“Faster than light” photons and rotating black holes

The effective action for QED in curved spacetime includes equivalence principle violating interactions between the electromagnetic field and the spacetime curvature. These interactions admit the possibility of superluminal yet causal photon propagation in gravitational fields. In this paper, we extend our analysis of photon propagation in gravitational backgrounds to the Kerr spacetime describing a rotating black hole. The results support two general theorems — a polarisation sum rule and a “horizon theorem”. The implications for the stationary limit surface bounding the ergosphere are also discussed.