The Event Horizon Telescope collaboration has reconstructed the shadow of a supermassive black hole.
Scientists have at last “seen” a black hole — and it’s beautiful.
Announcing the result at a National Science Foundation press conference in Washington, D.C., representatives of the Event Horizon Telescope collaboration unveiled a reconstructed image of the gargantuan black hole in the giant elliptical galaxy M87. The galaxy lies about 55 million light-years away in the constellation Virgo. The black hole itself is so large that light would take 1½ days to cross it.
“We have seen what we thought was unseeable,” said project director Sheperd Doeleman (Center for Astrophysics, Harvard & Smithsonian) during the press conference. “We have seen and taken a picture of a black hole. Here it is.”
Unmasking the Invisible
Capturing a black hole’s visage requires far more than just a point-and-shoot approach. The worldwide team of researchers, comprising 200 people from some 20 countries, constructed the black hole’s image using a technique called very long baseline interferometry (VLBI). VLBI combines the data from multiple radio telescopes scattered across the globe to create a virtual, Earth-sized dish, with a resolution equivalent to being able to read the date on a quarter in Los Angeles seen from D.C., Doeleman said.
Think of the old joke about a bunch of blindfolded scientists studying different parts of an elephant — one finds an ear, another the tail, and so forth. With VLBI, each blindfolded scientist represents the distance, or baseline, between two telescopes. But instead of sampling different body parts, each baseline observes a different scale of the elephant: One says it’s 10 feet high; another says there’s an ear here of such-and-such size; still another explores the fine texture of the elephant’s skin. As Earth turns, the baselines change, detecting different scales of the elephant. The scientists then piece these bits of information together into a coherent image.
Although the black hole image released today and reported in six papers in the Astrophysical Journal Letters is based on only four days of observations, EHT scientists spent years testing and installing equipment, working in the thin air of the remote Chilean desert, braving the cold of Antarctica. They built computer algorithms and developed simulations of what they might see. They did dry runs, agonizing over go/no-go weather conditions at eight telescopes at six geographic sites scattered from Hawai‘i to Spain and Arizona to the South Pole. “In VLBI, you really only get one shot,” said Dan Marrone (University of Arizona), who has flown repeatedly to the South Pole to retrofit the telescope there. “Everything has to be working exactly right.”
Then, in April 2017, they went for it.
As Earth turned, each telescope set its sights on M87 and the other targets, stockpiling data. By the end of the observing run, the observers had filled a half ton of hard drives with 5 petabytes of data — the equivalent of 5,000 years of MP3 files, or, Marrone quipped, “the entire selfie collection over a lifetime for 40,000 people.”
The team then flew these hard drives to Massachusetts and Germany, where the eight stations’ observations were fed into supercomputers and aligned to within trillionths of a second. “They have to be exactly right,” says Michael Johnson (Center for Astrophysics, Harvard & Smithsonian), who helped coordinate the imaging data analysis. “If they’re even a tiny bit off, you see nothing.”
The Shadow Knows
All these tribulations they tackled in order to detect the tiny silhouettes of distant supermassive black holes.
As gas swirls around a black hole and dives deeper into the pit the black hole creates in spacetime, it heats up, emitting light across the electromagnetic spectrum, from X-rays to radio, explains EHT astronomer Feryal Özel (University of Arizona). Very close to the black hole’s event horizon, these photons can become temporarily trapped, looping around and around the black hole in what’s called a photon ring before escaping and reaching our telescopes. As the glowing gas continues to fall in, though, it will plunge past the event horizon, and its light will never reach us. These effects combine to create what’s called the black hole’s “shadow”: a dark circle surrounded by a bright ring. It looks a bit like a glazed doughnut.
Once the researchers had calibrated their data, a subset of them (mostly young astronomers and computer scientists just starting their careers) split into four teams. “We told them, ‘Don’t talk to each other or anyone else,’” said Marrone. “‘Choose whichever imaging algorithms you think are best, and make images of these data.’”
“We went into a room, there were six or seven of us there,” says Johnson, “and we actually had the first picture 30 minutes later.”
The challenge isn’t making one image, he explains, but understanding its subtleties. The teams had to know all the potential images their algorithms might create and where the codes might lead them astray. After testing the countless alternatives, they all met and unveiled their four images — and all looked remarkably alike: four dark circles surrounded by ringlike structures.
Then the team members reorganized themselves, using what they’d learned to systematically attack each day’s data three different ways. Each method produced a slightly different image, but once again, the images were strikingly consistent with one another.
Combining these images into a single one took a long, long time, Johnson says. The researchers wrestled with how to convey what was sure versus what might be the byproduct of a single algorithm’s favorite bits. They finally decided to combine the four images and blur the result to match their instruments’ resolution. By doing so, they only showed the structure that appears using all four methods. “We stand behind basically every element” of this conservative image, he says.
And what an image it is. The width of the silhouette is about 40 microarcseconds — the size of a thumbnail seen from 40,000 miles away. “This is the first time that I saw this image,” said NSF Director and astrophysicist France Córdova, “and it did bring tears to my eyes.”
Around the Rabbit Hole
The team’s primary focus for today’s announcement was creating the image. But they have ascertained some of the underlying physics, too. Based on the size of the shadow, the researchers calculated the black hole contains 6.5 billion solar masses, a figure close to the larger of two contested values.
The black hole is spinning clockwise from our perspective; the bright crescent to the south is the boosted beam of gas moving toward us, while the dimmer north is where gas recedes from us. The data do not, however, reveal how quickly the black hole spins, because the shadow’s shape and size are independent of the spin except for the most extreme rotations.
The image also gives astrophysicists a huge boost of confidence in their theories about what happens near an accreting black hole. “I have to admit I was a little stunned that it matched so closely the predictions we had made,” said Avery Broderick (Perimeter Institute and University of Waterloo, Canada).
“Just the fact that our [simulations] came so close to images like the one we ended up getting for M87 already tells us that we’re on the right track for understanding accretion physics,” says Özel. “We could have been completely off.”
Soon, the researchers will start putting together maps of how the magnetic fields — crucial for powering jets like the one M87’s black hole spews forth — move near the event horizon. They also will be analyzing their observations of our own galaxy’s central black hole, Sagittarius A*. Although many expected today’s result to be about Sgr A*, not M87, our black hole will take more time: Because it’s about a thousandth the mass of M87’s black hole, Sgr A* is smaller, and gas whips around its circumference a thousand times faster. That means we see much faster changes in its light, making teasing apart the shadow signature more complex. “We knew it was a more turbulent child,” says Özel. “We have to apply special care.”
The team is already working on pushing to a slightly shorter wavelength than the 1.3 mm currently used, which will boost their resolution considerably. But M87 and Sgr A* are the only two black holes whose shadows we can detect with ground-based networks. To expand to a larger number of black holes, they’ll have to put radio telescopes in space. Adding geosynchronous orbits would lengthen baselines by more than six times Earth’s radius, enabling the EHT to see black hole shadows roughly one-tenth as wide as M87’s.
I first met Doeleman nearly 10 years ago, when writing my master’s thesis on black holes, and like many others I caught his visionary enthusiasm. Over the years, I’ve watched the team face many setbacks and frustrations. “I didn’t realize how hard it would be when we started out,” Doeleman says, thinking back on the journey. But he was indefatigable. “You know, there are just some projects that you have to keep pushing on, no matter what.” That they’ve seen a black hole’s shadow — when they could have just seen yet another blob, or something unexpected — is mesmerizing, humbling, and a testament to the years of everyone’s hard work. Or, as he puts it: “Sometimes you have to kiss a lot of frogs before you get the prince.”
The Event Horizon Telescope Collaboration. “First M87 Event Horizon Telescope Results. I-VI.” Astrophysical Journal. April 10, 2019.
Dimitrios Psaltis. “Testing General Relativity with the Event Horizon Telescope.” arXiv.org. June 26, 2018.
NSF press conference. April 10, 2019.
Update: The original version of this story stated the final image is the combination of the initial four images made; it's actually the combination of three images. The sixth paragraph of "The Shadow Knows" is a later addition to correct the explanation.