A comparison of superradiance and negative-phase-velocity phenomenons in   the ergosphere of a rotating black hole
  The propagation of electromagnetic plane waves with negative phase velocity (NPV) in the ergosphere of a rotating black hole has recently been reported. A comparison of NPV propagation and superradiance is presented. We show that, although both phenomenons involve negative energy densities, there are two significant differences between them. 
Sandi Setiawan (University of Edinburgh), Tom G. Mackay (University of   Edinburgh), Akhlesh Lakhtakia (Pennsylvania State U...