Edition: U.S. / Global

Space & Cosmos

A Black Hole Mystery Wrapped in a Firewall Paradox

This time, they say, Einstein might really be wrong.

Jim Wilson/The New York Times

Raphael Bousso.

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Jim Wilson/The New York Times

Leonard Susskind.

A high-octane debate has broken out among the world’s physicists about what would happen if you jumped into a black hole, a fearsome gravitational monster that can swallow matter, energy and even light. You would die, of course, but how? Crushed smaller than a dust mote by monstrous gravity, as astronomers and science fiction writers have been telling us for decades? Or flash-fried by a firewall of energy, as an alarming new calculation seems to indicate?

This dire-sounding debate has spawned a profusion of papers, blog posts and workshops over the last year. At stake is not Einstein’s reputation, which is after all secure, or even the efficacy of our iPhones, but perhaps the basis of his general theory of relativity, the theory of gravity, on which our understanding of the universe is based. Or some other fundamental long-established principle of nature might have to be abandoned, but physicists don’t agree on which one, and they have been flip-flopping and changing positions almost weekly, with no resolution in sight.

“I was a yo-yo on this,” said one of the more prolific authors in the field, Leonard Susskind of Stanford. He paused and added, “I haven’t changed my mind in a few months now.”

Raphael Bousso, a theorist at the University of California, Berkeley, said, “I’ve never been so surprised. I don’t know what to expect.”

You might wonder who cares, especially if encountering a black hole is not on your calendar. But some of the basic tenets of modern science and of Einstein’s theory are at stake in the “firewall paradox,” as it is known.

“It points to something missing in our understanding of gravity,” said Joseph Polchinski, of the Kavli Institute for Theoretical Physics in Santa Barbara, Calif., one of the theorists who set off this confusion.

Down this rabbit hole are many of the jazzy magical mysteries of modern physics: Black holes. The shortcuts through space and time called wormholes. Quantum entanglement, also known as spooky action at a distance, in which particles separated by light-years can still instantaneously appear to remain connected. The reward for going down this hole could be a new understanding of why we think we live in a universe with space and time at all, with suitably unpredictable consequences. After all, if Einstein hadn’t been troubled a century ago by logical inconsistencies in the Newtonian universe, we might not have GPS systems, which rely on his theory of general relativity to keep time, in our pockets today.

Falling Bodies

Black holes are the most extreme predictions of Einstein’s theory, which describes how matter and energy warp the geometry of space and time the way a heavy sleeper causes a mattress to sag. Too much matter and energy in one place could cause space to sag so far that the matter inside it would disappear as if behind a magician’s cloak, collapsing endlessly to a point of infinite density known as a singularity. Einstein thought that idea was ridiculous when it was pointed out to him at the time, in 1916, but today astronomers agree that the universe is speckled with such dark monsters, including beasts lurking in the hearts of most galaxies that are millions and billions of time more massive than the Sun. Many of them resulted from the collapse of dead stars.

General relativity is based on what Einstein later called his “happiest thought,” that a freely falling person would not feel his weight. It is known simply as the equivalence principle; it says that empty space looks the same everywhere and to everyone.

One consequence of this principle is that an astronaut would not feel anything special happening when he fell through the point of no return, known as the event horizon, into a black hole. Like a bungee jumper, he would feel weightless then and all the way until he hit the bottom, which could take seconds or years depending on how big the hole was, and he would be stretched like a noodle by tidal forces and then crushed into a speck. At the event horizon there would be “no drama,” in the lexicon — at least in the physical sense, as opposed to the intellectual trauma of knowing you were not ever going home. Things or people went in, they got crushed to infinite density and disappeared. That was the traditional view of black holes.