The universe's ultimate peep show
General relativity predicts that when a massive enough star dies, its gravity overwhelms all other forces, crushing it into a point of infinite density called a singularity. Usually, this catastrophic event is hidden from us. The singularity is cloaked behind an event horizon, a boundary from which not even light can escape. The combination of the singularity and its event horizon is what we call a black hole. This cosmic privacy shield is in Roger Penrose's 1969 "Cosmic Censorship Hypothesis," which posits that every singularity is decently clothed by an event horizon.
A "naked" singularity is a hypothetical gravitational singularity without an event horizon. If one existed, it would be a point of infinite density and spacetime curvature visible to the rest of the universe. This is a problem for physics. At a singularity, the known laws of general relativity break down, losing their predictive power. If such a place were visible, its unpredictable effects could ripple outwards, violating causality and creating a situation where the future state of the universe could not be predicted from the past.
A bet among physicists
The possibility of these cosmic spoilers was the subject of a scientific wager. In 1991, Stephen Hawking and Kip Thorne bet Caltech physicist John Preskill that naked singularities were impossible under classical general relativity. Hawking argued that some law of physics must prevent their formation, keeping the predictability of the universe. Preskill and Thorne wagered that there could be specific, albeit rare, conditions where a collapsing star could form a singularity without an event horizon.
In 1997, Hawking conceded the bet. His concession was based on computer simulations by physicist Matthew Choptuik. Choptuik's work on "critical collapse" showed that by precisely fine-tuning the initial conditions of a collapsing scalar field, it was mathematically possible to create a naked singularity. The setup is precise—like balancing a pencil on its sharpened point—and may not occur in nature, but its theoretical possibility meant Hawking had lost the bet on a technicality. He paid up, giving both Thorne and Preskill T-shirts embroidered with the message, "Nature abhors a naked singularity."