Santos and Crisford are running simulations now to test whether cosmic censorship is saved at exactly the limit where gravity becomes the weakest force in the model universe, as initial calculations suggest. Such an alliance with the better-established cosmic censorship conjecture would reflect very well on the weak gravity conjecture. And if weak gravity is right, it points to a deep relationship between gravity and the other quantum forces, potentially lending support to string theory over a rival theory called loop quantum gravity. The “unification” of the forces happens naturally in string theory, where gravity is one vibrational mode of strings and forces like electromagnetism are other modes. But unification is less obvious in loop quantum gravity, where space-time is quantized in tiny volumetric packets that bear no direct connection to the other particles and forces. “If the weak gravity conjecture is right, loop quantum gravity is definitely wrong,” said Nima Arkani-Hamed, a professor at the Institute for Advanced Study who co-discovered the weak gravity conjecture.

The new work “does tell us about quantum gravity,” said Gary Horowitz, a theoretical physicist at the University of California, Santa Barbara.

The Naked Singularities

In 1991, Preskill and Kip Thorne, both theoretical physicists at Caltech, visited Stephen Hawking at Cambridge. Hawking had spent decades exploring the possibilities packed into the Einstein equation, which defines how space-time bends in the presence of matter, giving rise to gravity. Like Penrose and everyone else, he had yet to find a mechanism by which a naked singularity could form in a universe like ours. Always, singularities lay at the centers of black holes—sinkholes in space-time that are so steep that no light can climb out. He told his visitors that he believed in cosmic censorship. Preskill and Thorne, both experts in quantum gravity and black holes (Thorne was one of three physicists who founded the black-hole-detecting LIGO experiment), said they felt it might be possible to detect naked singularities and quantum gravity effects. “There was a long pause,” Preskill recalled. “Then Stephen said, ‘You want to bet?’”

The bet had to be settled on a technicality and renegotiated in 1997, after the first ambiguous exception cropped up. Matt Choptuik, a physicist at the University of British Columbia who uses numerical simulations to study Einstein’s theory, showed that a naked singularity can form in a four-dimensional universe like ours when you perfectly fine-tune its initial conditions. Nudge the initial data by any amount, and you lose it—a black hole forms around the singularity, censoring the scene. This exceptional case doesn’t disprove cosmic censorship as Penrose meant it, because it doesn’t suggest naked singularities might actually form. Nonetheless, Hawking conceded the original bet and paid his debt per the stipulations, “with clothing to cover the winner’s nakedness.” He embarrassed Preskill by making him wear a T-shirt featuring a nearly-naked lady while giving a talk to 1,000 people at Caltech. The clothing was supposed to be “embroidered with a suitable concessionary message,” but Hawking’s read like a challenge: “Nature Abhors a Naked Singularity.”

The physicists posted a new bet online, with language to clarify that only non-exceptional counterexamples to cosmic censorship would count. And this time, they agreed, “The clothing is to be embroidered with a suitable, truly concessionary message.”

The wager still stands 20 years later, but not without coming under threat. In 2010, the physicists Frans Pretorius and Luis Lehner discovered a mechanism for producing naked singularities in hypothetical universes with five or more dimensions. And in their May paper, Santos and Crisford reported a naked singularity in a classical universe with four space-time dimensions, like our own, but with a radically different geometry. This latest one is “in between the ‘technical’ counterexample of the 1990s and a true counterexample,” Horowitz said. Preskill agrees that it doesn’t settle the bet. But it does change the story.