  Currently, a number of experiments are searching for vacuum magnetic
birefringence and dichroism, i.e. for dispersive and absorptive features in the
propagation of polarized light along a transverse magnetic field in vacuum. In
this note we calculate the Standard Model contributions to these signatures,
thereby illuminating the discovery potential of such experiments in the search
for new physics. We discuss the three main sources for a Standard Model
contribution to a dichroism signal: photon splitting, neutrino pair production
and production of gravitons.
