Photon Propagation in the Casimir Vacuum

A transformation that relates the Minkowskian space of the Quantum Electrodynamics (QED) vacuum between parallel conducting plates and the QED vacuum at finite temperature is obtained. From this formal analogy,the eigenvalues and eigenvectors of the photon self-energy for the QED vacuum between parallel conducting plates (Casimir vacuum) are found in an approximation independent form. It leads to two different physical eigenvalues and three eigenmodes. We also apply the transformation to derive the low energy photons phase velocity in the Casimir vacuum from its expression in the QED vacuum at finite temperature.