Can the EHT M87 results be used to test general relativity?

TL;DR

The paper argues that current EHT M87 observations cannot test general relativity due to astrophysical uncertainties, but future space-based interferometry could enable such tests by reaching astrophysics-independent predictions.

Contribution

It clarifies the limitations of current EHT data for testing GR and emphasizes the need for longer baselines to observe astrophysics-independent predictions.

Findings

Current observations are sensitive to accretion physics assumptions.

No definitive test of GR can be made with existing data.

Future space missions could enable astrophysics-independent tests.

Abstract

No. All theoretical predictions for the observational appearance of an accreting supermassive black hole, as measured interferometrically by a sparse Earth-sized array at current observation frequencies, are sensitive to many untested assumptions about accretion flow and emission physics. There is no way to distinguish a violation of general relativity (GR) from the much more likely scenario that the relevant "gastrophysical" assumptions simply do not hold. Tests of GR will become possible with longer interferometric baselines (likely requiring a space mission) that reach the resolution where astrophysics-independent predictions of the theory become observable.