Black holes are a tricky bunch. While Einstein’s theory of relativity predicts they’re common, tracking down the first one was quite a challenge. Unlike stars, black holes themselves don’t emit any light, so the only thing we can measure about them is their size and spin.
Today, we know black holes exist — we even have a picture of one! But scientists haven’t had definitive evidence of black holes for very long. The first one, Cygnus X-1, was discovered in 1964. But it still took nearly 30 years for the two leading black hole physicists, Stephen Hawking and Kip Thorne, to agree Cygnus X-1 was really a black hole.
The famous black hole bet
There was little doubt in Hawking’s mind that black holes existed. After all, they were one of the focuses of his career. But whether scientists could actually find one was another question entirely.
That lack of discovery, understandably, made Hawking a bit nervous — which is why he hedged his research efforts with the bet.
“This was a form of insurance policy for me. I have done a lot of work on black holes, and it would all be wasted if it turned out that black holes do not exist,” said Hawking in his book, A Brief History of Time. “But in that case, I would have the consolation of winning my bet…”
He goes on to say that when the bet was first placed in 1975, he and Thorne were 80 percent certain that Cygnus X-1 was a black hole. By 1988, Hawking reported they were both 95 percent certain. Yet, they agreed, the bet could only be settled when they were both 100 percent sure.
By 1990, further observation had been done of the system, building enough evidence that it contained a black hole that Hawking was forced to concede the bet. And, according to Thorne, Hawking waited for the opportune moment to admit defeat.
“I was in Moscow … when Stephen and his entourage, with help from my students, broke into my Caltech office and he thumbprinted his concession,” Thorne told Astronomy via email. “When I returned [a] few weeks later, I saw it and celebrated — as did he. It was wonderful that the observations had become so firm that we were both fully convinced Cygnus X-1 is a black hole and a massive star orbiting each other.”