The Shape of the Black Hole Photon Ring: A Precise Test of Strong-Field General Relativity

TL;DR

This paper introduces a novel method to test strong-field general relativity by analyzing the universal interferometric signature of the black hole photon ring, which is highly insensitive to source details and can be observed with space-based interferometry.

Contribution

It proposes using the shape of the black hole photon ring as a precise, source-profile independent test of Kerr geometry in strong gravity regimes.

Findings

Photon ring shape is insensitive to source profile.

Space-based interferometry can test Kerr nature to sub-sub-percent accuracy.

The method provides a direct probe of light bending near black holes.

Abstract

We propose a new test of strong-field general relativity (GR) based on the universal interferometric signature of the black hole photon ring. The photon ring is a narrow ring-shaped feature, predicted by GR but not yet observed, that appears on images of sources near a black hole. It is caused by extreme bending of light within a few Schwarzschild radii of the event horizon and provides a direct probe of the unstable bound photon orbits of the Kerr geometry. We show that the precise shape of the observable photon ring is remarkably insensitive to the astronomical source profile and can therefore be used as a stringent test of GR. We forecast that a tailored space-based interferometry experiment targeting M87* could test the Kerr nature of the source to the sub-sub-percent level.