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To what extend are we allowed to claim that the photon has some sort of mass, below some threshold. We certainly have no experimental evidence that the photon is completely massless, but, due to the fact that we can probe finitely small energies, one could argue that the mass of the photon can certainly not exceed some cutoff energy scale. This is as further as experiment takes us, right?

Theoretically, on the other hand, we know that gauge symmetry prohibits photons from having nonzero mass. But what if there is some sort of symmetry breaking, giving some negligibly small mass to the photon (spontaneously or not)? Is this a reasonable argument if one would like to claim that photons are massive?

What is the current understanding on the subject (both from experimental point of view and from theoretical point of view)?

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    $\begingroup$ Experiments are carried out from time to time. Last time I checked, the current upper limit was about $10^{-14}$ eV. $\endgroup$
    – Mauricio
    Commented Mar 13, 2023 at 12:20
  • $\begingroup$ Related: physics.stackexchange.com/q/613748 $\endgroup$
    – Mauricio
    Commented Mar 13, 2023 at 12:23
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    $\begingroup$ As for the theory, the Standard Model considers its mass to be 0. There are attempts to find theories with a massive photon but have not provided any experimental results. $\endgroup$
    – Mauricio
    Commented Mar 13, 2023 at 12:28
  • $\begingroup$ Thank you so much @Mauricio. As far as the related comment, the post you refer to does not discuss about theoretical predictions. And what about some symmetry breaking process, under which the photon acquires mass? Can something like that take place? $\endgroup$
    – schris38
    Commented Mar 13, 2023 at 12:30
  • $\begingroup$ Related: physics.stackexchange.com/a/669798/247642, physics.stackexchange.com/a/751376/247642 In general, any scientific theory must be falsifiable - i.e., it can be disproved by future evidence, and then we would have to modify our theories. $\endgroup$
    – Roger V.
    Commented Mar 13, 2023 at 12:34

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