Tuesday, February 23, 2010

Jan de Boer colloqium

Here's a popular-level online talk which goes over the current state of thinking on the whole fascinating mix of topics relating to black holes, string theory, "holography", and the Ads/CFT correspondence:


This is really some pretty remarkable stuff. About half the talk is "ancient history" from the 70's, 80's and 90's, not new anymore but still fascinating ( you can read about it also in the book "The Black Hole War"). The rest is on newer developments, particularly the application of string theory to quark/gluon physics and to high-energy superconductivity. This is a story still very much under development.

At the end he says something which I find highly dubious. He claims that a person falling in to a black hole would gradually lose consciousness as they hit the event horizon, and this, as far as I know, is not the accepted viewpoint at all. The generally accepted view is that the horizon is undetectable by someone falling across it. Indeed, we could be falling across one right now - perhaps for a huge black hole whose horizon is light years across - and we won't know the difference for millions of years until we start to approach the actual singularity at the heart of the hole.

But on the other hand, it is also generally accepted that if you watch someone falling into a hole from the outside, then you see them get closer and closer to the horizon but never actually fall in. And furthermore, the horizon has a temperature, although generally a low one. So from the outside it looks like a person should be encountering warm temperatures as they fall in, which might dissolve them or cook them or something.

This relates to the idea of "Black hole complementarity", according to which there two equally valid but complementary ways to look at a black hole: the view from outside, and the view falling in. But this seems to violate the principle, because the person falling in could be sending radio messages back home, and those messages would say, "situation normal, nothing to report". But if the infalling person actually sees a temperature and is getting cooked, then their messages would surely mention this fact.

So there is a conflict here, one which has been debated for several decades now, apparently without resolution. Personally I don't believe that the infalling observer would see anything, at least for big black holes. To me the view "from outside" seems pathological and highly suspect, because of the strong warping of time near the event horizon, relative to a distant spot.

That's my .02, but I've been wrong before!

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