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The growth of [[physics]] has brought not only fundamental changes in ideas about the [[nature|material world]], [[mathematics]] and [[philosophy]], but also, through [[technology]], a transformation of [[society]]. Physics is considered both a body of knowledge and the practice that makes and transmits it. The [[scientific revolution]], beginning about year [[1600]], is a convenient boundary between ancient thought and classical physics. Year [[1900]] marks the beginnings of a more modern physics; today, the [[science]] shows no sign of completion, as more issues are raised, with questions rising from the [[age of the universe]], to the nature of the [[vacuum]], to the ultimate nature of the properties of [[subatomic particle]]s. [[laws of physics|Partial theories]] are currently the best that physics has to offer, at the present time. The list of [[unsolved problems in physics]] is large; however,

:''"Outside the [[nucleus]], we seem to know it all."'' -- [[Richard Feynman]].

== Antiquity ==

Since antiquity, people have tried to understand the behavior of matter: why unsupported objects drop to the ground, why different materials have different properties, and so forth. Also a mystery was the character of the [[universe]], such as the form of the [[Earth]] and the behavior of celestial objects such as the [[Sun]] and the [[Moon]]. Several theories were proposed; most of them were wrong, but this is part of the nature of the scientific enterprise, and even modern theories of [[quantum mechanics]] and [[theory of relativity|relativity]] are merely considered "theories that haven't broken yet". Physical theories in antiquity were largely couched in [[philosophy|philosophical]] terms, and rarely verified by systematic experimental testing.

Typically the behaviour and nature of the world was explained by invoking the actions of [[gods]]. Around [[200 BC]], many [[Hellenic civilization|Greek]] philosophers began to propose that the world could be understood as the result

of [[nature|natural]] [[process]]es. Many also challenged traditional ideas presented in mythology, such as the origin of the human species (anticipating the ideas of [[Charles Darwin]]), although this falls into the history of [[biology]], not physics. The [[atomism|atomist]]s attempted to characterize the nature of matter, which anticipated work in our present day.

Due to the absence of advanced experimental equipment such as [[telescope]]s and accurate time-keeping devices, experimental testing of many such ideas was impossible or impractical. There were exceptions and there are [[anachronism]]s: for example, the [[Hellenic civilization|Greek]] thinker [[Archimedes]] derived many correct quantitative descriptions of mechanics and also hydrostatics when, so the story goes, he noticed that his own body displaced a volume of water while he was getting into a bath one day. Another remarkable example was that of [[Eratosthenes]], who deduced that the [[Earth]] was a sphere, and accurately calculated its circumference using the shadows of vertical sticks to measure the angle between two widely separated points on the Earth's surface. Greek mathematicians also proposed calculating the volume of objects like [[sphere]]s and [[cone]]s by dividing them into very thin disks and adding up the volume of each disk - anticipating the invention of [[integral calculus]] by almost two millennia.

Modern knowledge of these early ideas in physics, and the extent to which they were experimentally tested, is sketchy. Almost all direct record of these ideas was lost when the [[Library of Alexandria]] was destroyed, around [[400]] AD. Perhaps the most remarkable idea we know of from this era was the deduction by [[Aristarchus]] of Samos that the Earth was a planet that traveled around the Sun once a year, and rotated on its axis once a day (accounting for the seasons and the cycle of day and night), and that the stars were other, very distant suns which also had their own accompanying planets (and possibly, lifeforms upon those planets).

The discovery of the [[Antikythera mechanism]] points to a detailed understanding of movements of these astronomical objects, as well as a use of [[gear]]-trains that pre-dates any other known civilization's use of gears.

An early version of the steam engine, [[Hero of Alexandria|Hero]]'s [[aeolipile]] was only a curiosity which did not solve the problem of transforming its rotational energy into a more usable form, not even by gears. The [[Archimedes screw]] is still in use today, to lift water from rivers onto irrigated farmland. The simple machines were unremarked, with the exception (at least) of Archimedes' elegant proof of the law of the [[lever]]. Ramps were in use several millennia before Archimedes, to build the Pyramids.

Regrettably, this period of inquiry into the nature of the world was eventually stifled by a tendency to accept the ideas of eminent philosophers, rather than to question and test those ideas. [[Pythagoras]] himself is said to have tried to suppress knowledge of the existence of [[irrational numbers]], discovered by his own school, because they did not fit his number mysticism. For one thousand years following the destruction of the [[Library of Alexandria]], [[Ptolemy]]'s (not to be confused with the [[Egyptian Ptolemies]]) model of an Earth-centred universe with planets moving in perfect circular orbits was accepted as absolute truth.

:''We should mention physics outside Europe at this stage to make this history more balanced.''

===Middle-Eastern contributions to physics ===

Civilization eclipsed by the [[Roman Empire]], many Greek doctors began to practice medicine for the Roman elite, but sadly the physical sciences were not so well supported. Following the collapse of the Roman Empire, Europe entered the so-called [[Dark Ages]], and almost all scientific research ground to a halt. The rise of [[Christianity]] saw the suppression and destruction of most classical Greek philosophy (along with Greek and Roman art, literature and religious iconography) as heretical and pagan. In the Middle East, however, many Greek natural philosophers were able to find support for their work, and scholars built upon previous work in astronomy and mathematics while developing such new fields as alchemy (chemistry). For example, the Zorastrian scholar [[al-Khwarizmi]] gave his name to what we now call an [[algorithm]], and the word [[algebra]] is derived from ''al-jabr'', the beginning of the name of one of his publications in which he developed a system of solving quadratic equations, thus beginning Algebra, without which modern physics could hardly exist.

===Indian contributions to physics===

Modern physics can hardly be imagined without a system of arithmetic in which simple calculation is easy enough to make large calulations even possible. The positional [[number system]] and the concept of [[0 (number)|zero]] were first developed in [[India]] and were adopted by the [[Islam|Islamic]] empire.

===The Middle Ages===

The monk [[Roger Bacon]] conducted experiments into optics, although much of it was similar to what had been done and was being done at the time by Arab scholars. He did make a major contribution to the development of science in medieval Europe by writing to the [[Pope]] to encourage the study of natural science in university courses and compiling several volumes recording the state of scientific knowledge in many fields at the time. He described the possible construction of a [[telescope]], but there is no strong evidence of his having made one. He recorded the manner in which he conducted his experiments in precise detail so that others could reproduce and independently test his results - a cornerstone of the [[scientific method]], and a continuation of the work of researchers like [[Al Battani]].

The annexation of the last Muslim states on the [[Iberian Peninsula]] ([[1492]]) by the recently formed Hispanic monarchy, and the fall of [[Constantinople]] in hands of the Turks ([[1453]]) coincided with the dawn of the [[Renaissance]]. This "rebirth" of European culture was in part brought about by the re-discovery of those elements of ancient Greek, Indian, Chinese and Islamic culture preserved and further developed by Islam from the [[8th century|8th]] to the [[15th century|15th]] centuries, and translated by Christian Monks into Latin, such as the ''Almagest''.

==The scientific revolution==

The [[scientific revolution]] can be viewed as a flowering of the

Renaissance and the portal to modern civilization. It started with only a few researchers, evolving into an enterprise which continues to the present day. Starting with astronomy, the principles of natural philosophy crystallized into fundamental [[law of physics|laws of physics]] which were enunciated and improved in the succeeding centuries. By the 19th century, the sciences had segmented into multiple fields with specialized researchers and the field of physics, although logically pre-eminent, no longer could claim sole ownership of the entire field of scientific research.

=== 16th century ===

In the [[16th century]] [[Nicholas Copernicus]] revived the [[heliocentric]] model of the [[solar system]] devised by [[Aristarchus]] (which survives primarily in a passing mention in [[the Sand Reckoner]] of [[Archimedes]]). When this model was published at the end of his life, it was with a preface by [[Osiander]] that piously represented it as only a mathematical convenience for calculating the positions of planets, and not an account of the true nature of the planetary orbits.

In England [[William Gilbert]] (1544-1603) studied [[magnetism]] and published a seminal work, ''De Magnete'' (1600), in which he thoroughly presented his numerous experimental results.

=== 17th century ===

In the early [[17th century]] [[Johannes Kepler]] formulated a model of the solar system based upon the five [[Platonic solid]]s, in an attempt to explain why the orbits of the planets had the relative sizes they did. His access to extremely accurate astronomical observations by [[Tycho Brahe]] enabled him to determine that his model was inconsistent with the observed orbits. After a heroic seven-year effort to more accurately model the motion

of the planet [[Mars (planet)|Mars]] (during which he laid the foundations of modern [[integral calculus]]) he concluded that the planets follow not circular orbits, but [[ellipse|elliptical]] orbits with the Sun at one focus of the ellipse. This breakthrough overturned a millennium of dogma based on [[Ptolemy]]'s idea of "perfect" circular orbits for the "perfect" heavenly bodies. Kepler then went on to formulate his [[Laws of Kepler|three laws of planetary motion]]. He also proposed the first known model of planetary motion in which a force emanating from the Sun deflects the planets from their "natural" motion, causing them to follow curved orbits.

During the early [[17th century]], [[Galileo Galilei]] pioneered the use of experiment to validate physical theories, which is the key idea in the [[scientific method]]. Galileo's use of experiment, and the insistence of Galileo and Kepler that observational results must always take precedence over theoretical results (in which they followed the precepts of [[Aristotle]] if not his practice), brushed away the acceptance of dogma, and gave birth to an era where scientific ideas were openly discussed and rigorously tested. Galileo formulated and successfully tested several results in [[dynamics (mechanics)|dynamics]], including the correct law of accelerated motion, the parabolic trajectory, the relativity of unaccelerated motion, and an early form of the Law of [[Inertia]].

In [[1687]], [[Isaac Newton]] published the ''[[Philosophiae Naturalis Principia Mathematica|Principia Mathematica]],'' detailing two comprehensive and successful physical theories: [[Newton's laws of motion]], from which arise [[classical mechanics]]; and [[gravity|Newton's Law of Gravitation]], which describes the [[fundamental force]] of [[gravity]]. Both theories agreed well with experiment. Classical mechanics would be exhaustively extended by [[Joseph-Louis de Lagrange]], [[William Rowan Hamilton]], and others, who produced new formulations, principles, and results. The Law of Gravitation initiated the field of [[astrophysics]], which describes [[astronomy|astronomical]] phenomena using physical theories.

=== 18th century ===

From the [[18th century]] onwards, [[thermodynamics|thermodynamic]] concepts were developed by [[Robert Boyle]], [[Thomas Young (scientist)|Thomas Young]], and many others, concurrently with the development of the steam engine, onward into the next century. In [[1733]], [[Daniel Bernoulli]] used statistical arguments with classical mechanics to derive thermodynamic results, initiating the field of [[statistical mechanics]]. [[Benjamin Franklin]] conducted his researches into the nature of [[electricity]] in [[1752]]. In [[1798]], [[Benjamin Thompson]] demonstrated the conversion of unlimited mechanical work into heat; it would take the work of [[James Prescott Joule]] to demonstrate the [[conservation of energy]] in the next century.

=== 19th century ===

In a letter to the [[Royal Society]] in [[1800]], [[Alessandro Volta]] described his invention of the [[Battery (electricity)|electric battery]], thus providing for the first time the means to generate a constant electric current, and opening up a new field of physics for investigation.

In [[1847]] [[James Prescott Joule]] stated the law of conservation of [[energy]], in the form of heat as well as mechanical energy. However, the principle of conservation of energy had been suggested or enunciated in various forms by perhaps a dozen German, French, British and other scientists during the first half of the 19th Century.

The behavior of [[electricity]] and [[magnetism]] was studied by [[Michael Faraday]], [[Georg Ohm]], and others. Faraday, who began his career in chemistry working under [[Humphry Davy]] at the Royal Institution, demonstrated that electrostatic phenomena, the action of the newly discovered electric pile or battery, electrochemical phenomena, and lightning were all different manifestations of electrical phenomena. Faraday further discovered in 1821 that electricity can cause rotational mechanical motion, and in 1831 discovered the principle of electromagnetic induction, by which means mechanical motion is converted into electricity. Thus it was Faraday who laid the foundations for both the [[electric motor]] and the [[electric generator]].

In [[1855]], [[James Clerk Maxwell]] unified the two phenomena into a single theory of [[electromagnetism]], described by [[Maxwell's equations]]. A prediction of this theory was that [[light]] is an [[electromagnetic radiation|electromagnetic wave]]. A more subtle part of Maxwell's deduction was that the observed speed of light does not depend on the speed of the observer, a premonition of the development of [[special relativity]] by [[Albert Einstein]].

In [[1887]] the [[Michelson-Morley experiment]] is conducted and it is interpreted as counter to the general held theory of the day, that the [[Earth]] was moving through a "[[luminiferous aether]]". The development of what later became Einstein's [[Special relativity|Special Theory of Relativity]] provided a complete explanation which did not require an aether, and was consistent with the results of the experiment. [[Albert Abraham Michelson]] and [[Edward Morley]] are not convinced of the non-existence of the aether. Morely goes on to conduct experiments with [[Dayton Miller]].

In [[1887]], [[Nikola Tesla]] investigates [[X-ray]]s using his own devices as well as Crookes tubes. In [[1895]], [[Wilhelm Conrad Röntgen]] observes and analysies X-rays, which turned out to be high-frequency [[electromagnetic radiation]]. [[Radioactivity]] was discovered in [[1896]] by [[Henri Becquerel]], and further studied by the [[Pierre Curie|Pierre]] and [[Maria Sklodowska-Curie|Marie Curie]] and others. This initiated the field of [[nuclear physics]].

In [[1897]], [[J.J. Thomson]] studies the [[electron]], the elementary particle which carries electrical current in circuits. He deduces that [[cathode ray tube|cathode ray]]s existed and were negatively charged "''particles''", which he called "''corpuscles''".

=== 20th century ===

The beginning of the [[20th century]] brought the start of a revolution in physics. The long-held theories of Newton were shown not to be correct in all circumstances. Not only did [[quantum mechanics]] show that the laws of motion didn't hold on small scales, but even more disturbingly, [[general relativity]] showed that the fixed background of [[spacetime]], on which both [[Newtonian mechanics]] and [[special relativity]] depended, could not exist.

In [[1904]], Thomson proposed the first model of the [[atom]], known as the [[atom/plum pudding|plum pudding model]]. (The existence of the atom had been proposed in [[1808]] by [[John Dalton]].)

In [[1905]], Einstein formulated the theory of [[special relativity]], unifying space and time into a single entity, [[spacetime]]. Relativity prescribes a different transformation between [[inertial frame of reference|reference frames]] than classical mechanics, necessitating the development of relativistic mechanics as a replacement for classical mechanics. In the regime of low (relative) velocities, the two theories agree. In [[1915]], Einstein extended special relativity to explain gravity with the [[general relativity|general theory of relativity]], which replaces Newton's law of gravitation. In the regime of low masses and energies, the two theories agree. One principal result of general relativity is the [[gravitational collapse]] into [[black holes]], which was anticipated two centuries earlier, but elucidated by [[Robert Oppenheimer]]. Oppenheimer would later direct the [[Manhattan Project]] at [[Los Alamos National Laboratory|Los Alamos]]. Important exact solutions of [[Einstein's field equation]] were found by [[Karl Schwarzschild]] in 1915 and [[Roy Kerr]] only in 1963.

A ''[[variational principle]]'' is a principle in [[physics]] which is expressed in terms of the [[calculus of variations]]. According to [[Cornelius Lanczos]], any physical law which can be expressed as a variational principle describes an expression which is [[self-adjoint]]<sup id="fn_1_back">[[#fn_1|1]]</sup>. These expressions are also called [[Hermitian]]. Thus such an expression describes an [[invariant]] under a Hermitian transformation. [[Felix Klein]]'s [[Erlangen program]] attempted to identify such invariants under a group of transformations. On [[July 16]], [[1918]], before a scientific organization in [[Goettingen]], Klein read a paper written by [[Emmy Noether]], because she was not allowed to present the paper before the scientific organization herself. In particular, in what is referred to in physics as [[Noether's theorem]], this paper identified the conditions under which the [[Poincaré group]] of transformations (what is now called a [[gauge group]]) for [[general relativity]] define [[conservation law]]s. The relationship of these invariants (the symmetries under a group of transformations) and what are now called conserved currents, depends on a variational principle, or [[Action (physics)|action principle]]. Noether's papers made the requirements for the conservation laws precise.

[[David Hilbert]] had derived the same equation as the [[Einstein equation]] for general relativity within a period of the same few weeks as Einstein, in November 1915. The chief difficulty, which concerned Hilbert, was that the conservation of energy did not hold for a region subject to a gravitational field. (Byers' commentary{{fn|2}} notes that sometimes the objects which are needed to define conserved quantities are not [[tensor]]s, but [[pseudotensor]]s.{{fn|3}}) <!-- Hilbert's unified theory remained uncelebrated because of this difficulty. --> Noether's theorem remains right in line with current developments in physics to this day.

In [[1911]], [[Ernest Rutherford]] deduced from [[Rutherford scattering|scattering experiments]] the existence of a compact atomic nucleus, with positively charged constituents dubbed [[proton]]s. [[Neutron]]s, the neutral nuclear constituents, were discovered in [[1932]] by [[James Chadwick]].

Beginning in [[1900]], [[Max Planck]], [[Albert Einstein]], [[Niels Bohr]], and others developed [[quantum]] theories to explain various anomalous experimental results by introducing discrete energy levels. In [[1925]], [[Werner Heisenberg]] and [[Erwin Schrödinger]] formulated [[quantum mechanics]], which explained the preceding quantum theories. In quantum mechanics, the outcomes of physical measurements are inherently [[probability|probabilistic]]. The theory describes the calculation of these probabilities. It successfully describes the behavior of matter at small distance scales.

Quantum mechanics also provided the theoretical tools for [[condensed matter physics]], which studies the physical behavior of solids and liquids, including phenomena such as [[crystal]] structures, [[semiconductor|semiconductivity]], and [[superconductor|superconductivity]]. The pioneers of condensed matter physics include [[Felix Bloch]], who created a quantum mechanical description of the behavior of electrons in crystal structures in [[1928]].

In [[1929]], [[Edwin Hubble]] published his discovery that the speed at which galaxies recede positively correlates with their distance. This is the basis for understanding that the [[universe]] is expanding. Thus, the universe must have been smaller and therefore hotter in the past. By the [[1940]]s, researchers like [[George Gamow]] proposed the ''[[Big Bang]]'' theory{{fn|4}}, evidence for which was discovered in [[1964]]{{fn|5}}; [[Enrico Fermi]] and [[Fred Hoyle]] were among the doubters in the [[1940]]s and [[1950]]s. Hoyle had dubbed Gamow's theory the ''Big Bang'' in order to debunk it. Today, it is one of the principal results of [[cosmology]], with a well-accepted age of the universe.

During [[World War II]], physics became a major source of government funding and research on all sides of the conflict. Its importance in the technologies of [[radar]], [[rocketry]], [[operations research]], and [[anti-aircraft]] weapons was seen as paramount to the war efforts of both the [[Allies|Allied]] and [[Axis]] powers. Though physics had received some more funding after [[World War I]], this was dwarfed by the amount it received only a few decades later.

In [[1934]], the Italian physicist [[Enrico Fermi]] had discovered strange results when bombarding [[uranium]] with [[neutron]]s, which he believed at first to have created [[transuranic]] elements. In [[1939]], it was discovered by the chemist [[Otto Hahn]] and the physicist [[Lise Meitner]] that what was actually happening was the process of [[nuclear fission]], whereby the nucleus of uranium was actually being split into two pieces, releasing a considerable amount of energy in the process. At this point it became clear to a number of scientists independently that this process could potentially be harnessed to produce massive amount of energy, either as a civilian power source or as a weapon.

Both the Germans and the Americans pursued research in [[nuclear physics]] to assess the ability to create such a weapon in war. The [[German nuclear energy project]], led by Heisenberg, was unsuccessful, but the Allied [[Manhattan Project]] reached its goal. In America, a team led by Fermi achieved the first man-made [[nuclear chain reaction]] in [[1942]] in the world's first [[nuclear reactor]], and in [[1945]] the world's first nuclear explosive was detonated at [[Trinity Site]], north of [[Alamogordo, New Mexico]]. In August 1945, the USA dropped two [[nuclear weapon]]s on the Japaense cities of [[atomic bombings of Hiroshima and Nagasaki|Hiroshima and Nagasaki]], and official press reports gave (perhaps an unfair amount) of the credit to the physicists involved in the project. After the war, industrial governments would become the primary sponsors of physics. The scientific leader of the Allied project, theoretical physicist [[Robert Oppenheimer]], noted the change of the imagined role of the physicist when he noted in a speech that:

:"''In some sort of crude sense, which no vulgarity, no humor, no overstatement can quite extinguish, the physicists have known sin, and this is a knowledge which they cannot lose.''"

The terms of this new relationship with the military would be harshly set when Oppenheimer had his security clearance revoked in a much publicized hearing during the height of the [[McCarthy era]] under suspicions of his loyalty.

Though the process had begun with the invention of the [[cyclotron]] by [[Ernest O. Lawrence]] in the 1930s, physics in the postwar period entered into a phase of what historians have called "[[Big Science]]", requiring massive machines, budgets, and laboratories in order to test their theories and move into new frontiers. The primary patron of physics became state governments, who recognized that the support of "basic" research could often lead to technologies useful to both military and industrial applications (it was not until the post-Cold War [[1990s]] that the US Congress would fail to approve funding for a particle accelerator). Currently [[CERN]] still enjoys funding from the European community.

[[Quantum field theory]] was formulated in order to extend quantum mechanics to be consistent with special relativity. It achieved its modern form in the late [[1940s]] with work by [[Richard Feynman]], [[Julian Schwinger]], [[Sin-Itiro Tomonaga]], and [[Freeman Dyson]]. They formulated the theory of [[quantum electrodynamics]], which describes the electromagnetic interaction.

Quantum field theory provided the framework for modern [[particle physics]], which studies [[fundamental force]]s and elementary particles. In [[1954]], [[Yang Chen Ning]] and [[Robert Mills]] developed a class of [[gauge theory|gauge theories]], which provided the framework for the [[Standard Model]]. The Standard Model, which was completed in the [[1970s]], successfully describes almost all elementary particles observed to date.

At the same time, [[Stephen Hawking]] had discovered the spectrum of radiation emanating during the collapse of matter into [[black hole]]s; by [[2004]], even Hawking would admit that some [[Hawking radiation]] could escape a black hole.

=== Developments since 1990 ===

Attempts to unify [[quantum mechanics]] and [[general relativity]] made significant progress during the 1990s. At the close of the century, a [[Theory of everything]] was still not in hand, but some of its characteristics were taking shape. [[Loop quantum gravity]], [[string theory]], and [[black hole thermodynamics]] all predicted [[quantized]] [[spacetime]] on the [[Planck scale]].

=== Developments since 2000 ===

A new experiment demonstrated that [[speed of gravity|gravity propagates]] at approximately the [[speed of light]], confirming one prediction of [[general relativity]].

:''-- add stuff on convergence of superstring stuff to [[M-theory]]''

===Notes===

*{{fnb|1}}Cornelius Lanczos, ''The Variational Principles of Mechanics'' (Dover Publications, New York, 1986). ISBN 0-486-65067-7.

*{{fnb|2}}[http://www.physics.ucla.edu/~cwp/articles/noether.asg/noether.html E. Noether's Discovery of the Deep Connection Between Symmetries and Conservation Laws] by [[Nina Byers]]

*{{fnb|3}}A [[pseudotensor]] changes its sign under inversion by some transformation matrix. [http://mathworld.wolfram.com/Pseudotensor.html See note].

*{{fnb|4}}Alpher, Herman, and Gamow. ''Nature'' '''162''',774 (1948).

*{{fnb|5}}[http://nobelprize.org/physics/laureates/1978/wilson-lecture.pdf Wilson's [[1978]] Nobel lecture]

*[http://uk.arxiv.org/abs/physics/0310001 Indian physics]

See also: [[History of science and technology]]

{{Wikibookspar|Wikiversity|History of Physics}}

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