ELECTROMAGNETIC-FIELDS IN A THERMAL BACKGROUND

The one-loop effective action for a slowly varying electromagnetic fie ld is computed at finite temperature and density using a real-time for malism. We discuss the gauge invariance of the result. Corrections to the Debye mass from an electric field are computed at high temperature and high density. The effective coupling constant, defined from a pur ely electric weak-field expansion, behaves at high temperature very di fferently from the case of a magnetic field, and does not satisfy the renormalization group equation. The issue of pair production in the re al-time formalism is discussed and also its relevance for heavy-ion co llisions.