Reality and causality in quantum gravity modified electrodynamics

TL;DR

This paper analyzes how quantum gravity modifications affect electrodynamics, revealing that absorption is essential and dominates linear effects, with implications for causality and propagation in such theories.

Contribution

It provides a general framework for understanding propagation in quantum gravity modified electrodynamics, including explicit calculations of Green functions and refraction indices.

Findings

Absorption must be present in all models analyzed.

Linear effects in the correction parameter are dominated by absorption.

Propagation effects are quadratic in the correction parameter.

Abstract

We present a general description of the propagation properties of quantum gravity modified electrodynamics characterized by constitutive relations up to second order in the correction parameter. The effective description corresponds to an electrodynamics in a dispersive and absorptive non-local medium, where the Green functions and the refraction indices can be explicitly calculated. The reality of the electromagnetic field together with the requirement of causal propagation in a given referrence frame leads to restrictions in the form of such refraction indices. In particular, absorption must be present in all cases and, contrary to the usual assumption, it is the dominant aspect in those effective models which exhibit linear effects in the correction parameter not related to birefringence. In such a situation absorption is linear while propagation is quadratical in the correction parameter.