Amazing stuff! However, it may require a space based interferometer.
"A team of astrophysicists ... has developed an innovative technique to search for black hole light echoes. Their novel method, which will make it easier for the mass and the spin of black holes to be measured, represents a major step forward, since it operates independently of many of the other ways in which scientists have probed these parameters in the past.
The research ... introduces a method that could provide direct evidence of photons circling black holes due to an effect known as “gravitational lensing.” ..."
From the abstract:
"Light passing near a black hole can follow multiple paths from an emission source to an observer due to strong gravitational lensing. Photons following different paths take different amounts of time to reach the observer, which produces an echo signature in the image. The characteristic echo delay is determined primarily by the mass of the black hole, but it is also influenced by the black hole spin and inclination to the observer. In the Kerr geometry, echo images are demagnified, rotated, and sheared copies of the direct image and lie within a restricted region of the image. Echo images have exponentially suppressed flux, and temporal correlations within the flow make it challenging to directly detect light echoes from the total light curve. In this Letter, we propose a novel method to search for light echoes by correlating the total light curve with the interferometric signal at high spatial frequencies, which is a proxy for indirect emission. We explore the viability of our method using numerical general relativistic magnetohydrodynamic simulations of a near-face-on accretion system scaled to M87-like parameters. We demonstrate that our method can be used to directly infer the echo delay period in simulated data. An echo detection would be clear evidence that we have captured photons that have circled the black hole, and a high-fidelity echo measurement would provide an independent measure of fundamental black hole parameters. Our results suggest that detecting echoes may be achievable through interferometric observations with a modest space-based very long baseline interferometry mission."
Measuring Black Hole Light Echoes with Very Long Baseline Interferometry (open access)
Due to gravitational lensing, the photons from a single flash of light near a black hole follow winding paths. Some follow the trajectory of the blue line, where they take a direct path to the observer. Others orbit around the black hole once, following the path of the red dashed line. Others still orbit the black hole twice following the green dashed line. Because the different paths all have different time delays, the photons arrive one after another in sequence, and the original flash of light will appear to echo.
