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See Inside Scientific American Volume 310, Issue 4

What Stephen Hawking Really Meant When He Said There Are No Black Holes

A decades-old paradox returns
Artist rendition of a black hole
Artist rendition of a black hole



Credit: J. Schnittman NASA Goddard Space Flight Center, J. KROLIK Johns Hopkins University AND S. NOBLE Rochester Institute of Technology

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When Stephen Hawking was quoted earlier this year as saying that “there are no black holes,” he wasn't really talking about black holes. At least not about black holes as you or I imagine them—astrophysical objects that suck in everything, even light. Those, everyone agrees, are just as black as ever.

Hawking's quip instead concerns black holes in a highly theoretical sense. Like many other theorists, Hawking has been trying to understand a paradox eating at the heart of physics. The issue—often referred to as the black hole firewall paradox—implies that physicists might have to abandon (or deeply modify) quantum mechanics or Einstein's general theory of relativity, or both.

The firewall problem is related to a paradox that Hawking first pointed out in the 1970s. It concerns this question: What happens to information that falls into a black hole? The rules of quantum mechanics require that information can never be lost. Even burning a book doesn't destroy the information inside—it just scrambles it up. But black holes do seem to destroy information, sucking it past the event horizon, a point of no return.

The black hole information paradox stumped physicists for two decades. It appeared to be solved in the late 1990s, when researchers figured out that information could leak out of a black hole in the form of Hawking radiation. Then, in 2012, physicists at the University of California, Santa Barbara, found flaws in the previous solutions. They concluded that an event horizon is not, as previously thought, an ordinary place. Instead it is a wall of fire that prevents Hawking radiation on the outside from remaining intertwined on a quantum level with material inside.

Hawking's latest work is an attempt to offer an alternative solution. He proposes that a black hole has an “apparent” horizon in addition to its event horizon. The two are nearly always identical. Information can rise from inside the black hole to the apparent horizon. At that point, quantum effects can blur the boundary between the “apparent” and “event” horizons, sometimes allowing information to escape. Hence, black holes would not be strictly black, provided you had hundreds of trillions of years to watch them. But ultimately, what Hawking's paper means is that there is something fundamental about black holes we still do not understand.

This article was originally published with the title "The New Black Hole Battle."

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