Monday 28 January 2013

Black Hole Event Horizons & Quantum Entanglement

Question from Peter (A-level student) -

An aspect of physics which has always interested as well as confused me is
quantum entanglement. I believe that this is a part of the new theory,
perhaps incorrectly dubbed "burn up", for one of the theories of what
happens past the event horizon of a black hole. I would be greatly
appreciate it if you could pass this on to someone with the expertise
to explain what quantum entanglement is and how it affects this "burn
up" theory of black holes.

Quantum Entanglement - credit

Answer from Dr Marika Taylor - see her personal website below.

Your questions about burning up at the surface of a black hole and quantum entanglement were passed along to me to answer, since I work on black holes.

I think your questions must be in the context of some recent works on black holes, involving so-called firewall problem, see this popular article

which discusses these ideas.

The question of what happens at the horizon of a black hole has been hotly debated for close to forty years now, since Stephen Hawking discovered that black holes aren't really black, because quantum effects cause them to emit radiation. There's a nice heuristic picture of how this radiation gets created, which relates to the firewall and entanglement discussion - in quantum theory particles are continually created in pairs, live for a short time and then annihilate. When such pair creation happens near a black hole horizon, however, one member of the particle pair can fall behind the horizon where it gets trapped, and so can no longer annihilate with the other member of the pair. The particle outside the black hole horizon can escape off t infinity, far from the black hole horizon, where we could measure it with a detector. In this picture the particle behind the horizon and the particle which escape to infinity are said to be entangled, which really just means that they have complete information and knowledge of each other.

Soon after Hawking discovered black hole radiation, he realized an important consequence of his calculations - regardless of how the black hole was formed astrophysically the particles measured coming out of the black hole seemed to be the same! That means that what we measure doesn't seem to depend on what happened earlier, in the formation process of the black hole. However, physical measurements at a given time not depending on what happened at earlier times (technically, non-unitary evolution in time) is not what we expect for a physical theory. For about 25 years people debated whether black hole evolution was indeed non-unitary or not until various breakthroughs in building  a quantum theory of gravity (string theory) convinced people that actually black hole evolution indeed had to be unitary - Somehow we must actually be able to do measurements which tell us how the black hole was formed.

Right now the growing consensus is that the only way we will be able to recover enough information from the black hole to deduce how it was formed is if the description in terms of Enstein's theory of gravity isn't quite right, it must miss some important physics near the horizon. There is not however consensus about what physics is missing near the horizon. The authors mentioned in the article above think that the basic problem is in the entanglement of particles produced near the horizon. In other words they don't think it is true that the particle which fell behind the horizon and the particle which escapes to infinity actually have complete information about each other, they think that the one which falls behind the horizon gets strongly entangled with a gas of particles living there. They also think that this gas of particles is so hot that anything falling behind the horizon would be burnt up by it, hence the name firewall.

As I said the consensus is indeed that something happens behind the horizon, there is some gas of particles which record the history of the black hole's formation. But there is very little evidence for the firewall - the authors mentioned in the article above were the only people in a recent black hole conference to think that the firewall exists! A more popular viewpoint, which I have worked on, is the fuzzball picture. In this picture there would be a gas of particles behind the horizon telling us about how the black hole was formed but something passing behind the horizon wouldn't get burnt up, it would just interact weakly with this gas and then eventually escape back across the horizon, out of the black hole.

I hope this explanation helps in understanding what entanglement means in this context. Please feel free to ask if there is something I can explain further.

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