This is a concern I had after I had read the reply for a previous question I had: On the masslessness of the photon
So, I have been told that there are cases in which the gauge symmetry that preserves the masslessness of the photon breaks. Under what circumstances does that happen? And when it does, is the mass acquired by the photon consistent with the experimental evidence discussed in On the masslessness of the photon ? And finally, is the symmetry breaking spontaneous or not?
Thanks
