Black holes have been mysterious and elusive — until now. Astronomers using the Event Horizon Telescope (EHT) have, for the first time, photographed one.
“We’ve now seen the unseeable,” said Avery Broderick, a physicist at the University of Waterloo and the Perimeter Institute who was part of the international EHT research team. “Black holes are made real — they’re not just the scribblings on theorists’ chalkboards anymore, but they really are out there in the night.”
The image, which shows an orange ring around a round, black silhouette, is of the black hole at the centre of Messier 87 (M87), a galaxy 50 million light-years from Earth. This black hole is one of the most massive known: it’s six billion times more massive than our sun.
Black holes are so dense and have such strong gravity that anything that crosses their threshold — known as the event horizon — gets pulled into them, never to return. That includes both matter and light, making them black and invisible — and therefore very difficult to see and photograph.
An international team of more than 200 people spent more than a decade working to capture the image released today.
Black holes are made real — they’re not just the scribblings on theorists’ chalkboards anymore, but they really are out there in the night.– Avery Broderick , Perimeter Institute for Theoretical Physics
“Seeing the culmination of that effort was simply a marvelous moment,” Broderick said. “But at the same time I’m very excited about the future because this marks the beginning of a new era in astronomy, a new era of research into gravity. And we really just are standing at the threshold today.”
That’s because the images allow scientists to test Einstein’s general theory of relativity in ways that they never have before. On Earth, Newton’s laws of physics and the way they describe gravity work pretty well. But when gravity becomes extreme, Newton’s laws and general relativity predict very different things. That kind of extreme gravity doesn’t exist in our solar system, but it does in black holes.
“We’re able to probe general relativity in this region that has never been accessed before,” Broderick said. “That’s completely new and extremely powerful.”
So has Einstein’s theory passed the test so far?
“The answer is yes,” Broderick said.
M87’s black hole has long been intriguing for astronomers and astrophysicists. Not only is it incredibly massive, but it is also spewing a stream of particles outwards. These aren’t particles that have fallen inside a black hole, since nothing can escape once it has fallen in. Instead, particles are flung out just before crossing what is called the event horizon, the point of no return around a black hole, after which the black hole consumes whatever has fallen in and grows.
M87 was one of two targets for the EHT, the second being Sagittarius A*, a supermassive black hole at the centre of our galaxy, some 25,000 light-years away. This black hole has a mass of about 4.3 million times that of our sun.
Astronomers have been trying to directly image a black hole, but imaging something that is so far off and essentially invisible requires some out-of-the-box thinking.
Enter the EHT, a collection of eight telescopes that span the globe. Instead of having a telescope that measures perhaps a few metres or tens of metres across, astronomers now have telescopes that work in unison and become “Earth-sized.” This allows astronomers to collect data that provides an image of the black hole, though with some missing data.
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Breaking new scientific ground
Priya Natarajan, who was not involved with the research, studies the unseen. She is a theoretical astrophysicist and professor of astronomy and physics at Yale University in New Haven, Conn., whose primary area of research centres around dark matter, dark energy and black holes.
“Black holes are definitely much more important than we thought,” said Natarajan.
Specifically, they’re important in understanding their influence in and around the space they occupy, which hasn’t been understood.
“All the data that we have had so far has been through indirect imaging, through indirect inference,” she said.
in the dark and then suddenly you can see the face of the person.– Priya Natarajan , theoretical astrophysicist
Natarajan refers to M87’s black hole as an “ultramassive” black hole, one that is more than five billion times more massive than our sun — where it has “stunted” its growth meaning it can’t get any bigger.
“That is super cool,” Natarajan said. “That we happened to have had this one ultramassive black hole practically in our backyard.”
Natarajan says that thus far, indirect observations have stretched out to perhaps 1,000 to 100,000 times from the event horizon. But now the image takes astronomers right to the edge.
“It’s like you are seeing a silhouette in the dark and then suddenly you can see the face of the person,” she said.
Years in the making
Trying to image a black hole has taken a lot of time: 12 years with EHT involving hundreds of people from around the world. And it’s only come to be due to rapidly improving data storage and more telescopes that have been brought online.
Eight telescopes at six sites, each fitted with special equipment, imaged M87’s black hole in April 2017.
So why did it take two years to share the image?
“First, when you’re trying to break new ground, it behooves one to be very, very careful,” Broderick said. “Secondly … you often find yourself having to reinvent the wheel.”
Twice the team had to develop special software tools.
“The real question is why did it only take two years?” Broderick said.
Broderick, who has been with the EHT project for more than a decade, said that one of the most interesting things for him is observing the plasma that goes around the black hole: it changes on a time-scale of minutes to a week. He refers to the material as “fluff” — hot luminous plasma that has very little mass compared to the black hole itself.
“Every time we go back and look at these objects again, we’re getting a different version of the astrophysical fluff,” Broderick said.
And from this, they may acquire a wealth of knowledge.
The ‘ultimate paradox’
Natarajan is in awe of what this new announcement means.
“When you work in cosmology, there’s this ultimate paradox,” she says. “We are extremely significant because of all these systems of knowledge that we’ve created … and yet on the scale of the cosmos, we are really insignificant.”
Both scientists are anxious to obtain more knowledge and perhaps eventually unify quantum theory (the study of the very small, i.e., subatomic particles) and Einstein’s theory of general relativity (the study of the very big) — something that has eluded astrophysicists for years.
“You don’t know what’s under the rock until you turn it over,” Broderick said. “This is a voyage of exploration.”