By Daniel Stolte, University Communication
Discovering something a second time doesn’t usually make scientists jump out of their seats with excitement. But that is exactly what happened in the case of Sgr A* (pronounced “sadge-ay-star”), the second black hole pictured.
In 2019, the image of M87*, a supermassive black hole in a galaxy more than 50 million light-years from Earth, made the front pages of virtually every news outlet around the world. It was the first time an image of a black hole had ever been captured. On Thursday, the Event Horizon Telescope Collaboration presented the second image of such an object – this time a black hole at the center of our own Milky Way.
To the casual observer, the two images of a glowing orange ring surrounding a black shadow appear almost indistinguishable. But exactly this fact astonishes astrophysicists.
“I wish I could say it hasn’t gotten any better since we took the first image of a black hole three years ago, but this is actually better,” said a member of the EHT Science Council Feryal Ozel, professor of astronomy and physics and associate dean for research at the Steward Observatory of the UArizona College of Science. “We see a bright ring surrounding total darkness, the telltale sign of a black hole. Now we can affirm that we are looking straight to the point of no return.”
A love affair with black holes
Özel said she “fell in love” with Sgr A* 20 years ago. She was then a graduate student working on her dissertation at Harvard University when she decided to take on a challenge few thought possible: What would it take, she wondered, to actually look directly at a black hole? what would we see would we see something
Her research culminated in a seminal paper that she co-published in 2000 Dimitrios Psaltisa UArizona professor of astronomy and physics and senior investigator of the international Black hole PIRE project. In that work and a follow-up work published in 2001, she identified M87*, the first black hole ever imaged, and Sgr A* as the two ideal black holes that offered even a remote chance of being photographed by them. This helped lay the foundations for an Earth-sized observatory that is now the Event Horizon Telescope.
Because M87* is 1,500 times more massive but 2,000 times more distant than Sgr A*, the two appear about the same size in the sky. But despite the fact that they look almost identical, they are completely different beasts.
M87* has a mass of 6 billion suns and is gigantic in size. Our entire solar system would fit within its event horizon, also known as the black hole’s point of no return. In comparison, Sgr A* is only 25,000 light-years from Earth. With “only” 4 million solar masses, it is small enough to fit into the orbit of Mercury, the planet closest to the Sun. If the two black holes were lined up for a photo op, M87* would fill in the image while Sgr A* would disappear entirely. And while M87* voraciously devours surrounding matter, perhaps entire stars, and fires a beam of energetic particles that burns across its galaxy, Sgr A*’s appetite is minimal by comparison; According to the researchers, if it were a human, it would consume the equivalent of a grain of rice every million years.
One of the most fundamental predictions of Einstein’s theory of gravity, Psaltis said, is that the image of a black hole only scales with its mass. A black hole that is 1,000 times smaller than another has a very similar image that is only 1,000 times smaller. The same is not true for other objects, Psaltis explained.
“In general, small things usually look very different than big things, and that’s no coincidence,” he said. “There’s a good reason why an ant and an elephant look very different, as one has a lot more bulk to carry than the other.”
In other words, nature’s laws of size dictate that two objects that are very different sizes will usually look different. Black holes, on the other hand, scale without changing their appearance. If they were elephants, they would all look like elephants, whether they were as big as a typical elephant or as small as an ant.
Their stark simplicity is what makes the two black hole images so important, Psaltis explained, because they confirm what only theory had previously predicted: they appear to be the only objects in existence that obey only one law of nature — gravity.
“The fact that the light appears like a ring with the black shadow in it tells you it’s pure gravity,” Psaltis said. “All of this is predicted by Einstein’s general theory of relativity, the only theory in the cosmos that doesn’t care about scale.”
If scientists take a picture of a really small black hole of about 10 solar masses – which is not possible because even the Earth-sized EHT does not have the necessary resolving power – and compare it with M87* which has 6 billion solar masses, the two would therefore look very similar aboveSalt.
“Everywhere we look we should see donuts, and they should all look more or less the same,” he said, “and the reason for that, besides the fact that it confirms our prediction, is that nobody likes it. In physics, we tend to reject a world where things have no anchor point, no defined scale.”
The “Black Holes of Goldilocks”
Black holes are objects so alien that even Albert Einstein struggled to reconcile their existence. Their gravitational pull is so strong that not even light can escape, making them impossible to see by definition. The only reason astronomers were able to get these images is because they used radio telescopes that detect electromagnetic waves emitted by gas swirling around the black hole.
“If you looked at the black hole in space, you would see absolutely nothing,” Özel said. “The glow is in wavelengths that the eye can’t see.”
For this reason, M87* and Sgr A* were identified as the only viable targets for the Event Horizon Telescope in the paper written by Özel and Psaltis more than 20 years ago.
“You could say both are ‘Black Hole Goldilocks’,” said Özel. “Their surroundings are just right, and that’s why we can see them.”
For astrophysicists like Özel and Psaltis, black holes are natural laboratories, allowing them to test general relativity and perhaps even taking them closer to a theory that unifies gravity with quantum mechanics that has previously been elusive.
“The path to the image was not easy,” said Özel, who has been a member of the EHT Science Council since its inception and leads the modeling and analysis group. It took global collaboration, several years, petabytes of data, and more complicated algorithms than most previous scientific efforts to analyze and confirm the final picture of Sgr A*.
Going forward, the EHT collaboration is particularly interested in how black holes change over time, Özel said.
“If you looked at the source from one day to the next or one year to the next, how would that change and how much light would it emit at different wavelengths?” She said. “What could we predict about that? And how could we use our observations to understand the environment of this black hole?
“One of the most important points of this joint effort,” Özel said, “is to test general relativity and find out what its limit is, if there is one.”