Does the photon ring of M87* have the shape Kerr predicts?
In plain words
Light that circles a black hole several times forms a thin bright ring whose shape depends only on the black hole's mass, spin and our viewing angle. Measuring it would test the Kerr geometry directly but requires radio telescopes in space.
Precise statement
The $n = 1$ and $n = 2$ photon subrings of M87* produce universal interferometric signatures at long baselines (Johnson et al. 2020) and closely follow the Kerr critical curve, whose diameter stays within a few percent of $2\sqrt{27}\,G M/c^{2}$ for all spins and inclinations. Measure the $n = 1$ ring diameter and azimuthal shape to percent accuracy and test them against the Kerr critical curve. An answer is a measurement with baselines beyond about 20 G-lambda (space VLBI) and a stated consistency level.
What would settle it
Space-based very-long-baseline interferometry resolving the $n = 1$ subring of M87* and fitting its shape.
Status in the literature
Unverified note
Event Horizon Telescope images (2019-2024) resolve the emission ring but not the photon subring.