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Archive for December, 2008

The call of the wild.

I was without e-mail or the internet for a while falling in love with Africa. Now I’m back tending the fort and I find that I missed the very interesting discussion on AdS/CFT that happened in the meantime.  I’ll tell you more about South Africa later on, but let me tell you that petting the cheetah was awesome.

 

Also, this trip made me realize that the biggest threat to various endangered species is population growth which results in encroachment of habitat.

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One more thing

For the small minority of readers who are not entertained by ancient Jazz clips, here is a quick puzzle. Consider the quantum mechanics of a particle moving in a plane, paramatrized by coordinates x and y, and feeling the effect of the potential V(x,y) = x^2 y^2,  is the spectrum continuous or discrete?

(For a certain generation of string theorists this may bring back memories…)

The answer will come much later, as I am now hopping on a plane. Shame to leave really, looks like it is going to snow sometime soon.

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A short break

This week marks the end of the semester, and as an academic my life is organized by semesters. This is then the end of my first semester blogging. I have enjoyed many interesting discussions with many of our readers, mostly about quantum gravity . Many concerns I’ve had proven to be a non-issue, I’m pretty happy with the level of civility in our discussions, and with my current frequency of posting this hobby is not such a big burden on my time. So far so good,  I intend to continue for the time being.

But for the rest of the year I will be taking a break. First, as part of the promised theme of marital bliss, I will be travelling to a warm place in the southern hemisphere. Afterwards, combination of holidays and work, mostly for the grant agency I got sucked into, will probably mean sporadic to non-existent blolgging.

I hope David will be back to entertain you. In the meantime you can rely on Bill Evans, in one of my favorite pieces from him, originally from Miles Davis’ “Birth of the Cool” album, titled “Israel” (this is the most patriotic I ever get).

Update: As a bookmark for myself, if nothing else, here is another wonderful piece from the album “Explorations”, this one called “Beautiful Love”.

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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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