Towards gravitationally assisted negative refraction of light by vacuum
  Propagation of electromagnetic plane waves in some directions in gravitationally affected vacuum over limited ranges of spacetime can be such that the phase velocity vector casts a negative projection on the time-averaged Poynting vector. This conclusion suggests, inter alia, gravitationally assisted negative refraction by vacuum. 
Akhlesh Lakhtakia (Pennsylvania State University) and Tom G. Mackay   (University of Edinburgh)