LIGHT-PROPAGATION IN NONTRIVIAL QED VACUA - ART. NO. 025004

Within the framework of effective action QED, we derive the light cone condition for homogeneous nontrivial QED vacua in the geometric optic s approximation. Our result generalizes the ''unified formula'' sugges ted by Latorre, Pascual and Tarrach and allows for the calculation of velocity shifts and refractive indices for soft photons travelling thr ough these vacua. Furthermore, we clarify the connection between the l ight velocity shift and the scale anomaly. This study motivates the in troduction of a so-called effective action charge that characterizes t he velocity modifying properties of the vacuum. Several applications a le given concerning vacuum modifications caused by, e.g., strong field s, Casimir systems and high temperature. [S0556-2821(98)00114-3].