A couple people have written to tell me about the new BBC Faster Than the Speed of Light? documentary on superluminal neutrinos which evidently featured trademark hype from string theorist Mike Duff about how string theory could explain this. For better or worse, I don’t think I have access to the show from the US, although I’m sure that sooner or later it will arrive on our shores.
Update: Philip Gibbs has more here.
Update: The BBC program is now on Youtube, see here.
There’s a nice article this week in Nature about AdS/CMT, entitled String Theory Finds a Bench Mate. According to the article, the whole thing is (partly) my fault:
But in 2006, string theory took a public battering in two popular books: Not Even Wrong by Peter Woit, a mathematician at Columbia, and The Trouble With Physics by Lee Smolin, a physicist at the Perimeter Institute for Theoretical Physics in Waterloo, Canada. Both books excoriated the theory’s isolation from experiment.
“It’s hard to say whether the interest in condensed-matter applications is a direct response to those books because that’s really a psychological question,” says Joseph Polchinski, a string theorist at the Kavli Institute for Theoretical Physics in Santa Barbara. “But certainly string theorists started to long for some connection to reality.”
The main point of the story is to tell about what is probably the hottest topic in hep-th these days, attempts to use AdS/CFT to say something about some models in condensed matter physics. For some idea of what this is all about, see the review article What can gauge-gravity duality teach us about condensed matter physics? by Subir Sachdev, and take a look at the online talks from the KITP workshop Holographic Duality and Condensed Matter Physics.
The article does go into the history of this in some detail, including its roots in efforts to use AdS/CFT to say something about heavy-ion physics phenomena observed at RHIC (for the string theory promotional campaign surrounding this, see e.g. here). I had expected to see a lot about this topic when higher energy results from heavy-ion collisions at the LHC were released earlier this year, but it seems to have gone quiet, perhaps because of the kind of comparison of data with AdS/CFT predictions that Sabine Hossenfelder points out here:
As the saying goes, a picture speaks a thousand words, but since links and image sources have a tendency to deteriorate over time, let me spell it out for you: The AdS/CFT scaling does not agree with the data at all.
My knowledge of condensed matter theory is minimal, and the hype level surrounding string theory makes it hard to know whether to take at face value many of the claims being made. On general principles, this looks a bit more promising than the heavy-ion case, since there are many different kinds of systems one might look at, and the connections are more to QFT than to string theory. Experts quoted in the Nature article give opinions ranging from:
Polchinski admits that the condensed-matter sceptics have a point. “I don’t think that string theorists have yet come up with anything that condensed-matter theorists don’t already know,” he says. The quantitative results tend to be re-derivations of answers that condensed-matter theorists had already calculated using more mundane methods.
to condensed matter theorist Andrew Green’s:
“Maybe string theory is not a unique theory of reality, but something deeper — a set of mathematical principles that can be used to relate all physical theories,” says Green. “Maybe string theory is the new calculus.”
Time will tell whether this suffers the same fate as in the case of heavy ions.
The October issue of Discover magazine has a new feature, a column by Sean Carroll, whose inaugural effort is now on-line as Welcome to the Multiverse. Sean makes the argument that opposition to multiverse mania is due to people having too naive an idea about what science is. They don’t realize that testing those parts of a theory you can directly observe allows you to draw conclusions about those parts you can’t directly observe:
A lot of people, both inside and outside the scientific community, are viscerally opposed to the idea of other universes, for the simple reason that we can’t observe them—at least as far as we know. It’s possible that another universe bumped into ours early on and left a detectable signature in the cosmic background radiation; cosmologists are actively looking. But the multiverse might be impossible to test directly. Even if such a theory were true, the worry goes, how would we ever know? Is it scientific to even talk about it?
These concerns stem from an overly simple demarcation between science and nonscience. Science depends on being able to observe something, but not necessarily everything, predicted by a theory. It’s a mistake to think of the multiverse as a theory, invented by desperate physicists at the end of their imaginative ropes. The multiverse is a prediction of certain theories—most notably, of inflation plus string theory. The question is not whether we will ever be able to see other universes; it’s whether we will ever be able to test the theories that predict they exist.
Sean makes quite clear that multiverse mania is driven by string theory. Inflation is part of the story, but it’s not a fundamental theory by itself. All it can tell you is that your fundamental theory should have an inflaton field of some kind, with a potential satisfying certain properties. The big idea that justifies the multiverse is that:
In short, string theory predicts that the laws of physics can take on an enormous variety of forms, and inflation can create an infinite number of pocket universes. So the different laws of physics predicted by string theory might not be just hypothetical. They might really be out there somewhere among the countless parts of the multiverse. This is not a situation that cosmologists dreamed up in a flight of fancy; it is something we were led to by trying to solve problems right here in the universe we observe.
The problem that Sean doesn’t mention is one of circularity. Since you can’t observe anything about it directly, the multiverse must be justified in terms of another theory that can be tested and this is string theory. But if you talk to string theorists these days about how they’re going to test the unified theory that string theory is supposed to provide, their answer is that, alas, there is no way to do this, because of the multiverse. You see, the multiverse implies that all the things you would think that string theory might be able to predict turn out to be unpredictable local environmental accidents.
So, the multiverse can’t be tested, but we should believe in it since it’s an implication of string theory, but string theory can’t be tested because of the multiverse.
Until recently, string theorists would sometimes hold out hope that the LHC would see low-energy strings, extra dimensions, or supersymmetry, and that these discoveries would somehow pick out a predictive version of string theory from the landscape of the multiverse. This year’s data from the LHC has pretty much destroyed such hopes.
Sean ends with the inspirational admonition:
The proper scientific approach is to take every reasonable possibility seriously, no matter how heretical it may seem, and to work as hard as we can to match our theoretical speculations to the cold data of our experiments.
What’s going on in this story though is not a concerted effort to match theoretical speculation to experimental data, but something very different, a concerted effort to build a theoretical framework perfectly insulated from testability, and sell it to the rest of the physics community and the public, hoping no one notices the circularity.
Update: Besides the usual spam, this topic seems to attract mostly empty comments supposedly agreeing with me, and comment moderation is unusually annoying. I think I’ll turn off comments on this posting, and encourage people who want to discuss the topic to do so over at Cosmic Variance, where Sean has a posting devoted to this.
The latest New York Review of Books has an article by Steven Weinberg entitled Symmetry: A ‘Key to Nature’s Secrets’. It’s a bit unusual for the NYRB, since it is both scientifically more technical than usual for them (coming from a write-up of Weinberg’s talk at this conference), and doesn’t review any books. The printed version tells readers to go to the web version for footnotes, but some of these just note that things are being over-simplified. One of the footnotes is worse than useless: the editors have replaced x^3=x as an example of an equation with solutions that break a symmetry (x goes to -x) by “x 3 equals x”, an equation with the same symmetry but only a symmetric solution (x=0). The idea seems to have been to remove or replace any symbols in the equation that might upset people.
Weinberg tells the conventional story of how the Standard Model emerged during the 60s and early 70s out of the realization that non-abelian gauge symmetries were important and an understanding of what happens when symmetries are spontaneously broken. He tries to do some much more ambitious things, explaining the idea of “accidental symmetries” that are due to the limited number of possible renormalizable terms you can build out of a specified list of fields, but I’m not sure the typical reader of the NYRB is going to get much out of this.
The question of how to explain the notion of “symmetry” is an interesting one, and I thought a lot about it when writing Not Even Wrong, the book. To my mind, most such explanations mix up two conceptually distinct things: the group of symmetries (a group), and the action of the group on some other mathematical object (the representation: mathematically a homomorphism from the group to the group of automorphisms of something). It’s both the group and the representation that are important in the use of symmetries in physics, although often what is important is the trivial representation. From a mathematician’s point of view, the simplest representations to look at are unitary representations on a complex vector space, so the mathematical structure of quantum mechanics is very natural. To each symmetry generator you get a conserved quantity, and it appears in quantum mechanics as the thing you exponentiate (a self-adjoint operator) to get a unitary representation. In Weinberg’s piece, which aims at sophisticated issues in particle theory, the question of the basic relation of symmetries and conservation laws is relegated to a footnote which says only “For reasons that are difficult to explain without mathematics…”.
Weinberg ends with a landscape sort of picture, involving symmetries emerging only when a specific ground state emerges out of an initial chaotic inflation state. Philosophically this is a popular view of the future of the subject these days, but one that has so far led nowhere, and one that I think even in principle can never lead anywhere. Much more interesting would be to try and draw lessons from what has worked well in the past: exactly the gauge symmetries and spontaneous symmetry breaking phenomena that led to the standard model. We may very well soon find out there is no Higgs particle, turning this whole subject into a wide-open one. Future progress may come from exactly the same place as in the past: new ideas about how to exploit the mathematical structures inherent in quantum mechanical symmetries.
Update: The missing exponentiation in the on-line footnote has been fixed.
A few items with a European flavor:
Witten’s Hamilton Lecture will abandon string theory, however, in favour of knots, with a talk entitled: The Quantum Theory of Knots.
He may be there the previous day, when they hold the annual Hamilton Walk, commemorating Hamilton’s discovery of quaternions and inscription of the quaternion relations on a bridge.
In other mathematical physics coverage by the Irish Times, one of their columnists describes the interaction of the Irish revolutionary leader de Valera with Schrodinger and Dirac, speculating (humorously) that the three of them might have come up with an Irish “unified field theory.”
The graphic chosen years ago as the header for this blog is an event display from the UA1 detector in 1982, of historical importance since it was the first event found with a W candidate. To be honest, the reason it’s there is that I was looking for something quick to use at the interactions.org Imagebank, figuring these were graphics I could steal without getting sued. What I ended up with is a cropped, lower-resolution version of the much better image available here.
Even stripped of identifying info, UA1 experimentalist Jim Rohlf of course recognized it, and recently wrote me to tell me some more about it. He also tells me that he will soon be blogging at Quantum Diaries, and I look forward to seeing that. So, here’s the story behind that image:
The collision is Run 2958, Event 1279 and was the very first W candidate that was found. It was recorded in the Fall of 1982 with UA1. As a newly minted junior faculty member and CERN scientific associate, I was resident at CERN and the first round of the W event selection and analysis was completed during the CERN holiday shutdown. On 23 January 1983 we submitted the W discovery paper for publication (Phys. Lett. 122 B, 103 (1983)). The details of the events were given in this paper. Within a few days, I got a letter from Lev Okun who had become a good friend of mine due to his frequent visits to CERN and his great interest in the working details of our experiment. In this letter which was several pages long he referred to this event as a “monster” because it decayed in the “wrong direction” and asked if we could have made a measurement error. Then the obvious hit me instantly- nobody had thought of this before- we don’t measure the longitudinal momentum of the neutrino due to the singularity in the direction of beam pipe but we can solve it to a quadratic ambiguity knowing the W mass. Furthermore, I saw that the kinematics of a 80 GeV object being produced with a relatively low cm energy of 540 GeV gave a remarkable result: often one of the 2 solutions was kinematically forbidden and when it wasn’t, the two solutions were often close together. Therefore, we could solve for the longitudinal momentum of the neutrino and be able to transform to the rest frame of the W. Since the W was polarized because it was produced in proton-antiproton collisions, we could measure the angle of the decay wrt the spin direction. Very simple idea, but be the first to do it and it becomes interesting and fun. I immediately wrote this up as a UA1 internal note in which I acknowledged the contribution of Okun. This technique subsequently became a standard at the Tevatron and now at the LHC.
In the following months, we collected more data and the next international conference to come along was at Fermilab and I was told by Rubbia to give the talk which was published (J. Rohlf, “Physics at the Proton-Antiproton Collider,” Proceedings of the 12th International Conference on High Energy Accelerators, Fermilab, 619 (1983). I reported the first measurement direct observation of parity violation in (real) W decay and measurement of the spin. I attach a slide from a talk I gave at Fermilab 20 years later in 2003 where I pulled up some of my 1983 slides. (Notice I fit the W mass to 2% and got the right answer.) You can see the “monster” event in the bin at cos(theta) =-1. This W decayed in the wrong direction. We went on to collect about 300 W events in UA1. We never saw another one go in the wrong direction. We also could not find anything wrong with that original event 1279. So you see the event was “not even wrong”.
Update: A copy of the talk slide that Jim Rohlf refers to is here.
Update: Jim Rohlf’s blog at Quantum Diaries is now up here. His first blog entry is great, it’s about, independent of the Higgs issue, the fundamental problem the LHC hopes to investigate: what is causing electroweak symmetry breaking? He emphasizes that one way to study this is to try and see the self-interactions of Ws and Zs, which become strong at the TeV scale.
The next math department colloquium at Stanford will feature Lenny Susskind lecturing on p-adic numbers and cosmology, here’s the abstract:
The biggest conceptual problem of cosmology is called the measure problem. It has to do with the assignment of probabilities in an exponentially inflating universe, which falls apart into separate causally-disconnected regions. Neither I nor my friends had ever intended to learn about p-adic numbers until we realized how similar such a universe is to an endlessly growing tree-graph. The result has been some new insights from p-adic number theory into the measure problem and other puzzles of eternal inflation. Within the constraints of a one-hour lecture, I will explain as much of this as I can.
I’ve no idea what this is about, but I’m guessing that Susskind is somehow drawing inspiration from two facts:
It’s hard to believe that any of the special features of these mathematical structures will make the problems of eternal inflation go away, but who knows…
Coincidentally, I’ve spent a lot of time recently learning about the p-adics, with a very different motivation. The way these things come up in mathematics is that you can think of number theory as being about a space, the space of prime numbers. The p-adics appear naturally when you decide to ask what happens locally near one point (i.e. at one prime). P-adic integers correspond to power series expansions, p-adic numbers to Laurent series. Various people have thought about analogies between conformal field theories on a Riemann surface, where one also wants to focus on what happens at a point and use representation theory methods, and the Langlands program which does something similar in number theory. This is part of the geometric Langlands story, and has roots in a remarkable paper of Witten’s from 1988 entitled Quantum field theory, Grassmannians, and algebraic curves.
As I’ve mentioned before, this semester here at Columbia we have Harvard’s Dick Gross as Eilenberg lecturer, and he’s giving a wonderful series of lectures starting with local Langlands. I’m hoping at some point to put together what I’ve been learning about this and possible connections to QFT in some readable form, but at the moment things are still too speculative and hazy. In any case, no sign that these ideas are going to solve the problems of cosmology…
Update: The Susskind et al. paper on this topic is now out at the arXiv. A p-adic model is studied, but no reason is given to believe that it has anything to do with eternal inflation and cosmology.
Back in 2004 I made my first venture into Nobel Prize predictions, then decided to retire from that business. This year I came out of retirement with another prediction. After the posting, I consulted with experts who assured me that the right names were Perlmutter, Riess and Schmidt, something I thought I mentioned in a comment, but it appears that I didn’t, instead leaving this to Shantanu.
Congratulations to Perlmutter, Riess and Schmidt. The theoretical significance of their tour de force observational work remains still controversial, but it richly deserves the Nobel prize.
Prospect magazine has an excellent new article by Philip Ball on recent developments in the fundamental problem of the interpretation of quantum mechanics: why don’t we see superpositions? Most popular discussions of this seem to me to be stuck back in debates from the 1930s, and ignore the main question. QM is a simple, beautiful mathematical structure that works perfectly experimentally, the confusing question is that of how classical behavior emerges during a measurement process (typically involving huge numbers of degrees of freedom, making analysis difficult).
At the end of the article, Ball mentions one particularly intriguing set of ideas, due to Wojciech Zurek, about “quantum Darwinism”. For more about this, see Zurek’s survey here.
There’s a new preprint out by Steven Weinberg on Collapse of the State Vector. Weinberg claims that “There is now no entirely satisfactory interpretation of quantum mechanics”, and refers for more detail about this claim to Section 3.7 of his Lectures on Quantum Mechanics, a manuscript that is “to be published”. I’m definitely looking forward to this book when it comes out. The sort of thing that Weinberg examines in the preprint though, modifying QM to take into account wave-function collapse, is the kind of idea I’ve never found promising. Why modify QM, it works perfectly and is mathematically extremely aesthetically compelling? Better to keep QM as is, and closely examine one’s understanding of what the problem really is.
It had to happen. New Scientist managed to find a physicist willing to describe the OPERA result as “evidence for string theory”:
So if OPERA’s results hold up, they could provide support for the existence of sterile neutrinos, extra dimensions and perhaps string theory. Such theories could also explain why gravity is so weak compared with the other fundamental forces. The theoretical particles that mediate gravity, known as gravitons, may also be closed loops of string that leak off into the bulk. “If, in the end, nobody sees anything wrong and other people reproduce OPERA’s results, then I think it’s evidence for string theory, in that string theory is what makes extra dimensions credible in the first place,” Weiler says.
Update: hep-ph is chock-a-block with papers purporting to explain the OPERA results, using theoretical models of varying degrees of absurdity. There is however one much more sensible paper this evening, from Cohen and Glashow, which points out that superluminal neutrinos would produce electron-positron pairs via bremsstrahlung, and lose energy, which is not observed. This is also incompatible with Super-Kamionkande and IceCube data. No matter what sort of extra dimensions you introduce for the neutrinos to travel in, the OPERA claim seems to be in violent disagreement with other observations.
