On causality in nonlinear vacuum electrodynamics of the Pleba\'nski class

TL;DR

This paper analyzes the causality conditions in nonlinear vacuum electrodynamics within the Plebański class, establishing criteria for optical metrics to ensure causal light propagation for all background fields.

Contribution

It derives necessary and sufficient conditions on the Lagrangian or Hamiltonian to guarantee causality in nonlinear electrodynamics theories of the Plebański class.

Findings

Optical metrics depend on electromagnetic background fields.

Causality conditions are established for all background fields.

Examples illustrate the application of the criteria.

Abstract

We investigate the Pleba\'nski class of electrodynamical theories, i.e., theories of nonlinear vacuum electrodynamics that derive from a Lorentz-invariant Lagrangian (or Hamiltonian). In any such theory the light rays are the lightlike geodesics of two optical metrics that depend on the electromagnetic background field. A set of necessary and sufficient conditions is found whose fulfillment secures that the optical metrics are causal in the sense that the light rays are lightlike or timelike with respect to the underlying space-time metric. Thereupon we derive conditions on the Lagrangian, or the Hamiltonian, of the theory such that the causality conditions are satisfied for all background fields. (The allowed values of the field strength tensor are those for which the excitation tensor is finite and real.) The general results are illustrated with several examples.