All Black Holes Should Sport Light Rings

Theoretical astrophysicists predict that a glowing halo just outside the event horizon should surround all black holes.
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Black hole with disk

A NASA visualization of how a black hole warps light with its presence.

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Yuen Yiu, Staff Writer

(Inside Science) -- When the black hole Gargantuan first appeared onscreen in the 2014 blockbuster “Interstellar,” nobody had seen a black hole yet. Without a real-life image for reference, the visual effects artists who worked on the movie collaborated with astrophysicists to ensure their onscreen creation would look close to what the universe has in store for us.

In a tale of life imitating art imitating life, four years after “Interstellar” took home the 2015 Oscar for best visual effects, astronomers would capture the first actual image of a black hole. The image looked like a blurry picture of the very black hole they helped to create onscreen.

By now, you have probably seen the famous image of the supermassive black hole near the center of the galaxy Messier 87. The black hole, named M87* (pronounced “M87-star”), is billions of times heavier than our sun and more than 50 million light-years away. The fuzzy image of a glowing halo was the first to allow astrophysicists to verify their theoretical predictions. But what if the image wasn’t blurry? What if the image could be as sharp as “Interstellar” in 4K? What can such an image teach us about our universe?

Black holes give you rings

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Black hole closeup

An overview of the main observable features near a black hole according to a NASA visualization. Click here for a larger version.

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“A black hole, of course, is black. So, how can you see something that is black? You can see it only in contrast with light,” said Pedro Cunha, an astrophysicist from the Max Planck Institute for Gravitational Physics in Potsdam, Germany.

In the simulated image of a black hole shown above, there are two main features. The first is a thick band of light, known as the accretion disk, resembling the rings of Saturn. The disk is a band of hot glowing gas orbiting the black hole, but unlike Saturn’s rings, the far side of the band appears to have been warped both upward and downward to produce the band of light above and below the black hole. (This is labeled as “image of the disk’s far side” and “image of the disk’s underside” on the image.)

The other, perhaps less noticeable feature, is a much thinner ring of light closer to the center of the black hole, labeled as the “photon ring.”

“As you get close to the black hole, the gravity is so strong that light rays can bend around the black hole and close over themselves into circular orbits, or light rings,” said Cunha.

The photon ring, unlike the accretion disk, isn’t made of matter. It is closely related to the photon sphere, which is a shell of light encapsulating the black hole. The photons, most of them coming from the glowing accretion disk, travel on the skin of that sphere until they eventually fall into the black hole, never to be seen again, or escape from the orbit and travel to an observer far, far away.

The photon sphere represents the last possible points from which light can escape the gravitational pull of the black hole. It is intimately related to the event horizon of a black hole -- the boundary where even light cannot escape a black hole’s pull once it has crossed over --although the two are not the same thing.

“But there is a difference between what is there and what is observable. One thing is the orbits themselves, the other thing is what you see, and that's an important distinction,” said Cunha.

The reason we don’t see a black hole wrapped up in a glowing sphere of light is because we can only see the photons that have escaped the orbits around the black hole at the moment that they were flying directly at us, as they were being slingshot out of a circular cross section of the spherical shell, which looks like a ring of light to us.

Cunha is a co-author of a recent paper published in Physical Review Letters that explored the properties of light rings around black holes. Their results predict light rings exist for every type of black hole, a previous assumption that had not been thoroughly tested mathematically.

“The size and shape of this very, very thin ring encode the properties and the geometry of space-time,” said Cunha. “And that’s very interesting -- perhaps more interesting than even the accretion disk. Since the ring is a lot closer to the event horizon, which is essentially the edge of the black hole, if we can measure these light rings, we can learn about gravity in that regime which is not fully understood at the moment.”

So, how does this theoretical image of a black hole compare to what we can see with our current telescopes?

A forensic investigation of space-time

“The light ring is extremely thin. To have any information on the light ring itself will require an extremely high resolution. The kind of resolution we currently do not have,” said Luciano Rezzolla, an astrophysicist from Goethe University in Frankfurt, Germany. He was part of the Event Horizon Telescope collaboration that used a network of Earth-based radio telescopes to produce the famous image of the M87* black hole in 2019.

In order to produce the fuzzy image, a global network of telescopes had to have the resolution equivalent to seeing a Ping-Pong ball lying on the surface of the moon, from Earth.

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Black hole disk

The image of a black hole at the center of the Messier 87 galaxy.
Credit: Event Horizon Telescope Collaboration

Why don’t we see a belt around M87*?

In short, that is because from our perspective, we are staring at the poles of M87*. Imagine tilting Saturn until you are looking directly down at the face of the rings instead of the edge -- that is the angle we are viewing M87* from.

“If you were to look at M87* at an angle closer to its equator, it would look more similar to the one you see in ‘Interstellar,’ but with a much thicker and much less symmetric disk,” said Rezzolla.

“In ‘Interstellar,’ what you have is a very thin accretion disk, which we don't think is what happens in nature,” he said. “As plasma accretes around a black hole, we expect it to produce a doughnut, not a very thin pancake.”

“The resolution of a telescope essentially comes from its size, and we're already using a telescope that is as big as the Earth, by using an interferometric technique known as VLBI,” said Rezzolla. VLBI, or Very-Long-Baseline Interferometry, allows images collected by telescopes from far-flung corners of the Earth to combine their data into a single image.

“So, how would you get a telescope that is even bigger than the Earth? You can do that by going into space, by having radio telescopes mounted on satellites,” said Rezzolla. He is an author on a paper published in Proceedings of the International Astronomical Union that calculated that the resolution of the Event Horizon Telescope project can be improved about fivefold by employing two satellite radio telescopes -- not at all far-fetched with the current technology.

But for now, the Event Horizon Telescope collaboration is focusing on using its current setup to produce an image of the black hole at the heart of our own Milky Way galaxy, named Sagittarius A* (pronounced “Sagittarius A-star”). Astrophysicists noticed its presence by tracking the stars that were being tossed around by its massive gravitational pull over more than a decade.

“The data is far more complicated than M87* because Sagittarius A* is much smaller, and black holes with a certain mass bring with them a certain time scale,” said Rezzolla.

Because Sagittarius A* is smaller than M87*, the light features of the plasma around the black hole change more quickly, making it more complicated to analyze. “It's like taking a photo of a subject that’s moving all the time and in a very unpredictable way,” said Rezzolla.

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Yuen Yiu covers the Physics beat for Inside Science. He's a Ph.D. physicist and fluent in Cantonese and Mandarin. Follow Yuen on Twitter: @fromyiutoyou.