  We show in detail how the presence of a heat bath of photons effectively
gives charged particles in the final state of a decay process a
temperature-dependent mass, and changes the effective strength of the force
responsible for the decay. At low temperature, gauge invariance causes both
these effects to be largely cancelled by absorption of photons from the heat
bath and by stimulated emission into it, but at high temperature the
temperature-dependent mass is the dominant feature.
