The Event Horizon Telescope only recently gave us the first images of the environment immediately surrounding a black hole. Since then, it has been boosting the resolution and filling in the details of an environment dominated by the most extreme gravity in the Universe.
But which gravity are we talking about? Because of its incompatibility with quantum mechanics and our current inability to explain dark matter, people have proposed all sorts of variants of gravity that go beyond general relativity and clean up some of the physics’ awkwardness. It’s possible that the extreme environment near a black hole amplifies the differences among at least some of these hypotheses. So, a group of physicists decided to see whether any of those differences might be large enough that the next generation of telescopes might be able to rule out some potential replacements for relativity.
Searching for subtlety
Any replacement for general relativity faces an awkward challenge: General relativity does pretty well at explaining everything from the large-scale structure of the Universe to phenomena we can measure right here on Earth. So, any alternative theories would have to differ from relativity in very subtle ways that might be extremely difficult to detect.
But the environment around a black hole might be extreme enough to amplify some of these subtle differences. Its gravity is so extreme that a rotating black hole drags bits of space time along with it (a phenomenon called frame dragging), forcing light to travel in a bendy path on its way to Earth. And this is highly dependent on gravity. “General relativity predicts that the image of such a region will consist of a series of nested ring-like images where each ring is distinguished by the number of half-orbits [of the black hole] that photons make before reaching the observer,” the researchers write. “The accurate measurement structure of the photon rings and of their location represents the most compelling route to investigate gravity in the regime of strong but stationary curvature.”