- #1

JimJCW

Gold Member

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For discussion purpose, let’s assume the photon has a wavelength of 500 nm, close to the peak of the solar spectrum. During a journey of 1 Gly, for example, the number of CMB photons coming within a wavelength of the traveling photon is estimated to be,

This is a large number.

Because of the linearity of the classical Maxwell equations, it is generally assumed that the traveling photon does not interact with the CMB photons. However, the theory of quantum electrodynamics opens the possibility of photon-photon scattering via virtual electron-positron pairs in vacuum (see, for example,

**Two-photon physics**).

Does the photon travel freely? If not, over a long distance/time, even a slight interaction might accumulate a significant effect.