Editing Photon (section)

== Historical development ==
{{Main|Light}}
[[File:Young Diffraction.png|thumb|[[Thomas Young (scientist)|Thomas Young]]'s [[double-slit experiment]] in 1801 showed that light can act as a [[wave]], helping to invalidate early [[elementary particle|particle]] theories of light.<ref name=Halliday/>{{rp|964}}]]
In most theories up to the eighteenth century, light was pictured as being made of particles. Since [[Subatomic particle|particle]] models cannot easily account for the [[refraction]], [[diffraction]] and [[birefringence]] of light, wave theories of light were proposed by [[René Descartes]] (1637),<ref>{{cite book |last=Descartes |first=René |url={{google books |plainurl=y |id=difXAAAAMAAJ}} |title=Discours de la méthode (Discourse on Method) |publisher=Imprimerie de Ian Maire |year=1637 |isbn=978-0-268-00870-3 |language=fr |author-link=René Descartes}}</ref> [[Robert Hooke]] (1665),<ref>{{cite book |last=Hooke |first=Robert |url=http://digital.library.wisc.edu/1711.dl/HistSciTech.HookeMicro |title=Micrographia: or some physiological descriptions of minute bodies made by magnifying glasses with observations and inquiries thereupon&nbsp;... |publisher=[[Royal Society of London]] |year=1667 |isbn=978-0-486-49564-4 |location=London, UK |language=en-uk |author-link=Robert Hooke |access-date=2006-09-26 |archive-date=2008-12-02 |archive-url=https://web.archive.org/web/20081202101129/http://digital.library.wisc.edu/1711.dl/HistSciTech.HookeMicro |url-status=live }}</ref> and [[Christiaan Huygens]] (1678);<ref>{{cite book |last=Huygens |first=Christiaan |title=Traité de la lumière |title-link=Traité de la lumière |year=1678 |language=fr |author-link=Christiaan Huygens}}. An [[gutenberg:14725|English translation]] is available from [[Project Gutenberg]]</ref> however, particle models remained dominant, chiefly due to the influence of [[Isaac Newton]].<ref name="Newton1730">{{cite book |last=Newton |first=Isaac |url={{google books |plainurl=y |id=bSiTKcLf07UC}} |title=Opticks |publisher=Dover Publications |year=1952 |isbn=978-0-486-60205-9 |edition=4th |location=Dover, New York |at=Book II, Part III, Propositions XII–XX; Queries 25–29 |language=en |author-link=Isaac Newton |orig-year=1730}}</ref> In the early 19th century, [[Thomas Young (scientist)|Thomas Young]] and [[Augustin-Jean Fresnel|August Fresnel]] clearly demonstrated the [[Interference (wave propagation)|interference]] and diffraction of light, and by 1850 wave models were generally accepted.<ref>{{cite journal |last=Buchwald |first=J. Z. |url={{google books |plainurl=y |id=EbDw1lV_MKsC}} |title=The Rise of the Wave Theory of Light: Optical theory and experiment in the early nineteenth century |journal=Physics Today |publisher=University of Chicago Press |year=1989 |isbn=978-0-226-07886-1 |volume=43 |pages=78–80 |language=en-us |bibcode=1990PhT....43d..78B |doi=10.1063/1.2810533 |oclc=18069573 |issue=4}}</ref> [[James Clerk Maxwell]]'s 1865 [[Maxwell's equations|prediction]]<ref name="maxwell">{{cite journal |last=Maxwell |first=James Clerk |author-link=James Clerk Maxwell |year=1865 |title=A Dynamical Theory of the Electromagnetic Field |journal=[[Philosophical Transactions of the Royal Society]] |volume=155 |pages=459–512 |bibcode=1865RSPT..155..459M |doi=10.1098/rstl.1865.0008 |s2cid=186207827 |title-link=A dynamical theory of the electromagnetic field}} This article followed a presentation by Maxwell on 8&nbsp;December 1864 to the Royal Society.</ref> that light was an electromagnetic wave – which was confirmed experimentally in 1888 by [[Heinrich Hertz]]'s detection of [[radio|radio waves]]<ref name="hertz">{{cite journal |last=Hertz |first=Heinrich |author-link=Heinrich Hertz |year=1888 |title=Über Strahlen elektrischer Kraft |journal=Sitzungsberichte der Preussischen Akademie der Wissenschaften |language=de |volume=1888 |pages=1297–1307 |place=Berlin, Deutschland}}</ref> – seemed to be the final blow to particle models of light.

[[File:Light-wave.svg|thumb|upright=1.25|In 1900, [[James Clerk Maxwell|Maxwell's]] [[Maxwell's equations|theoretical model of light]] as oscillating [[electric field|electric]] and [[magnetic field]]s seemed complete. However, several observations could not be explained by any wave model of [[electromagnetic radiation]], leading to the idea that light-energy was packaged into ''quanta'' described by {{nobr| {{mvar|E {{=}} hν}}.}} Later experiments showed that these light-quanta also carry momentum and, thus, can be considered [[elementary particle|particles]]: The ''photon'' concept was born, leading to a deeper understanding of the electric and magnetic fields themselves.]]

The [[electromagnetic wave equation|Maxwell wave theory]], however, does not account for ''all'' properties of light. The Maxwell theory predicts that the energy of a light wave depends only on its [[intensity (physics)|intensity]], not on its [[frequency]]; nevertheless, several independent types of experiments show that the energy imparted by light to atoms depends only on the light's frequency, not on its intensity. For example, [[photochemistry|some chemical reactions]] are provoked only by light of frequency higher than a certain threshold; light of frequency lower than the threshold, no matter how intense, does not initiate the reaction. Similarly, electrons can be ejected from a metal plate by shining light of sufficiently high frequency on it (the [[photoelectric effect]]); the energy of the ejected electron is related only to the light's frequency, not to its intensity.<ref>"Frequency-dependence of luminiscence" pp. 276ff., §1.4 "photoelectric effect" in {{harvnb|Alonso|Finn|1968}}.</ref>

At the same time, investigations of [[black-body radiation]] carried out over four decades (1860–1900) by various researchers<ref name="Wien1911">{{cite web |last=Wien |first=W. |author-link=Wilhelm Wien |year=1911 |url=http://nobelprize.org/nobel_prizes/physics/laureates/1911/wien-lecture.html |title=Wilhelm Wien Nobel Lecture |website=nobelprize.org |access-date=2006-08-25 |archive-date=2011-07-15 |archive-url=https://web.archive.org/web/20110715190243/http://nobelprize.org/nobel_prizes/physics/laureates/1911/wien-lecture.html |url-status=live }}</ref> culminated in [[Max Planck]]'s [[Planck constant|hypothesis]]<ref name="Planck1901">
{{cite journal |last=Planck |first=Max |author-link=Max Planck |year=1901 |title=Über das Gesetz der Energieverteilung im Normalspectrum |journal=[[Annalen der Physik]] |language=de |volume=4 |issue=3 |pages=553–563 |bibcode=1901AnP...309..553P |doi=10.1002/andp.19013090310 |doi-access=free}} [https://web.archive.org/web/20080418002757/http://dbhs.wvusd.k12.ca.us/webdocs/Chem-History/Planck-1901/Planck-1901.html English translation]</ref><ref name="Planck1918">{{cite web |last=Planck |first=Max |author-link=Max Planck |year=1920 |title=Max Planck's Nobel Lecture |url=http://nobelprize.org/nobel_prizes/physics/laureates/1918/planck-lecture.html |publisher=nobelprize.org |access-date=2006-08-25 |archive-date=2011-07-15 |archive-url=https://web.archive.org/web/20110715190331/http://nobelprize.org/nobel_prizes/physics/laureates/1918/planck-lecture.html |url-status=live }}</ref> that the energy of ''any'' system that absorbs or emits electromagnetic radiation of frequency {{mvar|ν}} is an integer multiple of an energy quantum {{nobr| {{mvar|E}} {{=}} {{mvar|hν}} .}} As shown by [[Albert Einstein]],<ref name="Einstein1905"/><ref name="Einstein1909"/> some form of energy quantization ''must'' be assumed to account for the thermal equilibrium observed between matter and [[electromagnetic radiation]]; for this explanation of the photoelectric effect, Einstein received the 1921 [[Nobel Prize]] in physics.<ref>Presentation speech by [[Svante Arrhenius]] for the 1921 Nobel Prize in Physics, December 10, 1922. [http://nobelprize.org/nobel_prizes/physics/laureates/1921/press.html Online text] {{Webarchive|url=https://web.archive.org/web/20110904232203/http://www.nobelprize.org/nobel_prizes/physics/laureates/1921/press.html |date=2011-09-04 }} from [nobelprize.org], The Nobel Foundation 2008. Access date 2008-12-05.</ref>

Since the Maxwell theory of light allows for all possible energies of electromagnetic radiation, most physicists assumed initially that the energy quantization resulted from some unknown constraint on the matter that absorbs or emits the radiation. In 1905, Einstein was the first to propose that energy quantization was a property of electromagnetic radiation itself.<ref name="Einstein1905"/> Although he accepted the validity of Maxwell's theory, Einstein pointed out that many anomalous experiments could be explained if the ''energy'' of a Maxwellian light wave were localized into point-like quanta that move independently of one another, even if the wave itself is spread continuously over space.<ref name="Einstein1905" /> In 1909<ref name="Einstein1909">{{cite journal |last=Einstein |first=Albert |author-link=Albert Einstein |year=1909 |title=Über die Entwicklung unserer Anschauungen über das Wesen und die Konstitution der Strahlung |url=http://www.ekkehard-friebe.de/EINSTEIN-1909-P.pdf |journal=[[Physikalische Zeitschrift]] |language=de |volume=10 |pages=817–825 |access-date=2010-08-25 |archive-date=2011-06-07 |archive-url=https://web.archive.org/web/20110607135402/http://www.ekkehard-friebe.de/EINSTEIN-1909-P.pdf |url-status=live }} An [[wikisource:Translation:The Development of Our Views on the Composition and Essence of Radiation|English translation]] is available from [[Wikisource]].</ref> and 1916,<ref name="Einstein1916b">{{cite journal |last=Einstein |first=Albert |author-link=Albert Einstein |year=1916 |title=Zur Quantentheorie der Strahlung |journal=Mitteilungen der Physikalischen Gesellschaft zu Zürich |language=de |volume=16 |page=47}} Also ''Physikalische Zeitschrift'' (in German), '''18''', 121–128 (1917).</ref> Einstein showed that, if [[Planck's law]] regarding black-body radiation is accepted, the energy quanta must also carry [[momentum]] {{nobr|{{mvar| p {{=}} {{sfrac| h | λ }} }},}} making them full-fledged particles. 
[[File:Bohr-atom-PAR.svg|thumb|Up to 1923, most physicists were reluctant to accept that light itself was quantized. Instead, they tried to explain photon behaviour by quantizing only ''matter'', as in the [[Bohr model]] of the [[hydrogen atom]] (shown here). Even though these semiclassical models were only a first approximation, they were accurate for simple systems and they led to [[quantum mechanics]].]]

As recounted in [[Robert Millikan]]'s 1923  Nobel lecture, Einstein's 1905 predicted energy relationship was verified experimentally by 1916 but the local concept of the quanta remained unsettled.<ref name="Millikan1923">{{cite web |last=Millikan |first=Robert A. |author-link=Robert Millikan |year=1924 |title=Robert A. Millikan's Nobel Lecture |url=http://nobelprize.org/nobel_prizes/physics/laureates/1923/millikan-lecture.html |access-date=2006-08-25 |archive-date=2011-07-15 |archive-url=https://web.archive.org/web/20110715190254/http://nobelprize.org/nobel_prizes/physics/laureates/1923/millikan-lecture.html |url-status=live }}</ref>
Most physicists were reluctant to believe that electromagnetic radiation itself might be particulate and thus an example of wave-particle duality.<ref>{{cite journal |last=Hendry |first=J. |year=1980 |title=The development of attitudes to the wave–particle duality of light and quantum theory, 1900–1920 |journal=[[Annals of Science]] |volume=37 |issue=1 |pages=59–79 |doi=10.1080/00033798000200121}}</ref> 
Then in 1922 [[Arthur Compton]] experiment<ref name="Compton1923">{{cite journal |last=Compton |first=Arthur |author-link=Arthur Compton |year=1923 |title=A quantum theory of the scattering of X-rays by light elements |url=https://history.aip.org/history/exhibits/gap/Compton/Compton.html#compton1 |journal=[[Physical Review]] |language=en |volume=21 |issue=5 |pages=483–502 |bibcode=1923PhRv...21..483C |doi=10.1103/PhysRev.21.483 |doi-access=free |access-date=2020-11-08 |archive-date=2018-01-29 |archive-url=https://web.archive.org/web/20180129004433/https://history.aip.org/history/exhibits/gap/Compton/Compton.html#compton1 |url-status=live }}</ref> showed that photons carried momentum proportional to their [[wave number]] (1922) in an experiment now called [[Compton scattering]] that appeared to clearly support a localized quantum model. At least for Millikan, this settled the matter.<ref name="Millikan1923"/> Compton received the Nobel Prize in 1927 for his scattering work.

Even after Compton's experiment, [[Niels Bohr]], [[Hendrik Anthony Kramers|Hendrik Kramers]] and [[John C. Slater|John Slater]] made one last attempt to preserve the Maxwellian continuous electromagnetic field model of light, the so-called [[BKS theory]].<ref name="Bohr1924">{{cite journal |last1=Bohr |first1=Niels |author-link=Niels Bohr |last2=Kramers |first2=Hendrik Anthony |author2-link=Hendrik Anthony Kramers |last3=Slater |first3=John C. |author3-link=John C. Slater |year=1924 |title=The Quantum Theory of Radiation |journal=[[Philosophical Magazine]] |volume=47 |issue=281 |pages=785–802 |doi=10.1080/14786442408565262}} Also ''[[European Physical Journal|Zeitschrift für Physik]]'' (in German), '''24''', p. 69 (1924).</ref> An important feature of the BKS theory is how it treated the [[conservation of energy]] and the [[conservation of momentum]]. In the BKS theory, energy and momentum are only conserved on the average across many interactions between matter and radiation. However, refined Compton experiments showed that the conservation laws hold for individual interactions.<ref>{{Cite journal|last=Howard |first=Don |date=December 2004|title=Who Invented the "Copenhagen Interpretation"? A Study in Mythology |journal=[[Philosophy of Science (journal)|Philosophy of Science]] |language=en |volume=71 |issue=5 |pages=669–682 |doi=10.1086/425941 |issn=0031-8248 |jstor=10.1086/425941 |s2cid=9454552}}</ref> Accordingly, Bohr and his co-workers gave their model "as honorable a funeral as possible".<ref name="Pais1982"/> Nevertheless, the failures of the BKS model inspired [[Werner Heisenberg]] in his development of [[matrix mechanics]].<ref name="Heisenberg1932">{{cite web |last=Heisenberg |first=Werner |author-link=Werner Heisenberg |year=1933 |title=Heisenberg Nobel lecture |url=http://nobelprize.org/nobel_prizes/physics/laureates/1932/heisenberg-lecture.html |access-date=2006-09-11 |archive-date=2011-07-19 |archive-url=https://web.archive.org/web/20110719053050/http://nobelprize.org/nobel_prizes/physics/laureates/1932/heisenberg-lecture.html |url-status=live }}</ref>

By the late 1920, the pivotal question was how to unify Maxwell's wave theory of light with its experimentally observed particle nature. The answer to this question occupied Albert Einstein for the rest of his life,<ref name="Pais1982">{{cite book |last=Pais |first=A. |author-link=Abraham Pais |year=1982 |title=Subtle is the Lord: The science and the life of Albert Einstein |url=https://archive.org/details/subtleislordscie00pais |publisher=Oxford University Press |isbn=978-0-19-853907-0}}</ref> and was solved in [[quantum electrodynamics]] and its successor, the [[Standard Model]]. (See ''{{section link||Quantum field theory}}'' and ''{{section link||As a gauge boson}}'', below.)

A few physicists persisted<ref name="Mandel1976">
{{cite book |last=Mandel |first=Leonard |title=II the Case for and Against Semiclassical Radiation Theory |journal=Progess in Optics |publisher=North-Holland |year=1976 |isbn=978-0-444-10806-7 |editor=Wolf |editor-first=E. |series=[[Progress in Optics]] |volume=13 |pages=27–69 |language=en |bibcode=1976PrOpt..13...27M |doi=10.1016/S0079-6638(08)70018-0 |author-link=Leonard Mandel}}</ref> in developing semiclassical models in which electromagnetic radiation is not quantized, but matter appears to obey the laws of [[quantum mechanics]]. Although the evidence from chemical and physical experiments for the existence of photons was overwhelming by the 1970s, this evidence could not be considered as ''absolutely'' definitive; since it relied on the interaction of light with matter, and a sufficiently complete theory of matter could in principle account for the evidence. 

In the 1970s and 1980s photon-correlation experiments definitively demonstrated quantum photon effects.
These experiments produce results that cannot be explained by any classical theory of light, since they involve anticorrelations that result from the [[measurement in quantum mechanics|quantum measurement process]]. In 1974, the first such experiment was carried out by Clauser, who reported a violation of a classical [[Cauchy–Schwarz inequality]]. In 1977, Kimble ''et al.'' demonstrated an analogous anti-bunching effect of photons interacting with a beam splitter; this approach was simplified and sources of error eliminated in the photon-anticorrelation experiment of Grangier, Roger, & Aspect (1986);<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 |doi=10.1209/0295-5075/1/4/004 |bibcode=1986EL......1..173G |citeseerx=10.1.1.178.4356|s2cid=250837011 }}</ref> This work is reviewed and simplified further in Thorn, Neel, ''et al.'' (2004).<ref>{{cite journal |last1=Thorn |first1=J.J. |last2=Neel |first2=M.S. |last3=Donato |first3=V.W. |last4=Bergreen |first4=G.S. |last5=Davies |first5=R.E. |last6=Beck |first6=M. |year=2004 |title=Observing the quantum behavior of light in an undergraduate laboratory |journal=[[American Journal of Physics]] |volume=72 |issue=9 |pages=1210–1219 |doi=10.1119/1.1737397 |bibcode=2004AmJPh..72.1210T |url=http://people.whitman.edu/~beckmk/QM/grangier/Thorn_ajp.pdf |access-date=2009-06-29 |archive-date=2016-02-01 |archive-url=https://web.archive.org/web/20160201214040/http://people.whitman.edu/~beckmk/QM/grangier/Thorn_ajp.pdf |url-status=live }}</ref>