  If the photon possessed an electric charge or a magnetic moment, light waves
propagating through magnetic fields would acquire new quantum mechanical
phases. For a charged photon, this is an Aharonov-Bohm phase, and the fact that
we can resolve distant galaxies using radio interferometry indicates that this
phase must be small. This in turn constrains the photon charge to be smaller
that 10^(-32) e if all photons have the same charge and smaller than 10^(-46) e
if there are both positively and negatively charged photons. The best bound on
the magnetic moment comes from the observed absence of wavelength-independent
photon birefringence. Birefringence measurements, which compare the relative
phases of right- and left-circularly polarized waves, restrict the magnetic
moment to be less than 10^(-24) e cm. This is just a few orders of magnetude
weaker than the experimental bounds on the electron and neutron electric dipole
moments.
