Stephen Hawking owes University of Michigan physicist Gordon Kane $100.

Well, probably.

In 2000, Hawking bet Kane that the Higgs boson, a theoretical particle thought to do the important job of giving other particles (like electrons and protons) mass, would never be found. Tomorrow, researchers at the European Organization for Nuclear Research are expected to announce that the Higgs boson exists—well, probably. As the Associated Press reported yesterday, physicists have “a footprint and a shadow” of the Higgs, but “they don’t plan to use the word discovery.”   

The Higgs boson, named after University of Edinburgh physicist Peter Higgs, was first hypothesized in the 1960s. Since then, researchers have attempted to find evidence of the particle by smashing protons together in particle accelerators. The search has been fraught with claims—mostly by science journalists—that the particle’s discovery is just around the corner, while others have said that it will never be found. Hawking, for instance, predicted that tiny black holes would mask the existence of the Higgs. This, it is said, led to the bet with Kane.

The scientific wager is not Hawking’s first. In fact, he’s a notorious gambler. In 1975 he bet Caltech physicist Kip Thorne a one-year subscription to Penthouse magazine (Hawking was to receive four years of Private Eye) that Cygnus X-1, a massive unseen object in close orbit with a star thousands of light years from Earth, wasn’t a black hole. Hawking, it turned out, was wrong. In his bestselling book, A Brief History of Time, he reports that he indeed paid up, “to the outrage of Kip’s liberated wife.”

In a second black hole wager, Hawking and Thorne bet physicist John Preskill, also of Caltech, that information swallowed by a black hole is forever hidden from the universe. The loser was to give the winner an encyclopedia from which “information can be recovered at will.” Based on his own calculations, Hawking conceded in 2004, and gave Preskill Total Baseball: The Ultimate Baseball Encyclopedia, after Preskill turned down a book about cricket. This concession was controversial; Thorne has yet to concede. The outcome of the bet, he wrote in an e-mail, depends on aspects of the laws of quantum gravity that haven’t been entirely worked out.

Given the almost-discovery of the Higgs, will Hawking concede a third bet? Gordon Kane thinks so. “I’ll let Stephen decide when to pay off,” Kane wrote in an e-mail, but “I presume he will be convinced by the data tomorrow.”

If Hawking does concede this wager, he’s still left with at least one outstanding bet. While both at Cambridge in 2002, Hawking bet physicist Neil Turok that cosmologists would observe primordial gravitation waves—a finding that would help explain how the universe came into existence. Ten years later, the terms of the bet have yet to be settled, but, according to Turok, the physicists plan to “decide on an amount soon.”

Update, July 4th, 2012: Stephen Hawking spoke to the BBC this morning about the discovery of the Higgs boson and his bet with Gordon Kane. "It seems I have just lost $100," Hawking said.