I read somewhere that gauge symmetry prevents the photon from acquiring a mass. The argument seems to go that the 1-loop correction to the photon won't contain a term independent of the external momentum due to gauge invariance, so there is no need for a bare mass counter-term. So should that statement be modified to gauge symmetry prevents the photon from acquiring a bare mass? Can't you always set the renormalized mass equal to zero, even if gauge symmetry is lacking? Like a [itex]\phi^4[/itex] theory? Also, shouldn't the relationship between bare mass and renormalized mass be that they will always be proportional to each other, because there are no other parameters in the theory with dimensions of mass? Then it should follow that the bare mass can always be set to zero?