Not Even Wrong

If you’re interested in particle physics and not regularly reading Tommaso Dorigo’s blog, you should be. His latest posting reports on incendiary claims by Michael Dittmar of the CMS collaboration that recent Tevatron Higgs mass limits are wrong and not to be believed. According to Dittmar, the Tevatron is basically useless for looking for a SM Higgs, with only the future LHC experiments ever having a chance to see anything or produce real limits. You can look at the slides and the blog posting and make up your own mind. From what I can tell, Dittmar doesn’t make a strong enough case to show that the Tevatron results are wrong. It remains true of course that the statistical significance of the limits being set (“95% confidence level”), is right at the edge of what is normally taken as capable of seriously ruling something out.

In the latest New York Review of Books, Freeman Dyson, in context of a review of Frank Wilczek’s The Lightness of Being, engages in his own smackdown of particle physics at colliders. Here’s what Dyson has to say about the LHC, and colliders in general:

Wilczek’s expectation, that the advent of the LHC will bring a Golden Age of particle physics, is widely shared among physicists and widely propagated in the press and television. The public is led to believe that the LHC is the only road to glory. This belief is dangerous because it promises too much. If it should happen that the LHC fails, the public may decide that particle physics is no longer worth supporting. The public needs to hear some bad news and some good news. The bad news is that the LHC may fail. The good news is that if the LHC fails, there are other ways to explore the world of particles and arrive at a Golden Age. The failure of the LHC would be a serious setback, but it would not be the end of particle physics.

There are two reasons to be skeptical about the importance of the LHC, one technical and one historical. The technical weakness of the LHC arises from the nature of the collisions that it studies. These are collisions of protons with protons, and they have the unfortunate habit of being messy. Two protons colliding at the energy of the LHC behave rather like two sandbags, splitting open and strewing sand in all directions. A typical proton–proton collision in the LHC will produce a large spray of secondary particles, and the collisions are occurring at a rate of millions per second. The machine must automatically discard the vast majority of the collisions, so that the small minority that might be scientifically important can be precisely recorded and analyzed. The criteria for discarding events must be written into the software program that controls the handling of information. The software program tells the detectors which collisions to ignore. There is a serious danger that the LHC can discover only things that the programmers of the software expected. The most important discoveries may be things that nobody expected. The most important discoveries may be missed.

He goes on to somehow count Nobel prizes for experimental results in particle physics, with the conclusion:

The results of my survey are then as follows: four discoveries on the energy frontier, four on the rarity frontier, eight on the accuracy frontier. Only a quarter of the discoveries were made on the energy frontier, while half of them were made on the accuracy frontier. For making important discoveries, high accuracy was more useful than high energy. The historical record contradicts the prevailing view that the LHC is the indispensable tool for new discoveries because it has the highest energy.

His argument that proton collider physics is problematic because of the huge backgrounds and difficulty of designing triggers just states the reasons why these are complicated and difficult experiments. Despite the difficulties, they have produced a huge number of new physics results. He doesn’t give the details of how he is counting and categorizing Nobel Prize winning results, so that part of his argument is hard to evaluate.

In opposition to colliders, Dyson wants to make the case for passive detectors, with his main example Raymond Davis’s discovery that the neutrino flux from the sun is 1/3 what it should be. I don’t really see though why he sets up such experiments in opposition to high energy accelerator experiments. Right now many of them actually are accelerator experiments (for example MiniBoone), with an accelerator being used to produce a beam of neutrinos sent to the passive detector. Dyson’s point that if one is very smart and lucky one may get indirect evidence about physics at high energy scales from passive detectors looking at cosmic rays is valid enough, but there is no shortage of people trying to do this, and it is every bit as problematic as working with colliders. There are inherent reasons that such experiments can’t directly investigate the highest energies or shortest distance scales the way a collider experiment can. It’s extremely hard to come up with a plausible scenario in which cosmic ray experiments will give you any information about the big remaining mystery of particle physics, electroweak symmetry breaking.

While I agree with Dyson that the huge sales job to the public about a new golden age of physics coming out of the LHC is a mistake. I don’t see any reason to believe that if it fails cosmic ray experiments are going to get us to a golden age. If and when particle physics reaches a final energy frontier, with higher energies forever inaccessible to direct experiment, hopes for a golden age are going to rest on theory, not experiment, and recent experience with such hopes isn’t very promising.

Update: This Sunday the New York Times will have a profile of Dyson, see here.

Today is the fifth anniversary of the start of this blog, something that has caused me to go back and take a look at some of the early postings, and meditate a bit on what has happened during the past five years.

The first posting was content-free, just an experiment to see if the software worked. The inspiration for starting the blog included the examples of Jacques Distler’s Musings, which had been around for a while, and Sean Carroll’s Preposterous Universe, which he had just started. At the time I had finished writing the book Not Even Wrong, and was in the process of getting it published. The initial idea behind the blog was that it would be a place to comment on and share information with others about topics in math and physics that interested me, including following the on-going story of string theory, which plays a crucial role in the intersection of the two subjects.

A few days later, the first substantive posting was a discussion of a talk by David Gross at CUNY on The Coming Revolutions in Fundamental Physics. Gross had been giving similar talks for several years (you can see a version from 2001 here), and continues to do so to this day (in a few weeks, he’ll be at UC Davis, see here). I don’t see anything I’d want to change in my posting from five years ago, and find this in itself somewhat remarkable. One thing that I’m sure has changed in the more recent versions of the talk is that they don’t include the prediction that 2007-8 will see a headline in the New York Times about the discovery of supersymmetry at the LHC. One feature of many theorist’s talks in recent years has been consistently overly optimistic predictions about when results from the LHC will arrive.

The next posting was an attempt to balance the previous one with something positive and uncontroversial, a discussion of the importance of understanding electroweak symmetry breaking, along with speculation that this might end up having something to do with our still imperfect understanding of chiral gauge symmetry at a non-perturbative level. I found the reaction to this posting truly bizarre, and it gave an inkling of some of the strange chapters to come in what some started to refer to as the “string wars”. Over the years I’d heard from some people that quite a few string theory enthusiasts were convinced that the only possible explanation for skepticism was the ignorance of skeptics. String theory is certainly a remarkably complex and difficult subject, and many skeptics will freely admit to not understanding the subject well, but my own personal experience talking to string theorists was that they were well-aware that there were good reasons for skepticism. Over the years, even many experts who had worked on the subject had come to the conclusion that string theory unification was not as promising as initially hoped, and had moved on to work on different things.

A few months later, Harvard’s recently promoted faculty member Lubos Motl started up his blog, The Reference Frame, which kicked the pathological nature of the discussion of string theory up to a whole new level. By the way, it seems that the main character of one of the most popular shows on US television is based on Lubos, and there’s a campaign to get an Emmy for the actor portraying this character. You really couldn’t make stuff like this up.

Five years later, some things have definitely changed. String theory remains a very powerful political force in the theoretical physics community, but the very public debate over the problems of the subject has taken a huge toll. Perhaps the most accurate indicator of how an academic field is doing in the marketplace of ideas is how many universities are investing in tenure-track appointments in the field. At least in the US, the situation here for string theory is dire. I may be missing someone, but taking a look at the latest information about particle theory tenure-track positions in the US available here, I don’t see any string theorist even making it to the short lists. At least in the US these days, if you want a permanent position in particle theory, you need to be doing something in phenomenology or cosmology. From what I hear, a common situation in physics departments is that the argument for string theory that “let’s wait for the LHC results for vindication” has been taken to heart, with departments figuring that now is not the time to hire in string theory, deciding instead to wait a few years and see if it collapses completely or gets revived by whatever comes out of the LHC.

One sad aspect of all this is that it includes a generalized backlash against the use of sophisticated mathematical ideas in particle theory. Many physicists have drawn the conclusion from the failure of string theory that the problem was too much mathematics, rather than a wrong idea (even string theorists are moving away from mathematics: unlike many years, I see no mathematicians listed as speaking at Strings 2009). Maybe LHC results will point the way forward, but if not, and progress instead requires a deeper mathematical understanding of quantum field theory, the only place for people to get hired working on this will be mathematics, not physics departments, and this is a less than ideal situation for many reasons.

The devolution of string theory unification into pseudo-scientific argumentation about the multiverse is another cause for physics departments to shy away from the subject. This has also been deadly for the public perception of the subject. For this week’s example, see a story in the Boston Globe which compares the scientific status of string theory with that of alchemy:

And at the cutting edge of modern physics, string theory purports to offer a complete but possibly unprovable explanation of the universe based on 11 dimensions and imperceptibly tiny strings.

Alchemists wouldn’t recognize the mathematics behind the theory. But in its grandeur, in its claim to total authority, in its unprovability, they would surely recognize its spirit.

Searching the NSF physics awards database for the strings “multiverse” or “anthropic” turns up nothing, and I suspect that even the proponents of this research are well aware that their colleagues want nothing to do with it. For funding they may have to turn to other sources, including the Templeton Foundation, which recently financed a meeting at a resort in the Cayman Islands which brought together people from the world of business and philanthropy with an array of physicists, including the multiverse crowd. A report on the meeting, with some slides of presentations, is available here.

A somewhat related piece of news is that yesterday the Templeton Foundation announced that Bernard d’Espagnat is the latest winner of its $1.4 million Templeton Prize. d’Espagnat has a long career of serious work on the philosophy and interpretation of quantum mechanics, but what makes him eligible for the prize is having indulged in a certain amount of obscurantism concerning quantum mechanics, coupled with an indulgent attitude towards religion:

Classical physics developed by Isaac Newton believes it can describe the world through laws of nature that it knows or will discover. But quantum physics shows that tiny particles defy this logic and can act in indeterminate ways.

D’Espagnat says this points toward a reality beyond the reach of empirical science. The human intuitions in art, music and spirituality can bring us closer to this ultimate reality, but it is so mysterious we cannot know or even imagine it.

“Mystery is not something negative that has to be eliminated,” he said. “On the contrary, it is one of the constitutive elements of being.”
…
“I believe we ultimately come from a superior entity to which awe and respect is due and which we shouldn’t try to approach by trying to conceptualize too much,” he said. “It’s more a question of feeling.”

I’m looking forward to seeing what happens over the next five years. Surely we’ll finally start seeing results from the LHC and maybe they’ll re-invigorate particle physics. The wide variety of work on mathematics inspired by quantum field theory may also lead to progress of one sort or another. As ever, obscurantism and pseudo-science will find proponents, but I don’t think they’ll make much headway in the scientific community, even with funding from the wealthy. Undoubtedly things will happen that I can’t possibly imagine at this point. I hope that they’re positive things for mathematics and physics, or, at least, entertaining.

It’s becoming clear what the hot new topic in particle theory is these days: the use of twistor space methods to try and understand scattering amplitudes in Yang-Mills and gravity theories, especially the maximally supersymmetric versions. This evening on the arXiv there are two closely related papers on the topic: Scattering Amplitudes and BCFW Recursion in Twistor Space by Lionel Mason and David Skinner, and The S-Matrix in Twistor Space, by Arkani-Hamed and collaborators (there’s also a third, much more distantly related paper, this one). The Arkani-Hamed et al. paper gives an extensive discussion of motivation for this work in the introduction: the structure of scattering amplitudes for these theories is remarkably simple in twistor space, leading to the question of whether one can formulate the full theory directly in twistor space somehow, giving a different sort of holographic dual than the AdS/CFT one.

A very influential version of this idea goes back to Witten’s 2003 paper Perturbative Gauge Theory as a String Theory in Twistor Space, where the dual theory investigated was a topological string theory. This most recent work doesn’t appear to use topological string theory, although Arkani-Hamed et al. are rather cagey on the topic of what sort of twistor space theory is at issue. They promise a forthcoming paper entitled “Holography and the S-matrix”, with:

a completely different picture for computing scattering amplitudes at tree level than given by the BCFW formalism, that we strongly suspect is connected with a maximally holographic description of tree amplitudes that makes all the symmetries of the theory manifest but completely obscures space-time locality.

The history of using twistor-space to study gauge theory goes back a very long ways. Penrose started using twistor techniques to study gravity back in the mid-sixties, and after the 1975 discovery of self-dual solutions to the YM equations (instantons), it became apparent that twistor-space techniques could be used to solve them, turning the problem of solving these non-linear equations into a problem in algebraic geometry, that of constructing certain holomorphic vector bundles. Atiyah was among the mathematicians who got interested in this, and his beautiful 1979 lecture notes Geometry of Yang-Mills Fields remain a wonderful introduction to the mathematical side of the subject. While still a post-doc, Witten worked in this area, coming up in 1978 with an interpretation of the full YM equations using supersymmetry. Work on this topic was what brought Atiyah and other mathematicians into contact with physicists, including Witten, beginning a quite remarkable period of very successful interaction between the two camps.

Twistors were among the things I started thinking about at the end of my graduate school days in the mid-eighties, for a completely different reason, one that has nothing to do with the current interest in the subject. I was interested in the problem of how to put spinors on a lattice, and the twistor geometry story gives a beautiful way of thinking geometrically about spinors. This idea never really got anywhere, although I did notice some relations between the geometry of the standard model gauge groups and representations that I wrote about back in 1987 and still find quite remarkable. It will be interesting to see what new ideas emerge from this latest wave of interest in thinking about quantum field theory in twistor-space.

LHC media fever continues this year, with at least three books out or on the way:

The Quantum Frontier: The Large Hadron Collider by Fermilab experimentalist Don Lincoln.

Collider: The Search for the Worlds Smallest Particles by Paul Halpern.

and

The Large Hadron Collider by Lyn Evans, who knows a thing or two about the subject.

There’s also a documentary entitled Particle Fever being made about the LHC, produced by theorist David Kaplan, who “has discovered some of the most recognizable extensions to the standard model of elementary particles.” The film web-site has bios for five physicists who will feature prominently in the film: three theorists well-known for their work on large extra-dimensional models, one experimentalist from CMS, and one from ATLAS. The ATLAS experimentalist is described as “a leader in the search for extra dimensions.” I can’t find anything about the Higgs on the web-site, maybe they’ve already given up on that and left it to the Tevatron…

The descriptions of the theorists include “responsible for some of the wildest theories about the nature of gravity, cosmology and fundamental particles”, “a leader in the fields of supersymmetry, extra dimensions and new forces… has become a controversial figure by questioning experimentalists’ traditional methods of analyzing the data” and “the most likely to win a Nobel Prize after the LHC data is interpreted.” The experimentalist description is rather more modest (“has been involved in detector R&D and construction, software development and physics data analysis”), nothing about any possible Nobel prizes. If you had to choose whether to be a theorist or an experimentalist, the choice looks easy.

Steven Weinberg has a new preprint out entitled Living with Infinities, which is the written version of a recent talk given in memory of Gunnar Källén. Källén was a Swedish mathematical physicist, who died in a plane accident in 1968 at the age of 42. For more about him, see this by Ray Streater.

Weinberg begins by recalling his first first trip to the Bohr Institute in 1954, where he met Källén, who suggested a research problem involving an exactly solvable QFT model invented by TD Lee. The solvability of this model made it possible to use it for investigating renormalization outside of perturbation theory. Källén and Pauli showed that the model was non-unitary, Weinberg showed that it had states with complex energies.

In his talk, Weinberg describes Källén’s work during the 1950s investigating the question of how QED gets renormalized, outside the context of perturbation theory. Källén found an argument showing that at least one of the renormalization constants must be infinite but Weinberg notes that Källén never claimed the argument was rigorous, and concludes: “As far as I know, this issue has never been settled.” He goes on to give the now conventional Wilsonian description of the non-perturbative situation and the possibility that no non-trivial continuum limit exists. This question is now considered somewhat academic, since it is assumed that QED gets unified with other interactions and ultimately with gravity at energies below those at which the behavior of the coupling becomes problematic.

Weinberg states in his abstract that he will present his personal view on how the problem of infinities may ultimately be resolved. Here’s what he has to say about this:

My own view is that all of the successful field theories of which we are so proud — electrodynamics, the electroweak theory, quantum chromodynamics, and even General Relativity — are in truth effective field theories, only with a much larger characteristic energy, something like the Planck energy….

None of the renormalizable versions of these theories really describes nature at very high energy, where the non-renormalizable terms in the theory are not suppressed. From this point of view, the fact that General Relativity is not renormalizable in the Dyson sense is no more (or less) of a fundamental problem than the fact that non-renormalizable terms are present along with the usual renormalizable terms of the Standard Model. All of these theories lose their predictive power at a sufficiently high energy. The challenge for the future is to find the final underlying theory, to which the effective field theories of the standard model and General Relativity are low-energy approximations.
It is possible and perhaps likely that the ingredients of the underlying theory are not the quark and lepton and gauge boson fields of the Standard Model, but something quite different, such as a string theory. After all, as it has turned out, the ingredients of our modern theory of strong interactions are not the nucleon and pion fields of Källén’s time, but quark and gluon fields, with an effective field theory of nucleon and pion fields useful only as a low-energy approximation.
But there is another possibility. The underlying theory may be an ordinary quantum field theory, including fields for gravitation and the ingredients of the Standard Model…

He then goes on to describe the “asymptotic safety” scenario where the renormalized couplings approach a non-trivial fixed point as the energy cut-off is taken to infinity, a fixed point which presumably cannot be studied in perturbation theory, writing:

Other techniques such as dimensional continuation, 1/N expansions, and lattice quantization have provided increasing evidence that gravitation may be part of an asymptotically safe theory.

and referring to papers by Reuter/Saueressig, Percacci, and Litim (Percacci has a web-page about asymptotic safety here). He ends with the conclusion that, since string theory might not have any role in a fundamental theory, with only QFT needed to understand quantum gravity:

So it is just possible that we may be closer to the final underlying theory than is usually thought.

In these days of string landscape ideology, this possibility is an important one to keep in mind.

Lots of wonderful blog postings about math and physics out there worth reading, with a small sample including these:

From CERN, here’s a report from JoAnne Hewett about the summary session discussing the Chamonix workshop on the state of the LHC and plans for getting it up and running. The current schedule, which is tight, has collisions starting in November of this year, and running for nearly a year until late 2010. To accumulate an amount of data that would allow some significant new results (about 50pb^-1) should take six months or so, until mid-2010. The hope is to get to 2-300 pb^-1 later in 2010 before shutting down. This would not allow the LHC to do better than the Tevatron on the Higgs. For that, we’ll probably have to wait for data from the 2011 run. By the time this data is in, the Tevatron should have about 10fb^-1 to analyze, and may already have seen evidence of the Higgs.

Also at CERN, there has recently been a conference on the topic From the LHC to a future collider. Lots of interesting talks about future possiblities, including the ILC, CLIC, colliding LHC protons with electrons, as well as the possibility of a muon collider.

More and more areas of mathematics have a blog, here’s one for motivic homotopy.

Bert Schroer has updated two of his long articles that discuss both the sociology and conceptual framework of quantum string theory: String theory deconstructed, is dedicated to Philip Anderson and has a new section about history of the subject in Germany, and String theory and the crisis of particle physics, which is dedicated to Juergen Ehlers.

I hadn’t realized that Physics World has a blog. Among the latest entries are two reports (here and here) about Lenny Susskind’s recent talk to 700 people in Bristol about Darwin and the Cosmic Landscape. Susskind is still at it selling string theory and the multiverse to the public, no matter how unconvinced his colleagues may be:

Just as there is a vast landscape of biological designs, our best theories of physics imply an equally vast landscape of universe designs. String theory provides an analogue of DNA for the universe and modern cosmology makes use of a principle of mutation that creates a tremendously large multiverse.

It seems that

The central tenet of Susskind’s talk was that string theorists should look to Darwin because he “set the standard for what an explanation should be like”.

Funny, I always thought it was physics itself which set such a standard for the biological sciences, but I guess the idea now is to give up on that and have them be the gold standard.

While many theoretical physicists in their later years try and go for the Einstein look, according to one of the Physics World bloggers, Susskind is doing a good job of looking like Darwin.