Cosmic funhouse mirror: Black holes distort light to form multiple mirror images, which are really beautiful!

Black holes distort the universe to form multiple images

Illustration of a black hole (Image credit: Mark Glick/Science Image Library)

Imagine an entire galaxy reflected in a funny hall of mirrors, where each image of the galaxy becomes increasingly weird and distorted with each repetition. This is what the universe looks like near the event horizon of a black hole, one of the most distorted places in the universe.

While physicists previously had some ideas about what these regions look like, a new calculation accurately simulates what can be seen around a black hole, opening up potential new ways to test Einstein's theory of general relativity.

Round and round

The area near a black hole is really strange. Black holes treat everything equally, regardless of mass. Light is swallowed by the black hole's event horizon, and nothing can escape its immense gravitational pull.

But if you put a galaxy behind a black hole and look at it from the other side, you'll see a distorted image of the galaxy. That's because some of the light from the galaxy almost grazes the edge of the black hole but doesn't fall into it.

Because of the black hole's extreme gravity, such light is bent in the direction of your line of sight. Strangely, to you as an observer, the galaxy you see appears to be far away from the black hole, rather than directly behind it.

The gravity around a black hole is so strong, and space-time so warped, that at a certain distance, light itself can orbit the black hole. Some light from background galaxies can even get trapped, looping around the black hole forever.

However, the light needs to be at a precise distance from the black hole to be trapped in orbit. It can also hit the black hole at an angle that causes it to make one (or more) loops before finally escaping.

Looking at the edge of a black hole, your eye sees one image of a background galaxy from the deflected light. Then, you see a second image of the Milky Way. The light that successfully made one orbit before escaping is then observed from light that made two orbits, then three, and so on.

For decades, physicists have used simple estimates to show that each image is e2π times closer to the edge of a black hole than the last.

In this formula, e is the base of natural logarithms, which is approximately equal to 2.7182. Pi is another irrational number, approximately 3.14159, so e2π gives a number very close to 500. This means that each repetition of the same background object is about 500 times closer to the edge of the black hole than the last.

This diagram shows how light is mirrored by a background near the edge of a black hole.

Light from galaxies in the background surrounds the black hole, creating an endless 'mirror' image of the universe. (Image credit: Peter Lawson)

Take some effort

While physicists can get this simple result using pen and paper calculations, they're not sure the special factor of 500 is completely accurate if they look closely at the complex curvature of spacetime near a black hole.

In a new study, Albert Snepen, a graduate student at the Niels Bohr Institute at the University of Copenhagen in Denmark, used numerical methods to simulate the physics of light orbiting (and escaping) near a black hole. He confirmed that the factor of 500 remained constant in high-precision measurements. His results were published in the journal Scientific Reports.

“It’s beautiful to understand why these images repeat themselves in such an elegant way,” Sneppen said in a statement.

Snepen found that the factor of 500 only applies to simplified, motionless black holes. Black holes in the real universe rotate, which changes the way light orbits them — which in turn changes the distance at which the image appears.

"It turns out that when the black hole is spinning really fast, you no longer have to get 500 times closer to it, but much smaller," Sneppen said. "Each image is now only 50 or 5 times closer, or even down to only twice as close to the edge of the black hole."

Because the black hole's rotation distorts the spacetime around it, each successive image of a background object appears flatter. Thus, the most distant images would appear relatively undistorted, while the nearest images might be completely unrecognizable.

Enter the amusement park

Technically, there are an infinite number of repeating images of background objects, each one closer to the event horizon. In practice, humans will probably never be able to see them, because only a few of the images are resolvable even with the most powerful telescopes.

But these few will provide a powerful glimpse into the heart of general relativity, a mathematical theory that describes gravity.

In 2019, the Event Horizon Telescope — a disk-shaped network of telescopes spanning the globe — produced the first image of a black hole’s “shadow” cast on the gas and dust surrounding it. That telescope isn’t powerful enough to capture multiple funhouse mirror images of the background object, but future telescopes could.

Comparing the mirror images observed by the telescope with the data calculated by Snappen is an unprecedented test of general relativity. For example, if there is a supernova - a very powerful explosion of a dying star - behind the black hole, we will see that supernova explode multiple times. Each image will be delayed by a certain amount of time, depending on how many times it orbits the black hole, allowing researchers to compare their theory with reality.

We just have to be willing to stare into the void long enough.

BY:Paul Sutter

FY: Dong Meihui

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