Key Takeaways:

  • This confirms Einstein’s theory of general relativity, which predicts that massive objects warp spacetime and bend light.

  • The observed light is not directly emitted from behind the black hole, but rather reflected off the hot disc of matter swirling around it.

  • The black hole’s powerful magnetic field heats up the surrounding gas, producing high-energy electrons that generate X-rays.

  • Separating the reflected X-ray signal from other emissions is challenging.

  • By studying these echoes, astronomers can gain insights into the environment around black holes and how they grow.

Astrophysicists have seen light reflected from behind a black hole for the first time, confirming Einstein’s theory once more.

You may have heard that nothing – not even light – can escape a black hole, but this isn’t strictly true. Anything that passes through the event horizon is lost forever, but X-rays from the hot disc of matter circling the black hole can be seen from Earth with an incredible amount of power.

Not all of this light, though, is readily emitted.

Astrophysicist Dan Wilkins of Stanford University noticed unusual extra-X-ray flashes while observing the X-rays emerging from a supermassive black hole located 800 million light-years away at the center of a galaxy. They appeared as if they were echoes; they were smaller, appeared later, and had wavelengths distinct from the typical, brighter emissions.

These flashes appeared to be reflected from behind the black hole, which is an odd location for light to be coming from, according to a study led by Wilkins published in Nature.

Wilkins says, “We shouldn’t be able to see anything that’s behind the black hole because any light that goes into that black hole doesn’t come out.”

“The reason we can see that is because that black hole is warping space, bending light and twisting magnetic fields around itself.”

Like the surface of our Sun, a black hole’s extraordinarily powerful magnetic field arcs high above it as it spins and eventually becomes so tangled that the field lines break.

“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,” says Wilkins.

Some of the X-rays that are trying to escape the black hole’s strong gravitational pull wind up being pulled back, where they are reflected off the disc’s back and into space. Wilkins and his team see light because some of these “echoes” from behind the black hole are bent around it by intense gravity.

Building on findings from a study released that discovered “imprints” of such reflected light, astronomers have now directly observed light coming from behind a black hole for the first time.

Infographic describing how light echoes from behind a black hole.
Illustration of how light echoes from behind a black hole. Credit: ESA

Additionally, predictions derived from Einstein’s general theory of relativity are validated by these observations.

According to Queensland University of Technology astrophysicist Michael Cowley, who was not involved in the study, the theory indicates that massive objects have the ability to warp spacetime itself, causing light to travel along these bent paths.

The English astronomer Arthur Eddington made the first experimental proof of this theory in 1919 when he observed starlight bending around our Sun during a solar eclipse, according to Cowley.

By accounting for this effect, the authors could theoretically predict when the reflected X-ray signals should appear, as well as what they should look like.

However, as Curtin University astrophysicist James Miller-Jones, who was not involved in the research team, notes, it is difficult to see this effect.

He says, “It requires separating out from all the other emission in this active and high-energy region the disc’s response to being lit up by the X-ray flares from a region above the black hole.” The achievement of obtaining such a well-defined signal is impressive.

Cowley agrees: “This new work continues to bridge the divide between observational and theoretical research of active supermassive black holes and provide us with more insight into how they can generate such awesome amounts of power.”

Astronomers will be able to learn more about black holes by observing these kinds of signals.

According to astrophysicist Eric Thrane of Monash University and OzGrav, “one of the really cool things about this latest paper is that the researchers are able to probe the environment around a black hole devouring hot gas.”

They observe how this environment is ever-changing because it is dynamic. They calculated the black hole’s mass and the surrounding gas’s behavior using this information.

How the largest black holes in the universe became so massive so quickly is one of the fascinating mysteries of cosmology. I’m hoping that more research along these lines will eventually clarify how black holes expand over cosmic time.

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