Torsion nonminimally coupled to the electromagnetic field and birefringence

TL;DR

This paper investigates how nonminimal coupling of torsion to the electromagnetic field can induce birefringence and alter light speed in vacuum, expanding understanding of light propagation in torsioned spacetime.

Contribution

It demonstrates that nonminimally coupled torsion fields can cause vacuum birefringence and modify light speed, providing new insights into electromagnetic propagation in torsioned geometries.

Findings

Nonminimal torsion coupling can induce birefringence in vacuum.

Torsion affects the speed of light in a Friedmann cosmological model.

The optical metric analysis confirms birefringence effects due to torsion.

Abstract

In conventional Maxwell--Lorentz electrodynamics, the propagation of light is influenced by the metric, not, however, by the possible presence of a torsion T. Still the light can feel torsion if the latter is coupled nonminimally to the electromagnetic field F by means of a supplementary Lagrangian of the type l^2 T^2 F^2 (l = coupling constant). Recently Preuss suggested a specific nonminimal term of this nature. We evaluate the spacetime relation of Preuss in the background of a general O(3)-symmetric torsion field and prove by specifying the optical metric of spacetime that this can yield birefringence in vacuum. Moreover, we show that the nonminimally coupled homogeneous and isotropic torsion field in a Friedmann cosmos affects the speed of light.