The Physics reference article from the English Wikipedia on 24-Jul-2004
(provided by Fixed Reference: snapshots of Wikipedia from wikipedia.org)

Physics

See the real Africa
Physics (from Greek from φυσικός (physikos): natural, from φύσις (physis): Nature) is the science of Nature (or natural science) in the broadest sense. Physicists study the behaviour and interactions of matter and force. The laws of physics are generally expressed as mathematical relations.

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. Nevertheless, it is widely accepted among chemists and physicists that the laws of physics describe at the most fundamental level all chemical interactions.

In fact, many physicists take the position that physics is the only fundamental science. Their argument runs as follows: all sciences--biology, chemistry, geology, etc.--are concerned with matter; all matter is composed of atoms; physics describes the dynamics and internal configurations of atoms. Extension of this physico-centric view can result in profound philosophical consequences. For example, if one accepts that the human brain controls all human behavior, and if one accepts that the brain is composed entirely of atoms whose behavior is completely described by laws of physics, then one may reasonably question whether a person has the free will to control his behavior. Nevertheless it is not the task of physics to answer philosophical questions.

A common goal of theoretical physicists is to reduce the description of the physical world to a minimal set of laws governing a finite set of fundamental constituent elements in the universe. That the physical world can necessarily be completely reduced in such a way is unclear; one could conceive of a world comprised of an infinite variety of particles behaving in accordance with an infinite number of laws, or perhaps behaving entirely randomly on occasion. However, thanks to experimental physicists, physics have been remarkably successful to date at this reduction process, and the reduction trend is evident in the names of some of the proposed theories listed below.

Physics, like other sciences, is often subdivided into categories: theoretical physics and experimental physics or fundamental research and applied physics. Theoretical physicists seek new fundamental knowledge about the universe, using the observations of experimental physicists. Experimental physicists perform experiments designed to be able to decide which theory is true. Experimental physics often finds completely new phenomena with no existing theory, e.g. electromagnetism, radioactivity were discovered this way. Fundamental research quests for the basic structure of nature while applied physicists apply existing knowledge to analyze complex systems in order to use them in practical life and economy. Both fundamental research and applied research has theoretical and experimental aspects. As an example, a particularly fertile area of applied physics is solid-state physics, in which researchers use the more fundamental laws of quantum mechanics and electromagnetism to analyze the behavior of atoms that comprise a solid.

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. See also future directions, below. A more comprehensive list of physics topics is also available.

Overview of physics

Theories

Main article:
Theories of Physics

Central theories

Classical mechanics -- Thermodynamics -- Statistical mechanics -- Electromagnetism -- Special relativity -- General relativity -- Quantum mechanics -- Quantum field theory -- Standard Model -- Fluid dynamics

Proposed theories

Theory of everything -- Grand unification theory -- M-theory -- Loop quantum gravity -- Emergence -- Process Physics

Fringe theories

Cold fusion -- Dynamic theory of gravity -- Luminiferous aether -- Orgone energy -- Reciprocal System of Theory -- Steady state theory -- Time Cube -- Unified field theory -- Variable speed of light

Concepts

Matter -- Antimatter -- Elementary particle -- Boson -- Fermion

Symmetry -- Motion -- Conservation law -- Mass -- Energy -- Momentum -- Angular momentum -- Spin -- emergent structures

Time -- Space -- Dimension -- Spacetime -- Length -- Velocity -- Force -- Torque

Wave -- Wavefunction -- Quantum entanglement -- Harmonic oscillator -- Magnetism -- Electricity -- Electromagnetic radiation -- Temperature -- Entropy -- Physical information -- Vacuum energy -- Zero-point energy

Phase transitions -- Critical phenomena -- Self-organization -- Spontaneous symmetry breaking -- Superconductivity -- Superfluidity -- Quantum phase transitions

Fundamental forces

Gravitational -- Electromagnetic -- Weak -- Strong

Particles

Main article: Particless

Atom -- Electron -- Gluon -- Graviton -- Neutrino -- Neutron -- Quark -- Photino -- Photon -- Proton -- W and Z bosons -- Particle radiation -- Phonon -- Roton

Bosons -- Fermions -- Supersymmetry -- Higgs boson

Subfields of physics

Accelerator physics -- Acoustics -- Astrophysics -- Atomic, Molecular, and Optical physics -- Computational physics -- Condensed matter physics -- Cosmology -- Cryogenics -- Fluid dynamics -- Polymer physics -- Optics -- Materials physics -- Nuclear physics -- Plasma physics -- Particle physics (or High Energy Physics) -- Vehicle dynamics

Methods

Scientific method -- Physical quantity -- Measurement -- Measuring instruments -- Dimensional analysis -- Statistics--Scaling

Tables

List of physical laws -- Physical constants -- SI base units -- SI derived units -- SI prefixes -- Unit conversions

History

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

Related Fields

Astronomy -- Biophysics -- Cycles -- Electronics -- Engineering -- Geophysics -- Materials science -- Mathematical physics -- Medical physics -- Physical chemistry -- Physics of computation

A brief history of physics

Note: The following is a cursory overview of the development of physics. For a more detailed history, please refer to the main article on this subject, 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 which were wrong. These theories were largely couched in philosophical terms, and never verified by systematic experimental testing as is popular today. There were exceptions and there are anachronisms: for example, the Greek thinker Archimedes derived many correct quantitative descriptions of mechanics and hydrostatics.

During the early 17th 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. The Principia also included several theories in fluid dynamics. Classical mechanics was 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 Marie Curie, Pierre 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, Albert 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 at Trinity site, near Alamogordo, 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.

Future directions

As of 2004, research is progressing on a large number of fields of physics.

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.

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.

See unsolved problems in physics for a fuller treatment of this subject.

See the definition of physical.

Suggested reading and external links


General subfields within physics Edit
Classical mechanics | Condensed matter physics | Continuum mechanics | Electromagnetism |
General relativity | Particle physics | Quantum field theory | Quantum mechanics |
Solid state physics | Special relativity | Statistical mechanics | Thermodynamics