  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.
