# Soft photon

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In [particle physics](/source/Particle_physics), **soft photons** are [photons](/source/Photon) having [photon energies](/source/Photon_energy) much smaller than the energies of the particles participating in a particular [scattering](/source/Scattering) process, and they are not energetic enough to be detected. Such photons can be emitted from (or absorbed by) the external (incoming and outgoing) lines of charged particles of the [Feynman diagram](/source/Feynman_diagram) for the process. Even though soft photons are not detected, the possibility of their emission must be taken into account in the calculation of the [scattering amplitude](/source/Scattering_amplitude).[1]

Taking the soft photons into account multiplies the rate of a given process by a factor which approaches zero. However, there is also an [infrared-divergent](/source/Infrared_divergence) contribution to the scattering rate related to [virtual](/source/Virtual_photon) soft photons that are emitted from one of the external lines and absorbed in another (or in the same line). The two factors cancel each other, leaving a finite correction which depends on the sensitivity with which photons can be detected in the experiment.[1]

[Steven Weinberg](/source/Steven_Weinberg)'s [soft photon theorem](/source/Soft_graviton_theorem) simplifies the calculation of the contribution of soft photons to the scattering of particles.

## References

1. Kaku, Michio (1993). *Quantum Field Theory: A Modern Introduction*. New York: Oxford University Press. ISBN 0-19-507652-4., pages 177-184 and appendix A6

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