PHOTON PROPAGATION IN EINSTEIN AND HIGHER-DERIVATIVE GRAVITY

We derive the wave equation obeyed by electromagnetic fields in curved space-time. We find that there are Riemann and Ricci curvature coupli ng terms to the photon polarisation which result in a polarisation-dep endent deviation of the photon trajectories from null geodesics. Photo ns are found to have an effective mass in an external gravitational fi eld and their velocity in an inertial frame is, in general, less than c. A physically relevant consequence of the analysis is that the curva ture corrections to the propagation of electromagnetic radiation (in a n homogenous and isotropic space-time) keep the velocities subluminal provided the strong energy condition is satisfied. We further show tha t the claims of superluminal velocities in higher derivative gravity t heories are erroneous and arise due to the neglect of Riemann and Ricc i coupling terms in the wave equation of Einstein gravity. (C) 1998 El sevier Science B.V.