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Physics is  the most basic of the [[natural sciences]].  Physics deals with the fundamental constituents of matter and their interactions, as well as how matter is organized on all possible length and time scales. Physics aims to provide unified descriptions of the behavior of matter and energy, from fundamental principles as much as possible, while describing a wide-variety of phenoma. Physics has considerable overlap with other sciences and engineering fields, most notably biology, chemistry, mathematics, electrical engineering and materials science
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[[Physicist]]s study a wide range of physical phenomena, from [[quark]]s to galaxies, from individual [[atoms]] to macroscopic biological systems.
'''Physics''' (from the Greek ''physikos,'' nature) is the [[science]] of nature at its most fundamental form, and is the foundation of the [[natural sciences]]. From [[quark]]s to galaxies, from individual [[Atom_(science)|atoms]] to macroscopic biological systems, [[physicist]]s study a wide range of physical phenomena.


== Key Areas of Physics ==
Over the course of time, physical phenomena have been grouped together under specific branches. While it is widely believed that the whole of physics can be considered within a single unified theory, including all phenomena both at the level of [[quantum mechanics]] and on a scale as wide as the [[universe]], this has not yet been proven.
This is made clear especially by the fact that the current models developed by the different branches of physics give contradictory solutions to the same problem when they are combined. For example, the unification of [[general relativity]] and quantum mechanics has so far proven impossible due to creation of infinite values when looking at some properties of systems of subatomic particles. <!-- I am 90% sure this is an example but I am glad an expert will be checking it ~~~~ -->
<!-- an example would be nice, but I don't pretend to understand why relativistic QM hits problems. ~~~ -->
Nevertheless, these are considered as problems "to be solved". The central branches of physics are:


* [[Classical mechanics]] is a model of the physics of [[force]]s acting upon bodies, and the motion of those bodies. As opposed to quantum mechanics, classical mechanics is deterministic. Classical mechanics is usually regarded as a limit of quantum mechanics, although this has not been proven in general. Classical mechanics can be divided into two parts:
::Newtonian, after Isaac Newton and his laws of motion. This can be more generally formulated in Lagrangian mechanics, after Joseph-Louis Lagrange.
::Relativistic, due to [[Albert Einstein]] and his [[theory of relativity]]. This includes both special and general relativity, and addresses regimes in which Newtonian mechanics is no longer valid: when relative speeds of object are comparable to the speed of light, and for motion occurring near very massive objects.


* [[Quantum mechanics]] is the branch of physics treating [[Atom_(science) | atomic]] and subatomic systems and their interaction with [[radiation]] in terms of observable quantities. It is based on the observation that all forms of [[Energy_(science)|energy]] are released in discrete units or bundles called ''quanta''. Quantum theory typically permits only [[probability | probable]] or [[statistics | statistical]] calculation of the observed features of particles, understood in terms of [[wave function]]s. Quantum mechanics itself has several levels of approximation.


== Physics and Other disciplines ==
* [[Electromagnetism]], or electromagnetic theory, is the physics of the electromagnetic field: a field, encompassing all of [[space (physics)|space]], which exerts a [[force]] on those [[Elementary particle | particle]]s that possess the property of [[electric charge]], and is in turn affected by the presence and motion of such particles. Electromagnetism encompasses various real-world ''electromagnetic phenomena''. Electromagnetism, as taught in a typical undergraduate college physics curriculum, can be divided into several areas:
 
:: Electrostatics, the study of charged objects, the forces between them, and electric fields and potentials for charged objects at rest.
Discoveries in physics find applications throughout the other [[natural science]]s as they regard the basic principles of nature. Some of the phenomena studied in physics, such as the phenomenon of  [[conservation of energy]], are common to  all material systems. These are often referred to as [[law of physics | laws of physics]].
:: Magnetism, the study of forces between moving charged objects, the magnetic field, and electromagnetic induction.
 
:: Electric circuits, the study of electric potential difference and electric currents for various arrangements of circuit components.
Physics is often said to be the "fundamental science" (chemistry is sometimes included), because each of the other sciences ([[biology]], [[chemistry]], [[geology]], [[material science]], [[engineering]], [[medicine]] etc.) deals with particular types of material systems that obey the laws of physics. For example, chemistry is the science of matter (such as [[atom]]s and [[molecule]]s) and the [[chemical substance]]s that they form in the bulk. The structure, reactivity, and properties of a [[chemical compound]] are determined by the properties of the underlying molecules, which can be described by areas of physics such as [[quantum mechanics]] (called in this case [[quantum chemistry]]), [[thermodynamics]], and [[electromagnetism]].
:: Optics and electromagnetic waves, the study of how time-varying electromagnetic fields propagate through space.
 
Physics is closely related to  [[mathematics]], which provides the logical framework in which physical laws can be precisely formulated and their predictions quantified. Physical [[definitions]], [[model theory | models]] and [[theory | theories]] are invariably expressed using mathematical relations. A key difference between physics and mathematics is that because physics is ultimately concerned with descriptions of the material world, it tests its [[theories]] by [[observation]]s (called [[experiment]]s), whereas mathematics does not have such requirements. The distinction, however, is not always clear-cut. This large area of research intermediate between physics and mathematics isknown as [[mathematical physics]].
 
Physics is also closely related to [[engineering]] and [[technology]]. For instance, [[electrical engineering]] is the study of the practical application of [[electromagnetism]]. [[Statics]], a subfield of [[mechanics]], is responsible for the building of [[bridge]]s. Further, [[physicist]]s, or practitioners of physics, [[invention | invent]] and design processes and [[tool | device]]s, such as the [[transistor]], whether in [[basic research | basic]] or [[applied research]]. [[Experiment | Experimental]] physicists design and perform experiments with [[particle accelerator]]s, [[nuclear reactor]]s, [[telescope]]s, [[barometer]]s, [[synchrotron]]s, [[cyclotron]]s, [[spectrometer]]s, [[laser]]s, and other equipment.
 
 
 
== Central theories ==
 
While physics deals with a wide variety of systems, there are certain theories that are basis for physics. Each of these theorieshas been  experimentally tested numerous times and found correct as an approximation of nature (within a certain domain of validity).  These theories continue to be areas of active research; for instance, a remarkable aspect of classical mechanics known as [[chaos theory | chaos]] was discovered in the 20th century, three centuries after the original formulation of classical mechanics by [[Isaac Newton]] ([[1642]]–[[1727]]). These "central theories" are important tools for research into more specialized topics, and any physicist, regardless of his or her specialization, is expected to be literate in them.
 
* [[Quantum mechanics]] is the branch of physics treating [[atom | atomi]]c and [[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]]''. Quantum theory typically permits only [[probability | probable]] or [[statistics | statistical]] calculation of the observed features of subatomic particles, understood in terms of [[wave function]]s. Quantum mechanics itself has several levels of approximation.
 
* [[Classical mechanics]] is a model of the physics of [[force]]s acting upon bodies. As opposed to quantum mechanics, classical mechanics is deterministic. Classical mechanics is usually regarded as a limit of quantum mechanics, although this has not been proven in general. Classical mechanics can be divided into two parts: Newtonian, after Newton and his laws of motion, and relativistic, due to Einstein and his [[theory of relativity]].
 
* [[Electromagnetism]], or electromagnetic theory, is the physics of the [[electromagnetic field]]: a [[field (physics) | field]], encompassing all of [[space]], which exerts a [[force]] on those [[Elementary particle | particle]]s that possess the property of [[electric charge]], and is in turn affected by the presence and motion of such particles. Electromagnetism encompasses various real-world ''electromagnetic phenomena''.
 
*  [[Statistical mechanics]] and [[Thermodynamics]] are the branches of physics that deals with [[heat]], [[work]] and [[entropy]]. Thermodynamics is particularly concerned macroscopic energy and the effects of [[temperature]], [[pressure]], [[volume]], [[action (physics) | mechanical action]], and [[work]]. , Statistical mechanic  is the branch of physics that analyzes [[macroscopic]] [[thermodynamic system | systems]] by applying [[statistics | statistical principles]] to their microscopic constituents and, thus, connects the macroscopic viewpoint of thermodynamics with the atomic nature of matter descriped by either classical physics or quantum mechanics
 
 
=== Fields within physics ===
 
Physics can be subdivided in a variety of different manners; for teaching, for historical purposes, or  for research purposes.
 
Contemporary research in physics is divided into several subfields.  The following is based on the units the members of the American Physical Society organize themselves into in order of size. Although many physicists are members of multiple units
 
* [[Condensed matter physics]] is the study of the condensed phases;  [[solid]]s and [[liquid]]s, and how the properties of these phase arise from the properties and mutual interactions of the constituent [[atoms]].  


* [[Particles and fields |particle physics]], also known as "high-energy physics". This branch is concerned with the properties of subatomic particles much smaller than [[atoms]], including the [[elementary particle]]s from which all other units of matter are constructed.
* Statistical mechanics and [[Thermodynamics]] are the branches of physics that deal with [[heat]], [[work]] and [[entropy (thermodynamics)|entropy]]. Thermodynamics is particularly concerned with macroscopic [[Energy_(science)|energy]] and the effects of [[temperature]], [[pressure]], [[Volume (science)|volume]], [[action (physics)|mechanical action]], and [[work]].  Statistical mechanics is the branch of physics that analyzes macroscopic [[thermodynamic system | systems]] by applying [[statistics | statistical principles]] to their microscopic constituents and, thus, connects the macroscopic viewpoint of thermodynamics with the atomic nature of matter described by either classical physics or quantum mechanics.


* [[Atomic, molecular, and optical physics]]  (AMO physics) which deals with the behavior of individual [[atoms]] and molecules, and including the ways in which they absorb and emit [[light]]. Molecular physics is sometimes considered a branch of chemical physics. Laser science may be considered a subfield of AMO or as a separate field.
==Research and fields within physics ==
Physics can be subdivided in a variety of different manners; for teaching, for historical purposes, or  for research purposes. Contemporary research in physics is divided into many distinct subfields. An incomplete listing includes:


* [[Fluid dynamics ]] concerns itself with the study of the moving fluids; liquids or gases.
* [[Condensed matter physics]] is the study of the condensed phases of matter, [[Solid (state of matter)|solid]] and [[liquid]], and how the properties of matter in these phases arise from the properties and mutual interactions of the constituent [[Atom_(science)|atoms]]. More physicists study condensed matter physics than any other field.


* [[Plasma physics]] deals with the behavior of the [[plasma]] state of matter
*[[Particle physics]], also known as "high-energy physics". This branch is concerned with the properties of subatomic particles much smaller than [[Atom_(science)|atoms]], including the [[elementary particle]]s from which all other units of matter are constructed.


* [[Nuclear physics]]
*[[Astrophysics]] attempts to explain the physical workings of celestial objects and phenomena.


* [[Materials physics ]]
* [[Atomic, molecular, and optical physics]] (AMO physics) deals with the behavior of individual [[Atom_(science)|atoms]] and molecules, including the ways in which they absorb and emit [[light]]. Molecular physics is sometimes also considered a branch of chemical physics. Laser science may be considered a subfield of AMO or as a separate field.


* [[Computational physics]]
* [[Nuclear physics]] is the study of atomic nuclei. A [[nucleus]] is comprised of [[proton]]s and (usually) [[neutron]]s, and makes up about 99.97% of a typical atom's total mass.


*[[Astrophysics ]]
* Materials physics is the study of various physical properties of materials. Classifications of physical properties include, but are not limited to, thermal, electronic, magnetic, optical, and mechanical.


*[[Biological physics]] or biophysics. Many members of this field are actually biologists and chemists who are members of other societies. As evidenced by the size of the Biophysical Society, ~7000 members when compared to the biological physics unit of the APS , ~1800 members.
*Computational physics deals with numerically (as opposed to analytically) solving the equations that govern physical systems.


*[[Chemical physics]]
A number of fields of physics overlap strongly with other sciences: [[Biophysics]], [[Physical chemistry]] and [[Geophysics]] overlap  considerably with [[biology]], [[chemistry]] and [[geography]], but the focus is on the application of physics and physical techniques to problems within the other field.


== Classical, quantum and modern physics ==
=== Classical and quantum physics ===
{{further | [[Classical physics]], [[Quantum physics]], [[Modern physics]], [[Semiclassical]]}}
{{further | [[Classical physics]], [[Quantum physics]], [[Modern physics]], [[Semiclassical]]}}


Since the construction of [[quantum mechanics]] in the early twentieth century, it generally became evident to the physical community that it would be preferable for every known description of [[nature]] to be [[Quantization (physics) | quantized]], that is, to follow the [[postulate]]s of quantum mechanics. To this effect, all results that were not quantized are called ''classical'': this includes the [[Theory of relativity | special and general theories of relativity]]. Simply because a result is classical does not mean that it was discovered before the advent of quantum mechanics. Classical theories are, generally, much easier to work with and much research is still being conducted on them without the express aim of quantization. However, there exist problems in physics in which classical and quantum aspects must be combined to attain some approximation or limit that may acquire several forms as the passage from classical to quantum mechanics is often difficult — such problems are termed ''semiclassical''.
The distinction between classical and quantum theories is important in physics. Classical theories are generally valid despite not considering the quantum nature of things, but are ultimately an approximation to a deeper quantized truth; this approximation typically breaks down at extreme scales, particularly the subatomic. Some fundamental classical theories, such as [[Theory of relativity | relativity]] do not yet have full analogous quantum theories.


However, because relativity and quantum mechanics provide the most complete known description of fundamental interactions, and because the changes brought by these two frameworks to the physicist's world view were revolutionary, the term ''modern physics'' is used to describe physics which relies on these two theories. Colloquially, modern physics can be described as the physics of extremes: from systems at the extremely small ([[atom]]s, [[Atomic nucleus | nuclei]], [[Elementary particles | fundamental particles]]) to the extremely large (the [[Universe]]) and of the extremely fast ([[special relativity | relativity]]).
Both classical and quantum physics are active areas of research. However, there exist problems in physics in which classical and quantum aspects must be combined to attain some approximation or limit that may acquire several forms as the passage from classical to quantum mechanics is often difficult — such problems are termed ''semiclassical''.


== Theoretical and experimental physics ==
=== Theoretical and experimental physics ===
Most individual physicists specialize in either theoretical physics or experimental physics. There have been a few exceptions, such as  great [[Italy | Italian]] physicist [[Enrico Fermi]] (1901–1954), who made fundamental contributions to both theory and experimentation.


The culture of physics research differs from the other sciences in the separation of [[theory]] and [[experiment]]. Since the [[20th 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, though this is changing as of late.
Roughly speaking, theorists seek to develop theories, through mathematical and computational models, that can both describe and interpret existing experimental results and successfully predict future results, while experimentalists devise and perform experiments to explore new phenomena and test theoretical predictions. Although theory and experiment can be developed separately, they are strongly dependent on each other. Progress in physics frequently comes about when experimentalists make a discovery that existing theories cannot account for, necessitating the formulation of new theories, or when theorists make predictions that experimentalists test.


Roughly speaking, theorists seek to develop through [[Model (abstract) | abstractions]] and [[mathematical model]]s theories  that can both describe and interpret existing experimental results and successfully predict future results, while experimentalists devise and perform experiments to explore new phenomena and test theoretical predictions. Although theory and experiment are developed separately, they are strongly dependent on each other. However, theoretical research in physics may further be considered to draw from [[mathematical physics]] and [[computational physics]] in addition to experimentation. Progress in physics frequently comes about when experimentalists make a discovery that existing theories cannot account for, necessitating the formulation of new theories. Likewise, ideas arising from theory often inspire new experiments. In the absence of experiment, theoretical research can go in the wrong direction; this is one of the criticisms that has been leveled against [[M-theory]], a popular theory in high-energy physics for which no practical experimental test has ever been devised.
== Physics and Other disciplines ==
 
Physics finds applications throughout the other [[natural science]]s as they regard the basic principles of nature.  Physics is often said to be the "fundamental science", because the other sciences deal with material systems that obey the laws of physics. For example, chemistry is the science of matter (such as [[Atom_(science)|atom]]s and [[molecule]]s) and the chemical substances that they form in the bulk. The structure, reactivity, and properties of a [[chemical compound]] are determined by the properties of the underlying molecules, which can be described by areas of physics such as [[quantum mechanics]] (in the applied subfield of [[quantum chemistry]]), [[thermodynamics]], and [[electromagnetism]].  
=== Discredited theories ===
 
Scientific theories sometimes end up being discredited.  In some of these cases the theory was announced prematurely and gained press attention before being discredited.  Other times an established theory is overthrown and a new one erected in its place.  Some famous examples are:
 
* [[Cold fusion]] — Announced in a press conference in 1989 but never confirmed.
* [[Dynamic theory of gravity]] — Announced in a press release by [[Nikola Tesla]] in 1937 but never published.
* [[Steady state theory]] — An established theory of [[physical cosmology | cosmology]] in the early and middle [[20th century]], made obsolete by the success of [[Big Bang]] theory.
* [[Luminiferous aether]] — An established theory in the late [[19th century]], which was contradicted by observations and made "superfluous" by [[Theory of relativity | relativity]].
* [[Phlogiston theory]] — An established theory of the [[18th century]] that attributed [[combustion]] to the liberation of ''phlogiston'' from a material.
 
=== Phenomenology ===
 
[[Phenomenology (science) | Phenomenology]] is intermediate between experiment and theory. It is more abstract and includes more logical steps than experiment, but is more directly tied to experiment than theory. The boundaries between theory and phenomenology, and between phenomenology and experiment, are somewhat fuzzy and to some extent depend on the understanding and intuition of the [[scientist]] describing these. An example is Einstein's [[1905]] paper on the [[photoelectric effect]], "''[[Annus Mirabilis Papers#Photoelectric effect | On a Heuristic Viewpoint Concerning the Production and Transformation of Light]]''".
 
== Applied physics ==
 
[[Applied physics]] is physics that is intended for a particular technological or practical use, as for example in [[engineering]], as opposed to [[basic research]].  This approach is similar to that of [[applied mathematics]]. Applied physics is rooted in the fundamental truths and basic concepts of the physical sciences but is concerned with the utilization of scientific principles in practical devices and systems, and in the application of physics in other areas of science. "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. [http://www.stanford.edu/dept/app-physics/general/]
 
== History ==
{{main | History of physics}}
 
{{further | [[Famous physicists]], [[Nobel Prize in physics]]}}
 
[[Image:Francesco Hayez 001.jpg|thumb|150px|left|[[Aristotle]]]]
 
Since antiquity, people have tried to understand the behavior of [[matter]]: why unsupported objects drop to the ground, why different [[materials science | materials]] have different properties, and so forth. The character of the [[Universe]] was also a mystery, for instance 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 first theories were largely couched in [[philosophy | philosophical]] terms, and never verified by systematic experimental testing as is popular today. The works of [[Ptolemy]] and [[Physics (Aristotle) | Aristotle]], however, were also not always found to match everyday observations. There were exceptions and there are [[anachronism]]s - for example, [[Indian philosophy | Indian philosophers]] and [[Indian science and technology#Astronomy | astronomers]] gave many correct descriptions in [[atomism]] and [[astronomy]], and the [[Ancient Greece | Greek]] thinker [[Archimedes]] derived many correct quantitative descriptions of [[mechanics]] and [[hydrostatics]].
 
The willingness to question previously held truths and search for new answers eventually resulted in a period of major scientific advancements, now known as the [[Scientific Revolution]] of the late [[17th century]]. The precursors to the scientific revolution can be traced back to the important developments made in [[India]] and [[Persia]], including the [[ellipse | elliptical]] model of the planets based on the [[heliocentrism | heliocentric]] [[solar system]] of [[gravitation]] developed by [[Indian mathematics | Indian mathematician]]-astronomer [[Aryabhata]]; the basic ideas of [[atomic theory]] developed by [[Hindu]] and [[Jaina]] philosophers; the theory of light being equivalent to energy particles developed by the Indian [[Buddhist]] scholars [[Dignāga]] and [[Dharmakirti]]; the optical theory of [[light]] developed by [[Persian people | Persian]] [[Islamic science | scientist]] [[Alhazen]]; the [[Astrolabe]] invented by the Persian [[Mohammad al-Fazari]]; and the significant flaws in the [[Ptolemaic system]] pointed out by Persian scientist [[Nasir al-Din al-Tusi]].
 
As the influence of the [[Islam]]ic [[Caliph]]ate expanded to Europe, the works of Aristotle preserved by the [[Arab]]s, and the works of the Indians and Persians, became known in Europe by the [[12th century | 12th]] and [[13th century | 13th centuries]]. This eventually lead to the scientific revolution which culminated with the publication of the ''[[Philosophiae Naturalis Principia Mathematica]]'' in [[1687]] by the mathematician, physicist, alchemist and inventor Sir [[Isaac Newton]] ([[1643]]-[[1727]]).
 
[[Image:Galileo.arp.300pix.jpg|thumb|150px|left|[[Galileo]]]]
 
The Scientific Revolution is held by most historians (e.g., Howard Margolis) to have begun in [[1543]], when the first printed copy of [[Nicolaus Copernicus]]'s ''[[De Revolutionibus Orbium Coelestium | De Revolutionibus]]'' (most of which had been written years prior but whose publication had been delayed) was brought to the influential Polish astronomer from [[Nuremberg]].
 
[[Image:GodfreyKneller-IsaacNewton-1689.jpg|thumb|150px|right|[[Sir Isaac Newton]]]]
 
Further significant advances were made over the following century by [[Galileo Galilei]], [[Christiaan Huygens]], [[Johannes Kepler]], and [[Blaise Pascal]]. During the early [[17th century]], Galileo pioneered the use of experimentation to validate physical theories, which is the key idea in modern [[scientific method]]. Galileo formulated and successfully tested several results in [[dynamics (mechanics) | dynamics]], in particular the Law of [[Inertia]]. In [[1687]], [[Isaac Newton | Newton]] published the ''[[Philosophiae Naturalis Principia Mathematica | Principia]]'', detailing two comprehensive and successful physical theories: [[Newton's laws of motion]], from which arise [[classical mechanics]]; and [[gravity | 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 re-formulated and extended by [[Leonhard Euler]], French mathematician [[Joseph Louis Lagrange | Joseph-Louis Comte de Lagrange]], Irish mathematical physicist [[William Rowan Hamilton]], and others, who produced new results in mathematical physics. The law of universal gravitation initiated the field of [[astrophysics]], which describes [[astronomy | astronomical]] phenomena using physical theories.
 
After Newton defined [[classical mechanics]], the next great field of inquiry within physics was the nature of [[electricity]]. Observations in the [[17th century | 17th]] and [[18th century]] by scientists such as [[Robert Boyle]], [[Stephen Gray (scientist) | Stephen Gray]], and [[Benjamin Franklin]] created a foundation for later work. These observations also established our basic understanding of electrical charge and [[electric current | current]].
 
[[Image:James Clerk Maxwell.jpg|thumb|right|150px|[[James Clerk Maxwell]]]]
 
In [[1821]], the English physicist and chemist [[Michael Faraday]] integrated the study of [[magnetism]] with the study of electricity. This was done by demonstrating that a moving [[magnet]] induced an [[electric current]] in a [[Electrical conductor| conductor]]. Faraday also formulated a physical conception of [[electromagnetic field]]s. [[James Clerk Maxwell]] built upon this conception, in [[1864]], with an interlinked set of 20 equations that explained the interactions between [[electric field | electric]] and [[magnetic field]]s. These 20 equations were later reduced, using [[vector calculus]], to a set of [[Maxwell's equations | four equations]] by [[Oliver Heaviside]].
 
[[Image:Albert Einstein Head.jpg|thumb|left|175px|[[Albert Einstein]] in [[1947]]]]
 
In addition to other electromagnetic phenomena, Maxwell's equations also can be used to describe [[light]]. Confirmation of this observation was made with the [[1888]] discovery of [[radio]] by [[Heinrich Hertz]] and in [[1895]] when [[Wilhelm Roentgen]] detected [[X rays]]. The ability to describe light in electromagnetic terms helped serve as a springboard for [[Albert Einstein]]'s publication of the theory of [[special relativity]] in 1905. This theory combined classical mechanics with Maxwell's equations.
The theory of [[special relativity]] unifies space and time into a single entity, [[spacetime]]. Relativity prescribes a different transformation between [[inertial frame of reference | 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. Einstein built further on the special theory by including gravity into his calculations, and published his theory of [[general relativity]] in [[1915]].
 
One part of the theory of general relativity is [[Einstein's field equation]]. This describes how the ''stress-energy tensor'' creates curvature of [[spacetime]] and forms the basis of general relativity. Further work on Einstein's field equation produced results which predicted the [[Big Bang]], [[black hole]]s, and the [[expanding universe]]. Einstein believed in a static universe and tried (and failed) to fix his equation to allow for this. However, by [[1929]] [[Edwin Hubble]]'s astronomical observations suggested that the universe is expanding.
 
From the late [[17th century]] onwards, [[thermodynamics]] was developed by physicist and chemist [[Robert Boyle | Boyle]], [[Thomas Young (scientist) | Young]], and many others. In [[1733]], [[Daniel Bernoulli | Bernoulli]] used statistical arguments with classical mechanics to derive thermodynamic results, initiating the field of [[statistical mechanics]]. In [[1798]], [[Benjamin Thompson | Thompson]] demonstrated the conversion of mechanical work into heat, and in [[1847]] [[James Joule | Joule]] stated the law of conservation of [[energy]], in the form of heat as well as mechanical energy. [[Ludwig Boltzmann]], in the 19th century, is responsible for the modern form of statistical mechanics.
 
In [[1895]], [[Wilhelm Röntgen | Röntgen]] discovered [[X-ray]]s, which turned out to be high-frequency electromagnetic radiation. [[Radioactivity]] was discovered in [[1896]] by [[Henri Becquerel]], and further studied by [[Maria Sklodowska-Curie | Marie Curie]], [[Pierre Curie]], and others. This initiated the field of [[nuclear physics]].
 
In [[1897]], [[J.J. Thomson | Joseph J. Thomson]] discovered the [[electron]], the elementary particle which carries electrical current in [[electrical circuit | circuits]]. In [[1904]], he proposed the first model of the [[atom]], known as the [[atom/plum pudding | plum pudding model]]. (The existence of the atom had been proposed in [[1808]] by [[John Dalton]].)
 
These discoveries revealed that the assumption of many physicists that atoms were the basic unit of [[matter]] was flawed, and prompted further study into the structure of [[atom]]s.
 
[[Image:Ernest Rutherford.jpg|thumb|right|150px|[[Ernest Rutherford]]]]
 
In [[1911]], [[Ernest Rutherford]] deduced from [[rutherford scattering | scattering experiments]] the existence of a compact atomic nucleus, with positively charged constituents dubbed [[proton]]s. [[neutron | Neutrons]], the neutral nuclear constituents, were discovered in [[1932]] by [[James Chadwick | Chadwick]]. The equivalence of mass and energy (Einstein, 1905) was spectacularly demonstrated during [[World War II]], as research was conducted by each side into [[nuclear physics]], for the purpose of creating a [[nuclear weapon | 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 [[Enrico Fermi | Fermi]] achieved the first man-made [[nuclear chain reaction]] in [[1942]], and in [[1945]] the world's first [[nuclear weapon | nuclear explosive]] was detonated at [[Trinity site]], near [[Alamogordo]], [[New Mexico]].
 
In [[1900]], [[Max Planck]] published his explanation of [[blackbody radiation]]. This equation assumed that radiators are [[quantum | quantized]], which proved to be the opening argument in the edifice that would become [[quantum mechanics]]. By introducing discrete energy elvels, Planck, Einstein, [[Niels Bohr]], and others developed [[quantum]] theories to explain various anomalous experimental results. Quantum mechanics was formulated in [[1925]] by [[Werner Heisenberg | Heisenberg]] and in [[1926]] by [[Erwin Schrödinger | Schrödinger]] and [[Paul Dirac]], in two different ways that both explained the preceding heuristic quantum theories. In quantum mechanics, the outcomes of physical measurements are inherently [[probability | probabilistic]]; the theory describes the calculation of these probabilities. It successfully describes the behavior of matter at small distance scales. During the [[1920s]] Schrödinger, Heisenberg, and [[Max Born]] were able to formulate a consistent picture of the chemical behavior of matter, a complete theory of the electronic structure of the atom, as a byproduct of the quantum theory.
 
[[Image:Feynman-bongos.jpg|thumb|left|150px|[[Richard Feynman]]]]
 
[[Quantum field theory]] was formulated in order to extend quantum mechanics to be consistent with special relativity. It was devised in the late [[1940s]] with work by [[Richard Feynman]], [[Julian Schwinger]], [[Sin-Itiro Tomonaga]], and [[Freeman Dyson]]. They formulated the theory of [[quantum electrodynamics]], which describes the electromagnetic interaction, and successfully explained the [[Lamb shift]]. Quantum field theory provided the framework for modern [[particle physics]], which studies [[fundamental force]]s and elementary particles.
 
[[Chen Ning Yang]] and [[Tsung-Dao Lee]], in the [[1950s]], discovered an unexpected [[asymmetry]] in the decay of a [[subatomic particle]]. In [[1954]], Yang and [[Robert Mills (physicist) | Robert Mills]] then developed a class of [[gauge theory | gauge theories]] which provided the framework for understanding the nuclear forces. The theory for the [[strong nuclear force]] was first proposed by [[Murray Gell-Mann]]. The [[electroweak force]], the unification of the [[weak nuclear force]] with electromagnetism, was proposed by [[Sheldon Lee Glashow]], [[Abdus Salam]] and [[Steven Weinberg]] and confirmed in [[1964]] by [[James Watson Cronin]] and [[Val Fitch]]. This led to the so-called [[Standard Model]] of particle physics in the [[1970s]], which successfully describes all the elementary particles observed to date.


Quantum mechanics also provided the theoretical tools for [[condensed matter physics]], whose largest branch is [[solid state physics]]. It studies the physical behavior of solids and liquids, including phenomena such as [[crystal structure]]s, [[semiconductor | semiconductivity]], and [[superconductor | superconductivity]]. The pioneers of condensed matter physics include [[Felix Bloch]], who created a quantum mechanical description of the behavior of electrons in crystal structures in [[1928]]. The transistor was developed by physicists [[John Bardeen]], [[Walter Houser Brattain]] and [[William Bradford Shockley]] in [[1947]] at [[Bell Labs | Bell Telephone Laboratories]].
Physics is closely related to  [[mathematics]], which provides the logical framework in which physical laws can be precisely formulated and their predictions quantified. Physical [[definitions]], [[model theory | models]] and [[theory | theories]] are invariably expressed using mathematical relations. A key difference between physics and mathematics is that because physics is ultimately concerned with descriptions of the material world, it tests its [[theories]] by [[observation]]s (called [[experiment]]s), whereas mathematics does not have such requirements. The distinction, however, is not always clear-cut. This large area of research intermediate between physics and mathematics is known as [[mathematical physics]].


The two themes of the [[20th century]], general relativity and quantum mechanics, appear inconsistent with each other. General relativity describes the [[universe]] on the scale of [[planet]]s and [[solar system]]s while quantum mechanics operates on sub-atomic scales. This challenge is being attacked by [[string theory]], which treats [[spacetime]] as composed, not of points, but of one-dimensional objects, [[string theory | strings]]. Strings have properties like a common string (e.g., [[Tension (mechanics) | tension]] and [[oscillation | vibration]]). The theories yield promising, but not yet testable results. The search for experimental verification of string theory is in progress.
Physics is also closely related to [[engineering]] and [[technology]]. For instance, [[electrical engineering]] is the study of the practical application of [[electromagnetism]]. Statics, a subfield of [[mechanics]], is responsible for the building of [[Bridge (civil engineering)|bridge]]s. Further, [[physicist]]s, or practitioners of physics, invent and design processes and [[tool | device]]s, such as the [[Electronic switch#Transistor|transistor]], whether in [[basic research | basic]] or [[applied research]]. [[Experiment | Experimental]] physicists design and perform experiments with [[particle accelerator]]s, [[nuclear reactor]]s, [[telescope]]s, barometers, synchrotrons, cyclotrons, spectrometers, [[laser]]s, and other equipment.
 
[[Image:WYP2005 logo.gif|right|130px]]
 
The United Nations declared the year [[2005]], the centenary of Einstein's [[annus mirabilis]], as the [[World Year of Physics]].
 
== Future directions ==
{{main | Unsolved problems in physics}}


== Current research directions ==
Research in physics is progressing constantly on a large number of fronts, and is likely to do so for the foreseeable future.
Research in physics is progressing constantly on a large number of fronts, and is likely to do so for the foreseeable future.
Some current directions include:


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 computer]]s.
In [[condensed matter physics]], the biggest unsolved theoretical problem is the explanation for [[superconductivity|high-temperature superconductivity]]. Strong efforts, largely experimental, are being put into making workable [[spintronics]] and [[quantum computer]]s.


In particle physics, the first pieces of experimental evidence for physics beyond the [[Standard Model]] have begun to appear. Foremost amongst these are indications that [[neutrino]]s 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 accelerator]]s will begin probing energy scales in the [[TeV]] range, in which experimentalists are hoping to find evidence for the [[Higgs boson]] and [[supersymmetry | supersymmetric particles]].
In [[particle physics]], the first pieces of experimental evidence for physics beyond the [[Standard Model]] have begun to appear. Foremost amongst these are indications that [[neutrino]]s 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 accelerator]]s will begin probing [[Energy_(science)|energy]] scales in the [[TeV]] range, in which experimentalists are hoping to find evidence for the [[Higgs boson]] and [[supersymmetry|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, have not yet borne fruit. The current leading candidates are [[M-theory]], [[superstring theory]] and [[loop quantum gravity]].
Theoretical attempts to unify [[quantum mechanics]] and [[general relativity]] into a single theory of [[quantum gravity]], a program ongoing for over half a century, have not yet borne fruit. The current leading candidates are [[M-theory]], [[superstring theory]] and [[loop quantum gravity]].


Many [[astronomy | astronomical]] and [[physical cosmology | cosmological]] phenomena have yet to be satisfactorily explained, including the existence of [[GZK paradox | ultra-high energy cosmic rays]], the [[baryon asymmetry]], the [[accelerating universe | acceleration of the universe]] and the [[galaxy rotation problem | anomalous rotation rates of galaxies]].
Many [[astronomy | astronomical]] and [[physical cosmology | cosmological]] phenomena have yet to be satisfactorily explained, including the existence of GZK paradox | ultra-high energy cosmic rays, the [[baryon asymmetry]], the [[accelerating universe | acceleration of the universe]] and the [[galaxy rotation problem | anomalous rotation rates of galaxies]].
 
Although much progress has been made in high-energy, [[quantum]], and astronomical physics, many everyday phenomena, involving [[complex systems | complexity]], [[chaos]], or [[turbulence]] are still poorly understood. Complex problems that seem like they could be solved by a clever application of dynamics and mechanics, such as the formation of sandpiles, nodes in trickling [[water]], the shape of water [[droplet]]s, mechanisms of [[surface tension]] [[catastrophe theory | catastrophes]], or self-sorting in shaken heterogeneous collections are unsolved. These complex phenomena have received growing attention since the 1970s for several reasons, not least of which has been the availability of modern [[mathematics | mathematical]] methods and [[computers]] which enabled [[complex systems]] to be modeled in new ways. The [[interdisciplinary]] [[relevance]] of complex physics has also increased, as exemplified by the study of [[turbulence]] in [[aerodynamics]] or the [[observation]] of [[pattern]] [[formation]] in [[biology | biological]] systems. In 1932, [[Horace Lamb]] correctly prophesied:
<blockquote>
''I am an old man now, and when I die and go to heaven there are two matters on which I hope for enlightenment. One is quantum electrodynamics, and the other is the turbulent motion of fluids. And about the former I am rather optimistic.''
</blockquote>
 
== See also ==
{{portal | Physics}}
 
* [[E=mc² | E = mc²]]
* [[Classical physics]]
* [[Glossary of classical physics]]
* [[List of basic physics topics]]
* [[List of physics topics]]
* [[Unsolved problems in physics]]
* [[Philosophy of physics]]
* [[Physics (Aristotle)]] - an early book on physics, which attempted to analyze and define motion from a philosophical point of view
* [[Perfection#Perfection in physics and chemistry | Perfection in physics and chemistry]]
* [[Mathematics]]
* [[Astronomy]]
* [[Chemistry]]
* [[Engineering]]
 
== Notes ==
 
<references />
<!-- Dead note "big bang": -->
 
* Alpher, Herman, and Gamow. ''Nature'' '''162''',774 (1948). [http://nobelprize.org/physics/laureates/1978/wilson-lecture.pdf Wilson's [[1978]] Nobel lecture]
<!-- Dead note "parity": -->
 
* [http://cwp.library.ucla.edu/Phase2/Wu,_Chien_Shiung@841234567.html C.S. Wu's contribution to the overthrow of the conservation of parity]
<!-- Dead note "gauge theories": -->
 
* Yang, Mills [[1954]] ''[[Physical Review]]'' '''95''', 631; Yang, Mills 1954 ''Physical Review'' '''96''', 191.
 
== Further reading ==
 
=== Popular reading ===
 
* {{cite book | author=[[Stephen Hawking | Hawking, Stephen]] | title=[[A Brief History of Time]] | publisher=Bantam | year=1988 | id=ISBN 0-553-10953-7}}
 
* {{cite book | author=[[Richard Feynman | Feynman, Richard]] | title=Character of Physical Law | publisher=Random House | year=1994 | id=ISBN 0-679-60127-9}}
 
* {{cite book | author=[[Roger Penrose | Penrose, Roger]] | title=Road to Reality: A Complete Guide to the Laws of the Universe | publisher=Knopf | year=2004 | id=ISBN 0-679-45443-8}}
 
* {{cite book | author=[[Brian Greene | Greene, Brian]] | title=[[The Elegant Universe | The Elegant Universe: Superstrings, Hidden Dimensions, and the Quest for the Ultimate Theory]] | publisher=Vintage | year=2000 | id=ISBN 0-375-70811-1}}
 
* {{cite book | author=[[Michio Kaku | Kaku, Michio]] | title=[[Hyperspace (book) | Hyperspace: A Scientific Odyssey Through Parallel Universes, Time Warps, and the 10th Dimension]] | publisher=Anchor | year=1995 | id=ISBN 0-385-47705-8}}
 
* {{cite book | author=Walker, Jearl | title=The Flying Circus of Physics | publisher=Wiley | year=1977 | id=ISBN 0-471-02984-X}}
 
* {{cite book | author=Leggett, Anthony | title=The Problems of Physics | publisher=Oxford University Press | year=1988 | id=ISBN 0-19-289186-3}}
 
* {{cite book | author=Rogers, Eric | title=Physics for the Inquiring Mind: The Methods, Nature, and Philosophy of Physical Science | publisher=Princeton University Press | year=1960 | id=ISBN 0-691-08016-X}}
 
* {{cite book | author=Coward, David | title=An Advanced Guide to Physics | publisher=Bantam | year=1988 | id=ISBN 0-681-08016-X}}
 
=== University-level textbooks ===
 
==== Introductory ====
 
* {{cite book | author=Feynman, Richard; Leighton, Robert; Sands, Matthew | title=[[The Feynman Lectures on Physics | Feynman Lectures on Physics]] | publisher=Addison-Wesley | year=1989 | id=ISBN 0-201-51003-0}}
 
* {{cite book | author=Feynman, Richard | title=Exercises for Feynman Lectures Volumes 1-3 | publisher=Caltech | year= | id=ISBN 2-35648-789-1}}
 
* {{cite book | author=Knight, Randall | title=Physics for Scientists and Engineers: A Strategic Approach | publisher=Benjamin Cummings | year=2004 | id=ISBN 0-8053-8685-8}}
 
* {{cite book | author=Resnick, Robert; Halliday, David; Walker, Jearl | title=Fundamentals of Physics}}
 
* {{cite book | author=Hewitt, Paul | title=Conceptual Physics with Practicing Physics Workbook (9th ed.) | publisher=Addison Wesley | year=2001 | id=ISBN 0-321-05202-1}}
 
* {{cite book | author=Giancoli, Douglas | title=Physics: Principles with Applications (6th ed.) | publisher=Prentice Hall | year=2005 | id=ISBN 0-13-060620-0}}
 
* {{cite book | author=Serway, Raymond A.; Jewett, John W. | title=Physics for Scientists and Engineers (6th ed.) | publisher=Brooks/Cole | year=2004 | id=ISBN 0-534-40842-7}}
 
* {{cite book | author=Tipler, Paul | title=Physics for Scientists and Engineers: Mechanics, Oscillations and Waves, Thermodynamics (5th ed.) | publisher=W. H. Freeman | year=2004 | id=ISBN 0-7167-0809-4}}
 
* {{cite book | author=Tipler, Paul | title=Physics for Scientists and Engineers: Electricity, Magnetism, Light, and Elementary Modern Physics (5th ed.) | publisher=W. H. Freeman | year=2004 | id=ISBN 0-7167-0810-8}}
 
* {{cite book | author=Wilson, Jerry; Buffa, Anthony | title=College Physics (5th ed.) | publisher=Prentice Hall | year=2002 | id=ISBN 0-13-067644-6}}
 
* {{cite book | author=Schiller, Christoph | title=Motion Mountain: The Free Physics Textbook | year=2005 | url=http://www.motionmountain.net/}}
 
* {{cite book | author=H. C. Verma | title=Concepts of Physics | publisher=Bharti Bhavan | year=2005 | id=ISBN 81-7709-187-5}}
 
==== Undergraduate ====
 
* {{cite book | author=Thornton, Stephen T.; Marion, Jerry B. | title=Classical Dynamics of Particles and Systems (5th ed.) | publisher=Brooks Cole | year=2003 | id=ISBN 0-534-40896-6}}
 
* {{cite book | author=Griffiths, David J. | authorlink = David Griffiths | title=Introduction to Electrodynamics (3rd ed.) | publisher=Prentice Hall | year=1998 | id=ISBN 0-13-805326-X}}
 
* {{cite book | author=Wangsness, Roald K. | title=Electromagnetic Fields (2nd ed.) | publisher=Wiley | year=1986 | id=ISBN 0-471-81186-6}}
 
* {{cite book | author=Fowles, Grant R. | title=Introduction to Modern Optics | publisher=Dover Publications | year=1989 | id=ISBN 0-486-65957-7}}
 
* {{cite book | author=Hecht, Eugene | title=Optics (4th ed.) | publisher=Pearson Education | year=2001 | id=ISBN 0-8053-8566-5}}
 
* {{cite book | author=Schroeder, Daniel V. | title=An Introduction to Thermal Physics | publisher=Addison Wesley | year=1999 | id=ISBN 0-201-38027-7}}
 
* {{cite book | author=Kroemer, Herbert; Kittel, Charles | title=Thermal Physics (2nd ed.) | publisher=W. H. Freeman Company | year=1980 | id=ISBN 0-7167-1088-9}}
 
* {{cite book | author=Griffiths, David J. | authorlink = David Griffiths |  title=Introduction to Quantum Mechanics (2nd ed.) | publisher=Prentice Hall | year=2004 | id=ISBN 0-13-805326-X}}
 
* {{cite book | author=Liboff, Richard L. | title=Introductory Quantum Mechanics | publisher=Addison-Wesley | year=2002 | id=ISBN 0-8053-8714-5}}
 
* {{cite book | author=[[David Bohm | Bohm, David]] | title=Quantum Theory | publisher=Dover Publications | year=1989 | id=ISBN 0-486-65969-0}}
 
* {{cite book | author=Eisberg, Robert; Resnick, Robert | title=Quantum Physics of Atoms, Molecules, Solids, Nuclei, and Particles (2nd ed.) | publisher=Wiley | year=1985 | id=ISBN 0-471-87373-X}}
 
* {{cite book | author=Taylor, Edwin F.; [[John Archibald Wheeler | Wheeler, John Archibald]] | title=Spacetime Physics: Introduction to Special Relativity (2nd ed.) | publisher=W.H. Freeman | year=1992 | id=ISBN 0-7167-2327-1}}
 
* {{cite book | author=Taylor, Edwin F.; Wheeler, John Archibald | title=Exploring Black Holes: Introduction to General Relativity | publisher=Addison Wesley | year=2000 | id=ISBN 0-201-38423-X}}
 
* {{cite book | author=Schutz, Bernard F. | title=A First Course in General Relativity | publisher=Cambridge University Press | year=1984 | id=ISBN 0-521-27703-5}}
 
* {{cite book | author=Bergmann, Peter G. | title=Introduction to the Theory of Relativity | publisher=Dover Publications | year=1976 | id=ISBN 0-486-63282-2}}
 
* {{cite book | author=Tipler, Paul; Llewellyn, Ralph | title=Modern Physics (4th ed.) | publisher=W. H. Freeman | year=2002 | id=ISBN 0-7167-4345-0}}
 
* {{cite book | author=Griffiths, David J. | authorlink = David Griffiths |  title=Introduction to Elementary Particles | publisher=Wiley, John & Sons, Inc | year=1987 | id=ISBN 0-471-60386-4}}
 
* {{cite book | author=Perkins, Donald H. | title=Introduction to High Energy Physics | publisher=Cambridge University Press | year=1999 | id=ISBN 0-521-62196-8}}
 
* {{cite book | author=[[Bogdan Povh | Povh, Bogdan]] | title=Particles and Nuclei: An Introduction to the Physical Concepts | publisher=Springer-Verlag | year=1995 | id=ISBN 0-387-59439-6}}
 
* {{cite book | author=Menzel, Donald Howard | title=Mathematical Physics | publisher=Dover Publishications | year=1961 | id=ISBN 0-486-60056-4}}
 
* {{cite book | author=Joos, Georg; Freeman, Ira M. | title=Theoretical Physics | publisher=Dover Publications | year=1987 | id=ISBN 0-486-65227-0}}
 
==== Graduate ====
 
* {{cite book | author=Landau, L. D.; Lifshitz, E. M. | title=Course of Theoretical Physics | publisher=Butterworth-Heinemann | year=1976 | id=ISBN 0-7506-2896-0}}
 
* {{cite book | author=Morse, Philip; Feshbach, Herman | title=Methods of Theoretical Physics | publisher=Feshbach Publishing | year=2005 | id=ISBN 0-9762021-2-3}}
 
* {{cite book | author=Arfken, George B.; Weber, Hans J. | title=Mathematical Methods for Physicists (5th ed.) | publisher=Academic Press | year=2000 | id=ISBN 0-12-059825-6}}
 
* {{cite book | author=Goldstein, Herbert | title=Classical Mechanics | publisher=Addison Wesley | year=2002 | id=ISBN 0-201-65702-3}}
 
* {{cite book | author=Jackson, John D. | title=Classical Electrodynamics (3rd ed.) | publisher=Wiley | year=1998 | id=ISBN 0-471-30932-X}}
 
* {{cite book | author=[[Lev Landau | Landau, L. D.]]; Lifshitz, E. M. | title=Mechanics and Electrodynamics, Vol. 1 | publisher=Franklin Book Company, Inc | year=1972 | id=ISBN 0-08-016739-X}}
 
* {{cite book | author=Huang, Kerson | title=Statistical Mechanics | publisher=Wiley, John & Sons, Inc | year=1990 | id=ISBN 0-471-81518-7}}
 
* {{cite book | author=Merzbacher, Eugen | title=Quantum Mechanics | publisher=Wiley, John & Sons, Inc | year=1998 | id=ISBN 0-471-88702-1}}
 
* {{cite book | author=Peskin, Michael E.; Schroeder, Daniel V. | title=Introduction to Quantum Field Theory | publisher=Perseus Publishing | year=1994 | id=ISBN 0-201-50397-2}}
 
* {{cite book | author=[[Kip Thorne | Thorne, Kip S.]]; Misner, Charles W.; Wheeler, John Archibald | title=Gravitation | publisher=W.H. Freeman | year=1973 | id=ISBN 0-7167-0344-0}}
 
* {{cite book | author=Wald, Robert M. | title=General Relativity | publisher=University of Chicago Press | year=1984 | id=ISBN 0-226-87033-2}}
 
* {{cite book | author=[[Steven Weinberg | Weinberg, Steven]] | title=Gravitation and Cosmology: Principles and Applications of the General Theory of Relativity | publisher=John Wiley & Sons | year=1972 | id=ISBN 0-471-92567-5}}
 
==== History ====
 
* {{cite book | author=Cropper, William H. | title=Great Physicists: The Life and Times of Leading Physicists from Galileo to Hawking | publisher=Oxford University Press | year=2004 | id=ISBN 0-19-517324-4}}
 
* {{cite book | author=[[George Gamow | Gamow, George]] | title=The Great Physicists from Galileo to Einstein | publisher=Dover Publications | year=1988 | id=ISBN 0-486-25767-3}}
 
* {{cite book | author=Heilbron, John L. | title=The Oxford Guide to the History of Physics and Astronomy | publisher=Oxford University Press | year=2005 | id=ISBN 0-19-517198-5}}
 
* {{cite book | author=Weaver, Jefferson H. (editor) | title=The World of Physics | publisher=Simon and Schuster | year=1987 | id=ISBN 0-671-49931-9}} A selection of 56 articles, written by physicists. Commentaries and notes by [[Lloyd Motz]] and Dale McAdoo.
 
== External links ==
 
; General
 
 
* [http://math.ucr.edu/home/baez/physics/ Usenet Physics FAQ]. A [[FAQ]] compiled by sci.physics and other physics newsgroups.
* [http://www.nobel.se/physics Website of the Nobel Prize in Physics].
* [http://www.physicstoday.org Physics Today] - Your daily physics news and research source
 
 
; Organizations
 
* [http://www.aip.org/index.html AIP.org] Website of the [[American Institute of Physics]]
* [http://www.iop.org IOP.org] Website of the [[Institute of Physics]]
* [http://www.aps.org APS.org] Website of the [[American Physical Society]]
* [http://www.spsnational.org SPS National] Website of the [[Society of Physics Students]]
 


{{Physics-footer}}
Although much progress has been made in high-energy, quantum, and astronomical physics, many everyday phenomena, involving complexity, chaos, or turbulence are still poorly understood. Complex problems that seem like they could be solved by a clever application of dynamics and mechanics, such as the formation of sand piles, nodes in trickling [[water]], the shape of water droplets, mechanisms of [[surface tension]] catastrophes, or self-sorting in shaken heterogeneous collections are unsolved. These complex phenomena have received growing attention since the 1970s for several reasons, not least of which has been the availability of modern [[mathematics | mathematical]] methods and [[computers]] which enabled complex systems to be modeled in new ways. The interdisciplinary relevance of complex physics has also increased, as exemplified by the study of turbulence in aerodynamics or the observation of pattern [[formation]] in [[biology | biological]] systems.


{{FundamentalForces}}
Two rapidly-growing applied fields to which physics makes contributions are [[biophysics]] and [[nanotechnology]].


{{Natural sciences-footer}}
== Attribution ==
<!-- The parental lineage categories are incomplete without physics listed in them. Physics was a blatant ommission, and makes those lists look rather unprofessional, considering the other fields are listed. -->
{{WPAttribution}}


[[Category:Natural sciences]]
== References ==[[Category:Suggestion Bot Tag]]
[[Category:Physical sciences]]
[[Category:Physics |Physics ]]
[[Category:School subjects]]

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Physics (from the Greek physikos, nature) is the science of nature at its most fundamental form, and is the foundation of the natural sciences. From quarks to galaxies, from individual atoms to macroscopic biological systems, physicists study a wide range of physical phenomena.

Key Areas of Physics

Over the course of time, physical phenomena have been grouped together under specific branches. While it is widely believed that the whole of physics can be considered within a single unified theory, including all phenomena both at the level of quantum mechanics and on a scale as wide as the universe, this has not yet been proven. This is made clear especially by the fact that the current models developed by the different branches of physics give contradictory solutions to the same problem when they are combined. For example, the unification of general relativity and quantum mechanics has so far proven impossible due to creation of infinite values when looking at some properties of systems of subatomic particles. Nevertheless, these are considered as problems "to be solved". The central branches of physics are:

  • Classical mechanics is a model of the physics of forces acting upon bodies, and the motion of those bodies. As opposed to quantum mechanics, classical mechanics is deterministic. Classical mechanics is usually regarded as a limit of quantum mechanics, although this has not been proven in general. Classical mechanics can be divided into two parts:
Newtonian, after Isaac Newton and his laws of motion. This can be more generally formulated in Lagrangian mechanics, after Joseph-Louis Lagrange.
Relativistic, due to Albert Einstein and his theory of relativity. This includes both special and general relativity, and addresses regimes in which Newtonian mechanics is no longer valid: when relative speeds of object are comparable to the speed of light, and for motion occurring near very massive objects.
  • Quantum mechanics is the branch of physics treating atomic and 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 quanta. Quantum theory typically permits only probable or statistical calculation of the observed features of particles, understood in terms of wave functions. Quantum mechanics itself has several levels of approximation.
  • Electromagnetism, or electromagnetic theory, is the physics of the electromagnetic field: a field, encompassing all of space, which exerts a force on those particles that possess the property of electric charge, and is in turn affected by the presence and motion of such particles. Electromagnetism encompasses various real-world electromagnetic phenomena. Electromagnetism, as taught in a typical undergraduate college physics curriculum, can be divided into several areas:
Electrostatics, the study of charged objects, the forces between them, and electric fields and potentials for charged objects at rest.
Magnetism, the study of forces between moving charged objects, the magnetic field, and electromagnetic induction.
Electric circuits, the study of electric potential difference and electric currents for various arrangements of circuit components.
Optics and electromagnetic waves, the study of how time-varying electromagnetic fields propagate through space.

Research and fields within physics

Physics can be subdivided in a variety of different manners; for teaching, for historical purposes, or for research purposes. Contemporary research in physics is divided into many distinct subfields. An incomplete listing includes:

  • Condensed matter physics is the study of the condensed phases of matter, solid and liquid, and how the properties of matter in these phases arise from the properties and mutual interactions of the constituent atoms. More physicists study condensed matter physics than any other field.
  • Particle physics, also known as "high-energy physics". This branch is concerned with the properties of subatomic particles much smaller than atoms, including the elementary particles from which all other units of matter are constructed.
  • Astrophysics attempts to explain the physical workings of celestial objects and phenomena.
  • Atomic, molecular, and optical physics (AMO physics) deals with the behavior of individual atoms and molecules, including the ways in which they absorb and emit light. Molecular physics is sometimes also considered a branch of chemical physics. Laser science may be considered a subfield of AMO or as a separate field.
  • Materials physics is the study of various physical properties of materials. Classifications of physical properties include, but are not limited to, thermal, electronic, magnetic, optical, and mechanical.
  • Computational physics deals with numerically (as opposed to analytically) solving the equations that govern physical systems.

A number of fields of physics overlap strongly with other sciences: Biophysics, Physical chemistry and Geophysics overlap considerably with biology, chemistry and geography, but the focus is on the application of physics and physical techniques to problems within the other field.

Classical and quantum physics

Further information: Classical physics, Quantum physics, Modern physics, Semiclassical

The distinction between classical and quantum theories is important in physics. Classical theories are generally valid despite not considering the quantum nature of things, but are ultimately an approximation to a deeper quantized truth; this approximation typically breaks down at extreme scales, particularly the subatomic. Some fundamental classical theories, such as relativity do not yet have full analogous quantum theories.

Both classical and quantum physics are active areas of research. However, there exist problems in physics in which classical and quantum aspects must be combined to attain some approximation or limit that may acquire several forms as the passage from classical to quantum mechanics is often difficult — such problems are termed semiclassical.

Theoretical and experimental physics

Most individual physicists specialize in either theoretical physics or experimental physics. There have been a few exceptions, such as great Italian physicist Enrico Fermi (1901–1954), who made fundamental contributions to both theory and experimentation.

Roughly speaking, theorists seek to develop theories, through mathematical and computational models, that can both describe and interpret existing experimental results and successfully predict future results, while experimentalists devise and perform experiments to explore new phenomena and test theoretical predictions. Although theory and experiment can be developed separately, they are strongly dependent on each other. Progress in physics frequently comes about when experimentalists make a discovery that existing theories cannot account for, necessitating the formulation of new theories, or when theorists make predictions that experimentalists test.

Physics and Other disciplines

Physics finds applications throughout the other natural sciences as they regard the basic principles of nature. Physics is often said to be the "fundamental science", because the other sciences deal with material systems that obey the laws of physics. For example, chemistry is the science of matter (such as atoms and molecules) and the chemical substances that they form in the bulk. The structure, reactivity, and properties of a chemical compound are determined by the properties of the underlying molecules, which can be described by areas of physics such as quantum mechanics (in the applied subfield of quantum chemistry), thermodynamics, and electromagnetism.

Physics is closely related to mathematics, which provides the logical framework in which physical laws can be precisely formulated and their predictions quantified. Physical definitions, models and theories are invariably expressed using mathematical relations. A key difference between physics and mathematics is that because physics is ultimately concerned with descriptions of the material world, it tests its theories by observations (called experiments), whereas mathematics does not have such requirements. The distinction, however, is not always clear-cut. This large area of research intermediate between physics and mathematics is known as mathematical physics.

Physics is also closely related to engineering and technology. For instance, electrical engineering is the study of the practical application of electromagnetism. Statics, a subfield of mechanics, is responsible for the building of bridges. Further, physicists, or practitioners of physics, invent and design processes and devices, such as the transistor, whether in basic or applied research. Experimental physicists design and perform experiments with particle accelerators, nuclear reactors, telescopes, barometers, synchrotrons, cyclotrons, spectrometers, lasers, and other equipment.

Current research directions

Research in physics is progressing constantly on a large number of fronts, and is likely to do so for the foreseeable future. Some current directions include:

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 these 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, have not yet borne fruit. The current leading candidates are M-theory, superstring theory and loop quantum gravity.

Many astronomical and cosmological phenomena have yet to be satisfactorily explained, including the existence of GZK paradox | ultra-high energy cosmic rays, the baryon asymmetry, the acceleration of the universe and the anomalous rotation rates of galaxies.

Although much progress has been made in high-energy, quantum, and astronomical physics, many everyday phenomena, involving complexity, chaos, or turbulence are still poorly understood. Complex problems that seem like they could be solved by a clever application of dynamics and mechanics, such as the formation of sand piles, nodes in trickling water, the shape of water droplets, mechanisms of surface tension catastrophes, or self-sorting in shaken heterogeneous collections are unsolved. These complex phenomena have received growing attention since the 1970s for several reasons, not least of which has been the availability of modern mathematical methods and computers which enabled complex systems to be modeled in new ways. The interdisciplinary relevance of complex physics has also increased, as exemplified by the study of turbulence in aerodynamics or the observation of pattern formation in biological systems.

Two rapidly-growing applied fields to which physics makes contributions are biophysics and nanotechnology.

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== References ==

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