Gravitation and Electromagnetic Wave Propagation with Negative Phase Velocity

Gravitation has interesting consequences for plane wave propagation in a vacuum. The propagation of plane waves with phase velocity directed opposite to the the time-averaged Poynting vector is investigated for a generally curved spacetime. Conditions for such negative-phase-velocity (NPV) propagation are established in terms of the metric components for general and special cases. The negative energy density implications of NPV propagation are discussed.