Such an observation might seem not just difficult but outright impossible, given black holes famously eat up any light that goes near them.
But the new study used an unusual effect where light “echoes” around the black hole, such that scientists can see it from the other side.
The detection came after scientists examined X-rays that were being thrown out into the universe by a supermassive black hole that sits in the middle of another galaxy, 800 million light years away.
Those X-rays kept to a strange pattern: after the bright, flaring X-rays, there would be smaller and later ones, which appeared in different “colours” than those more obvious ones.
The X-rays looked as if they were being reflected from the other side of the black hole, noticed Stanford University astrophysicist Dan Wilkins. But such an event would seem highly unusual, given black holes are more famous for destroying light than being a source of it.
“Any light that goes into that black hole doesn’t come out, so we shouldn’t be able to see anything that’s behind the black hole,” he said in a statement.
But it was another, less famous characteristic of black holes that meant those X-rays could be seen. “The reason we can see that is because that black hole is warping space, bending light and twisting magnetic fields around itself,” Wilkins explained.
The discovery of light from the other side of a black hole was predicted by Einstein’s theory of general relativity. But it has never actually happened before, and the discovery marks the first time that light has ever been directly observed coming from behind a black hole.
The research began with a slightly different aim of a more common light formed by a black hole: the corona which wraps around the outside of it, formed as material falls in. That corona is among the brightest continuous sources in the universe, flinging out X-ray light that can be then be used to analyse the black hole itself.
Researchers think that the corona starts when gas falls into the black hole and is heated up to a temperature of millions of degrees. It is so hot that the electrons are ripped from the atoms, and it forms huge arcing and twirling magnetic fields that break apart.
“This magnetic field getting tied up and then snapping close to the black hole heats everything around it and produces these high energy electrons that then go on to produce the X-rays,” said Wilkins.
But it was those smaller flashes that surprised him – and went on to produce the newly announced research, as well as the first ever glimpse at the other side of a black hole.
“I’ve been building theoretical predictions of how these echoes appear to us for a few years,” said Wilkins. “I’d already seen them in the theory I’ve been developing, so once I saw them in the telescope observations, I could figure out the connection.”
The study, ‘Light bending and X-ray echoes from behind a supermassive black hole’, is published in Nature today.