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Archive for December 3rd, 2008

An ode to spam filters

10 minutes to the condensed matter seminar! (remember that email from last week?).

9 minutes to the condensed matter seminar! (there’s also an announcement on the bulletin board…).

8 minutes to the condensed matter seminar! (…and the department website).

7 minutes to the condensed matter seminar! (it should be a good one, speaker is a good guy).

6 minutes to the condensed matter seminar! (and it will take place at the usual room, in case someone is wondering).

5 minutes to the condensed matter seminar! (afterwards we’ll go for lunch, would anyone like to join?).

4 minutes to the condensed matter seminar! (too bad, but would you like to meet the speaker?).

3 minutes to the condensed matter seminar! (he’s a really good guy. Really.).

2 minutes to the condensed matter seminar! (we’ll take him out for dinner).

1 minute to the condensed matter seminar! (would you like to join?)

Condensed matter seminar! (next week we’ll have a really good one, in the usual place and time, we’ll email you the details).

(with apologies to the delightful original).

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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.

(more…)

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