Archive for the ‘Quantum Gravity’ Category

A rather hot topic of modern research in quantum gravity has hit the news stands. This is the “firewall” business that got started in Santa Barbara.

Dennis Overbye has a piece on it in the New York times. Have fun reading it.

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Right now I’m in the midst of a program I helped to organize (and I’m still organizing) at the KITP. The program deals with the question of how to use numerical methods from lattice and gravity to make inroads into interesting (usually very hard) questions about quantum field theory (and quantum gravity) and the dynamics of the strong interactions at finite temperature (like in the heavy ion collisions).


We’ve had a lot of great talks about a wide variety of topics. Personally, I really liked the talk by Phillipe DeForcrand on the sign problem. The main reason I like it is because he had really simple examples that illustrate what the sign problem is all about. You can find it here.

And if you want to see what we’ve been hearing about, you can go here and see the full list of talks so far.

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Here is an announcement of a program I will be organizing at the KITP from Jan 17 thru March 9 2012. It is a program on numerical methods for gravity and QFT. The web page of the program is located here.

Here is the image I made to illustrate the program: it is generated by taking a set of modes in a box with a UV cutoff. Then amplitudes are seeded for these modes with random numbers and phases multiplied by the typical quantum uncertainty on each mode. The result is a picture like the one below.

It is also fun to animate it.

Right now I have to start chasing people and reminding them that the (first) deadline for applications is coming soon (April 30th).

In the meantime stay tuned.

Image of Fluctuations of quantum fields

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I happened onto an article on the New York Times abut Erik Verlinde’s take on gravity as an Entropic force. The article was written by Dennis Overbye who most of the time does  a good job of covering high energy physics.  Erik’s work dates from earlier this year and can be found here. To tell the truth, I don’t understand what he’s trying to say in that paper and to me it feels like it’s almost certainly wrong.

However, I don’t want to discuss that paper. What I want to discuss is the following  provocative quote

“We’ve known for a long time gravity doesn’t exist,” Dr. Verlinde said, “It’s time to yell it.”

I don’t believe this is taken out of context,  so we should take it at face value. The statement is obviously wrong, so it sounds like ultra-post-modern pap and makes all physicists working on the subject of quantum gravity look like crazy mad men. I’m sure this sells newspapers, but that is not the point.

When asked for a sound byte can’t people at least say something that is correct and not just provocative?

The proper way to write that statement is that “Gravity is not really a fundamental force “, which is more correct and does not deny gravity its proper place as something that has been observed in nature, however it is less catchy. If we apply the same criteria as used in the above construction, all of the following statements are also correct:

  • Hydrodynamics does not exist (it only happens for collections of atoms, but not for individual ones)
  • Space and time do not exist (often used when talking about quantum gravity being emergent from somewhere else)
  • All emergent phenomena do not exist (they are not fundamental after all).
  • I do not exist (I’m an emergent phenomenon).

Reminds me of discussions I have read before at Backreaction, here and see also  here in the Discover magazine about time not existing.

You should also read the following from Asymptotia: But is it real? and also a discussion on What is fundamental, Anyway?

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New evidence against cosmic censorship in five dimensions was announced today in the arxiv. The evidence is numerical, but it is done by a very good group of people working on numerical general relativity: Luis Lehner and Frans Pretorius. They argue that in the black string Gregory Laflamme instability the system develops `singularities in finite time’ as seen from the outside. This is due to the way the system evolves ins a self-similar way.

This is important because for these systems one can not ignore quantum gravity effects for  a ‘generic’ set of  initial conditions. For a long time the system of the black string Gregory-Laflamme instability was believed to provide a counterexample for cosmic censorship, but various technical results made it hard to prove and showed that the problem was rather difficult to approach.

Once again, higher dimensions show that they are very different from four dimensions when it comes to the study of Einstein’s equations. The first statement that went out was the ‘no hair’ theorem when people found all kinds of super-tubes, black rings,  etc where one could have many stationary solutions to Einstein’s equations (with some matter) with the same quantum numbers and with black objects involved in their description.

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The Gravity Research Foundation announced the results of the 2010 competition. Here are the results. At UCSB we discussed the prize-winning paper by Mark van Raamsdonk today. It was a very lively discussion and we thought it was a great paper to read. Mark’s paper provided some very tantalyzing evidence that entanglement seems to play a very important role in building up geometry.

On another note, a paper by Daniel Green, Zohar Komargodski, Nathan Seiberg,
Yuji Tachikawa, and Brian Wecht
appeared today. They solve a problem in four dimensional supersymmetric conformal field theories on counting how many marginal deformations there are. As a byproduct, they also solve the problem in 3-d field theories with N=2 Supersymmetry. The paper is beautiful and it is a huge improvement on the work by Leigh and Strassler on the subject many years ago. After reading it I was kicking myself because ‘I could have done it’ (I was interested in the problem and I knew many of the facts. I just didn’t put them together. But if I had thought hard about it I probably could have, although the paper would read rather differently). It’s not surprising that these authors at the Institute for Advanced Study found the solution and that it is written in the particular way that it is written since they have been studying very carefully the superfield formulation of supersymmetric theories in four dimensions. Lubos also commented on the paper.

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At some point I promised that I was going to write about my  most recent paper. So here is my promotion. In a sense, that paper is an exercise to understand what does it mean to have quantum gravity in a setup of emergent geometry: this is a situation where geometry is not there a priori, but it is extracted from some collective behavior of a system. I don’t want to go into semantics of what emergence means. For our purposes it is something that is extracted from a non-trivial procedure in systems with a lot of degrees of freedom, where we extract stuff that involves all degrees of freedom simultaneously in a non-trivial  way. The system is quantum mechanical, and therefore there are quantum fluctuations and whatnot, and the whole purpose of our study is to measure some property that can be associated with a distance, but taking into account that the measurement will give you some type of probability distribution on some variable that is supposed to be geometric. Instead of getting something where this is all done analytically, we did it by computer simulations and ran it like an experiment. To top it off, the research was done with an undergraduate student, who ran the simulations and did some of the basic data analysis of the numbers we got.


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