ABSTRACT
There is a way of giving gauge field quanta a mass, which is by ‘spontaneously breaking’ the gauge (i.e. local) symmetry. This is the topic of this chapter. The chapter begins by noting an elementary (classical) argument for why a gauge field quantum cannot have mass. The first serious challenge to the then widely held view that electromagnetic gauge invariance requires the photon to be massless was made by Schwinger. The treatment conveys much of the essential physics behind the phenomenon of ‘photon mass generation’ in a superconductor. In particular, it suggests rather strongly that a second field, in addition to the electromagnetic one, is an essential element in the story.
