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Thursday, August 22, 2013

Black hole firewalls and all that

Here's the Times coverage of the so-called "firewall paradox" in black hole physics. For my take on it see this paper and these additional comments. There is a meeting at KITP on this topic taking place starting this week -- see here for talk videos.
NYTimes: ... 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. ...
Via Sean Carroll, here are Joe Polchinski's slides from a firewall talk at Caltech.

My claim is that (see slide 29) the b which forms a pure state with b_E is not the same as the b which forms a pure state with b'. The latter b is an excitation relative to the vacuum state of a particular decoherent spacetime (background geometry) whereas the former b is a component of the global radiation state, summing over all spacetimes. The Equivalence Principle (no drama) can only be applied to one geometry at a time, whereas unitarity (purity) only applies to the global state, including all the branches.

If I am correct, then the main benefit from this firewall discussion will be the realization that unitarity only holds after summing over all spacetime geometries of the evaporating black hole. Most theorists seem to think it will hold (at least approximately) on each geometry separately.

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