ABSTRACT:
It is experimentally well-established that under some circumstances, classical electromagnetic radiation may propagate with superluminal group velocities and superluminal phase velocities. But it is usually claimed that these superluminal velocities do not correspond to "superluminal propagation" of the sort we should worry about. In this paper, I will argue against this standard interpretation. I will suggest that classical electromagnetic radiation in a dielectric medium provides an example of "Geroch matter," that is, of matter whose associated causal cones are wider than the metric lightcones. These widened light cones are associated with the superluminal velocity (suitably understood) of a discontinuity propagating in the wave, which is precisely the notion of "signal velocity" developed in Sommerfeld's seminal work on wave propagation and relativity theory. I will conclude that there is a robust sense in which superluminal propagation is possible in classical electromagnetism---but that this does not signal a conflict with relativity, properly conceived.