CMB@50

Posted on June 14, 2015 by woit

Last week Princeton hosted what seems to have been a fascinating conference, celebrating the 50th anniversary of studies of the CMB. Hopefully videos and slides will be posted, but one can get some idea of the highlights of the talks from live tweeting that was going on, that is gathered together here. The third day of the conference featured a panel where sparks flew on the topics of inflation and the multiverse, including the following:

Neil Turok: “even from the beginning, inflation looked like a kluge to me… I rapidly formed the opinion that these guys were just making it up as they went along… Today inflation is the junk food of theoretical physics… Inflation isn’t radical enough – it’s too much a patchwork. It all rests on rare initial conditions… Akin to solving electron stability with springs… all we have is proof of expansion, not that the driving force is inflation… “because the alternatives are bad you must believe it” isn’t an option that I ascribe to, and one that is prevalent now… inflation is pretty but we should encourage young to think about its problems & be creative (not just do designer inflation)

David Spergel: papers on anthropics don’t teach us anything – which is why it isn’t useful.. sometimes we need to surrender (to anthropics) but that time is not yet now.

Slava Mukhanov: inflation is defined as exponential expansion (physics) + non-necessary metaphysics (Boltzmann brains etc)… we should separate inflation from the landscape… exponential inflation is very useful, the rest [of the metaphysical stuff] is not for scientific discussion… In most papers on initial conditions on inflation, people dig a hole, jump in, and then don’t succeed in getting out… unfortunately now we have three new indistinguishable inflation models a day – who cares?

Paul Steinhardt: inflation is a compelling story, it’s just not clear it is right… I’d appreciate that astronomers presented results as what they are (scale invariant etc) rather than ‘inflationary’… Everyone on this panel thinks multiverse is a disaster.

Roger Penrose: inflation isn’t falsifiable, it’s falsified… BICEP did a wonderful service by bringing all the Inflation-ists out of their shell, and giving them a black eye.

Marc Davis: astronomers don’t care about what you guys are speculating about at all (mulitiverses, pre-big bang, etc).

I was encouraged by Steinhardt’s claim that “Everyone on this panel thinks multiverse is a disaster.” (although I think he wasn’t including moderator Brian Greene). Perhaps as time goes on the fact that “the multiverse did it” is empty as science is becoming more and more obvious to everyone.

Posted in Multiverse Mania | 15 Comments

A View from an Ex-String Theorist

Posted on June 9, 2015 by woit

Every so often I’ve taken a look at something about theoretical physics on Reddit, generally ending up not spending much time there. One reason was that I realize I’m already spending more of my life than is healthy arguing with people about string theory and the like, so better to avoid a new venue for that. The temptation to respond is strong when one sees someone mischaracterizing one’s opinions, but I’ve generally been able to resist temptation at that site.

Today I happened to come across a really wonderful discussion there though, and wanted to draw attention to it, even though it’s from a year ago. It’s entitled A View from an Ex-String Theorist and consists of a long piece by someone who has recently left string theory, as well as some answers to questions asked by others. If you want to understand what string theory looks like these days to good theorists who are working on it, read what “No_More_Strings” has to say.

No_More_Strings explains very well the difficult job situation in the field, and the effects this has. With a lot of very smart people and almost no jobs, postdocs are in no position to take the time to try and learn something new that isn’t a “hot topic”, or try and work on an unpopular idea that might take years to go anywhere. This is a huge part of the story of why this field is in trouble, and the situation seems to have just gotten worse since I wrote about it in my book over 10 years ago.

The suggestion that “string theorists” should stop calling what they do “string theory” is an excellent one. No_More_Strings explains how smart people in the field are not working on string unification, but have moved on to different things with little relation to quantized strings. Giving up the name would be a good first step to allowing people to think of what they are doing in a less narrow way. If you didn’t have to start every grant application by explaining that you’re motivated by “our best hope for a theory of everything”, you might find it easier to work on something quite different, with no relation at all to quantized strings.

I can’t quite resist correcting a couple things mentioning me. No, I don’t think string theorists are stupid. No, I don’t think that Witten “singlehandedly destroyed the study of “real” physics” (the last one isn’t the fault of No_More_Strings).

Posted in Uncategorized | 38 Comments

A Crisis at the (Western) Edge of Physics

Posted on June 9, 2015 by woit

The New York Times had an op-ed piece this weekend by Adam Frank and Marcelo Gleiser, entitled A Crisis at the Edge of Physics. They make some of the usual criticisms of string theory and the multiverse, ending with

Are superstrings and the multiverse, painstakingly theorized by hundreds of brilliant scientists, anything more than modern-day epicycles?

I mostly agree, although I don’t think they make clear what the real problem is, that these theories predict nothing and explain nothing. In contrast, epicycles were a quite useful, well tested model that was highly predictive and approximately correct. If we had modern day epicycles, that would be a huge advance…

Last Friday in his concluding talk at a Nordita conference on particle physics and cosmology, Michael Turner gave his take on the multiverse:

Most important discovery since Copernicus?
Is it science? (not testable)
Many true believers (left coast) and not enough doubters.

He makes clear his opinion on these questions with this graphic:

62727833

and I think this expresses well the majority opinion of the physics community. A major question here is whether the problem of pseudo-scientific multiverse mania is one of the “edge” of physics (the “left edge”, as Turner notices and was discussed here), or whether it has infected the center. Some days I’m quite discouraged to see how widespread this is, other days it seems to me that we may finally be getting over this. There’s only so long you can get media attention for your empty but easy to understand new “Copernican revolution” before people lose interest and move on to something else. Perhaps we’re getting to that point. I think this was the first year that the World Science Festival here in New York didn’t have a program promoting the multiverse, and maybe that’s a sign of change.

For quite a few years now, there have been few scientific talks trying to use a multiverse to do calculations at serious string theory conferences (see for instance this week’s String Pheno 2015, or Strings 2015 later this month), with the multiverse mainly appearing in promotional talks to the public. Maybe the public is finally getting bored and starting to adopt the point of view that Turner’s graphic suggests (and that I think the physics community should get behind).

Update: Physics Today has an opinion piece entitled Could the evolution of theoretical physics harm public trust in science? This addresses an issue I don’t think some theorists realize the seriousness of. If you start arguing that conventional notions of testability don’t matter, this can be a very dangerous thing to do in an environment where public trust in science is an issue. Put differently, if physicists publicly promote the pursuit of speculative ideas in an ideological framework that can never be falsified, they create a real danger of a public perception that science is just one more ideology.

Posted in Multiverse Mania | 23 Comments

The West Coast Metric is the Wrong One

Posted on June 5, 2015 by woit

I’m trying to get back to blogging about quantum mechanics slogans, this one is about relativistic quantum mechanics. I’m hoping it will stir up more trouble than my last East vs. West one.

If you’re doing calculations in relativistic quantum field theory, you typically handle the Minkowski nature of space-time by introducing an indefinite signature metric, and there are two possible choices:

  • Mostly minus signs (for the spatial components), positive sign for the time direction. This is commonly known as the “West Coast metric”, I’m guessing because Feynman used it.
  • Mostly plus signs (for the spatial components), negative sign for the time direction. This is commonly known as the “East Coast metric”, I’m guessing because Schwinger used it.

While I was educated on the East Coast, most courses I took and textbooks I used favored the West Coast metric, and that’s very much true of more recent textbooks (I don’t know of a recent one using the East Coast convention). When I started on this project I used the West Coast convention. After a while though, I finally found this conceptually more and more confusing, and switched to the East Coast convention. As time has gone on, I’ve become more and more convinced that this is the right convention to choose, that the West Coast convention is just a mistake, and a source of conceptual confusion in the subject.

Here are some reasons:

  • With the East Coast convention, the treatment of spatial coordinates is just like in the non-relativistic case. In the West Coast convention, as far as space goes, you have decided to work with a negative definite metric, which is a quite misguided thing to do for obvious reasons.
  • With the East Coast convention, if you do what you always do to make a QFT well-defined, analytically continue to imaginary time, you end up working with the standard Euclidean metric in four dimensions. In the West Coast version, you end up with a negative definite metric, again a bad idea (thanks to Peter Orland for emphasizing this to me). You could instead do your analytic continuation by analytically continuing all three of the space coordinates, also a really bad idea.
  • With the East Coast convention, the Clifford algebra Cliff(3,1) is the algebra of real four by four matrices. In other words, you can choose your gamma-matrices to be real matrices and work with a real spinor representation (the Majorana representation). Going with the West Coast, Cliff(1,3) is the algebra of two by two quaternionic matrices, a confusing thing to work with. Ignoring that, what physicists end up doing is working with gamma matrices that are pure imaginary, which is highly confusing (odd powers of gamma matrices are pure imaginary, even ones real). According to Figueroa-O’Farrill, in this case you are working with:

    pseudo-Majorana spinors – a nebulous concept best kept undisturbed.

    To be fair, for this problem you can do what he does, and just change the sign in your definition of the Clifford algebra.

  • Weinberg’s quantum field theory textbook uses the East Coast convention.

One reason this issue came to mind is that I’ve been trying to understand (not very successfully…) Schwinger’s old papers on Euclidean quantum field theory, where he makes some quite interesting claims. Schwinger used the East Coast convention for this, and as explained above, it’s only with this convention that you get something sensible after analytic continuation in time. There is very little literature following up on Schwinger’s arguments, I’m suspecting partly because in the West Coast convention following Schwinger’s arguments becomes virtually impossible.

The problem here is part of a more general problem, that I think most physicists don’t appreciate the mathematical concept of “real structure” or complexification. Given any formulas, they’re happy to just start using complex numbers, even when the quantities involved are real, with the idea that only at the end, when you get observable numbers, do you need to impose some reality condition. For an example of this, one often finds in qft texts claims that sound very strange to mathematicians, I have in mind especially things like

The mathematically sophisticated say that the algebra SO(3,1) is isomorphic to SU(2) X SU(2) (Tony Zee, QFT in a nutshell, page 113 first edition).

One problem here is that Zee is using the standard math notation for the Lie group to denote the Lie algebra, an unfortunately common practice. But the real problem is that the two Lie algebras are only isomorphic if you complexify, as real Lie algebras they are quite different. If you have always from the beginning complexified everything, this distinction doesn’t make sense to you, but it is often an important one if you want to really understand what is going on in some calculation involving spinors. For another example, there’s

the 3D rotation algebra, which has multiple names so(3)=sl(2,R)=so(1,1)=su(2), due to multiple Lie groups having the same algebra. So we have shown that so(3,1)=su(2) + su(2) (Matthew Schwartz, QFT and the Standard Model, Page 162).

Schwartz properly distinguishes by notation between the group and the Lie algebra, so is in better shape than Zee, but, again, so(3,1)=su(2) + su(2) is only true for complexified Lie algebras. The statement “so(3)=sl(2,R)=so(1,1)=su(2)” suffers from a typo (so(1,1) should be so(2,1)), but again an identification is being made that only makes sense if you have complexified everything. What’s really true here is that you have SL(2,R) a double cover of SO(2,1), and sl(2,R)=so(2,1), as well as SU(2) a double cover of SO(3), with su(2)=so(3). But SU(2) is a very different Lie group than SL(2,R) (although they share the complexification SL(2,C)).

So, my modest proposal is that the HEP community should just admit that the West Coast convention was a mistake, and rewrite all the textbooks (Weinberg doesn’t have to…).

Update: A commenter tells me there is at least one recent textbook with the right convention, Srednicki’s.

Checking some books, I remembered one other intriguing recent choice, that of Michael Dine, who wrote the first half of his book (the QFT part) in the West Coast metric, but the second half (the string theory part) in the East Coast metric.

Update: For those interested in how to translate back and forth between Coasts in the two-spinor notation, I noticed that Dreiner, Haber and Martin have written review papers, with a line in the tex that lets you choose which Coast. See here and here.

Posted in Favorite Old Posts | 61 Comments

LHC Run 2

Posted on June 2, 2015 by woit

Run 2 of the LHC is about to start with first stable beams scheduled for Wednesday morning, Geneva time). If you’re up (I’ll be asleep) you can watch a live webcast, or watch what is going on here. The current plan is 3 bunches/beam Wednesday, 13 bunches/beam Friday, and 48 bunches/beam over the weekend.

Tomorrow will also be an LHCC meeting, which you can also watch live. It will include reports from the experiments, and a status report about the machine which should give the latest details about the planned schedule for ramping up the intensity over the next couple months.

For the best advice about what to look for in coming months, see Jester’s summary here. First new results may well be about gluinos.

This week there’s a workshop going on at Nordita. On Thursday Gordon Kane will explain how string theory predicts that the LHC will see superpartners soon. I gather his claim is that gluinos are at 1.5 TeV, just above the Run I limits of around 1.4 TeV, so a sure thing for Run II. Of course, back in 1997 he was claiming they were at around 250 GeV, just above Run I limits, a sure thing for Run II (but that was the Tevatron…).

Update: Kane has very specific string theory predictions for Run 2: gluinos at 1.5 TeV, winos at 620 GeV (+/- 10%). So, I guess string theory is going to finally be tested by the LHC over the next year or so…

Posted in Experimental HEP News | 43 Comments

The Nash Musings

Posted on May 28, 2015 by woit

Since the news of the tragic death recently of John Nash and his wife in an automobile accident last weekend, some of those who were at Princeton during the same time I was (late 70s, early 80s) have been exchanging emails of their memories of Nash from that time. It turns out that two of them, Mark Schneider and Steven Bottone, had transcribed some of Nash’s blackboard writings in a notebook. Here’s Steve’s account of this:

Back in May 1979, just after our general examination, Mark Schneider and I decided we would write down some of the writings that Nash left on the blackboards around Jadwin. Mark sat in a wheeled desk chair with a steno notebook in hand and I wheeled him from board to board as he transcribed the musings. I believe if we had done this even six months earlier we would have had a lot more interesting samples of Nash’s writings, but I think he was already starting to tail off on his output by May 1979. As far as I know, we were the only Princetonians who thought of compiling any of this. I have made a PDF out of the musings.

Since I suspect quite a few people are curious about these blackboard writings (including me, they mostly had ended by the time I got to Princeton in late 1979), Mark and Steve have allowed me to make the pdf available, see the link above. Mark points out that they didn’t actually see Nash writing these, so his authorship is an assumption. Steve notes that the handwriting was distinctive, and that the general assumption was that Nash was the author.

Update: Princeton has now made available publicly Nash’s graduate school records, see here.

Posted in Uncategorized | 22 Comments

This Week’s Hype

Posted on May 27, 2015 by woit

I’m busy with other things, so no possible way I can keep up with the claims about string theory flooding the media for some reason these days. It’s hard enough to find the time to read all of this, much less write something thoughtful about it… One obvious point to make though is that none of it acknowledges the obvious: the widely promoted idea that we can get a unified theory and explain the Standard Model by using a theory of strings has turned out to be an empty one. The result of tens of thousands of papers and more than 30 years of work is that all the evidence is that if you can get something this way that looks at all like the Standard Model, you can get anything. Normally when that happens you simply acknowledge the problem and give up, but for some reason that hasn’t happened. Instead of a description of this straightforward situation, the public gets the following:

  • Frank Close describes the situation as

    In recent years, however, many physicists have developed theories of great mathematical elegance, but which are beyond the reach of empirical falsification, even in principle. The uncomfortable question that arises is whether they can still be regarded as science. Some scientists are proposing that the definition of what is “scientific” be loosened, while others fear that to do so could open the door for pseudo-scientists or charlatans to mislead the public and claim equal space for their views…

    Is physics moving towards an era in which elegance will suffice and into the domain of theories that are beyond the reach of experimental proof? Or will empirical evidence remain the arbiter of science?

    Close correctly identifies the problematic nature of multiverse pseudo-science, but misses the basic facts about the string theory landscape. This is not a theory of “great mathematical elegance”, quite the opposite, and there is no such thing developed “in recent years”. If you go back 30 years, there were then claims of “elegant” string theory models, but those never worked out. KKLT is the opposite of “elegant”.

  • Clifford Johnson takes the Close piece as a starting point to explain his own view. He lacks interest in string unification, thinks that string theory should be thought of as a “method” for solving problems. He doesn’t really explain though why it is a “method” that deserves so much more attention than any number of other methods used in physics. He also doesn’t acknowledge that, besides the huge amount of TOE hype (which he and other string theorists often appear on TV to promote), the hype problem for the string theory “method” may be just as bad. For example, he was quite proud of his efforts to promote string theory as the method to understand heavy-ion physics (see here and here), but that’s something that really hasn’t worked out very well.
  • Over at Starts With a Bang, by Sabine Hossenfelder, there’s Will the LHC be able to test String Theory: Definitely maybe. The article actually does a very good job of explaining why the answer is “no”, so I have no idea why the misleading headline. The fact that the AdS/CFT heavy ion predictions haven’t worked out is explained, with the comment that

    The LHC, thus, has already tested string theory!

    and that this failed, which might be a more accurate headline.

  • There’s a long interview with Sean Carroll at the Edge website. He’s quite defensive about the multiverse, claims it’s a prediction of our best theories, and gives his usual characterization of multiverse critics as zealots unable to understand the idea of an indirect test

    But certain zealous colleagues of mine are saying that because you can’t see the other universes in the multiverse or because you can’t see the little super strings moving around, these theories are not falsifiable and, therefore, should not count as science.

    He’s also defensive about string theory, there the argument is

    Either we will bring it down to earth and connect it to the world we see or people will lose interest. People cannot maintain this optimistic idea that we’re going to get the right theory of quantum gravity, the theory of everything, if it’s literally decades and decades of people writing down equations and never predicting the experimental outcome of anything. But we’re not there yet. It would be a terrible shame if we gave up on string theory when maybe next year someone will figure out how to bring it in connection to observations, or maybe ten years from now it will happen. This is how science works, and this is it at work.

    The problem here is that it is “literally decades and decades of people writing down equations and never predicting the experimental outcome of anything”, and no matter how many decades of this go on, someone can always argue that “maybe next year”. He’s avoiding the very real issue at the center of things: why hasn’t string theory been held to account for its failures the way any normal speculative scientific idea is supposed to?
    As far as his current interests go, from what he says, he seems to be losing interest in the multiverse, which soon may no longer be a hot topic, and moving into research into complexity and consciousness.

Posted in This Week's Hype | 22 Comments

John Nash 1928-2015

Posted on May 24, 2015 by woit

I was sorry to hear this morning that John Nash and his wife Alicia died yesterday in a car crash (news story here). They were in a taxi on the New Jersey Turnpike, heading home from the airport after a trip to Norway where Nash was awarded the Abel Prize.

Nash’s mathematical career was cut short by the onset of mental illness, which he then struggled with for many years. Sylvia Nasar’s A Beautiful Mind is a wonderful biography, doing a great job of accurately portraying Nash’s life, including the role of the mathematics community in its various parts. The movie version is another story, especially in the way it shows Nash’s mathematical achievements as somehow being due to his delusions, when what really happened is that the onset of delusional thinking is what made it no longer possible for him to continue doing research at the highest level.

During the years I was a graduate student in Princeton, Nash was often to be seen, especially in the mathematics/physics library, and I talked to him a few times. The first time was when he stopped me one day, told me he had seen my name on the physics department picture board, and was curious about the origin of my last name. While I had heard stories about Nash, that he was mentally ill, spent his time writing delusional things on the hallway blackboards, he seemed fine to me. This was a period (early 1980s) when he had stopped writing on the blackboards and was successfully dealing with the illness. I was very glad to see how later on he was able to lead a more normal life and enjoy the recognition he deserved.

Update: The New York Times has an excellent long obituary of Nash this morning, presumably mostly prepared before his death.

Posted in Obituaries | 20 Comments

Various News

Posted on May 19, 2015 by woit
  • First test collisions at 6.5 TeV/beam at the LHC are tentatively scheduled for Thursday morning.
  • At CERN today there’s a workshop about the Higgs Machine Learning Challenge.
  • Also on the topic of LHC data analysis news, Tommaso Dorigo announces the award of a grant for the AMVA4NewPhysics project.
  • Sabine Hossenfelder has a review, slideshow and discussion of the Dawid book on “String theory and the scientific method” (which I wrote about here and here).
    Much of the discussion is about the “No Alternatives” argument, but at this point I don’t even see how it applies here. The Landscape shows that string theory unification is a failed program, which rules it out. As for whether “gravitation is due to the spin two massless mode of a superstring” is the only alternative, these days my impression is that many prominent theorists are pursuing alternatives, that gravity is supposed to be an “emergent” phenomenon coming from something else.
  • For those sticking to the 1984 point of view though, there’s a workshop on some interesting mathematics that’s part of that story (super geometry, super moduli spaces) going on at the Simons Center.
  • Last week on Jeopardy (see here), no one got this question:

    Nima Arkani-Hamed is using this number dimension, the next one beyond time, to rock the physics world.

    I wouldn’t have either…

  • For a bit of mathematics history, you might want to read Beilinson on Gelfand’s seminar.
  • Natalie Wolchover at Quanta keeps on coming up with interesting physics stories not seen anywhere else, last week covering news about ultra-high energy cosmic rays.

Update: Successful test with first collisions at 13 TeV this morning at the LHC, see here.

Posted in Uncategorized | 12 Comments

Lurie and Categorifying the Fourier Transform

Posted on May 18, 2015 by woit

Before I turn to the main topic of this posting, a lecture by Jacob Lurie, I’d like to point to something else involving him, a comment and posting at Mathematics Without Apologies, a blog you should be following anyway. On the topic of the usefulness of “proof assistants”, I liked Lurie’s point that a major problem with this is:

Working in a formal system, more or less by definition, means that you can’t ignore steps which are routine and focus attention on the ones that contain the fundamental ideas.

But if you want to discuss this, it should be over there, the topic of this posting is something very different.

Last week I noticed that Lurie had given a talk at Harvard on “Categorifying Fourier Theory”, which is available here. I enjoyed watching it, ending up quite intrigued by the abstract picture he was painting, but rather discouraged by the lack of any example that would give insight into what it might be useful for. Neglecting to mention the example that explains why an abstract theorem is useful is unfortunately all-too-common practice among mathematicians. Perhaps in this case with Dick Gross, Jean-Pierre Serre and John Tate in the front row, he felt it unnecessary. Luckily though, he gave the same talk recently in Arizona, and there (in the question session) did give a fascinating motivating example.

His starting point was the Fourier transform, which one can generalize to any abelian group G, and think of as identifying complex functions on G with complex functions on the dual (or character) group G^. The standard Fourier transform is the case G=G^=R, Fourier series are the case G=U(1), G^=Z. He then went on to discuss two levels of abstraction, or categorification of this. The first identifies a representation of G on a vector space V with a function from G^ to vector spaces (the isotypic decomposition of the representation). The second identifies representations of G on categories with representations of G^ on categories.

It was this equivalence of representations on categories that was his main result, for which in Arizona he gave the example of G the group of invertible Laurent series. The idea is that this group can be identified with its dual group G^ (in some sense as algebraic groups), using the Weil symbol (for a definition and context, see here). Lurie’s claim that was new to me was that the equivalence in this case is essentially the GL(1) version of the general local geometric Langlands conjecture, which is supposed to be an equivalence of two representations on categories, for more general (non-abelian) groups G.

At least for me, understanding of some sophisticated mathematical phenomenon really starts when I understand the simplest example of the phenomenon. For the number field case of Langlands theory, my initial efforts to understand the subject didn’t lead anywhere until I realized that maybe it was best to first think about the local version, which was a statement about representation theory that I could make some sense of. I was hopeful that thinking about the simplest case of that, the abelian case, would give great insight, found though that the Abelian case is already quite non-trivial (local class field theory). For the geometric Langlands case, I found that the discussion of the local version in Edward Frenkel’s book was very helpful, but I always wondered about the abelian case. Now I’m hopeful that the abelian story is something that although I’ve never seen it, is well-understood, and that a helpful reader will point me to a reference.

Another reason for being interested in this particular topic is that it has some connection to the relationship between Langlands theory and QFT that first got me interested in all of this. Back in 1987 Witten wrote some fascinating papers giving an abstract formulation of free fermion theories on Riemann surfaces (see here and here) with tantalizing connections to what later became geometric Langlands. In this work the group of invertible Laurent series and the Weil symbol play a central role. There was also later work by Takhtajan on this, see here and here. I wonder why the most recent version of the last reference deletes the material on the multiplicative group case, which is the one Lurie mentions.

Posted in Langlands | 12 Comments