In many ways, the late astrophysicist Stephen Hawking resembled the very phenomena he spent so much of his life conceptualizing: black holes. Like a black hole, Hawking was a massive presence who drew all around him into his personal orbit. Where black holes collect light, Hawking collected luminaries—his circle of intellectual friends and acquaintances was as energetic and bright as they come, including Illinois-born physicist and screenwriter Leonard Mlodinow, Caltech Nobelist Kip Thorne, and British Astronomer Royal Martin Reese.
Yet while black holes are known for destroying and compressing everything that has the misfortune to fall within their range, Hawking was always astonishingly creative. He expanded the collective imagination of humanity in a way few others have managed, helping scientists and non-scientists alike to better grasp the workings of the universe. In the wake of his death, scores of Hawking’s longtime collaborators stepped back from the daily whir of research to reflect on the impacts he had made on their lives.
Among his many collaborations, Hawking had an outsized influence on an unprecedented global astronomy project called the Event Horizon Telescope (EHT), whose development was led in part by the Harvard-Smithsonian Center for Astrophysics. The mission of the EHT is deceptively simple: create the first-ever image of a real black hole. Its target, some four million times the mass of our Sun, sits at the very heart of our galaxy, the Milky Way. The ongoing mission, which involves a large network of telescopes whose observations will be composited, spans dozens of academic institutions across the globe.
At a distance of 26 light years, it may seem a wonder that we can detect such an object at all. But black holes are a lot less dark and lifeless than most people would expect. “You would think that a black hole, which swallows light, wouldn’t be visible,” says EHT director Sheperd Doeleman. “But in fact, there’s so much gas and dust trying to get into it that, through friction and heating, the gas around the black hole shines with a temperature of hundreds of billions of degrees. They wind up being some of the brightest things in the sky.”
EHT director Sheperd Doeleman says Stephen Hawking played a dual role in shaping the project. On the one hand, Hawking’s technical findings remind EHT scientists that, whatever their preconceptions of what this black hole will look like, they have to be ready for anything. “He really showed some of the seminal results that led to our understanding that black holes can emit radiation,” Doeleman says. “Until then, they’d been these drains of the universe that were a one-way door. And he rsolidified the mathematics and the thinking behind the fact that they appear to be able to evaporate and radiate all the energy that they have swallowed. So, in that sense, they become quite different objects.”
At the same time, the macroscopic example of Hawking, a man who managed to assemble seminal theories on black hole dynamics from the confines of his electric wheelchair, is a source of profound inspiration for the EHT team. “When things get hard,” Doeleman says, “I think of somebody like Stephen Hawking and I think, keep going.” In 2016, Hawkings gave the inaugural lecture for the Black Hole Initiative at Harvard.
As Doeleman notes, Hawking is perhaps best known for establishing mathematically the existence of radiation leakage in black holes—now termed “Hawking radiation” in his honor. Originally suggested by Jacob Bekenstein but fleshed out and concretized by Hawking in a paper of 1975, the notion that quantum effects brought on by the immense gravity of black holes could actually allow some heat to escape was a game-changer in the field.
Black holes had long been supposed to be perfect consumers: the consensus view was that what happens in a black hole stays in a black hole. Hawking’s finding that black holes could have temperatures and let heat slip away implied that they were not as cut off from the universe as we had thought. We now know that black holes, astronomers like to say, are “messy eaters” that suck in gas and then burp out radiation.
“This idea of black holes radiating is the chink in the armor,” Doeleman says. “It says that there’s something inside the black hole that can be made manifest outside the black hole.”
To ensure that the telescope will accurately capture what it sees no matter how bizarre or off-the-wall the data is, EHT imaging scientist Katie Bouman is spearheading intensive tests of the network and its associated software. At this point, the EHT has proven its reliability in generating faithful composite images (even in a simulation scenario where the object at the center of the Milky Way looks exactly like Frosty the Snowman). So, all in all, the team is feeling pretty confident in their adaptability.
Another EHT contributor, the Harvard theoretical physicist Andrew Strominger, has a more personal Hawking connection. Strominger, back when he was an MIT grad student, had been one of the very first among his peers to recognize the brilliance of Hawking’s Cambridge PhD thesis. Tracking Hawking’s work in the years following, Strominger became fascinated by the British physicist’s efforts to link the grand effects of gravitational spacetime distortion with the miniscule machinery of quantum mechanics—work which dovetailed beautifully with his own areas of interest.
Before long, the he and Hawking were collaborating on theories of wormholes. The two met face-to-face nearly every year of the past three decades, kicking around ideas and collaborating on original research. Most recently, they had been picking apart the so-called “No Hair Theorem,” a widely accepted mathematical formulation that seems to imply that only a handful of basic black hole types can exist in the real universe, for a series of groundbreaking papers.
What Hawking and Strominger discovered, and were eagerly working to formalize right up until the moment of Hawking’s passing, was that the theorem was, in Strominger’s words, “a precise mathematical answer to the wrong physical question. And if you carefully ask the question in the right way, there is in fact an infinite variety of black holes.” What’s more, he says it could be possible to avoid the infamous “information paradox” of information being lost forever in a black hole if the black hole’s shape is taken to be a unique reflection of the information it has consumed.
Strominger recalls fondly a recent academic retreat at which he and Hawking “had some very exciting discussions, which went on till late at night and continued through the rest of the workshop. We were continuing until he passed away.” On the occasion of Hawking’s death, Strominger published an exuberant remembrance of the 76-year-old’s heart, humor and humility in Scientific American, writing, “I will miss the fun almost as much as the physics. But it is hard to say where one began and the other ended.”
Doeleman, who has always thought of himself and the others on the EHT team as followers in the intellectual footsteps of Stephen Hawking, says his one regret is that Hawking did not live to see the EHT make its landmark observations. “I like to think that he was waiting for some of the results from the EHT,” Doeleman says. “It would have been a great joy to share them with him, and we’re all sad we’re not going to get that chance. But we soldier on with his memory very close in mind.”
Strominger, too, says he’s disappointed that Hawking will not be around to see the Event Horizon Telescope and the other modern descendants of his ideas blossom into fruition. But he takes heart in the fact that Hawking will be inspiring future scientists—as Hawking inspired him—for generations to come.
“Stephen’s passions for life and for science were inseparable,” Strominger says. “They were really two sides of the same coin. He reminded many that there’s more to reality than their own selves. What he focused on was discovering scientific truth, which is an indescribable, exhilarating voyage to try to understand the reality of the universe we live in.”