The event horizon of a black hole is the invisible line-in-the-sand across which you can never return. Once anything passes through the event horizon, even light itself, it can no longer return to the universe. The black hole’s gravity is just too strong within that region.
Streams of gas fall to their dooms, plunging into black holes, locked away from the universe forever. In their final moments, these gassy shreds send out one last flare of light, some of the brightest emissions in the universe.
In a new study, physicists looked at specific features of that light to figure out the closest you can get to a black hole without having to work hard to prevent disaster — a threshold called the innermost stable circular orbit or ISCO.
This boundary, ISCO, is a firm prediction of Einstein’s general theory of relativity, the same theory that predicts the existence of black holes in the first place. Despite the success of general relativity in predicting and explaining phenomena across the universe, and our sure knowledge that black holes are real, we’ve never been able to verify the existence of the ISCO and whether it conforms to the predictions of general relativity.
But the gas that falls to its doom may provide a way for us to verify that existence.