  We consider the gravitational correction to (electronic) vacuum polarization
in the presence of a gravitational background field. The Dirac propagators for
the virtual fermions are modified to include the leading gravitational
correction (potential term) which corresponds to a coordinate-dependent fermion
mass. The mass term is assumed to be uniform over a length scale commensurate
with the virtual electron-positron pair. The on-mass shell renormalization
condition ensures that the gravitational correction vanishes on the mass shell
of the photon, i.e., the speed of light is unaffected by the quantum field
theoretical loop correction, in full agreement with the equivalence principle.
Nontrivial corrections are obtained for off-shell, virtual photons. We compare
our findings to other works on generalized Lorentz transformations and combined
quantum-electrodynamic gravitational corrections to the speed of light which
have recently appeared in the literature.
