Editing Photon (section)

=== Experimental checks on photon mass ===
Current commonly accepted physical theories imply or assume the photon to be strictly massless. If photons were not purely massless, their speeds would vary with frequency, with lower-energy (redder) photons moving slightly slower than higher-energy photons. Relativity would be unaffected by this; the so-called speed of light, ''c'', would then not be the actual speed at which light moves, but a constant of nature which is the [[upper bound]] on speed that any object could theoretically attain in spacetime.<ref>{{cite journal|author=Mermin, David|title=Relativity without light|doi=10.1119/1.13917|journal=American Journal of Physics|date=February 1984|volume=52|issue=2|pages=119–124|bibcode=1984AmJPh..52..119M }}</ref> Thus, it would still be the speed of spacetime ripples ([[gravitational waves]] and [[graviton]]s), but it would not be the speed of photons.

If a photon did have non-zero mass, there would be other effects as well. [[Coulomb's law]] would be modified and the [[electromagnetic field]] would have an extra physical [[degree of freedom]]. These effects yield more sensitive experimental probes of the photon mass than the frequency dependence of the speed of light. If Coulomb's law is not exactly valid, then that would allow the presence of an [[electric field]] to exist within a hollow conductor when it is subjected to an external electric field. This provides a means for precision [[Tests of electromagnetism|tests of Coulomb's law]].<ref>{{cite journal|last1=Plimpton|first1=S.|last2=Lawton|first2=W.|title=A Very Accurate Test of Coulomb's Law of Force Between Charges|journal=Physical Review|volume=50|page=1066|year=1936|doi=10.1103/PhysRev.50.1066|bibcode=1936PhRv...50.1066P|issue=11 }}</ref> A null result of such an experiment has set a limit of {{nowrap|''m'' ≲ {{val|e=-14|u=eV/c2}}}}.<ref>{{cite journal|last1=Williams|first1=E.|last2=Faller|first2=J.|last3=Hill|first3=H.|title=New Experimental Test of Coulomb's Law: A Laboratory Upper Limit on the Photon Rest Mass|journal=Physical Review Letters|volume=26|page=721|year=1971|doi=10.1103/PhysRevLett.26.721|bibcode=1971PhRvL..26..721W|issue=12}}</ref>

Sharper upper limits on the mass of light have been obtained in experiments designed to detect effects caused by the galactic [[magnetic vector potential|vector potential]]. Although the galactic vector potential is large because the galactic [[magnetic field]] exists on great length scales, only the magnetic field would be observable if the photon is massless. In the case that the photon has mass, the mass term {{sfrac|1|2}}''m''{{sup|2}}''A''{{sub|''μ''}}''A''{{sup|''μ''}} would affect the galactic plasma. The fact that no such effects are seen implies an upper bound on the photon mass of {{nowrap|''m'' &lt; {{val|3|e=-27|u=eV/c2}}}}.<ref>{{cite journal |last1=Chibisov |first1=G. V. |year=1976 |title=Astrophysical upper limits on the photon rest mass |journal=Soviet Physics Uspekhi |volume=19 |issue=7 |page=624 |bibcode=1976SvPhU..19..624C |doi=10.1070/PU1976v019n07ABEH005277}}</ref> The galactic vector potential can also be probed directly by measuring the torque exerted on a magnetized ring.<ref>{{cite journal|last1=Lakes|first1=Roderic|title=Experimental Limits on the Photon Mass and Cosmic Magnetic Vector Potential|journal=Physical Review Letters|volume=80|page=1826|year=1998|doi=10.1103/PhysRevLett.80.1826|bibcode=1998PhRvL..80.1826L|issue=9}}</ref> Such methods were used to obtain the sharper upper limit of {{val|1.07|e=-27|u=eV/c2}} ({{val|e=-36|ul=Da}}) given by the [[Particle Data Group]].<ref name=amsler>{{cite journal |last1=Amsler |first1=C |last2=Doser |first2=M |last3=Antonelli |first3=M |last4=Asner |first4=D |last5=Babu |first5=K |last6=Baer |first6=H |last7=Band |first7=H |last8=Barnett |first8=R |last9=Bergren |display-authors=8 |first9=E |title=Review of Particle Physics⁎ |journal=[[Physics Letters B]] |volume=667 |issue=1–5 |page=1 |year=2008 |doi=10.1016/j.physletb.2008.07.018 |bibcode=2008PhLB..667....1A |url=http://scipp.ucsc.edu/%7Ehaber/pubs/Review_of_Particle_Physics_2014.pdf |hdl=1854/LU-685594 |s2cid=227119789 |hdl-access=free |access-date=2017-10-26 |archive-date=2020-06-01 |archive-url=https://web.archive.org/web/20200601115825/http://scipp.ucsc.edu/%7Ehaber/pubs/Review_of_Particle_Physics_2014.pdf |url-status=live}} [http://pdg.lbl.gov/2009/tables/contents_tables.html Summary Table] {{webarchive |url=https://web.archive.org/web/20100109093036/http://pdg.lbl.gov/2009/tables/contents_tables.html |date=2010-01-09 }}</ref>

These sharp limits from the non-observation of the effects caused by the galactic vector potential have been shown to be model-dependent.<ref>{{cite journal |last1=Adelberger |first1=Eric |last2=Dvali |first2=Gia |last3=Gruzinov |first3=Andrei |title=Photon-Mass Bound Destroyed by Vortices |journal=Physical Review Letters |volume=98 |issue=1 |page=010402 |year=2007 |pmid=17358459 |doi=10.1103/PhysRevLett.98.010402 |bibcode=2007PhRvL..98a0402A |arxiv=hep-ph/0306245 |s2cid=31249827 }}</ref> If the photon mass is generated via the [[Higgs mechanism]] then the upper limit of {{nowrap|''m'' ≲ {{val|e=-14|u=eV/c2}}}} from the test of Coulomb's law is valid.
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NOTE BEFORE DELETION:Don't confuse absorption and remission with a single photon. This is untrue, an overstretch, and may confuse:
Photons inside [[superconductors]] develop a nonzero [[effective mass (solid-state physics)|effective rest mass]]; as a result, electromagnetic forces become short-range inside superconductors.<ref>{{cite book
|last=Wilczek |first=Frank
|title=The Lightness of Being: Mass, Ether, and the Unification of Forces
|journal=Physics Today
|volume=62
|issue=4
|year=2010
|publisher=Basic Books
|page=212
|isbn=978-0-465-01895-6
|url={{google books |plainurl=y |id=22Z36Qoz664C|page=212}}
|bibcode=2009PhT....62d..61W
|doi=10.1063/1.3120899
}}</ref>
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