Here are various things of interest that accumulated while I was away:
Last week there was a conference in Florence on the early history of string theory, some of the talks are available here.
Lots of blogging activity among Fields Medalists: there’s a lot worth reading in Terry Tao’s reporting on a series of lectures by Yau at UCLA here, here and here. At the blog of fellow Fields Medalist Alain Connes, there’s mention of on a recent conference at Vanderbilt (slides here), as well as a report from Connes about a recent conference on the philosphical ideas of Wolfgang Pauli. Finally, yet another Fields Medalist, Richard Borcherds, has a blog. Not only do Fields Medalists like to have blogs it seems, but they also like to use them to discuss physics…
Some other blogs I’ve run across include A Strange Universe, from gravitational wave physicist Warren Anderson, which includes his Dire Straits inspired Papers For Nothing. Also John Armstrong’s The Unapologetic Mathematician, which has a lot of expository material, and Julie Rehmeyer’s MathTrek, a blog at the Science News web-site.
Steven Weinberg has canceled a planned public talk at an event to be held in conjunction with PASCOS 2007 at Imperial College in London in July, an event celebrating the 50th anniversary of the arrival of Abdus Salam at Imperial. In a letter to Mike Duff (available here), Weinberg says that he is boycotting the event in response to news of a boycott of Israel by the British National Union of Journalists, due to his belief that there is no possible explanation of this other than widespread anti-Semitism in Britain “especially in the intellectual establishment” or “a desire to pander to the growing Muslim minority in Britain.” Note: any attempts to use mention of this news to justify attempts to carry on the Israeli-Palestinian conflict in the comment section of my blog will be ruthlessly suppressed.
As usual, Tommaso Dorigo is doing a great job of making current collider physics actually seem exciting and interesting. He’s now spreading rumors of a 4-5 sigma excess of multi-b-jet events being seen by D0. What does CDF data show? He’s keeping his mouth shut about that… Maybe there will be some excitement at the upcoming summer conferences…
There’s a new Harvard College magazine about math, run by undergraduates called The Harvard College Mathematics Review. The first issue contains an article by Noam Elkies about the abc conjecture, and one by Dennis Gaitsgory about how not to teach linear algebra.
Via Mathephysique, here’s an interview with theorist Edouard Brezin.
I keep running across more and more web-sites of theory groups that are putting up material from their theory seminar talks. The latest is the HEFTI Seminar Archive at Davis.
Mike Hopkins gave a Distinguished Lecture Series in Toronto recently. Only audio from the talks is available on-line, and I can attest that forcing someone to try and follow a talk they are interested in like Mike’s The Topological WZW Space of Conformal Blocks just by listening to the audio without always being able to tell what he is writing on the board is just cruel.
Via the n-Category Cafe, notes from a recent conference in Kyoto on Link (also known as Khovanov) homology and categorification. Lots of interesting talks to read, but I’m especially fond of the abstract of Dror Bar-Natan’s talk, which begins:
I’m over forty, I’m a full professor, and it’s time that I come out of the closet. I don’t understand quantum groups and I never did.
Now I don’t feel so bad. Also, for inspiration, check out his Dream Map.
Update: A Chicago network news show has a recent segment about Fermilab and the hunt for the Higgs.
Update: The D0 rumor has made it to Slate.
Here’s an interesting link in response to Wienberg’s decision and providing an other side to the story:
http://counterpunch.com/jensen06012007.html
Regards
Hi,
I’m not able to read your entire site, but I have a general question about your critical stance toward string theory. My superficial understanding has it that there are many possible vacua of the “theory,” each corresponding to different physics, and we don’t know which is the “true” vacuum, so one says the theory makes no predictions and cannot be falsified.
However, I thought I had come across schemes that claimed to reproduce the standard model with very few parameters. (I don’t remember details; the one author I can remember is Dmitri Nanopoulos; this was in the late 90’s). Are these claims false? And if not, doesn’t it count as huge progress to go from a 19 free-parameter theory to one with few parameters and a theory of quantum gravity to boot?
Thanks!
“Are these claims false?”
Yes.
No one has been able to use string theory to calculate any of the standard model parameters in terms of the others. If you don’t believe this, try and find a string theory prediction of the one still unknown SM parameter (the Higgs Mass) in terms of the others, or a string theory prediction of anything that will be seen at the LHC.
It would be huge progress if string theory (or any other theory) was able to reduce the number of free parameters in the SM, but this has not been done. Any claims otherwise are intentionally misleading.
Peter,
Your statements are not entirely correct. The model that we present in hep-th/0703280 is very close to reproducing the three-generation MSSM, including the standard particle masses and mixings. We can make predictions of the superparticle spectrum and Higgs mass with essentially only five parameters, the moduli VEVs and the gravitino mass. We can restrict the range of these parameters by demanding that the relic neutralino density is in the range to produce the observed dark matter density. To be fair, this is only one vacuum among many and we haven’t completely addressed the issue of moduli stabilization, however the model is testable in that either this vacuum will correctly predict the pattern of superpartner and Higgs masses or it won’t. By the way, the model generically predicts the Higgs mass to be in the range 117-121 GeV.
Just want to point this out. Hopefully, this doesn’t trigger another round of angry post.
Cheers,
Eric
dear Eric, you do not have any string prediction for the Higgs mass. You just ASSUME tanBeta = 46 (page 3 of your paper) because you know that, according to a computation done 15 years ago in quantum field theory, this gives m_higgs = 91 GeV + (top loop correction) such that the Higgs mass is slighly above the LEP bound m_HIggs > 115 GeV. Had you ASSUMED a lower tanBeta, you would have obtained a Higgs mass that contradicts the LEP bound.
Alas, M, your comment is at the very essence of the problem that plagues HEP today. What is the predictiveness of a model (say Supersymmetry, or string theory, or whatever) that contains (many) new parameters that can be adjusted to provide whatever outcome one wants ?
I remember years ago, when many expected SUSY would be discovered in a discrepancy in the sides of the CKM triangle… The Bs mixing delta M was expected to be 30/ps or so. Now it is 18+-1/ps, the triangle closes beautifully and what do susy theorists do ? They cook up a “MFV scheme” to keep away from inconsistencies. M is for “minimal”, to imply they have grown accustomed to walking on tiptoes to avoid perturbing the beautiful consistency of the SM.
I wonder how much sense that does. Given the large phase space allowed to SUSY parameters years ago (masses of SUSY particles at 80-100 GeV, large effects in BR through loops, etcetera), and a “flat prior” assumption for their values, wouldn’t it be reasonable to compute the “confidence level” of SUSY today ? We would find we have excluded it at well more than 95% confidence level.
Mind you, I do not usually give too much weight to “95% confidence levels”. They are not very meaningful if you do not specify the probability distribution you have integrated. For instance, the direct limit for the higgs at 114 by LEP II is very strong, because if you go even slightly below 114 the probability becomes zero quickly – they would not have missed the higgs at 112. An example of the other kind is the “upper 95% cl limit” from indirect fits of SM observables at 166 GeV: even 200 GeV is not wildly unlikely – it just entails one or two wrong measurements of electroweak parameters.
My bottomline is: a theory which can be tuned to give just about any possible outcome is not a theory, it is an exercise. Not worth investing a large part of a budget of funding agencies!
And I would like to add something to my previous comment. I remember that in the late eighties and early nineties, when the top quark had not been found yet, theoretical predictions for its mass used to follow closely the lower limits from experiments: some theoretical calculations indicated a mass of x+-x/3, then a direct limit would arrive at 1.2*x, and a new prediction then would be published at (1.5+-0.3)*x. New collider data would then exclude masses lower than 1.6*x, and guess what, new predictions would foresee masses around 1.8*x…
It looks like some theorists love to have a bet out which makes it possible for them to dream hitting the jackpot. I am not implying these are not serious scientists – just that the game is a bit silly.
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Eric,
Your model, like all such things, not only doesn’t reproduce the SM with fewer parameters, but actually adds more undetermined parameters. And, in any case, it isn’t a “prediction of string theory” anyway.
Cool Dire Straights song/link.
Many of you are familiar with Johnny Cash’s song “San Quentin,” which he perfomred in front of the inmates of San Qunetin, during one of his famous prison concerts.
It has recently come to light that he also traveled the world, singing to those who were placed in non-tenured postdocoral/lectureship postitions by string theorists–those everlasting prisons for today’s freethinkers.
The Man in Black sang,
String Theory, you’ve been livin’ hell to me
You’ve hosted me since nineteen eighty three
I’ve seen ’em come and go and I’ve seen them die
And long ago I stopped askin’ why
String Theory, I hate every inch of you.
You’ve cut me and have scarred me thru an’ thru.
And I’ll walk out a wiser weaker man;
Mister Congressman why can’t you understand.
String Theory, what good do you think you do?
Do you think I’ll be different when you’re through?
You bent my heart and mind and you may my soul,
And your m-brane walls turn my blood a little cold.
String Theory, may you rot and burn in hell.
May your walls fall and may I live to tell.
May all the world forget you ever stood.
And may all the world regret you did no good.
String Theory, you’ve been livin’ hell to me.
Johnny Cash said to the exiled postdocs, “I was thinking about you guys yesterday. I’ve been here three times before, and I think I understand how you feel about some of the things. It’s none of my business how you feel about some of the things, and I don’t give a damn about how you feel about some of the things. But anyway, I tried to put myself in your place and I believe this is how I would feel about String Theory” — Johnny Cash @ The San Quentin String Theory Conference
Exoneration. – Asked whether she agreed with the characterization of the present as the “age of the fakers,” somebody insisted on the caveat that the popular rendering of string theory can be a source for subjective cosmological mysticism, along the lines of, say, a romance novel. Thus, it would not be fair to refer to string theorists as fakers, if they were marketing it like dime store romance novels.
Professor in Black,
I’m sure that this Johnny Cash ballad will take its place alongside that other classic of the genre, “He ain’t heavy, he’s my supersymmetric partner.”
Ain’t no selectron, at less than 10 TeV
Ain’t no squark that a detector can see
But I won’t give up on supersymmetry:
Just need more time, and more money.
(… or something like that)
> A Chicago network news show has a recent segment about
> Fermilab and the hunt for the Higgs.
Umm… very nice. When did that annoying style of explaining everything through movie-teaser soundbites become so prevalent? “Shake the Higgs loose”? Oh well.
And there’s an allusion to the ILC at the end! Let’s not get carried away though, the Real World has a way of crushing dreams.
“As usual, Tommaso Dorigo is doing a great job of making current collider physics actually seem exciting and interesting.”
Thanks for the heads up.
Thanks very much for the helpful responses from different perspectives, and the bittersweet lyrics. I appreciate the specific examples without overwhelming technical detail.
So, to press a little further, I think I understand the point that the various string-derived particle spectra represent putative vacuum states that might not really be the true and stable ground state of String Theory, so we may want to avoid calling such theories “predictions” of string theory. But if we can find one or many of these theories that describe known physics and unify all the forces–can we afford to discard them? We would perhaps need to stop thinking of string theory as a fundamental theory, but isn’t just having a unified description a major step forward? Aside from counting free parameters, if we have a theory that’s in some sense arbitrary, like the standard model, but it does unify all the forces…isn’t it superior (even if we KNOW it’s not the true vacuum of the theory)?
Doesn’t string theory unify the forces, if considered as merely a phenomenological description rather than a fundamental theory? Forgive my ignorance, but do we actually have any other theory that does this?
Finally, since you were all so straightforward, I have to demonstrate my ignorance again to ask this–is there any other theory of quantum gravity that is finite or renormalizable?
Muchos thanks!
Mike,
The true ground state of string theory is presumably known, it’s one of the supersymmetric ones like flat 10d space-time. Problem is that this ground state looks nothing like the real world. What string theorists are doing now is looking at metastable possible ground states that might look more like the real world. But they are extremely complicated and exist in huge number and variety. The bottom line is that string theory is not able to predict anything at all. To claim that you have “unified” physics with a theory that doesn’t predict anything just makes no sense.
People now think N=8 supergravity might be finite, and there’s a whole research program pursuing another idea about quantum gravity (loop quantum gravity) which arguably has been as successful as string theory in terms of coming up with a quantum gravity theory.
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Peter,
When you say string theory is not able to predict anything at all, I’m taking you to mean that we have no principled way of choosing among the huge number of ground states that (might) look like the real world. But are you saying that even if we pick one of those adequate ground states we don’t have a unified description of the the forces? Because the parameters are still too unconstrained? Or is it because we can’t tell if any single one of the promising ground states is “adequate”?
I don’t quite see why it makes no sense to think of each “adequate” quasi-ground state as a consistent unified description of all the forces. I mean, we can’t derive the weak mixing angle in the standard model but we were pretty happy to get the weak interaction unified with the electromagnetic, right?
If our situation is that we have too many adequate theories, isn’t that better than none?
Thanks for your patience!
Mike,
There are two huge problems here, one of practice, one of principle:
1. In practice, one doesn’t know what the full, non-perturbative string theory is, so one is working with various approximations and assumptions. As a result, for each conjectural metastable ground state, all one can get are various crude approximate “predictions”, you can’t actually extract any accurate predictions of SM parameters.
2. In principle, even if you had a full theory and could do reliable, accurate calculations in it, the number and variety of these states is so huge that no one has a credible proposal about how to get a prediction of anything out of studying them.
In electroweak unification, sure you can’t predict the Weinberg angle, but you can predict a huge number of other things and test these predictions. The problem with superstring theory unification is not that there are things it currently can’t predict, but that it can’t predict anything at all. Again, trying to sell people on how wonderful your unified theory is, when it can’t predict a single thing about anything, is obviously problematic.
Hi Peter,
I’m thinking of “predictions” as being an orthogonal dimension to “unification.” If I accept that these theories (or “schemes”) make zero predictions, do they still give me a unified description of the fundamental forces? Even if we have to fit every single parameter to experiment, don’t we have something we didn’t have before, namely, a way of seeing all the forces as arising out of one kind of stuff?
Sorry if I’m sounding like a not-too-bright broken record. You seem to feel that a unified theory is not a unified theory if it doesn’t make any predictions. But isn’t there a difference between a unified description as I’m imagining exists in string theory, as opposed to the mere aggregate of interactions that we have in the standard model?
Maybe your point is that if you can’t do reliable calculations in any of these theories, then you don’t have any unified description to speak of. Are you saying something to the effect that, even if I set all the parameters, we don’t know how to calculate cross-sections and whatnot? So the indeterminacy is not just in the parameters, but somehow the theory turns to mush if you try to calculate anything even given the parameters?
In that case, would it not even qualify as a theory of quantum gravity? Am I wrong to believe there is a consistent quantum theory of gravity in there somewhere? (“No, Mike, you’re NOT EVEN wrong…”)
I do appreciate the importance of being able to predict things…but I also tend to see unification as valuable in itself.
Mike,
As far as I’m concerned, you’re just not doing science unless you can test your theory against experiment, i.e. make predictions with it and check them. One of the main reasons we want a “unified” theory is that by relating different parts of physics we should get more predictions we can check, not less. A “unified” theory that can’t predict anything is just not a scientific theory.
just to add a new stuff not included in your blog, there’s a good article by Smolin in New York Review of Books, about some new books on Einstein.
tty,
Thanks for mentioning that, I was going to include it on this list, but seem to have missed it. There’s a detailed posting about this and discussion at Cosmic Variance.
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Peter,
I think you’ve just about convinced me. It would seem that if one’s putative unification doesn’t constrain the different sectors by relating them to each other, then nothing really got unified. If I think of an example where the unification seems substantive but no new predictions are made, I’ll get back to you…
But–is it a finite theory of quantum gravity? Or again, because it is so unconstrained, do you say there is no theory there?
Hi Peter,
about PASCOS: I will attend the conference for a couple of days (so what, your readers are entitled to ask). I will be presenting CDF tests of the Standard Model (so what ?). Nothing, just to mention that I will try to blog about what I hear there. I think I will not understand much of the stringy talks, but at least I hope to give some report of the cosmology part.
Cheers,
T.
Mike,
String theory really isn’t a finite theory of quantum gravity. It only gives the terms in a perturbation series, and it is these that are supposed to be finite. The series doesn’t converge (it’s presumed to be a divergent, but asymptotic series). Conjecturally, there is some non-perturbative finite theory, to which this series is asymptotic. But, even if this conjecture is true, the problem is that the evidence then is that you’ll end up with not one theory of quantum gravity, but 10^500….
Sweet!
Gotta love that.
What does divergent but asymptotic mean? Can you really be divergent and at the same time get asymptotically close to a finite number?
Mike,
Asymptotic series are ones that become arbitrarily accurate for arbitrarily small coupling, also they get closer to the true answer as you compute more terms up to some point, where they start getting worse. Depending on how small your coupling is, in some cases they can give quite accurate results, even if there is an inherent limit to this accuracy. In QED the perturbation series is presumably just asymptotic, but quite accurate. In string theory you have no idea since you don’t know what the coupling is. But, when you only have an asymptotic series, it’s misleading to claim you have a finite theory.
Aha. Why would QED be “presumably” only asymptotic with a coupling of 1/137? Because you know things change at the Planck scale (or before)?
Thanks Peter.
Mike,
There’s an old argument (due to Dyson), that in QED the way things depend on the coupling constant [tex] \alpha [/tex] can’t be an analytic function at [tex] \alpha=0 [/tex] since the vacuum is unstable for even infinitesimal negative coupling constant. Only for analytic functions will you get the power-series converging to the true answer. So, you expect to only have an asymptotic series in QED, this is independent of how big the coupling is. Since the coupling is quite small, even though the series is only asymptotic, at low order one expects it to be very accurate (which it is). More detailed arguments imply that the series expansion should get better and better up to order around 137, only for higher order calculations than that does it start to get worse. So, for all practical purposes, the series expansion is fine.
Interesting.
Thanks!