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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)?

Mauricio
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schris38
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    Experiments are carried out from time to time. Last time I checked, the current upper limit was about $10^{-14}$ eV. – Mauricio Mar 13 '23 at 12:20
  • Related: https://physics.stackexchange.com/q/613748/ – Mauricio Mar 13 '23 at 12:23
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    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. – Mauricio Mar 13 '23 at 12:28
  • 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? – schris38 Mar 13 '23 at 12:30
  • Related: https://physics.stackexchange.com/a/669798/247642, https://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. – Roger V. Mar 13 '23 at 12:34
  • @schris38 that's a different question. You can make one about the predictions of a broken symmetry photon. – Mauricio Mar 13 '23 at 12:39
  • Okay @Mauricio, I will do that and edit this question back the way it was – schris38 Mar 13 '23 at 12:52
  • Possible duplicates: https://physics.stackexchange.com/q/4700/2451 and links therein. – Qmechanic Mar 13 '23 at 15:19

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