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Stephen Hawking shakes up theory (again): Black holes are actually gray

Image: Stephen Hawking
Physicist Stephen Hawking says there's not a sharply defined event horizon at the edge of a black hole, and thus it would be wrong to say that a black hole is truly black.David Parry / PA via AP file

British physicist Stephen Hawking earned worldwide attention for his surprising claims about black holes, and he's doing it again with a new paper claiming that "there are no black holes."

Actually, Hawking isn't denying the existence of the massive gravitational singularities that lurk at the center of many galaxies, including our own Milky Way. He's just saying the classical view of a black hole as an eternal trap for everything that's inside, even light, is wrong. In his revised view, black holes are ever so slightly gray, with a chaotic and shifting edge rather than a sharply defined event horizon.

"The absence of event horizons mean that there are no black holes — in the sense of regimes from which light can't escape to infinity," Hawking writes in a brief paper submitted to the preprint database. "There are, however, apparent horizons which persist for a period of time."

Hawking's paper, titled "Information Preservation and Weather Forecasting for Black Holes," has kicked off a new round in the long-running debate over black holes and what happens to the stuff that falls into them. Theoretical physicists, including Hawking, have gone back and forth on this issue, known as the information paradox.

Back and forth over black holes
For decades, Hawking contended that the information that disappeared inside a black hole was lost forever. Then, in 2004, he reversed course and said the information would slowly be released as a mangled form of energy. That switch led him to pay off a bet he had made with another physicist about the fate of information in a black hole.

More recently, other physicists have suggested that there was a cosmic firewall dividing the inner region of a black hole's event horizon from the outside, and that anything falling through the event horizon would be burnt to less than a crisp. But that runs counter to the relativistic view of black holes, which holds that there should be no big difference in the laws of physics at the event horizon.

To resolve the seeming paradox, Hawking says that black holes would have "apparent horizons" — chaotic, turbulent regions where matter and energy are turned into a confusing mess. "There would be no event horizons and no firewalls," he says. Everything in a black hole would still be there, but the information would be effectively lost because it gets so scrambled up.

"It will be like weather forecasting on Earth. ... One can't predict the weather more than a few days in advance," Hawking writes.

Protests and jests
Hawking's paper wasn't peer-reviewed, but his peers are already weighing in on the accuracy of the black hole weather report.

"It is not clear what he expects the infalling observer to see," Joseph Polchinski, a pro-firewall physicist at the University of California at Santa Barbara, told New Scientist. "It almost sounds like he is replacing the firewall with a chaos-wall, which could be the same thing."

"The idea that there are no points from which you cannot escape a black hole is in some ways an even more radical and problematic suggestion than the existence of firewalls," Raphael Bousso, a theoretical physicist at the University of California at Berkeley, said in Nature's online report on Hawking's paper. "But the fact that we’re still discussing such questions 40 years after Hawking’s first papers on black holes and information is testament to their enormous significance."

If the "no black holes" quote is taken out of context, it makes Hawking's claim sound kind of ridiculous — and Andy Borowitz, a humorist at The New Yorker, has turned that take into an Onion-like jab at members of Congress. "If black holes don't exist, then other things you scientists have been trying to foist on us probably don't either, like climate change and evolution," Borowitz writes in one faux quote.

Fortunately, we're getting to the point where we won't have to take any theorist's word for the existence of black (or gray) holes. Astronomers are preparing to watch a huge cloud of gas fall into the black hole at the center of our galaxy — and over the next decade, they're planning to follow through on the Event Horizon Telescope, a campaign aimed at direct observation of the galactic black hole's edge.

As for Hawking, it just so happens that this is a big month: He turned 72 years old a couple of weeks ago, and he appears to be keeping active despite his decades-long struggle with amyotrophic lateral sclerosis. And this week marks the television premiere of "Hawking," a PBS documentary about the good doctor's life and work. For still more about the world's best-known physicist, check out his recently published memoir, "My Brief History."

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Alan Boyle is's science editor. Connect with the Cosmic Log community by "liking" the NBC News Science Facebook page, following @b0yle on Twitter and adding +Alan Boyle to your Google+ circles. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.