# What is the current limit for a photon's rest mass?

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).

• "A few eV" is an enormous amount of energy when you're talking about experimental limits.
– J...
Feb 11, 2021 at 13:38
• Considering that we've measured plenty of photons with a total mass less than a milli-electronVolt, it seems rather obvious that the rest mass is way lower than that! Feb 11, 2021 at 15:11
• Ooh, give me some slack! According to the answer I was only off by a factor of 10 to the power of 18 or so. Not too bad really ;-) Feb 11, 2021 at 16:00
• @J... Note that "a few eV" is the current upper limit for the mass of the heaviest neutrino.
– rob
Feb 11, 2021 at 18:32
• @rob In the context of photons, I should say. Obviously something as stupendously exotic and elusive as the tau neutrino, with MeV scale energy, will have larger uncertainties. In terms of photons, a few eV is more than the total energy of all visible and longer photons.
– J...
Feb 11, 2021 at 20:43

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.