In chatting with colleagues after a talk this week, Joe Polchinski said he’d love to fall into a black hole. Most theoretical physicists would.
It’s not because they have some peculiar death wish or because science funding prospects are so dark these days. They are just insanely curious about what would happen. Black holes are where the known laws of physics come into their most direct conflict.
The worst trouble is the black hole information paradox that Stephen Hawking loosed upon the world in 1976. Polchinski and his colleagues have shown that the predicament is even worse than physicists used to think.
I first heard about their brainstorm while visiting the Kavli Institute for Theoretical Physics in Santa Barbara this spring, and the team—Polchinski and fellow Santa Barbarans Don Marolf, Ahmed Almheiri, and James Sully—wrote it up over the summer. Polchinski blogged about it a few months ago, and another theorist who helped to usher in the idea, John Preskill,did so last week.
Polchinski’s talk to the New York University physics department drew a standing-room-only crowd, not a single person snuck out early, and he was still fending questions an hour after it ended.
Almost as much has been written about Hawking’s original paradox (includingby me) as about the fiscal cliff, so I’ll jump straight to the new version. Step #1 of the argument is what Polchinski and his co-authors call the “no-drama” principle. According to current theories of physics, a black hole is mostly just empty space. Its perimeter or “event horizon” is not a material surface, but just a hypothetical location that marks the point of no return. Once inside, you are gripped too tightly by gravity ever to get back out.
By then, falling at nearly the speed of light, you have a few seconds to look around before you reach the very center and get crushed into oblivion. But nothing noticeable should happen at the moment of crossing. One of Einstein’s great insights was that observers who are freely falling—whether into a black hole or toward the ground—don’t feel the force of gravity, since everything around them is falling, too. As they say, it’s not the fall that kills you; it’s the sudden stop at the end.
An outside observer knows you’re doomed, but likewise doesn’t think anything untoward happens upon passing through the event horizon. Indeed, this observer never sees anything actually cross over. Because of a kind of gravitational mirage, things seem to slow down and freeze in time. All the stuff piling up at the horizon forms a ghostly membrane, which obeys the usual laws of physics and has conventional properties such as viscosity and electrical conductivity.
Step #2 is to relate these two viewpoints. To the infalling observer, space looks like a vacuum, and in quantum theory, a vacuum is a very special state of affairs. It is a region of space that is empty of particles. It is not a region that is empty of everything. There’s no getting rid of the electromagnetic field and other fields. (If you could, the region would not merely be empty, but nonexistent.)
A particle is nothing more or less than a vibration one of these fields, and what makes a vacuum a vacuum is that all the possible vibrations cancel one another precisely, leaving the fields becalmed. To maintain this finely balanced condition, the vibrations must be thoroughly quantum-entangled with one another.
To the outgoing observer, the horizon (or membrane) cleaves space in two, and the vibrations no longer appear to cancel out. It looks like there are particles flying off in every direction. This is perfectly compatible with the infalling observer’s viewpoint, since the fields are what is fundamental and the presence of particles is a matter of perspective. To put it differently, emptiness is a holistic property in quantum physics—true for a region of space in its entirety, but not for individual subregions.
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