# Gravity and handedness of photons

## TL;DR

This paper explores how strong gravitational fields can induce a quantum optical activity in electromagnetic radiation, causing polarization changes over time due to a quantum anomaly related to electric-magnetic duality.

## Contribution

It introduces the concept that spacetime geometry can act as an optically active medium for photons, revealing a quantum effect linked to duality anomalies in Maxwell theory.

## Key findings

- Spacetime geometry influences photon polarization via quantum effects.
- Polarization states evolve over time without electromagnetic sources.
- The effect is rooted in a duality anomaly in Maxwell's equations.

## Abstract

Vacuum fluctuations of quantum fields are altered in presence of a strong gravitational background, with important physical consequences. We argue that a non-trivial spacetime geometry can act as an optically active medium for quantum electromagnetic radiation, in such a way that the state of polarization of radiation changes in time, even in the absence of electromagnetic sources. This is a quantum effect, and is a consequence of an anomaly related to the classical invariance under electric-magnetic duality rotations in Maxwell theory.