ESA/V. Beckmann (NASA-GSFC)

Revolving door?
New theoretical work suggests that the gravitational grip of black holes does not last forever.

Are Black Holes Two-Way Streets?

Black holes are just about the least friendly places in the universe. According to Einstein's theory of general relativity, they're so powerful that they warp space and time, and they've condensed so much matter and energy into a tiny point called a singularity that nothing, not even light, can escape. Getting sucked down a black hole should be a one-way trip. But is it?

Stephen Hawking thought so. Back in the 1970s, the eminent physicist hypothesized that a black hole eventually--over time scales lasting trillions of years--would evaporate into nothingness. The problem for Hawking's idea was that it clashed with quantum mechanics, of which one of the primary tenets is that information cannot be lost. Hawking could not reconcile the conflict, and a few years ago he recanted his position on information loss.

Now, physicists from Pennsylvania State University in State College have shown that Hawking was right to change his mind. Delving into a cousin of quantum mechanics called quantum gravity, Abhay Ashtekar and colleagues Victor Tavares and Madhavan Varadarajan calculate that singularities cannot exist. According to relativity, a singularity is essentially a frontier where spacetime ends. As such, nothing should be able to escape it. But complex calculations by Ashtekar's team show that singularities are not allowed by quantum gravity. That means that although the center of a black hole may be very, very dense, it's not so dense that it traps information forever. "Quantum spacetime doesn't end at a singularity," Ashtekar says.

The findings, reported in the 20 May issue of Physical Review Letters, are good news for quantum mechanics, because they support the idea that information cannot disappear permanently. But, by calling singularities into question, they spell trouble for relativity. If black holes are not singularities, then the continuum of spacetime described by Einstein must be only an approximation, says Ashtekar. That's not necessarily a bad thing. "[It] opens the door to a lot of new explorations," Ashtekar says. "They may lead to physics beyond Einstein."

The team's work is particularly fascinating because it provides a mathematical basis for actually looking into black holes, says astronomer Kimberly Weaver of NASA's Goddard Space Flight Center in Greenbelt, Maryland. "Black holes are such mysteries that this may be the only way we're going to be able to know what's going on inside them," she says. Weaver says astronomers will be looking for evidence that black holes evaporate. If so, "we might be able to see information coming out, and that would be really exciting."

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