Nobody had ever seen a black hole. Not because telescopes weren’t powerful enough in the ordinary sense, but because the object researchers wanted to photograph was 55 million light-years away and so compact that capturing it would be, as one scientist put it, like trying to photograph an orange sitting on the surface of the Moon. On April 10, 2019, a team of scientists managed it anyway, and a 29-year-old computer scientist’s reaction to seeing the image resolve for the first time became almost as famous as the photo itself.
Building a Telescope the Size of a Planet
No single telescope on Earth could ever be built large enough to resolve something so small and so far away. So the Event Horizon Telescope collaboration didn’t build one, they networked eight radio telescopes scattered across the globe, from the South Pole to Spain, and linked them together to function as a single virtual instrument roughly the size of Earth itself. Each telescope captured its own slice of data simultaneously, and the real challenge became stitching those fragments into a coherent picture.
That’s where Katie Bouman came in. As a graduate student in computer science and artificial intelligence at MIT, she led the development of an algorithm called CHIRP, Continuous High-resolution Image Reconstruction using Patch priors, specifically designed to reconstruct an image from the EHT’s sparse, noisy data. “We can’t build an Earth-sized telescope,” she later explained. “So instead, what we did is we built a computational Earth-sized telescope.”
Four Petabytes, Shipped on Hard Drives
The scale of the data problem was almost absurd. The EHT’s observing campaign generated roughly four petabytes of data, so much that it couldn’t practically be transmitted over the internet and instead had to be physically shipped, hundreds of pounds of hard drives flown to processing centers including MIT’s Haystack Observatory. To make sure the eventual image reflected reality rather than some quirk of a single algorithm, the team split into four independent groups, each working with different imaging methods, deliberately kept isolated from each other’s results to avoid any unconscious bias.
After roughly a month of separate work, the four teams finally compared notes. “That was the happiest moment I’ve ever had,” Bouman said, “when I saw all the other teams had images that were very similar.” Every group, working independently, had converged on the same glowing, asymmetric ring.
A Photo of a Photo
When the final image was unveiled to the world on April 10, 2019, a colleague happened to photograph Bouman’s own reaction as the ring resolved on her screen, hands pressed to her face, visibly overwhelmed. That image spread across the internet nearly as fast as the black hole photo itself, turning a previously unknown graduate student into an unexpected public face of the discovery. Bouman was quick to redirect the attention, noting that over 200 researchers had contributed to the project, roughly 40 of them women, and that no single person deserved sole credit for a genuinely collective achievement.
If you’re curious about the second black hole the same collaboration went on to image, our profile of Sagittarius A*, the supermassive black hole at the center of our own Milky Way, covers the EHT’s follow-up target, one that proved even harder to photograph precisely because Earth sits inside the same galaxy as its subject.
What the Ring Actually Shows
The glowing ring in the image isn’t the black hole itself, black holes are by definition invisible, since nothing, not even light, escapes them. What the EHT captured instead was the black hole’s shadow, a dark silhouette cast against the bright, superheated disk of gas swirling just outside its event horizon. “Years ago, we thought we would have to build a very large space telescope to image a black hole,” said Paul Hertz, NASA’s astrophysics division director. “By getting radio telescopes around the world to work in concert like one instrument, the EHT team achieved this, decades ahead of time.”
For more on the discovery, check out the full announcement from NASA and Bouman’s own account from CNBC’s coverage.