# Alfonso Salomón Acevedo Juárez

## How birefringence arise from nonlinear electrodynamics

We analyze the propagation of light signals in the context of nonlinear electrodynamics. As a general feature of the non linear theories the superposition principle is no longer satisfied, on electromagnetic theory this is because light propagation is influenced by the electromagnetic background. We present a simple derivation of the two light cones that arise if the Lagrangian depends nonlinearly on the electromagnetic invariants. The analysis is based on the algebraic properties of the electromagnetic tensor $f_{\mu \nu}$. We show that the problem can be treated as a Sturm-Liouville problem where the eigenvalues are related to the principal null directions. It turns out that, in the presence of the background field, the propagation can be slower or faster than that of light.