The fear, Strominger said, was that the image would only reveal information about the swirling, glowing stuff, which is mostly gases heated to billions of degrees and back-lit by light looping around and around and around. The black hole, “the prize in the middle,” as Strominger called it, casts a shadow on that jetsam, gifting the scientists with a few nebulous clues about its mass.
The photo brought theorists — like Strominger — together with observers like Michael Johnson, first author on the EHT team’s recent study published in Science Advances and an astrophysicist with the Black Hole Initiative. “It’s an example of how a new result can prompt these unexpected, new collaborations,” Johnson said.
Together, the interdisciplinary team discovered the doughnut is not just one doughnut but a collection of sub-rings of light bending in thinner loops the closer they get to the black hole’s event horizon — a boundary around the hole, which is the point of no return where matter and even light disappear to no one knows where. Right outside that is what the team calls the photon shell. “In that region,” Lupsasca said, “gravity is not strong enough to capture the light rays forever, but it’s strong enough to deflect them so much that they go around in circles.” Nothing can bend light like a black hole.
As Strominger feared, an image of just one of those rings gives information about the inferno outside the black hole but not much about the “prize,” he said. But the team has a solution for that: With an image of a second ring, they can compare the black hole at two different periods of time. Each ring is like a mirror image of the black hole at different points in its history, not unlike the rings of a tree. So, like the infinite reflections of a person surrounded in mirrors, the stacked images provide enough data to learn the black hole’s essential properties: mass and spin.
The rings also betray how the black hole warps space-time. Their individual brightness, thickness, and shape depend on the monster’s manipulation of its surrounding geography. In reality, the rings are far from perfect circles — like a bug caught in the gravitational pull of a drain, each one weaves a warped path around the black hole’s bizarre landscape.
Here’s where things get stranger: A black hole hoards images of the past. Light is composed of photons, and each one carries a bit of the image of whatever it hits. So when you see a tree the light hits the tree, bounces to your eye, and your brain eventually pulls it all together like a mosaic. Light stuck in a black hole’s gravitational pull can loop once, twice, or an infinite number of times, depending on its angle of approach, Lupsasca said. Those that finally escape in the direction of Earth carry a reflection of what the universe looked like when they entered the black hole’s pull. The longer light was held captive, the earlier in the past their image shows.
“As we peer into these rings, first, second, third, etc., we are looking at light from all over the visible universe; we are seeing farther and farther into the past, a movie, so to speak, of the history of the visible universe,” said Peter Galison, the Joseph Pellegrino University Professor of the History of Science and of Physics, in the Black Hole Initiative’s press release.