Kerr black holes as circular polarizers

TL;DR

This paper investigates how extremal Kerr black holes can act as circular polarizers, causing measurable splitting of polarized light beams that could influence retrolensing observations from Earth.

Contribution

It demonstrates that Kerr black holes can split polarized light beams by measurable angles, highlighting polarization as a key factor in gravitational lensing observations.

Findings

Polarized beams are split by up to 10^{-3} radians.

Separation of polarized beams can reach about 10^{12} meters.

Polarization changes as the observer moves, affecting retrolensing signals.

Abstract

We study the retrograde second caustics of extremal Kerr black holes, where the intensity of the light beam is infinitely magnified. We find that the caustics of different polarized beams are split by as much as $10^{-3}$rad by an external black hole for a suitable range of parameters. A lensing black hole at several lys away separates the polarized beams about $10^{12}$m apart. This splitting is larger than the radius of the Earth. Therefore, an observer on Earth would see different circularly polarized light according to their location. The polarization will change while the detector is wandering around. Thus, the polarization of light beams can be an important quantity in retrolensing observations.