amazing helmets and scenes that might be for the theatre.
Leonardo, like most other well-known artists of his time, had servants, young students and older assistants in his workshop. One of his young students was a boy whose name was Gian Giacomo Caprotti da Oreno. He was a handsome boy with beautiful long golden curls. He looked perfect as an artist's model for an angel. But he was such a difficult and dishonest boy that Leonardo called him "Salai" or "Salaino" which means "the little devil". Leonardo wrote in his notebook that Salai was very greedy, that he was a liar and that he had stolen things from the house at least five times. Salai stayed in Leonardo's household for thirty years as a pupil and a servant.
 Gran Cavallo 
Leonardo's most important work for Duke Ludovico was to make a huge statue of the previous ruler, Francesco Sforza, on horseback. He started with the horse. After studying horses and drawing designs, he made a huge horse of clay. It was called the "Gran Cavallo". It was going to be cast in bronze, and it was going to be the biggest bronze horse made in more than a thousand years. Unfortunately, the bronze horse was never made. In 1494, Ludovico had the bronze made into cannons because the French army was invading Milan. The huge clay horse was still standing when the French army invaded again in 1499. It was used for target practice and completely destroyed.
 The Virgin of the Rocks 
While Leonardo was working for Duke Ludovico, he had two important painting commissions. One was an oil painting to go in a big altarpiece for the Confraternity of the Immaculate Conception. Leonardo did the painting twice. He left one with the monks in Milan, and took the other painting to France. It is now displayed in the Louvre Museum. Both paintings are called the Virgin of the Rocks. They show a scene of the Virgin Mary and the child Jesus in a rocky mysterious landscape. Mary and Jesus are meeting with John the Baptist. There is a story (which is not in the Bible but is part of Christian tradition) about how the baby John and the baby Jesus met on the road to Egypt. In this scene John is praying and the baby Jesus raises his hand to bless John. The paintings have a strange eerie light with soft deep shadows. In the background is a lake and mountains in the mist. No paintings like this had ever been done before.
 The Last Supper 
Leonardo's other important painting in Milan is even more famous: The Last Supper. The painting shows the last meal shared by Jesus with his disciples, before his capture and death. Leonardo chose to paint the moment when Jesus said "one of you will betray me". Leonardo tells the story of the surprise and upset that this caused to the twelve followers of Jesus. He shows this through the actions and faces of the people in the painting. Some are talking, some have stood up, and some are raising their hands in horror.
The novelist Matteo Bandello saw Leonardo at work. Bandello wrote that on some days he would paint from morning till night without stopping to eat. Then for three or four days he would not paint at all. He would often just stand and look at the painting. Vasari said that the prior of the convent was very annoyed. He asked Ludovico to tell Leonardo to work faster. Vasari said that Leonardo was worried because he did not think that he could paint the face of Jesus well enough. Leonardo told the Duke that he might use the face of the prior as his model for Judas, the traitor.
When it was finished, everyone that saw it said that the painting was a masterpiece. But Leonardo had not used proper fresco for the painting. He had used tempera over gesso, which is not usually used for wall painting. Soon the painting started to grow mold and flake off the wall. In a hundred years it was "completely ruined".
Even though in some places the paint has fallen right off the wall, the painting is so popular that it is printed and copied more that any other religious painting in the world.
 Working life 1499–1513 
In 1499, Ludovico il Moro was overthrown. Leonardo left Milan with his servant Salai and a friend, Luca Pacioli, a mathematician. They went to Venice, where Leonardo worked as a military architect and engineer. Because Venice is a city on many islands, Leonardo tried to think of ways to defend the city from a naval attack.
In 1500, Leonardo went back to Florence, taking his "household" of servants and apprentices with him. The monks from the monastery of The Holy Annunciation gave Leonardo a home and a large workshop. In 2005 when some buildings which were used by the Department of Military Geography were being restored, the restorers discovered that part of the building used to be Leonardo's studio.
 The Virgin and Child with St Anne and John the Baptist 
Leonardo started work on a new painting. He drew a large "cartoon" (a drawing that is a plan for the painting). The cartoon showed the Virgin Mary sitting on the knee of her mother, St Anne. Mary holds the baby Jesus in her arms. Jesus stretches out his hands to his young cousin John the Baptist. Vasari says that everyone was so amazed by the beautiful drawing that "men and women, young and old" came in large groups to see it "as if they were attending a great festival". The drawing is now in the National Gallery, London. Even though it is old, faded, and kept in a dark room, people go to the gallery to sit in front of it every day.
 The Battle of Anghiari 
In 1502 and 1503, Leonardo worked for Cesare Borgia, a powerful noble who was the son of Pope Alexander VI. Leonardo travelled around Italy with Borgia as a military architect and engineer. Late in 1503, Leonardo returned to Florence. He rejoined the Guild of St Luke.
He was given a very important commission. The Signoria (Town Council) of the City of Florence wanted two large frescos painted on the walls of the most important room of the Signoria Palace. Michelangelo was to paint The Battle of Cascina and Leonardo was to paint The Battle of Anghiari. Leonardo began the project by studying and drawing the faces of angry men and fighting horses. These drawings can still be seen in his notebooks. But unfortunately, this was to be another failure for Leonardo. When he painted the picture on the wall, instead of using fresco, he mixed the paints with oil. The paint would not dry. Leonardo lit some fires to dry it, and the painting melted. Peter Paul Rubens drew a copy of the middle part. After a time, the town council covered it up and got somebody else to paint the wall. Michelangelo did not finish his painting either, because the Pope called him to Rome.
 Mona Lisa 
In about 1503 Leonardo began painting the portrait of a woman known as Mona Lisa, the most famous portrait that has ever been painted. He continued working on it for many years. It is a small picture, painted in oil on a wooden panel. It shows the face, upper body and hands of a woman. She is very plainly dressed. For a portrait, a woman would usually put on her best clothes and jewellery. Mona Lisa has a dark dress and a fine black veil over her head. Leonardo often left symbols in his paintings that give clues about the person. The unusual thing about this picture is the smile. The smile is the clue to her name: Mona Lisa Giacondo. Giacondo means "the joking one". (Mona is short for Madonna which means "My Lady".)
The reason why the painting is so famous is that it seems to be full of mystery. Mona Lisa's eyes look out at the viewer. But no-one can guess what she is thinking. Her eyes and her mouth seem to be smiling. This is very unusual in a portrait painting. Most people in portraits look very serious. It is hard to tell what Mona Lisa's exact expression is. When a person wants to read another person's feelings, they look at the corners of their mouth and eyes. But Leonardo has painted soft shadows in the corners of Mona Lisa's mouth and eyes, to disguise her expression. The soft shadows are also found on the sides of her face, her neck and hands. The way that Leonardo uses shadow is called "sfumato" (which is an Italian word for "smoke"). Vasari said that the picture was so beautifully painted that every other artist who looked at it thought that they could never paint so well.
 Working life, 1506–1516 
In 1506, Leonardo went back to Milan with his pupils, and lived in his own house in Porta Orientale. D'Oggione made several copies of the Last Supper. Luini made a copy of the Virgin of the Rocks. Boltraffio (and the others) painted many Madonna and Child pictures which can still be seen in art galleries and churches. One of pupils was a young nobleman called Count Francesco Melzi. Melzi never became a very good painter, but he loved Leonardo and stayed with him until the day he died.
In September 1513 Leonardo went to Rome and lived there until 1516. He lived in the Vatican. The three greatest painters of the High Renaissance, Leonardo, Michelangelo and Raphael were all working in Rome at the same time. Even though their names are often said together as if they were friends, they were not. Leonardo at this time was in his sixties, Michelangelo was middle-aged. He was not friendly to either Leonardo or Raphael. Raphael was a very clever young painter who learnt a lot by looking at the pictures painted by Leonardo and Michelangelo. But neither of them was ever his teacher.
In October 1515, King Francis I of France captured Milan. On December 19, there was a meeting of Francis I and Pope Leo X, in Bologna. Leonardo went to the meeting with Pope Leo. Leonardo made an amazing toy to entertain King Francis. It was a life-sized mechanical lion that could walk. It had doors in its chest which opened, and a bunch of lilies came out. Lilies were the royal symbol of the French Kings.
 Old age, 1516–1519 
In 1516, Francis I invited Leonardo to go to France with him. He gave Leonardo a beautiful house called Clos Lucé (sometimes called "Cloux"). It is near the king's palace, Chateau Amboise. Leonardo spent the last three years of his life at Clos Lucé, with his faithful friend and apprentice, Count Melzi. The king gave Leonardo a pension of 10,000 scudi. One of the last paintings that Leonardo did was a picture of John the Baptist. His model was Salai, with his beautiful long curling hair.
When Leonardo was dying, he asked for a priest to come, so that he could make his confession and receive Holy Communion. Leonardo died at Clos Lucé, on May 2, 1519. King Francis had become a close friend. Vasari says that the King held Leonardo's head in his arms as he died. In his will, he asked that sixty beggars should follow his casket in procession. He was buried in the Chapel of the Chateau Amboise.
Leonardo had never married and had no children of his own. In his will, he left his money, his books and most of his paintings to Count Melzi. Leonardo also remembered his other pupil Salai and his servant Battista di Vilussis, who each received half of Leonardo's vineyards near Milan. Leonardo left to his serving woman a black cloak with a fur edge. Salai was the owner of Leonardo's most famous oil painting, the Mona Lisa. He still owned it a few years later when he died, after fighting in a duel.
King Francis said: "There had never been another man born in the world who knew as much as Leonardo, not so much about painting, sculpture and architecture, as that he was a very great philosopher."
 Drawings 
Leonardo did not paint very many pictures. But he drew hundreds of quick sketches, plans, maps and detailed drawings. This is how he recorded all the interesting things that he saw, studied and thought about. Some of Leonardo's drawings are "studies" for paintings. In these drawings Leonardo planned the things he was going to paint. Some studies are plans for whole paintings. One of these paintings is the large beautiful drawing of the Madonna and Child with St Anne and St John the Baptist that is now in the National Gallery, London. Many of the studies show "details" that Leonardo wanted to get just right. One study shows a very detailed perspective drawing of the ruined buildings in the background of the painting of the Magi. Other studies show hands, faces, drapery, plants, horses and babies. The earliest drawing by Leonardo that has a date on it, is a Landscape of the Arno Valley, 1473, which shows the river, the mountains, Montelupo Castle and the farmlands beyond it in great detail.
 Leonardo's notebooks 
Leonardo studied things throughout his life. He did not go to university to study. He studied by observing things in the world around him, to see how they were made and how they worked. He drew things that he saw and discoveries he made into his notebooks. Many of his notebooks are now in museums. There are about 13,000 pages of notes and drawings in his notebooks, which are mostly scientific studies.
Leonardo's notebooks are hard to read because he wrote backwards in "mirror writing". Some people think that perhaps he was trying to keep his work secret. This is not true. Leonardo wrote (and sometimes drew) with his left hand. In those days pens were made from a quill (a large feather) that was cut with a pen-knife on the end. It is hard for a left-handed person to write with a quill in the ordinary way, but quite easy to write backwards.
It is likely that Leonardo planned to publish the studies in his notebooks. He organized many pages carefully, with one study taking up the front and back of each page. There is a page with drawings and writing about the human heart and a page about the womb and the fetus. One page shows drawings of the muscles of a shoulder and another page shows how an arm works.
The notebooks were not published in Leonardo's lifetime. After he died, they were divided between different people who had known him. They are nearly all in museums or libraries such as Windsor Castle, the Louvre, and the British Library. The Biblioteca Ambrosiana (a library) in Milan has the twelve-volume Codex Atlanticus.
 Studies 
Some of the things that Leonardo studied are: 
 The geology of the Earth, with its mountains, valleys, rivers and rocks.
 The anatomy of the human body with its skeleton, muscles, veins and internal organs. Leonardo was given dead bodies by a hospital. He dissected thirty dead bodies and carefully drew many of the parts. His drawings of bones and muscles were to help other artists to paint the human body properly.
 The anatomy of horses, cows, dogs, and bears.
 The expressions on human faces.
 The flight of birds .
 The weather and its phenomena.
 The way that water flows.
 The botany of plants.
 Light, shadows, mirrors and lenses.
 Perspective and the way to make things look near or far.
 The geometry of solid objects. He drew many careful pictures which were used by the mathematician Luca Pacioli in a book called De Divina Proportione.
 Designs and inventions 
Many of the drawings and notes in Leonardo's notebooks are designs, plans and inventions.
Some of the things that Leonardo designed are: 
 Costumes for parades, carnivals and theatre. These were probably for Duke Federico's court. They include armour, and a ferocious dragon.
 War machines such as an armour-plated tank, an enormous cross bow and a horrible horse-driven leg-chopper. None of these things were ever made in Leonardo's lifetime.
 Dams and canals for rivers.
 A wooden bridge that could be carried flat on wagons and unfolded and put together at the river.
 Flying things with wings that flapped, a helicopter, a parachute and a hang glider. One of Leonardo's servants was injured, trying out the hang glider. The parachute has been made and tested in modern times, and it does work.
 Church (building) and castles. It is possible that the Castle of Locarno, in the south of Switzerland was designed by Leonardo. No other building that he designed was built.
 Leonardo's studies, designs and inventions


Evolution: Evolution is a biological process. It is how living things change over time and how new species develop. The theory of evolution explains how evolution works, and how living and extinct things have come to be the way they are. 
The theory of evolution is an essential idea in biology. Theodosius Dobzhansky, a well-known evolutionary biologist, said: "Nothing in biology makes sense except in the light of evolution".
Evolution has been happening since life started on Earth and is happening now. Evolution is caused mostly by natural selection. Living things are not identical to each other. Even living things of the same species look, move, and behave differently to some extent. Some differences make it easier for living things to survive and reproduce. 
Differences may make it easier to find food, hide from danger, or give birth to offspring which survive. The offspring will have some of the things which made it easier for their parents to have and raise them. Over time, these good differences continue and are spread through the population. Many generations pass and living things change enough to become new species.
Individuals who have differences that make it harder to find food, have offspring or avoid being eaten are likely not to have offspring at all and so will not be parents of future generations.
It is known that living things have changed over time, because their remains can be seen in the rocks. These remains are called 'fossils'. They prove that the animals and plants of today are different from those of long ago. The older the fossils, the bigger the differences from modern forms. Evolution is what made this happen. Evolution is a fact, and there is a lot of evidence that shows it is true. Scientists continue to study evolution to learn more about it.
Comparing DNA sequences helps scientists group living things based on how similar their DNA is. In 2010, a study compared DNA to family trees of evolution. This showed that all life comes from a common ancestor. Now, there is clear proof that life is connected through evolution.
 Evidence 
The evidence for evolution is given in a number of books. Some of this evidence is discussed here.
 Fossils show that change has occurred 
The realization that some rocks contain fossils was a very important event in natural history. There are three parts to this story:
1. The realization that things in rocks which looked organic actually were the altered remains of living things. This was settled in the 16th and 17th centuries by Conrad Gessner, Nicolaus Steno, Robert Hooke and others.
2. The realization that many fossils represented species which do not exist today. It was Georges Cuvier, the comparative anatomist, who proved that extinction occurred and that different strata contained different fossils.p108
3. The realization that early fossils were simpler organisms than later fossils. Also, the later the rocks, the more like the present day are the fossils.
"The most convincing evidence for the occurrence of evolution is the discovery of extinct organisms in older geological strata... The older the strata are...the more different the fossil will be from living representatives... that is to be expected if the fauna and flora of the earlier strata had gradually evolved into their descendants. Ernst Mayr p13
 Geographical distribution 
Where species live is a topic which fascinated both Charles Darwin and Alfred Russel Wallace. When new species occur, usually by the splitting of older species, this takes place in one place in the world. Once it is established, a new species may spread to some places and not others.
 Australasia 
Australasia has been separated from other continents for many millions of years. In the main part of the continent, Australia, 83% of mammals, 89% of reptiles, 90% of fish and insects, and 93% of amphibians are endemic. Its native mammals are mostly marsupials like kangaroos, bandicoots, and quolls. By contrast, marsupials are today totally absent from Africa and form a small portion of the mammalian fauna of South America, where opossums, shrew opossums, and the monito del monte occur (see the Great American Interchange).
The only living representatives of primitive egg-laying mammals (monotremes) are the echidnas and the platypus. They are only found in Australasia, which includes Tasmania, New Guinea, and Kangaroo Island. These monotremes are totally absent in the rest of the world. On the other hand, Australia is missing many groups of placental mammals that are common on other continents (carnivora, artiodactyls, shrews, squirrels, lagomorphs), although it does have indigenous bats and rodents, which arrived later.
The evolutionary story is that placental mammals evolved in Eurasia, and wiped out the marsupials and monotremes wherever they spread. They did not reach Australasia until more recently. That is the simple reason why Australia has most of the world's marsupials and all the world's monotremes.
 Evolution of horses 
The evolution of the horse family (Equidae) is a good example of the way that evolution works. The oldest fossil of a horse is about 52 million years old. It was a small animal with five toes on the front feet and four on the hind feet. At that time, there were more forests in the world than today. This horse lived in woodland, eating leaves, nuts and fruit with its simple teeth. It was only about as big as a fox.
About 30 million years ago the world started to become cooler and drier. Forests shrank; grassland expanded, and horses changed. They ate grass, they grew larger, and they ran faster because they had to escape faster predators. Because grass wears teeth out, horses with longer-lasting teeth had an advantage.
For most of this long period of time, there were a number of horse types (genera). Now only one genus exists: the modern horse, Equus. It has teeth which grow all its life, hooves on single toes, great long legs for running, and the animal is big and strong enough to survive in the open plain. Horses lived in western Canada until 12,000 years ago, but all horses in North America became extinct about 11,000 years ago. The causes of this extinction are not yet clear. Climate change and over-hunting by humans are suggested.
So, scientists can see that changes have happened. They have happened slowly over a long time. How these changes have come about is explained by the theory of evolution.
 Hawaiian Drosophila (fruit flies)
In about 6500 sqmi km2 on, the Hawaiian Islands have the most diverse collection of Drosophila flies in the world, living from rainforests to mountain meadows. About 800 Hawaiian fruit fly species are known.
Genetic evidence shows that all the native fruit fly species in Hawaii have descended from a single ancestral species that came to the islands, about 20 million years ago. Later adaptive radiation was caused by a lack of competition and a wide variety of vacant niches. Although it would be possible for a single pregnant female to colonise an island, it is more likely to have been a group from the same species.
 Distribution of Glossopteris 
The combination of continental drift and evolution can explain what is found in the fossil record. Glossopteris is an extinct species of seed fern plants from the Permian period on the ancient supercontinent of Gondwana.
Glossopteris fossils are found in Permian strata in southeast South America, southeast Africa, all of Madagascar, northern India, all of Australia, all of New Zealand, and scattered on the southern and northern edges of Antarctica.
During the Permian, these continents were connected as Gondwana. This is known from magnetic striping in the rocks, other fossil distributions, and glacial scratches pointing away from the temperate climate of the South Pole during the Permian.p103
 Common descent 
When biologists look at living things, they see that animals and plants belong to groups which have something in common. Charles Darwin explained that this followed naturally if "we admit the common parentage of allied forms, together with their modification through variation and natural selection".p402p456
For example, all insects are related. They share a basic body plan, whose development is controlled by master regulatory genes. They have six legs; they have hard parts on the outside of the body (an exoskeleton); they have eyes formed of many separate chambers, and so on. Biologists explain this with evolution. All insects are the descendants of a group of animals who lived a long time ago. They still keep the basic plan (six legs and so on) but the details change. They look different now because they changed in different ways: this is evolution.
It was Darwin who first suggested that all life on Earth had a single origin, and from that beginning "endless forms most beautiful and most wonderful have been, and are being, evolved".p490 Evidence from molecular biology in recent years has supported the idea that all life is related by common descent.
 Vestigial structures 
Strong evidence for common descent comes from vestigial structures.p397 The useless wings of flightless beetles are sealed under fused wing covers. This can be simply explained by their descent from ancestral beetles which had wings that worked.p49
Rudimentary body parts, those that are smaller and simpler in structure than corresponding parts in ancestral species, are called vestigial organs. Those organs are functional in the ancestral species but are now either nonfunctional or re-adapted to a new function. Examples are the pelvic girdles of whales, halteres (hind wings) of flies, wings of flightless birds, and the leaves of some xerophytes (e.g. cactus) and parasitic plants (e.g. dodder).
However, vestigial structures may have their original function replaced with another. For example, the halteres in flies help balance the insect while in flight, and the wings of ostriches are used in mating rituals and aggressive displays. The ear ossicles in mammals are former bones of the lower jaw.
"Rudimentary organs plainly declare their origin and meaning..." (p262). "Rudimentary organs... are the record of a former state of things, and have been retained solely through the powers of inheritance... far from being a difficulty, as they assuredly do on the old doctrine of creation, might even have been anticipated in accordance with the views here explained" (p402). Charles Darwin.
In 1893, Robert Wiedersheim published a book on human anatomy and its relevance to man's evolutionary history. This book contained a list of 86 human organs that he considered vestigial. This list included examples such as the appendix and the 3rd molar teeth (wisdom teeth).
The strong grip of a baby is another example. It is a vestigial reflex, a remnant of the past when pre-human babies clung to their mothers' hair as the mothers swung through the trees. Human babies' feet curl up when they are sitting down, while primate babies can grip with their feet as well. All primates except modern man have thick body hair which an infant can grasp, unlike modern humans. The grasp reflex allows the mother to escape danger by climbing a tree using both hands and feet.
Vestigial organs often have some selection against them. The original organs take resources to build and maintain. If they no longer have a function, reducing their size improves fitness. There is direct evidence of selection. Some cave crustacea reproduce more successfully with smaller eyes than do those with larger eyes. This may be because the nervous tissue dealing with sight now becomes available to handle other sensory input.p310
 Embryology 
From the eighteenth century, it was known that embryos of different species were much more similar than the adults. In particular, some parts of embryos reflect their evolutionary past. For example, the embryos of land vertebrates develop gill slits like fish embryos. Of course, this is only a temporary stage, which gives rise to many structures in the neck of reptiles, birds, and mammals. The proto-gill slits are part of a complicated system of development: that is why they persisted.
Another example is the embryonic teeth of baleen whales. They are later lost. The baleen filter is developed from different tissue, called keratin. Early fossil baleen whales did actually have teeth as well as the baleen.
A good example is the barnacle. It took many centuries before natural historians discovered that barnacles were crustacea. Their adults look so unlike other crustacea, but their larvae are very similar to those of other crustacea.
 Artificial selection 
Charles Darwin lived in a world where animal husbandry and domesticated crops were vitally important. In both cases, farmers selected individuals for breeding that had desirable characteristics and prevented the breeding of individuals with less desirable characteristics. The eighteenth and early nineteenth centuries saw growth in scientific agriculture. Some of that growth was due to artificial breeding.
Darwin discussed artificial selection as a model for natural selection in the 1859 first edition of his work On the Origin of Species, in Chapter IV: Natural selection: 
"Slow though the process of selection may be, if feeble man can do much by his powers of artificial selection, I can see no limit to the amount of change... which may be effected in the long course of time by nature's power of selection".p109
Nikolai Vavilov showed that rye, originally a weed, came to be a crop plant by unintentional selection. Rye is a tougher plant than wheat: it survives in harsher conditions. Having become a crop like wheat, rye was able to become a crop plant in harsh areas, such as hills and mountains.
There is no real difference in the genetic processes underlying artificial and natural selection, and the concept of artificial selection was used by Charles Darwin as an illustration of the wider process of natural selection. There are practical differences. Experimental studies of artificial selection show that "the rate of evolution in selection experiments is at least two orders of magnitude (that is 100 times) greater than any rate seen in nature or the fossil record".p157
 Artificial new species 
Some have thought that artificial selection could not produce new species. It now seems that it can.
New species have been created by domesticated animal husbandry, but the details are not known or not clear. For example, domestic sheep were created by hybridisation, and no longer produce viable offspring with Ovis orientalis, one species from which they are descended. Domestic cattle, on the other hand, can be considered the same species as several varieties of wild ox, gaur, yak, etc., as they readily produce fertile offspring with them.
The best-documented new species came from laboratory experiments in the late 1980s. William Rice and G.W. Salt bred fruit flies, Drosophila melanogaster, using a maze with three different choices of habitat such as light/dark and wet/dry. Each generation was put into the maze, and the groups of flies that came out of two of the eight exits were set apart to breed with each other in their respective groups.
After thirty-five generations, the two groups and their offspring were isolated reproductively because of their strong habitat preferences: they mated only within the areas they preferred, and so did not mate with flies that preferred the other areas.
Diane Dodd was also able to show how reproductive isolation can develop from mating preferences in Drosophila pseudoobscura fruit flies after only eight generations using different food types, starch, and maltose.
Dodd's experiment has been easy for others to repeat. It has also been done with other fruit flies and foods.
 Observable changes 
Some biologists say that evolution has happened when a trait that is caused by genetics becomes more or less common in a group of organisms. Others call it evolution when new species appear.
Changes can happen quickly in smaller, simpler organisms. For example, many bacteria that cause disease can no longer be killed with some antibiotic medicines. These medicines have only been in use since the 1940s, and at first, they worked extremely well. The bacteria have evolved so that they are less affected by antibiotics. The drugs killed off all the bacteria except a few which had some resistance. These few resistant bacteria reproduced, and their offspring had the same drug resistance.
The Colorado beetle is famous for its ability to resist pesticides. Over the last 50 years it has become resistant to 52 chemical compounds used in insecticides, including cyanide. This is natural selection sped up by artificial conditions. However, not every population is resistant to every chemical. The populations only become resistant to chemicals used in their area.
 History 
Although there were a number of natural historians in the 18th century who had some idea of evolution, the first well-formed ideas came in the 19th century. Four biologists are considered the most important.
 Lamarck 
Jean-Baptiste de Lamarck (1744–1829), a French biologist, claimed that animals changed according to natural laws. He said that animals could pass on traits they had acquired during their lifetime to their offspring, using inheritance. Today, his theory is known as Lamarckism. Its main purpose is to explain adaptations by natural means. He proposed a tendency for organisms to become more complex, moving up a ladder of progress, plus use and disuse.
Lamarck's idea was that a giraffe's neck grew longer because it tried to reach higher up. This idea failed because it conflicts with heredity (Mendel's work). Mendel made his discoveries about half a century after Lamarck's work.
 Darwin 
Charles Darwin (1809–1882) wrote his On the Origin of Species in 1859. In this book, he put forward much evidence that evolution had occurred. He also proposed natural selection as the way evolution had taken place. But Darwin did not understand genetics and how traits were actually passed on. He could not accurately explain what made children look like their parents.
Nevertheless, Darwin's explanation of evolution was fundamentally correct. In contrast to Lamarck, Darwin's idea was that the giraffe's neck became longer because those with longer necks survived better.p177/9 These survivors passed their genes on, and in time the whole species got longer necks.
 Wallace 
Alfred Russel Wallace OM FRS (1823–1913) was a British naturalist, explorer, biologist, and social activist. He proposed a theory of natural selection at about the same time as Darwin. His idea was published in 1858 together with Charles Darwin's idea.
 Mendel 
An Austrian monk called Gregor Mendel (1822–1884) bred plants. In the mid-19th century, he discovered how traits were passed on from one generation to the next.
He used peas for his experiments: some peas have white flowers and others have red ones. Some peas have green seeds and others have yellow seeds. Mendel used artificial pollination to breed the peas. His results are discussed further in Mendelian inheritance. Darwin thought that the inheritance from both parents blended together. Mendel proved that the genes from the two parents stay separate, and may be passed on unchanged to later generations.
Mendel published his results in a journal that was not well-known, and his discoveries were overlooked. Around 1900, his work was rediscovered. Genes are bits of information made of DNA which work like a set of instructions. A set of genes are in every living cell. Together, genes organise the way an egg develops into an adult. With mammals, and many other living things, a copy of each gene comes from the father and another copy from the mother. Some living organisms, including some plants, only have one parent, so get all their genes from them. These genes produce the genetic differences that evolution acts on.
 Darwin's theory 
Darwin's On the Origin of Species has two themes: the evidence for evolution, and his ideas on how evolution took place. This section deals with the second issue.
 Variation 
The first two chapters of the Origin deal with variations in domesticated plants and animals, and variations in nature.
All living things show variation. Every population which has been studied shows that animals and plants vary as much as humans do.p90 This is a great fact of nature, and without it evolution would not occur. Darwin said that, just as man selects what he wants in his farm animals, so in nature the variations allow natural selection to work.
The features of an individual are influenced by two things, heredity and environment. First, development is controlled by genes inherited from the parents. Second, living brings its own influences. Some things are entirely inherited, others partly, and some not inherited at all.
The colour of eyes is entirely inherited; they are a genetic trait. Height or weight is only partly inherited, and language is not at all inherited. The fact that humans can speak is inherited, but what language is spoken depends on where a person lives and what they are taught. Another example: a person inherits a brain of somewhat variable capacity. What happens after birth depends on many things such as home environment, education, and other experiences. When a person is an adult, their brain is what their inheritance and life experience have made it.
Evolution only concerns the traits which can be inherited, wholly or partly. The hereditary traits are passed on from one generation to the next through genes. A person's genes contain all the characteristics that they inherit from their parents. The accidents of life are not passed on. Each person lives a somewhat different life, which increases the differences.
Organisms in any population vary in reproductive success.p81 From the point of view of evolution, 'reproductive success' means the total number of offspring which live to breed and leave offspring themselves.
 Inherited variation 
Variation can only affect future generations if it is inherited. Because of the work of Gregor Mendel, we know that much variation is inherited. Mendel's 'factors' are now called genes. Research has shown that almost every individual in a sexually reproducing species is genetically unique.p204
Genetic variation is increased by gene mutations. DNA does not always reproduce exactly. Rare changes occur, and these changes can be inherited. Many changes in DNA cause faults; some are neutral or even advantageous. This gives rise to genetic variation, which is the seed corn of evolution. Sexual reproduction, by the crossing over of chromosomes during meiosis, spreads variation through the population. Other events, like natural selection and drift, reduce variation. A population in the wild always has variation, but the details are always changing.p90
 Natural selection 
Evolution mainly works by natural selection. What does this mean? Animals and plants which are best suited to their environment will, on average, survive better. There is a struggle for existence. Those who survive will reproduce and create the next generation. Their genes will be passed on, and the genes of those who did not reproduce will not. This is the basic mechanism which changes the characteristics of a population and causes evolution.
Natural selection explains why living organisms change over time, and explains the anatomy, functions, and behavior that they have. It works like this:
 All living things have such fertility that their population size could increase rapidly forever.
 However, population sizes do not increase forever. Mostly, population sizes remain about the same.
 Food and other resources are limited. Therefore, there is competition for food and resources.
 No two individuals are alike. Therefore, they will not have the same chances to live and reproduce.
 Much of this variation can be inherited. Parents pass traits to their children through their genes.
 The next generation can only come from those that survive and reproduce. After many generations of this, the population will have more helpful genetic differences, and fewer harmful ones. Natural selection is really a process of elimination.p117 The elimination is being caused by the relative fit between individuals and the environment they live in.
 Selection in natural populations 
There are now many cases where natural selection has been proved to occur in wild populations. Almost every case investigated of camouflage, mimicry and polymorphism has shown strong effects of selection.
The force of selection can be much stronger than was thought by the early population geneticists. The resistance to pesticides has grown quickly. Resistance to warfarin in Norway rats (Rattus norvegicus) grew rapidly because those that survived made up more and more of the population. Research showed that, in the absence of warfarin, the resistant homozygote was at a 54% disadvantage to the normal wild type homozygote.p182 This great disadvantage was quickly overcome by the selection for warfarin resistance.
Mammals normally cannot drink milk as adults, but humans are an exception. Milk is digested by the enzyme lactase, which switches off as mammals stop taking milk from their mothers. The human ability to drink milk during adult life is supported by a lactase mutation which prevents this switch-off. Human populations have a high proportion of this mutation wherever milk is important in the diet. The spread of this 'milk tolerance' is promoted by natural selection, because it helps people survive where milk is available. Genetic studies suggest that the oldest mutations causing lactase persistence only reached high levels in human populations in the last ten thousand years. Therefore, lactase persistence is often cited as an example of recent human evolution. As lactase persistence is genetic, but animal husbandry a cultural trait, that is gene–culture coevolution.
 Adaptation 
Adaptation is one of the basic phenomena of biology. Through the process of adaptation, an organism becomes better suited to its habitat.
Adaptation is one of the two main processes that explain the diverse species we see in biology. The other is speciation (species-splitting or cladogenesis). A favourite example used today to study the interplay of adaptation and speciation is the evolution of cichlid fish in African rivers and lakes.
When people speak about adaptation they often mean something which helps an animal or plant survive. One of the most widespread adaptations in animals is the evolution of the eye. Another example is the adaptation of modern horses' teeth to grinding grass. Camouflage is another adaptation; so is mimicry. The better-adapted animals are the most likely to survive and reproduce successfully (natural selection).
An internal parasite (such as a fluke) is a good example: it has a very simple bodily structure, but still the organism is highly adapted to its particular environment. From this we see that adaptation is not just a matter of visible traits: in such parasites, critical adaptations take place in the life cycle, which is often quite complex.
 Limitations 
Not all features of an organism are adaptations.p251 Adaptations tend to reflect the past life of a species. If a species has recently changed its life style, a once valuable adaptation may become useless, and eventually become a dwindling vestige.
Adaptations are never perfect. There are always tradeoffs between the various functions and structures in a body. It is the organism as a whole that lives and reproduces, therefore it is the complete set of adaptations that is passed on to future generations.
 Genetic drift and its effect 
In populations, there are forces that add variation to the population (such as mutation), and forces that remove it. Genetic drift is the name given to random changes which remove variation from a population. Genetic drift gets rid of variation at the rate of 1/(2N) where N = population size.p29 It is therefore "a very weak evolutionary force in large populations".p55
Genetic drift explains how random chance can affect evolution in surprisingly big ways, but only when populations are quite small. Overall, its action is to make the individuals more similar to each other, and hence more vulnerable to disease or to chance events in their environment.
 Drift reduces genetic variation in populations, potentially reducing a population’s ability to survive new selective pressures.
 Genetic drift acts faster and has more drastic results in smaller populations. Small populations usually become extinct.
 Genetic drift may contribute to speciation (starting a new species) if the small group does survive.
 Bottleneck events: when a large population is suddenly and drastically reduced in size by some event, the genetic variety will be very much reduced. Infections and extreme climate events are frequent causes. Occasionally, invasions by more competitive species can be devastating. ♦ In late 1800s, hunting reduced the Northern elephant seal to only about 20 individuals. Although the population has rebounded, its genetic variability is much less than that of the Southern elephant seal.♦ Cheetahs have very little variation. We think the species was reduced to a small number at some recent time. Because it lacks genetic variation, it is in danger of infectious diseases.
Founder events: these occur when a small group buds off from a larger population. The small group then lives separately from the main population. The human species is often quoted as having been through such stages. For example, when groups left Africa to set up elsewhere (see human evolution). Apparently, we have less variation than would be expected from our worldwide distribution. Groups that arrive on islands far from the mainland are also good examples. These groups, by virtue of their small size, cannot carry the full range of alleles to be found in the parent population.
 Species 
How species form is a major part of evolutionary biology. Darwin interpreted 'evolution' (a word he did not use at first) as being about speciation. That is why he called his famous book On the Origin of Species.
Darwin thought most species arose directly from pre-existing species. This is called anagenesis: new species by older species changing. Now we think most species arise by previous species splitting: cladogenesis.
 Species splitting 
Two groups that start the same can become very different if they live in different places. When a species gets split into two geographical regions, a process starts. Each adapts to its own situation. After a while, individuals from one group can no longer reproduce with the other group. Two separate species have evolved from one.
A German explorer, Moritz Wagner, during his three years in Algeria in the 1830s, studied flightless beetles. Each species is confined to a stretch of the north coast between rivers which descend from the Atlas mountains to the Mediterranean. As soon as one crosses a river, a different but closely related species appears. He wrote later:
"... a [new] species will only [arise] when a few individuals [cross] the limiting borders of their range... the formation of a new race will never succeed... without a long continued separation of the colonists from the other members of their species".
This was an early account of the importance of geographical separation. Another biologist who thought geographical separation was critical was Ernst Mayr.
One example of natural speciation is the three-spined stickleback, a sea fish that, after the last ice age, invaded freshwater, and set up colonies in isolated lakes and streams. Over about 10,000 generations, the sticklebacks show great differences, including variations in fins, changes in the number or size of their bony plates, variable jaw structure, and colour differences.
The wombats of Australia fall into two main groups, common wombats and hairy-nosed wombats. The two types look very similar, apart from the hairiness of their noses. However, they are adapted to different environments. Common wombats live in forested areas and eat mostly green food with lots of moisture. They often feed in the daytime. Hairy-nosed wombats live on hot dry plains where they eat dry grass with very little water or nutrition in it. Their metabolic rate is slow and they sleep most of the day underground.
When two groups that started the same become different enough, then they become two different species. Part of the theory of evolution is that all living things started the same, but then split into different groups over billions of years.
 Modern evolutionary synthesis 
This was an important movement in evolutionary biology, which started in the 1930s and finished in the 1950s. It has been updated regularly ever since.
The synthesis explains how the ideas of Charles Darwin fit with the discoveries of Gregor Mendel, who found out how we inherit our genes. The modern synthesis brought Darwin's idea up to date. It bridged the gap between different types of biologists: geneticists, naturalists, and palaeontologists.
When the theory of evolution was developed, it was not clear that natural selection and genetics worked together. But Ronald Fisher showed that natural selection would work to change species. Sewall Wright explained genetic drift in 1931.
Evolution and genetics: evolution can be explained by what we know about genetics, and what we see of animals and plants living in the wild.
Thinking in terms of populations, rather than individuals, is important. The genetic variety existing in natural populations is a key factor in evolution.
Evolution and fossils: the same factors which act today also acted in the past.
Gradualism: evolution is gradual, and usually takes place by small steps. There are some exceptions to this, notably polyploidy, especially in plants.
Natural selection: the struggle for existence of animals and plants in the wild causes natural selection. The strength of natural selection in the wild was greater than even Darwin expected.
Genetic drift can be important in small populations.
The rate of evolution can vary. There is very good evidence from fossils that different groups can evolve at different rates, and that different parts of an animal can evolve at different rates.p292, 397
 Some areas of research 
 Co-evolution 
Co-evolution is where the existence of one species is tightly bound up with the life of one or more other species.
New or 'improved' adaptations which occur in one species are often followed by the appearance and spread of related features in the other species. The life and death of living things is intimately connected, not just with the physical environment, but with the life of other species.
These relationships may continue for millions of years, as it has in the pollination of flowering plants by insects. The gut contents, wing structures, and mouthparts of fossilized beetles and flies suggest that they acted as early pollinators. The association between beetles and angiosperms during the Lower Cretaceous period led to parallel radiations of angiosperms and insects into the late Cretaceous. The evolution of nectaries in Upper Cretaceous flowers signals the beginning of the mutualism between hymenoptera and angiosperms.
 Tree of life 
Charles Darwin was the first to use this metaphor in biology. The evolutionary tree shows the relationships among various biological groups. It includes data from DNA, RNA and protein analysis. Tree of life work is a product of traditional comparative anatomy, and modern molecular evolution and molecular clock research.
The major figure in this work is Carl Woese, who defined the Archaea, the third domain (or kingdom) of life. Below is a simplified version of present-day understanding. 
 Macroevolution 
Macroevolution: the study of changes above the species level, and how they take place. The basic data for such a study are fossils (palaeontology) and the reconstruction of ancient environments. Some subjects whose study falls within the realm of macroevolution:
 Adaptive radiation, such as the Cambrian Explosion.
 Changes in biodiversity through time.
 Mass extinctions.
 Speciation and extinction rates.
 The debate between punctuated equilibrium and gradualism.
 The role of development in shaping evolution: heterochrony; hox genes.
 Origin of major categories: cleidoic egg; origin of birds.
It is a term of convenience: for most biologists it does not suggest any change in the process of evolution.p87 For some palaeontologists, what they see in the fossil record cannot be explained just by the gradualist evolutionary synthesis. They are in the minority.
 Altruism and group selection 
Altruism – the willingness of some to sacrifice themselves for others – is widespread in social animals. As explained above, the next generation can only come from those who survive and reproduce. Some biologists have thought that this meant altruism could not evolve by the normal process of selection. Instead a process called "group selection" was proposed. Group selection refers to the idea that alleles can become fixed or spread in a population because of the benefits they bestow on groups, regardless of the alleles' effect on the fitness of individuals within that group.
For several decades, critiques cast serious doubt on group selection as a major mechanism of evolution.
In simple cases it can be seen at once that traditional selection suffices. For example, if one sibling sacrifices itself for three siblings, the genetic disposition for the act will be increased. This is because siblings share on average 50% of their genetic inheritance, and the sacrificial act has led to greater representation of the genes in the next generation.
Altruism is now generally seen as emerging from standard selection. The warning note from Ernst Mayr, and the work of William Hamilton are both important to this discussion.
 Hamilton's equation 
Hamilton's equation describes whether or not a gene for altruistic behaviour will spread in a population. The gene will spread if rxb is greater than c:
where:
  is the reproductive cost to the altruist,
  is the reproductive benefit to the recipient of the altruistic behavior, and
  is the probability, above the population average, of the individuals sharing an altruistic gene – the "degree of relatedness".
 Sexual reproduction 
At first, sexual reproduction might seem to be at a disadvantage compared with asexual reproduction. In order to be advantageous, sexual reproduction (cross-fertilisation) has to overcome a two-fold disadvantage (takes two to reproduce) plus the difficulty of finding a mate. Why, then, is sex so nearly universal among eukaryotes? This is one of the oldest questions in biology.
The answer has been given since Darwin's time: because the sexual populations adapt better to changing circumstances. A recent laboratory experiment suggests this is indeed the correct explanation.
"When populations are outcrossed genetic recombination occurs between different parental genomes. This allows beneficial mutations to escape deleterious alleles on its original background, and to combine with other beneficial alleles that arise elsewhere in the population. In selfing populations, individuals are largely homozygous and recombination has no effect".
In the main experiment, nematode worms were divided into two groups. One group was entirely outcrossing, the other was entirely selfing. The groups were subjected to a rugged terrain and repeatedly subjected to a mutagen. After 50 generations, the selfing population showed a substantial decline in fitness (= survival), whereas the outcrossing population showed no decline. This is one of a number of studies that show sexuality to have real advantages over non-sexual types of reproduction.
 What evolution is used for today 
An important activity is artificial selection for domestication. This is when people choose which animals to breed from, based on their traits. Humans have used this for thousands of years to domesticate plants and animals.
More recently, it has become possible to use genetic engineering. New techniques such as 'gene targeting' are now available. The purpose of this is to insert new genes or knock out old genes from the genome of a plant or animal. A number of Nobel Prizes have already been awarded for this work.
However, the real purpose of studying evolution is to explain and help our understanding of biology. After all, it is the first good explanation of how living things came to be the way they are. That is a big achievement. The practical things come mostly from genetics, the science started by Gregor Mendel, and from molecular and cell biology.
 Evolution gems 
In 2010 the journal Nature selected 15 topics as 'Evolution gems'. These were:
 Gems from the fossil record 
Land-living ancestors of whales
From water to land (see tetrapod)
The origin of feathers (see origin of birds)
The evolutionary history of teeth
The origin of vertebrate skeleton
 Gems from habitats 
Natural selection in speciation
Natural selection in lizards
A case of co-adaptation
Differential dispersal in wild birds
Selective survival in wild guppies
Evolutionary history matters
 Gems from molecular processes 
Darwin's Galapagos finches
Microevolution meets macroevolution
Toxin resistance in snakes and clams
Variation versus stability
Nature is the oldest scientific weekly journal. The link downloads as a free text file, complete with references. The idea is to make the information available to teachers.
 Responses to the idea of evolution 
 Debates about the fact of evolution 
The idea that all life evolved had been proposed before Charles Darwin published On the Origin of species. Even today, some people still discuss the concept of evolution and what it means to them, their philosophy, and their religion. Evolution does explain some things about our human nature. People also talk about the social implications of evolution, for example in sociobiology.
Some people have the religious belief that life on Earth was created by a god. In order to fit in the idea of evolution with that belief, people have used ideas like guided evolution or theistic evolution. They say that evolution is real, but is being guided in some way.
There are many different concepts of theistic evolution. Many creationists believe that the creation myth found in their religion goes against the idea of evolution. As Darwin realised, the most controversial part of the evolutionary thought is what it means for human origins.
In some countries, especially in the United States, there is tension between people who accept the idea of evolution and those who do not accept it. The debate is mostly about whether evolution should be taught in schools, and in what way this should be done.
Other fields, like cosmology and earth science also do not match with the original writings of many religious texts. These ideas were once also fiercely opposed. Death for heresy was threatened to those who wrote against the idea that Earth was the center of the universe.
Evolutionary biology is a more recent idea. Certain religious groups oppose the idea of evolution more than other religious groups do. For instance, the Roman Catholic Church now has the following position on evolution: Pope Pius XII said in his encyclical Humani Generis published in the 1950s:
"The Church does not forbid that (...) research and discussions (..) take place with regard to the doctrine of evolution, in as far as it inquires into the origin of the human body as coming from pre-existent and living matter," Pope Pius XII Humani Generis
Pope John Paul II updated this position in 1996. He said that Evolution was "more than a hypothesis":
"In his encyclical Humani Generis, my predecessor Pius XII has already said that there is no conflict between evolution and the doctrine of the faith regarding man and his vocation. (...) Today, more than a half-century after (..) that encyclical, some new findings lead us toward the recognition of evolution as more than an hypothesis. In fact it is remarkable that this theory has had progressively greater influence on the spirit of researchers, following a series of discoveries in different scholarly disciplines," Pope John Paul II speaking to the Pontifical Academy of Science
The Anglican Communion also does not oppose the scientific account of evolution.
 Using evolution for other purposes 
Many of those who accepted evolution were not much interested in biology. They were interested in using the theory to support their own ideas on society.
 Racism 
Some people have tried to use evolution to support racism. People wanting to justify racism claimed that certain groups, such as black people, were inferior. In nature, some animals do survive better than others, and it does lead to animals better adapted to their circumstances. With humans groups from different parts of the world, all evolution can say is that each group is probably well suited to its original situation. Evolution makes no judgements about better or worse. It does not say that any human group is superior to any other.
 Eugenics 
The idea of eugenics was rather different. Two things had been noticed as far back as the 18th century. One was the great success of farmers in breeding cattle and crop plants. They did this by selecting which animals or plants would produce the next generation (artificial selection). The other observation was that lower class people had more children than upper-class people. If (and it's a big if) the higher classes were there on merit, then their lack of children was the exact reverse of what should be happening. Faster breeding in the lower classes would lead to the society getting worse.
The idea to improve the human species by selective breeding is called eugenics. The name was proposed by Francis Galton, a bright scientist who meant to do good. He said that the human stock (gene pool) should be improved by selective breeding policies. This would mean that those who were considered "good stock" would receive a reward if they reproduced. However, other people suggested that those considered "bad stock" would need to undergo compulsory sterilization, prenatal testing and birth control. The German Nazi government (1933–1945) used eugenics as a cover for their extreme racial policies, with dreadful results.
The problem with Galton's idea is how to decide which features to select. There are so many different skills people could have, you could not agree who was "good stock" and who was "bad stock". There was rather more agreement on who should not be breeding. Several countries passed laws for the compulsory sterilisation of unwelcome groups. Most of these laws were passed between 1900 and 1940. After World War II, disgust at what the Nazis had done squashed any more attempts at eugenics.
 Algorithm design 
Some equations can be solved using algorithms that simulate evolution. Evolutionary algorithms work like that.
 Social Darwinism 
Another example of using ideas about evolution to support social action is social Darwinism. Social Darwinism is a term given to the ideas of the 19th century social philosopher Herbert Spencer. Spencer believed the survival of the fittest could and should be applied to commerce and human societies as a whole.
Again, some people used these ideas to claim that racism, and ruthless economic policies were justified. Today, most biologists and philosophers say that the theory of evolution should not be applied to social policy.
 Controversy 
Some people disagree with the idea of evolution. They disagree with it for a number of reasons. Most often these reasons are influenced by or based on their religious beliefs instead of science. People who do not agree with evolution usually believe in creationism or intelligent design.
Despite this, evolution is one of the most successful theories in science. People have discovered it to be useful for different kinds of research. None of the other suggestions explain things, such as fossil records, as well. So, for almost all scientists, evolution is not in doubt.


Albert Einstein: May 2019
Albert Einstein (14 March 1879 – 18 April 1955) was a German-American Jewish scientist. He worked on theoretical physics. He developed the theory of relativity. He won the Nobel Prize in Physics in 1921 for theoretical physics.
His most famous equation is in which E is for Energy, m for mass, c is the speed of light is therefore Energy equals mass multiplied by the speed of light squared.
At the start of his career, Einstein didn't think that Newtonian mechanics was enough to bring together the laws of classical mechanics and the laws of the electromagnetic field. Between 1902 and 1909 he made the theory of special relativity to fix it.
Einstein also thought that Isaac Newton's idea of gravity was not completely correct. So, he extended his ideas on special relativity to include gravity. In 1916, he published a paper on general relativity with his theory of gravitation.
In 1933, Einstein was visiting the United States but in Germany, Adolf Hitler and the Nazis came to power (this is before World War II). Since Einstein was Jewish, he did not go back to Germany because of Hitler’s anti-Semitic laws. He lived in the United States and became an American citizen in 1940. On the beginning of World War II, he and Leó Szilárd sent a letter to President Franklin D. Roosevelt explaining to him that Germany was in the making an Atomic bomb; so Einstein and Szilard recommended that the U.S. should also make one. This led to the Manhattan Project, and the U.S. became the first nation in history to create and use the atomic bomb (not on Germany but on Japan). Einstein and other physicists like Richard Feynman who worked on the Manhattan Project later regretted that the bomb was used on Japan.
Einstein lived in Princeton and was one of the first members invited to the Institute for Advanced Study, where he worked for the rest of his life.
He is now thought to be one of the greatest scientists of all time.
His contributions helped lay the foundations for all modern branches of physics, including quantum mechanics and relativity.
 Life 
 Early life 
Einstein was born in Ulm, Württemberg, Germany, on 14 March 1879. His family was Jewish, but was not very religious. However, later in life Einstein became very interested in his Judaism. Einstein did not begin speaking until he was 4 years old. According to his younger sister, Maja, "He had such difficulty with language that those around him feared he would never learn". When Einstein was around 4 years old, his father gave him a magnetic compass. He tried hard to understand how the needle could seem to move itself so that it always pointed north. The needle was in a closed case, so clearly nothing like wind could be pushing the needle around, and yet it moved. So in this way Einstein became interested in studying science and mathematics. His compass gave him ideas to explore the world of science.
When he became older, he went to school in Switzerland. After he graduated, he got a job in the patent office there. While he was working there, he wrote the papers that first made him famous as a great scientist.
Einstein married with a 20-year-old Serbian woman Mileva Marić in January 1903.
In 1917, Einstein became very sick with an illness that almost killed him, fortunately he survived. His cousin Elsa Löwenthal nursed him back to health. After this happened, Einstein divorced Mileva on 14 February 1919, and married Elsa on 2 June 1919.
 Children 
Einstein's first daughter was Lieserl Einstein. She was born in Novi Sad, Vojvodina, Austria-Hungary on January 27, 1902. She spent her first years in the care of Serbian grandparents because her father Albert did not want her to be brought to Switzerland, where he had a job offer at the patent office. Some historians believe she died from scarlet fever.
Einstein's two sons were Hans Albert Einstein and Eduard Tete Einstein. Hans Albert was born in Bern, Switzerland in May 1904. He became a professor in Berkeley (California). Eduard was born in Zürich, Switzerland in July 1910. He died at 55 years old of a stroke in the Psychiatric University Hospital Zurich "Burghölzli" . He had spent his life in and out of hospitals due to his schizophrenia.
 Later life 
In spring of 1914, he moved back to Germany, and became ordinary member of the Prussian Academy and director of a newly established institute for physics of the Kaiser-Wilhelm-Gesellschaft. He lived in Berlin and finished the General Theory of Relativity in November 1915. In the Weimar Republic, he was politically active for socialism and Zionism. In 1922, he received the Nobel prize for Physics for his explanation of the photoelectric effect in 1905. He then tried to formulate a general field theory uniting gravitation and electromagnetism, without success. He had reservations about the quantum mechanics invented by Heisenberg (1925) and Schrödinger (1926). In spring of 1933, Einstein and Elsa were traveling in the US when the Nazi party came to power. The Nazis were violently antisemitic. They called Einstein's relativity theory "Jewish physics," and some German physicists started polemics against his theories. Others, like Planck and Heisenberg, defended Einstein.
After their return to Belgium, considering the threats from the Nazis, Einstein resigned from his position in the Prussian Academy in a letter from Oostende. Einstein and Elsa decided not to go back to Berlin and moved to Princeton, New Jersey in the United States, and in 1940 he became a United States citizen.
Before World War II, in August 1939, Einstein at the suggestion of Leó Szilárd wrote to the U.S. president, Franklin D. Roosevelt, to say that the United States should invent an atomic bomb so that the Nazi government could not beat them to the punch. He signed the letter. However, he was not part of the Manhattan Project, which was the project that created the atomic bomb.
Einstein, a Jew but not an Israeli citizen, was offered the presidency in 1952 but turned it down, stating "I am deeply moved by the offer from our State of Israel, and at once saddened and ashamed that I cannot accept it." Ehud Olmert was reported to be considering offering the presidency to another non-Israeli, Elie Wiesel, but he was said to be "very not interested".
He did his research on gravitation at the Institute for Advanced Study at Princeton, New Jersey until his death on 18 April 1955 of a burst aortic aneurysm. He was still writing about quantum physics hours before he died.
He was awarded the Nobel Prize in Physics.
 Photoelectric effect 
In 1905 he came up with a theory that light was made of small particles called photons Einstein referred to photons as light quant in his paper. Using this theory he was able to explain the photoelectric effect. The formula relating the energy and frequency of a photon is . This means that higher frequency light has more energy per photon.
The photoelectric effect happens when light shining on a metal surface causes it to emit electrons. The difficulty for the classical wave theory was to explain why this effect only seems to occur for high frequency light such as UV, but not lower frequency such as red or infrared.
Einstein showed that, since higher frequency light has photons with more energy, it has a greater chance of forcing electrons out of the metal.
Einstein was also able to explain other phenomena with photons, such as fluorescence and ionization. In 1921 he was awarded the Nobel Prize for this discovery.
 Theory of relativity 
The theory of special relativity was published by Einstein in 1905, in the paper On the Electrodynamics of Moving Bodies. It says that both distance measurements and time measurements change near the speed of light. This means that as one get closer to the speed of light (nearly 300,000 kilometres per second), lengths appear to get shorter, and clocks tick more slowly. Einstein said that special relativity is based on two ideas. The first is that the laws of physics are the same for all observers that are not moving in relation to each other.
Things going in the same direction at the same speed are said to be in an inertial frame.
People in the same "frame" measure how long something takes to happen. Their clocks keep the same time. But in another "frame" their clocks move at a different rate. The reason this happens is as follows. No matter how an observer is moving, if he measures the speed of the light coming from that star it will always be the same number.
Imagine an astronaut were all alone in a different universe. It just has an astronaut and a spaceship. Is he moving? Is he standing still? Those questions do not mean anything. Why? Because when we say we are moving we mean that we can measure our distance from something else at various times. If the numbers get bigger we are moving away. If the numbers get smaller we are moving closer. To have movement you must have at least two things. An airplane can be moving at several hundred kilometers per hour, but passengers say, "I am just sitting here."
Suppose some people are on a spaceship and they want to make an accurate clock. At one end they put a mirror, and at the other end they put a simple machine. It shoots one short burst of light toward the mirror and then waits. The light hits the mirror and bounces back. When it hits a light detector on the machine, the machine says, "Count = 1," it simultaneously shoots another short burst of light toward the mirror, and when that light comes back the machine says, "Count = 2." They decide that a certain number of bounces will be defined as a second, and they make the machine change the seconds counter every time it has detected that number of bounces. Every time it changes the seconds counter it also flashes a light out through a porthole under the machine. So somebody outside can see the light flashing every second.
Every grade school child learns the formula d=rt (distance equals rate multiplied by time). We know the speed of light, and we can easily measure the distance between the machine and the mirror and multiple that to give the distance the light travels. So we have both d and r, and we can easily calculate t. The people on the spaceship compare their new "light clock" with their various wrist watches and other clocks, and they are satisfied that they can measure time well using their new light clock.
Now this spaceship happens to be going very fast. They see a flash from the clock on the space ship, and then they see another flash. Only the flashes do not come a second apart. They come at a slower rate. Light always goes at the same speed, d = rt. That is why the clock on the spaceship is not flashing once a second for the outside observer.
Special relativity also relates energy with mass, in Albert Einstein's E=mc2 formula.
 Mass-energy equivalence 
E=mc2, also called the mass-energy equivalence, is one of the things that Einstein is most famous for. It is a famous equation in physics and math that shows what happens when mass changes to energy or energy changes to mass. The "E" in the equation stands for energy. Energy is a number which you give to objects depending on how much they can change other things. For instance, a brick hanging over an egg can put enough energy onto the egg to break it, but a feather can not.
There are three basic forms of energy: potential energy, kinetic energy, and rest energy. Two of these forms of energy can be seen in the examples given above, and in the example of a pendulum.
A cannonball hangs on a rope from an iron ring. A horse pulls the cannonball to the right side. When the cannonball is released it will move back and forth as diagrammed. It would do that forever except that the movement of the rope in the ring and rubbing in other places causes friction, and the friction takes away a little energy all the time. If we ignore the losses due to friction, then the energy provided by the horse is given to the cannonball as potential energy. (It has energy because it is up high and can fall down.) As the cannonball swings down it gains more and more speed, so the nearer the bottom it gets the faster it is going and the harder it would hit you if you stood in front of it. Then it slows down as its kinetic energy is changed back into potential energy. "Kinetic energy" just means the energy something has because it is moving. "Potential energy" just means the energy something has because it is in some higher position than something else.
When energy moves from one form to another, the amount of energy always remains the same. It cannot be made or destroyed. This rule is called the "conservation law of energy". For example, when you throw a ball, the energy is transferred from your hand to the ball as you release it. But the energy that was in your hand, and now the energy that is in the ball, is the same number. For a long time, people thought that the conservation of energy was all there was to talk about.
When energy transforms into mass, the amount of energy does not remain the same. When mass transforms into energy, the amount of energy also does not remain the same. However, the amount of matter and energy remains the same. Energy turns into mass and mass turns into energy in a way that is defined by Einstein's equation, E = mc2.
The "m" in Einstein's equation stands for mass. Mass is the amount of matter there is in some body. If you knew the number of protons and neutrons in a piece of matter such as a brick, then you could calculate its total mass as the sum of the masses of all the protons and of all the neutrons. (Electrons are so small that they are almost negligible.) Masses pull on each other, and a very large mass such as that of the Earth pulls very hard on things nearby. You would weigh much more on Jupiter than on Earth because Jupiter is so huge. You would weigh much less on the Moon because it is only about one-sixth the mass of Earth. Weight is related to the mass of the brick (or the person) and the mass of whatever is pulling it down on a spring scale – which may be smaller than the smallest moon in the solar system or larger than the Sun.
Mass, not weight, can be transformed into energy. Another way of expressing this idea is to say that matter can be transformed into energy. Units of mass are used to measure the amount of matter in something. The mass or the amount of matter in something determines how much energy that thing could be changed into.
Energy can also be transformed into mass. If you were pushing a baby buggy at a slow walk and found it easy to push, but pushed it at a fast walk and found it harder to move, then you would wonder what was wrong with the baby buggy. Then if you tried to run and found that moving the buggy at any faster speed was like pushing against a brick wall, you would be very surprised. The truth is that when something is moved then its mass is increased. Human beings ordinarily do not notice this increase in mass because at the speed humans ordinarily move the increase in mass is almost nothing.
As speeds get closer to the speed of light, then the changes in mass become impossible not to notice. The basic experience we all share in daily life is that the harder we push something like a car the faster we can get it going. But when something we are pushing is already going at some large part of the speed of light we find that it keeps gaining mass, so it gets harder and harder to get it going faster. It is impossible to make any mass go at the speed of light because to do so would take infinite energy.
Sometimes a mass will change to energy. Common examples of elements that make these changes we call radioactivity are radium and uranium. An atom of uranium can lose an alpha particle (the atomic nucleus of helium) and become a new element with a lighter nucleus. Then that atom will emit two electrons, but it will not be stable yet. It will emit a series of alpha particles and electrons until it finally becomes the element Pb or what we call lead. By throwing out all these particles that have mass it has made its own mass smaller. It has also produced energy.
In most radioactivity, the entire mass of something does not get changed to energy. In an atomic bomb, uranium is transformed into krypton and barium. There is a slight difference in the mass of the resulting krypton and barium, and the mass of the original uranium, but the energy that is released by the change is huge. One way to express this idea is to write Einstein's equation as:
E = (muranium – mkrypton and barium) c2
The c2 in the equation stands for the speed of light squared. To square something means to multiply it by itself, so if you were to square the speed of light, it would be 299,792,458 meters per second, times 299,792,458 meters per second, which is approximately 
(3•108)2 =
(9•1016 meters2)/seconds2=
90,000,000,000,000,000 meters2/seconds2
So the energy produced by one kilogram would be:
E = 1 kg • 90,000,000,000,000,000 meters2/seconds2
E = 90,000,000,000,000,000 kg meters2/seconds2
or
E = 90,000,000,000,000,000 joules
or
E = 90,000 terajoule
About 60 terajoules were released by the atomic bomb that exploded over Hiroshima. So about two-thirds of a gram of the radioactive mass in that atomic bomb must have been lost (changed into energy), when the uranium changed into krypton and barium.
 BEC 
The idea of a Bose-Einstein condensate came out of a collaboration between S. N. Bose and Prof. Einstein. Einstein himself did not invent it but, instead, refined the idea and helped it become popular.
 Zero-point energy 
The concept of zero-point energy was developed in Germany by Albert Einstein and Otto Stern in 1913. It is the lowest possible energy a quantum mechanical system can have, even at absolute zero temperature. It's the energy that remains in a system due to inherent quantum fluctuations and cannot be reduced further.
 Momentum, mass, and energy 
In classical physics, momentum is explained by the equation:
p = mv
where
p represents momentum
m represents mass
v represents velocity (speed)
When Einstein generalized classical physics to include the increase of mass due to the velocity of the moving matter, he arrived at an equation that predicted energy to be made of two components. One component involves "rest mass" and the other component involves momentum, but momentum is not defined in the classical way. The equation typically has values greater than zero for both components:
E2 = (m0c2)2 + (pc)2
where
E represents the energy of a particle
m0 represents the mass of the particle when it is not moving
p represents the momentum of the particle when it is moving
c represents the speed of light.
There are two special cases of this equation.
A photon has no rest mass, but it has momentum. (Light reflecting from a mirror pushes the mirror with a force that can be measured.) In the case of a photon, because its m0 = 0, then:
E2 = 0 + (pc)2
E = pc
p = E/c
The energy of a photon can be computed from its frequency ν or wavelength λ. These are related to each other by Planck's relation, E = hν = hc/λ, where h is the Planck constant (6.626×10−34 joule-seconds). Knowing either frequency or wavelength, you can compute the photon's momentum.
In the case of motionless particles with mass, since p = 0, then:
E02 = (m0c2)2 + 0
which is just
E0 = m0c2
Therefore, the quantity "m0" used in Einstein's equation is sometimes called the "rest mass." (The "0" reminds us that we are talking about the energy and mass when the speed is 0.) This famous "mass-energy relation" formula (usually written without the "0"s) suggests that mass has a large amount of energy, so maybe we could convert some mass to a more useful form of energy. The nuclear power industry is based on that idea.
Einstein said that it was not a good idea to use the classical formula relating momentum to velocity, p = mv, but that if someone wanted to do that, he would have to use a particle mass m that changes with speed:
mv2 = m02 / (1 – v2/c2)
In this case, we can say that E = mc2 is also true for moving particles.
 The General Theory of Relativity 
The General Theory of Relativity was published in 1915, ten years after the special theory of relativity was created. Einstein's general theory of relativity uses the idea of spacetime. Spacetime is the fact that we have a four-dimensional universe, having three spatial (space) dimensions and one temporal (time) dimension. Any physical event happens at some place inside these three space dimensions, and at some moment in time. According to the general theory of relativity, any mass causes spacetime to curve, and any other mass follows these curves. Bigger mass causes more curving. This was a new way to explain gravitation (gravity).
General relativity explains gravitational lensing, which is light bending when it comes near a massive object. This explanation was proven correct during a solar eclipse, when the sun's bending of starlight from distant stars could be measured because of the darkness of the eclipse.
General relativity also set the stage for cosmology (theories of the structure of our universe at large distances and over long times). Einstein thought that the universe may curve a little bit in both space and time, so that the universe always had existed and always will exist, and so that if an object moved through the universe without bumping into anything, it would return to its starting place, from the other direction, after a very long time. He even changed his equations to include a "cosmological constant," in order to allow a mathematical model of an unchanging universe. The general theory of relativity also allows the universe to spread out (grow larger and less dense) forever, and most scientists think that astronomy has proved that this is what happens. When Einstein realized that good models of the universe were possible even without the cosmological constant, he called his use of the cosmological constant his "biggest blunder," and that constant is often left out of the theory. However, many scientists now believe that the cosmological constant is needed to fit in all that we now know about the universe.
A popular theory of cosmology is called the Big Bang. According to the Big Bang theory, the universe was formed 15 billion years ago, in what is called a "gravitational singularity". This singularity was small, dense, and very hot. According to this theory, all of the matter that we know today came out of this point.
Einstein himself did not have the idea of a "black hole", but later scientists used this name for an object in the universe that bends spacetime so much that not even light can escape it. They think that these ultra-dense objects are formed when giant stars, at least three times the size of our sun, die. This event can follow what is called a supernova. The formation of black holes may be a major source of gravitational waves, so the search for proof of gravitational waves has become an important scientific pursuit.
 Beliefs 
Many scientists only care about their work, but Einstein also spoke and wrote often about politics and world peace. He liked the ideas of socialism and of having only one government for the whole world. He also worked for Zionism, the effort to try to create the new country of Israel.
In his final days, Einstein faced a crucial decision. Doctors offered surgery to treat his condition, but he chose a different path. He believed in living life naturally, saying, “I want to go when I want to go. It is tasteless to prolong life artificially.” With these words, Einstein showed us the dignity in accepting life’s natural cycle. On January 3, 1954, Einstein sent the following reply to Gutkind: "The word God is for me nothing more than the expression and product of human weaknesses, the Bible a collection of honourable, but still primitive legends which are nevertheless pretty childish. .... For me the Jewish religion like all other religions is an incarnation of the most childish superstitions." In 2018 his letter to Gutkind was sold for $2.9 million.
Even though Einstein thought of many ideas that helped scientists understand the world much better, he disagreed with some scientific theories that other scientists liked. The theory of quantum mechanics discusses things that can happen only with certain probabilities, which cannot be predicted with more precision no matter how much information we might have. This theoretical pursuit is different from statistical mechanics, in which Einstein did important work. Einstein did not like the part of quantum theory that denied anything more than the probability that something would be found to be true of something when it was actually measured; he thought that it should be possible to predict anything, if we had the correct theory and enough information. He once said, "I do not believe that God plays dice with the Universe."
Because Einstein helped science so much, his name is now used for several different things. A unit used in photochemistry was named for him. It is equal to Avogadro's number multiplied by the energy of one photon of light. The chemical element Einsteinium is named after the scientist as well. In slang, we sometimes call a very smart person an "Einstein."
 Criticism 
Most scientists think that Einstein's theories of special and general relativity work very well, and they use those ideas and formulas in their own work. Einstein disagreed that phenomena in quantum mechanics can happen out of pure chance. He believed that all natural phenomena have explanations that do not include pure chance. He spent much of his later life trying to find a "unified field theory" that would include his general relativity theory, Maxwell's theory of electromagnetism, and perhaps a better quantum theory. Most scientists do not think that he succeeded in that attempt.


India: yes
July 2025
June 2024
July 2022 
India (hi भारत 85% Bhārat Gaṇarājya) officially the Republic of India, also known as Hindustān, is a country in South Asia. It is the seventh-largest country by area. It is also the most populated country in the world. It is the world's largest democracy by number of people since 1947.Metcalf Metcalf 2012 327 : "Even though much remains to be done, especially in regard to eradicating poverty and securing effective structures of governance, India's achievements since independence in sustaining freedom and democracy have been singular among the world's new nations."Fisher 2018 184–185 : "Since 1947, India's internal disputes over its national identity, while periodically bitter and occasionally punctuated by violence, have been largely managed with remarkable and sustained commitment to national unity and democracy."
India is a peninsular country. It has the Indian Ocean to the south, the Arabian Sea in the southwest, the Bay of Bengal in the southeast, and the Himalayas up north. It has six neighbours: Pakistan in the northwest;The Government of India also gives Afghanistan as a neighbouring country. It claims all of Kashmir as the part of India. But Pakistan disputes this claim. Pakistan controls the region of Kashmir bordering Afghanistan. Source: China, Nepal and Bhutan in the north; and Bangladesh and Myanmar in the east. Sri Lanka and the Maldives are nearby to the south. Its Andaman and Nicobar Islands share water borders with Indonesia, Myanmar and Thailand.
Humans came to the Indian subcontinent from Africa more than 55,000 years ago. They have lived there for long time. At first, they had lived in the subcontinent as hunter-gatherers. The Indian subcontinent is the second most diverse region after Africa. Humans began to create settlements in the subcontinent 9,000 years ago, on the western banks of the Indus River. The settlements became parts of the Indus Valley Civilisation in the third millennium BCE. By 1200 BCE, Sanskrit, an Indo-European language, spread to India from the northwest. The first presence of Sanskrit is found in the hymns (songs of worship) of the Rigveda. The hymns were spread from one person to another orally, not by any book. They show the early forms of Hinduism. The Indo-Aryan languages replaced the Dravidian languages in the northern and western regions of India. By 400 BCE, the caste system was developed within Hinduism. Buddhism and Jainism were also developed in India at the same time.
India has been a federal republic since 1950. Its government is a democratic parliamentary system. It is a multilingual (multiple languages) and multicultural (multiple cultures) society. The capital city of India is New Delhi. India has the second largest military force in the world and is also a nuclear weapon state. India's economy became the world's fastest growing in the G20 developing nations during 2014, replacing the People's Republic of China. India's literacy and wealth are also rising.
India has the fourth largest economy by nominal GDP, the third largest by GDP (PPP) and is one of the fastest growing major economy. According to New World Wealth, India is the fifth richest country in the world with a total individual wealth of $12.6 trillion. However, it still has many social and economic issues, for example poverty, pollution, social equality, religious extremism, terrorism and corruption. India has reduced its rate of poverty. But its economic inequality has increased. 
India is a founding member of the World Trade Organisation (WTO), and has signed the Kyoto Protocol. It is also a member of the G20 developing nations. India has its own space agency (ISRO). It has done much research throughout the Solar System. It has sent spacecraft to the Moon and Mars. Indian movies, music and spiritual teachings are becoming more important in global culture. Sources describe it as a potential superpower, because of its rising economy and increase in global influence. India is a country with nuclear weapons. It also has a high rank in military expenditure. It has disputes over Kashmir with its neighbours, Pakistan and China, since the middle of the 20th century. 
India has the fourth largest number of spoken languages per country in the world, only behind Papua New Guinea, Indonesia, and Nigeria. Most of Indians follow Hinduism at 80%, but people of different religions such as Buddhism, Sikhism and Islam also live there.
Origin of the name
The Oxford English Dictionary (third edition - 2009) says that the name "India" comes from the Classical Latin name India. It was originally used for the Indian subcontinent and the areas to its east. Latin took the name from Hellenistic Greek India (Ἰνδία), from ancient Greek Indos (Ἰνδός) and then from Old Persian Hinduš. The Old Persian name was used for the eastern province of the Achaemenid Empire. The name has a relation with the Sanskrit word sindhu. It means "river", especially the Indus River.Thieme 1970 447–450 The ancient Greeks called Indians as Indoi (GRC Ἰνδοί), which means "the people of the Indus".Kuiper 2010 86
The name Bhārat (hi भारत; ˈbʱaːɾət pron hi-Bharat.ogg) is found in both Indian epic poetry and the Constitution of India.Clémentin-Ojha 2014 It is used in different Indian languages in different forms. Bhārat is a modern form of the older name Bharātavarṣa (sa भारतवर्ष). It original meaning was the northern part of India.Singh 2017 253 It has become a very popular name for India since the middle of the 19th century.Clémentin-Ojha 2014Barrow 2003
Hindustān (ɦɪndʊˈstaːn Hindustan.ogg) is a Middle Persian name for India. It became popular by the 13th century. It is used widely since the Mughal Empire.
 History 
 Ancient India 
In 300 BCE, a king named Chandragupt Maurya built the Maurya Empire and under Ashoka united most of the Indian subcontinent under a centralized state.
One of the oldest languages of the world, Tamil, was born in today's India. It is more than 3000 years old. 
 Indian Empires & dynasties 
The Gupta Empire managed to reunite large parts of the Indian subcontinent after the collapse of the Mauryan Empire. The Gupta period is traditionally considered India's Golden age as there were great developments in culture, religion and Mathematics. 
There were also many other Indian dynasties (empires) such as the Chalukyas, Cholas, Pallavas, and Pandyas. Southern India at that time was famous for its science, art, and writing. 
The Cholas of Thanjavur were pioneers at war in the seas and influenced Malaya, Borneo, Cambodia. The influence of Cholas are still noticeable in Southeast Asia.
The Vijayanagara empire was another significant Indian empire. 
In the 16th century India came under Mughal rule. The Taj Mahal was built during the Mughal period. 
As Mughal rule weakened other empires like the Maratha empire and Sikh empire replaced it. 
 British Raj 
In the 1600s, India came under European colonization, and by 1856 the British controlled most of India.
British Colonial exploitation resulted in the deaths of millions of Indians due to starvation and famine. The British also introduced railways and banned Widow burning.
In the early 1900s, millions of people peacefully started to protest against British rule. One of the people who led the freedom movement was Mahatma Gandhi, who only used peaceful tactics, including a way called "ahimsa", which means "non-violence". 
On 15 August 1947, India peacefully became free and independent from the British Empire. India's constitution was founded on 26 January 1950. Every year, on this day, Indians celebrate Republic Day. The first official leader (Prime Minister) of India was Jawaharlal Nehru.
 After independence 
After 1947, India had a socialist planned economy. It is one of the founding members of the Non-Aligned Movement and the United Nations. It has fought many wars since independence from Britain, including wars in 1947-48, 1965, 1971, and 1999 with Pakistan and in 1962 with China. India also fought a war to capture Goa, a Portuguese-built port and a city that was not a part of India until 1961. The Portuguese refused to give it to India, so India used force and defeated the Portuguese. 
India’s early general elections saw the Congress Party, led by Jawaharlal Nehru, dominate until his death in 1964. Lal Bahadur Shastri briefly took over, followed by Indira Gandhi, who won elections in 1967 and 1971. After declaring a state of emergency in 1975, public dissatisfaction led to the Congress losing power in 1977, with the Janata Party taking over. However, this government lasted only about two years, with Morarji Desai and Charan Singh serving as prime ministers. 
Congress returned to power in 1980, and after Indira Gandhi’s assassination in 1984, her son Rajiv Gandhi won a landslide election. The party lost again in 1989, with a short-lived National Front coalition government under V.P. Singh and Chandra Shekhar. By 1991, no party secured a majority, but Congress formed a minority government led by P.V. Narasimha Rao.
India performed nuclear tests in 1974 and 1998. It is one of the few countries that have nuclear bombs. 
Since 1991, India has been one of the fastest-growing economies in the world.
Geography
India is the seventh biggest country in the world by land. It is the main part of the Indian subcontinent. The countries next to India are Pakistan, Bangladesh, Myanmar, China, Bhutan and Nepal. It is also near Sri Lanka and the Maldives, two island countries. The Andaman and Nicobar Islands, a union territory of India, is near Thailand, Indonesia and Myanmar.
 Terrain 
There are many mountains in the northern part of India. The most famous mountain range there is the Himalayas, which have some of the tallest mountains in the world. 
There are many rivers in India. The main rivers are the Ganges, the Brahmaputra, the Yamuna, the Godavari, the Kaveri, the Narmada, and the Krishna.
 Coastline 
India is a peninsula, which means that it is surrounded on three sides by water. In the west is the Arabian Sea, in the south is the Indian Ocean, and in the east is the Bay of Bengal.
India's total coastline is 7517 km mi -2 on off long. The mainland's coastline is 5423 km mi -2 on off long. The Andaman, Nicobar and Lakshadweep islands have 2094 km mi -2 on off long coastlines in total.Kumar Pathak Pednekar Raju 2006 
Based on Indian naval hydrographic charts, 43% of the mainland coast are sandy beaches, 11% are rocky shores and cliffs, and 46% are mudflats or marshy shores.Kumar Pathak Pednekar Raju 2006
 Climates 
India has different climates. In South India, the climate is mainly tropical, which means it can get very hot in summer and cool in winter. The northern part, though, has a cooler climate, called subtropical. The mountainous regions can be alpine. The Himalayas, in the alpine climate region, can get extremely cold. 
The Himalayas block cold Central Asian winds from blowing into the Indian subcontinent. It keeps the most of the subcontinent warmer than most places at same latitudes.Posey 1994 118Wolpert 2003 4 
There is very heavy rainfall along the west coast and in the Eastern Himalayan foothills. The west, though, is drier. 
 Monsoon 
Because of some of India's deserts, the entire country gets rain for four months of the year. That time (usually around June to September) is called the monsoon. It happens because the deserts attract water-filled winds from the Indian Ocean, which give rain when they come into India. 
When the monsoon rains come late or not so heavily, droughts (when the land dries out because there is less rain) are possible.
Politics
1
 250
 Rashtrapati Bhavan Wide New Delhi India.jpg
 The Rashtrapati Bhavan in New Delhi is the official residence (house where a person lives) of the President of India.
 Glimpses of the new Parliament Building, in New Delhi (2).jpg
 The Parliament House (Sansad Bhavan) in New Delhi is the place where the Parliament of India meets. 
India is a parliamentary republic with a multi-party system.Burnell Calvert 1999 125 It is the largest democracy in the world by the number of people. It has sixnational parties, for example the Indian National Congress (INC) and the Bharatiya Janata Party (BJP). It also has more than 50regional parties.Election Commission of India The Congress is known as centre in Indian political culture, while the BJP is known as right-wing.Malik Singh 1992 318–336Banerjee 2005 3118 The Congress was the majority in the Parliament from 1950 to the end of the 1980s. From the end of the 1980s, the BJPSarkar 2007 84 and the powerful regional parties are getting more seats in the Parliament over time. This forced the national parties to create coalition governments.Chander 2004 117
Government
India is ruled under the Constitution of India. It is the country's highest document of law. It came into effect on 26 January 1950.Pylee 2003a 4 Its original form said that India would be a "sovereign, democratic republic". In 1971, the statement was changed to "sovereign, socialist, secular, democratic republic".Dutt 1998 421 
India has been said to be a "quasi-federal" form of government. That means a strong federal government with weak state governments.Wheare 1980 28 The federal government is often called the "union government" or the "central government". But after political, economic and social changes at the end of the 1990s, the government became federal.Echeverri-Gent 2002 19–20Sinha 2004 25
 Branches of government 
The union government is divided into three parts: the legislature (the one that make laws), the executive (the one that applies laws), and the judiciary (the one that makes sure that the laws are obeyed). All three parts are in New Delhi, the capital city of India.
The legislature of India is called the Parliament (hi संसद hi ISO Sansad). It is divided into two houses: the upper house Rajya Sabha (Council of States); and the lower house Lok Sabha (House of the People).Gledhill 1970 127 The Rajya Sabha has 245 members. They remain members for six years.Sharma 2007 161 Most members are elected indirectly by the legislatures of state and union territories.Sharma 2007 162 The Lok Sabha has 545 members. They remain members for five years. They are elected directly by the people's vote.Sharma 2007 143
The executive is made up of the President, the Vice President, the Prime Minister and the Union Council of Ministers. The President is the head of state of India.Sharma 2007 31 The presidents are elected by an electoral college for a period of five years. The electoral college is made up of members of central and state legislatures.Sharma 2007 138Gledhill 1970 112 The Prime Minister is the head of government of India. The President can choose the Prime Minister, who has most of the power.Sharma 2007 162 The President has less power than the Prime Minister. The Union Council of Ministers helps the Prime Minister. It is similar to a cabinet in many countries.
The judicial branch is made up of three types of courts of law: the Supreme Court, the 24 High Courts and a number of trial courts.Neuborne 2003 478 The Chief Justice of India is the head of the Supreme Court. The members of the court have the power to stop a law being passed by Parliament if they think that the law contradicts (opposes) the Constitution.Sripati 1998 423–424 They can make any government action invalid if it contradicts the Constitution.Pylee 2003b 314
Divisions
For administration purposes, India has been divided into smaller pieces. Most of these pieces are called states; others are called union territories. In total, there are twenty-eight states and eight union territories.
States and union territories differ in the way they are represented. Most union territories are ruled by administrators (called Lieutenant Governors) sent by the central government. All the states (and the territories of Delhi and Puducherry) elect their local government themselves. 
States:
Union territories:
 Military 
The Indian Armed Forces is the country's military. It is made up of an Army, Navy and Air Force. There are other parts like Paramilitary and Strategic Nuclear Command. Its Commander-in-Chief is the President of India, but the Ministry of Defence manages the military. 
In 2010, the Indian Armed Forces had 1.32 million active personnel, making it one of the largest militaries in the world.
The Indian Army is becoming more modern by buying and making new weapons. It is also building defenses against missiles of other countries. In the years 2018-2022, India imported more arms than any other nation in the world. Since its independence in 1947, India fought four wars with Pakistan and a war with China.
 National symbols 
 Emblem 
The national emblem of India shows four lions standing back-to-back. The lions symbolize power, pride, confidence, and courage. Only the government can use this emblem, according to the State Emblem of India (Prohibition of Improper Use) Act, 2005.
 Name 
The name India comes from the Greek word Indus. This came from the word sindhu, which, over time, turned into Hind, Hindi, or Hindu. 
The preferred endonym (the name given to the country by its own people) is "Bhārat" in Hindi and other Indian languages. This differs with names that outsiders use for the country.
 Symbols 
Some of the national symbols are:
 National anthem: Jana Gana Mana
 National song: Vande Mataram
 National animal: Tiger
 National bird: Peacock
 National flower: Lotus
 National tree: Banyan
 National river: Ganges (Ganga)
 National Aquatic Animal: Ganges River Dolphin
 National fruit: Mango
 National heritage animal: Elephant
 National heritage bird: Indian eagle
 Border disputes 
Different countries disagree about where India's borders lie. For example, India claims Jammu and Kashmir as an Indian state. However, Pakistan and China do not recognise this area as part of India. Similarly, the Republic of India does not recognise the Pakistani and Chinese parts of Kashmir.
In 1914, British India and Tibet agreed on the McMahon Line, as part of the Simla Accord. Indians and Tibetans see this line as the official border. However, in July 1914, China withdrew from the agreement. Neither mainland China nor Taiwan recognize that Arunachal Pradesh belongs to India. According to them, it is a part of South Tibet, which belongs to China.
 Economy 
 Growth 
India's economy is among the world's fastest growing. It is the 7th largest in the world with a nominal GDP of $2,250 billion (USD). In terms of PPP, the economy is 3rd largest (worth US$8.720 trillion). The growth rate is 8.25% for fiscal year 2010. However, that is still $3678 (considering PPP) per person per year. 
 Sectors 
India's economy is based on:
 Service sector: 43%
 Industries: 41%
 Information technology: 7%
 Farming: 7%
 Outsourcing: 2%.
India's economy is diverse. Major industries include automobiles, cement, chemicals, consumer electronics, food processing, machinery, mining, petroleum, pharmaceuticals, steel, transportation equipment, and textiles.
 Poverty 
Despite its economic growth, India continues to suffer from poverty. Over one quarter of the population (27.5%) was living in poverty in 2004–2005. In 2007-2008, 80.4% of the population lived on less than US $2 a day. By 2009, this number had decreased to 68%.
 People 
There are 1.4 billion people living in India. In 2023, India passed China to become the world's most populous country. About 65% of Indians live in rural areas, or land set aside for farming. The largest cities in India are Mumbai, Kolkata, Delhi, Chennai, Bangalore, Hyderabad, and Ahmedabad. Hindi and English are Official languages of India. India has 23 officially recognised languages. Altogether, 1,625 languages are spoken in India.
 Languages 
 Language families 
There are many different languages and cultures in India. There are two main language families in India, the Indo-Aryan and the Dravidian languages. About 69% of Indians speak an Indo-Arayan language, and about 26% speak a Dravidian language. Other languages spoken in India come from the Austro-Asiatic group. Around 5% of the people speak a Tibeto-Burman language.
 Hindi & English 
Hindi is the official language in India with the largest number of speakers. It is the official language of the union. Native speakers of Hindi represented about 41% of the Indian population in the 2001 Indian census. 
English is also used, mostly for business and in administration. It has the status of a 'subsidiary official language'. 
 Other languages 
The constitution also recognises 21 other languages - either because many people speak them, or because they are very important for Indian culture. There may be as many as 1,652 different dialects in India.
In the south of India, many people speak Kannada, Telugu, Tamil and Malayalam. In the north, many people speak Punjabi, Bengali, Gujarati, and Marathi, Odia, and Maithili.
India has 23 official languages. Its constitution lists the name of the country in each of the languages. Hindi and English (listed in boldface) are the "official languages of the union" (Union meaning the Federal Government in Delhi); Tamil, Sanskrit, Telugu, Kannada, Malayalam, and Odia are officially the "classical languages of India."
 Culture 
Religion in India
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 Religion
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Cave paintings from the Stone Age are found across India. They show dances and rituals and suggest there was a prehistoric religion. During the Epic and Puranic periods, the earliest versions of the epic poems Ramayana and Mahabharata were written from about 500–100 BCE, although these were orally transmitted for centuries before this period. Other Indian Stone Age sites apart from Pakistan are in modern India, such as the Bhimbetka rock shelters in central Madhya Pradesh and the Kupgal petroglyphs of eastern Karnataka, contain rock art showing religious rites and evidence of possible ritualised music.
Several modern religions are linked to India, namely modern Hinduism, Jainism, Buddhism and Sikhism. All of these religions have different schools (ways of thinking) and traditions that are related. As a group they are called the Eastern religions. The Indian religions are similar to one another in many ways: The basic beliefs, the way worship is done and several religious practices are very similar. These similarities mainly come from the fact that these religions have a common history and common origins. They also influenced each other.
The religion of Hinduism is the main faith followed by 79.80% of people in the Republic of India; Islam – 14.23%; Christianity – 2.30%; Sikhism – 1.72%; Buddhism – 0.70% and Jainism – 0.37%.
 Technology 
India sent a spacecraft to Mars for the first time in 2014. That made it the fourth country and first Asian country to do so, successfully. It was called the Mars Orbiter Mission.
ISRO launched 104 satellites in a single mission to create a world record. India became the first nation in the world to have launched over a hundred satellites in one mission. That was more than the 2014 Russian record of 37 satellites in a single launch.
This historic event of Chandrayaan-III successfully landed on the South Pole of the moon, being the first one to do so.
 Pop culture 
India has the largest movie industry in the world. The Hindi film industry is known as Bollywood, and is mainly based in Bombay, now known as Mumbai. Other industries include Tollywood, Kollywood, Sandalwood, Mollywood, Jollywood, Dhollywood, etc. It makes 1,000 movies a year, about twice as many as Hollywood.
 Sports 
 Cricket 
Cricket is the most popular sport in India. The Indian cricket team won the 1983 and 2011 Cricket World Cup and the 2007 ICC World Twenty20. They shared the 2002 ICC Champions Trophy with Sri Lanka and won the 2013 ICC Champions Trophy and 2025 ICC Champions Trophy. 
Cricket in India is controlled by the Board of Control for Cricket in India or BCCI. Domestic tournaments are the Ranji Trophy, the Duleep Trophy, the Deodhar Trophy, the Irani Trophy, and the Challenger Series. There is also the Indian cricket league and Indian premier league Twenty20 competitions.
 Other sports 
Indians have excelled in hockey. They have also won eight gold, one silver, and two bronze medals at the Olympic Games. 
Tennis has become popular due to the victories of the India Davis Cup team. 
Association football is also a popular sport in northeast India, West Bengal, Goa and Kerala. The Indian national football team has won the South Asian Football Federation Cup many times. 
Chess, which originated in India, is also becoming popular. The number of Indian Grandmasters has increased recently. 
Traditional sports include kabaddi, kho kho, and gilli-danda, which are played throughout India.