The Black Hole Photon Ring I: A Science Case

What does a black hole look like? The first images of the supermassive black holes M87* and Sgr A* display a bright ring encircling the event horizon, which appears as a dark patch in its surrounding emission. But Einstein's theory of general relativity predicts that within this image there also lies a thin ''photon ring'' consisting of multiple mirror images of the main emission. These images arise from photons that orbited around the black hole multiple times, probing the warped space-time geometry just outside its horizon. The photon ring carries an imprint of the strong gravity in this region and encodes fundamental properties of the black hole. In this talk, I will discuss a recent proposal to measure this predicted (but not yet observed) ring and how it could provide a precise test of general relativity. The proposed measurement will be the target of a next-generation space-VLBI mission proposed to fly within the next decade.