Did Stephen Hawking Owe Kip Thorne a Subscription to ‘Penthouse’?

Last week, the world lost one of the greatest scientific minds in history when Stephen Hawking passed away at the age of 76.

Hawking was known for his groundbreaking work in cosmology, and his work on black holes in particular. He fundamentally changed the way we thought about the universe, but even the most brilliant scientists make mistakes sometimes and Hawking was no exception. Nevertheless, his conviction in his own theories was unwavering, as evidenced by his tendency to place bets on the nature of astrophysical phenomena.

While perusing Wikipedia for an entirely unrelated reason, I came across a wiki dedicated to scientific wagers. These are bets made between scientists on the outcome of some uncertain statement that can only be settled by the scientific method. While reading the article, I noticed Hawking had made a lot of scientific wagers in his day and had lost most of the bets listed in the wiki. But one particular Hawking wager caught my eye.

Read More: The Visionary Legacy of Stephen Hawking

According to the wiki, Hawking had made a bet with Nobel Prize-winning physicist Kip Thorne in 1974 about whether a recently discovered cosmic object called Cygnus X-1 was a black hole. If Hawking was right and the object was not a black hole, Thorne agreed to get him a four-year subscription to Private Eye (basically the British equivalent of The Onion). If Thorne was right and it did turn out to be a black hole, Hawking would have to get him a subscription to the softcore porn magazine Penthouse.

Prior to the 1960s, black holes were just a theoretical object predicted by Einstein’s theory of general relativity. In 1964, two suborbital rockets were launched from New Mexico that carried Geiger counters as their payloads in order to map x-ray emissions in the cosmos. During this flight, researchers detected a source of x-ray emissions that remains one of the strongest ever detected from Earth. This object, called Cygnus X-1, was considered to be a strong candidate for the first detection of a black hole, but a lack of evidence for decades following this discovery left it as an open question.

Today, cosmologists believe that Cygnus X-1 is almost certainly a black hole. It’s an interesting topic if you’re a space nerd, but what I really wanted to know is whether Hawking ever paid up after losing the bet.

The wiki didn’t say whether the bet was settled, so I reached out to Thorne to see if he ever got his subscription to Penthouse. I didn’t receive a response. In fact, the only evidence that the wager even happened was an obscure 1997 straight-to-VHS documentary called Black Holes that features Hawking and Thorne.

After a bit of searching I managed to track down a copy of the film on YouTube and at long last found the answer to my question.

“In 1990, Stephen Hawking happened to be visiting Los Angeles and he broke into my office and thumb printed off on this bet,” Thorne recalls in the video. Although the status of Cygnus X-1 was an open question in the 70s, by the 90s mounting evidence had forced Hawking to concede the wager.

The handwritten bet between Stephen Hawking and Kip Thorne. Stephen Hawking’s fingerprint acknowledging he lost the bet can be seen in the lower right of the card. Image: YouTube

The bet was recorded in a handwritten note scrawled on a piece of card which is shown in the film. It read: “Whereas Stephen Hawking has a large investment in general relativity and black holes and desires an insurance policy, and whereas Kip Thorne likes to live dangerously without an insurance policy, therefore be it resolved that Stephen Hawking bets 1 year’s subscription to ‘Penthouse’ as against Kip Thorne’s wager of a 4-year subscription to “Private Eye,” that Cygnus X-1 does not contain a black hole of mass above the Chandrasekhar limit.”

“I had given Thorne a subscription to Penthouse, much to his wife’s disgust,” a smiling Hawking says in the film.

So there you have it: The first formal acknowledgement that black holes really exist in nature was sealed with a subscription to a softcore porn rag by two of the world’s leading cosmologists.