Physics
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:''This is a discussion of a present category of science. For the work by [[Aristotle]], see [[Physics (Aristotle)]]. For a history of the science, see [[History of physics]]. For the etymology of the word "physics," see [[physis]] (φύσις)''.
[[Image:Meissner effect p1390048.jpg|thumb|right|A [[magnet]] levitating above a [[high-temperature superconductor]] demonstrates the [[Meissner effect]].]]
'''Physics''' is the [[science]] of ''[[matter]]''<ref>
[[R. P. Feynman]], [[R. B. Leighton]], [[M. Sands]] (1963), ''[[The Feynman Lectures on Physics]]'', ISBN 0-201-02116-1 Hard-cover. p.1-1 Feynman begins with the [[atomic theory|atomic hypothesis]], as his most compact statement of all scientific knowledge: "If, in some cataclysm, all of scientific knowledge were to be destroyed, and only one sentence passed on to the next generations ..., what statement would contain the most information in the fewest words? I believe it is ... that ''all things are made up of atoms -- little particles that move around in perpetual motion, attracting each other when they are a little distance apart, but repelling upon being squeezed into one another. ...''" vol. '''I''' p. I-2
</ref>
and its ''[[Motion (physics)|motion]],''<ref>
[[James Clerk Maxwell]] (1876), ''Matter and Motion''. Notes and appendices by [[Joseph Larmor]]. "'''Physical science''' is that department of knowledge which relates to the order of nature, or, in other words, to the regular succession of events". p.1
</ref><ref>
"Give me matter and motion, and I will construct the universe." --[[Rene Descartes]] (1596-1650)
</ref> as well as ''[[space]]'' and ''[[time]]''.<ref>[http://www.fnal.gov/pub/inquiring/matter/index.html Inquiring Minds<!-- Bot generated title -->]</ref><ref>
E.F. Taylor, [[John Archibald Wheeler|J.A. Wheeler]] (2000), ''Exploring Black Holes: Introduction to General Relativity, ISBN 0-201-38423-X Hard-cover. Back cover: "Spacetime tells matter how to move; mass tells spacetime how to curve."
</ref>
It uses concepts such as [[energy]], [[force]], [[mass]], and [[charge (physics)|charge]]. Physics is an [[experiment]]al [[science]],<ref>
H.D. Young & R.A. Freedman, ''University Physics with Modern Physics'': 11th Edition: International Edition (2004), Addison Wesley. Chapter 1, section 1.1, page 2 has this to say: "Physics is an ''experimental'' science. Physicists observe the phenomena of nature and try to find patterns and principles that relate these phenomena. These patterns are called physical theories or, when they are well established and of broad use, physical laws or principles."
</ref> creating [[Theoretical physics|theories]] that are tested against [[Experimental physics|observations]]. Broadly, it is the general scientific analysis of [[nature]], with a goal of understanding how the universe behaves.<ref>
Steve Holzner, ''Physics for Dummies'' (2006), Wiley. Chapter 1, page 7 says: "Physics is the study of your world and the world and universe around you." See [http://www.amazon.com/gp/reader/0764554336 Amazon Online Reader: Physics For Dummies (For Dummies(Math & Science))], last viewed 24 Nov 2006.
</ref>
Physics is [[History of physics|one of the oldest academic disciplines]], and through its modern subfield of [[astronomy]], it may be the oldest of all.<ref>
Evidence exists that the earliest civilizations dating back to beyond 3000 BC, such as the [[Sumer]]ians, [[Ancient Egyptians]], and the [[Indus Valley Civilization]], all had a predictive knowledge and a basic understanding of the motions of the Sun, Moon, and stars.
</ref> [[Experimental physics]] began in the [[Middle Ages]] and eventually [[Scientific Revolution|emerged as a modern science]] during the [[early modern period]].<ref>
[[Ibn al-Haytham|Alhazen]]'s ''[[Book of Optics]]'' (1021) and [[Francis Bacon]]'s ''[[Novum Organum]]'' (1620) were critical in the [[history of scientific method|development of scientific method]].</ref> Those who work professionally in the field are known as [[physicist]]s.
Advances in physics often translate to the technological sector, and sometimes influence the other sciences, as well as [[mathematics]] and [[philosophy]]. For example, advances in the understanding of [[electromagnetism]] have led to the widespread use of electrically driven devices (televisions, computers, home appliances etc.); advances in [[thermodynamics]] led to the development of motorized transport; and advances in [[mechanics]] motivated and benefitted from the development of [[calculus]], [[quantum chemistry]], and the use of instruments such as the [[electron microscope]] in [[microbiology]]. The "[[Atomic Age]]" is also an important concept in philosophy and [[historical analysis]].
Today, physics is a broad and highly developed subject. Research is often divided into four subfields: [[condensed matter physics]]; [[atomic, molecular, and optical physics]]; [[particle physics|high-energy physics]]; and astronomy and [[astrophysics]]. Most physicists also specialize in either [[Theoretical physics|theoretical]] or [[Experimental physics|experimental]] research, the former dealing with the development of new theories, and the latter dealing with the experimental testing of theories and the discovery of new phenomena. Despite important discoveries during the last four centuries, there are a number of [[unsolved problems in physics]], and many areas of active research.
== Branches of physics == <!-- [[Mechanical engineering]] links here -->
[[Image:physicsdomains.jpg|right|thumb|370px|Domains of physics theories]]<!--[[Image:Modernphysicsfields.svg|right|thumb|380px|Domains of physics theories]]-->Although physics encompasses a wide variety of phenomena, the fundamental branches of physics are [[classical mechanics]], [[electromagnetism]] (including [[optics]]), [[Theory of relativity|relativity]], [[thermodynamics]], and [[quantum mechanics]]. Each of these theories has been tested in numerous experiments and proven to be an accurate model of nature within its domain of validity. For example, [[classical mechanics]] correctly describes the motion of objects in everyday experience, but it breaks down at the atomic scale, where it is superseded by [[quantum mechanics]], and at speeds approaching the [[speed of light]], where [[special relativity|relativistic]] effects become important. While these theories have long been well-understood, they continue to be areas of active research — for example, a remarkable aspect of classical mechanics known as [[chaos theory]] was developed in the 20th century, three centuries after the original formulation of mechanics by Isaac Newton (1642–1727). The basic theories form a foundation for the study and research of more specialized topics.
A [[Physics/Theories|table of these theories]], along with many of the concepts they employ, is available.
===Classical mechanics===
{{main|Classical mechanics}}
[[Image:Polispasto4.jpg|thumb|right|200px|A [[pulley]] uses the principle of [[mechanical advantage]] so that a small force over a large distance can lift a heavy weight over a shorter distance.]]
[[Classical mechanics]] is a model of the physics of [[force]]s acting upon bodies. It is often referred to as "Newtonian mechanics" after [[Isaac Newton]] and his [[Newton's laws of motion|laws of motion]]. Mechanics is subdivided into [[statics]], which models objects at rest, [[kinematics]], which models objects in motion, and [[dynamics (mechanics)|dynamics]], which models objects subjected to forces. The classical mechanics of continuous and deformable objects is [[continuum mechanics]], which can itself be broken down into [[solid mechanics]] and [[fluid mechanics]] according to the state of matter being studied. The latter, the mechanics of [[liquid]]s and [[gas]]es, includes [[hydrostatics]], [[hydrodynamics]], [[pneumatics]], [[aerodynamics]], and other fields. Mechanical Statics deals with objects at rest. Mechanical kinematics deals with objects in motion. Mechanical dynamics deals with motion of objects subject to forces.
Classical mechanics produces accurate results within the domain of everyday experience. It is superseded by [[Theory of relativity|relativistic mechanics]] for systems moving at large velocities near the [[speed of light]], [[quantum mechanics]] for systems at small distance scales, and [[relativistic quantum field theory]] for systems with both properties. Nevertheless, classical mechanics is still useful, because it is much simpler and easier to apply than these other theories, and it has a very large range of approximate validity. Classical mechanics can be used to describe the motion of human-sized objects (such as [[top]]s and [[baseball (ball)|baseball]]s), many astronomical objects (such as planets and galaxies), and certain microscopic objects (such as organic molecules).
An important concept of mechanics is the identification of conserved [[energy]] and [[momentum]], which lead to the [[Lagrangian mechanics|Lagrangian]] and [[Hamiltonian mechanics|Hamiltonian]] reformulations of Newton's laws. Theories such as [[fluid mechanics]] and the [[kinetic theory]] of gases result from applying classical mechanics to macroscopic systems. A relatively recent result of considerations concerning the dynamics of nonlinear systems is [[chaos theory]], the study of systems in which small changes in a variable may have large effects. [[Newton's law of universal gravitation]], formulated within classical mechanics, explained [[Kepler's laws of planetary motion]] and helped make classical mechanics an important element of the [[Scientific Revolution]].
===Electromagnetism===
{{main|Electromagnetism}}
{{see also|Optics}}
[[Image:Magnet0873.png|thumb|left|250px|Magnetic lines of force of a bar magnet shown by iron filings on paper]]
[[Electromagnetism]] describes the interaction of charged particles with electric and magnetic [[field (physics)|field]]s. It can be divided into [[electrostatics]], the study of interactions between [[Electric charge|charge]]s at rest, and [[electrodynamics]], the study of interactions between moving charges and [[radiation]]. The classical theory of electromagnetism is based on the [[Lorentz force]] law and [[Maxwell's equations]].
[[Electrostatics]] is the study of phenomena associated with charged bodies at rest. As described by [[Coulomb’s law]], such bodies exert [[force]]s on each other. Their behavior can be analyzed in terms of the concept of an [[electric field]] surrounding any charged body, such that another charged body placed within the field is subject to a force proportional to the magnitude of its own charge and the magnitude of the field at its location. Whether the force is attractive or repulsive depends on the [[Magnet#Magnetic poles|polarity]] of the charge. Electrostatics has many applications, ranging from the analysis of phenomena such as thunderstorms to the study of the behavior of electron tubes.
[[Electrodynamics]] is the study of phenomena associated with [[Electric charge|charge]]d bodies in motion and varying [[electric field|electric]] and [[magnetic field]]s. Since a moving charge produces a magnetic field, electrodynamics is concerned with effects such as magnetism, electromagnetic [[radiation]], and [[electromagnetic induction]], including such practical applications as the [[electric generator]] and the [[electric motor]]. This area of electrodynamics, known as classical electrodynamics, was first systematically explained by [[James Clerk Maxwell]], and Maxwell’s equations describe the phenomena of this area with great generality. A more recent development is [[quantum electrodynamics]], which incorporates the laws of [[Quantum field theory|quantum theory]] in order to explain the interaction of electromagnetic radiation with matter. [[Dirac]], [[Heisenberg]], and [[Pauli]] were pioneers in the formulation of quantum electrodynamics. Relativistic electrodynamics accounts for [[special relativity|relativistic]] corrections to the motions of charged particles when their speeds approach the speed of light. It applies to phenomena involved with [[particle accelerators]] and electron tubes carrying high [[voltages]] and [[Electric current|current]]s.
Electromagnetism encompasses various real-world electromagnetic [[phenomena]]. For example, [[light]] is an oscillating [[electromagnetic field]] that is radiated from accelerating charged particles. Aside from [[gravity]], most of the forces in everyday experience are ultimately a result of electromagnetism.
The principles of electromagnetism find applications in various allied disciplines such as [[microwave]]s, [[Antenna (radio)|antenna]]s, electric machines, [[satellite communication]]s, bioelectromagnetics, [[Plasma (physics)|plasma]]s, [[nuclear physics|nuclear]] research, [[fiber optic]]s, electromagnetic interference and compatibility, electromechanical energy conversion, [[radar]] [[meteorology]], and [[remote sensing]]. Electromagnetic devices include [[transformer]]s, electric [[relay]]s, [[radio]]/[[TV]], [[telephone]]s, [[electric motor]]s, [[transmission line]]s, [[waveguide]]s, [[optical fiber]]s, and [[laser]]s.
===Relativity===
{{main|Special relativity|General relativity}}
[[Image:Cassini-science-br.jpg|thumb|right|250px|High-precision test of general relativity by the [[Cassini-Huygens|Cassini]] space probe (artist's impression): [[radio]] signals sent between the Earth and the probe (green wave) are [[Shapiro effect|delayed]] by the warping of [[space and time]] (blue lines).]]
[[Theory of relativity|Relativity]] is a generalization of classical mechanics that describes fast-moving or very massive systems. It includes [[special relativity|special]] and [[general relativity]].
The theory of [[special relativity]] was proposed in 1905 by [[Albert Einstein]] in his article "[[Annus Mirabilis Papers#Special relativity|On the Electrodynamics of Moving Bodies]]". The title of the article refers to the fact that special relativity resolves an inconsistency between [[Maxwell's equations]] and classical mechanics. The theory is based on [[postulates of special relativity|two postulates]]: (1) that the mathematical forms of the [[Physical law|laws of physics]] are invariant in all [[inertial frame of reference|inertial systems]]; and (2) that the [[speed of light]] in a [[vacuum]] is constant and independent of the source or observer. Reconciling the two postulates requires a unification of [[space]] and [[time]] into the frame-dependent concept of [[spacetime]].
Special relativity has a variety of surprising consequences that seem to violate common sense, but all have been experimentally verified. It overthrows [[Newtonian physics|Newtonian notions of absolute space and time]] by stating that distance and [[time]] depend on the observer, and that time and space are perceived differently, depending on the observer. The theory leads to the assertion of change in [[mass]], [[dimension]], and [[time]] with increased [[velocity]]. It also yields the equivalence of [[matter]] and [[energy]], as expressed in the [[mass-energy equivalence]] formula '''''E'' = ''mc''<sup>2</sup>''', where ''c'' is the speed of light in a vacuum. Special relativity and the [[Galilean relativity]] of Newtonian mechanics agree when velocities are small compared to the speed of light. Special relativity does not describe gravitation; however, it can handle accelerated motion in the absence of gravitation.<ref> Change in velocity of light causes apparent changes in mass,dimension and time.
{{Citation | last = Taylor | first = Edwin F.
| last2 = Wheeler | first2 = John Archibald | author2-link = John Archibald Wheeler
| title = Spacetime Physics | publisher = W.H. Freeman and Company | year = 1966
| location = San Francisco | isbn = 0-7167-0336-X}}
See, for example, ''The Relativistic Rocket'', Problem #58, page 141, and its worked answer. </ref>
[[General relativity]] is the [[Geometry|geometrical]] [[theory]] of [[gravitation]] published by [[Albert Einstein]] in 1915/16.<ref name=Ein1915>{{cite journal|last=Einstein| first=Albert| authorlink = Albert Einstein| date=[[November 25]], [[1915]]| title=Die Feldgleichungen der Gravitation| journal=Sitzungsberichte der Preussischen Akademie der Wissenschaften zu Berlin| pages=844–847 | url=http://nausikaa2.mpiwg-berlin.mpg.de/cgi-bin/toc/toc.x.cgi?dir=6E3MAXK4&step=thumb | accessdate=2006-09-12}}</ref><ref name = Ein1916> {{cite journal| last = Einstein| first = Albert| title = The Foundation of the General Theory of Relativity| journal = Annalen der Physik| volume = | issue = | pages = | date = 1916| publisher = | url = http://www.alberteinstein.info/gallery/gtext3.html| format = [[PDF]]| id = | accessdate = 2006-09-03 }} </ref> It unifies [[special relativity]], [[Newton's law of universal gravitation]], and the insight that gravitation can be described by the [[curvature]] of [[space]] and [[time]]. In general relativity, the curvature of [[space-time]] is produced by the [[energy]] of matter and radiation. General relativity is distinguished from other [[Metric (mathematics)|metric]] [[:Category:Theories of gravitation|theories of gravitation]] by its use of the [[Einstein field equations]] to relate space-time content and space-time curvature. Local [[Hendrik Lorentz|Lorentz]] Invariance requires that the manifolds described in GR be 4-dimensional and Lorentzian instead of [[Riemannian manifold|Riemannian]]. In addition, the principle of [[general covariance]] forces that mathematics be expressed using [[tensor calculus]].
The first success of general relativity was in explaining the anomalous [[perihelion]] [[precession]] of [[Mercury (planet)|Mercury]]. Then in 1919, [[Arthur Stanley Eddington|Sir Arthur Eddington]] announced that observations of [[star]]s near the [[eclipse]]d [[Sun]] confirmed general relativity's prediction that massive objects bend [[light]]. Since then, many other [[tests of general relativity|observations and experiments]] have confirmed many of the [[#Predictions|predictions of general relativity]], including [[gravitational time dilation]], the [[gravitational redshift]] of light, [[Shapiro delay|signal delay]], and [[gravitational radiation]]. In addition, numerous observations are interpreted as confirming one of general relativity's most mysterious and exotic predictions, the existence of [[black hole]]s.
===Thermodynamics and statistical mechanics===
{{main|Thermodynamics|Statistical mechanics}}
[[Image:Triple expansion engine animation.gif|thumb|right|320px|Typical '''[[thermodynamic system]]''' - heat moves from hot (boiler) to cold (condenser) and [[Work (thermodynamics)|work]] is extracted]]
[[Thermodynamics]] studies the effects of changes in [[temperature]], [[pressure]], and [[volume]] on [[physical system]]s at the [[macroscopic]] scale, and the transfer of energy as [[heat]].<ref name="Perrot" >{{cite book | author=Perrot, Pierre | title=A to Z of Thermodynamics | publisher=Oxford University Press | year=1998 | id=ISBN 0-19-856552-6}}</ref><ref>{{cite book | author=Clark, John, O.E.| title=The Essential Dictionary of Science | publisher=Barnes & Noble Books | year=2004 | id=ISBN 0-7607-4616-8}}</ref> Historically, thermodynamics developed out of need to increase the [[thermodynamic efficiency|efficiency]] of early [[steam engine]]s.<ref>{{cite book | last = Clausius | first = Ruldolf | title = On the Motive Power of Heat, and on the Laws which can be deduced from it for the Theory of Heat | publisher = Poggendorff's ''Annalen der Physick'', LXXIX (Dover Reprint) | year = 1850 | id = ISBN 0-486-59065-8}}</ref>
The starting point for most thermodynamic considerations are the [[laws of thermodynamics]], which postulate that [[energy]] can be exchanged between physical systems as heat or [[Mechanical work|work]].<ref>{{cite book | author=Van Ness, H.C. | title=Understanding Thermodynamics | publisher=Dover Publications, Inc. | year=1969 | id=ISBN 0-486-63277-6}}</ref> They also postulate the existence of a quantity named [[entropy]], which can be defined for any system.<ref>{{cite book | author=Dugdale, J.S. | title=Entropy and its Physical Meaning | publisher=Taylor and Francis | year=1998 | id=ISBN 0-7484-0569-0}}</ref> In thermodynamics, interactions between large ensembles of objects are studied and categorized. Central to this are the concepts of [[System (thermodynamics)|system]] and [[Surroundings (thermodynamics)|surroundings]]. A system is composed of particles, whose average motions define its properties, which in turn are related to one another through [[Equation of state|equations of state]]. Properties can be combined to express [[internal energy]] and [[thermodynamic potentials]], which are useful for determining conditions for [[Dynamic equilibrium|equilibrium]] and [[spontaneous process]]es.
[[Statistical mechanics]] analyzes [[macroscopic]] [[thermodynamic system|systems]] by applying [[statistics|statistical principles]] to their microscopic constituents. It provides a framework for relating the microscopic properties of individual atoms and molecules to the macroscopic or bulk properties of materials that can be observed in everyday life. [[Thermodynamics]] can be explained as a natural result of statistics and mechanics (classical and quantum) at the microscopic level. <!-- In particular, it can be used to calculate the thermodynamic properties of bulk materials from the spectroscopic data of individual molecules.--> In this way, the [[gas laws]] can be derived, from the assumption that a gas is a collection of individual particles, as hard spheres with [[mass]]. Conversely, if the individual particles are also considered to have [[Electric charge|charge]], then the individual accelerations of those particles will cause the emission of [[light]]. It was these considerations which caused [[Max Planck]] to formulate his law of [[blackbody radiation]],<ref>
[[Max Planck]] (1925), ''A Survey of Physical Theory'' derives his law of blackbody radiation in the notes on pp. 115-116, ISBN 0-486-67867-9
</ref> but only with the assumption that the spectrum of radiation emitted from these particles is not continuous in frequency, but rather quantized.<ref>
''[[Feynman Lectures on Physics]]'', vol '''I''' p. 41-6, ISBN 0-201-02010-6
</ref>
===Quantum mechanics===
{{main|Quantum mechanics}}
[[Image:HAtomOrbitals.png|thumb|left|250px|The first few [[hydrogen atom]] [[Electron configuration|electron orbital]]s shown as cross-sections with color-coded [[probability amplitude|probability density]]]]
[[Quantum mechanics]] is the branch of physics treating [[atom]]ic and [[subatomic particle|subatomic]] systems and their interaction with [[radiation]] in terms of [[observable]] quantities. It is based on the observation that all forms of energy are released in discrete units or bundles called "[[quantum|quanta]]". Remarkably, quantum theory typically permits only [[probability|probable]] or [[statistics|statistical]] calculation of the observed features of subatomic particles, understood in terms of [[wavefunction]]s. The [[Schrödinger equation]] plays the role in quantum mechanics that [[Newton's laws of motion|Newton's laws]] and [[conservation of energy]] serve in classical mechanics — i.e., it predicts the future behavior of a [[dynamical system|dynamic system]] — and is a [[wave equation]] in terms of the wavefunction which predicts analytically and precisely the probability of events or outcomes.
According to the older theories of [[classical physics]], energy is treated solely as a continuous phenomenon, while matter is assumed to occupy a specific region of space and to move in a continuous manner. According to the quantum theory, energy is held to be emitted and absorbed in tiny, discrete amounts. An individual bundle or packet of energy, called a quantum (pl. quanta), thus behaves in some situations much like particles of matter; particles are found to exhibit certain [[wave]]like properties when in motion and are no longer viewed as localized in a given region but rather as spread out to some degree. For example, the light, or electromagnetic radiation, emitted or absorbed by an atom has only certain [[frequency|frequencies]] (or [[wavelength]]s), as can be seen from the [[atomic spectra|line spectrum]] associated with the chemical element represented by that atom. The quantum theory shows that those frequencies correspond to definite energies of the light quanta, or [[photon]]s, and result from the fact that the [[electron]]s of the atom can have only certain allowed energy values, or levels; when an electron changes from one allowed level to another, a quantum of energy is emitted or absorbed whose frequency is directly proportional to the energy difference between the two levels.
The formalism of quantum mechanics was developed during the 1920s. In 1924, [[Louis de Broglie]] proposed that not only do light waves sometimes exhibit particle-like properties, as in the photoelectric effect and atomic spectra, but particles may also exhibit wavelike properties. Two different formulations of quantum mechanics were presented following de Broglie’s suggestion. The [[wave mechanics]] of [[Erwin Schrödinger]] (1926) involves the use of a mathematical entity, the wave function, which is related to the probability of finding a particle at a given point in space. The [[matrix mechanics]] of [[Werner Heisenberg]] (1925) makes no mention of wave functions or similar concepts but was shown to be mathematically equivalent to Schrödinger’s theory. A particularly important discovery of the quantum theory is the [[uncertainty principle]], enunciated by Heisenberg in 1927, which places an absolute theoretical limit on the accuracy of certain measurements; as a result, the assumption by earlier scientists that the physical state of a system could be measured exactly and used to predict future states had to be abandoned. Quantum mechanics was combined with the theory of relativity in the formulation of [[Paul Dirac]] (1928), which, in addition, predicted the existence of [[antiparticles]]. Other developments of the theory include [[quantum statistics]], presented in one form by Einstein and [[Satyendranath Bose|S. N. Bose]] (the [[Bose-Einstein statistics]]) and in another by Dirac and [[Enrico Fermi]] (the [[Fermi-Dirac statistics]]); [[quantum electrodynamics]], concerned with interactions between charged particles and electromagnetic fields; its generalization, [[quantum field theory]]; and [[quantum electronics]]. The discovery of quantum mechanics in the early 20th century revolutionized physics, and quantum mechanics is fundamental to most areas of current research.
== Research ==
=== Theory and experiment===
The culture of physics research differs from most sciences in the separation of [[theory]] and [[experiment]]. Since the twentieth century, most individual physicists have specialized in either [[theoretical physics]] or [[experimental physics]]. The great [[Italy|Italian]] physicist [[Enrico Fermi]] (1901–1954), who made fundamental contributions to both theory and experimentation in [[nuclear physics]], was a notable exception. In contrast, almost all the successful theorists in [[biology]] and [[chemistry]] (e.g. American [[quantum chemistry|quantum chemist]] and [[biochemistry|biochemist]] [[Linus Pauling]]) have also been experimentalists, although this is changing as of late.
Theorists seek to develop [[mathematical model]]s that both agree with existing experiments and successfully predict future results, while experimentalists devise and perform experiments to test theoretical predictions and explore new phenomena. Although theory and experiment are developed separately, they are strongly dependent upon each other. Progress in physics frequently comes about when experimentalists make a discovery that existing theories cannot explain, or when new theories generate experimentally testable predictions. Theorists working closely with experimentalists frequently employ [[Phenomenology (science)|phenomenology]].
[[Theoretical physics]] is closely related to [[mathematics]], which provides the language of physical theories, and large areas of mathematics, such as [[calculus]], have been invented specifically to solve problems in physics. [[Theorist]]s may also rely on [[numerical analysis]] and [[simulation#computer simulation|computer simulations]], which play an ever richer role in the formulation of physical models. The fields of [[mathematical physics|mathematical]] and [[computational physics]] are active areas of research. Theoretical physics has historically rested on [[philosophy]] and [[metaphysics]]; electromagnetism was unified this way.<ref>
See, for example, the influence of [[Immanuel Kant|Kant]] and [[Johann Wilhelm Ritter|Ritter]] on [[Hans Christian Ørsted|Oersted]].
</ref> Thus physicists may speculate with multidimensional spaces and [[Many-worlds interpretation|parallel universes]], and from this, hypothesize theories.
[[Experiment]]al physics informs, and is informed by, [[engineering]] and [[technology]]. Experimental physicists involved in [[basic research]] design and perform experiments with equipment such as [[particle accelerator]]s and [[laser]]s, whereas those involved in [[applied research]] often work in industry, developing technologies such as [[MRI|magnetic resonance imaging (MRI)]] and [[transistor]]s. Feynman has noted that experimentalists may seek areas which are not well explored by theorists.
===Research fields===
Contemporary research in physics can be broadly divided into [[condensed matter physics]]; [[atomic, molecular, and optical physics]]; [[particle physics]]; [[astrophysics]]; [[geophysics]] and [[biophysics]]. Some physics departments also support research in [[Physics education]]. Since the twentieth century, the individual fields of physics have become increasingly [[specialization|specialized]], and today most physicists work in a single field for their entire careers. "Universalists" such as [[Albert Einstein]] (1879–1955) and [[Lev Landau]] (1908–1968), who worked in multiple fields of physics, are now very rare.<ref>
Yet, universalism is encouraged in the culture of physics. For example, the [[World Wide Web]], which was innovated at [[CERN]] by [[Tim Berners-Lee]], was created in service to the computer infrastructure of CERN, and was/is intended for use by physicists worldwide. The same might be said for [[arXiv.org]]
</ref> A table of the major fields of physics, along with their subfields and the theories they employ, can be found [[Physics/Subfields|here]].
===Condensed matter===
{{main|Condensed matter physics}}
[[Image:Bose Einstein condensate.png|right|thumb|350px|Velocity-distribution data of a gas of [[rubidium]] atoms, confirming the discovery of a new phase of matter, the [[Bose–Einstein condensate]]]]
[[Condensed matter physics]] is the field of physics that deals with the macroscopic physical properties of [[matter]]. In particular, it is concerned with the "condensed" [[phase (matter)|phases]] that appear whenever the number of constituents in a system is extremely large and the interactions between the constituents are strong. The most familiar examples of condensed phases are [[Solid-state physics|solids]] and [[liquid]]s, which arise from the bonding and [[electromagnetic force]] between [[atom]]s. More exotic condensed phases include the [[superfluid]] and the [[Bose-Einstein condensate]] found in certain atomic systems at very low [[temperature]], the [[superconductivity|superconducting]] phase exhibited by [[conduction electron]]s in certain materials, and the [[ferromagnet]]ic and [[antiferromagnet]]ic phases of [[spin (physics)|spin]]s on [[crystal lattice|atomic lattices]].
Condensed matter physics is by far the largest field of contemporary physics. Much progress has also been made in theoretical condensed matter physics. By one estimate, one third of all American [[physicist]]s identify themselves as condensed matter physicists. Historically, condensed matter physics grew out of [[solid-state physics]], which is now considered one of its main subfields. The term ''condensed matter physics'' was apparently coined by [[Philip Warren Anderson|Philip Anderson]] when he renamed his research group — previously ''solid-state theory'' — in 1967. In 1978, the Division of Solid State Physics at the [[American Physical Society]] was renamed as the Division of Condensed Matter Physics.<ref name=dcmp_governance>{{cite web | url = http://dcmp.bc.edu/page.php?name=governance_history | title = Division of Condensed Matter Physics Governance History | accessdate = 2007-02-13}}</ref> Condensed matter physics has a large overlap with [[chemistry]], [[materials science]], [[nanotechnology]] and [[engineering]].
===Atomic, molecular, and optical===
{{main|Atomic, molecular, and optical physics}}
[[Image:Military laser experiment.jpg|thumb|250px|A military scientist operates a [[laser]] on an [[optical table]].]]
[[Atom]]ic, [[Molecule|molecular]], and [[Optics|optical]] physics (AMO) is the study of [[matter]]-matter and [[light]]-matter interactions on the scale of single [[atom]]s or structures containing a few atoms. The three areas are grouped together because of their interrelationships, the similarity of methods used, and the commonality of the [[energy]] scales that are relevant. All three areas include both [[classical physics|classical]] and [[quantum physics|quantum]] treatments; they can treat their subject from a microscopic view (in contrast to a macroscopic view).
[[Atomic physics]] studies the [[electron]] hull of [[atom]]s. Current research focuses on activities in quantum control, cooling and trapping of atoms and ions, low-temperature collision dynamics, the collective behavior of atoms in weakly interacting gases (Bose-Einstein Condensates and dilute Fermi degenerate systems), precision measurements of fundamental constants, and the effects of electron correlation on structure and dynamics. Atomic physics is influenced by the [[Atomic nucleus|nucleus]] (see, e.g., [[hyperfine splitting]]), but intra-nuclear phenomenon such as [[nuclear fission|fission]] and [[nuclear fusion|fusion]] are considered part of [[high energy physics]].
[[Molecular physics]] focuses on multi-atomic structures and their internal and external interactions with matter and light. [[Optical physics]] is distinct from [[optics]] in that it tends to focus not on the control of classical light fields by macroscopic objects, but on the fundamental properties of [[optical field]]s and their interactions with matter in the microscopic realm.
===High energy/particle physics===
{{main|Particle physics}}
[[Image:CMS Higgs-event.jpg|thumb|A simulated event in the CMS detector of the [[Large Hadron Collider]], featuring the appearance of the Higgs boson.]]
[[Particle physics]] is the study of the [[elementary particle|elementary]] constituents of [[matter]] and [[energy]], and the interactions between them. It may also be called "high energy physics", because many elementary particles do not occur naturally, but are created only during high energy [[collision]]s of other particles, as can be detected in [[particle accelerator]]s.
Currently, the interactions of elementary particles are described by the [[Standard Model]]. The model accounts for the 12 known particles of matter that interact via the [[strong nuclear force|strong]], [[weak nuclear force|weak]], and [[electromagnetism|electromagnetic]] [[fundamental force]]s. Dynamics are described in terms of matter particles exchanging messenger particles that carry the forces. These messenger particles are known as [[gluon]]s, [[W boson|W<sup>−</sup> and W<sup>+</sup>]] and [[Z boson]]s, and the [[photon]]s, respectively. The Standard Model also predicts a particle known as the [[Higgs boson|Higgs]] [[boson]], the existence of which has not yet been verified.
===Astrophysics===
{{main|Astrophysics|Physical cosmology}}
[[Image:Hubble ultra deep field.jpg|thumb|250px|left|The deepest visible-light image of the [[universe]], the [[Hubble Ultra Deep Field]]]]
[[Astrophysics]] and [[astronomy]] are the application of the theories and methods of physics to the study of [[stellar structure]], [[stellar evolution]], the origin of the [[solar system]], and related problems of [[Physical cosmology|cosmology]]. Because astrophysics is a broad subject, astrophysicists typically apply many disciplines of physics, including mechanics, electromagnetism, statistical mechanics, thermodynamics, quantum mechanics, relativity, nuclear and particle physics, and atomic and molecular physics.
Astrophysics developed from the ancient science of astronomy. Astronomers of early civilizations performed methodical observations of the night sky, and astronomical artifacts have been found from much earlier periods. After centuries of developments by Babylonian and Greek astronomers, western astronomy lay dormant for fourteen centuries until [[Nicolaus Copernicus]] modified the [[Ptolemaic system]] by placing the sun at the center of the universe. [[Tycho Brahe]]'s detailed observations led to [[Kepler's laws of planetary motion]], and [[Galileo]]'s [[telescope]] helped the discipline develop into a modern science. Isaac Newton's theory of universal gravitation provided a physical, dynamic basis for Kepler's laws. By the early 19th cent., the science of celestial mechanics had reached a highly developed state at the hands of [[Leonhard Euler]], [[Joseph Louis Lagrange|J. L. Lagrange]], [[Pierre Simon de Laplace|P. S. Laplace]], and others. Powerful new mathematical techniques allowed solution of most of the remaining problems in classical gravitational theory as applied to the solar system. At the end of the 19th century, the discovery of [[spectral line]]s in sunlight proved that the chemical elements found in the Sun were also found on Earth. Interest shifted from determining the positions and distances of stars to studying their physical composition (see [[stellar structure]] and [[stellar evolution]]). Because the application of physics to astronomy became increasingly important throughout the 20th century, the distinction between [[astronomy]] and [[astrophysics]] has faded.
The discovery by [[Karl Jansky]] in 1931 that radio signals were emitted by celestial bodies initiated the science of [[radio astronomy]]. Most recently, the frontiers of astronomy have been expanded by space exploration. Perturbations and interference from the earth’s atmosphere make space-based observations necessary for [[infrared astronomy|infrared]], [[ultraviolet astronomy|ultraviolet]], [[gamma-ray astronomy|gamma-ray]], and [[X-ray astronomy]]. The [[Hubble Space Telescope]], launched in 1990, has made possible visual observations of a quality far exceeding those of earthbound instruments; earth-bound observatories using telescopes with [[adaptive optics]] will now be able to compensate for the turbulence of [[Earth]]'s [[atmosphere]].
[[Physical cosmology]] is the study of the formation and evolution of the universe on its largest scales. Albert Einstein’s theory of relativity plays a central role in all modern cosmological theories. In the early 20th century, [[Edwin Hubble|Hubble]]'s discovery that the universe was expanding, as shown by the [[Hubble diagram]], prompted rival explanations known as the [[steady state]] universe and the [[Big Bang]]. The Big Bang was confirmed by the success of [[Big Bang nucleosynthesis]] and the discovery of the [[cosmic microwave background]] in 1964. The Big Bang model rests on two theoretical pillars: Albert Einstein's general relativity and the [[cosmological principle]]. Cosmologists have recently established a [[Lambda-CDM model|precise model]] of the evolution of the universe, which includes [[cosmic inflation]], [[dark energy]] and [[dark matter]].
===Applied physics===
{{main|Applied Physics}}
[[Applied physics]] is a general term for physics which is intended for a particular [[Utility|use]]. ''Applied'' is distinguished from ''pure'' by a subtle combination of factors such as the motivation and attitude of researchers and the nature of the relationship to the technology or science that may be affected by the work.<ref>[http://www.stanford.edu/dept/app-physics/ Stanford Applied Physics Department Description]</ref> It usually differs from [[engineering]] in that an applied physicist may not be designing something in particular, but rather is using physics or conducting physics research with the aim of developing new technologies or solving a problem. The approach is similar to that of [[applied mathematics]]. Applied physicists can also be interested in the use of physics for scientific research. For instance, people working on [[accelerator physics]] might seek to build better particle detectors for research in theoretical physics.
Physics is used heavily in [[engineering]]. For example, [[statics]], a subfield of [[mechanics]], is used in the building of [[bridge]]s or other structures, while [[acoustics]] is used to design better concert halls. An understanding of physics is important to the design of realistic [[flight simulator]]s, video game [[physics engine]]s, and movies.
===Physics Education===
{{main|Physics education}}
[[Physics education]] refers both to the methods currently used to teach physics, and to an area of pedagogical research that seeks to improve those methods. Historically, physics has been taught at the high school and university level primarily by the lecture method, together with laboratory exercises aimed at verifying concepts taught in the lectures.
==References==
<references/>
== Further reading ==
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* A large number of textbooks, popular books, and webpages about physics are available for [[Physics (further reading)|further reading]].
* [[List of important publications in physics|Important publications in physics]]
===Organizations===
* [http://www.aip.org/index.html AIP.org] is the website of the [[American Institute of Physics]]
* [http://www.iop.org/ IOP.org] is the website of the [[Institute of Physics]]
* [http://www.aps.org/ APS.org] is the website of the [[American Physical Society]]
* [http://www.spsnational.org/ SPS National] is the website of the American [[Society of Physics Students]]
* [http://www.cap.ca/ CAP.ca] is the website of the [[Canadian Association of Physicists]]
* [http://www.eps.org/ EPS.org] is the website of the [[European Physical Society]]
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