Not Even Wrong

A new preprint by Tom Banks is out, about his idea of “Cosmological Supersymmetry Breaking”. One notable aspect of the paper is a new terminology to describe Weinberg’s “prediction” about the cosmological constant. Since the term “anthropic principle” normally applied to this has acquired a bad odor as it becomes clear it is not science, Banks decides to come up with a different name for the argument. He refers to it as the “galactothropic principle of Weinberg”. Let’s see if this catches on…

The latest issue of Scientific American is devoted to articles about Einstein and his legacy. One article in the magazine doesn’t really have much to do with Einstein and I believe would make him gag if he were still around. The article, entitled “The String Theory Landscape” is by Raphael Bousso and Joe Polchinski. In it they claim credit for the pseudo-scientific idea of “explaining” the value of the cosmological constant by the existence of the “landscape” and the anthropic principle. It’s sad to see this nonsense being purveyed by the most respected and well-known popular science publication in the US.

For something more sensible about the anthropic principle, see a recent column from Nature.

Larry Yaffe’s comments about string theory reflect well mainstream opinion in the particle physics community. On matters of fact I think what he has to say is pretty accurate, but I disagree with some of his statements that reflect not facts but scientific judgements. Of his positive comments about string theory, the ones about its impact on mathematics and about AdS/CFT are right on target. For an interesting talk explaining the status of attempts to use AdS/CFT to say something about QCD, see Larry’s colleague Matt Strassler’s talk this month opening a workshop in Santa Barbara (don’t miss the heated exchange at the end of the talk about whether or not this is all just supergravity).

Larry’s comments about “compelling hints” that there is something “deep and meaningful” to string theory and that it has provided “partial insights” into conceptual problems in quantum gravity are hard to to argue with. But while these hints seem to point in the direction of the existence of an interesting 11 dimensional supersymmetric theory, they provide no evidence that it has anything to do with the standard model. Quite the opposite, the evidence of the “landscape” suggests that any attempts to relate such a theory to the real world produce a framework that is completely vacuous, and can never explain anything (or, equivalently, can explain absolutely anything you choose).

The one place where I think I really disagree with Larry is his claim that, indisputably, “string theory is the most promising framework we have for combining quantum mechanics and gravity”. This “most promising framework” locution has been around now for nearly twenty years. It was justifiable when people were just starting to try and understand the implications of superstring theory, but the failure of twenty years of effort by thousands of very talented physicists has to be taken into account. The fact is that despite all this effort, string theorists still don’t have a consistent theory of 4-dimensional quantum gravity and prospects are not promising that this situation is going to change anytime soon.

As part of this “most promising” comment, Larry has critical things to say about loop quantum gravity. I’m no expert on this myself, but, like many theorists, he seems to me to be holding string theory and loop quantum gravity to quite different standards. Lee Smolin recently wrote to me and Larry to respond to Larry’s comments, he allowed me to reproduce his e-mail here:

“Dear Peter and Larry,

Thanks for the comments, most of which I agree with. But in case either of you are interested, Larry’s comments about loop quantum gravity do not reflect the real results.

A side effect of the sociology of string theory seems to be that there is as much ignorance of the genuine results concerning loop quantum gravity and other approaches to quantum gravity as there is overhype in string theory. It is fascinating that, just there are results that are believed to be true in string theory, despite never having been shown, there are results that have been shown, in some cases rigorously, in lqg, about which many people seem not to have heard about, in spite of being published 5-10 years ago on the archive and in the standard journals.

To combat this I wrote a recent review hep-th/0408048 which I would gently suggest reading before making public pronouncements about the status of the field. There are also good reviews on the rigorous side by Ashtekar and Lewandowski and by Thiemann, as well as two textbooks in press from CUP, one from Rovelli and one more rigorous from Thiemann.

Larry says of LQG that it “has not been shown to have anything to do with gravity. Does it have a large-volume limit? Does it have long distance dynamics…”

Can I mention some of the results that show that lqg quite definitely is a quantum theory of gravity, with details and referenceds in the paper? Larry, if you think any of these results are wrong, please tell us on what step of what calculation or proof someone made an error. Otherwise, we invite you to study the results and the methods by which they were gotten. You might surprise yourself by coming to agree with us, after all this is just quantum gauge theory, but in a diffeomorphism invariant setting rather than on a background manifold.

A key result is the LOST uniqueness theorem which shows that for d >=2 the hilbert space LQG is based on is the UNIQUE quantization of a gauge field that carries a unitary rep of the diffeo group, in which both the wilson loop and non-abelian electric flux operator are well defined operators. (see the paper and references for the precise statement).

Given that GR and supergravity are well understood to have configuration spaces defined as configurations of gauge fields mod diffeos, to which the theorem applies, this implies that the hilbert space used is uniquely suited to the quantization of those theories.

It is further shown that the hamiltonian contraint of GR for d=3+1 is rigorously defined on the hilbert space of diffeo constraints, allowing exact solutions to all the quantum constraints to be constructed.

As far as the path integral is concerned, using the method of spin foam models, based on the observation that GR and supergravity in all dimensions are constrained topological field theory, leads to rigorously defined path integral measures corresponding to the quantization of these theories. There are in addition rigorous UV finiteness results. There are also results that establish correspondences with Regge calculus in various limits.

These results all are quite sufficient to establsih that these theories are precisely the quantization of GR or supergravity. Surely this has something to do with gravity.

Regarding the low energy limit, more explicitly, there are several classes of candidate ground states that have the property that 1) measurements of coarse grained geometrical operators agree with classical flat or deSitter spacetime, up to small fluctuations. 2) small excitations of the gravitational degrees of freedom, which satisfy the constraints to linear order in l_Planck/wavelength have two spin two massless degrees of freedom per momentum mode (i.e. the gravitons are recovered for wavelengths long in planck units, again showing this is a quantization of gravity.) 3) after coupling to any standard matter field, excitations of the ground state yield a cutoff version of the quantum matter field theory on the classical background, cutoff at the planck scale because of the finiteness of quantum geometry.

The classes of states these statements characterize are a) coherent states, b) eigenvalues of coarse grained 3-geometry (sometimes called weave states) and c) for non-zero cosmological constant, the Kodama state.

I would think that finding explicit states with these properties proves that at least linearized gravity and effective field theories correspodning to qft on background manifolds is recovered. Certainly this is again something to do with gravity.

In addition, we can mention 1) the black hole results, which give an exact description of the quantum geometry of an horizon, in agreement with all semiclassical results and 2) the loop quantum cosmology results, which again recover all results of semiclassical quantum cosmology and go beyond them in the context of a rigorously defined framework. Again, some things to do with gravity.

See my paper for complete references.

I’m sorry for the tone, but one loses patience after 15 years. We have always been careful to state results precisely with full qualifications and never to overclaim. By now there are sufficient results that I think the many good people who are working hard in this field deserve to have their results much better known.

As always, I and my (rapdily growing number of) colleagues are happy to talk to anyone and go anywhere to explain the results and the methods by which they were gotten. Indeed, the number of invitations for talks at places that previously expressed no interest previously is growing. I’d certainly be glad to recommend good speakers who could educate your department about the state of art in quantum gravity.

Thanks,

Lee

For many years now, the highest priority of experimental particle physicists for a next-generation accelerator project has been a new electron-positron linear accelerator. The last high energy electron-positron collider, LEP, reached a total energy of 209 Gev before being shutdown in 2000. To get to higher energies than LEP, a ring isn’t viable because synchrotron radiation losses go as the fourth power of energy, so a ring with much higher energies than LEP would use an intolerable amount of electric power.

A linear collider, where you build two linear accelerators and collide their beams together, doesn’t have the synchrotron radiation problems (although the electric power demands are still a problem since the beams you accelerate only can collide once, not many times like in a storage ring). There have been several competing designs for a linear collider, with one of the main difference in the designs being whether the RF accelerating structures are superconducting (“cold”) or room temperature (“warm”). These designs all are for a machine that would start out with a total energy of 500 Gev and ultimately reach 1 Tev. A committee was formed called the “International Technology Recommendation Panel” (ITRP), and it has issued a press release announcing its decision today. The ITRP came down on the superconducting side; this is a design mainly developed at DESY in Hamburg as part of the TESLA project.

The German government has decided to use the TESLA technology to construct a free-electron X-ray laser at DESY called XFEL. They have done this in a way that would allow XFEL to ultimately be upgraded to a linear collider at DESY, but have put off any decision about whether to actually fund and build such a machine.

The ITRP decision will allow work on a final design for the linear collider to begin, but the trickiest questions still lie ahead. Where will the thing be built and who is going to pay for it? The order of magnitude of the cost is $5 billion and the general assumption is that this will be an international collaboration. Besides the possibility of siting it at DESY in Germany, sites that have been discussed in the US mainly are at Fermilab in Illinois (being pushed by Fermilab), or somewhere in California (being pushed by SLAC). Even the most optimistic time scales for designing, funding and building a linear collider don’t have it running until late in the next decade. More realistic might be the mid 2020’s. The question of where the machine is located is crucial to the long-term future of the SLAC and Fermilab laboratories. If it is at their site or nearby they have an assured future, if not their future becomes much more problematic. CERN has its own design for an even higher energy linear collider called “CLIC”, but CERN’s funds for the forseeable future are committed to constructing and funding the LHC, as well as possible future upgrades of that machine.

There’s a bewildering array of web-sites with information about this, including the new International Linear Collider Communication and linearcollider.org ones, another one at SLAC, one at Fermilab (which seems kind of out of date..) and Michael Peskin’s home page. This last one contains links to many talks by Peskin about the physics to be done by a linear collider as well as a web page of links to other information about the linear collider.

The ITRP decision was announced at one of the year’s biggest high energy physics conferences, the ICHEP being held now in Beijing. The web site for that conference contains many talks giving the latest results from experimental groups around the world. Unfortunately, as far as I can tell there’s nothing very earth-shattering being reported.

My old friend and Princeton roommate Nathan Myhrvold has written an excellent piece about the anthropic principle and the Smolin-Susskind debate that has just been posted on the Edge web-site. It seems to me to summarize the issues very clearly.

After getting his Ph.D. in quantum gravity at Princeton, Nathan went to work as a post-doc with Stephen Hawking, and one of the topics he worked on involved a possible mechanism for explaining the small size of the cosmological constant. Nathan left physics and joined with some of my other friends from grad school days to start a software company near Berkeley that they called “Dynamical Systems”. They soon sold the company and themselves to Microsoft, where Nathan ended up in the position of Chief Technical Officer. He periodicaly reminds me that if I had taken up one of his many offers to come work with them back in the mid-eighties, I could be obscenely wealthy too. At the time I remember it seemed clear that it was much smarter to stay as a postdoc at Stony Brook, being paid enough to live on and able to think about whatever I wanted, than to go to California to work twenty hour days writing operating system code and getting paid in worthless pieces of paper.

Oh, well.

John Brockman at his “Edge” web-site has put up an exchange between Smolin and Susskind about the “multiverse” and the anthropic principle. This includes the page and a half paper by Susskind that was rejected by the arXiv. Susskind seems quite willing to give up the idea that a physical theory should be falsifiable, so his response to Smolin’s argument that his use of the anthropic principle is not falsifiable is basically “Yeah, and so what?”.

To Smolin’s claim that non-falsifiable theories aren’t really science, Susskind answers by listing several prominent physicists (Weinberg, Polchinski, Linde, Rees), their titles, affiliations and prizes they have won. He then announces that since these prominent people agree with him and think the anthropic principle is science, Smolin should just shut up. I don’t know about other people, but one reason I went into physics was that it was supposed to be a subject where issues could be decided by rational argumentation, not appeals to authority. That doesn’t seem to be the case anymore.

Jeff Harvey sent in a comment correcting me on a point of history in my last posting. I’d read somewhere about Green and Schwarz fed-exing their paper to Witten, and had assumed this was their idea. Harvey, who was at Princeton at the time, recalls that it was Larry Yaffe who brought news of Green and Schwarz’s result from the Aspen Workshop to Witten at Princeton, and Witten was the one who asked Green and Schwarz to fed-ex him the paper.

Harvey also strongly disagrees with the statement that people were not impressed by the work independently of Witten. He was at Princeton at the time, so knows far more about what attitudes there were. For those who weren’t at Princeton or at Aspen, news of Green and Schwarz’s paper and Witten’s arrived more or less at the same time (they were published in the 13 and 20 December issues of Physics Letters B, the preprints were circulating in October). At that time any new paper by Witten was a major event, especially one in which he took up a new topic. I stand by my recollection that for people I was talking to at the time, the fact that Witten was working on the subject overshadowed the Green-Schwarz result itself.

Update:

This morning I tracked down my original source for what happened at Aspen. It is John Schwarz’s article entitled “Superstrings – a Brief History”, published in the proceedings of a conference on the history of particle physics held at Erice in 1994. The volume is entitled “History of Original Ideas and Basic Discoveries in Particle Physics” and contains many things very much worth reading. Schwarz describes in detail what happened at Aspen, ending with the following remarks:

“But still, given our previous experiences, neither of us had any idea of how sudden and enthusiastic the response of the physics world would be. In my opinion this was largely due to the influence of Edward Witten, who immediately grasped the implications of our result. Without that, string theory would probably have emerged much more gradually. As soon as our letter came off the printer we sent Witten a copy by Federal Express. (This was before the days of TeX and email!). I am told that the next day everyone in Princeton was studying it. Our letter on anomaly cancellation was submitted September 10, 1984. The deluge began 18 days later with Witten’s letter suggesting ways to compactify SO(32) superstrings to get anomaly-free theories in four dimensions.”

This is what led me to believe that it was Schwarz and Green’s idea to Fed Ex Witten their paper. Presumably Harvey is right that he had asked them to do this. But that’s about the only thing that I think I got wrong in the original posting.

Today a symposium is being held in Aspen to celebrate the twentieth anniversary of the “First Superstring Revolution”. The canonical story of this “Revolution” is that twenty years ago this month, on a dark and stormy night at a workshop in Aspen, Michael Green and John Schwarz completed a calculation showing that gauge anomalies canceled in a specific superstring theory, thus changing physics forever. The more complete story of what happened at that time goes more or less as follows:

By 1984 Witten had been taking some interest in superstring theory for a while, giving a talk on the subject in April 1983 at a conference on Grand Unified Theories, but not publishing anything about it himself. In 1983 at the Shelter Island conference he had shown that the popular unification idea of the time, using supergravity on higher-dimensional spaces and the Kaluza-Klein mechanism, could not give the kind of asymmetry between left and right handed particles that occurs in the standard model (this has had a revival in M-theory, which these days invokes singular compactification spaces to get around Witten’s no-go theorem). The failure of supergravity ideas had gotten him interested in superstring theory, but he was concerned about the issue of gauge and gravitational anomalies in the theory, anomalies that he worried would render the theory inconsistent. In his 1983 paper about gravitational anomalies with Luis Alvarez-Gaume, they noted at the end that these anomalies canceled in the supergravity theory in ten dimensions that is the low energy limit of the type II superstring. This theory didn’t allow for low-energy gauge theories so wasn’t useful for unification. Witten suspected that the type I superstring (which could be used for unification) would have insurmountable problems with anomalies, but the Green-Schwarz calculation showed that these anomalies could be canceled for a specific choice of gauge group.

Evidently Green and Schwarz talked to others about their new result at the 1984 Aspen workshop, but I would suspect that no one was very impressed (if anyone who was there knows differerently, I’d be interested to hear about it), since superstring theory was generally considered pretty much a far-out, highly unlikely idea. Green and Schwarz were well aware that their only real hope for getting attention was to get Witten interested, so on September 10th they sent him a copy of their paper via Fed Ex (this was before e-mail) at the same time they sent it off to Physics Letters B. Witten immediately went to work full time on superstring theory, with his first paper on the subject arriving at Physics Letters B on September 28th. I think this is really the point at which one should date the First Superstring Revolution.

Witten was at the height of his influence, and the news that he was now working on superstring theory spread very quickly through the particle theory community. I had just finished my graduate work at Princeton and was starting a post-doc at the Institute for Theoretical Physics at Stony Brook when I heard the news. Over the next six months to a year I remember hearing from a couple colleagues who had gone down to Princeton to talk about their work with Witten, only to hear from him that, while what they were doing was all well and good, the future was in superstring theory, so they should drop what they were doing and start working on that.

My own attitude was that it didn’t look like a very promising idea. It was a complicated theory and didn’t really explain anything at all about the standard model. I figured that there would be a lot of smart people working on it for a while and within a year or two either they would get somewhere with the idea and it would be clear I had been wrong, or they wouldn’t, and everyone would lose interest. Neither I nor anyone else could conceivably have guessed that 20 years later superstring theory would still not explain anything about the standard model, but would completely dominate particle theory.

One reason for this is the string theory hype machine continues in high gear. The University of Chicago has issued a press release telling us that “growing numbers of physicists see superstring theory as their best chance” to formulate a theory of everything. Jeff Harvey is quoted as saying that “It’s an intellectual enterprise that’s extremely exciting and vigorous and full of ideas”, which is very different than what I hear string theorists telling me in private.

A couple conferences going on this week have already put some of the talks online.

SLAC has a summer school each year, aimed more at experimentalists than theorists. This year’s topic is “Nature’s Greatest Puzzles” and there are quite a few interesting talks already online there.

The Michigan Center for Theoretical Physics is hosting this year’s String Phenomenology 2004 conference. The “Landscape” seems to be a big topic; two online talks are Michael Douglas’s, which is more or less the same as his one at Strings 2004 a few weeks ago, and Michael Dine’s. Dine seems optimistic that the Landscape will lead to predictions, saying

“If we adopt the anthropic viewpoint, we may be lead to predictions – perhaps the first predictive framework for string theory”

Before taking this too seriously, one should note that Dine has been giving review talks about “superstring phenomenology” and claiming that predictions are right around the corner since before most of our incoming students at Columbia were born (see his 1986 Erice lectures, no, they’re not online, this was way before the arXiv).

And I’m headed off soon for a short vacation out of the range of the internet, back late next week.