Asymmetric Light Bending in the Equatorial Kerr Metric

TL;DR

This paper investigates the complexities of light bending near rotating black holes within the Kerr metric, clarifying interpretative ambiguities in strong gravitational lensing relevant for testing general relativity.

Contribution

It analyzes the subtleties in interpreting bending angles for prograde and retrograde photon trajectories around Kerr black holes, resolving contradictions in existing literature.

Findings

Clarified the interpretation of bending angles in Kerr spacetime

Identified sources of confusion in previous analogies

Provided insights for using gravitational lensing to test relativity

Abstract

The observation of the bending of light by mass, now known as gravitational lensing, was key in establishing general relativity as one of the pillars of modern physics. In the past couple of decades, there has been increasing interest in using gravitational lensing to test general relativity beyond the weak deflection limit. Black holes and neutron stars produce the strong gravitational fields needed for such tests. For a rotating compact object, the distinction between prograde and retrograde photon trajectories becomes important. In this paper, we explore subtleties that arise in interpreting the bending angle in this context and address the origin of seemingly contradictory results in the literature. We argue that analogies that cannot be precisely quantified present a source of confusion.