Shadow Implications: What does measuring the photon ring imply for gravity?

Understand

With the imaging and characterization of the horizon-scale images of M87* and Sgr A* by the Event Horizon Telescope (EHT), it has become possible to resolve the near-horizon region of astrophysical black holes.

• As a result, there has been considerable interest in the implications of the measurement of the shadow size, i.e., the asymptotic photon ring.
• We explore the general implications of such a measurement, identifying what is and, more importantly, is not constrained by such measurements, with applications to EHT and future instruments.
• We consider a general spherically symmetric metric, which effectively applies for a polar observer (appropriate for M87*) in the slow rotation limit.