2003/02/27 by G. M. Shore, G.M. Shore, Shore, G. M.
Physics and Astronomy · #FOS: Physical sciences #General Relativity and Quantum Cosmology (gr-qc) #High Energy Physics - Theory (hep-th) #Quantum Mechanics and Applications #Quantum optics and atomic interactions #gr-qc #hep-th
paper · pdf · doi:10.48550/arxiv.gr-qc/0302116
22 pages, 5 figures; prepared for the proceedings of the conference `Time and Matter', Venice, August 2002
arxiv created 2003/02/27 · openalex publication_date 2003/02/27 · arxiv updated 2009/11/30 · openalex created_date 2025/10/10 · openalex updated_date 2026/07/28
The causal properties of curved spacetime, which underpin our sense of time in gravitational theories, are defined by the null cones of the spacetime metric. In classical general relativity, it is assumed that these coincide with the light cones determined by the physical propagation of light rays. However, the quantum vacuum acts as a dispersive medium for the propagation of light, since vacuum polarisation in QED induces interactions which effectively violate the strong equivalence principle (SEP). For low frequencies the phenomenon of gravitational birefringence occurs and indeed, for some metrics and polarisations, photons may acquire \it superluminal phase velocities. In this article, we review some of the remarkable features of SEP violating superluminal propagation in curved spacetime and discuss recent progress on the issue of dispersion, explaining why it is the high-frequency limit of the phase velocity that determines the characteristics of the effective wave equation and thus the physical causal structure.