Global and local perspectives of gravitationally assisted   negative-phase-velocity propagation of electromagnetic waves in vacuum
  Consistently with the Einstein equivalence principle and using an electromagnetic formulation first suggested by Tamm, we show that a local observer cannot observe negative-phase-velocity (NPV) propagation of electromagnetic waves in vacuum, whereas a global observer can appreciate that phenomenon. Using the specific example of the Kerr metric, we also demonstrate the possibility of NPV propagation within the ergosphere of a rotating black hole. 
Akhlesh Lakhtakia (Pennsylvania State University), Tom G. Mackay   (University of Edinburgh) and Sandi Setiawan (University ...