  It is now well-known that vacuum polarisation in QED can lead to superluminal
low-frequency phase velocities for photons propagating in curved spacetimes. In
a series of papers, we have shown that this quantum phenomenon is dispersive
and have calculated the full frequency dependence of the refractive index,
explaining in detail how causality is preserved and various familiar results
from quantum field theory such as the Kramers-Kronig dispersion relation and
the optical theorem are realised in curved spacetime. These results have been
criticised in a recent paper by Akhoury and Dolgov arXiv:1003.6110 [hep-th],
who assert that photon propagation is neither dispersive nor necessarily
causal. In this note, we point out a series of errors in their work which have
led to this false conclusion.
