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Accepted · first revision · record

Title: Does the photon ring of M87* have the shape Kerr predicts?
Plain: 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: 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.
Settled by: Space-based very-long-baseline interferometry resolving the $n = 1$ subring of M87* and fitting its shape.
Kind: well-posed
Answer type: yes-no
References: (empty)
Literature status: open
Status note: Event Horizon Telescope images (2019-2024) resolve the emission ring but not the photon subring.
Posed since: 2020