Editing Photon (section)

=== Contributions to the mass of a system ===
{{See also|Mass in special relativity|Mass in general relativity}}
The energy of a system that emits a photon is ''decreased'' by the energy <math>E</math> of the photon as measured in the rest frame of the emitting system, which may result in a reduction in mass in the amount <math>{E}/{c^2}</math>. Similarly, the mass of a system that absorbs a photon is ''increased'' by a corresponding amount. As an application, the energy balance of nuclear reactions involving photons is commonly written in terms of the masses of the nuclei involved, and terms of the form <math>{E}/{c^2}</math> for the gamma photons (and for other relevant energies, such as the recoil energy of nuclei).<ref>E.g., section 10.1 in {{Cite book |last=Dunlap |first=R. A. |title=An Introduction to the Physics of Nuclei and Particles |publisher=[[Cengage Learning#Brands/imprints|Brooks/Cole]] |year=2004 |isbn=978-0-534-39294-9 |language=en}}</ref>

This concept is applied in key predictions of [[quantum electrodynamics]] (QED, see above). In that theory, the mass of electrons (or, more generally, leptons) is modified by including the mass contributions of virtual photons, in a technique known as [[renormalization]]. Such "[[radiative correction]]s" contribute to a number of predictions of QED, such as the [[anomalous magnetic dipole moment|magnetic dipole moment]] of [[lepton]]s, the [[Lamb shift]], and the [[hyperfine structure]] of bound lepton pairs, such as [[muonium]] and [[positronium]].<ref>Radiative correction to electron mass section 7–1–2, anomalous magnetic moments section 7–2–1, Lamb shift section 7–3–2 and hyperfine splitting in positronium section 10–3 in {{Cite book |last1=Itzykson |first1=C. |url=https://archive.org/details/quantumfieldtheo0000itzy |title=Quantum Field Theory |last2=Zuber |first2=J.-B. |publisher=McGraw-Hill |year=1980 |isbn=978-0-07-032071-0 |url-access=registration}}</ref>

Since photons contribute to the [[stress–energy tensor]], they exert a [[gravity|gravitational attraction]] on other objects, according to the theory of [[general relativity]]. Conversely, photons are themselves affected by gravity; their normally straight trajectories may be bent by warped [[spacetime]], as in [[gravitational lens]]ing, and [[gravitational redshift|their frequencies may be lowered]] by moving to a higher [[potential energy|gravitational potential]], as in the [[Pound–Rebka experiment]]. However, these effects are not specific to photons; exactly the same effects would be predicted for classical [[electromagnetic radiation|electromagnetic waves]].<ref>E.g. sections 9.1 (gravitational contribution of photons) and 10.5 (influence of gravity on light) in {{Cite book|last1=Stephani|first1=H.|url={{google books |plainurl=y |id=V04_vLQvstcC|page=86}}|last2=Stewart|first2=J.|pages=86 ff, 108 ff|title=General Relativity: An Introduction to the Theory of Gravitational Field|isbn=978-0-521-37941-0|publisher=Cambridge University Press|year=1990}}</ref>