King of Infinite Space

Posted on October 19, 2006 by woit

Back when I was a graduate student trying to figure out how to define and calculate topological charge in lattice gauge theory, at one point I went over to the math department to ask some people I knew if they had any idea about how to calculate the volumes of spherical tetrahedra. I was taken to the math department lounge to consult with the master of 3 dimensions, Bill Thurston. Thurston explained to me that this could be done by breaking the tetrahedra into “double-rectangular tetrahedra”, whose volumes were then expressed in terms of the angles defining them using something called Schläfli functions, defined back in 1860. This experience helped cure me of my prejudice that modern mathematicians were probably ignorant of the older more concrete mathematics of the 19th century.

Thurston also pointed me to a more modern reference for this, a paper by H.S.M. Coxeter from 1935 entitled The Functions of Schläfli and Lobatschefsky. I ultimately found a much simpler way of computing topological charges, but I always wondered about this early 20th century mathematician, whose parents had given him a set of initials reminiscent of a British naval vessel. Later on in life, I learned a bit about some important algebraic constructions called Coxeter groups, and also heard that there was an active mathematician in Toronto named Donald Coxeter. I assumed that there were at least two and maybe three mathematicians named Coxeter out there, perhaps relatives.

It turns out that these are all the same Coxeter (the M. is for MacDonald), and there’s a very nice new biography of him that has recently appeared, writtten by Siobhan Roberts and entitled King of Infinite Space. Coxeter only died quite recently, in 2003 at the age of 96, and Roberts was able to get to know him while writing the book. It contains a wealth of information about pieces of mathematical history I was not aware of, often buried in the very extensive footnotes.

Coxeter’s main interest was in “classical” geometry, the geometry of figures in two and three dimensional space and he wrote a very popular and influential college-level textbook on the subject, Introduction to Geometry. Much of this subject can be thought of as group theory, thinking of these figures in terms of their discrete symmetry groups. This subject has always kind of left me cold, perhaps mainly because these groups play little role in the kind of physics I’ve been interested in, where what is important are continuous Lie groups, both finite and infinite-dimensional, not the kind of 0-dimensional discrete groups that Coxeter mostly investigated.

One theme of the book is to set Coxeter, as an exemplar of the intuitive, visual and geometric part of mathematics, up against Bourbaki, exemplifying the formal, abstract and algebraic. Bourbaki is blamed for the New Math, and I certainly remember being subjected by the French school system in the late sixties to an experimental math curriculum devoted to things like set theory and injective and surjective mappings. On the other hand, I also remember a couple years later in the U.S. having to sit through a year-long course devoted to extraordinarily boring facts about triangles, giving me a definite sympathy for the Bourbaki rallying cry of “A bas Euclide! Mort aux triangles!”. To this day, both of these seem to me like thoroughly worthless things to be teaching young students.

Actually Bourbaki and Coxeter ended up having a lot in common. They both pretty much ignored modern differential geometry, that part of mathematics that has turned out to be the fundamental underpinning of modern particle physics and general relativity. Coxeter’s most important work probably was the notion of a Coxeter group, which turns out to be a crucial algebraic construction, and ended up being a main topic in some of the later Bourbaki textbooks. A Coxeter group is a certain kind of group generated by reflections, and Weyl groups are important examples. Coxeter first defined and studied them back in the 1930s, part of which he spent in Princeton. Weyl was there at the same time giving lectures on Lie groups, and used Coxeter’s work in his analysis of root systems and Weyl groups.

Coxeter groups and associated Coxeter graphs pop up unexpectedly in all sorts of mathematical problems, and Roberts quotes many mathematicians (including Ravi Vakil, Michael Atiyah and Edward Witten) on the topic of their significance. There are quite a few places where one can learn more about this. These include various expository pieces by John Baez (see for example here, based to some extent on this), as well as a web-site set up by Bill Casselman. The AMS Notices had an interesting series of articles about Coxeter and his work, written shortly after his death. The proceedings of a recent conference at the Fields Institute in Toronto entitled The Coxeter Legacy – Reflections and Projections have recently been published. In a couple weeks there will be a special program in Princeton about Coxeter, aimed at the general public.

One reason I’d started reading the book about Coxeter was to get away from thinking about string theory, but this was definitely not a success, since the book contains a rather extensive discussion of string theory. Coxeter was aware of string theory, it seems it reminded him of Jabberwocky, and he’s quoted as follows:

It’s like reading about a part of mathematics that you know is beautiful, but that you don’t quite understand. Like string theory. That’s as much a mystery to me as it is to anyone else who can’t make head nor tails of the eleventh or sixteenth dimension.

Roberts quotes Witten (who she says is known as the “pope of strings”) about the possible relevance of Coxeter groups and E(10) to string theory. She describes string theory in somewhat skeptical terms:

But rumblings are that if a bigger breakthrough doesn’t occur soon, and in the form of streams of empirical evidence, string theory will at best be a branch of mathematics or philosophy, but not part of physics.

She quotes Amanda Peet as proposing that string theory become “a faith-based initiative”, and Susskind as “There’s nothing to do except hope the Bush administration will keep paying us.”

Update: Siobhan Roberts has set up a web-site for the book, and she tells me that she’ll soon be starting up a blog there.

Update: There’s a very good expository paper by Igor Dolgachev that discusses Coxeter groups, and generally the way reflection groups appear in algebraic geometry.

Posted in Book Reviews | 29 Comments

The String Wars

Posted on October 18, 2006 by woit

I noticed recently that George Johnson will be journalist in residence and giving a talk on Friday at the KITP in Santa Barbara about “The String Wars”. Somehow I don’t really think that it’s a good thing that this is now being perceived as a “war”. Johnson is the author of an excellent biography of Murray Gell-Mann and writes for the New York Times.

For controversy on the East coast, tonight the Center for Science Writings at Stevens Institute for Technology in Hoboken will be hosting a panel discussion and debate on The End of Science?, featuring John Horgan and Michio Kaku.

This week’s New Yorker has a couple letters to the editor responding to their recent article about the string theory controversy. One points out that particle theory and quantum gravity is not all there is to theoretical physics. The second is by Lisa Randall, and mainly concerned with claiming that there is now a healthy interaction going between string theory and phenomenology, with most particle physicists eagerly awaiting the LHC.

Update: Today’s New York Times has an Op-Ed piece entitled The Universe on a String by my Columbia colleague Brian Greene, in which he responds to recent criticism of string theory. As you might guess, Brian’s piece doesn’t really convince me to change my mind (as my book and Lee Smolin’s don’t seem to have convinced him).

Brian mentions the possibility of seeing supersymmetry or extra dimensions at the LHC, and possible effects of quantum gravity in the CMB, but acknowledges that these are not definitive predictions of string theory that can be used to falsify it. He also mentions the recent attempts to apply AdS/CFT to heavy ion physics, but these don’t address the use of string theory as an idea about unification.

He deals with the landscape only by making an argument I’ve heard him make before: that just having a unified theory of gravity and particle physics would be a big accomplishment, even it this theory didn’t explain any of the things about the standard model that one would like it to explain. Besides the fact that string theory still doesn’t provide a fully consistent unified theory (since it has no non-perturbative formulation), I’ve always found this point of view problematic. If string theory can’t make any definitive predictions about particle physics, it’s very unclear that one can ever test it, which is a huge problem.

Brian does, unlike some string theorists, acknowledge that it’s possible that string theory is wrong and will have to be abandoned, in particular if “future studies reveal an insuperable barrier to making contact with experimental data”. My argument is that if string theorists accept the existence of the Landscape, such an insuperable barrier appears. He describes string theory critics as calling for research on string theory to be dropped, which really isn’t accurate. Neither Smolin nor I have ever called for this, rather our argument is that research into alternatives to string theory needs to be encouraged.

Update: The George Johnson talk is now available here. It seems that many of the string theorists at the KITP are not very happy about my book and Smolin’s, although it’s unclear if any of them have read either of the books. Amanda Peet claimed that both books have many errors (invoking the NYT review by Tom Siegfried), while Johnson repeatedly told her that it would be a good idea for her to actually read one of the books. She also kept claiming that there is “a backstory” that explains why Smolin wrote his book, but she was dissuaded from elaborating on this when someone pointed out that the talk was on video and would be on the web.

The experience of watching the talk was pretty odd, since Johnson began by connecting to my blog and discussing the fact that I was discussing his upcoming talk. I watched a lot of the talk during commercials of an episode of Numb3rs, and during this episode “Larry” the physicist was working on calculations involving branes, and playing hooky from a string theory conference.

Update: Davide Castelvecchi has put up an interview with George Johnson on his web-site.

Update: Clifford Johnson and Lubos Motl have their own takes on the KITP video.

Update: It appears that there will be a second talk by George Johnson about this, String Wars 2. After the first one, I’m having trouble figuring out why anyone at KITP thought a second one would be a good idea.

Posted in Favorite Old Posts, Uncategorized | 97 Comments

Links

Posted on October 12, 2006 by woit

This evening a very interesting paper appeared on the arXiv, entitled Instantons Beyond Topological Theory I by E. Frenkel, Losev and Nekrasov. The authors are studying theories with a topological sector (supersymmetric quantum mechanics and 2d sigma models on a Kahler manifold, N=2 supersymmetric YM in 4d), but are interested in sectors of the theories that are not purely topological. I’m looking forward to reading the paper over the next few days, but it is a bit daunting. This paper is nearly 100 pages long, and it is only part I of three parts, and actually just the simplest part, that involving quantum mechanics.

HEPAP is meeting today and tomorrow, here’s the agenda. From the slides of the talk about NASA, the budget situation there for fundamental science missions doesn’t look good, and there is discussion of the upcoming NRC committee charged with figuring out which of the “Beyond Einstein” missions to allow to go forward. At Dynamics of Cats, Stein Sigurosson has been writing about this in terms of the missions being sent to Thunderdome, only one to emerge alive.

Slides from the talks last month at the conference in honor of Nigel Hitchin’s 60th birthday are available.

Joe Lykken has a nice review article about the standard model, in which he notes:

There is only one diagonal Yukawa coupling that is of order one, and that is the top quark Yukawa. But even this case is mysterious. The top Yukawa is not really of order one: it is equal to one! For example, using the 2005 combined Tevatron value for the pole mass of the top quark, the corresponding Yukawa coupling is 0.99 +/- 0.01. The entire particle physics community has chosen (so far) to regard this fact as a 1 per cent coincidence. I should point out that similar percent level equalities, e.g. supersymmetric gauge coupling unification or the ratio of the total mass-energy density of the universe to the critical density, have spawned huge theoretical frameworks bolstered by thousands of papers.

Difference is that, as far as I know, nobody has an idea why this Yukawa coupling should be one. Maybe this is a big clue…

Over at Backreaction, there’s an excellent posting about Does String Theory Explain Heavy Ion Physics?, one of the very few places to find a non-overhyped discussion of this topic.

Davide Castelvecchi has a well-done review of my book at his sciencewriter.org web-site.

At this week’s physics colloquium at Penn, Andre Brown reports that Robert Cahn emphasized that “half the particles needed for supersymmetry have already been discovered.” He also recalled a quote from another colloquium about supersymmetry: “Supersymmetry has stood the test of time. There is no evidence for supersymmetry.”

Update: A couple people have pointed out the following rather accurate cartoon.

Posted in Uncategorized | 70 Comments

Rube Goldberg’s Instruction Manual

Posted on October 11, 2006 by woit

Since string theory first became popular in 1984-5, attempts to connect it to particle physics have suffered from various problems. One of the most severe of these goes under the name of “moduli stabilization”. Six dimensional Calabi-Yau manifolds come in families, parametrized by “moduli”. The dimensions of these moduli spaces can be of order 100 or so.

Naively it might appear that string theories are characterized by a choice of a topological class of Calabi-Yaus (no one knows if the number of these is finite or infinite), and then a choice of each of the 100 or so parameters that fix the size and shape of the Calabi-Yau. According to the standard string theory ideology, this is not the right way to think, instead there is really only one string theory, with different moduli values corresponding to different states. The moduli parameters are supposed to be dynamical elements of the theory, not something parametrizing different theories.

The problem with this is that if you promote the moduli to dynamical fields, they naively correspond to massless fields, and thus give new long-range forces. So you have to explain away why we don’t see 100 or so different kinds of long-range forces, and the experimental bounds on such forces are very good. Some kind of dynamics must be found that will “stabilize moduli”, giving them a non-trivial potential. The moduli fields will then be fluctuations about the minima of this potential. If the quadratic piece of the potential is large enough, their mass will be high enough to have escaped observation.

One needs a potential with non-trivial minima, and has to ensure that the dynamics is not such that the moduli will run off to infinity. In recent years, ways of achieving this have been found that typically involve “flux compactifications”, i.e. choosing non-trivial fluxes through the topologically non-trivial holes in the Calabi-Yau. On the one hand, this seems to provide a long-standing solution to the problem of how to stabilize the Calabi-Yau, on the other hand, it appears that there is an exponentially large number of possible minima. This is the origin of the “Landscape” and the associated claims of 10500 or more possible vacuum states for string theory.

The constructions involved are famously exceedingly complex and ugly, with Susskind referring to them as “Rube Goldberg machines”, and one of their creators, Shamit Kachru, the “Rube Goldberg architect”. Very recently a new Reviews of Modern Physics article by Kachru and Douglas called Flux Compactification has appeared. It can be thought of as a manual describing how to construct and count these Rube Goldberg machines.

Many string theorists had long hoped that whatever method was found to stabilize moduli would have only a small number of solutions. Then, in principle one would get only a small number of possible models of particle physics for each topological class of Calabi-Yaus. If the number of these was finite and not too large (the known number of constructions is something like 105-106), then to see if string theory could make contact with particle physics, one would just have to do a moderately large number of calculations, check them against the real world, and hope that one matched. If it did, it would then be highly predictive.

The existence of the flux compactifications with stabilized moduli described in the Douglas-Kachru article has convinced many string theorists that this old dream is dead. Some have tried to claim that this is a good thing, that the exponentially large number of states allows the existence of ones with anomalously small cosmological constants, and thus an anthropic explanation of its value. The problem then becomes one of how to ever extract any prediction of anything from string theory. Small CCs are achieved by very delicate cancellations, and it appears to be a thoroughly calculationally intractable problem to even identify a single state with small enough CC.

Many string theorists are now claiming that this is not really a big deal. So what if there are lots and lots of string theory vacua, it’s just like the fact that there are lots and lots of 4d QFTs! For arguments of this kind, see recent comment threads here and here. There’s something fishy about this argument, since discussion of flux compactifications has from the beginning focused on whether it is possible to use them to make predictions, whereas no one ever was worrying about whether (renormalizable) QFTs were predictive or not.

The source of the problem lies in the combination of the large numbers of string theory vacua with their Rube Goldberg nature. Consistent 4d QFTs are characterized by a limited set of data (gauge groups, fermion and scalar representations, coupling constants), and it has turned out that among the simplest possible choices of such data lies the Standard Model. String theorists commonly describe the Standard Model as “ugly”, but it is among the simplest possible 4d QFTs, and is extremely simple and beautiful compared to something like the flux compactification constructions. One could hope that while flux compactifications are inherently rather complicated, one of the simpler ones might correspond to the real world. As far as I know there’s no evidence at all for this, such a hope appears to have nothing behind it besides pure wishful thinking. Some string theorists like Douglas and Kachru don’t seem to think this is possible, focussing instead on statistical counts of more and more complicated flux compactifications, hoping to find not a simple one that will work, but a statistical enhancement of certain complicated ones that would pick them out.

4d QFT is a predictive framework not because the number of possible such QFTs is small, but because our universe is described extremely accurately by one of a small number of the simplest of such QFTs. A few experiments are sufficient to pick out the right QFT, and then an infinity of predictions follow.

Is the QFT framework falsifiable? One could imagine that things had worked out differently, that instead of the Standard Model predictions being confirmed, each time a new experimental result came in, one could only get agreement with experiment by adding new fields and interactions to the model. It might very well be that the QFT framework could not be falsified, since one could always evade falsification by adding complexity. This happens very often with wrong ideas: they start with a simple model, experimental results disagree with this, but can be matched by making the model more complicated. As new experiments are done, if the original idea is wrong, it doesn’t get simply falsified, but the increasing complexity of the models needed to match experiment sooner or later causes people to give up on the whole idea.

This is very much what has happened with string theory. The simple models that got people excited about the idea of string theory unification don’t agree with experiment, with the moduli stabilization problem just one example. It appears one can solve the problem, but it’s a Pyrrhic victory: one is forced into working with a class of models so vast and so complicated that one can get almost anything, and never can extract any real predictions.

Douglas and Kachru do address the question of whether one can ever hope to get predictions out of this class of models, but their answer is that they can’t think of any plausible way of doing so. They mention various things that people have tried, but none of these ideas seem to work. The best hope was that counting vacua with different supersymmetry breaking scales would lead to a statistical prediction of this scale, but this has not worked out for reasons that they describe. In the end they conclude:

For the near term, the main goal here is not really prediction, but rather to broaden the range of theories under discussion, as we will need to keep an open mind in confronting the data.

This acknowledges that no predictions from this framework seem to be possible, and that continuing work in this area just keeps producing yet wider and wider classes of these Rube Goldberg machines. They are suggesting basically giving up not on string theory, which would be the usual scientific conclusion in this circumstance, but instead to for now give up on the theorist’s traditional goal of making testable predictions. They advocate not giving up on string theory no matter how bad things look, instead just continuing as before, hoping against hope that an experimental miracle will occur. Maybe astronomers will find evidence for cosmic superstrings, maybe the LHC will see strings or something that matches up with characteristics of one of the Rube Goldberg models. There’s not the slightest reason to believe this will happen other than wishful thinking, which has now been promoted to a new program for how to do fundamental science.

Update: Via Lubos, for those who don’t know what a Rube Goldberg machine is, two examples are here and here.

Posted in Uncategorized | 38 Comments

Corrections…

Posted on October 8, 2006 by woit

I’m well aware that there’s far too much these days on this blog about the controversy over string theory, but two things have appeared today in the press about this that aren’t accurate, and I can’t resist using this platform to issue corrections. Readers who have had enough of this are warned to move on to some other blog with fresher material.

The Observer (the Sunday version of the British newspaper the Guardian) has an article today by Robin McKie, entitled String theory: Is it science’s ultimate dead end? On the whole, the article is a well-written piece about the controversy over string theory. I talked to McKie on the phone, and he quotes me as saying something that is probably an abbreviated version of what I actually said.

‘Too many people have been overselling very speculative ideas,’ said Woit – author of Not Even Wrong – last week. ‘String theory has produced nothing.’

The first part of this quote is fine, but “String theory has produced nothing” is not what I think, and presumably was part of some longer statement. String theory has certainly produced some very interesting mathematics, as well as some promising ideas about strongly coupled gauge theories. It has produced nothing useful about unification and how to get beyond the standard model.

The McKie piece also has some strong quotes in defense of string theory from David Gross, Samjaye Ramgoolam and Michael Green:

‘String theory is on the right path,’ said David Gross, of the University of California, Santa Barbara, and another Nobel prize winner. ‘But this path is quite long. Further breakthroughs are required.’

I’m kind of wondering why he claims that definitely string theory is on the right path. Perhaps he also had some caveats that got dropped.

‘said Sanjaye Ramgoolam, of Queen Mary, University of London. ‘There are a number of ways that we could prove – or disprove – string theory. For example, Europe’s new Large Hadron Collider may well be powerful enough to provide evidence that suggests we are on the right road.’

This kind of invocation of the LHC as being able to prove or disprove string theory always strikes me as less than honest.

According to Green:

“There is no alternative to string theory. It is the only show in town – and the universe.’

Again, perhaps some caveats have been dropped here.

The second piece with inaccuracies that appeared today is a review of my book and Lee Smolin’s in the LA Times by K.C. Cole. It’s entitled Strung Along and is basically a hit-job on me and Smolin. Some of the things in it are so dishonest and incompetent as to be pretty hilarious:

In fact, many statements about string theory in these books are plain wrong… To say, as Woit does, that fundamental mysteries about neutrinos are being ignored will come as news to the dozens of physicists who’ve been working on these problems for years.

At first I couldn’t figure out why she was attributing to me the insane statement that “fundamental mysteries about neutrinos are being ignored”, but after taking a look at all the references to neutrinos in the book, I finally figured it out. On page 93 of the US edition I write, after giving a description of the things the standard model leaves unexplained, including a parameter count that ignores neutrino masses:

One complication that has been ignored so far involves neutrinos.

and then go on to explain about the experimental evidence for neutrino masses. The “ignored so far” obviously means “ignored so far in this chapter”, not “fundamental mysteries about neutrinos are being ignored” by physicists. This recalls some of the hilarities in Lubos’s review of my book. It’s absolutely amazing that a supposedly serious journalist would do this kind of thing.

There are plenty more claims in the review that are pretty much the opposite of reality:

To mathematician Peter Woit and physicist Lee Smolin, however, the search for beauty is ruining physics.

Actually my view is quite the opposite: what’s ruining physics is pursuing very unbeautiful theories (Susskind is fond of calling them “Rube Goldberg machines”) for which there is no experimental evidence.

I’ve never met Cole and she knows nothing about me personally, but she seems intent on painting me and Lee as embittered failures:

Woit, and Smolin… write mostly about how string theory has ruined their careers.

I don’t think there’s anything in Smolin’s book about how string theory has ruined his career (and he’s had quite a successful one). As for me, there’s no such sentiment expressed in the book and my feelings about this are quite the opposite. If it weren’t for string theory, most likely my academic career would have led at best to a job at a not very good institution in a place I really wouldn’t be very happy living. Because of string theory I moved into mathematics early on, and have ended up with an academic position I’m extremely happy with, living in my favorite place in the world. String theory didn’t “ruin my career”, it made a very happy one possible.

As I said, I don’t know Cole, so I don’t know why she decided to write this kind of dishonest hit-job. Perhaps it has something to do with her professional association with string theorist Clifford Johnson at USC. I’ve long suspected that Clifford was the author of the referee report for Cambridge which compared doubting string theory to doubting the theory of evolution, and constructed evidence that I didn’t know what I was talking about by taking a sentence in my manuscript out of context and changing a word. One is often wrong about such guesses, probably I’ll never know…

Update: Amazing how quickly one finds out things one thinks one will never know. Over at Clifford Johnson’s blog, Capitalist Imperialist Pig asked him if he was the referee who tried to stop Cambridge University Press from publishing my book. His answer: “that’s all just silly and irrelevant”. OK, now I know…

The funny thing about this is that Clifford has been bitterly complaining about the fact that the book is being marketed and publicized to a wide audience, but it appears that he was the one who stopped it from being published a couple years ago in a form where it would have reached many fewer people. Priceless.

Update: Thanks to “Another Grad Student”, who in the comment section over at Clifford Johnson’s blog did a better job than I could of explaining to him why I was no longer bothering to respond to his endlessly condescending, sneering and dishonest comments. Anyone who thinks there is anything to the accusations Johnson and Distler are making about me over there is encouraged to read for themselves some of the many comment threads where I have tried to have serious discussions with them.

More substantively, it’s clearly a waste of one’s time to try and debate these issues with someone who is on record as claiming that criticizing string theory is like criticizing the theory of evolution.

Update:  Clifford Johnson has denied being the CUP referee in question, or having anything to do with the Cole “review”, saying here that he has not even read the book.  My apologies to him for incorrect suggestions made in this posting, and my misunderstanding of his later comments.

Posted in Not Even Wrong: The Book | 65 Comments

Schroer’s “Samizdat”

Posted on October 6, 2006 by woit

Bert Schroer has a new version of his paper that was discussed here earlier this year, now with the amended title String theory and the crisis in particle physics (a Samizdat on particle physics). He claims that the version reflects a change in viewpoint due to his participation in this and other weblogs, and I believe he would like the opportunity to discuss this further here. There’s also a posting about this at the weblog of Risto Raitio.

Update: Schroer, agreeing with his critics that his paper had too many typos, has sent me a corrected version, which is available here, for use until the arXiv version gets updated. He also agrees that an “s” should be a “z” in Samizdat…

Update: Schroer has a new paper out, which contains a review of AQFT and a discussion of light-front holography, with further comments on the relation to the Maldacena conjecture.

Posted in Uncategorized | 122 Comments

Navier-Stokes Equation Progress?

Posted on October 5, 2006 by woit

Penny Smith, a mathematician at Lehigh University, has posted a paper on the arXiv that purports to solve one of the Clay Foundation Millenium problems, the one about the Navier-Stokes Equation. The paper is here, and Christina Sormani has set up a web-page giving some background and exposition of Smith’s work. I should emphasize that I know just about nothing about this kind of mathematics, but I’m reporting on this here for two reasons:

1. It looks plausible that this really is important.

2. Penny Smith tells me that she is a regular reader of this weblog.

Update: There’s an informative news article about this on the Nature web-site.

Posted in Uncategorized | 176 Comments

Controversy, Controversy….

Posted on October 4, 2006 by woit

There’s almost too much to keep track of the last couple days on the string theory controversy front:

Burton Richter of SLAC has a Reference Frame piece in the latest Physics Today entitled Theory in particle physics: Theological speculation versus practical knowledge. Richter shares my point of view that the Landscape studies currently popular in string theory are not science:

To me, some of what passes for the most advanced theory in particle physics these days is not really science. When I found myself on a panel recently with three distinguished theorists, I could not resist the opportunity to discuss what I see as major problems in the philosophy behind theory, which seems to have gone off into a kind of metaphysical wonderland. Simply put, much of what currently passes as the most advanced theory looks to be more theological speculation, the development of models with no testable consequences, than it is the development of practical knowledge, the development of models with testable and falsifiable consequences (Karl Popper’s definition of science)…

The anthropic principle is an observation, not an explanation… I have a very hard time accepting the fact that some of our distinguished theorists do not understand the difference between observation and explanation, but it seems to be so…

What we have is a large number of very good people trying to make something more than philosophy out of string theory. Some, perhaps most, of the attempts do not contribute even if they are formally correct.

The issue of Nature that just came out today has an article about the controversy by Geoff Brumfiel with the title Theorists snap over string pieces: Books spark war of words in physics. He describes Lubos Motl’s reviews of the Smolin book and mine on the Amazon web-site, and quotes Polchinski and Susskind. The reaction of string theorists to the books is said to be:

Few in the community are, at least publicly, as vitriolic as Motl. But many are angry and struggling to deal with the criticism. “Most of my friends are quietly upset,” says Leonard Susskind, a string theorist at Stanford University in California.

and

The books leave string theorists such as Susskind wondering how to approach such strong public criticism. “I don’t know if the right thing is to worry about the public image or keep quiet,” he says. He fears the argument may “fuel the discrediting of scientific expertise”.

Susskind will be giving a public lecture October 17 at UC Davis on String Theory, Physics and the “Megaverse”.

Polchinski avoids the problems associated with the failure of string theory as a unified theory, and promotes in a somewhat overhyped way the idea that string theory explains the RHIC data.

Finally, Smolin makes an offer to string theorists that I feel I should try and match, hoping they will read his book to better understand exactly what he has to say:

If they don’t want to buy it, tell them to get in touch with me and I’ll send them a copy.

One thing Brumfiel gets a bit wrong is that my problem with string theory is not quite what he says “a fear that the field is becoming too abstract and is focusing on aesthetics rather than reality.” The problems I see are rather different, with mathematical abstraction one of the few tools still available to theorists trying to make progress.

The same issue of nature contains an editorial Power and Particles lustily repeating much of the standard hype about string theory, noting that there are problems, but ending with:

Critical-mindedness is integral to all scientific endeavour, but the pursuit of string power deserves undaunted encouragement.

The editorialist definitely does not seem to be of the opinion that alternatives also deserve to be encouraged.

Finally, lots of reviews of Lee Smolin’s book:

Unburdened by proof by George Ellis, also in Nature. Ellis takes the opposite point of view from the Nature editorialist, calling for more research on alternatives to string theory.

A loopy view by Michael Duff, in Nature Physics. Duff is extremely hostile to Smolin’s book, sneering at Smolin and claiming that his book will “leave the reader rooting for strings” (funny, but this doesn’t seem to have been its effect on most reviewers…). Duff agrees that there are problems with string theory, but claims that the problems Smolin correctly identifies are exactly the ones that he himself first identified back in 1987. String theorists like Duff seem torn between claiming that criticisms of string theory are crackpot nonsense, and that they themselves made them first. He goes on to furiously attack various straw men, accusing Smolin of “denying that any progress has been made!” (something I don’t think Smolin does at all), and answering the criticism that string theory makes no predictions despite more than twenty years of effort by discussing how theories that did make predictions have sometimes taken a long time to be confirmed (or remain unconfirmed).

The string theorists were scammed! by Peter Shor on Amazon.

The Trouble With Physics by Sean Carroll at Cosmic Variance. If I can find the time, I may write about some of my problems with this review as a comment over there.

Posted in Uncategorized | 41 Comments

Falsifying String Theory: Not

Posted on October 1, 2006 by woit

Back in April a paper appeared on the arXiv from string theorist Jacques Distler and collaborators that made a rather outrageously overhyped claim to have found a way to “falsify string theory”. The paper was entitled Falsifying String Theory Through WW Scattering, and was discussed extensively here. After the Wall Street Journal published an article in June about the problems of string theory, Distler wrote them to complain that the article was incorrect, because he and his collaborators had shown that string theory was falsifiable.

I had heard that this paper was going to be refereed, and was wondering whether a referee would really let the authors get away with the outrageous claim of their title. Well, it appears that the answer is no. A new version of the paper is now on the arXiv, with a new title: Falsifying Models of New Physics via WW Scattering. The abstract, which originally claimed that violations of the bounds they described “would falsify string theory” has now been modified to no longer make this claim; the new language is “would falsify generic models of string theory”.

The paper has acquired a new co-author and been extensively rewritten. I’m assuming many of the changes were made to satisfy a referee. Besides changing the misleading, overhyped title, criticisms of earlier work embedded in one reference have been removed, and nine new references to earlier work have been added.

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The String Vacuum Project

Posted on September 30, 2006 by woit

Last week at the KITP, Keith Dienes gave a talk on A Statistical Study of the Heterotic Landscape. He gave a good idea of the state of the art of the investigation of the Landscape, focusing on one special type of models, heterotic models. The results he presented gave statistical distributions for just two very crude aspects of these compactifications, their gauge groups and cosmological constants. These models remain highly unrealistic, since the cosmological constants are of order the Planck scale and the compactifications are not stable.

The models studied have gauge groups of rank 22, and while many of them contain the standard model SU(3)xSU(2)xU(1), they also contain many more gauge group factors, with typically not one, but about seven SU(2) factors. These models, with their instabilities, far too large gauge groups and cosmological constants, are extremely far from anything like the standard model. It’s not at all clear what the point is in enumerating them and studying their statistics, but Dienes describes in detail various problems that arise with the whole concept of generating “random” models of this kind and trying to get sensible statistical distributions. He also looks for correlations between gauge groups and cosmological constants, finding that at small cosmological constant one is somewhat more likely to get many factors in the gauge group (although in his case, both the gauge group and the cosmological constant are very different than in the real world).

Despite the very crude state of these calculations, Dienes reports that a group of 17 prominent string theorists have banded together to form the “String Vacuum Project”, with the goal over the next few years of accumulating a database of 10s of billions of string models, with the hope of finding within this mountain of data about 100 models that have crude features of the standard model. I don’t at all see what the point of this is, but it certainly is a computationally intensive project that could keep many people occupied for a long time. It also appears to be just the beginning, with the longer term goal being to devote the next decades to expanding from 10s of billions farther into the 10^500 or whatever exorbitantly large number is thought to be the number of all string models.

The String Vacuum Project submitted a proposal to the NSF last year, which seems to have been turned down, and they appear to be planning to resubmit the proposal. They have a Wiki, with all sorts of details about the project. Most recent additions to the Wiki are from Bert Schellekens in August, who discusses a proposed “String Vacuum Markup Language” (SVML) format, with links to a web-page that produces data in this format for certain sorts of models. There’s also a European String Vacuum Project web-site.

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