The velocities of light in modified QED vacua

TL;DR

This paper reviews how external conditions like magnetic fields and Casimir boundaries alter light velocities in QED vacua, revealing superluminal phase and group velocities and discussing implications for causality and vacuum structure.

Contribution

It provides an overview of light propagation in modified QED vacua, highlighting recent findings of superluminal velocities in Casimir setups and analyzing their physical implications.

Findings

Light velocities can exceed c in Casimir vacua.

Superluminal modes occur at low frequencies (omega<<m).

Implications for causality and vacuum properties are discussed.

Abstract

QED vacua under the influence of external conditions (background fields, finite temperature, boundary conditions) can be considered as dispersive media whose complex behaviour can no longer be described in terms of a single universal vacuum velocity of light c. Beginning in the early 1950's (J.S. Toll), quantum field theoretic investigations have led to considerable insight into the relation between the vacuum structure and the propagation of light. Recent years have witnessed a significant growth of activity in this field of research. After a short overview, two characteristic situations are discussed: the propagation of light in a constant homogeneous magnetic field and in a Casimir vacuum. The latter appears to be particularly interesting because the Casimir vacuum has been found to exhibit modes of the propagation of light with phase and group velocities larger than c in the low frequency domain omega<<m where m is the electron mass. The impact of this result on the front velocity of light in a Casimir vacuum is discussed by means of the Kramers-Kronig relation.