2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/193984A covariant gauge independent derivation of the generalized dispersion relation of electromagnetic waves in a medium with local and linear constitutive law is presented. A generalized photon propagator is derived. For Maxwell constitutive tensor, the standard light cone structure and the standard Feynman propagator are reinstated.High Energy Physics - Theorytext