Peppered throughout the cosmos are mysterious monstrosities where gravity is so strong, nothing – not even light – can escape. The extreme gravity of these black holes imparts the ultimate invisibility cloak, hiding them from telescopes and preventing astronomers from seeing them for over a century.
But an international team of researchers have finally managed to capture an image. On April 10, 2019, they released the first-ever glimpse of an elusive black hole. With eight linked telescopes positioned around the globe, the group produced its silhouette: a black disc embraced by a glowing crescent of artificial orange color in the inky expanse of space.
“This is going to be an iconic image,” says Sheperd Doeleman, an astronomer at Harvard University and Smithsonian’s Center for Astrophysics, director of the Event Horizon Telescope project and Moore Foundation grantee. Doeleman expects the golden ring to one day sit among other famed photos from the history of space such as the Earth rising over the moon.
Developing the science
An English rector named John Mitchell proposed the concept of a black hole in 1783, which he coined “dark stars.” Though his initial notion was to use light to determine the gravitational pull from a star, – and thus its mass – he imagined a case where gravity was too strong to allow light to escape. But Mitchell did little to promote his ideas and they faded to obscurity.
Then, over 100 years later, Albert Einstein first predicted what would happen in a black hole’s vicinity with his theory of general relativity in 1915. His theory suggested that massive objects, such as stars, distort space-time and we perceive this as gravity. Think of gravity like a ball on a trampoline. The larger the ball, the deeper the dent in the fabric, and smaller objects on the trampoline will fall toward it.
The next year, physicist and astronomer Karl Schwarzschild solved the first analytical solutions to the equations of general relativity, implying the existence of black holes. For decades, astronomers sought answers about these extreme cosmic structures. Yet no one knew what they looked like in real life.
That changed in the 1970s, when scientists generated both a mathematical calculation and a computer simulation of a black hole. But a picture of a real, live structure remained out of reach. Current telescopes weren’t sophisticated enough to achieve the needed resolution.
There are two types of black holes. Stellar black holes are three to several tens of times the size of the Sun and form when gigantic stars collapse at the end of their lives. Supermassive black holes, on the other hand, are millions or billions of times larger. These live in the center of all large galaxies and, thanks to their gargantuan size, are the only types that Earth-bound telescopes had hopes of viewing.
Part of the problem was the distance between telescopes and black holes. Even though these structures are massive, vast regions of space extend in between. Seeing a supermassive black hole from Earth – such as Sagittarius A* (Sgr A*) in the center of the Milky Way of Messier 87 (M87) in the center of the Virgo cluster – is like trying to read the date on a quarter in Washington D.C. from Los Angeles.
But Doeleman and his colleagues knew that, theoretically at least, they could see the shadow of a black hole as particles of light swirled around its boundary drawn in by its immense gravitational force. This border around the black hole is called the event horizon and is the point from which nothing can escape its grip. And to see a black hole’s event horizon, scientists rely on a technique called very-long-baseline-interferometry (VLBI).
Making the image
Black holes emit a ton of radio waves. Radio waves – which are often used for communication – can stream the considerable distance between a black hole and Earth for scientists to measure with telescopes. To see a black hole via these waves, however, the telescope needs to be as big as the Earth.
Rather than build a telescope that would displace residents from half the planet, researchers can employ VLBI to link much smaller existing telescopes. Each one is synchronized with the others to record data at exactly the same time. Researchers can later combine the data to calculate what they were looking at.
In the early 2000s, Doeleman and his team created new instruments that could collect such data simultaneously from multiple telescopes. It led to what he calls a “breakthrough detection” that was published in Nature in 2008. With three telescopes, the group saw something in the center of the Milky Way that was only a few times the size of the Sgr A* event horizon.
“That just knocked our socks off,” Doeleman says. “This was the moment when, I think, the whole community looked and said ‘Wow, this is possible.’”
The next decade was filled with a flurry of telescope expansions and improvements to capture the first black hole image. In 2013, Doeleman received a grant from a Moore Foundation grant to build much faster electronic components for data processing and recording. He immediately hired a squad of post-doctoral fellows, including Laura Vertatschitsch, who worked on the instrumentation, and Michael Johnson, who focused on analyzing the data.
The team developed the Event Horizon Telescope – an array of eight synchronized radio telescopes designed to take the first images of supermassive black holes. Researchers built a new “brain” for the Earth-sized radio telescope, which enabled them to record and process an immense amount of data from linked arrays around the globe. They then developed the systems needed to compress the data into an image. In April 2017, the “brain” was put into practice in a span of unbelievably good weather. The next two years were dedicated to interpreting the results.
“As you’re doing it, there’s always this sense of terror like, ‘what if I’m completely misinterpreting my data?’” says Johnson, now a member of the Smithsonian Astrophysical Observatory.
But the crew found ways to make sure their results were real. Four teams worked with the data – blind to what their counterparts were doing. All four came up with essentially the same image: the circular event horizon of a black hole that is 6.5 billion times the size of the Sun and 55-million light-years away from Earth. Their results were published in a series of six papers in a special edition of The Astrophysical Journal Letters in April 2019.
Running extensive tests and coming up with the final image was like a marathon, Johnson says. “I think the nice thing is that sort of utter exhaustion at pulling this all together is wonderfully counterbalanced by the ability to finally share it with the world,” he says.
Lights, camera, gravity!
The image is only the beginning, says Avi Loeb, chair of astronomy at Harvard University, founding director of Harvard’s Black Hole Initiative and Moore grantee. Next is a movie.
Black holes are not stationary objects. They are dynamic and a movie would capture what a black hole looks like in real-time, says Loeb, who simulated black hole images over the past two decades and motivated the Event Horizon Telescope. He is also interested to learn where black holes come from and what lessons can be gleaned about gravity.
In the meantime, the first still image of a black hole has captivated the world. And it provides unique ways to study gravitational forces.
“I view this as a handshake across 100 years,” Doeleman says. “We’re trying to connect with Einstein and Schwarzschild – and through them Newton – to give us a new window into gravity.”
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