Over a century ago, Albert Einstein predicted that the gravitational pull of black holes were so strong that they should bend light right around them.
Black holes don’t emit light, they trap it; and ordinarily, you can’t see anything behind a black hole.
But it seems Einstein’s theory was right.
For the first time, astronomers have caught a glimpse of light being reflected — or “echoing” — from behind a supermassive black hole, 800 million light years away from Earth.
These “echoes” were in the form of X-ray flashes, according to a study published on Thursday in Nature.
While scientists have seen light bending around a black hole before, this is the first time they have been able to see the phenomenon happening from the other side.
“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,” said study co-author Dan Wilkins, an astrophysicist at Stanford University.
The anatomy of a cosmic beast
With their enormous gravitational pull, black holes chew up anything that comes too close to their event horizon, the region where not even light can escape their clutches.
Surrounding these cosmic beasts is the accretion disk, where gas and dust spiral towards oblivion.
As material gets sucked into the black hole, it releases a plume of super-hot particles called the corona, which emits X-ray flares.
“Supermassive black holes are objects of extreme density up to billions of times more massive than our own sun,” said Michael Cowley, an astrophysicist at the Queensland University of Technology, who was not involved in the study.
The X-ray flares are generated when the black hole’s giant magnetic field gets tangled up in its spin.
“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,” Dr Wilkins said.
Catching hidden light
Dr Wilkins and his team were studying these X-ray flares spewing out from the supermassive black hole at the centre of a galaxy called I Zwicky 1.
Using NASA’s Nuclear Spectroscopic Telescope Array and the European Space Agency’s XMM-Newton telescope, they saw the expected bright X-ray flashes — but there was also something strange going on.
The team detected fainter bursts of X-rays that had different wavelengths to the larger ones, indicating that they had bounced off the accretion disk from behind the black hole.
This occurs when some X-rays manage to slip past the black hole’s massive gravitational pull, only to get sucked back in.
Some of these escapee X-rays reflect off the back of the accretion disk and are bent around the black hole by its formidable gravity.
It’s this phenomenon that allowed Dr Wilkins and his team to detect these X-ray ‘”echoes” from the other side.
“I’ve been building theoretical predictions of how these echoes appear to us for a few years,” Dr Wilkins said.
Discovery provides insight into supermassive black holes
In addition to proving Einstein’s theory of relativity is right once again, the discovery could also help astronomers better understand supermassive black holes, Dr Cowley said.
“Their result provides further support for Albert Einstein’s general theory of relativity, which continues to hold up well after more than 100 years since publication,” Dr Cowley said.
Tamara Davis, an astrophysicist at the University of Queensland, added that while we’ve seen gravity bending starlight around the sun during an eclipse, it’s exciting to see it happening around a black hole.
“It is very impressive that they have managed to interpret the flares they’ve seen as light that has lapped the black hole,” said Professor Davis, who was not involved in the study.
“The exciting thing about this observation is that it is light bent around a black hole.
“It tells us what the material around the black hole is doing, at a resolution we couldn’t ever hope to achieve with conventional telescopes.”