Physics (from Greek from φυσικός (phusikos): natural, from φύσις (fysis): nature) is the science of Nature in the broadest sense. Physicists study the behaviour and interactions of matter and energy, which are referred to as physical phenomena. Theories of physics are generally expressed as mathematical relations. Well-established theories are often referred to as physical laws or laws of physics; however, like all scientific theories, they are ultimately provisional.
Physics is very closely related to the other natural sciences, particularly chemistry, the science of molecules and the chemical compounds that they form in bulk. Chemistry draws on many fields of physics, particularly quantum mechanics, thermodynamics and electromagnetism. However, chemical phenomena are sufficiently varied and complex that chemistry is usually regarded as a separate discipline.
Below is an overview of the major subfields and concepts in physics, followed by a brief outline of the history of physics and its subfields. A more comprehensive list of physics topics is also available.
Table of contents
Central Theories Classical mechanics -- Thermodynamics -- Statistical mechanics -- Electromagnetism -- Special relativity -- General relativity -- Quantum mechanics -- Quantum field theory -- Standard Model
Subfields of Physics Astrophysics -- Atomic, Molecular, and Optical physics -- Computational physics -- Condensed matter physics -- Cryogenics -- Fluid dynamics -- Polymer physics -- Optics -- Materials physics -- Nuclear physics -- Plasma physics -- Particle physics (or High Energy Physics) -- Vehicle dynamics
Note: The following is a cursory overview of the development of physics. For a more detailed history, please refer to History of physics.
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. These theories were largely couched in philosophical terms, and never verified by systematic experimental testing. There were exceptions: for example, the Greek thinker Archimedes derived many correct quantitative descriptions of mechanics and hydrostatics.
During the late 16th century, Galileo pioneered the use of experiment to validate physical theories, which is the key idea in the scientific method. Galileo formulated and successfully tested several results in dynamics, in particular the Law of Inertia. In 1687, Newton published the Principia Mathematica, detailing two comprehensive and successful physical theories: Newton's laws of motion, from which arise classical mechanics; and 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 Lagrange, Hamilton, and others, who produced new formulations, principles, and results. The Law of Gravitation initiated the field of astrophysics, which describes astronomical phenomena using physical theories.
From the 18th century onwards, thermodynamics was developed by Boyle, Young, and many others. In 1733, Bernoulli used statistical arguments with classical mechanics to derive thermodynamic results, initiating the field of statistical mechanics. In 1798, Thompson demonstrated the conversion of mechanical work into heat, and in 1847 Joule stated the law of conservation of energy, in the form of heat as well as mechanical energy.
The behavior of electricity and magnetism was studied by Faraday, Ohm, and others. In 1855, 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 wave.
In 1895, Roentgen discovered X-rays, which turned out to be high-frequency electromagnetic radiation. Radioactivity was discovered in 1896 by Henri Becquerel, and further studied by Pierre Curie and Marie Curie and others. This initiated the field of nuclear physics.
In 1897, Thomson discovered the electron, the elementary particle which carries electrical current in circuits. In 1904, he proposed the first model of the atom, known as the plum pudding model. (The existence of the atom had been proposed in 1808 by 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 reference frames than classical mechanics; this necessitated 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 theory of relativity, which replaces Newton's law of gravitation. In the regime of low masses and energies, the two theories agree.
In 1911, Rutherford deduced from scattering experiments the existence of a compact atomic nucleus, with positively charged constituents dubbed protons. Neutrons, the neutral nuclear constituents, were discovered in 1932 by Chadwick.
Beginning in 1900, Planck, Einstein, Bohr, and others developed quantum theories to explain various anomalous experimental results by introducing discrete energy levels. In 1925, Heisenberg and 1926, Schrödinger and Dirac formulated quantum mechanics, which explained the preceding quantum theories. In quantum mechanics, the outcomes of physical measurements are inherently 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, semiconductivity, and superconductivity. The pioneers of condensed matter physics include Bloch, who created a quantum mechanical description of the behavior of electrons in crystal structures in 1928.
During World War II, research was conducted by each side into nuclear physics, for the purpose of creating a nuclear bomb. The German effort, led by Heisenberg, did not succeed, 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, and in 1945 the world's first nuclear explosive was detonated in Alamagordo, New Mexico.
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 Feynman, Schwinger[?], Tomonaga[?], and 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 forces and elementary particles. In 1954, Yang[?] and Mills[?] developed a class of 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.
As of 2003, research is progressing on a large number of fields of physics.
In particle physics, the first pieces of experimental evidence for physics beyond the Standard Model have begun to appear. Foremost amongst this are indications that neutrinos have non-zero mass. These experimental results appear to have solved the long-standing solar neutrino problem in solar physics. The physics of massive neutrinos is currently an area of active theoretical and experimental research. In the next several years, particle accelerators will begin probing energy scales in the TeV range, in which experimentalists are hoping to find evidence for the higgs boson and supersymmetric particles.
Theoretical attempts to unify quantum mechanics and general relativity into a single theory of quantum gravity, a program ongoing for over half a century, has yet to bear fruit. The current leading candidates are M-theory and loop quantum gravity.
Many astronomical phenomena have yet to be explained, including the existence of ultra-high energy cosmic rays and the anomalous rotation rates of galaxies. Theories that have been proposed to resolve these problems include doubly-special relativity, modified Newtonian dynamics, and the existence of dark matter. In addition, the cosmological predictions of the last several decades have been contradicted by recent evidence that the expansion of the universe is accelerating.
In condensed matter physics, the biggest unsolved theoretical problem is the explanation for high-temperature superconductivity. Strong efforts, largely experimental, are being put into making workable spintronics and quantum computers.
See unsolved problems in physics for a fuller treatment of this subject.