Throwing light on cosmic censorship.

Throwing Light on Cosmic Censorship

Einstein's general theory of relativity provides a framework for understanding how gravity drives the collapse of massive stars or generates the tidal forces that tear apart colliding galaxies. But the equations are so difficult to solve that theorists are just beginning to explore the complex sorts of behavior encompassed by general relativity.

Using a supercomputer to solve Einstein's equations, Stuart L. Shapiro and Saul A. Teukolsky of Cornell University in Ithaca, N.Y., have now uncovered evidence that the gravitational collapse of certain three-dimensional distributions of matter leads to the formation of a "naked" singularity--an exposed point in space where physical quantities such as density and gravitational force become infinite. When such infinities occur, theorists can no longer solve the equations to predict the future course of gravitational collapse, and the theory of relativity breaks down.

"This could be one of the most important results we have found with our supercomputer calculations to date," Shapiro says. "If it holds up, it could in fact be very significant for relativity theory." He and Teukolsky report their findings in the Feb. 25 PHYSICAL REVIEW LETTERS.

Singularities associated with gravitational collapse appear frequently in the solutions of Einstein's equations, even though nature doesn't countenance such bizarre features. What saves relativity theory is that these singularities normally sit at the centers of black holes -- collapsed regions where the gravitational force is so strong that not even light can escape.

Because nothing that happens within a black hole could ever influence anything outside, the presence of a singularity there doesn't matter. Relativity theory would still work everywhere outside the black hole. For this reason, theorists believed they could safely ignore any such well-hidden nuisances that surfaced in their solutions to Einstein's equations.

In 1969, mathematical physicist Roger Penrose of Oxford University in England enshrined this idea in his "cosmic censorship" hypothesis, stating that singularities would always be found inside black holes. There are no naked singularities, he claimed.

To test this notion, Shapiro and Teukolsky computed the gravitational collapse of assorted three-dimensional balls of particles. They discovered that compact balls collapsed to become singularities enveloped in black holes. However, sufficiently large, nonspherical distributions collapsed to singularities without the formation of censoring black holes, leaving the singularities naked to the rest of the universe (see illustrations).

This violation of cosmic censorship represents a potential disaster for general relativity, Shapiro says. It exemplifies a situation in which relativity theory clearly fails to model the physical world, in which no such singularities appear.

Physicists now face either the formidable task of salvaging cosmic censorship or the dismaying prospect of modifying a landmark theory. "If cosmic censorship really goes out the window -- and one would need more work to really nail that down--then one would need to revise the mathematical equations for relativistic gravity," Shapiro says.

Shapiro and Teukolsky are studying the effects of the object's spin and are seeking to determine whether treating matter as a fluid rather than as a distribution of particles would change their results.

"Numerical work is probably the only way you're going to be able to determine whether or not these naked singularities form, but you have to be extremely careful," says David W. Hobill of the University of Illinois at Urbana-Champaign. "We have a long way to go before we really understand what's happening."