Coupling Electromagnetism to Torsion: Black Holes and Spin-Charge Interactions

TL;DR

This paper introduces a novel non-minimal coupling in Riemann--Cartan geometry, leading to exact black hole solutions that reveal new interactions between torsion, spin, and electromagnetic charges, affecting black hole properties.

Contribution

It presents the first exact vacuum solutions with non-minimal electromagnetic coupling in torsionful gravity, generalizing Reissner--Nordström and BTZ black holes with novel charge interactions.

Findings

Exact four-dimensional black hole solutions with torsion and electromagnetic coupling.

Effective charge can be negative, influencing gravitational effects.

New interactions between torsion, spin, and electric/magnetic charges.

Abstract

The coupling between matter fields and gravity, encoded in the geometry of spacetime, can be realized in various ways. Most commonly, a minimal coupling principle is employed, meaning that all matter fields, except spinors, couple only to the spacetime metric, while spinors additionally couple to the spacetime connection. Non-minimal couplings between matter fields and spacetime curvature can arise, for example, from quantum field theory on curved spacetime through renormalization corrections, in gauge theories of gravity, and in effective field theories. In this article, we consider a non-minimal coupling $F^{\mu\nu}\tilde{R}_{\mu\nu}$ between the field strength tensor of the electromagnetic field $F_{\mu\nu}$ and the antisymmetric part of the Ricci tensor $\tilde{R}_{[\mu\nu]}$ in Riemann--Cartan geometry, which is based on a general metric-compatible connection with torsion. We find an exact four-dimensional vacuum solution that generalizes the Reissner--Nordstr{\"o}m black hole from Einstein--Maxwell and reveals new interactions between the intrinsic torsion-spin charge and the electric charge. Qualitatively, this solution exhibits two distinct features: the effective charge is not constrained to be positive, and the sign of the electric charge influences its gravitational effects. We also derive slowly rotating solutions in three dimensions, representing a generalized slowly rotating BTZ black hole solution with couplings among the magnetic and electric charges, the angular momentum, and the intrinsic torsion-spin charge.