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. 
Tom G. Mackay (University of Edinburgh), Akhlesh Lakhtakia   (Pennsylvania State University), Sandi Setiawan (University of ...