``Faster than Light'' Photons in Gravitational Fields -- Causality, Anomalies and Horizons

TL;DR

This paper investigates superluminal photon propagation in gravitational fields, revealing conditions under which causality is preserved despite apparent faster-than-light travel, and establishing the stability of black hole horizons in quantum electrodynamics.

Contribution

It introduces two theorems linking photon velocity shifts to matter energy-momentum and confirms horizons remain true horizons in QED, addressing causality concerns.

Findings

Superluminal photon propagation can occur without violating causality.

The horizon theorem confirms black hole horizons are stable in QED.

A polarisation sum rule relates velocity shifts to the energy-momentum tensor.

Abstract

A number of general issues relating to superluminal photon propagation in gravitational fields are explored. The possibility of superluminal, yet causal, photon propagation arises because of Equivalence Principle violating interactions induced by vacuum polarisation in QED in curved spacetime. Two general theorems are presented: first, a polarisation sum rule which relates the polarisation averaged velocity shift to the matter energy-momentum tensor and second, a `horizon theorem' which ensures that the geometric event horizon for black hole spacetimes remains a true horizon for real photon propagation in QED. A comparision is made with the equivalent results for electromagnetic birefringence and possible connections between superluminal photon propagation, causality and the conformal anomaly are exposed.