Physicist, Startup Founder, Blogger, Dad

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.


David Coughlin said...

Preskill is tweeting with the hashtag #fuzzorfire

Rastus Odinga-Odinga said...

A really interesting question: how many people *really* believe that firewalls exist? My feeling is that those people are a tiny minority. Thank God. The fact that any such people exist is a jarring reminder that the standard of the General Relativity courses nearly everywhere is utterly abysmal.

Carson Chow said...

So your argument is basically that accepting Many Worlds resolves the firewall paradox as well as the black hole information problem. Sounds plausible to me. Are you headed to Santa Barbara?

steve hsu said...

Actually, in some sense the BH information problem is *posed* within a many worlds framework -- does the quantum state of the BH + radiation + environment evolve unitarily (pure state to pure state), or do BHs cause pure states to evolve to mixed (Hawking's original claim)?

Unfortunately I can't make it to the meeting -- my day job interferes with doing physics :-( But I'm getting ready to post a sequel to my earlier paper since this subject remains unresolved.

steve hsu said...

Surprisingly many! In fact a significant number of theorists whose brainpower I admire have (a) looked carefully at AMPS and (b) can't find the problem. Among the people who share my perspective I can only count Nomura and co. and Lubos!

I don't think it's a problem with understanding of GR. The subtle aspect is really in the QM part.

Carson Chow said...

Is it a theorem that if you start with a pure state of particle(s) and a black hole and evolve that unitarily you will end up with a system on which any continuous branch that an observer lives on is mixed in the sense that the BH forces you to marginalize over the rest of the universe states. I probably don't have the language right but the black hole automatically radiates to all the decohered worlds so Hawking is correct that an isolated BH can never evolve unitarily but the universe does.

Rastus Odinga-Odinga said...

Looking carefully at AMPS, and not being able to find the problem, are *not* the same thing as *really* believing in the existence of firewalls! My attitude, and I *hope* the attitude of most people, is that firewalls can't exist: it's just a matter of finding the problem[s] in AMPS. My comment about GR was based on the way people talk about "no drama" at the event horizon, as if there were a clear alternative. I'm afraid that people think, "Oh well, let's give up the equivalence principle, after all that's just old-fashioned junk anyway." The truth is that accepting the real existence of firewalls involves not just giving up some dusty old "principle": it means giving up on the idea that spacetime is a differentiable manifold even far away from singularities. Just about any other alternative makes more sense!

steve hsu said...

Not sure I understand, but I think you mean that to a particular semiclassical observer it appears the BH destroys information whereas a global observer (who can see the all branches of the wavefunction of the universe) sees unitary evolution. If so, I agee :-)

Note there is an exact parallel to ordinary QM measurement (e.g., Stern-Gerlach), which *appears* to collapse the wavefn (first to a mixed state, and then, probabilistically, to a final outcome), whereas a global observer (at least in many worlds) can see that no collapse actually happened.

steve hsu said...

Well, I agree that giving up EP so quickly is crazy.

But I do think if ~30 really top theorists (check out the workshop participants at KITP) have thought hard about it for ~1 year and can't sort it out then it's nontrivial ...

Carson Chow said...

What's the precise mathematical definition of a semi-classical observer? I'm pretty sure what you say is true as far as I understand it but is it at theorem status? Here is my trivial understanding of QM so please correct me because it will help me to understand this better. Consider a a state vector for the entire universe that evolves through a unitary transformation. Now consider some subset of this state that corresponds to a BH and particles outside of the BH. My interpretation of your comments is that I cannot evolve this subset unitarily as it will necessarily involve other elements of the state vector. In other words, I cannot block diagonalize a BH state entangled with some other isolated subset. The BH is automatically entangled with the entire universe. So the resolution of the firewall paradox is that purity for the BH is lost if it is treated without the rest of the universe.

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