After the news of Hawking’s death, I emailed several astrophysicists who study black holes about their reactions. For them, the thought that Hawking will not see the potential photographic evidence of a black hole is heart-wrenching.
“For some people, seeing really is believing, so we were hoping to get something in time,” said Shep Doeleman, an astronomer at the Harvard-Smithsonian Center for Astrophysics who is leading the effort to photograph a black hole, known as the Event Horizon Telescope, or EHT. “It would have been nice to have it for him. I like to think—the whole team likes to think—that he would be enthused by what we’re up to.”
In the 1970s, Hawking upended theories for explaining black holes—but he didn’t actually mean to. “For a long time, people thought that black holes were these infinite sinks, like a roach motel—when things went in, they didn’t come back out again,” Doeleman said.
Hawking believed much the same. So when the Israeli theoretical physicist Jacob Bekenstein, at the time a Princeton graduate student, suggested black holes have entropy—the disorder of a system, measured using thermodynamics—and therefore have temperatures, Hawking set about to prove Bekenstein wrong. He paired insights from small-scale physics (quantum mechanics) with those of large-scale physics (Einstein’s theory of gravity).
After many hours of running through calculations, he concluded that Bekenstein was correct. Hawking determined that black holes actually spew out radiation particles, and even eventually shrink. These particles were named Hawking radiation.
“Hawking’s work demonstrated that one of the fundamental assumptions of physics must break down near a black hole,” said Vincent Fish, a research scientist at MIT’s Haystack Observatory, and a member of the EHT team. “The debate over what exactly that is continues to this day.”
EHT won’t be able to photograph this radiation. “Hawking radiation is a teeny, tiny trickle of light compared to what needs to happen around a black hole to make it visible to astronomers,” said Sera Markoff, an astrophysicist at the University of Amsterdam, and another EHT member.
Nor is the project capable of actually seeing the interior of the black hole. EHT is going after what’s known as the event horizon. This is the invisible boundary thought to surround every black hole, a point of no return beyond which light cannot escape. EHT uses high-frequency radio telescopes powerful enough to peer through walls of hot gas and take the sharpest images possible of this mysterious barrier. If the team got the shot they wanted last year, they will see the black hole in silhouette, illuminated by the glow of warped light as it gets sucked into the event horizon.
Still, getting this close to a black hole would allow astrophysicists to test whether Hawking’s—and Einstein’s—theories hold up in such an extreme and stupefying part of the universe. Hawking’s work was dizzyingly complicated, but the physicist was known for translating it into digestible descriptions for the public. “His work was both esoteric and accessible,” said Geoff Bower, an EHT member and a scientist at the Academia Sinica Institute of Astronomy and Astrophysics who is based in Hawaii. “Only last week, I found myself explaining to observatory visitors the process of Hawking radiation and how that reveals the paradox of a universe described by quantum mechanics and general relativity.”