Physics

**Physics** (Greek *phusis*: nature) is the science of Nature in the broadest sense. Physicists study the behavior of matter across space and time, and its interactions through forces. Physical theories are generally expressed as mathematical relations. Well-established theories are often referred to as *physical laws*.

Physics is 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 thermodynamics and electromagnetism; however, chemical phenomena are sufficiently varied and complex that chemistry is usually regarded as a separate discipline.

### Fields of Physics

Below is an overview of major subfields and concepts in physics. See /Schemes for alternative presentations.

**Central Theories**
Classical mechanics -- Thermodynamics -- Statistical mechanics -- Electromagnetism -- Special relativity -- General relativity -- Quantum mechanics -- Quantum electrodynamics -- Quantum chromodynamics

**Proposed Theories**
Theory of everything -- Grand unified theory -- String theory -- M-theory

**Concepts**
Matter -- Antimatter -- Mass -- Energy -- Momentum -- Angular momentum -- Time -- Force -- Torque -- Wave -- Harmonic oscillator -- Magnetism -- Electricity -- Electromagnetic radiation -- Temperature -- Entropy

**Fundamental Forces**
Gravity -- Electromagnetic interaction -- Weak nuclear force -- Electroweak force -- Strong nuclear force

**Particles**
Atom -- Proton -- Neutron -- Electron -- Quark -- Photon -- Gluon -- W boson -- Z boson -- Graviton -- Neutrino

**Subfields of Physics**
Astrophysics -- Atomic physics -- Computational physics -- Condensed matter physics -- Cryogenics -- Fluid dynamics -- Polymer physics -- Optics -- Materials physics -- Nuclear physics -- Plasma physics -- Particle physics (or High Energy Physics)

**Methods**
Scientific method -- Instrumentation -- Experimental methods -- Physical quantity -- Measurement -- Dimensional analysis -- Probability and Statistics

**Tables**
Physical constants -- SI base units -- SI derived units -- SI prefixes -- Unit conversions

**History**
History of Physics -- Famous Physicists -- \
Nobel Prize in physics

**Related Fields**
Mathematical physics -- Astronomy and Astrophysics -- Materials science -- Electronics

(*To help develop a list of the most basic topics in Physics, please see Physics basic topics.*)

### A Ridiculously Brief 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, Archimedes, derived many correct quantitative descriptions of mechanics and hydrostatics.

The use of experiment to validate physical theories was pioneered by Galileo, who obtained several successful results in dynamics. With the publication of the Principia Mathematica in 1687, Newton formulated two comprehensive and successful physical theories: Newton's laws of motion, from which arise classical mechanics; and Newton's law of gravitation, which describes to a good approximation the fundamental force of gravity. Newton's theories agreed well with experiment. Classical mechanics would be exhaustively extended by Lagrange, Hamilton, and others, producing new formulations, principles, and results. The law of gravitation initiated the field of astrophysics, which describes astronomical phenomena using physical theories.

From the 17th century onwards, various thermodynamic results were developed by Boyle, Young, and many others. In 1733, Bernoulli pioneered the use of statistical arguments applied to classical mechanics to derive thermodynamic results; this initiated 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 consequence of this theory was that light is an electromagnetic wave.

In 1897, Thompson discovered the electron, the elementary particle that 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 1901, Planck, Einstein, Bohr, and others developed quantum theories to explain various anomalous experimental results by introducing discrete energy levels. In 1925, Heisenberg and Schrodinger 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.

The extension of quantum mechanics to special relativity is quantum field theory, which achieved its modern form in the 1940s from work by Feynman, Schwinger, Tomonaga, and Dyson. The first successful quantum field theory was quantum electrodynamics, which describes the electromagnetic interaction.

Quantum field theory provides the framework for modern particle physics, which studies fundamental forces and elementary particles. In 1954, Yang and Mills developed a class of gauge theories; this provided the framework for the "Standard Model", which classifies all observed elementary particles to date. The Standard Model was completed in the 1970s.

**Suggested Reading:**

* Feynman, *The Character of Physical Law*, MIT Press, 1965

* Feynman, Leighton, Sands, *The Feynman Lectures on Physics*, Reading Mass., Addison-Wesley 1963

* Eric Weisstein, *Treasure Troves of Physics*, http://www.treasure-troves.com/physics/. Online Physics encyclopedic dictionary.

/Talk