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Even light can't escape black holes, but now we know it can shine behind them

By Elizabeth Rayne

Ever since the unprecedented image of the M87 black hole broke the internet, scientists have been trying to see more. They just did.

Nothing escapes a black hole. Even the M87 image being retweeted everywhere was not showing the black hole itself — which is impossible to see into — but the accretion disc of light and gas swirling around an invisible mouth into which that light would fall and eventually vanish forever. That is how powerful the gravity of these enigmatic objects is. But if photons are doomed once they pass the point of no return, then what was that glow coming from behind a black hole?

Astrophysicist Dan Wilkins of Stanford University, who recently led a study published in Nature, found out what causes this strange phenomenon that was revealed through ESA’s XMM-Newton space telescope and NASA’s NuStar array. The gravity that means doom for captured particles also bends light light nothing else.

“Black holes are the most extreme manifestation of gravity in our Universe,” Wilkins told SYFY WIRE. “We were able to observe extreme light bending, and saw the echoes of X-rays from gas that should be hidden behind the black hole from our viewpoint. This light was bent all the way around the outside of the black hole so that we were able to see these echoes.”

What Wilkins and his team are now calling “luminous echoes” further prove Einstein’s theory of General Relativity. Under General Relativity, massive objects such as stars and planets have enough gravitational force to bend the space and time around them. This is the reason kids in towel capes everywhere are disappointed when they jump off the sofa and crash instead of flying. The curving of spacetime on Earth explains why they can’t soar like Superman. Because the Moon is stronger and curves the time and space around it less, Apollo astronauts were able to take a literal giant leap without a problem.

The curvature of spacetime also means light is unable to travel past a massive object without being bent. This is why stars behind the Sun appear to be in slightly different positions than they actually are; the Sun is massive enough to bend their light and convince observers those stars are shining from somewhere else. From micro-particles to huge asteroids, anything that attempts to travel in a straight line is eventually going to be forced to follow the curvature of space when it passes an object that bends spacetime around it.

“We were able to confirm that the prediction of how light bends around a black hole holds true even at the extremes of the theory, where gravity is strong and the space is warped to a high degree around a black hole,” said Wilkins.

Warped spacetime made it possible to notice something that was never caught before, which showed itself as Wilkins and his team were studying X-ray flares being emitted from the corona of black hole 800 million light years away, which is 10 million times more massive than the Sun. A black hole’s corona is made up of scorching hot particles in the part of the accretion disc closest to the event horizon. The coronal X-rays reflected, or echoed, off the disc, and faint flashes of light now known as luminous echoes appeared behind them.

What is even more sci-fi about this is that the echoes could be observed in different colors. Not all particles in an accretion disc are made of the same elements, which means that their X-ray flashes will also differ, and the color of an echo depends on the wavelength produced by an X-ray. The distance at which an echo also affects what color it shows up as. As the wavelength of a flare (and its echo) stretches further, it will shift towards a different part of the spectrum.

Finding out where in the accretion disc each echo came from involved putting together the shift in color with how long it took the echo to reach Earth after an X-ray flare occurred. These observations could eventually help scientists get an idea of what is around black holes, most of which are too far away and therefore appear too small for imaging. M87 happened to be an anomaly.

Wilkins believes echoes could even tell us more about the birth of the universe. Because gas that is devoured by black holes, the extreme energy released can power some of the most luminous objects that exist. The brightest object ever found is an enormous dust cloud that would be invisible to us, because most of its light is infrared, but human vision is too limited for us to see an unedited image. This cloud is 300 trillion times brighter than the Sun.

“Understanding what is happening just outside supermassive black holes to power some of the brightest light sources in the Universe is a vital component in understanding how the structure of the Universe and the galaxies as we know them came to be,” he said.

Whether or not they want to, black holes seem to be slowly coming out of hiding.

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