Kerr spacetime geometric optics for vortex beams

TL;DR

This paper explores how light passing near a rotating black hole can acquire orbital angular momentum and polarization changes, using geometric optics analogies to describe Kerr spacetime as an optical medium, with implications for gravitational lensing observations.

Contribution

It introduces a geometric optics framework for Kerr spacetime that explains OAM acquisition and polarization effects in gravitational lensing, supported by data analysis of M87*.

Findings

Kerr spacetime induces orbital angular momentum in passing light.

Polarization experiences gravitational Faraday rotation.

Data from M87* supports the theoretical predictions.

Abstract

We apply the analogy between gravitational fields and optical media in the general relativistic geometric optics framework to describe how light can acquire orbital angular momentum (OAM) when it traverses the gravitational field of a massive rotating compact object and the interplay between OAM and polarization. Kerr spacetimes are known not only to impose a gravitational Faraday rotation on the polarization of a light beam, but also to set a characteristic fingerprint in the orbital angular momentum distribution of the radiation passing nearby a rotating black hole (BH). Kerr spacetime behaves like an inhomogeneous and anisotropic medium, in which light can acquire orbital angular momentum and spin-to-orbital angular momentum conversion can occur, acting as a polarization and phase changing medium for the gravitationally lensed light, as confirmed by the data analysis of M87* black hole.