Not Even Wrong

David Goss wrote to tell me that Physics Web has an article about physics weblogs. The theorists quoted are Sean Carroll, Paul Cook and Dave Bacon.

There’s an article by Michael Green in the latest Nature Physics reporting on the recent 23rd Solvay conference (mentioned here and here). Green notes that “Much of the discussion focused on string theory” but that “the structure of string theory is still so badly understood that it does not yet deserve to be described as a ‘theory’ at all — it is more a ‘work in progress’.” He describes the discussion at Solvay about anthropic explanations of the CC as “lively”, but that Polchinski et. al. have yet to carry the day: “there is a strong body of opinion that holds that it may be premature to decide which parameters are environmental within string theory, as the structure of the theory is still poorly understood and will surely hold further surprises.”

The same issue of Nature has an essay by Lawrence Krauss entitled Anthropic Fever where he starts by explaining the standard Landscape story that fundamental physics is really an “environmental science”, but then goes on to write:

But I have been wondering whether things might actually be much worse. It could be that many different combinations of laws could allow life to form, and that it is a pure accident, not favoured by any particular probabilistic explanation, that the constants of nature result in the combinations we experience in our Universe. Or, it could be that the mathematical formalism is so complex that the ground states of the theory might not be mathematically determinable, even in principle.

Whether or not nature is ultimately ‘undecidable’ in this strong sense, these ideas point to a possible future for particle physics that is very different from the past. Fundamental physics might not be restricted by any underlying grand mathematical structure that would ‘explain’ why the Universe is the way it is. It’s a possibility that I hope will be wrong, but it’s a possibility nonetheless.

This kind of worry seems to me completely misplaced. There’s not a shred of evidence for it, and the only reason people have been engaging in this kind of speculation is that string theory led them down this sorry path. Finding a trackless swampland at the end of the path doesn’t mean that physics is destined to spend the rest of eternity mucking about in a swamp, it is far more likely that a wrong turn was taken quite a ways back.

Last night a write-up by Nati Seiberg of his rapporteur talk at Solvay appeared at the arXiv, entitled Emergent Spacetime. It does a very good job of laying out the reasons that people often say that string theory suggests that our standard concept of “space” needs to be revised, that perhaps space is an “emergent” concept. Personally I don’t think the different reasons that Seiberg lays out add up to a very convincing case. For one thing, string theorists have been trying to come up with a new “stringy” version of space for twenty years now, without much success at all. They are still far from anything like a consistent proposal of what this new idea about space will be, and the various evidence given by Seiberg is rather incoherent, leading to different kinds of generalizations of space, not pointing to any one of them in particular.

The past two days there was a conference at Harvard on Black holes, topological strings, and invariants of holomorphic submanifolds, which also included some lectures in memory of Raoul Bott by Sir Michael Atiyah, S. T. Yau, and Dan Freed. Lubos has reports on two talks there, one by Robbert Dijkgraaf about a hoped for “Universal Wave Function”, a Hartle-Hawking kind of wave-function that would give the amplitude for the universe to be in various parts of the Landscape. The second was by Frederik Denef who spoke on “D-brane ground states, multicentered black holes, DT/GW correspondence, and the OSV conjecture [or: why OSV is probably right].” Lubos reports on a conversation with Frederik about his two forthcoming papers with Michael Douglas on computational complexity and the Landscape. It seems that what Denef-Douglas show is that, even if everything one would like to calculate is in principle calculable, the problem of identifying a specific string theory background realizing anthropically small values of the CC is NP-hard. This means that in practice you’ll never be able to do what landscapeologists would like to do: use the observed values of the CC and maybe some other standard model parameters to identify a tolerably small number of backgrounds, then use the properties of these backgrounds to make predictions. I believe it is this possibility that Krauss is alluding to in his quote above about how the “ground states of the theory might not be mathematically determinable, even in principle.”

Update: The last paragraph has been modified to better reflect reality. In its initial version I had assumed from Lubos’s blog entry that these computational complexity issues had been what Frederik’s talk was about.

Update: Polyakov also has a new preprint based on remarks at the Solvay conference, entitled Beyond Space-time. It’s a mixed bag, mostly about various ideas related to AdS/CFT, as well as the cosmological constant. He begins with critical but not completely dismissive comments about anthropic arguments:

Another danger is to get distracted by non-dynamical anthropic arguments, which recently acquired some popularity. I find the anthropic principle irrelevant. It is unlikely to uncover fundamental ideas and equations governing the universe. But, in spite of these misanthropic remarks, I believe that in special cases anthropic arguments may be appropriate.

At the end of the paper, he characterizes the various topics he has discussed as follows:

As it is clear from the list of the references below, these ideas (except for the gauge/strings correspondence) did not attract any attention. Perhaps they don’t deserve it. My best hope, however, is that some of them may serve as small building blocks of the future theory, the vague contours of which we can
discern at the horizon.

One of Fermilab’s recent colloquia was by James Rozensweig of UCLA on the topic of Reinventing the Accelerator for the High-Energy Frontier. Video and Powerpoint slides of the talk are now available.

Current accelerator technologies are up against very fundamental physical limits. In the case of circular proton colliders like the LHC, the fundamental limiting factor is the strength of the magnets and the size of the ring. The LHC uses a 27km ring and 8.36 Tesla superconducting magnets, and the energy scales linearly with the magnet strength and ring size. So one could get beams an order of magnitude more energetic than those in the LHC by using a 270km ring, but the cost of such a thing is likely to be prohibitive. One could also try and design higher field magnets, but the current record for this kind of magnet is only about 16 Tesla.

For circular electron colliders, the limiting factor is the energy loss to synchrotron radiation and these energy losses scale as the fourth power of the energy. LEP was probably the highest energy collider of this kind anyone is ever likely to build, since it already was using an amount of power a sizable fraction of that of the city of Geneva. One could try and use muons, which are much heavier so synchrotron radiation is not a problem, but they decay quickly so there are lots of problems with storing them in a collider.

These considerations mean that there is only one viable route to much higher energies, a linear collider, and this is the path that the ILC project is pursuing. What limits the energy in a linear collider like the ILC is the combination of the energy gradient one can achieve and the length of the machine. The superconducting RF cavities being studied for use in the ILC are inherently limited to gradients of less than 40-50 MV/m, with something like 35 MV/m a likely realistic number. With this gradient, to get up to a TeV in energy would require a length of about 33 km, about at the outer limits of what is possibly affordable. Realistically, to get to higher energies than this, one needs to find some way to get much higher energy gradients.

Rozensweig’s talk covers this material, but goes on to discuss various exotic new technologies that in principle can provide these much higher gradients. He describes progress on a long list of these, the most advanced of which is the CLIC project at CERN which uses the wake-field from a drive beam (a second accelerator). Much more exotic are various proposals involving lasers and plasma waves, some of which have been used to achieve gradients of 40 GV/m over short distances in the laboratory.

So, now all one has to do is to achieve a stable, high luminosity beam and make this work over kilometers not centimeters…. Not going to happen any time soon, but the distant future of high energy physics may depend on this kind of technology.

Update: I should also have mentioned here an article on this topic in the current (February) Scientific American entitled Plasma Accelerators.

Thanks to Wolfgang Beirl for the news that there’s an exciting new string theory blog, called The Official String Blog.

This month, the New York Times book review has been the place to follow the latest debates about what is going on in particle theory. This started with an essay by John Horgan on January 1, which drew letters to the editor from Lisa Randall on January 15, and Lawrence Krauss and Leonard Susskind on January 22 (this last letter was discussed here).

The January 15 issue also had a not very positive review of Susskind’s recent book (discussed here). In today’s (January 29) issue, Burton Richter has a letter commenting on and contrasting the recent work of Randall and Susskind. He’s positive on Randall, since he sees her as trying to come up with testable predictions, but about Susskind he has the following to say:

Susskind and the Landscape school have given up. To them the reductionist voyage that has taken physics so far has come to an end. Since that is what they believe, I can’t understand why they don’t take up something else — macramé, for example.

Richter is an emeritus Stanford professor, ex-director of SLAC, and won a Nobel prize in 1976 for his role in the “November Revolution”: the discovery at SPEAR in November 1974 of the “Psi” particle, a bound state of a charmed and anti-charmed quark (also found by a group at Brookhaven led by Sam Ting, who called it the “J” particle). Since he is emeritus, presumably Richter doesn’t attend Stanford physics department faculty meetings anymore, which is too bad, since I for one would love to see Susskind, Richter and Robert “string theory is like a 50-year old woman trying to camouflage her flaws by wearing way too much lipstick” Laughlin debating department hiring policy.

On the same page as Richter’s letter, there’s an ad for a book called “Reality Check”, by David L. Weiner. I don’t know anything about the book but the advertising blurb goes like this:

It turns out that the ape-like mechanisms that remain in our brains not only can create mental turmoil if we don’t meet their primitive expectations, but their penchant for pecking order and status can create far-out realities we think are absolutely true. These may cause us to inflict unwarranted harm on others, limit our own potential, or both.

Seems to me this book might explain some of the reaction to the recent interview in Discover magazine.

I’d been wondering what’s up with Witten and his ongoing work on geometric Langlands. He has been giving talks about this since last summer, and in the past has always quickly produced a paper (often a quite long one) once he has some new result like this that he’s publicly talking about. It had surprised me that it was taking him unusually long to get this written up, but now comes news from Anton Kapustin (via Lubos) that Witten is working on a document 3-400 pages long. This length would certainly explain why it is taking longer than usual, and surely the end result will be something quite interesting. The Kapustin rumor also claims that whatever this 300-400 page thing is that Witten is working on, it’s not a paper. Mysterious… The obvious guess is that it will actually be a book.

The German weekly Die Zeit has an article this week by Max Rauner about string theory, the Landscape, and the controversy over whether this is science or not. My German is rather shaky, but as far as I can tell it’s an intelligent summary of the controversy, emphasizing Susskind and his new book, and quoting many of the usual suspects. The same issue also has an interview with philosopher of science Martin Carrier about the question of whether or not string theory is a science.

Update: Eli Rabett has put up a translation of the article into English on his blog.

There’s an extended version of the interview of me by Susan Kruglinski in the February issue of Discover magazine that is now available, for free, on-line.

Before anybody starts yelling about AdS/CFT or topological strings when they read the headline “No one has a plausible idea about how string theory can explain anything”, I’ll just point out that, yes, it’s certainly plausible that some day string theory will explain something about QCD, and it already has explained some things in mathematics. The headline is a summary of some things I say in the interview, and in context it should have been clear I was talking about the use of string theory to predict anything not already predicted by the standard model.

Update: Harvard string theorist Lubos Motl has posted his commentary on the Discover article. If you read the comment section there, keep in mind that he is deleting comments from anyone who disagrees with him. I encourage anyone new to the current controversy over string theory to read what I have to say, read what Lubos has to say, and carefully look into the scientific issues involved to make your own judgement about what is going on here.

Lawrence Krauss and Leonard Susskind have a letter to the editor in this week’s New York Times Sunday Book Review, complaining about John Horgan’s NYT Book Review essay Einstein Has Left the Building of a couple weeks back. For some discussion of an earlier Susskind-Horgan exchange about another NYT piece of Horgan’s, see here and here.

Krauss and Susskind write that “Horgan evidently sees the two of us as being on opposite sides of an imagined controversy”, but that “the fact is that there is little of substance that we disagree about.” They accuse Horgan of thinking “that reconciling quantum mechanics with Einstein’s theory of gravity is a frivolous pursuit”, whereas “both of us feel that reconciling the conflict between gravity and quantum mechanics is one of the deepest problems in modern physics.” They comment about extra dimensions:

As for Horgan’s bête noire of physics — higher dimensions, or what he refers to as “hyperspace theories” — he writes: “But pursuers of this ‘theory of everything’ have wandered into fantasy realms of higher dimensions with little or no empirical connection to our reality.” That both of the present writers recognize that additional degrees of freedom of one sort or another are needed to characterize the physics of elementary particles may come as a surprise to Horgan. What the two of us may disagree about is what may be the likely physical and mathematical basis of this fact. But we both recognize that the mathematical spaces that we already deal with in describing the quantum theory of matter are in a certain sense more mathematically exotic than simple possible extra physical dimensions.

It seems to me that Krauss and Susskind are creating a straw man to attack here, not really dealing with Horgan’s actual criticisms, the full text of which was:

Especially as represented by best sellers like “A Brief History of Time,” by Stephen Hawking, and “The Elegant Universe,” by Brian Greene, physics has also become increasingly esoteric, if not downright escapist. Many of physics’ best and brightest are obsessed with fulfilling a task that occupied Einstein’s latter years: finding a “unified theory” that fuses quantum physics and general relativity, which are as incompatible, conceptually and mathematically, as plaid and polka dots. But pursuers of this “theory of everything” have wandered into fantasy realms of higher dimensions with little or no empirical connection to our reality. In his new book “Hiding in the Mirror: The Mysterious Allure of Extra Dimensions, from Plato to String Theory and Beyond,” the physicist Lawrence Krauss frets that his colleagues’ belief in hyperspace theories in spite of the lack of evidence will encourage the insidious notion that science “is merely another kind of religion.”

I don’t see Horgan here criticizing the attempt to quantize gravity as “frivolous”. His criticism of physicists as having “wandered into fantasy realms of higher dimensions with little or no empirical connection to our reality”, is a justifiable one that deserves to be seriously addressed. Krauss and Susskind’s comment that Horgan would be surprised that both of them think that new degrees of freedom will be needed to characterize elementary particle physics doesn’t seem to have any basis in fact. Horgan isn’t making broad claims that physicists shouldn’t look for new degrees of freedom, he is very specifically referring to the use of extra space-time dimensions.

Krauss and Susskind at least implicitly take Horgan to task for referrring to such extra dimensions as “hyperspace”. He may well have picked this up from Michio Kaku who wrote a book with this title. By the way, tonight Kaku will be appearing on the Art Bell “Coast to Coast” radio program, a program which is mostly concerned with UFOs and the like. If Krauss and Susskind want an example of the kind of theoretical physics research that Horgan is bothered by, they could check out this radio program.

Theorist Richard Szabo has had part of an evening he spent last week at the bar in a Bloomsbury hotel memorialized in today’s Observer Magazine.