Before you start yelling, I know that a photon according to theory has zero rest mass. But it is sort of difficult to measure zero, so what is the current best measurement upper limit of this zero? A few electronvolts? (The question is sort of similar to that it seems like the rest mass of the neutrino-s are not exactly zero although we believed that for a long time).
Photons – Current Limit for a Photon’s Rest Mass: What is It?
massMeasurementsphotons
Best Answer
The limit in the 2020 Particle Data Group Summary is $m_\gamma < 10^{-18}\mathrm{eV} \approx 10^{-27}m_\text{proton}$, based on a 2007 analysis of the magnetohydrodynamics of the solar wind. The PDG reviews are excellent reading on these kinds of questions.