For Einstein's theory of gravity to hold water, it must be impossible to glimpse the heart of a black hole--a point where the force of gravity is infinite, called a singularity--even from a prime viewing spot inside. A few years ago, however, some physicists had found a case where this "strong cosmic censorship" seemed to be violated. Now the censors are back in force: Researchers report in the 20 April Physical Review Letters that the offending hole would trap light in such a way as to forever hide the singularity from view.
Whether a singularity can be seen depends on the behavior of light inside a black hole. When light enters a black hole, its frequency increases, or gets "blue shifted," because of the black hole's enormous gravity. Normally the light gains so much energy from this blue shift that it creates a new singularity, which, loosely speaking, gobbles up all the light. As a result, an unfortunate astronaut who had fallen into the black hole would plow into the original singularity without ever seeing it, says Eric Poisson, a physicist at the University of Guelph in Ontario, Canada. However, in a special case (a black hole of a particular mass, charge, and spin), advocates argued, the blue shift was not strong enough to hide the singularity: The astronaut would be able to see it before being sucked in.
A team led by Patrick Brady, a physicist at the California Institute of Technology in Pasadena, took a closer look at this special case. The researchers found that the previous work had neglected an additional source of blue-shifted light--the edge of the black hole, which reflects light back toward the center. This additional blue-shifted light is usually negligible, but the researchers found that in this special case it was enough to tip the balance in favor of cloaking the singularity and preserving strong cosmic censorship.
"There was only one [known] example [in which cosmic censorship] failed, and now that's gone," says Eanna Flanagan, a physicist at Cornell University in Ithaca, New York. He and Brady say the question of whether a singularity could be seen under any circumstances will be resolved only with more realistic computer simulations of what actually happens inside a black hole. The equations are simply too complex to solve on paper, they say.