Not Even Wrong

Roger Penrose has a new book out in England, called “The Road to Reality”. It is 1000 pages long and is now ranked number 17 on Amazon’s UK site. There’s a review of the book here.

It sounds like the book contains all sorts of things, including some of the material about string theory that Penrose has presented in public talks various places about “Fashion, Faith and Fantasy in Modern Physical Theories”. One version of these talks is available online from Princeton. By “Fashion” Penrose is referring to string theory, and he considers the question of why it is so fashionable. String theory has been heavily sold as “beautiful” and to some extent Penrose seems to go along with this, but his invoking of the term “fashion” indicates an awareness of how problematic notions of “beauty” can be. The latest fashionable clothes are heavily promoted for their beauty, although after a few years, when they become unfashionable, this beauty is no longer so obvious.”Beauty” is very often a social construct, with many people willing to agree that something is “beautiful” if everyone around them is saying so. Recall the story of the emperor and his fashionable outfit.

I’ve never understood these claims that string theory is “beautiful”, and was again struck by this when I got ahold of a copy of Barton Zwiebach’s new book on string theory aimed at undergraduates called A First Course in String Theory. Most of the first 270 pages of the book are devoted to working out in detail the quantization of the bosonic string in light-cone gauge, and I find it hard to believe that anyone finds this a beautiful subject. It is mathematically rather complicated and not that interesting, and has no real connection to any observable physics.

Later on in the book Zwiebach does devote a fair amount of space to trying to connect string theory to the standard model, mainly using the construction of intersecting D6 branes. At the end of this section, he acknowledges that this construction is truly hideous and looks all too much like a Ptolemaic use of epicycles on epicycles to explain planetary motion, saying “the models seem contrived, at least in the sense that they are engineered to give the physics that we observe, rather than obtained naturally as the simplest solutions of string theory”. He quotes Alfonso the Wise (1221-1284) as having said the following about Ptolemaic epicycles:

“Had I been present at the creation, I would have given some useful hints for the better ordering of the universe.”

He tries to end on a more optimistic note, hoping that some deeper meaning of string theory will emerge with more work, quoting Maimonides as follows:

“In the realm of Nature, there is nothing purposeless, trivial or unnecessary”

which begs the question of whether string theory is part of the “realm of Nature”.

Susskind has posted a new preprint entitled “Cosmic Natural Selection”. It’s just two pages of various attacks on Lee Smolin’s theory of cosmological (not cosmic) natural selection. Smolin’s theory is described in detail in his book “The Life of the Cosmos”, published seven years ago. As far as I know the terminology of “Landscape” entered physics in this book, where Smolin adopts the term “fitness landscape” from evolutionary biology.

Why is Susskind all of a sudden taking an interest in Smolin’s old theory? Well, last Tuesday a preprint by Smolin appeared entitled “Scientific Alternatives to the Anthropic Principle” in which he gives a detailed criticism of the anthropic principle as unscientific. The next day Susskind tried to post a response to Smolin, consisting of a 3 page paper, half of which was just a quote from a summary of his argument that Smolin had sent to him. The one and a half pages that Susskind himself wrote were pretty much incoherent, and showed no sign that he had bothered to actually read Smolin’s article.

This was such a bizarre document that someone responsible for the arXiv actually refused to accept it. I’ve heard of several non-mainstream physicists who have had problems getting their articles accepted, but this is the first time I’ve heard of this happening to a well-known mainstream physicist. Whoever did this was doing Susskind a huge favor, but he then immediately forwarded a copy of the paper via e-mail to a long distribution list.

So that’s why Susskind’s latest begins “In an unpublished note I criticized Smolin’s theory of cosmological selection…” Note that he is not addressing any of Smolin’s criticism of his own work as unscientific, instead he is attacking Smolin’s own speculative ideas. I’ve personally experienced this kind of thing from more than one string theory fanatic. They don’t respond to criticism of what they are doing (because they don’t have much of a response), and instead forcefully and incoherently attack one for any speculative comments one may have made.

This just gets weirder and weirder all the time….

Now string theory is being used to peddle Verizon DSL service (courtesy of Lubos Motl). According to the writers of the advertising copy :

“String Theory: The so-called unified theory is gaining credibility among young scientists”

Right.

Gregg Easterbrook is a senior editor at the New Republic, and gives every indication of being a complete moron. His column about Hawking’s recent talk on black hole information loss is a masterpiece of anti-intellectualism. He appears to believe that the problem with physics is that, in trying to understand the big bang, ideas have been invoked that “don’t do especially well on the common-sense test”.

Only someone with zero common-sense would think that common-sense is going to explain the state and dynamics of the universe billions of years ago when conditions were utterly unlike those in which humans evolved and continue to exist.

Jacques Distler has something interesting about the prospects for producing black holes at the LHC. This has often been promoted as one of the most exciting possibilities for new physics from the LHC. Evidently it turns out that cosmic-ray experiments currently in progress also are sensitive to this kind of hypothetical black hole prediction. Depending on one’s assumptions about the minimal mass of a black hole that can be cleanly distinguished, either all or almost all of the range of new fundamental gravity scales accessible to the LHC will have been covered by the cosmic-ray experiments. So, if there is a new unexpected gravity scale in the range of a few Tev, this is very likely to first be seen in cosmic-rays, not at the LHC.

These Tev-scale gravity models have gotten a lot of attention in recent years, much of it in the form of claims of string-theory “predictions” that could be tested at the LHC. A few days ago I was talking to one of my colleagues, who believed, based on hearing a talk by Nima Arkani-Hamed, that string theory made predictions about what would happen at the LHC. Of course this is nonsense. There are lots of models you can construct involving string theory and any gravity scale you want. Past experiments rule out the possibility of a gravity scale up to the range of 100’s of Gev-1 Tev, but there is no reason other than wishful thinking and a desire to have something to say to people who point out that string theory makes no predictions, to believe that there is a gravity scale just a bit too high to have been seen at the Tevatron, but observable at the LHC. Such an assumption actually ruins what string theorists consider the major success of the whole string theory/supersymmetry picture, the fact that the 3 coupling constants in the standard model nearly come together at an energy somewhat below the Planck mass.

Personally I’ve always found the current “Large Extra Dimensions” Tev-scale gravity models to be just hideous and completely unmotivated. On the other hand, the one thing we know is that the electroweak-breaking scale is in this region, and since we don’t yet know what is causing this breaking, it is not completely inconceivable that it has something to do with gravity. The argument that the scale of quantum gravity is the Planck scale is not water-tight. It is based on the assumption that whatever generates the Einstein-Hilbert action as the effective low-energy action for gravity produces it with a magnitude of order one. If instead it comes with an exponential factor, the underlying gravity scale could be quite different than the Planck scale.

Besides some enthusiasts whose talks give the impression that Tev-scale gravity is a prediction of string theory, most string theorists believe that this is something unlikely to occur (Distler is one of these). One of the weirder things I’ve encountered in arguing with prominent string theorists is that they like to say that the Tev-scale gravity models are one of the major achievements in string theory in recent years, while at the same time saying they don’t believe in these models. What’s up with claiming as a point in favor of your theory that it “predicts” something you don’t believe?

Lee Smolin has a new preprint discussing the “anthropic principle”. He argues that one standard form of the anthropic principle that has been invoked by proponents of the “Landscape” is not falsifiable and he gives an eloquent explanation of the importance of falsifiability for a shared scientific enterprise. He also discusses the “prediction” of the rough magnitude of the cosmological constant that supposedly uses the anthropic principle and is due to Weinberg. He points out that this argument really isn’t an anthropic one, since it is independent of the existence of intelligent life. It just relies on showing that there is a relation between the cosmological constant and the existence of gravitationally bound structures. Then, since we see galaxies, we know something about the cosmological constant.

One of Smolin’s concerns is to show that his theory of “cosmological natural selection” (discussed in his book “The Life of the Cosmos”), while being a theory of a “multiverse” just like the string theory Landscape, is different in that it is potentially falsifiable, unlike some recent anthropic arguments.

He states well the predicament that theoretical physics finds itself in, with the tactic that worked so well throughout the 20th century, that of searching for unification by exploiting symmetry, no longer having much success. While I agree with most of what he has to say in this preprint, I’m more optimistic than him that future progress through new ideas about unification and the exploitation of symmetry is still possible. My point of view is more that the reason the last twenty years have seen no progress of this kind is that virtually all the field’s effort has gone into pursuing one very speculative and not very promising idea about unification, ignoring other possible lines of research.

Alexander Polyakov is one of the most prominent figures in theoretical physics and one of the most well-known string theorists at Princeton. He has written a review of his career and of his efforts to understand the relation between gauge theory and string theory. His penultimate paragraph goes as follows:

“In my opinion, string theory in general may be too ambitious. We know too little about string dynamics to attack the fundamental questions of the ‘right’ vacua, hierarchies, to choose between anthropic and misanthropic principles, etc. The lack of control from the experiment makes going astray almost inevitable. I hope that gauge/string duality somewhat improves the situation. There we do have some control, both from experiment and from numerical simulations. Perhaps it will help to restore the mental health of string theory.”

Seems to me he’s saying that, while using string theory to understand gauge theory is sensible, those claiming that it provides a theory of everything have gone nuts. I wonder what his colleagues at Princeton think of this.

Hawking gave his widely anticpated talk in Dublin today and reports are on CNN and all sorts of other places in the media. Sean Carroll has managed to get ahold (via Dennis Overbye of the New York Times) of a transcript.

Here’s the part where he summarizes his argument:

“I assume the evolution is given by a Euclidean path integral over metrics of all topologies. The integral over topologically trivial metrics, can be done by dividing the time interval into thin slices, and using a linear interpolation to the metric in each slice. The integral over each slice, will be unitary, and so the whole path integral will be unitary.

On the other hand, the path integral over topologically non trivial metrics, will lose information, and will be asymptotically independent of its initial conditions. Thus the total path integral will be unitary, and quantum mechanics is safe.”

His argument is in Euclidean quantum gravity, which he describes as “the only sane way to do quantum gravity non-perturbatively”, something which some might disagree with. What he seems to be arguing is that, while it is true you get information loss in the path integral over metrics on a fixed non-trivial black hole topology, you really need to sum over all topologies. When you do this you get unitary evolution from the trivial (no black hole) topology and the non-trivial topologies give contributions that are independent of the initial state and don’t contribute to the initial-final state amplitude.

I guess what this means is that he is claiming that, sure, if you knew you really had a black hole, then there would be a problem with unitarity, but in quantum gravity you don’t ever really know that you have a black hole, you also have to take into account the amplitude for not actually having one and when you properly do this the unitarity problem goes away.

He has some proposal for doing some kind of calculation that implements his proposal using the AdS/CFT correspondence.

My copy of the proceedings of the conference in honor of Graeme Segal’s 60th birthday finally arrived and I’ve been spending some enjoyable time reading parts of it. To me, the most interesting contributions were the ones by Ben-Zvi and Frenkel, Dijkgraaf, Moore, Stolz and Teichner, Teleman and Witten. Unfortunately, Dijkgraaf’s beautiful paper about how matrix integrals give you Gromov-Witten invariants of Calabi-Yau manifolds doesn’t seem to be available on-line. Neither is Witten’s very interesting paper, which is about explaining the SL(2,Z) symmetry seen in N=4 SSYM in four dimensions in terms of the existence of a six-dimensional superconformal theory.

The Stolz and Teichner paper is quite interesting . They are pursuing the idea that conformal field theories provide geometrical representatives of elliptic cohomology classes. Segal and Mike Hopkins worked on this a bit in the late 80s, with no conclusive results. Recently Hopkins has reformulated the whole elliptic cohomology story in terms of a new cohomology theory he calls “topological modular forms”. He gave a beautiful series of talks about this at the Segal Conference; this isn’t written up in the proceedings, but was for the 2002 ICM. For a more expository version of the ideas of Stolz and Teichner, see Teichner’s survey talk at a conference in Santa Barbara last summer.

Finally, the proceedings volume contains Segal’s wonderful unfinished manuscript “The Definition of Conformal Field Theory”, together with nine pages of very interesting comments about what he was trying to do then, what he would do differently now, and what had kept him from finishing the manuscript. The main problem seems to have been that he was unable by his methods to explicitly construct the “modular functor” that one should get out of WZW models, so for this reason the crucial chapter 11 on WZW models remains unwritten.

His comments begin with:

“The manuscript that follows was written fifteen years ago. On balance, though, conformal field theory has evolved less quickly than I expected, and to my mind the difficulties that kept me from finishing the paper are still not altogether elucidated.”