String Theory for Undergraduates

Posted on July 5, 2006 by woit

I hadn’t realized how many of the physics departments at the top universities in the US have instituted undergraduate string theory courses.  The only one I was aware of was MIT’s 8.251, String Theory for Undergraduates, taught by Barton Zwiebach, who developed a textbook for the course, A First Course in String Theory. 

Maybe now that there’s a textbook, that is what has caused other institutions to follow suit.  Caltech has Physics 134, String Theory, and Carnegie-Mellon has Physics 33-652, An Introduction to String Theory.  Stanford goes its competitors one better by having two undergraduate courses in string theory: Physics 153A, Introduction to String Theory I, and Physics 153B, Introduction to String Theory II. This last course even promises to explain to students how string theory is connected to particle physics. 

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Various Weirdness

Posted on July 5, 2006 by woit

Must be something in the air, lots of weird things going on recently:

My book has been officially non-endorsed by the people at Axes & Alleys.

I finally realized why Lubos was carrying on about how if any errors in my book had been corrected it was because of him. Evidently one of the university presses I sent it to decided he was an appropriate reviewer for the book, and sent it to him to referee. I assume the review was as loony as the one he put on Amazon, but I never saw it, and have no idea whether it convinced the publisher to turn down my book. I do wonder which if any string theorist suggested Lubos to the publisher as an appropriate referee. The “free marketplace of ideas”, indeed…

Lubos has put up a paranoid rant about how someone just told him that I “made amazon.co.uk erase all reviews” of the book except for the 5-star ones. This is complete nonsense. When his review appeared there and I first saw it over a month ago, I did hit the “report as inappopriate” link at the bottom of the review, but that’s all I ever did about this, and I never saw any other negative reviews except his. Actually, I think his review was responsible for several people posting positive reviews in response to it (thanks folks!). I have no idea why Amazon UK recently deleted his review. Perhaps lots of people hit the “report as inappropriate” link, perhaps someone there just read it and recognized it for what it is. I wrote a comment on Lubos’s blog explaining this, but it was immediately deleted (and has now been added to the Censored Comments From the Reference Frame section of this blog). It’s pretty hilarious how exercised he is about censorship.

Over at the arXiv in hep-ph, a few days ago there was a paper from Tom Banks entitled Remodeling the Pentagon After the Events of 2/23/06. Somehow, Banks seems to be comparing the appearance of the paper of Intriligator, Seiberg and Shih about metastable SUSY breaking to the events of 9/11, and the following “neo-conservative revolution” in the US. Banks had a SUSY model containing a “Pentagon” (a “new strongly interacting SU(5) super-QCD with 5 flavors of pentaquark”), which he now enhances with metastable SUSY breaking to get what he describes as “a lean and mean, stripped down version of the Pentagon, suitable for rapid deployment to solve all of the problems of the supersymmetric standard model” (a footnote warns about the Pentagon’s propensity for hyperbole).

A commenter here pointed to another paper on hep-ph, from last night, entitled Neighboring Valley in the String Landscape. Pretty much pure science fiction, although it did make me realize that just about any Landscape paper could be improved by doing what this author did, including an impressive color graphic of the earth de-materializing.

Over at hep-th, there’s a new paper last night, entitled Generalized Flux Vacua. Using a new construction, the authors find an infinite number of solutions that are supposed to be consistent backgrounds for string theory. This pulls the plug on the arguments from a couple weeks ago by Acharya and Douglas that the Landscape should be finite (after imposing varous cutoffs). The authors also claim that this drains some of the swampland promoted recently by Ooguri and Vafa. They do note that, although string theory is a completely precise and rigorous framework, it’s impossible to tell whether the backgrounds they describe really are consistent vacua for string theory (because of, among other things, possible non-perturbative effects):

This somewhat surprising result seems to contradict recent predictions regarding properties of the string landscape, though as we will discuss there are some reasons why the solutions we find may not correspond to stable nonperturbative vacua in a complete string theory framework.

Posted in Not Even Wrong: The Book, Uncategorized | 29 Comments

Many Worlds In One

Posted on July 2, 2006 by woit

Alex Vilenkin has a new popular book out about cosmology, entitled Many Worlds In One. It’s mainly about the extremely speculative end of cosmology, and much of it is devoted to explaining the author’s ideas on eternal inflation, creating the universe by tunneling out of nothing, and the anthropic landscape, together with stories about how he came to these ideas. It contains various amusing anecdotes, especially about Alan Guth. Sean Carroll is credited with the following story:

One of the leading superstring theorists, Joseph Polchinski, once said that he would quit physics if a nonzero cosmological constant were discovered. Polchinski realized that the only explanation for a small cosmological constant would be the anthropic one, and he just could not stand the thought.

He also describes the reaction to his anthropic arguments back during the years when these were not all the rage like they are now:

After one of my seminars, a prominent Princeton cosmologist rose from his seat and said, “Anyone who wants to work on the anthropic principle – should.” The tone of his remark left little doubt that he believed all such people would be wasting their time.

Vilenkin’s book covers much the same ground as Susskind’s, although from the point of view of a cosmologist, not a particle physicist. A huge amount is made of the supposed anthropic “prediction” of the value of the cosmological constant (any news of the rumor from Sean Carroll of new work by prominent Princeton cosmologist Paul Steinhardt showing this is bunk?). Unlike Susskind, Vilenkin at least doesn’t seem to be on a campaign to attack the “Popperazi” and convert everyone to anthropics, but he demonstrates a similar lack of concern for the fact that the ideas he is discussing don’t lead to much if anything in the way of a testable experimental prediction.

Here’s his scientifc program for 21st century physics, which he hopes will be spent working on the anthropic landscape:

First, we will need to map the landscape. What kinds of vacua are there, and how many of each kind? We cannot realistically hope to obtain a detailed characterization of all 10500 vacua, so some kind of statistical description will be necessary. We will also need to estimate the probabilities for bubbles of one vacuum to form amidst another vacuum. The we will have all the ingredients to develop a model of an eternally inflating universe with bubbles inside bubbles inside bubbles… Once we have this model, the principle of mediocrity can be used to determine the probablility for us to live in one vacuum or other.

Unfortunately for this research program, it has yet to even begin to get off the ground, and there are very good arguments that it can never succeed. There are an infinite number of possible vacua, and trying to make this finite so one can do statistics requires putting in cutoffs, with results then strongly depending on the cutoff. The large numbers of these vacua make any attempts to identify ones that agree with the real world computationally completely intractable. Even if one could do this, all evidence is that one would end up with broad statistical distributions for many of the parameters of the standard model, providing no useful prediction of what new experiments will see, or any insight into why these parameters have the values that they do.

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Some Links

Posted on June 29, 2006 by woit

The Cao-Zhu paper at the Asian Journal of Mathematics that is supposed to have a complete proof of the Poincare and geometrization conjectures is still not available, but the introduction to the paper has been posted there.

If you’re not getting enough string theory bashing today, head over to John Horgan’s Scientific Curmudgeon blog, where he has a posting entitled Pulling the Plug on Strings. It contains a wide selection of string-puns (or whatever you call such things), and he has decided to refer to string theory advocates as “yarn-heads” and braniacs. For his trouble, his comment section is under assault by the usual suspects. There’s also this site, containing a graphic mentioned here before which I refuse to admit to finding funny. The proprietors have an interesting way of dealing with the comment section.

Sabine Hossenfelder has an excellent posting on Science and Democracy.

This past week I’ve spent some time at the 26th International Colloquium on Group Theoretical Methods in Physics, being held here in New York at the CUNY Graduate Center. It was ably organized by Sultan Catto, who somehow convinced me to give a short talk on the blog and the book, one where I think I disappointed people by keeping string-bashing to a minimum. I enjoyed seeing people at the conference, and there were some good talks, including one by Greg Moore on his recent work with Dan Freed and Graeme Segal (see here and here).

Urs Schreiber has been putting his notes on-line about elliptic cohomology. Lots of interesting material, but his comment that he expects the landscape of superstring theories to be equal to the spectrum of elliptic cohomology sounds frightening. Maybe he means a different landscape…

I was quite sorry to hear of the recent death of Irving Kaplansky. Kaplansky was an algebraist, and director of MSRI when I was there in 1988-89. At the time I wasn’t much interested in algebra, so didn’t talk to him about math, but he was responsible for making MSRI a really wonderful place to work.

Update: The slides from Yau’s talk at Strings 2006 are now available here.

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Zwiebach Letter to the WSJ

Posted on June 29, 2006 by woit

Over at his web-site, Lubos has posted a letter to the Wall Street Journal from Barton Zwiebach. The letter seriously misrepresents the current state of string theory in several ways:

string theory is an extraordinarily precise and rigorous framework where facts can be proven beyond doubt and computations give unequivocal answers.

All that is needed to confirm string theory is finding one solution that describes our universe. All that is needed to rule out string theory is showing that no solution describes our universe. An answer must exist.

Zwiebach gives the impression that there are rigorously well-defined, “extraordinarily precise” equations that characterize string theory, and all that is needed now is to solve the technical problem of finding the solutions to these equations and seeing if one of them agrees with what we observe. This is simply not true. Since we don’t know what non-perturbative string theory is or what its equations are, all equations used by string theorists to generate solutions are not “extraordinarily precise”, but are explicitly approximations, often very crude ones, whose reliability is unknown. If you look at the debate over the landscape, you will find that not only do most string theorists not believe that the solutions involved are “facts [that] can be proven beyond doubt”, many of them believe these are not real solutions to the full unknown theory at all.

Even if one does believe in the rigorous nature of the landscape solutions, Zwiebach’s claim that all one has to do is examine them to see if they agree with nature is again highly misleading. If there are 101000 or more of these solutions, all evidence is that identifying which of them have desired properties (e.g. the correct CC) is an inherently computationally intractable problem. Even if one could do this, the class of solutions that agree with all known values of the parameters characterizing the standard model seems likely to be so large that no new testable predictions would be possible.

Zwiebach also claims:

String theory has explained, for example, why black holes have entropy and temperature.

This is what Hawking did back in 1974 with a semi-classical calculation. Any theory of quantum gravity should reproduce this. What string theory adds to Hawking’s calculation is a long story, but if we manage to observe a black hole any time soon and it behaves as Hawking predicted, he’s the one who is going to get a Nobel prize for explaining “why black holes have entropy and temperature”, not string theorists.

I had been wondering what the response from string theorists would be to the public dissemination of arguments from Smolin and me about string theory. The response seen here from Distler et. al. and Zwiebach is not at all what I expected. Most serious string theorists I talk to take the reasonable attitude that string theory is still so poorly understood that it cannot be confronted with experiment, even in principle. Many publicly say that we still don’t know what “string theory” is. Zwiebach seems to believe that there now exist “extraordinarily precise” equations, with solutions that will give “unequivocal answers”. Whether this is true is a well-defined question. All he has to do is explain what these equations are, and let’s see if the string theory community will really stand by this definition and let string theory be judged accordingly.

Finally, perhaps the most surprising aspect of Zwiebach’s letter is the form in which he has chosen to distribute it. I and many people have been wondering what Lubos Motl’s colleagues in Cambridge think of the way he is defending their subject. Now at least one of them has made clear that he is fine with this and willing to encourage it.

Update: The usual response from Lubos Motl/Bill O’Reilly: an endless rant about how stupid people who disagree with him are, completely ignoring the scientific questions at issue.

Posted in Not Even Wrong: The Book | 59 Comments

Responses To WSJ Article

Posted on June 27, 2006 by woit

The lack of any serious (Lubos doesn’t count) response to my book from string theorists since its appearance in the UK a month or so ago has begun to surprise me a bit. I was also suprised at how weak the defense of string theory was that the Wall Street Journal’s Sharon Begley put in her recent article. It seems that she did talk to some string theorists, but couldn’t get much usable from them (one supposedly told her that the best argument for string theory was the anthropic landscape). Peskin’s argument that string theory’s biggest success is its “explanation” of the number of generations was evidently the best he could come up with, although it’s obviously very weak.

I’ve heard that the WSJ has gotten some correspondence about this, with Jacques Distler writing in to complain about the article. In one letter from him (also signed by his two collaborators), he claims that Smolin is wrong to say that string theory is not falsifiable, since Distler has a recent paper called Falsifiying String Theory Through WW Scattering.

This paper was discussed extensively here. You can make up your own mind about it, but it’s undeniable that the calculations in the paper don’t involve string theory at all (Distler’s two co-authors are not string theorists). Pretty amazing trick to show that a theory is falsifiable without actually using the theory at all.

There are two obvious problems with the claim in the title of the paper, the first is that when one says a theory is falsifiable, one is talking about the characteristic predictions of the theory, and that’s not what the paper is about. The second is that “string theory” is an ill-defined term, and many versions of “string theory” don’t satisfy the assumptions of the paper (one of the co-authors admits this in the comment section). To fudge his way around this, in the letter to the WSJ, Distler refers to “the canonical definition of string theory” as opposed to “string theory”, although he provides no reference to what this is. Putting “canonical definition” and “string theory” into Google doesn’t turn up anything relevant.

It will be interesting to see if a referee can be found who will go along with allowing the “falsifiying string theory” claim. Most physicists, string theorist and not, that I’ve talked to about this think it’s way out of bounds. It seems to me pretty amazing that Distler would choose to take this case to the Wall Street Journal.

We’ll also see if the WSJ publishes the letter, if not I’ve asked one of the co-authors if they’ll let me publish it here.

I could certainly do a better job of defending string theory than the people Begley talked to. The strongest argument string theorists have is clearly the one that they have “the most promising approach to quantum gravity”. The problem with this is that there are plenty of people who disagree, especially those who do LQG. This is the reason that Distler has been on an anti-LQG campaign throughout the blogosphere recently. His latest posting is about this, with comment section featuring the always incoherent Lubos Motl, and the trademark Distlerian sarcastic sneering at people he disagrees with, e.g. the following comment on a paper by Thomas Thiemann:

I suppose that it is only Thomas’s natural modesty that prevented him from submitting this paper for the Clay prize.

In the first comment, someone quotes from my book something I have to say about the issue being discussed in this posting. For the record, I’m no expert on LQG, and can’t judge exactly how close they are to having a fully satisfactory quantum gravity theory, but my impression is that what they are doing is a more promising approach to quantum gravity than string theory, and the fact that they are convincing more people about this is what is getting Distler and others very worked up. I also don’t think either LQG or string theory has made any headway on the problems of the standard model, although several orders of magnitude more effort have gone into the string theory approach.

Since Distler has a whole posting and ongoing discussion in his comment section about this and I’m no expert, if you want to discuss LQG, its problems and prospects, or comparisons to string theory, please do it there, not here.

Posted in Not Even Wrong: The Book | 73 Comments

Talks at Strings 2006 Now Available

Posted on June 26, 2006 by woit

Slides from the talks at Strings 2006 are now available. I’ve spent a little while looking through them today, and am sorry I don’t have time to say much about them. Lots of more or less the same thing as in earlier years, and many talks devoted to complicated arguments designed to, as Eva Silverstein writes, populate, probe or constrain the Landscape. Nothing remotely like a plausible idea about how to ever get a prediction of anything out of this, but also virtually no anthropic arguments. I assume this was due to the iron fist of organizer David Gross.

I was pleasantly surprised to see that there were quite a few mathematically very interesting talks. Besides many talks of the sort that have been common in recent years dealing with the topological string, there were several that had nothing to do with string theory, but involved interesting mathematics related to QFT, and many of these had to do with work Witten is involved with, so they may get some attention. These were:

1. Witten’s own talk on Gauge Theory And The Geometric Langlands Program.

2. Kapustin’ talk on the same topic, entitled Topological reduction of supersymmetric gauge theories and S-duality.

3. Gukov’s talk on Surface Operators in Gauge Theory and Categorification, where he mentions that some of what he discusses is based on on-going joint work with Witten.

4. Nikita Nekrasov’s talk on Beyond Morse Theory.

So, as far as physics goes, the organizers are not allowing any talks on alternatives to string theory (the only mention of LQG seems to have been Dijkgraaf’s making fun of it), but they are willing to allow mathematical talks on QFT that are not related to string theory, especially since this is the field that Witten seems to be doing a lot of work in.

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Yarn Theory

Posted on June 25, 2006 by woit

A couple people wrote in this morning to tell me about today’s Doonesbury, which features slacker Zipper Harris (who has a blog) trying to impress a woman he last saw in sophomore year of college. He’s still undecided about his major, but tells her:

I’m thinking physics.  Yarn theory.

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NEW in the WSJ

Posted on June 23, 2006 by woit

Today’s Wall Street Journal has an article by Sharon Begley entitled Has String Theory Tied Up Better Ideas In Field of Physics? (sorry, subscription required for on-line version). The summary of the article goes

After two decades in which string theory has been the doyenne of best-seller lists and the dominant paradigm in particle physics, some critics say it may be tying up better ideas.

and in some sense it covers for the public in simplified form the discussion going on over at Cosmic Variance at the moment. It quotes Lee Smolin and me as critics, with Mike Peskin as the only defender of string theory. Peskin acknowledges the compactification problem, and then makes only a strikingly weak argument for string theory: that it can claim some success because it explains the number of generations. This “explanation” in terms of the topology of the Calabi-Yau is no explanation at all, since you can get any value for this number you want. I don’t see how changing the parametrization of your ignorance from that of a single natural number to that of the topology of a Calabi-Yau is any improvement.

I assume that any moment now Lubos will be producing a rant comparing Sharon Begley to one sort of animal or another.

There’s also a long review of Not Even Wrong from John Walker over at his blog called Fourmilog. He does a good job of laying out the controversial argument of the book, and we seem to be mostly in agreement, although I don’t share his conviction that government funding is a major source of the problem.

Update: I just heard that official publication date in the US is September 8, although books should be for sale a bit earlier than this. Lubos has a posting about the WSJ article, less of a rant than some of his recent ones. He has restored his anti- Sean Carroll screed, which was a classic that many people were sorry to see suppressed.

Update: A commenter points out that the WSJ article is available on-line here. There’s also a mention at Slashdot. The extensive discussion there is heavily anti-string theory which is an interesting sign of the times, but also convinces me that the idea of moderating this kind of discussion by having participants vote on which comments are worthwhile just doesn’t work at all.

Update: There’s commentary on this from science writer David Appell here, who pretty much gets the situation right.

Posted in Not Even Wrong: The Book | 26 Comments

This Week’s Hype

Posted on June 22, 2006 by woit

The Economist has an article this week entitled To catch a gravitational wave. It’s about the proposed LISA satellite experiment designed to measure gravitational waves, with a much greater sensitivity than LIGO. According to the article, what would you guess is one of the main goals of the LISA experiment? Exactly, like most other ambitious experiments, it will solve the problem of how to test string theory:

[LISA] could allow scientists to examine the validity of string theory, which says that there are more than four dimensions to space-time and that the extra dimensions are hidden. String theory has come under fire because its predictions have so far proved untestable. The normal version has it that these dimensions are curled up in strings that are smaller than the known elementary particles. However, in some versions strings form very long “superstrings” that stretch across the universe. These superstrings form loops and vibrate, radiating gravitational waves; they can also crack like whips, sending bursts of gravitational waves towards Earth. “Seeing direct evidence of strings would be as important as discovering that the world is made of atoms,” claims Craig Hogan, an astronomer at the University of Washington, who is a member of the international science team for LISA.

The writer appears to be a bit confused about what a superstring is, guessing that it is a really big string (a cosmic string). This is presumably all based on the idea promoted by Joe Polchinski that it is in principle possible to come up with superstring theory models with cosmic scale superstrings, whose effects would be visible through gravitational lensing and gravitational waves. As far as I can tell, this is just another case of the phenomenon that one can get pretty much anything one wants out of string theory, and there’s no reason at all to expect cosmic strings with just the right properties to have been invisible so far, but visible through gravitational wave effects measurable by LIGO or LISA.

Two years ago there was a press release about this from UCSB quoting Polchinski as saying

the gravitational signatures from cosmic strings are remarkable because they are potentially visible even from the early stages of LIGO! That means ‘potentially visible’ over the next year or two.

LIGO hasn’t seen anything, so time was up for this nearly two weeks ago but I haven’t noticed any UCSB press releases reporting that things haven’t worked out.

There was some excitement a year or so ago when a group claimed that an astronomical object might be a single galaxy lensed by a cosmic string. Turned out to just be a pair of nearby galaxies.

LISA is tentatively scheduled for launch nearly ten years from now, so it will be a while before this particular “test of string theory” brings in any results. This past week the 6th Annual International LISA Symposium was held in Maryland.

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