The Photon Ring: An Illumination of Spacetime

The more than decade-long project of the Event Horizon Telescope Collaboration (EHTC) led, in 2019, to the first image of a black hole, M87*, the huge elliptical galaxy 55 million light years away. In many ways, the image was, in its formation and evidentiary status, revelatory of how we assess objectivity. Now we—a growing team—are considering expanding the linked network of the EHT to space—a possibility imaginable with recent technological breakthroughs. The Black Hole Explorer (BHEX) would be a radio telescope satellite, joined through very long baseline interferometry, to the EHT terrestrial network. It would extend the virtual telescope diameter (and resolution) by a factor of ten, allowing us to peer inside the luminous donuts imaged of M87*, and from them extract the trace of orbiting light itself, a nested series encircling the black hole. Freed from the complexities of the billion-degree plasma orbiting M87*, observing the photon ring would offer a direct glimpse of spacetime itself in the neighborhood of a black hole. In principle, light from anywhere in the universe (anywhere visible from the black hole), could get caught in this succession of orbits—philosophically (not practically)—here would be the ultimate storehouse of memory. More practically: a proposed SMEX mission to launch within a decade to measure directly black hole spin and unravel aspects of astrophysical jets that can shape where stars can form.