  Various decay processes, such as the decay of a spin-1 particle into two
photons or the gravitational decay of a spin-1/2 fermion, are forbidden in the
vacuum by a combination of requirements, including angular momentum
conservation, Lorentz invariance and gauge invariance. We show that such
processes can occur in a medium, such as a thermal background of particles,
even if it is homogeneous and isotropic. We carry out a model-independent
analysis of the vertex function for such processes in terms of a set of form
factors, and show that the amplitude can be non-zero while remaining consistent
with the symmetry principles mentioned above. The results simulate Lorentz
symmetry violating effects, although in this case they arise from completely
Lorentz-invariant physics.
