Physics (from Greek fromφυσικός (physikos): natural, from φύσις(physis): Nature ) is the study of energy and its interaction with matter (see chemistry , biology ). Because of the primacy of energy in thehistory of the universe, because all matter must interact with energy to express its properties and engage in transformations,and because energy is the key player when matter is decomposed into its most basic parts, physics is often considered to be thefundamental science.
Types of physics research
Theoretical physicists seek to deduce laws of the universe using the observations of experimental physicists. Experimentalphysicists perform experiments designed to be able to decide which of the proposed theories is true. Experimental physics oftenfinds completely new phenomena with no existing theory; electromagnetism and radioactivity were discoveredthis way. New fields of physics are often developed in response to contradictory or unexplainable phenomena observed inexperiment. Occasionally new fields of physics begin as theory before they receive experimental confirmation (such as the theory of relativity or many proposed theories such as M-theory .)
Fundamental research quests for an understanding of the laws in practice while applied physicists apply existing knowledge toanalyze complex systems in order to use them in practical life, economy , or furtherphysics research. Both fundamental research and applied research has theoretical and experimental aspects. As an example, aparticularly 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 .
The culture of physics research is distinctive from the other sciences in that theory is readily accepted as a specialization;while theorists exist in biology and chemistry , the most successful theorists have generally also been experimentalists, and (sometimes overt)prejudice against pure theorists exists in these sciences.
Fields of study in physics
Accelerator physics Acoustics Astrophysics Atomic, Molecular, and Opticalphysics Computational physics Condensed matter physics Cosmology Cryogenics Fluid dynamics Polymer physics Optics Materials physics Nuclear physics Plasma physics Particle physics (or HighEnergy Physics) Vehicle dynamics
Main article: Theories of Physics
The Role of Physics?
Note: The following is a cursory overview of the development of physics. For a more detailed history, please refer to themain article on this subject, History of physics .
Since antiquity, people have tried to understand the behavior of matter : whyunsupported 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 celestialobjects such as the Sun and the Moon . Several theorieswere 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 wereexceptions 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 testedseveral results in dynamics , in particular the Law of Inertia . In 1687 , Newton published the Principia Mathematica , detailing two comprehensive and successfulphysical theories: Newton's laws of motion , fromwhich arise classical mechanics ; and Newton's Law of Gravitation , which describes the fundamental force of gravity . Both theories agreed well withexperiment. The Principia also included several theories in fluiddynamics . Classical mechanics was exhaustively extended by Lagrange , Hamilton , andothers, 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 withclassical 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 ofconservation 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 , andothers. 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 proposedthe 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 andtime into a single entity, spacetime . Relativity prescribes a differenttransformation between reference frames thanclassical mechanics; this necessitated the development of relativistic mechanics as a replacement for classical mechanics. In theregime of low (relative) velocities, the two theories agree. In 1915 , Einstein extendedspecial relativity to explain gravity with the general theory ofrelativity , 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 acompact 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, includingphenomena 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 detonatedat Trinity site , near Alamogordo , New Mexico .
Quantum field theory was formulated in order to extendquantum 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 electromagneticinteraction.
Quantum field theory provided the framework for modern particlephysics , which studies fundamental forces and elementaryparticles. 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 2004 , research is progressing on a large number of fields ofphysics.
In condensed matter physics , the biggestunsolved theoretical problem is the explanation for high-temperature superconductivity . Strong efforts, largely experimental, are being putinto 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 tohave solved the long-standing solar neutrino problem insolar physics. The physics of massive neutrinos is currently an area of active theoretical and experimental research. In the nextseveral years, particle accelerators will begin probingenergy 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 bearfruit. The current leading candidates are M-theory and loop quantum gravity .
Many astronomical phenomena have yet to be explained, including the existenceof ultra-high energy cosmic rays and the anomalous rotation rates of galaxies . Theories that have beenproposed 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 byrecent evidence that the expansion of the universe isaccelerating .
In the rush to solve high-energy, quantum, and astronomical physics, quite a bit of quotidian physics have been left behind.Complex problems that seem like they could be solved by a clever application of dynamics and mechanics, like the formation ofsandpiles, nodes in trickling water, the shape of water droplets, mechanisms of surface tension catastrophes, or self-sorting inshaken heterogeneous collections, still remain largely open for characterization.
See unsolved problems in physics for afuller treatment of this subject.
See the definition of physical .
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