Math and Physics, Summer 2011

Posted on June 9, 2011 by woit

This week in Philadelphia the String-Math 2011 conference is going on, planned as the first of a series, with String-Math 2012 next summer in Bonn. Slides of the talks are appearing here. There’s also supposed to be video, but the saved video seems to require some sort of UPenn login, and I’ve not been able to get the streaming video to work. The public talk by Cumrun Vafa puts out the classic message that strings have come to the rescue of physics, unifying QM and gravity, and that:

Smooth geometry of strings seems to explain all known interactions (at least in principle)

The techinical talks cover a lot of ground, much of it having little to do with string theory. Michael Douglas’s talk surveys problems related to finding non-perturbative formulations of quantum field theory that one might hope to say something precise about, but it contains a lot more questions than answers. I’m most curious about David Ben-Zvi’s talk tomorrow, so hope that slides or video of that will be available.

The circle of ideas relating gauge theories, geometric Langlands, TQFTs and representation theory will be getting even more attention than the mathematics of string theory this summer. In a couple weeks will begin a two-part program at Luminy and then Cargese on Double Affine Hecke Algebras, the Langlands Program, Affine Flag Varieties, Conformal Field Theory, Super Yang-Mills Theory. I don’t know who the author is, but some person or group has written up for the occasion a wonderful summary of the current activity in these and related fields of mathematics, see here. Next month, the KITP will be hosting a program on Nonperturbative Effects and Dualities in QFT and Integrable Systems that will cover some of the same topics.

In some other unrelated news, if you understand French, you can listen to an interesting set of interviews with Pierre Cartier here. Finally, it was announced recently that my colleague Richard Hamilton is sharing this years $1 million Shaw Prize for Mathematics with Demetrios Christodoulou. Congratulations Richard!

Posted in Langlands | 10 Comments

New Look

Posted on June 9, 2011 by woit

I’ve updated the blog a bit, to a newer, widgetized theme. Functionality should be the same as before. The only issue I’m aware of now is that the “Archives” widget adds an annoying character below its header which I can’t get rid of. This widget may also be the source of problems some people are having (due to some combination of Javascript and incompatibility with latest WordPress version), it may get replaced…

Please let me know of any other issues.

Posted in Uncategorized | 13 Comments

This Week’s Rumor

Posted on June 8, 2011 by woit

A couple months ago CDF made the New York Times by releasing results claiming to see a resonance in the invariant mass spectrum of two jets produced together with a W. Last week they released a new analysis with twice as much data, claiming the signal was still there, now at a statistical significance of nearly 5 sigma.

I recently wrote about this here, explaining the reasons for being skeptical, despite the high statistical significance. One very good reason for being a skeptic is that CDF’s Tommaso Dorigo doesn’t believe this is real, going so far as to put his money where his mouth is, offering a $100 wager to back up his arguments. The crucial question in everyone’s mind has been whether D0, CDF’s competition and sister detector at Fermilab, would see the same thing in its data. If there’s really something there, D0 should see it.

This Friday there will be a Wine and Cheese talk at Fermilab, where the D0 results will be unveiled, and you can watch this as a live video stream here. But, as one might expect, now that the D0 result is ready to be revealed, people do things like leave print jobs on printers, etc., causing well-sourced rumors to spread. Blogs such as this one seem to be a place where such information tends to end up, so I can report a rumor (based on excellent sources) that Tommaso is right. D0 will report on Friday that there’s nothing there, that they find no evidence for a dijet resonance in the region from 110-170 GeV. They reject the CDF hypothesis of a resonance with a cross section of 4 pb at a significance level of over 4 sigma.

In other news, the LHC is running very well, with the official goal of this week being to reach an integrated luminosity of 1 fb-1, something that had been the official goal for the entire year (although, unofficially, 2-3 fb-1 is more like it). Right now, they’re around .8 fb-1. This sort of luminosity should finally start to allow in coming months results that either rule out a Higgs in the region it is expected or see first indications if it is there.

At the KITP, this week marks the start of a program on The First Year of the LHC. Unfortunately for theorists, the only result of data from the first year of the LHC has been to shoot down some of their favorite models, ruling out for instance a large amount of the parameter space where supersymmetry was expected to be found, making the most popular theoretical idea of the last thirty years significantly less popular. The first talk held at the KITP program was this afternoon, and it dealt not with the LHC data, but with the supposed CDF resonance (it appears that news of the D0 result hadn’t yet made it to Santa Barbara).

Update: The KITP talk is now available here.

Update: The D0 PRL submission that has been circulating privately for the past few days is supposed to be available at 9am Friday Fermilab time here. See here for other material to be released publicly today.

Update: Now that the D0 results are officially out, as usual your best bet for informed explanation is to check out what Jester and Tommaso have to say.

Posted in Experimental HEP News | 34 Comments

The Quantum Story

Posted on June 8, 2011 by woit

Jim Baggott’s The Quantum Story: A History in 40 Moments is now out here and I’ve been starting to see it in bookstores. I read most of it a year or two ago when he sent me a draft of the manuscript asking if I’d take a look at it, and very much enjoyed getting the chance to see it then. If you’re looking for an excellent popular level physics book to read, I highly recommend that you consider this one, which should be accessible to just about anyone, no matter what their background.

The topic of the book is the story of quantum physics in general, told historically with a structure of 40 vignettes. The first three chapters cover mainly events of the dramatic period of the mid-1920s to early-1930s during which physicists uncovered the basic structures of quantum theory and struggled to make some sense of them. The next two take on the late-40s to mid-70s during which a long succession of discoveries about elementary particle physics drove theoretical progress on quantum field theory and gauge theory, culminating in the Standard Model falling into place in 1973. The material Baggott works with here has been the topic of many other books, but he does a wonderful job of putting it together in a fast-paced but very clear and entertaining narrative. Along the way, the individual stories he tells often contain fascinating details I’d never heard before, even though I thought this was a subject I knew all too well.

The next to last chapter starts with the 1950s and David Bohm, picking up the thread of later debates and discoveries related to the general problem of the interpretation of quantum mechanics. It brings this story up to date, explaining some of the current questions that are still being debated. The final chapter gives an appropriately short discussion of speculative ideas in quantum gravity and string theory that have dominated theoretical research for the past few decades, making clear that they’re a long ways yet from the solid science that’s the main topic of the book.

The LHC and the search for the Higgs make up a final epilogue or 41st vignette, accurately describing the high expectations and drama that surrounds the final period of the long wait for new data that is finally coming to an end this year. The story of quantum theory is not a finished one, and we all hope that very soon we’ll get some clues as to where it will go next.

Posted in Book Reviews | 3 Comments

This Week’s Hype

Posted on June 1, 2011 by woit

The latest New Scientist has a much larger dose of M-theory/multiverse hype than I’ve seen in one place in quite a while. There’s a four-part series on M-theory (here, here, here and here) by Mike Duff. It tells the story of the progress of modern physics over the past century according to the dominant ideology: general relativity, Kaluza-Klein extra dimensions, super-symmetry, superstrings, branes, ending in the apotheosis of M-theory more than fifteen years ago. For the current state of affairs, Duff describes his “M-theory” predictions about the real world (that 4 qubits can be entangled 31 different ways, something discussed here). He ends with the M-theory multiverse and the following comments on whether this can ever be tested:

So is M-theory the final theory of everything? In common with rival attempts, falsifiable predictions are hard to come by. Some generic features such as supersymmetry or extra dimensions might show up at collider experiments or in astrophysical observations, but the variety of possibilities offered by the multiverse makes precise predictions difficult.

Are all the laws of nature we observe derivable from fundamental theory? Or are some mere accidents? The jury is still out.

In my opinion, many of the key issues will remain unresolved for quite some time. Finding a theory of everything is perhaps the most ambitious scientific undertaking in history. No one said it would be easy.

Here he makes it clear that, at least while he’s still around and enjoying academic prominence because of M-theory, there’s no danger it will face any sort of test it might fail. He answers critics of M-theory by claiming that its failures don’t matter. It’s the dominant paradigm, and will reign as such until someone comes up with a different theory of everything that isn’t a failure.

Elsewhere in the magazine, there’s a fawning article about the recent Bousso-Susskind paper (see here):

TWO of the strangest ideas in modern physics – that the cosmos constantly splits into parallel universes in which every conceivable outcome of every event happens, and the notion that our universe is part of a larger multiverse – have been unified into a single theory. This solves a bizarre but fundamental problem in cosmology and has set physics circles buzzing with excitement, as well as some bewilderment.

No critics of the idea were located by the writer, with the discussion on blogs described as:

The paper has caused flurry of excitement on physics blogs and in the broader physics community. “It’s a very interesting paper that puts forward a lot of new ideas,” says Don Page, a theoretical physicist at the University of Alberta in Edmonton, Canada. Sean Carroll, a cosmologist at the California Institute of Technology in Pasadena and author of the Cosmic Variance blog, thinks the idea has some merit. “I’ve gone from a confused skeptic to a tentative believer,” he wrote on his blog. “I realized that these ideas fit very well with other ideas I’ve been thinking about myself!”

Somehow Lubos’s “they’re on crack” take on the subject was missed.

Finally, the significance of all of this is summarized in an editorial which argues that Bousso-Susskind finally pulls the plug on religion and replaces it with science:

Cosmologists can now begin to take God seriously, precisely because they can explain him (or her) away.

Posted in Multiverse Mania, This Week's Hype | 47 Comments

Quick Links

Posted on June 1, 2011 by woit
  • The big news of the past couple days has been the release of more data by CDF which continues to show a bump in the invariant mass of two jets produced with a W. Resonaances gives an excellent description of this and its possible significance. Tommaso Dorigo remains a skeptic.

    I can’t do better than the two of them on this story, but here’s my summary take on the situation:

    With the new data, this can no longer be written off as a statistical fluke. 3 sigma you can argue away as such a fluke, but not 5 sigma.

    The main reason to be skeptical though hasn’t been the statistical significance, but the possibility that this is due to bad modeling of the background. The signal is being extracted from a huge background, so a small misunderstanding of the background could be its cause. If this is the case, the new data changes nothing, you expect to continue to see the effect as more data is analyzed.

    The fact that Tommaso is a skeptic carries a lot of weight, since he works on the CDF experiment and understands the problems well. In general, experimentalists want the experiments they work on to make great discoveries, so tend to be optimists about their own results. When someone is skeptical about a result of their own experiment, that should give one pause.

    What would really make the case for new physics here more compelling would be if the result is confirmed by one of the other experiments (DO at Fermilab, CMS or ATLAS at the LHC) that should be able to see the same effect if it is there. These groups have a certain motivation to not just confirm their competition’s discovery (raising the question of why they didn’t find this first), but to convincingly shoot it down. This posting by Pauline Gagnon of ATLAS says that they see nothing in their 2010 data. One expects that D0 is hard at work and should soon release whatever they have found. ATLAS and CMS should also be hard at work looking at the much larger 2011 data samples. We’ll know soon the results, but the public comments of Dorigo and Gagnon don’t sound to me like those they would be making if they knew their experiments had preliminary confidential results confirming the CDF anomaly.

    Finally, while there are lots of theory papers out already with supposed models explaining this, none are really compelling. This is not an experimental result with an obviously attractive theoretical explanation.

  • Abstruse Goose has commentary on SUSY here.
  • In gossip of the mathematics world, it looks like Princeton (the IAS) has stolen away number theorist Richard Taylor from Harvard.
  • Video of Graeme Segal’s Felix Klein lectures this spring at Bonn on quantum field theory are now available, and well worth watching.
  • Other interesting video available is Greg Moore’s lectures on geometry, topology and QFT at Rutgers last fall.
  • For pictures from this years Physics of the Universe Summit, see here. Any info beyond the transparencies caught in the pictures seems to be private.
  • The journal Foundations of Physics will be putting out an issue on “Forty Years of String Theory”. So far articles intended for this have appeared on the arXiv from Dean Rickles, Steven Gubser, and, last night, Steven Giddings. The Giddings contribution is entitled Is string theory a theory of quantum gravity?, and provides an unusually hype-free discussion of the relevance of gauge/gravity duality to hopes to use string theory to understand quantum gravity, writing:

    While string theory addresses some problems of quantum gravity, its ability to resolve these remains unclear. Answers may require new mechanisms and constructs, whether within string theory, or in another framework.

Posted in Experimental HEP News, Uncategorized | 9 Comments

Cycles of Time

Posted on May 27, 2011 by woit

Today’s Wall Street Journal has a review I wrote of Sir Roger Penrose’s new book Cycles of Time. The review is aimed at a much wider audience than this blog, and is the product of substantial editing to get its length down and make it as readable as possible for as many people as possible, so here are some supplementary remarks.

I should make it clear that I’m not at all convinced by what Penrose is proposing. He needs the distant future of the universe to be conformally invariant, and this requires all particles to be massless. As far as we know the electron is completely stable, with unchanging mass, and this will always ruin conformal invariance. Penrose himself notes the problem. For this to be overcome, whatever our ultimate understanding is of how particles get mass must change so that these masses go to zero in the future. It’s also seems to me that the conformal anomaly of QCD will always be a problem, with quantization and the renormalization group always breaking conformal invariance and giving a mass scale, indefinitely far into the future.

The other main problem is the one shared by most “pre-big-bang” ideas: how do you ever test them? Penrose and a collaborator last year created a stir by claiming to see in the CMB patterns of the sort he argues might be expected from black hole decays late in an era before the Big Bang, but it’s not clear there’s a real prediction here, and others who have redone this analysis say they see nothing.

Attempts to get a Big Bang in our future as well as our past generally strike me as motivated by a very human desire to see in the global structure of the universe the same cyclic pattern of death and rebirth that govern human existence. To me though, deeper understanding of the universe leads to unexpected structures, fascinating precisely because of how alien they are to human concerns and experience. Just because we might find a cold, empty universe an unappealing future doesn’t mean that that’s not where things are headed.

The book is in many ways an unusual document. It includes an extensive appendix working out some of the details of the mathematics of his proposal. In some sense he has managed to get a trade publisher to put out a highly technical discussion of a speculative idea inside the covers of a popular book, instead of going the usual route of publishing this in a refereed journal. The only references I can find to other places where he has written some of this up are to chapters in this book and this one, as well as this contribution to a conference proceeding. The technical idea behind this, that the hypothesis of the vanishing of the Weyl curvature in the early universe leads to possible cosmological models that can be extended past the Big Bang singularity he attributes to this paper of K.P. Tod. There’s a nice recent exposition of this by Tod here.

So, I’m not convinced by the speculation about the far future, and for an evaluation of the ideas about extending back through the big bang singularity you’ll need someone more expert about cosmology than me. These topics are very clearly labeled in the book as speculative, without support from other physicists or any experimental evidence. The bulk of the book though is other material providing a background and context for the speculation, and it is this which I think makes it most valuable as a popular book. Penrose is a wonderful, elegant and clear writer, and he covers a lot of ground about physics beautifully here. Most remarkable are the illustrations, by far the best visual representations of a range of important ideas that I know of. Physicists and mathematicians work with lots of internal pictures in their minds representing important aspects of the concepts they are investigating, but very rarely do they have the technical skill to grasp some of the essence of these pictures and get them down on paper. Even more rarely do they make it into wide distribution in print, so I’m glad to see that happen here.

Posted in Book Reviews | 44 Comments

Cosmological Interpretations of Quantum Mechanics

Posted on May 19, 2011 by woit

It seems that there’s now a new burgeoning field bringing together multiverse studies and interpretational issues in quantum mechanics. Last year Aguirre, Tegmark and Layzer came out with with Born in an Infinite Universe: a Cosmological Interpretation of Quantum Mechanics, which claimed:

This analysis unifies the classical and quantum levels of parallel universes that have been discussed in the literature, and has implications for several issues in quantum measurement theory… the analysis suggests a “cosmological interpretation” of quantum theory in which the wave function describes the actual spatial collection of identical quantum systems, and quantum uncertainty is attributable to the observer’s inability to self-locate in this collection.

Last month there was Nomura’s Physical Theories, Eternal Inflation, and Quantum Universe where “a picture that the entire multiverse is a fluctuation in the stationary, fractal “mega-multiverse,” in which an infinite sequence of multiverse productions occurs” is invoked and:

Our framework provides a fully unified treatment of quantum measurement processes and the multiverse. We conclude that the eternally inflating multiverse and many worlds in quantum mechanics are the same.

Most recently, tonight’s arXiv listing has Bousso and Susskind’s The Multiverse Interpretation of Quantum Mechanics:

We argue that the many-worlds of quantum mechanics and the many worlds of the multiverse are the same thing, and that the multiverse is necessary to give exact operational meaning to probabilistic predictions from quantum mechanics.

I confess that I’m having trouble making sense of any of these papers. According to Bousso and Susskind, if I want to understand how quantum mechanics describes some simple, local physical system and what happens when I do measurements of it, I need to sign on to the theory of eternal inflation and the multiverse:

We will offer some principles that we believe are necessary for a consistent interpretation of quantum mechanics, and we will argue that eternal inflation is the only cosmology which satisfies those principles.

In the case of many string theory papers, one’s problems understanding their claims could often be attributed to the highly complex and sophisticated mathematical framework involved. These papers are mostly long sections of verbiage, sometimes with pictures. My inability to make sense of them must have some other source…

Update: Lubos has an explanation of the Bousso-Susskind paper: “they’re on crack”.

Update: I suppose one could have guessed that Sean Carroll would be a fan of this. In his book he argues that the way to understand the second law of thermodynamics and the arrow of time is to invoke cosmology and the multiverse, now he seems happy to do the same thing with the interpretation of quantum mechanics. The ideas seems to be that to understand some local quantum mechanical phenomenon, you need to use cosmology and think about the horizon that is part of the deSitter geometry. I don’t find this argument any more plausible than the arrow of time one.

It does seem like this is now being promoted as the hot topic in theoretical physics, with Sean and others organizing a conference partially devoted to this at Perimeter this summer.

Posted in Multiverse Mania | 64 Comments

Particle Theory Job Market 2011

Posted on May 19, 2011 by woit

By now the hiring season for tenure-track jobs is pretty much over, and for the field of particle theory some idea of the results is available at the Theoretical Particle Physics Jobs Rumor Mill. As has been usual for the last few years, most jobs are going to phenomenologists. Remarkably, it seems that no jobs at all are going to string theorists so far this year. The final number of jobs is yet to be determined, with ten people so far getting job offers. It looks like the total number of jobs in the field will remain at the low level typical of the last three years since the recession hit in 2008.

Erich Poppitz has been compiling statistics based on the Rumor Mill data, and has the results through 2010 here. Job numbers rose to a level of 20-25 jobs/year from 2000-2007, from a low level of 10 jobs/year back in the 1990s. This phenomenon is generally attributed to jobs becoming available as the generation that was tenured during the huge expansion of universities in the 1960s finally started to retire. The recession has brought those numbers down to 15 (2008), 9 (2009) and 14 (2010). In recent years (since about 2004) he counts about a fifth of the jobs as going to string theorists. If that number does go to zero this year, that would be the first time this has happened since the numbers became available, and I would guess all the way back to shortly after string theory first became popular in 1984-5.

Posted in Uncategorized | 21 Comments

Hawking and the Google Zeitgeist

Posted on May 16, 2011 by woit

Today is the first day of this year’s Google Zeitgeist gathering of high-powered world leaders and thinkers (described here, it seems that Google either doesn’t believe in having a web presence, or it’s a secret one. Anyway, my attempts to Google it have failed). One of the headline speakers is Stephen Hawking, and the Guardian reports that:

His talk will focus on M-theory, a broad mathematical framework that encompasses string theory, which is regarded by many physicists as the best hope yet of developing a theory of everything.

M-theory demands a universe with 11 dimensions, including a dimension of time and the three familiar spatial dimensions. The rest are curled up too small for us to see.

Evidence in support of M-theory might also come from the Large Hadron Collider (LHC) at Cern, the European particle physics laboratory near Geneva.

One possibility predicted by M-theory is supersymmetry, an idea that says fundamental particles have heavy – and as yet undiscovered – twins, with curious names such as selectrons and squarks.

Confirmation of supersymmetry would be a shot in the arm for M-theory and help physicists explain how each force at work in the universe arose from one super-force at the dawn of time.

They got him to respond to some questions posed in advance, and one of them has the Guardian’s Ian Sample puzzled. Hawking’s answer to “What is the value in knowing “Why are we here?'” was:

The universe is governed by science. But science tells us that we can’t solve the equations, directly in the abstract. We need to use the effective theory of Darwinian natural selection of those societies most likely to survive. We assign them higher value.

Sample’s reaction to this:

On reading it I had one of those familiar, sinking moments of realisation that my brain is so spectacularly inferior to the interviewee’s that all I can do is hold up my hands and say: “Huh?”

At best I might have an inkling of what this means, but I am by no means sure. In this situation, it might take a while to clarify the answer, but other bright minds out there might well be able to unravel it for me and anyone else who might be interested. If you can help, post your thoughts below and put me out of my misery.

To me, it feels as though he is referring to the idea that there are many possible universes and that we can use Darwinian ideas of natural selection to work out which might be most hospitable to life as we know it, and because they are habitable in some sense we value them more highly. That’s my best guess, but I have minimal confidence in it being right.

I will do my best to clarify the answer this week.

My contribution to explaining this is that I see two possibilities:

1. Hawking has signed on to Lee’s Smolin’s ideas about cosmological natural selection.

2. Hawking has realized that once you’ve decided to trade in science for pseudo-science and head down the Multiverse Mania path, there’s no longer any point in worrying about whether what you say makes sense or not, and is behaving appropriately.

Posted in Multiverse Mania | 49 Comments