Editing Photon (section)

{{short description|Elementary particle or quantum of light}}
{{About|the elementary particle or quantum of light}}
{{Infobox particle
 |bgcolour = 
 |name = Photon
 |num_types =
 |composition = [[Elementary particle]]
 |statistics = [[Bosonic]]
 |group = [[Gauge boson]]
 |generation = 
 |interaction = [[Electromagnetism|Electromagnetic]], [[gravity]]
 |theorized = [[Albert Einstein]] (1905) <br/> The name "photon" is generally attributed to [[Gilbert N. Lewis]] (1926)
 |discovered = 
 |symbol={{math|γ}}
 |mass={{nowrap|0 (theoretical value)}}<br/>
   {{nowrap|&lt; {{val|1|e=-18|ul=eV/c2}} (experimental limit)}}<ref name="Particle_table_2009">{{cite journal
 |last1           = Amsler
 |first1          = C.
 |display-authors = etal
 |collaboration   = [[Particle Data Group]]
 |year            = 2008
 |title           = Review of Particle Physics: Gauge and Higgs bosons
 |journal         = [[Physics Letters B]]
 |volume          = 667
 |issue           = 1
 |page            = 1
 |bibcode         = 2008PhLB..667....1A
 |doi             = 10.1016/j.physletb.2008.07.018
 |hdl             = 1854/LU-685594
 |s2cid           = 227119789
 |hdl-access      = free
 |url             = http://pdg.lbl.gov/2009/tables/rpp2009-sum-gauge-higgs-bosons.pdf
 |access-date     = 2010-04-09
 |archive-date    = 2018-12-25
 |archive-url     = https://web.archive.org/web/20181225235527/http://pdg.lbl.gov/2009/tables/rpp2009-sum-gauge-higgs-bosons.pdf%0A
 |url-status      = live
}}</ref>
 |mean_lifetime = Stable<ref name="Particle_table_2009"/>
 |decay_particle = 
 |electric_charge = 0
<br/>{{nowrap|&lt; {{val|1|e=-35|ul=e}}}}{{px2}}<ref name="Particle_table_2009"/>
 |color_charge = No
 |spin = 1&nbsp;[[reduced Planck constant|''ħ'']]
 |num_spin_states = +1&nbsp;''ħ'',&nbsp; −1&nbsp;''ħ''
 |parity = −1<ref name="Particle_table_2009"/>
 |g_parity = 
 |c_parity = −1<ref name="Particle_table_2009"/>
 |r_parity = 
 |condensed_symmetries=''[[Weak isospin|I]]''(''[[Total angular momentum|J]]''<sup> [[Parity (physics)|P]][[C parity|C]]</sup>) = 0, 1 (1<sup>−−</sup>)<ref name="Particle_table_2009"/>
}}

A '''photon''' ({{etymology|grc|''{{wikt-lang|grc|φῶς}}'', ''{{wikt-lang|grc|φωτός}}'' ({{grc-transl|φῶς, φωτός}})|light}}) is an [[elementary particle]] that is a [[quantum]] of the [[electromagnetic field]], including [[electromagnetic radiation]] such as [[light]] and [[radio wave]]s, and the [[force carrier]] for the [[electromagnetic force]]. Photons are [[massless particle]]s that can move no faster than the [[speed of light]] measured in vacuum. The photon belongs to the class of [[boson]] particles.

As with other elementary particles, photons are best explained by [[quantum mechanics]] and exhibit [[wave–particle duality]], their behavior featuring properties of both [[wave]]s and [[particle]]s.<ref>{{cite book |last1=Joos |first1=George  |date=1951 |title=Theoretical Physics |page=679 |publisher=Blackie and Son Limited  |location=London and Glasgow }}</ref> The modern photon concept originated during the first two decades of the 20th century with the work of [[Albert Einstein]], who built upon the research of [[Max Planck]]. While Planck was trying to explain how [[matter]] and electromagnetic radiation could be in [[thermal equilibrium]] with one another, he proposed that the energy stored within a [[material]] object should be regarded as composed of an [[integer]] number of discrete, equal-sized parts. To explain the [[photoelectric effect]], Einstein introduced the idea that light itself is made of discrete units of energy. In 1926, [[Gilbert N. Lewis]] popularized the term ''photon'' for these energy units.<ref name="www.aps.org">{{cite web |url=https://www.aps.org/publications/apsnews/201212/physicshistory.cfm |title=December 18, 1926: Gilbert Lewis coins "photon" in letter to Nature |website=www.aps.org |language=en |access-date=2019-03-09 |archive-date=2019-05-02 |archive-url=https://web.archive.org/web/20190502171300/https://www.aps.org/publications/apsnews/201212/physicshistory.cfm |url-status=live }}</ref><ref>{{cite web |url=https://www.atomicheritage.org/profile/gilbert-n-lewis |title=Gilbert N. Lewis |website=Atomic Heritage Foundation |language=en |access-date=2019-03-09 |archive-date=2015-04-16 |archive-url=https://web.archive.org/web/20150416123637/https://www.atomicheritage.org/profile/gilbert-n-lewis |url-status=live }}</ref><ref name="kragh">{{cite arXiv |last=Kragh |first=Helge |date=2014 |title=Photon: New light on an old name |eprint=1401.0293 |class=physics.hist-ph }}</ref> Subsequently, many other experiments validated Einstein's approach.<ref name="compton-lecture">{{cite book |last1=Compton |first1=Arthur H. |title=From Nobel Lectures, Physics 1922–1941 |publisher=Elsevier Publishing Company |year=1965 |location=Amsterdam |chapter=X-rays as a branch of optics |orig-year=12 Dec 1927 |chapter-url=https://www.nobelprize.org/uploads/2018/06/compton-lecture.pdf |access-date=3 January 2019 |archive-date=12 May 2024 |archive-url=https://web.archive.org/web/20240512231537/https://www.nobelprize.org/uploads/2018/06/compton-lecture.pdf |url-status=live }}</ref><ref>{{cite journal |last1=Kimble |first1=H.J. |last2=Dagenais |first2=M. |last3=Mandel |first3=L. |year=1977 |title=Photon Anti-bunching in Resonance Fluorescence |url=https://authors.library.caltech.edu/6051/1/KIMprl77.pdf |journal=[[Physical Review Letters]] |volume=39 |issue=11 |pages=691–695 |bibcode=1977PhRvL..39..691K |doi=10.1103/PhysRevLett.39.691 |access-date=2019-01-03 |archive-date=2020-11-25 |archive-url=https://web.archive.org/web/20201125123348/https://authors.library.caltech.edu/6051/1/KIMprl77.pdf |url-status=live }}</ref><ref>{{cite journal |last1=Grangier |first1=P. |last2=Roger |first2=G. |last3=Aspect |first3=A. |year=1986 |title=Experimental Evidence for a Photon Anticorrelation Effect on a Beam Splitter: A New Light on Single-Photon Interferences |journal=[[EPL (journal)|Europhysics Letters]] |volume=1 |issue=4 |pages=173–179 |bibcode=1986EL......1..173G |citeseerx=10.1.1.178.4356 |doi=10.1209/0295-5075/1/4/004 |s2cid=250837011 }}</ref>

In the [[Standard Model]] of [[particle physics]], photons and other elementary particles are described as a necessary consequence of physical laws having a certain [[Symmetry (physics)|symmetry]] at every point in [[spacetime]]. The intrinsic properties of particles, such as [[electric charge|charge]], [[invariant mass|mass]], and [[Spin (physics)|spin]], are determined by [[gauge symmetry]]. The photon concept has led to momentous advances in experimental and theoretical physics, including [[laser]]s, [[Bose–Einstein condensation]], [[quantum field theory]], and the [[probability amplitude|probabilistic interpretation]] of quantum mechanics. It has been applied to [[photochemistry]], [[two-photon excitation microscopy|high-resolution microscopy]], and [[fluorescence resonance energy transfer|measurements of molecular distances]]. Moreover, photons have been studied as elements of [[quantum computer]]s, and for applications in [[optical imaging]] and [[optical communication]] such as [[quantum cryptography]].