Editing Photon (section)

=== Antiparticle annihilation ===
{{Main | Annihilation | Electron–positron annihilation}}
The collision of a particle with its antiparticle can create photons. In free space at least ''two'' photons must be created since, in the [[center of momentum frame]], the colliding antiparticles have no net momentum, whereas a single photon always has momentum (determined by the photon's frequency or wavelength, which cannot be zero). Hence, [[momentum|conservation of momentum]] (or equivalently, [[translational invariance]]) requires that at least two photons are created, with zero net momentum.<ref name=Griffiths2008>{{cite book |last=Griffiths |first=David J. |year=2008 |title=Introduction to Elementary Particles |edition=2nd revised |publisher=WILEY-VCH |isbn=978-3-527-40601-2}}</ref>{{rp|64–65}} The energy of the two photons, or, equivalently, their frequency, may be determined from [[Conservation law (physics)|conservation of four-momentum]].

{{anchor |antiphoton}}Seen another way, the photon can be considered as its own antiparticle (thus an "antiphoton" is simply a normal photon with opposite momentum, equal polarization, and 180° out of phase). The reverse process, [[pair production]], is the dominant mechanism by which high-energy photons such as [[gamma ray]]s lose energy while passing through matter.<ref>{{harvnb|Alonso|Finn|1968|loc=Section 9.3}}.</ref> That process is the reverse of "annihilation to one photon" allowed in the electric field of an atomic nucleus.

The classical formulae for the energy and momentum of [[electromagnetic radiation]] can be re-expressed in terms of photon events. For example, the [[radiation pressure|pressure of electromagnetic radiation]] on an object derives from the transfer of photon momentum per unit time and unit area to that object, since pressure is force per unit area and force is the change in [[momentum]] per unit time.<ref>{{cite book |last1=Born |first1=Max |url={{google books |plainurl=y |id=NmM-KujxMtoC}} |title=Atomic Physics |last2=Blin-Stoyle |first2=Roger John |last3=Radcliffe |first3=J. M. |date=1989 |publisher=Courier Corporation |isbn=978-0-486-65984-8 |language=en |section=Appendix&nbsp;XXXII}}</ref>