Comments on: Looking for Revolutions in Physics

By: Democratic Vote Squashes Anthropic Principle | Cosmic Variance

Democratic Vote Squashes Anthropic Principle | Cosmic Variance — Wed, 03 Aug 2005 00:52:04 +0000

[...] We have already heard about the panel discussion on the Next Revolution in String Theory at Strings 2005 from Clifford and Jacques. Today, it made the New York Times. It shouldn’t be a surprise that the NYT had a slightly different spin. The NYT paid some homage to the concept that by their nature revolutions can’t be predicted. Progress in science occurs when scientists work on questions that puzzle them rather than when they try to guess which questions might have the biggest payoff. Most of the NYT article was devoted to The Vote. [...]

By: The Blog as a Sharp Tool for Research | Cosmic Variance

The Blog as a Sharp Tool for Research | Cosmic Variance — Sat, 23 Jul 2005 23:41:45 +0000

[...] Well, a number of things did come up as interesting and fun to try, and the following is one of them. I’ve already said on this blog that our particular field (string theory and related topics) could do with more ways of having discussion, both general and specific. We have already accelerated the primary way in which we exchange research results (revolutionizing scientific publishing in the process) by establishing the Archive (see writing about this by Paul Ginsparg), and it undeniably helped drive the field’s rapid developments in the middle 90s while also democratising it by enabling serious papers from the traditional large and famous institutions to be seen on everybody’s computer screen right alongside the serious papers from smaller less well known institutions, often within minutes or hours of the completion of the work. [...]

By: Friday Random Ten: iPod Tells the Future of String Theory? | Cosmic Variance

Fri, 22 Jul 2005 11:35:05 +0000

[...] In view of the varying opinion about the state of string theory, regarded by some as the last, best hope for fundamental physics (going to get myself into trouble there), discussed wonderfully and I hope continually on this very blog (e.g. here and here), it might be worth sitting back and letting the pod do its thing, and tell us the future. [...]

By: Robert

Robert — Fri, 22 Jul 2005 09:33:21 +0000

I should have said “gravity might be like any other power non-renormalizable field theory”. Luckily, in particle physics we usually don’t have to deal with those as the bad terms are nearly zero by RG flow.

By: Moshe Rozali

Moshe Rozali — Thu, 21 Jul 2005 17:40:38 +0000

Robert:

Gravity is a fine classical field theory, but it is easy to see it cannot be treated as a QFT, since it cannot be renormalized. Just try to calculate almost anything with the usual formalism of QFT, you’ll get nonsense.
If you redefine what you mean by either gravity or QFT, all bets are off.

There are less mechanical ways to state the problem, for example the lack of decoupling in gravity. Decoupling is at the heart of properly defining QFT via RNG flows etc., but in gravity heavy objects cannot be treated as “fast” variables a la Born-Oppenheimer, because they are simultaneously strongly coupled, exactly because they are massive.

One issue in QFT which may be interesting is its applications to highly dynamical situations, such as non-equilibrium processes. I have a feeling that a lot of the Lorentzian issues one encounters in string theory lately will already be manifested there, and there is probably a useful body of knowledge out there.

By: Robert

Robert — Thu, 21 Jul 2005 12:11:45 +0000

I think the reason one might think that some are convinced that there are no field theory questions left and all is just strings (and friends) is this very succesfull approach of the last decade to reformulate field theory questions in terms of branes and then use geometry (of one kind or the other) to attack these problems. A lot of mileage has been gained this way but this of course does not mean that these are not intrinsic field theory questions and that not all questions can be approached this way.

As nobody else starts I’ll take the lead to list a couple of field theory questions: And I start with a non-field theory question: Is gravity just a field theory? I.e. is there really an intrinsic difference between gravity and ordinary field theories or is it just another field theory with spin 2 fields and a large gauge invariance. People often take it for granted that there is some special magic with gravity but I am not yet convinced this is the case. For example these peopl e point out that it is remarkabel that AdS/CFT connects a field theory with a gravitational theory. But it seems one could also come up with similar purely field theoretic examples. Maybe you are going to say that gravity has holography but in that case please define holography precisely and explain why this cannot be a property of a field theory.

There are of course millions of questions to understand theories without susy and non-holomorphic properties of N=1 theories beyond the first couple of orders of pertubation theory, confinement being just one of them.

I think what is not such an interesting question (and I believe even many of the more formal people agree) is to show that there are interesting field theories in the mathematical sense (e.g. Whiteman or Haag-Kastler axioms): My understanding is that the only examples where this has been worked out are free theories and rational CFTs in 2D but I think nobody has doubts that 4D gauge theories (maybe supersymmetric) fulfill these (or some reasonably adopted) axioms: You could start from the lattice (once you solved the problem of having chiral fermions and susy in this case) and then attempt to prove that the continuum limit exists and has nice properties (covariance, locality, positivity). Technically, this is a very very hard problem but I doubt that anybody believes that there serious problems there.

I have the impression that there are a lot of interesting things still to be learned from the connection between field theories and topology (and these might again have brane interpretations) but maybe Peter can comment on this.

Please comment and continue!

By: Moshe Rozali

Moshe Rozali — Wed, 20 Jul 2005 20:12:57 +0000

Arun- nice lectures are at hep-th/0504194, for a new calculation in pure QCD using these methods look at hep-ph/0507005.

Thanks for the reply, Clifford, I agree with everything you say. I should say welcome as well- it is already clear that one has no chance of getting anything done at the face of 5 (!) of you guys…

By: Aaron Bergman

Aaron Bergman — Wed, 20 Jul 2005 20:09:25 +0000

I assume Moshe's referring to the twistor stuff, starting with Witten's paper and following with various papers by some subset of Volovich, Spradlin and Roiban, the collected works of Freddy Cachazo, and the stuff by Dixon et al. And probably plenty more than that. You can also check out Freddy's slides at Strings 05.

By: Arun

Arun — Wed, 20 Jul 2005 19:58:08 +0000

“It is clear to me that QFT is not a finished subject, even in the last year there was enormous progress in perturbative (!) QFT.”

Moshe, link to some representative papers, please!

By: Clifford

Clifford — Wed, 20 Jul 2005 17:49:27 +0000

Hi Moshe, Welcome, and also to Robert and Aaron and Jack (I should have said so earlier). You’re very correct of course. Why not discuss it? Maybe there are others lurking who can point up some of their favourite things. For what it’s worth, and by way of amplification of the point you’ve made (and because there are non-experts listening), I’ll say that the work that has been happening in both string theory and field theory, perturbative or non-perturbative, has led us to realize that the key tools that we have used to formulate and calculate with are clearly not in any sense fundamental in determining what the theory “is”. We saw it very clearly non-perturbatively with dualities such as S-duality and its cousins in field theory, and very spectacularly with strong/weak coupling dualities in string theory, which relate completely different theories, often in spacetimes of different dimensions. The weak form AdS/CFT (connecting gravity in 5D to strongly coupled gauge theory in 4D) combined these things into a clean and stunning example, and droe the point further home: The Lagrangian and Feynman diagrams don’t tell you the whole story. But in retrospect it was already beginning to be apparent perturbatively in gauge theory, because of the huge redundancy that exists there in formulating computations in terms of Feynman diagrams, (and further, the philosophy of effective field theory and RG flow was itself suggestive). But with the program of computing in these completely new ways given by the twistor program using spectacularly fewer diagrams, etc, the perturbative reliance on Feynman diagrams as a “definition” of what field theory is also seems very shaky. These are interesting times. But I know that you know all that….but this maybe gets the ball rolling…..

-cvj