SPEED OF LIGHT IN NONTRIVIAL VACUA

We unify all existing results on the change of the speed of low-energy photons due to modifications of the vacuum? finding that it is given by a universal constant times the quotient of the difference of energy densities between the usual and modified vacua over the mass of the e lectron to the fourth power. Whether photons move faster or slower tha n c depends only on the lower or higher energy density of the modified vacuum, respectively. Physically, a higher energy density is characte rized by the presence of additional particles (real or virtual) in the vacuum whereas a lower one stems from the absence of some virtual mod es. We then carry out a systematic study of the speed of propagation o f massless particles for several field theories up to two loops on a t hermal vacuum. Only low-energy massless particles corresponding to a m assive theory show genuine modifications of their speed while remainin g massless. All other modifications are mass related, or running mass- related We also develop a formalism for the Casimir vacuum which paral lels the thermal one and check that photons travel faster than c betwe en plates.