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Archive for the ‘Quantum Gravity’ Category

There will be a school on quantum gravity this summer in Morelia, Mexico, from June 23rd to July the 3rd. Don Marolf, the chair of the event asked me to promote the event. The deadline for applications is January 31st (the end of this week).

There is also going to be a workshop on future directions in lattice gauge theory, this summer at CERN. This is of interest to me, but I don’t know if I will be able to make it or not.

Also, the deadline for the Aspen Center for Physics summer 2010 session is by the end of this week.

In the fall there will also be an interesting workshop on aspects of the AdS/CFT correspondence at the Galileo Galilei Institute for Theoretical Physics in Florence. Again, very interesting, but I don’t know if I will be able to make it.

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I’m organizing a workshop in late September, together with Jun Nishimura and Toby Wiseman, taking place in London. We’re very close to the limit of participants that we can afford. The workshop will be on Numerical approaches to AdS/CFT, large N and (quantum) gravity. We’re going to put a lot of people who have not all met each other yet in a closed room, we’re going to `lock them in’ so to speak, and hope that fantastically new collaboratiosn and ideas will spring out of this.

Mostly, we will try to figure out what has been done in this area already and hope to get a road map of what can be done, what should be done and how much resources does it need in order to be done.

For more information, here is the website.

Below is the conference poster, which I cobbled up from some old photos of Big Ben, some photoshop magic and some creative license with masks trying to make it look cool. Of course, my co-organizers corrected all the mispelled names that I mistyped and it is likely that we saved a bundle on ‘artwork’ commisioning.  I really have no clue how much these things cost.

Conference Poster

Conference Poster

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Today on the arxiv Oswaldo Zapata wrote an essay on issues about fact and belief systems in superstring theory. Naturally, Peter Woit decided that this was really important and wrote a whole article about it. Here I will collect a few recollections that serve as a rebuttal/complement to some of those discussions. Mostly, I feel implicated by some of this discussion since at least one of my works (together with Juan Maldacena and Horatiu Nastase) is mentioned as changing the history of an idea from belief to fact.

The whole issue is to quantify the following statement: the AdS/CFT correspondence is a true fact.

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We now have a few working examples of a microscopic theory of quantum gravity, all come with specific boundary conditions (like any other equation in physics or mathematics), but otherwise full background independence. In particular, all those theories include quantum black holes, and we can ask all kinds of puzzling questions about those fascinating objects. Starting with, what is exactly a black hole?

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Fundamental physics has a strong appeal to the imagination. There ought to be some underlying structure to our theories of physics, something beautiful and intuitive that explains the parameters of the standard model, unifies gravity and quantum mechanics, explains puzzles like the black hole information paradox, and probably has a few bonus surprises in store for us. This is probably one of the main reasons many people with strong expertise in diverse disciplines take an active interest, read blogs and popular books on the subject, and sometime try to lend a hand and help.

It is natural for a person with an intimate knowledge of some theoretical structure to try to apply it to the main few questions of fundamental physics. Could this fundamental theory involve spin chains, Turing machines, cellular automata, or your favorite (sort of) Lie super algebra? This looks much more likely if you devoted your career to studying the intricacies of these structures. There is of course nothing wrong with that, nobody has any idea what the fundamental theory of our universe looks like, the issues are difficult and we can use all the help we can get. To facilitate such help I’ll offer some unsolicited advice: Nature is relativistic, and this fact is crucially important!

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Back in 2001, in a truly beautiful paper, Juan Maldacena formulated a version of Hawking’s information paradox, which has the added advantage that it could be discussed and analyzed in the context of a complete background independent theory of quantum gravity, namely that of the AdS/CFT correspondence.

This variant is similar to the original paradox, formulated for black holes surrounded by flat space, in that it displays a sharp conflict between properties of black holes in classical General Relativity, and basic postulates of quantum mechanics. Alas, it is also different in many crucial ways from the original paradox. Despite that, Juan’s proposed resolution to his paradox seems to have led to Hawking’s arguments, who managed to convince himself (though I think it is fair to say not too many others, unless they were already convinced) that information is not lost after all in the process of black hole formation and evaporation.

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Comments by Giotis on my previous post, about emergent gravity, reminded me about one of the tricky points in gauge-gravity dualities such as the AdS/CFT correspondence. This is the understanding of them as dualities between one theory that lives in the “bulk” spacetime (say quantum gravity on five dimensional AdS space) and another that lives on the boundary of that spacetime (four dimensional supersymmetric gauge theory in that case). This is kind of correct, if you know precisely what you mean, but it can also lead you to scratching your head if you don’t.

The tricky part in thinking about dualities is internalizing the idea that the two sides are actually two descriptions of one and the same object.  So, the gauge theory has the same properties as quantum gravity in five dimensional space, it is a five dimensional quantum gravity theory. Like any other description of this quantum theory, it has a bulk and a boundary, it has gravitational forces and propagating gravitons, it has black holes forming and evaporating, etc., etc. … All of those are realized in a somewhat unfamiliar language, more suited for the quantum rather than the classical theory, but all the same they are still there.  It’s a good mental exercise to phrase gravitational properties of the theory in terms of the gauge theory variables, it forces you to distinguish physics from language. So, I’ll do that here by discussing bulk and boundary of spacetime in the gauge theory language.

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