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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).

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    $\begingroup$ 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 ;-) $\endgroup$
    – ghellquist
    Commented Feb 11, 2021 at 16:00

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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.

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    $\begingroup$ I know this is a little fast, but I will mark this as accepted answer due to the reference to PDG reviews, that I have been able to miss up to now. Thank you! $\endgroup$
    – ghellquist
    Commented Feb 11, 2021 at 11:36
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    $\begingroup$ To give some kind of handle on how low this number is its interesting to see that a (zero-mass) photon with 10^-18 electron volts of energy would have a wavelength about ten times longer than the Earth-Sun distance. Thus I believe that if photons DID have a rest mass at about that limit a photon would need to have a wavelength at least 10AU before its rest-mass energy surpassed its kinetic energy. $\endgroup$
    – Dast
    Commented Feb 11, 2021 at 18:38

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