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So I stumbled upon this fragment from wikipedia's photon page

"

**Current commonly accepted physical theories imply or assume the photon to be strictly massless, but this should be also checked experimentally. If the photon is not a strictly massless particle, it would not move at the exact speed of light in vacuum,**"

*c*Of course I was a bit surprised, so I googled and found this https://www.princeton.edu/~romalis/PHYS312/Coulomb Ref/TuCoulomb.pdf

"Up to now, no experiment has

proved the photon rest mass to be nonzero. However, an

experiment that fails to find a finite photon mass does not

prove definitely that the mass is zero. The limits on the photon

mass have approached ever more closely the ultimate limit

determined by the uncertainty principle. So, nobody can assert

that the next experiment will not reveal evidence of a definite,

nonzero mass"

Can someone please explain this more a bit ?