The experimental upper bound on photon mass is 10^-18 eV. The photons near visible spectrum have about 10^-3 eV, which means their relative deviation from c is of order 10^-30. Gamma would be even closer. I don’t think mass of photon is measurable via speed of light.
The experimental upper bound on photon mass is 10^-18 eV. The photons near visible spectrum have about 10^-3 eV, which means their relative deviation from c is of order 10^-30. Gamma would be even closer. I don’t think mass of photon is measurable via speed of light.