Light cone condition fos a thermalized QED vacuum - art. no. 105033

Within the QED effective action approach, we study the propagation of low-f requency light at finite temperature. Starting from a general effective Lag rangian for slowly varying fields whose structure is solely dictated by Lor entz covariance and gauge invariance, ne derive the light cone condition fo r light propagating in a thermalized QED vacuum. As an application, we calc ulate the velocity shifts, i.e., refractive indices of the vacuum, induced by thermalized fermions to one loop. We investigate various temperature dom ains and also include a background magnetic field. While low-temperature ef fects to one loop are exponentially damped by the electron mass, there exis ts a maximum velocity shift of - delta nu(max)(2) = alpha/3 pi in the inter mediate-temperature domain T similar to m. [S0556-2821(99)01622-7].