LIGHT-PROPAGATION IN NONTRIVIAL QED VACUA

Within the geometric optics approximation, we derive the light cone co ndition for a class of homogeneous non-trivial QED vacua using the eff ective action approach. Our result generalizes the ''unified formula'' suggested by Latorre, Pascual and Tarrach which turns out to be a low -energy limit. We calculate the velocity shifts induced by electromagn etic fields and finite temperature and especially analyze the high-ene rgy limit consistent with the loop approximation. (C) 1998 Elsevier Sc ience B.V. All rights reserved.