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[[File:CollageFisica.jpg|upright=1.35|thumb|Various examples of physical phenomena]]

'''Physics''' is a branch of [[science]]. It is one of the most fundamental [[Discipline (study)|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 [[Ancient Greek language|Greek]] word ἡ φύσις, meaning "nature".<ref name="feynmanleightonsands1963-atomic">At the start of ''The Feynman Lectures on Physics'', [[Richard Feynman]] offers the [[Atomic theory|atomic hypothesis]] as the single most important scientific concept, that ''all things are made up of atoms – little particles that move around in perpetual motion, attracting each other when they are a little distance apart, but repelling upon being squeezed into one another'' ..."</ref> 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".<ref>[[James Clerk Maxwell|Maxwell J.C.]] 1878. ''Matter and motion''. Van Nostrand, p9. {{ISBN|0-486-66895-9}}</ref>

[[Astronomy]], 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]].

Physics is very important in the development of new [[Technology|technologies]], such as [[fixed-wing aircraft|airplanes]], [[television]]s, [[computer]]s and [[nuclear weapon]]s. [[Mechanics]], a branch of physics, helped develop the mathematical field of [[calculus]].

Modern physics connects ideas about the four laws of [[symmetry]] and conservation of [[energy]], [[momentum]], [[charge]], and [[parity]].

== History ==
=== Ancient astronomy ===
Astronomy is 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.<ref>Aaboe A. 1991. ''Mesopotamian mathematics, astronomy, and astrology''. The Cambridge Ancient History. Volume III (2nd ed). Cambridge University Press. {{ISBN|978-0-521-22717-9}}</ref> 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 [[Pre-socratic philosophers|a movement of philosophers]] replaced [[superstition]] with [[Naturalism (philosophy)|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 [[Aristotle|Aristotelian]] physics during the [[Islamic Golden Age]]. One main contribution was to observational astronomy. Some, like [[Ibn Sahl]], [[Al-Kindi]], [[Alhazen|Ibn al-Haytham]], [[Kamāl al-Dīn al-Fārisī|Al-Farisi]] and [[Avicenna]], worked on [[optics]] and [[vision]]. In ''[[Book of Optics|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]].<ref>Dijksterhuis E.J. 1986. ''The mechanization of the world picture: Pythagoras to Newton''. Princeton, New Jersey: Princeton University Press. {{ISBN|978-0-691-08403-9}}</ref> [[Galileo Galilei|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 [[Newton's laws of motion|three laws of motion]] and his [[Newton's law of universal gravitation|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.<ref>Ben-Chaim M. 2004. ''Experimental philosophy and the birth of empirical science: Boyle, Locke and Newton''. Aldershot: Ashgate. {{ISBN|0-7546-4091-4}}</ref>

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 [[Photoelectric effect|metal releases electrons when you shine a light at it]], and why [[Black-body radiation|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]].<ref>Einstein, Albert and Infeld, Leopold 1938. ''The evolution of physics: from early concept to relativity and quanta''. Cambridge University Press. A non-mathematical account.</ref><ref>Feynman R.P; Leighton R.B. & Sands M. 1963. ''The Feynman Lectures on Physics''. 1. {{ISBN|0-201-02116-1}}</ref>

== Definition ==

Physics is the study of [[energy]] and [[matter]] in [[Space (physics)|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 quantity|physical quantities]] in terms of these [[units of measurement|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 '''S'''ystème '''I'''nternational), 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 (unit)|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 [[Force (physics)|forces]] that make them move. For example, [[velocity]] and [[acceleration]] are used by physics to show how things move. Also, [[physicists]] study the [[Force (physics)|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 [[star]]s, [[planet]]s and [[galaxy|galaxies]] but could also study small pieces of matter, such as [[atom]]s and [[electron]]s.They may also study [[sound]], [[light]] and other [[wave]]s. As well as that, they could examine [[energy]], [[heat]] and [[radioactivity]], and even [[Space (physics)|space]] and [[time]].
Physics not only helps people understand how objects move, but how they change form, how they make [[noise]], how [[heat|hot]] or [[cold]] they will be, and what they are made of at the [[atom|smallest level]].

== Physics and mathematics==
Physics is a [[quantity|quantitative]] science because it is based on [[measure|measuring]] with [[number]]s. Mathematics is used in physics to make [[model]]s that try to [[prediction|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 equlibrium 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 [[Force (physics)|forces]], and [[Space (physics)|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 [[galaxy|galaxies]] and even larger things. There are only four [[force]]s that appear to operate over this whole range. However, even these four [[force]]s ([[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 [[:wikt:collect|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 law]]s. 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.<ref>An equation (e.g., f = m a) is called a "law" when there are clear empirical results that substantiate it.</ref>

=== 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 relativity|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 [[Empiricism|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 [[List of Jupiter's moons|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 accelerator]]s, [[laser]]s, and important industrial applications such as [[transistor]]s and [[magnetic resonance imaging]] have come from applied research.

== Physicists ==
===Prominent theoretical physicists===
Famous ''theoretical physicists'' include
{{div col|rules=yes|colwidth=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 (physicist)|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 (physicist)|Michael Berry]] (1941– )
*[[Bertrand Halperin]] (1941–)
*[[Stephen Hawking]] (1942–2018 )
*[[Alexander Markovich Polyakov|Alexander Polyakov]] (1945–)
*[[Gerardus 't Hooft]] (1946– )
*[[Jacob Bekenstein]] (1947–)
*[[Robert Laughlin]] (1950–)
{{div col end}}

==Related pages==
* [[American Physical Society]]
* [[Astronomy]]
* [[Energy]]
* [[Matter]]
* [[Time]]

== References ==
{{reflist}}

== Other websites ==
{{commonscat}}

* [https://physicsqphobia.blogspot.com/2019/01/the-length-breadth-and-thickness-of.html Units and Measurements Question ]

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[[Category:Physics| ]]