Solar constraints on new couplings between electromagnetism and gravity

TL;DR

This paper investigates how nonminimal couplings between electromagnetism and gravity, specifically torsion in a metric-affine gauge theory, can be constrained using solar spectropolarimetric observations, setting new limits on the coupling constant.

Contribution

It introduces constraints on nonminimal electromagnetic-gravity couplings in a metric-affine gravity theory using solar polarization data.

Findings

Strong constraints on the coupling constant k: k^2 < (2.5 km)^2

Polarization phase differences can be used to test gravity theories

Solar observations provide competitive bounds on new couplings

Abstract

The unification of quantum field theory and general relativity is a fundamental goal of modern physics. In many cases, theoretical efforts to achieve this goal introduce auxiliary gravitational fields, ones in addition to the familiar symmetric second-rank tensor potential of general relativity, and lead to nonmetric theories because of direct couplings between these auxiliary fields and matter. Here, we consider an example of a metric-affine gauge theory of gravity in which torsion couples nonminimally to the electromagnetic field. This coupling causes a phase difference to accumulate between different polarization states of light as they propagate through the metric-affine gravitational field. Solar spectropolarimetric observations are reported and used to set strong constraints on the relevant coupling constant k: k^2 < (2.5 km)^2.