Thursday, December 7, 2006

String Theory Debate Continues

You know These would come. Here is a rebuttal by Peter Woit:

This month’s American Scientist has a review entitled All Strung Out? of The Trouble With Physics and Not Even Wrong by prominent string theorist Joe Polchinski, and he has posted a slightly edited version of the review with some explanatory footnotes at Cosmic Variance. I assume there will be a lot of discussion of it over there, perhaps with Polchinski participating, so, even though I wanted to write some sort of response here, I’ll leave comments off and encourage people to discuss this over at CV.

First of all I should say that I was quite pleased to see Polchinski’s review. While I disagree with much of it, it’s a serious and reasonable response to the two books, the kind of response I was hoping that they would get, opening the possibility of a fruitful discussion. Unless I’ve missed something, the only review by a string theorist to appear in a publication so far has been Susskind’s almost purely ad hominem one in the Times Higher Education Supplement. There also are two other (not published in conventional media) reviews by string theorists that I know of, a serious one by Aaron Bergman, and a nutty one by Lubos Motl that Princeton University Press paid him to write for some mysterious reason. I’ve heard that several publications have had a hard time finding a string theorist willing to write a review of the books, which I guess is not too surprising. It’s not obviously a rewarding task to involve oneself in a controversy that has become highly contentious, and where some of the main points at issue involve very real and serious problems with the research program one has chosen to pursue.

Much of Polchinski’s review refers specifically to Smolin’s arguments; some of it deals with the endless debate over “background independence”, and the “emergent” nature of space-time in string theory vs. loop quantum gravity. I’ll leave that argument to others.

Polchinski notes that I make an important point out of the lack of a non-perturbative formulation of string theory and criticizes this, referring to the existence of non-perturbative definitions based on dualities in certain special backgrounds. The most well-known example of this is AdS/CFT, where it appears that one can simply define string theory in terms of the dual QFT. This gives a string theory with the wrong number of large space-time dimensions (5), and with all sorts of unphysical properties (e.g. exact supersymmetry). If it really works, you’ve got a precisely well-defined string theory, but one that has a low-energy limit completely different than the standard model in 4d that we want. This kind of string theory is well-worth investigating since it may be a useful tool in better understanding QCD, but it just does not and can not give the standard model. The claim of my book is not that string theories are not interesting or sometimes useful, just that they have failed in the main use for which they are being sold, as a unified theory of particle physics and gravity.

The lack of any progress towards this goal of a unified theory over the past 32 years (counting from the first proposal to use strings to do unification back in 1974) has led string theorists to come up with various dubious historical analogies to justify claiming that 32 years is not an unusual amount of time to investigate a theory and see if it is going to work. In this case Polchinski argues that it took about 50 years to get from the first formulation of QED to a potentially rigorous non-perturbative version of the theory (using lattice gauge theory). The problem with this analogy is of course that in QED non-perturbative effects are pretty much irrelevant, with perturbation theory describing precisely the physics you want to describe and can measure, whereas with string theory the perturbative theory doesn’t connect to the real world. When QED was first written down as a perturbative theory, the first-order terms agreed precisely with experimental results, and if anything like this were true of string theory, we wouldn’t be having this discussion. For the one theory where non-perturbative effects are important, QCD, the time lag between when people figured out what the right theory was, and when its non-perturbative formulation was written down, was just a few months (Wilson was lecturing on lattice gauge theory in the summer of 1973, having taken up the problem earlier in the year after the discovery of asymptotic freedom).

Polchinski agrees that the key problem for string theory is its inability to come up with predictions about physics at observable energies. He attributes this simply to the fact that the Planck energy is so large, but I think this is misleading. The source of the problem is not really difficulties in extrapolating from the Planck scale down to low energy, but in not even knowing what the theory at the Planck scale is supposed to be (back to that problem about non-perturbative string theory…).

Weinberg’s anthropic argument for the size of the cosmological constant is described by Polchinski as a possible “prediction” of string theory, and he recommends Susskind’s book as a good description of the latest views of string theorists. I’ve been far too rude to Polchinski in the past in expressing my views about this “anthropic landscape” philosophy, so I won’t go on about it here. He neglects to mention in his review that many of his most prominent colleagues in the string theory community are probably closer in their views on this subject to mine and Smolin’s than to his, and that our books are the only ones I know of that explain the extremely serious problems with the landscape philosophy.

Recently string theorists have taken to pointing to attempts to use AdS/CFT to say something about heavy-ion physics as a major success of string theory, and Polchinski also does this. I’m no expert on this subject, but those who are like Larry McLerran have recently been extremely publicly critical of claims like the one here that “Physicists have found that some of the properties of this plasma are better modeled (via duality) as a tiny black hole in a space with extra dimensions than as the predicted clump of elementary particles in the usual four dimensions of spacetime.” My impression is that many experts in this subject would take strong exception to the “better” in Polchinski’s claim.

Finally, about the “sociological” issues, Polchinski disagrees about their importance, believing they are less important than scientific judgments, but I’m pleased to see that he does to some extent acknowledge that there’s a serious question being raised that deserves discussion in the theoretical physics community: “This convergence on an unproven idea is remarkable. Again, it is worth taking a step back and reflecting on whether the net result is the best way to move science forward, and in particular whether young scientists are sufficiently encouraged to think about the big questions of science in new ways. These are important issues — and not simple ones.”

Again, my thanks to him for his serious and highly reasonable response to the two books.

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