A Brief History of String Theory

Posted on March 26, 2014 by woit

On the long plane flight to Italy I had the chance to read the recently published A Brief History of String Theory: From Dual Models to M-theory by philosopher of science Dean Rickles. The book deals with the history of string theory, beginning with its origins in the Veneziano model of strong interactions, and ending in the mid-90s with M-theory and the “Second Superstring Revolution”. It’s a good serious scientific history, explaining in technical detail exactly how the theory developed, with good explanations of the high points of crucial papers, together with some of the story of how they came about. While I’ve spent a lot of time in the past reading about much of this history, I learned a lot from the book, about string theory as well as other topics in particle physics that interacted with it. I’m strongly of the opinion that if you want to really understand a subject, you need to understand its history, so anyone who wants to really master string theory would do well to spend some time with this book.

There is something quite unusual about this though as a work of history, since while this subject is 45 years old, it is quite unclear how to evaluate its significance as science (arguments seem to still rage about this…). I can’t think of any other topic in modern science which has been the subject of such intense activity, with no one sure of how to evaluate it nearly a half century later. More succinctly, is this the history of a brilliant insight into the physical world or is it the history of a misguided failure? In the introduction Rickles worries that historians of science will find it too “Whiggish”, but maybe more of a problem is not knowing what the right final end-point will be. To a large degree Rickles adopts the point of view of many prominent string theorists, that this is a success story, whatever its problems might seem to be. My own point of view is different of course, and I’d claim that in recent years the viewpoint of the physics community as a whole has shifted, with this looking less and less like a success story and more and more like something else.

I can’t do justice to all that’s in the book, but for personal reasons I do want to focus on one part of the story and how Rickles treats it, one where I have a significant disagreement with him, and one that points out well the basic problem faced by this kind of history. The issue is the 1984 “First Superstring Revolution”, generally dated to the Green-Schwarz anomaly cancellation calculation of that summer. Rickles does a good job of explaining the background of this. He emphasizes that this didn’t come out of nowhere, that the issue of the problems posed by such chiral anomalies had been identified by Witten and others as of great importance in constructing unified theories.

What should one make of the significance of the discovery by Green and Schwarz of anomaly cancellation for SO(32) in type I string theory? The story of string theory as a success is that this convinced theorists that string theory was a very promising road to unification and unleashed a revolution. But I remember this differently (I had just finished by Ph.D. at Princeton and taken up a postdoc at Stony Brook). The idea that anomaly cancellation predicted a specific gauge group and dimension was obviously attractive, but the fact that the prediction was for the wrong gauge group (SO(32)) and the wrong dimension (10 space-time dimensions) looked to me (and many others) like a deadly problem. The flurry of activity leading to the heterotic string, E8, and Calabi-Yau compactifications was an impressive use of mathematical technology, but there was no sign of the Standard Model coming out of this in any natural way. It looked all too likely that this wasn’t explaining anything about particle physics, just parametrizing the choices of possible unified gauge theories in a very complicated way. Yes, there was also a theory of gravity there, but it was not obviously an attractive one.

Earlier today I was watching the video of John Schwarz’s general talk about string theory at the Simons Center yesterday. Schwarz gives much the same promotional talk he and other have given many times over the last 20 years (with little change since the addition of M-theory), making claims for success of exactly the sort that inform the point of view of the Rickles book. At the end of his talk, Dusa McDuff asked about parity violation in other parts of M-theory and Schwarz explained that the original 1984 motivation from Type I anomaly cancellation has long been abandoned:

Nowadays we have enough tricks up our sleeve that we can get parity violation out of anything.

There are now all sorts of ways of getting “string vacua” that might give a unified theory, many not using anomaly cancellation. The supposed breakthrough of 1984 is looking much more like a red herring, and the question of historical interest shifts from “how was this brilliant breakthrough accomplished?” towards “why did so many people not realize this obviously wasn’t going to work?”

Rickles on page 162 explicitly takes issue with the comments in my book emphasizing the important influence of Witten at this point, mischaracterizing me (and Smolin) as claiming it was “almost as if that community [theorists] had no decision-making power of its own”. This is far from any thing I think or wrote (a big section of my book explained the excellent reasons why any sensible person would take Witten’s opinions seriously). What I wrote was

By itself the news that gauge anomalies cancel in a version of type I superstring theory would probably not have had so dramatic an effect on the particle theory community, but the news that Witten was now devoting all his attention to this idea spread among theorists very quickly.

and I still think that’s quite accurate. Ten years ago I wrote about this in detail on the blog (see here, here and here), including a first-hand version of the story from Larry Yaffe, who was at Aspen and was the one who told Witten about the Green-Schwarz result. He reports:

Concerning reaction to the Green-Schwarz result, my recollection is that there was relatively little immediate buzz about it at Aspen. John had a fairly diffident style of presentation, and I don’t recall anyone jumping up and saying ‘this will change the course of physics!’. As best as I can reconstruct my own reaction, it seemed like a technically slick calculation and a nice result but it wasn’t, of course, addressing any of the conceptually hard questions about quantum gravity, and it seemed very far removed from the practical concerns of particle physics.

and

I think the speed with which others in the particle theory community jumped into string theory had a lot to do with Ed’s involvement and proselytizing, but I expect that even without his involvement, interest in string theory would have steadily grown, albeit slower.

Rickles ends his detailed history with M-theory, with the latter part of the book summarizing the recent history, again pretty much from the point of view of a string theory proponent, one on the defensive. I think he gets the multiverse issue quite wrong, characterizing the anthropic multiverse vs. search for a unique unified theory dichotomy among string theorists as:

It is more likely that the two stances will continue in parallel, as they appear to have done for some time, defined more by the personalities of those adopting them than by the physics.

I don’t think this has anything to do with personalities. The problem is that the anthropic multiverse point of view predicts nothing, and those unhappy with it are not unhappy because they have a personality that leads them to want or believe in uniqueness, but because they’re aware you need to make predictions to be doing science. But, one can’t expect historians to get right current events…

Posted in Book Reviews | 11 Comments

B-mode News

Posted on March 25, 2014 by woit

While I was away on vacation, the big news in physics was the BICEP2 result on B-modes in the CMB. Maybe it’s just as well I wasn’t available to blog about this, since inflation and cosmology aren’t at all my field of expertise. Now that some of the dust has settled from the media blitz though, I do think it’s worth while to write something here, since there are some aspects of the story where the media coverage could use some extra perspective.

First of all, there’s the obligatory caveat about this result not being definitive, which most coverage by scientists has included. To my non-expert eye, looking at the main graph reproduced everywhere, if you subtract the gravitational lensing background you get something which is much larger at higher values of l than it is supposed to be if it were coming from primordial gravitational waves. But, I’m the wrong person to be evaluating this, you should read what the experts have to say, with some examination of the issue from Peter Coles here and Sesh Nadathur here and here. The great thing about this is that all you should need is a little bit of patience to see it resolved, with data coming in from Planck later this year and sooner or later from BICEP3 and other experiments as well. If this is a red-herring, we should know that within a relatively short time-frame.

Assuming that there really is a primodial gravitational wave signal, this is something that has long been predicted by inflationary models, so is a significant extra piece of evidence for some sort of inflationary scenario. I’m not the right person to try and explain the details of this, or even to point you to the best review articles, but some things you might want to look at are John Preskill’s derivation of the prediction here, and many people are pointing to Daniel Baumann’s lectures here. On the plane back from Italy I was looking for other reasons at an excellent introductory QFT textbook by Alvarez-Gaume and Vazquez-Mozo, which turns out to have a section (6.5) devoted to this calculation.

For the implications of this kind of confirmation of inflation, one obvious question is what it means for string theory. The standard argument from string theorists is that its testability problems arise because we can only do relatively low energy experiments, that at high enough energies, such as those of the very early universe, it would be testable (not true, since string theory is capable of giving you pretty much anything you want at any energy). The press coverage of what BICEP2 means for string theory is pretty comical [to be clear, the comedy is provided not by journalists, but by physicists and a “theory” that can explain anything], with Nature telling us:

The BICEP2 results will also send some string theorists back to the drawing board, says Frank Wilczek, a theoretical physicist and Nobel laureate at MIT. String theory posits that elementary particles are made of tiny vibrating loops of energy. Efforts to combine string theory with cosmology have led to inflationary models that generate gravitational waves with energies much lower than the level detected by BICEP2, he says.

Theoretical physicist Eva Silverstein of Stanford says she disagrees that string theory-based models of inflation are in any sort of trouble. “There is no sense in which we are forced to start over,” she says. She adds that in fact a separate class of theories that involve both axions and strings now look promising.

Linde agrees. “There is no need to discard string theory, it is just a normal process of learning which versions of the theory are better,” he says.

New Scientist has a string theorist making the usual claim that finally, string theory is testable, showing up those bloggers who say it isn’t. The idea is that the BICEP2 results don’t confirm inflation, but something completely different:

Picture the cosmos as a rolled-up piece of paper held in place with rubber bands, says Robert Brandenberger at McGill University in Montreal, Canada, who was part of a team that came up with the model in 1989.

The paper is a nine-dimensional universe, and the rubber bands are vibrating strings. If two strings meet, their edges can form a single, twisted loop. That would release three dimensions of space and one of time, which can then swell to the scales we see in the universe today. This process can account for the tiny density variations seen in the CMB and strong gravitational waves – no inflation required.

The BICEP2 results slightly favour this model. If Planck sees the same signal, it could be the first observational evidence for string theory. “For string theorists this is very important,” says Brandenberger. “Opponents can no longer say string theory does not connect with data.”

While Brandenberger argues that string theory is testable because it predicts inflation is wrong, Science has Scott Dodelson arguing that string theory is testable because it predicts various versions of inflation:

Moreover, Dodelson says, theories of quantum gravity, such as string theory, predict modifications to the shape of the inflaton energy landscape. So if that landscape can be measured precisely, he suggests, physicists might finally put string theory—long mocked as an untestable “theory of anything”—to a concrete test.

Then of course there’s Michio Kaku who at NBC News explains:

“Inflation simply says there was a bang, and it expanded rapidly, but it doesn’t say what the fuse was,” Kaku said. “Nobody can say they know what the fuse is.”

Kaku, a string theorist, says that string theory could provide the answer … or answers. The cosmic parameters for string theory suggest that the number of possible universes could amount to around 10 to the 500th power. That’s a 1 with 500 zeroes after it. Such a scenario offers so many possibilities for parallel universes that in some of them, “Elvis Presley is still alive,” Kaku joked.

Besides watching the string cosmology clown-show, I’ve not followed at all closely the huge amount of work done by theorists in recent decades on various ways to get inflationary scenarios, so don’t have anything well-informed to say about how the BICEP2 results will affect this area. One thing to watch will be a conference next week at Perimeter (thanks to a commenter here for pointing this out).

For some background on why I haven’t paid much attention to this, I should explain some history. Back in 1980 when Alan Guth’s work on inflation first came out, I was a graduate student and did pay close attention to what was going on. The arguments from Guth and others for inflation as an explanation for several otherwise hard to understand aspects of cosmology (the horizon problem, flatness problem) were (and are) compelling. Even better, the idea motivating Guth at the time was that the fields responsible for inflation would be those that broke the GUT symmetries, so grand unified particle physics models would explain aspects of cosmology, and cosmological observations might tell us more about GUTs. All in all, this was a very attractive idea.

Over the years though, no evidence for GUTs emerged and it became clear that GUTs didn’t actually provide very much in the way of explanatory power about the Standard Model. Lots of work was done on inflationary models, but these models just typically invoked a single conjectural scalar field (the “inflaton”), with its relationship to anything known in particle physics a mystery. Earlier CMB data gave some hints of further evidence for inflation, and now the BICEP2 data provides yet more significant evidence, so there’s lots of reasons to take seriously the idea of inflationary scenarios. The models getting some confirmation though seem to be very simple ones, with a single inflaton field and a very simple potential. This is great news for the general idea of inflation, but still leaves the whole subject with pretty much no convincing explanation of anything about particle physics, and with a minimal connection to quantum gravity (although one intriguing new BICEP2 paper I did notice was this one).

The sad thing about this whole subject though is how some people involved in it have reacted to its problems making connection with particle physics, by throwing in their lot with the multiverse as an explanation for the failure of string theory. The multiverse functions here as an all-purpose excuse for not being able to explain anything about particle physics, with the argument being made that particle physics is fundamentally something just random and inexplicable, different at different points of the multiverse.

The standard move of the people doing this is to point to the fact that in the simple models getting some confirmation, “eternal inflation” can give you lots of copies of our universe, all with the same physics. This is advertised as “evidence for the multiverse”, with no mention of the fact that, to the extent this is true, it’s evidence for what Tegmark calls a “Type I multiverse” (all the same physics), not a “Type II multiverse” (different physics in different universes, making our physics unpredictable). Several physicists in recent years have been engaged in a vigorous publicity campaign based on confusing this issue, and the BICEP2 results found them hard at work. There’s Max Tegmark here and here, Sean Carroll at the New York Times, and Andrei Linde and Alan Guth everywhere (see for example here, here and here).

Luckily not all of the press coverage is dominated by this, with the better science journalists doing a good job of ignoring it and focusing on the real story (a good example is Dennis Overbye here and here).

For some other press coverage of the “BICEP2 implies Multiverse” story, there’s Fox News, which has Dr. William Lane Craig explaining how this is proof the scriptures are true. Claims are also being made by The Bosnian Royal Family for having priority over Andrei Linde in this proof of the Multiverse.

Update: On Twitter, Peter Coles comments that “Perhaps there is a part of the multiverse in which the #BICEP2 results provide evidence for a multiverse, but I don’t think we live there.”

Posted in Multiverse Mania | 48 Comments

Rainy Day Posting

Posted on March 23, 2014 by woit

My spring break vacation is not quite over but, after 10 days of spectacularly beautiful weather, it’s now raining hard here today and I’ve got some time indoors to write something. First some quick links to things I’ve seen in my short periods of recent internet access (leaving the BICEP2 story for after I get back Tuesday …):

  • I don’t often link to things at Tommaso Dorigo’s blog, since my advice is that you should just always follow it since it’s the best HEP experiment blog to be found. His latest has news of an impressive CMS limit on the Higgs width, something that I had never realized could be done. This should get a lot more attention than it has gotten; it’s a great example of experimental cleverness, getting at a seemingly impossible measurement in an indirect way.

    And, seriously, I’m not just saying this since Tommaso recently showed me around Venice…

  • For another, very different, blog you should be following, there’s my friend Mathbabe, who has a simultaneously amusing and disturbing take on Princeton, which addresses the question of why it produces graduates like this one.

    From what I can tell, Princeton seems to be little changed since the time I spent there more than thirty years ago, and at the time it seemed devoted to staying much like the place of thirty years before that. Something that hasn’t changed is the vanishingly small number of women, with even fewer at the IAS on the other side of the golf course (and if you want to argue about why that is, please do it somewhere else).

    One thing I did enjoy about Princeton was getting to know some of my fellow students. In other HEP news, one of them, Jon Bagger, has just been appointed director at TRIUMF.

  • The recent HEPAP meeting seems to have had some unusual activity from the DOE in response to Laurence Yaffe’s recent complaints about large cuts to theory grants. This included a presentation specifically about HEP Theory funding, but reading it I still don’t see the explanation for why, as Yaffe claimed, cuts in theory group funding seem to be much more widespread than in other areas (see page 9 of this presentation).

    The DOE/HEP presentation had a specific warning against discussion on blogs of funding problems, I’d guess specifically aimed at Yaffe:

    Intense discussion in the community around the sociological issues can easily be mistaken by decision makers as disputes over the P5 plan, so please be careful to frame discussion points properly, especially when discussing issues we face with others outside the field.
    – Blogging, posting on public websites are a de facto public conversation
    …
    ‘Bickering scientists get nothing’

  • Scott Aaronson has a review of Max Tegmark’s Our Mathematical Universe, which argues that the main claim the book is designed to promote is empty, but everyone should read it:

    I think everyone interested in math, science, or philosophy should buy the book and read it. And I still think the MUH is basically devoid of content, as it stands.

Posted in Uncategorized | 15 Comments

Spring Break

Posted on March 11, 2014 by woit

I’m leaving for a spring break vacation in Northern Italy tomorrow afternoon, and will shut down comments while I’m away. Back in two weeks. Some events that will occur while I’m away that might be of interest:

  • The 2014 Templeton Prize will be awarded about the time I land in Milan.
  • There will be a HEPAP meeting this week, likely to have discussion of US HEP funding issues discussed here recently.
  • Next week will be the 10th anniversary of the blog. So far 1350 postings, 37,540 comments posted (and about 250,000 spam comments…). When I get back maybe I’ll write something to mark the occasion.
Posted in Uncategorized | Comments Off on Spring Break

Multiverse Mania at Wikipedia

Posted on March 9, 2014 by woit

The quality of Wikipedia entries about mathematics is often quite good, but unfortunately the same cannot be said for their entries about physics. I happened to take a look today at the Wikipedia entry for Multiverse, which is an outrageously one-sided promotional piece for pseudo-science.

It’s hard to know where to start with a document like this, and I’ve neither the time nor the Wikipedia expertise to start trying to edit it to something sensible (at this point I’d suggest that the most sensible edit would be to remove the whole thing).

I include just a couple of random examples of problems with the entry. The “criticism” section has little actual criticism, just some mild comments from Ellis and Davies, together with positive quotes from them about the multiverse as a research program. Nothing from Gross or Steinhardt, for instance. Much of the “criticism” section is actually defense of the multiverse through claims about experimental evidence from Mersini-Houghton that I don’t think anyone except her takes seriously. Other claims of experimental evidence are completely outrageous, for instance we read that “Recent research has indicated the possibility of the gravitational pull of other universes on ours.[22]” where reference [22] is to a Planck collaboration paper which states the exact opposite (“There is no detection of bulk flow”).

There’s a good case to be made that I pay too much attention to popular media nonsense about the multiverse. Unfortunately Wikipedia is taken a lot more seriously by the public than magazine stories. At this very moment, hundreds of high school students may be copying material out of it for their assigments…

Update: Some people have written to tell me about the appearance of the multiverse in the new Cosmos program that started last night. I saw just 20 minutes of the end of the program, missed that part. Presumably Tyson will deal with this in more detail in a later episode, so I’ll wait to write more about this then.

Posted in Multiverse Mania | 43 Comments

Various and Sundry

Posted on March 5, 2014 by woit
  • It seems to be too early for April Fool’s day, and yet the arXiv has Dark Matter as a Trigger for Periodic Comet Impacts by Lisa Randall and Matt Reece, a preprint described as “Accepted by Physical Review Letters, 4 figures, no dinosaurs.” The Register has a story: Dark matter killed the dinosaurs, boffins suggest.

    Also recently at the arXiv in a similar “too early for April 1” category is Crossing Stocks and the Positive Grassmannian I: The Geometry behind Stock Market, which deals with the “stockmarkethedron”, also known as the Geometrical Jewel at the Heart of Finance.

  • The president’s FY2015 budget request is out, with news for HEP not so good: a 6.6% cut proposed in DOE HEP funding. No details about the NSF budget, but the proposal is basically for flat funding (an overall cut of .03% in the research budget). The NSF is proposing one big increase, 13.5% for management. This is just an initial proposal from the administration, with the possibility of something different ultimately emerging from Congress.
  • The particle physics documentary Particle Fever opens here in New York at Film Forum tonight, with appearances tonight and this weekend by the director and “physicists from the film”. There’s a review in today’s New York Times.

    I saw the film last fall at the New York Film festival and wrote about it here, with the summary:

    most of it I thought was fantastically good and I really hope it finds distribution and gets widely seen. On the other hand, some of it I thought was a really bad idea.

    The film is a very inspiring inside look at the LHC experimental search for and discovery of the Higgs. My misgivings were about the theoretical framing of the story, which was the Arkani-Hamed point of view that this is all about two alternatives: SUSY or the multiverse. The NYT review shows that these misgivings were quite justified, with the reviewer’s summary of what they learned about the significance of the Higgs from the film:

    While the discovery of the Higgs may not have immediate consequences for the way we live, or applications in the world of technology and industry, its implications, according to “Particle Fever,” could hardly be more profound. Through most of the film, the scientists are awaiting a specific bit of data, a single number that will either vindicate a theory of the universe known as supersymmetry or suggest the possibility of multiple universes.

    The differences between these two outcomes seem very stark. In the first case, more particles are likely to be found, contributing to a detailed and orderly picture of the nature of things. In the second, the Standard Model will be thrown into chaos, and the stability of the universe itself may be called into question. It won’t be the end of the world, but for some theorists, it will feel that way.

    Mr. Kaplan is hoping for supersymmetry. His friend and sometime table tennis partner, Nima Arkani-Hamed of the Institute for Advanced Study in Princeton, is in the multiverse camp.

    Physicists often get outraged when they feel journalists badly misrepresent science to the public. Will they get equally outraged when it is physicists doing the misrepresenting?

  • For some insight into the current concerns of particle theorists, you can watch some of the videos at last week’s KITP conference. In particular, there’s Matt Strassler’s talk, where he got all Peter Woit and argued that “one could make the argument” that not seeing SUSY (or anything else stringy) at the LHC “would be significant circumstantial evidence against string theory as a description of nature” and that just seeing the SM at the LHC would be “circumstantial evidence against effective quantum field theory as a complete description of known particle physics”. This got him an argument from Gross about his insufficient enthusiasm for a 100 TeV collider. Gross then also got all Peter Woit, arguing that the failure of the “naturalness” argument for new physics was no big deal since it wasn’t a very good argument to begin with (I get all sorts of grief when I do this..).
    The conference ended with a session of people trying to predict the future of the field 30 years hence. This was mostly pretty discouraging, with a lot of people envisioning more of the same: endless generalities about quantum gravity, firewalls etc. Prominent by its absence was any role of mathematics in theoretical physics, with only Greg Moore speaking up for the question of the significance of now popular 6d superconformal theories, and Nati Seiberg mentioning that connections of the field to mathematics were a good thing.

    Lots of talks mentioned people’s good experiences working with and interacting with Polchinski, who seems to be a very nice guy. I’ve never met him personally, but people have speculated to me that he had something to do with the decision of the arXiv to block links to my blog (he was unhappy about my characterization of his Scientific American article promoting the multiverse). What the truth is about that particular story I suppose I’ll never know.


Update: Another review of Particle Fever leads with this explanation of the main point they got from the film:

Stakes come no higher than in Particle Fever, a dazzling, dizzying documentary about nothing less than whether we exist in a coherent universe of ordered, even beautiful laws — or whether, as Princeton physicist Nima Arkani-Hamed theorizes, our universe is one of an infinite set of other universes defined by a chaotic mash-up of unstable, inexplicable, random conditions.

Update: Reddit has a live Q and A with physicists involved in the film. Savas Dimopoulos (described as “considered the most likely to have a theory confirmed by the LHC”) argues for the multiverse and tells questioners that “We may know about whether Nature prefers the Multiverse or the more traditional (super)symmetry path after the second run of the LHC which will start in a year.” Arkani-Hamed also gives the multiverse argument, also claiming “I envy anyone who is jumping into fundamental physics as a grad student today!”. No theorists in sight who might think there’s more significance to the negative LHC results about SUSY than “must be the multiverse”.

Update: Reddit the next day hosted a live Q and A with Michio Kaku. He there explains to the public that:

The best theory comes from string theory, which states that dark matter is nothing but a higher vibration of the string. We are, in some sense, the lowest octave of a vibrating string. The next octave is dark matter….

The next big accelerator might be the ILC in Japan, a linear collider which might be able to probe the boundaries of string theory…

In the coming decades, I hope we find evidence of dark matter in the lab and in outer space. This would go a long way to proving the correctness of string theory, which is what I do for a living. That is my day job. So string theory is a potentially experimentally verifiable theory.

Seems that well-known theorists going on Reddit to mislead the public is now a daily phenomenon…

Posted in Multiverse Mania, Uncategorized | 25 Comments

All Hail Resonaances

Posted on February 26, 2014 by woit

I’m delighted to see Jester back in action, providing great material on the current state of HEP physics, with, over the past week and a half:

  • A sober look at the sparse prospects for near-term (i.e. 2014) input from experiment, with the Planck CMB polarization results one of the few things for which there are significant expectations.
  • An equally sober look at the problem of making the case for a 100 TeV collider, given that this will be the first time people don’t have a “no-lose theorem” showing that something new has to turn up in the new energy range being explored (of course the argument that it’s an unexplored new energy range remains an excellent one by itself for the exploration). About the arguments Arkani-Hamed is making, Jester has:

    Nima’s idea that we need a 100 TeV collider to prove that SUSY fine-tuning is larger than 0.01% is good. As a joke to relax the atmosphere. Certainly, the case for the new collider can be made stronger than that.Some ideas that are being bandied around are precision Higgs physics, double Higgs production, rare Higgs and top decays, non-perturbative electroweak effects, or WW scattering. These topics can be made more concrete and several more items can be added to the list.

  • To give us all some hope, he has some news about a possible astrophysical X-ray spectrum signal that could conceivably be evidence for a sterile neutrino dark matter candidate. Right-handed neutrino fields fit naturally into the SM pattern of fundamental fields, but with zero SU(3)xSU(2)xU(1) charges. That such fields have something to do with dark matter looks more promising than the SUSY or axion proposals of introducing a new and different sector of fields. My knowledge of neutrino physics isn’t what it should be, so I’d be curious to hear of good references about the sterile neutrino dark matter issue.

Update: For some idea of the case being made for a larger collider, one might want to take a look at talks in Beijing a few days ago, where there’s a proposal for the Chinese to build it. Talks at a conference are here, and last Sunday there was a big event featuring Yau, Gross, Witten, Arkani-Hamed, ‘t Hooft, Maiani and Incandela, video here. On the whole people seem to be pretty much sticking to making the generic case for high energy, not promising superpartners or extra dimensions this time.

Posted in Uncategorized | 19 Comments

Quantum Mechanics, The Theoretical Minimum

Posted on February 25, 2014 by woit

In recent years Leonard Susskind has been giving an excellent series of lectures on basic ideas of theoretical physics, under the title The Theoretical Minimum. The general idea seems to be to provide something in between the usual sort of popular book about physics (which avoids equations and tries to give “intutitive” explanations in ordinary language) and conventional undergraduate-level textbooks. Such textbooks generally assume college-level multi-variable calculus, differential equations and linear algebra, and often skip lots of detail and motivation, assuming that the book is a supplement to a standard course of lectures.

For Susskind’s lectures, you mostly just need high-school level mathematics, up to some some basic differential calculus, as well as two by two matrices. Actually though, if you’ve never seen matrices and very simple linear algebra, this is a good place to learn some basics examples of this subject.

A year ago the first book version of some of the lectures appeared as The Theoretical Minimum, with George Hrabovsky writing up Susskind’s lectures on classical mechanics. I wrote a little bit about the book here, and was quite impressed by the way it managed to give the details of the formalism of Hamiltonian mechanics, while sticking to as simple and concrete mathematics and calculational tools as possible.

Today is publication day for the next volume, Quantum Mechanics: The Theoretical Minimum, which is a joint effort this time with Art Friedman. It’s even better than the first volume, taking on a much more difficult subject. About the first two-thirds of the book sticks to the simplest possible quantum system, one with a two-dimensional state space. The linear algebra needed is developed from scratch and Susskind works out at a very leisurely pace all the details of what the quantum picture of reality looks like in this simplest context. There’s a lot about what “entanglement” really is, and this part ends up with an introduction to Bell’s theorem.

The last third of the book is a quicker-paced trip through the usual material about wave-functions and the Schrödinger equation, ending up with the details for the harmonic oscillator potential.

“The Theoretical Minimum” phrase is a reference to Landau, but it’s a good characterization of this book and the lectures in general. Susskind does a good job of boiling these subjects down to their core ideas and examples, and giving a careful exposition of these in as simple terms as possible. If you’ve gotten a taste for physics from popular books, this is a great place to start learning what the subject is really about.

I only noticed one mistake in the book, on its back cover, where one of the blurbs is attributed to a Professor of Mathematics at Columbia, when I know for a fact that his actual title there is “Senior Lecturer”. Susskind does have a bit of history of getting this point wrong, but probably the fault here lies with the publisher.

Update: Nature has a review here.

Posted in Book Reviews | 20 Comments

More Quick Links

Posted on February 24, 2014 by woit

First, a couple of examples of recent progress in mathematics

  • Terry Tao has some new ideas about the Navier-Stokes equation. See his blog here, a paper here, and a story by Erica Klarreich at Quanta here.
  • I’ve been hoping to find more time to learn enough to write something intelligible about a major new advance: Peter Scholze’s recent work on the p-adic geometry of Shimura varieties and results linking torsion classes and Galois representations. I’m still far from being up to that task, but Scholze’s Marston Morse lectures at the IAS are a good place to start (see here, here and here). Last week MSRI hosted a very successful week-long “Hot Topics” program on this, see here.
  • For more IAS talks, see “Cross-disciplinary” talks last week by Witten, Seiberg and Maldacena.
  • Nature has a story about a recent discovery by Cormac O’Raifertaigh and collaborators of an unpublished manuscript by Einstein containing a “steady-state” cosmological model.
  • A computer scientist has identified more than 120 papers published in supposedly peer-reviewed conference proceedings that were all randomly generated gibberish produced by the program SCIgen. No, these didn’t appear in bogus “open access” publications, but in subscription publications from Spring and the IEEE. What’s going on is described as a “spamming war at the heart of science”.

    • Update: One more, for those of you not getting enough multiverse. Today’s Washington post has an op-ed from Bush speech-writer Michael Gerson (at one point the ninth most influential evangelical Christian in the US, if you believe Wikipedia and Time). The title is Physics is Enjoying a Golden Age (also available here). Gerson thinks physics is in a Golden Age because he has just read Tegmark’s book and is very excited that physics has now become metaphysics, with room for God again:

    The point here is not that Tegmark’s theories are broadly accepted, only that such theories are no longer considered absurd. Physics has seen the return of the unseen — parallel universes, infinitesimal strings, floating and colliding branes — that are reasonably inferred without being physically observed. I can think of other creative forces in that category. Not for centuries has physics been so open to metaphysics, or more amenable to an ancient attitude: a sense of wonder about things above and within.

    Posted in Multiverse Mania, Uncategorized | 9 Comments

    US HEP Theory Funding News

    Posted on February 20, 2014 by woit

    Laurence Yaffe has gathered some information about DOE funding of US HEP theory groups, showing sharp drops (average 23%) in such funding for groups reviewed in FY2013 and FY2014. These drops imply serious reductions in the numbers of theory graduate students supported and in the number of postdoc positions funded. To get an idea of the reaction to these numbers, the file name is calamity.pdf. Sean Carroll has a blog posting here, also with the “Calamity” title.

    There’s something very odd though about these numbers, with no explanation available from Carroll or Yaffe, who both interpret the situation as a sharp reduction in US government support for HEP theory. If you look at the numbers for total amount of funding, there’s no evidence of the 23% drop (for sources of numbers, see references in Yaffe’s document, the DOE HEP budgets here, and HEPAP presentations here). The recent Congressional agreement on a FY2014 budget puts the DOE HEP total at $798 million, significantly higher than FY2013 ($750 million) which was reduced by the sequester, and higher than FY2012 ($770 million) and FY2011 ($776 million). Of these amounts, the amount going to theory seems stable at around $50-$55 million/year (exact numbers depend on exactly what you include, Yaffe quotes FY12 and FY13 at an identical $51.3 million/year).

    So, the “calamity” of collapse of government support for theory is somehow taking place despite no collapse in the amount of money budgeted for this. What is going on? I’d love to hear from someone who understands this. The only explanation I’ve seen is that this is a temporary phenomenon having something to do with how the budgeting process works. Note that with the way typical multi-year grants are made, the DOE is promising to provide money several years in the future, despite the current US budgeting environment, where budgets are typically set not in advance, but often very late. The current fiscal year’s budget was set last month, although the fiscal year started Oct. 1. This was considered a huge success… One conjecture is that the DOE has been promising less to theory groups, partly because they only recently found out about the good FY2014 result, and partly because of a policy change to budget for future year outlays in current years. If this is the source of the calamity, it should quickly disappear as the FY2014 money comes in and the transition to new budgeting ends. Yaffe examines some other possibilities for explanations, but without anything conclusive.

    The main concern raised is about the effects of a reduction in the number of US HEP theory grad students and postdocs. Perhaps more worrying than this though should be the trend of numbers of permanent research positions in the subject, which Erich Poppitz has gathered here. These numbers come from the Theoretical Particle Physics Jobs Rumor Mill, and show a constant level of 11 jobs/year for the past three years, about half the level of the years 2000-2007. The number of jobs posted this year is so far only 13, which can be compared to around 20/year in recent years. It’s still early in the season, so more jobs may appear, and more may get filled than in recent years, but the trend of US institutions hiring significantly fewer permanent people in HEP theory is clear. Given this trend, there’s a reasonable argument to be made that numbers of grad students and postdocs should also be reduced. There’s no evidence though that some decision has been made to do this, or that this is the reason for the numbers Yaffe is looking at.

    Comments from anyone well-informed about this are strongly encouraged, while comments from people who just want to make the usual arguments about government funding for science rules/sucks will be deleted.

    Update: More commentary at Ted Bunn’s blog. He sees this as possibly an implementation of changes at DOE designed to “decrease the effect of historical inertia”, which might be moving money from historically well-funded major groups to others.

    Possibly all that’s going on though is just the “bridging” problem mentioned on slide 7 of this presentation. A decision was made to start grants later than in the past on April 1 (with the idea that typically it’s not going to be until that deep in the fiscal year that Congress has got its act together and given DOE a budget number). This meant though that money had to be taken out of upcoming grants to cover extending current ones until April 1 when new ones might start. The number on that slide reflects a 25% charge to new grants to cover this, suspiciously close to Yaffe’s 23% number.
    The same slide though does point out that this is a temporary problem: “will be better in 2015”. If that’s all it is, and grad student/postdoc number are slated to go back up a couple years from now, maybe “calamity” isn’t the right word here.

    Update: For more from Yaffe, see here.

    Posted in Uncategorized | 18 Comments