The new conceptions of black holes eliminate the event horizon altogether. The basic idea is that there does, in fact, exist a force that could halt the collapse of a star when all else fails. That force is gravity itself. In matter with certain properties, gravity switches from being an attractive force to a repulsive force. Such a material, going by the name "dark energy," is thought to be driving the acceleration of cosmic expansion.

Last year physicists Pawel O. Mazur of the University of South Carolina and Emil Mottola of Los Alamos National Laboratory reasoned that a pocket of the stuff might freeze out, like ice crystals, during the collapse of a star. The result, which they call a gravastar, would look like fried ice cream: a crust of dense but otherwise ordinary matter stabilized by a curious interior. The crust replaces what would have been the event horizon.

Another proposal goes further. It conjectures not only that dark energy would freeze out but that relativity would break down altogether. The idea comes from a dark-horse contender for quantum gravity, the proponents of which are struck by the resemblance between the basic laws of physics and the behavior of fluids and solids (also known as condensed matter). In many ways, the equations of sound propagation through a moving fluid are a dead ringer for general relativity; sound waves can get trapped in the fluid much as light gets trapped in a black hole. Maybe spacetime is literally a kind of fluid.

What makes this approach so interesting is that the behavior of condensed matter is collective. The details of individual molecules hardly matter; the system's properties emerge from the act of aggregation. When water freezes, the molecules do not change, but the collective behavior does, and the laws that apply to liquids no longer do. Under the right conditions, a fluid can turn into a superfluid, governed by quantum mechanics even on macroscopic scales. Chapline, along with physicists Evan Hohlfeld, Robert B. Laughlin and David I. Santiago of Stanford University, has proposed that a similar process happens at event horizons. The equations of relativity fail, and new laws emerge. "If one thinks of spacetime as a superfluid, then it is very natural that in fact something physical does happen at the event horizon--that is, the classical event horizon is replaced by a quantum phase transition," Chapline says.