Editing Photon (section)

=== As a gauge boson ===
{{Main|Gauge theory}}

The electromagnetic field can be understood as a [[gauge field]], i.e., as a field that results from requiring that a gauge symmetry holds independently at every position in [[spacetime]].<ref name="Ryder">{{cite book |last=Ryder |first=L. H. |url={{google books |plainurl=y |id=nnuW_kVJ500C}} |title=Quantum field theory |publisher=Cambridge University Press |year=1996 |isbn=978-0-521-47814-4 |edition=2nd |location=England |language=en-uk}}</ref> For the [[electromagnetic field]], this gauge symmetry is the [[Abelian group|Abelian]] [[unitary group|U(1) symmetry]] of [[complex number]]s of absolute value 1, which reflects the ability to vary the [[complex geometry|phase]] of a complex field without affecting [[observable]]s or [[real number|real valued functions]] made from it, such as the [[energy]] or the [[Lagrangian (field theory)|Lagrangian]].

The quanta of an [[gauge theory|Abelian gauge field]] must be massless, uncharged bosons, as long as the symmetry is not broken; hence, the photon is predicted to be massless, and to have zero [[electric charge]] and integer spin. The particular form of the [[electromagnetic interaction]] specifies that the photon must have [[Spin (physics)|spin]] ±1; thus, its [[helicity (particle physics)|helicity]] must be <math>\pm \hbar</math>. These two spin components correspond to the classical concepts of [[circular polarization|right-handed and left-handed circularly polarized]] light. However, the transient [[virtual photon]]s of [[quantum electrodynamics]] may also adopt unphysical polarization states.<ref name="Ryder" />

In the prevailing [[Standard Model]] of physics, the photon is one of four gauge bosons in the [[electroweak interaction]]; the [[W and Z bosons|other three]] are denoted W<sup>+</sup>, W<sup>−</sup> and Z<sup>0</sup> and are responsible for the [[weak interaction]]. Unlike the photon, these gauge bosons have [[invariant mass|mass]], owing to a [[Higgs mechanism|mechanism]] that breaks their [[special unitary group|SU(2) gauge symmetry]]. The unification of the photon with W and Z gauge bosons in the electroweak interaction was accomplished by [[Sheldon Glashow]], [[Abdus Salam]] and [[Steven Weinberg]], for which they were awarded the 1979 [[Nobel Prize]] in physics.<ref name="Glashow">[http://nobelprize.org/nobel_prizes/physics/laureates/1979/glashow-lecture.html Sheldon Glashow Nobel lecture] {{Webarchive|url=https://web.archive.org/web/20080418033045/http://nobelprize.org/nobel_prizes/physics/laureates/1979/glashow-lecture.html |date=2008-04-18 }}, delivered 8 December 1979.</ref><ref name="Salam">[http://nobelprize.org/nobel_prizes/physics/laureates/1979/salam-lecture.html Abdus Salam Nobel lecture] {{Webarchive|url=https://web.archive.org/web/20080418033106/http://nobelprize.org/nobel_prizes/physics/laureates/1979/salam-lecture.html |date=2008-04-18 }}, delivered 8 December 1979.</ref><ref name="Weinberg">[http://nobelprize.org/nobel_prizes/physics/laureates/1979/weinberg-lecture.html Steven Weinberg Nobel lecture] {{Webarchive|url=https://web.archive.org/web/20080418033111/http://nobelprize.org/nobel_prizes/physics/laureates/1979/weinberg-lecture.html |date=2008-04-18 }}, delivered 8 December 1979.</ref> Physicists continue to hypothesize [[grand unification theory|grand unified theories]] that connect these four gauge bosons with the eight [[gluon]] gauge bosons of [[quantum chromodynamics]]; however, key predictions of these theories, such as [[proton decay]], have not been observed experimentally.<ref>E.g., chapter 14 in {{cite book|last=Hughes|first=I.S.|title=Elementary particles|edition=2nd|publisher=Cambridge University Press|year=1985|isbn=978-0-521-26092-3|url=https://archive.org/details/elementarypartic00hugh}}</ref>