This is what a black hole looks like.
A black hole isn’t really a hole. It’s an object in space with incredible mass packed into a very small area. All that mass creates such a huge gravitational tug that nothing can escape a black hole, including light.
The newly imaged supermassive monster lies in a galaxy called M87. A world-spanning network of observatories called the Event Horizon Telescope, or EHT, zoomed in on M87 to create this first-ever picture of a black hole.
“We have seen what we thought was unseeable,” Sheperd Doeleman said April 10 in Washington, D.C. “We have seen and taken a picture of a black hole,” he reported at one of seven concurrent news conferences. Doeleman is EHT’s director. He also is an astrophysicist at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. Results from his team’s work appear in six papers in the Astrophysical Journal Letters.
The concept of a black hole was first hinted at back in the 1780s. The mathematics behind them came from Albert Einstein’s 1915 general theory of relativity. And the phenomenon got its name “black hole” in the 1960s. But until now, all “pictures” of black holes have been illustrations or simulations.
“We’ve been studying black holes so long, sometimes it’s easy to forget that none of us have actually seen one,” France Córdova said in the Washington, D.C., news conference. She is director of the National Science Foundation. Seeing a black hole “is a Herculean task,” she said.
That’s because black holes are famously hard to see. Their gravity is so extreme that nothing, not even light, can escape across the boundary at a black hole’s edge. That edge is known as the event horizon. But some black holes, especially supermassive ones dwelling in galaxies’ centers, stand out. They gather bright disks of gas and other material that surrounds the black hole. The EHT image reveals the shadow of M87’s black hole on its accretion disk. That disk looks like a fuzzy, asymmetrical ring. It unveils for the first time the dark abyss of one of the universe’s most mysterious objects.
“It’s been such a buildup,” Doeleman said. “It was just astonishment and wonder… to know that you’ve uncovered a part of the universe that was off limits to us.”
The much-anticipated big reveal of the image “lives up to the hype, that’s for sure,” says Priyamvada Natarajan. This astrophysicist at Yale University, in New Haven, Conn., is not on the EHT team. “It really brings home how fortunate we are as a species at this particular time, with the capacity of the human mind to comprehend the universe, to have built all the science and technology to make it happen.”
Einstein was right
The new image aligns with what physicists expected a black hole to look like based on the theory of general relativity by Albert Einstein. That theory predicts how spacetime is warped by the extreme mass of a black hole. The picture is “one more strong piece of evidence supporting the existence of black holes. And that, of course, helps verify general relativity,” says Clifford Will. He’s a physicist at the University of Florida in Gainesville, who is not on the EHT team. “Being able to actually see this shadow and to detect it is a tremendous first step.”
Studies in the past have tested general relativity by looking at the motions of stars or gas clouds near a black hole, but never at its edge. “It’s as good as it gets,” Will says. Tiptoe any closer and you’d be inside the black hole. And then you’d be unable to report back on the results of any experiments.
“Black hole environments are a likely place where general relativity would break down,” says EHT team member Feryal Özel. She is an astrophysicist who works at the University of Arizona in Tucson. So testing general relativity in such extreme conditions could reveal things that don’t seem to support Einstein’s predictions.
However, she adds, just because this first image upholds general relativity “doesn’t mean general relativity is completely fine.” Many physicists think that general relativity won’t be the last word on gravity. That’s because it’s incompatible with another essential physics theory, quantum mechanics. This theory describes physics on very small scales.
The new image provided a new measurement of the size and heft of M87’s black hole. “Our mass determination by just directly looking at the shadow has helped resolve a longstanding controversy,” Sera Markoff said in the Washington, D.C., news conference. She’s a theoretical astrophysicist at the University of Amsterdam in the Netherlands. Estimates made using different techniques have ranged between 3.5 billion and 7.22 billion times the mass of the sun. New EHT measurements show that the mass of this black hole is about 6.5 billion solar masses.
The team also has figured out the behemoth’s size. Its diameter stretches 38 billion kilometers (24 billion miles). And the black hole spins clockwise. “M87 is a monster even by supermassive black hole standards,” Markoff said.
EHT trained its sights on both M87’s black hole and Sagittarius A*. That second supermassive black hole sits at the center of our galaxy, the Milky Way. But, the scientists found it easier to image M87’s monster, even though it’s about 2,000 times as far away as Sgr A*.
M87’s black hole sits about 55 million light-years from Earth in the constellation Virgo. But it’s also about 1,000 times as massive as the Milky Way’s giant. Sgr A* only weighs the equivalent of roughly 4 million suns. M87’s extra heft nearly compensates for its greater distance. The size it covers in our sky “is pretty darn similar,” says EHT team member Özel.
Because M87’s black hole is bigger and has more gravity, gases swirling around it move and vary in brightness more slowly than they do around Sgr A*. And here’s why that’s important. “During a single observation, Sgr A* doesn’t sit still, whereas M87 does,” says Özel. “Just based on this ‘Does the black hole sit still and pose for me?’ point of view, we knew M87 would cooperate more.”
With more data analysis, the team hopes to solve some long-standing mysteries about black holes. These include how M87’s black hole spews such a bright jet of charged particles many thousands of light-years into space.
This first image is like the “shot heard round the world” that kicked off the American Revolutionary War, says Avi Loeb. He is an astrophysicist at Harvard University in Cambridge, Mass. “It’s very significant. It gives a glimpse of what the future might hold. But it doesn’t give us all the information that we want.”
The team does not yet have a picture of Sgr A*. But the researchers were able to collect some data on it. They are continuing to analyze those data in the hopes of adding to a new gallery of black hole portraits. Since the appearance of that black hole changes so quickly, the team is having to develop new techniques to analyze the data from it.
“The Milky Way is a very different galaxy from M87,” Loeb notes. Studying such different environments could reveal more details of how black holes behave, he says.
The next look at the M87 and Milky Way behemoths will have to wait, though. Scientists got a lucky stretch of good weather at all eight sites that made up the Event Horizon Telescope in 2017. Then there was bad weather in 2018. (Water vapor in the atmosphere can interfere with the telescope’s measurements.) Technical difficulties cancelled this year’s observing run.
The good news is that by 2020, EHT will include 11 observatories. The Greenland Telescope joined the consortium in 2018. The Kitt Peak National Observatory outside Tucson, Ariz., and the NOrthern Extended Millimeter Array (NOEMA) in the French Alps will join EHT in 2020.
Adding more telescopes should allow the team to extend the image. That would let EHT better capture the jets that spew from the black hole. The researchers also plan to make observations using light having a slightly higher frequency. That can further sharpen the image. And even bigger plans are on the horizon — adding telescopes that orbit Earth. “World domination is not enough for us. We also want to go to space,” Doeleman quipped.
These extra eyes may be just what’s needed to bring black holes into even greater focus.
Staff writer Maria Temming contributed to this story.