The Council for the Advancement of Science Writing is holding a New Horizons in Science conference right now in Austin. This morning Steven Weinberg gave a talk, now available online, with the title Higgs, dark matter and supersymmetry, what the Large Hadron Collider will tell us. He described the Higgs as something definitely expected, supersymmetry as a much more speculative possibility, but had nothing to say about string theory during the talk. In the question session, Tom Siegfried of Science News asked him about why he hadn’t mentioned string theory, and what its prospects now were, 25 years after first being heavily promoted to the press. Weinberg answered:
It’s developed mathematically, but not to the point where there is any one theory, or to the point that even if we had one theory we would know how to do calculations to predict things like the mass of the electron, or the masses of the quarks. So, I would say, although there has been theoretical progress it’s been, I find it disappointing. One of the hopes would be that the LHC would provide a clue to something we’re missing in superstring theory and I think there supersymmetry is the most likely place to look.
One of the troubles with superstring theory is that although in a sense the theorists think there is only one theory, there are an infinite number of approximate solutions of it and we don’t know which one corresponds to our world. But at least in a large variety of the solutions of superstring theory there is supersymmetry visible at low energies, and if we see supersymmetry at low energies, superstring theorists may be able to derive from it some kind of clue as to how to solve these theories. But I haven’t talked about it in this lecture because I don’t see how that would work, it would be.. I mean I couldn’t say that that was likely with any degree of sincerity, and certainly the LHC and any other accelerator that we can imagine being built will not get up to energies which are high enough so that we can directly see the structures that are described by superstring theory, the strings or the D-branes or whatever it is. Those will not be accessible at the LHC, so any clue we get will be very indirect.
I myself, well I was working on superstring theory in the 80s and gave it up because I… I moved into cosmology, which in the last couple of decades has had the excitement that elementary particle physics had in the 60s and 70s, a wonderful coming together of theory and observation. Cosmology now reminds me of the excitement that I felt when I was younger and doing particle physics.. and it’s a pity that superstring hasn’t developed better. I still think it’s the best hope we have, I don’t know of anything else. My own work very recently has been trying to develop an alternative to superstring theory as a way of making sense out of quantum gravity at very high energies. But even though I’m working on this I still find superstring theory more attractive, but not attractive enough…
Siegfried gives an account of the talk here. It includes a new remarkably convoluted and misleading way of referring to the fact that string theory predicts nothing at all about observable physics:
But despite a quarter century of intense effort, superstring theory has not produced a cohesive and clear guide to testing its fit with all the observable features of physical existence.
Well, it’s a shame that in such conferences speakers do not have a tie-microphone. Hanging a hand microphone is so much dated, and so much painful for old people…
Whatever, wehn I was a young student, I attended in Paris a conference by Alfred Kastler, about the origin of quantum theory. I must say, what he told us then was so much unexpected and unheard of, that I am still muling his lesson, thirty five years after. Weinberg is a too well known teacher and Noble price, what he can say is much less original… (putting away the fact that I find his books about QFT ones of the less pedagogical, except for references)
“Siegfried gives an account of the talk here. It includes a new remarkably convoluted and misleading way of referring to the fact that string theory predicts nothing at all about observable physics”
Sigfried’s conspicuous lack of—to put it delicately—intellectual candor, so far as string theory is concerned at least, is the main reason that I let my subscription to Science News lapse. If the periodical is going to be turned into just another mouthpiece for string theory ‘irrational exuberance’, what’s the point of supporting it? His skewed choice of coverage (including one particularly contentless last-page essay by some guy whose name I’ve succeeded in forgetting that made some empty dismissive comments about Lee Smolin’s book) shows that he’s pretty much just a shill for an agenda in science that has become far more a matter of ideology than a real research program—one that is willing to commit itself to gaining some battle-tested results, as Wheeler put it, before claiming ANY degree of success.
But I’m glad to see that SW has finally said something publicly about his doubts. I wonder how the latter will affect his pronouncements about the anthropic principle and the predictive possibilities of physics as a science—he was pretty bullish on the AP for a while, as I recall…
Bob,
i think you are mixing things up a bit here. afaik, Weinberg *invented* the anthropic principle (or to put it better was the first one to point out how it can be applied to particle physics/string theory). with so many things that he has pioneered, he really does not need to advertise or push anything (except his book maybe 🙂 ). i always found him extremely honest and lucid in his statements.
just look how he is talking about the asymptotic safety scenario in this very citation given here. he invented that too, you know.
” with so many things that he has pioneered, he really does not need to advertise or push anything (except his book maybe ).”
Chris—
My impression was that Brandon Carter and John Barrow were the ones who introduced the AP into the discourse in physics, and that Weinberg’s comments invoking the AP were connected with the landscape interpretation of string theory, which seemed to be the only way to give a realistic interpretation to the vast number of possible vacuum states implied by the theory (or program, or whatever the right term for it is). I could be quite mistaken about that, for sure. But I wasn’t thinking of SW’s promoting or advertising the AP so much as perhaps dropping it altogether. After all, if you no longer take string theory seriously as a promising direction in pursuing a ‘final theory’, then a lot of the motivation for the landscape, and hence the AP, disappears along with it. And it certainly sounds as if Weinberg no longer has anything like the view of ST he was articulating earlier. So it makes me wonder if we’ll be hearing anything further about the AP from him, or if he instead views the whole AP/landscape quagmire as a dead end.
“i always found him extremely honest and lucid in his statements.”
I wouldn’t disagree at all with this. It’s Siegfried who I find profoundly, um, *disingenuous* in the way he uses SCIENCE NEWS to try to minimize/deny the foundational problems that ST faces (whether it wants to or not).
Bob,
My impression from reading Weinberg’s recent papers is that he still believes that the multiverse is the only available explanation for the cosmological constant problems. Note that the multiverse is an independent concept from the string theory landscape. But the latter does give an explicit construction for the former.
What I found interesting about Weinberg’s talk and comments was that, whatever his views on the multiverse, AP, extra dimensions, string theory landscape, etc. might be, he decided not to mention these at all in his talk. He takes a very different approach than many other prominent figures in theoretical physics, who think it’s a good idea to get the press and the public “excited” by feeding them all sorts of highly speculative nonsense. Because of this latter approach, people like Siegfried go out and write books based on such nonsense, and have a completely misguided view of the subject. Also as a result, both questioners wanted to ask Weinberg about string theory, which he hadn’t talked about, not about the solid science he did talk about.
Bob,
I think the Science News piece you refer to is this one:
http://www.sciencenews.org/view/generic/id/45196/title/Comment__Five_problems_in_physics_without_the_definite_article
which described Lee Smolin’s book as “silly attacks on string theory”, but which I’d describe as a silly attack on Lee Smolin…
Peter—yes, that’s the one. I’d already pretty much decided that in good conscience I couldn’t really continue supporting SN, and it’s been in the back of my mind for a while that if Siegfried is pursuing this kind of partisan agenda in one branch of science, what sort of illusions of consensus is he trying to promote in biology, psychology and so on?…
…and to PhysicsPhile: yes, point taken, except that the AP seems to have been most aggressively flogged of late in connection with the string landscape business, and reading Peter’s citation of SW’s evident disillusionment with ST suggests that maybe the drunken orgy phase of ST—and its sociological domination of the field in terms of grantsmanship, hiring and the like—is about to come to an end, if even high-profile advocates such as Weinberg are having second and third thoughts.
“Also as a result, both questioners wanted to ask Weinberg about string theory, which he hadn’t talked about, not about the solid science he did talk about.”
An old, wise professor of mine in graduate school once commented bleakly that you can’t kill a bad idea. That’s doubly true I think if giving up the bad idea leaves you in a relatively unsatisfactory, frustrating situation given your ambitious hopes for a real working GUT (which is presumably why Siegried and others like him want to keep the party going).
But is it too much to hope that the various bits and pieces we’ve been reading about here (David Tong’s comments, Weinberg’s frank acknowledgement that ST has, in terms of real physics, gone nowhere, and the rest) are a sign that the town drunk is beginning to sober up at last?
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what did he say about DM?
Please blog your thoughts about Green’s new post.
I’m interested in what you think about it!
http://www.guardian.co.uk/science/2009/oct/20/stephen-hawking-michael-green-cambridge
Cormac,
You can watch the video, but from what I remember Weinberg just made conventional points, explaining some of the astrophysical evidence, the possible connection to supersymmetry, and the possibility of production of dark matter particles at the LHC.
Michael,
Green seems like a reasonable choice for the post. While I complain about string theory and the faddish way it has been pursued, it’s worth noting that there’s a major difference between Green and most other string theorists: he was doing important work moving the field forward at a time that this was a very unfashionable thing to be doing. He and Schwarz deserve a lot of respect for that.
One odd thing I saw mention of somewhere is that Green is only 4 years younger than Hawking, so presumably will be stepping down from this position rather soon after taking it.
PP said
*…impression…recent papers is that he still believes that the multiverse is the only available explanation for the cosmological constant problems…*
Impressions of what someone still believes—rather than what they actually say—can certainly be tricky. I couldn’t find anywhere that he actually says that, in his ten papers from the last four years. A lot has happened during that time, so earlier than that would not seem exactly “recent”.
http://arxiv.org/find/grp_physics/1/au:+Weinberg_S/0/1/0/2006,2007,2008,2009/0/1
Maybe you can point out for us which of these papers actually says that he still believes such and such. I could easily have missed a reference.
If on the other hand you are using your intuition to guess another person’s state of belief, without his explicit statement, that’s…well…different. Tells us more about you than about Weinberg.
We should then probably be discussing your own ideas, rather than projecting them on him.
The two *most* recent papers of Weinberg, both from 2009, and his July 6 talk opening a conference at Cern, all cited extensively some work by Reuter, Percacci, and others on the “asymptotic safety” treatment of gravity—allowing Newton’s G and the c.c. to run with the renormalization group flow. According to a paper of Reuter et al that Weinberg cited, this offers another way to understand the cosmological constant. I don’t know what Weinberg thinks or believes, but he explicitly said in his Cern talk that his current research focus is on asymptotic safety (a concept he introduced in 1976) and its relevance to cosmology.
On the other hand, I’ve seen nothing from him recently either of multiverse or anthropic argumentation. If you have please provide a reference.
Marcus,
In his comments at the end of this talk, Weinberg mentions that he is working on asymptotic safety, but says “I still find string theory more attractive”. It’s a reasonable interpretation of this to think that he hasn’t changed the views about the CC, multiverse, string theory landscape that he wrote about in “living in the multiverse” back in 2005. If he has come to the realization that there is something fundamentally misguided about most multiverse research, he hasn’t said so publicly, as far as I know.
In any case, I think, as shown in this talk, Weinberg is careful to keep in mind the distinction between what is solid science, with real evidence behind it, and what is highly speculative, and thus likely to be wrong. And all explanations of the size of the CC now available are in the latter category, no matter which of them is your favorite.
Peter, Weinberg is careful, but not in all statements. For example, he gives talks since 30 years saying that the essence of nature is its symmetry (repeating what Heisenberg said). Symmetry is surely important, but whether it is of fundamental importance is questionable. Weinberg’s stress on symmetry and its role was the reason that so many researchers looked for larger and larger symmetries (GUTs, supersymmetry, string theory). It is not at all clear that this is the right approach for fundamental physics – especially since none of these symmetries has shown up. The search for larger and larger symmetries is usually taken for granted, due to Weonberg’s constant stress on the issue – but is it the correct approach? Maybe the LHC can give us a hint about this fundamental issue.
If the LHC does not find any new symmetry, then a complete change of mind might be required. Maybe there is no larger symmetry in nature, apart from those that are known?
Frank,
Weinberg is far from the only one who thinks symmetry is a fundamental principle. This was a very fruitful idea throughout the 20th century. My own view is that it will continue to be one, but one has to keep in mind that new ways to exploit symmetry principles may need to be found. The way gauge symmetry is realized in the electroweak theory is a good example, it’s rather subtle and still not completely understood non-perturbatively.
So, if Weinberg is pushing symmetry as a principle, I’m with him. But to get something new it’s likely to require something much more non-trivial than just replacing SU(3) by SU(5).
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Peter, I understand what you say: but people have tried GUTs, supersymmetry, quantum groups, BRST, and so many other ideas. None worked. It is quite courageous to think that more symmetries – apart from the gauge symmetries of the standard model – play a role in nature. Thousands of people have looked for more general symmetries, but none has been found. So the question arises: is a larger symmetry really needed?
Frank,
My own point of view is just that there are fundamental questions about how to handle things like gauge symmetry in QFT that remain poorly understood. They’re deep questions both about mathematics and about physics, so worth understanding. It’s true that such efforts haven’t yet led to any breakthroughs in physics, and maybe they never will. But, historically, looking for new ways of exploiting symmetry principles has been very fruitful, I don’t you think you need to be a wild-eyed optimist to suspect this might still lead somewhere interesting.
And, it’s not like there are a lot of other ideas out there which are working out…
I think one can make a strong case that the asserted symmetries that have been most fruitful are those that express a principle of indifference or non-observability, for which empirical support was already in hand. Special relativity is the obvious case in point.
Asserting that “nature loves mathematical symmetry” strikes me as simply missing the point, and research programs based on such an assumption are likely to get lost in a mathematical labyrinth. Of course, if one loves mathematical structure for its own sake, one may not care to escape from that labyrinth. [ 🙂 ]
Peter, you said,
“In his comments at the end of this talk, Weinberg mentions that he is working on asymptotic safety, but says ‘I still find string theory more attractive’. In any case, I think, as shown in this talk, Weinberg is careful to keep in mind the distinction between what is solid science, with real evidence behind it, and what is highly speculative, and thus likely to be wrong. And all explanations of the size of the CC now available are in the latter category, no matter which of them is your favorite.”
It seems to me your observation that his work on the reason for the value of the small CC is probably wrong is somewhat misleading. For one thing it ignores the fact that there is a vital difference between the CC being small and “zero” which was assumed until 10 years ago. The CC is now simply small and the current estimate is about 0.7 .
Much of the work in particle physics, including the SM assumed a zero CC, though it wasn’t really part of the SM. This new value of the CC really does require a rethinking of physics and I think it really is a gross exageration to just minimize it and say asymtotic safety “still” has nothing to do with it. When Stephen Weinberg says he finds string theory more satisfying than this new direction, to me it just means he would rather have not had the CC turn out to be non-zero. But now that it is new directions are required because the facts have changed. That doesn’t mean he wouldn’t have preferred the previous fact, i.e. the CC = 0, to still be in force.
Eric,
I see no reason to believe that Weinberg’s preference for string theory over asymptotic safety as an explanation for quantum gravity has anything to do with the CC. In any case, he explicitly says that string theory, while better than known alternatives, is not attractive enough an idea to be convincing.
The point I was trying to make is that Weinberg, unlike an increasingly large number of people in theoretical physics, understands very clearly the difference between solid ideas which we have good reason to believe are true, and highly speculative ones that are probably wrong. He was carefully avoiding going on about the latter to this group of journalists.
Chris W.
Sorry, but that’s just not true. Most uses of symmetry principles in physics involve using the symmetry group to decompose states into non-trivial representations of the group. The action of the symmetry group is very much observable.
Actually, most well-known observables in QM are the infinitesimal generators of symmetry group actions on states (momentum, angular momentum, charge). Non-observability is only relevant in the case of local gauge symmetry, where you expect states to transform trivially (although even in this case, the situation is much more complicated).
Eric Habegger wrote:
It seems to me your observation that his work on the reason for the value of the small CC is probably wrong is somewhat misleading. For one thing it ignores the fact that there is a vital difference between the CC being small and “zero” which was assumed until 10 years ago. The CC is now simply small and the current estimate is about 0.7.
To be more precise: observations require, apart from the usual radiation, baryonic and dark matter components, a “dark energy” component, which in dimensionless terms requires a energy density of 0.7 (relative to the critical energy density). One can treat it as a fluid and see that it leads to a “repulsive” gravitational effect. There are several possibilities for this effect. In one of them, usually regarded as the simplest one, is not a fluid, but the cosmological constant. In this case, the theory introduces a length scale related to the dark energy density. There is a dimensionless combination of that length scale, G, c and the Plank constant, and it is this dimensionless combination that is required (observationally) to be of the order 10^{-123}. What people believed previously was, despite of being a puzzling fact per se, that this dimensionless combination was exactly zero. However, what current observations require is that this number is today extremely small in fact, but not zero. This requires a big fine tuning and consequently a big problem to solve.
To sum up my previous comment: to begin with, for many decades, the cosmological constant was thought to be exactly zero (although no one had an explanation for that), then came observations leading to a large energy density component, to which several explanations have been offered. The cosmological constant came as one of them, but then it introduces a natural dimensionless combination which is extremely small.
Christine,
If you want some other source for the dark energy, you still have to explain why the cosmological constant is then zero as vacuum fluctuations and phase transitions imply it should be 10^{many} orders higher. The nice thing about the multiverse/landscape (not necessarily string) explanation is it kills these two birds with one stone. But I do agree with Peter that with our present state of knowledge it is still not a testable explanation. Never the less it is the only one available so I think it shouldn’t be treated as irrelevant.
Physicsphile,
I agree that if for some reason the dark energy problem “goes away” (whether observationally or by some alternative well-tested explanation not involving the CC) , it is clear that it will still be necessary to understand why the CC is zero. This problem existed before the dark energy problem, and may be independent of it.
As I see it, the CC problem must be explained through quantum gravity; classically the CC can be gauged away, this tells something. I do not think that the multiverse/landscape solves the problem. Conceptually, many more problems arise than vanish. You are entitled to disagree on this.
Please enough about the CC. This has nothing to do with Weinberg’s talk, where he didn’t mention this topic at all, for good reason…
Peter,
Just a suggestion, but of course you run your blog as you find correct. If you thought that the CC was off-topic, why have you not cut the matter from the very root? I thought it was reasonable to make a few clarifications about someone else’s comment about the CC, so I took my time to write here, feeling that it was a constructive thing to do. It’s not the first time that this happens. I know this is difficult to manage, since I have a blog myself. But at the same time it is hard to receive a “enough of this!” at one’s face, when one is just trying to be constructive on a matter *already* accepted and published in the comment’s section.
Hi Christine,
I wasn’t criticizing your clarifications, I just wanted to discourage other people from pursuing this topic further here.
It’s natural for a discussion to evolve away from the original topic, but I really want to stop comment threads here from evolving into a general physics discussion board, unrelated to the postings. The main reason for this is simply that moderating such discussions is difficult, time-consuming and unrewarding work, something I don’t have the time or interest to be doing. It’s hard enough to figure out how to sensibly moderate comments that are on-topic…
I think it would be a good idea to focus on what SW actually said, and not try to mindread about what he does or does not “still believe”. The discussion has been interesting, especially Christine’s comment, but we could waste our opportunity to learn, if we don’t pay adequate attention to the actual soundbites. Some are just repeats of what we’ve heard before, but others could indicate a shift of perspective or a change in the zeitgeist weather.
Here are some essential snippets, some oldish, some newish:
==excerpts==
It’s developed mathematically, but not to the point where there is any one theory, …
… although in a sense the theorists think there is only one theory, there are an infinite number of approximate solutions of it and we don’t know which one corresponds to our world.
… in a large variety of the solutions of superstring theory there is supersymmetry visible at low energies, and if we see supersymmetry at low energies, superstring theorists may be able to derive from it some kind of clue …
… I don’t see how that would work, it would be.. I mean I couldn’t say that that was likely with any degree of sincerity, …
I myself, well I was working on superstring theory in the 80s and gave it up because I… I moved into cosmology, which in the last couple of decades has had the excitement that elementary particle physics had in the 60s and 70s, a wonderful coming together of theory and observation…
… My own work very recently has been trying to develop an alternative to superstring theory as a way of making sense out of quantum gravity at very high energies. But even though I’m working on this I still find superstring theory more attractive, but not attractive enough…
==endquote==
What mainly impresses me is a certain earthy openness, he’s willing to expose his hunches—feelings about what is exciting now (why cosmology rather than particle physics) and what is likely or not likely (deriving clue-help for string from appearance of susy at low energy).
Hello, Peter. Could you devote an entry in the blog to comment this recent “development”? I’m really interested in your opinion about it. Thanks!
j.
Already done, see
http://www.math.columbia.edu/~woit/wordpress/?p=2172