As particle physicists eagerly await results from the LHC, many theorists are already promoting interpretations of what they hope it will find. This week’s Chronicle of Higher Education has a cover story on the LHC entitled The Machine at the End of the Universe (see associated articles here and here). In it, Gordon Kane enthusiastically describes the LHC as “It is certainly the most important experiment of any kind in the past century, without qualification” and “the most important thing ever in our quest to understand the fundamental laws of nature and the universe.”
The question that the LHC will actually address, that of electroweak symmetry breaking, doesn’t get much attention. Instead, the focus is on supersymmetry, extra dimensions and string theory. While noting that there’s no evidence for string theory, the article reports:
The new collider could change that, says Joseph D. Lykken, a physicist at Fermi National Accelerator Laboratory. “Either the discovery of supersymmetry or extra dimensions is a triumph of string theory.” While such a finding would not conclusively show that string theory is correct, it would provide a first crucial experimental test, he says.
At New Scientist this week, there’s another article about the LHC, entitled Awaiting a messenger from the multiverse. In it, Savas Dimopoulos explains that there’s already quite a lot of experimental evidence against weak-scale supersymmetry:
“After a lot of experiments there has not been any hint of SUSY,” says Dimopoulos. For each individual one of those predictions, he says, you can find plausible explanations for why they are not seen. “But by the time you look at the whole package of ‘things that should have happened but didn’t’, you start getting a somewhat baroque structure.”
Dimopoulos instead promotes his joint work with Nima Arkani-Hamed on the idea of “split-supersymmetry”, where the supersymmetry breaking scale is very high, so can’t explain the hierarchy problem. One possible experimental signature of such models would be a long-lived gluino. They promote the idea that such a thing would be a “Messenger From the Multiverse”, the idea being that if supersymmetry doesn’t explain the hierarchy problem, the explanation must be the anthropic landscape:
That powerful piece of evidence would have dizzying implications. “It would be a strong indication that there is a string landscape or a multiverse,” says Dimopoulos. “I think the majority of opinion would come around to that point of view.”
One aspect of this argument is that it also works if no gluino is seen. If no superpartners at all are found at the LHC, and thus supersymmetry can’t explain the hierarchy problem, by the Arkani-Hamed/Dimopoulos logic this is strong evidence for the anthropic string theory landscape. Putting this together with Lykken’s argument, the LHC is guaranteed to provide evidence for string theory no matter what, since it will either see or not see weak-scale supersymmetry.
The New Scientist article does explain that this kind of argument for anthropics has its critics:
However, anthropic arguments remain controversial, and despite the authors’ heavyweight reputations, split supersymmetry is no exception. “I’m not a big fan of it,” says John Ellis, a theoretical particle physicist at the CERN laboratory, where the LHC is being prepared for its first run later this year. His criticism is that the anthropic approach gives you too much freedom to answer any troublesome question in physics. “At some point you might just as well say ‘let’s fine-tune everything’ and go home,” Ellis says.
Theorist Frank Wilczek at the Massachusetts Institute of Technology is also unhappy with the idea of split SUSY. “I think it’s a logical possibility, but if we really have to appeal to anthropic considerations I think that’s a big retreat. It would mean the explanatory power of theoretical physics would be limited.”
…
And even if the LHC does find Dimopoulos’s stopped gluinos, not everyone will be persuaded that arguments based on the multiverse are good science. “My opinion of anthropic reasoning is likely to remain unprintable,” says Ellis.
Update: Forgot to add one more piece of LHC-related news. France is now joining the US with its own national LHC web-site: LHC-France. Because of the Gallic fondness for cartoons, it includes a section Le LHC en BD.
Is string theory becoming more like a mathematical framework like algebra? Or is it still something that should only be considered as a possible theory of the physical world?
The way I keep hearing about string theory being able to be almost anything and that any experiment would only be providing evidence for the theory is getting me confused.
There’s nothing confusing about how to interpret the phenomenon of a theory that can explain almost anything. It’s what’s known as a failed theory.
It is interesting to note that the major topic of electroweak symmetry breaking (EWSB) remains somehow overshadowed by the strong desire to discover evidence for SUSY and extra-dimensions. Many people tend to forget that the lack of evidence for SUSY at LHC may carry serious implications for the validity of the Higgs mechanism.
Peter,
I’m wondering why you very frequently post links to New Scientist.
My early impression is that you promote advocate a conservative, cautious science, where claims should be qualified and skepticism is best.
I’m a former subscriber to New Scientist (3-4 years back), and I didn’t renew because I found every issue engaged in ultra-sensationalism. While I found their coverage of politics, the environment, and the social sciences interesting, these are fields where my knowledge is shallow. In physics, and in astronomy, it was always my impression they went for the dramatic. The next Einstein is just around the corner, and he’s going to establish contact with aliens by traveling to the multiverse through his LHC time machine. Or something like that.
I remember a cover story where people were claiming that pendulums oscillate differently during solar eclipses because of the ether. Right.
As there are zillions of different publications it’s most probably you specifically chose to read NS as opposed to being compelled to read it due to a lack of options. I’m perplexed by your focus on NS as it does not strike me as a Woit-like publication.
David,
New Scientist is definitely a mixed bag, with a lot of reasonable stories about science (as well as book reviews, opinion columns, etc), but also a fair number of overly sensationalistic stories about physics. Most of these I just ignore, although sometimes I can’t resist making fun of some of the sillier ones that invoke string theory.
But this story is an example where I think the magazine is actually straightforwardly reporting about research claims being made by leaders of the physics community. It’s not the reporter who is putting out sensationalistic nonsense, but two of the most well-known researchers in the field, from Stanford (Dimopoulos) and Harvard/IAS (Arkani-Hamed). Sure, NS stories about how some obscure researcher has an idea for a time machine are just silly and should be ignored by everyone. But when leading figures in particle theory start going on about how the LHC is going to prove the existence of a multiverse, I can see why NS would cover this, and here they seem to have done so responsibly and pretty accurately. The sensationalist nonsense isn’t the fault of the NS writer or editor.
In this particular case, I also thought the quotes from two other leading figures (Wilczek and Ellis) were highly newsworthy, given the ongoing controversy about the multiverse among theorists. Wilczek has had some sympathy for and worked on some anthropic sorts of explanations, here he was critical of this one, which was interesting. The fact that John Ellis’s view of anthropic reasoning is “unprintable”, and that he says it will remain so even in the face of claims from the anthropic crowd to have experimental evidence was definitely news to me.
Hi PW, no I am not a string theorist. Quite convenient to label anyone who doesn’t agree with you with the same label. I have a problem with a string theorist at the IRS… Anyway I used – some while ago – to work on theoretical elementary particle physics (QCD related) but turned since then to more condensed matters… So nothing at stake here except that I am quite upset that a single person can mislead numerous non-experts without any rebuttal from leading scientists (I am not claiming to be one). This explains my anonimity, even a hint that one looks at your blog is highly suspect in any serious physics environment (and no I am not talking about string theory departments).
I think that you mistake my point. You continously attack a main field of research. But whenever a concerned person takes his or her time to give you a serious answer – which unavoidably is technical – you manage to change the topic or you start writing your standard paragraphs (actually this is not only on your blog, we did meet in person and that confirmed the statemenet I just made). A few examples for the non-expert readers who might still believe that you are a leading scientist: reread the thread after “Wonders of supergravity etc.” some items below here. If you go to the Musings blog by Distler or Asymptotia by Johnson, you should make a search on “Woit” and read the discussions. In at least half a dozen instances you manage to completely avoid answering very concrete technical/scientific questions, instead always reiterating the stuff we’ve read many times by now. In fact doing this I see that for many years you are alluding to your slowly progressing research. It remains a fact (give me a counter example if I am wrong) that since 1989 you did not produce a single (published) scientific result. At the same time the innuendo in your blog is such that any uninformed person can only get the idea that you are an active scientist. In my book this is called cheating. Anyway, a few threads below you announce that you are finishing up a project. I am looking forward to see the result of 19 years of hard work… So long, I am out of here for good…
“Lucy”
You’re not hiding behind anonymity because you fear damage to your reputation if someone finds out you have looked at the “Not Even Wrong” blog. You are doing this because you are dishonest. Your anonymous comment comes from the same block of network addresses as those of an identifiable string theorist who has posted critical comments here (and who, when you google him, the second link that turns up is one about how dishonest he is, on matters unrelated to science). Maybe this is you, maybe it’s someone else from the same institution, but looking at the names of physicists working there, there’s no one answering the description you give of yourself, in particular the only person I’ve personally met there is a senior string theorist (not the one who commented here on the blog).
So, I’m convinced that you’re just a liar, using anonymity to lie and engage in character assassination with impunity. As for your characterization of my exchanges with Distler or Clifford Johnson, people can read them for themselves and make up their own minds.
If you have anything else to say, you better do it under your own name.
Peter:
Good comeback to “Lucy”. I read your book and understand that basically the SM was formulated in the time frame 1973-1974. Since then experimental results have been confirmed some parameter values. Other than neutrinos having mass (accepted fact?) have there been in other deviations from the SM?
Now there is continuing reference to leading particle physicists, other than the recent Nobel prize winners Gross, Politzer, & Wilczek (2004), ‘T Hooft & Veltman (1999) and Glashow,Salam & Weinberg (1979) has there been any particular HEP physicist that really stands out? Not about speculative ideas such as ST but an idea that really attempts to address an issue with the SM? You seem to have great respect for Witten but can you cite anything that Ed has doen that would merit a Nobel? Who is your leading canidate for work done since 1973?
p. falor,
Besides neutrino masses, no definite deviations from the SM have shown up at accelerators. From astrophysics, there’s evidence for “dark matter” of a sort that doesn’t seem to fit into the SM.
Since 1973, no theorist has come up with a big breakthrough in experimentally testable HEP theory. That’s why there have been no Nobel prizes for work done since then in this area. Many people have done significant work that helps to better understand QFT. A lot of this is from Witten, whose investigations of the relations of QFT and math won him the Fields medal, a recognition in math even harder to get than a Nobel prize in physics. The work of Witten’s that I can think of that is related to experiment and might be worthy of a Nobel prize is his early 80s work on large N, current algebra and the “skyrmion”. This is a quite beautiful approximate model for the low-energy behavior of QCD. Other people also contributed to this though, and the goal of this work, a good 1/N expansion for QCD, still hasn’t been achieved.
p. falor and Peter,
I wish to add few comments on deviations from SM. It is important to remind ourselves that, while SM is remarkably consistent with experimental observations, it is considered incomplete for the following main reasons:
a) SM does not include the contribution of gravity and gravitational corrections to both QFT and renormalization group equations; b) SM does not fix the large number of free parameters that enter the theory (in particular the spectra of masses, gauge couplings and fermion mixing angles); c) SM has a gauge hierarchy problem, which requires fine-tuning; d) SM postulates that the origin of electroweak symmetry breaking is the Higgs mechanism, whose confirmation is sought at LHC and ILC. The number and physical attributes of the Higgs boson are neither explained by SM nor fixed from first principles, e) SM does not clarify the origin of its underlying gauge group and why quarks and leptons occur in certain representations of this group, f) SM does not explain why the weak interactions are chiral, that is, why the force transmitted by the triplet of massive vector bosons is sensitive to handedness g) SM does not explain the origin of CP violation and the underying cause of the lepton anomalous magnetic moment.
The Wilczek quote is particularly ironic because his latest paper, arXiv:0807.1726, is about making predictions using anthropic reasoning. I think this reflects the general situation: no one likes it, but there are reasons to believe that anthropic selection plays a role in determining why our universe is the way it is, and if so then we have to learn to deal with it.
Sulfur Surfer,
The problem with anthropic reasoning has nothing to do with whether one “likes it”, but with whether it can be used to do real science, or is just being used as an excuse for propping up failed ideas, in such a way that even though they don’t work, they can never be shown to be wrong.
I take Wilczek’s point of view to be that some uses of anthropic argumentation are a legitimate part of model-building, and that’s what he’s trying out. Interesting to see that it appears that he doesn’t see the Dimopoulos/Arkani-Hamed argument as falling into this category. Perhaps all there is to it is that he recognizes the obvious fallacy in the argument that: “If supersymmetry doesn’t explain the weak scale, it must be the string theory anthropic landscape”.
For more from Wilczek about his point of view on this, a good source is:
http://arxiv.org/abs/hep-ph/0512187
where he warns:
“let me lament our prospective losses, if we adopt anthropic or statistical selecton arguments too freely…”
and notes
“Resort to anthropic reasoning involves plenty of pain, as I’ve lamented, but so far the gain has been relatively meagre, to say the least.”
Peter,
Just a thought from a refugee from the ’60’s. “Lucy in the Sky with Diamonds” is a very good Beatles song that turns out to be about LSD. I wonder if “Lucy” realizes this.
Maybe I should have signed this “Yellow Submarine,” which is about amphetamines.
> Putting this together with Lykken’s argument, the LHC is
> guaranteed to provide evidence for string theory no matter
> what, since it will either see or not see weak-scale
> supersymmetry.
String theory is a framework for constructing UV complete theories while having gravity. It is really a paradigm in that sense, like QFT was for theories without gravity. ANY experiment at low energies is “evidence” for QFT. That hardly means that a specific QFT is not falsifiable.
The real problem is that constructing models with the stringy paradigm is hard because we don’t understand the theory well enough. That would be a valid and perhaps fatal criticism for string theory as a useful theory. But comments like the above are merely strawmen.
somebody,
The above was not a criticism of string theory, but of string theory partisans, at least those who think it’s a good idea to go to the press with bogus claims that the LHC will test string theory or the the anthropic multiverse.
Peter,
I’m just curious what your position on string theory would be in the case that either the superpartners are found or there is evidence of large extra dimensions such as black holes or KK modes? Would you consider this evidence in favor of string theory?
Eric,
Depends exactly what you see, but, in general, no. For instance, it might very well be that some variant of N=8 supergravity gives a finite quantum theory of gravity, which is not a string theory. If supersymmetry shows up at the LHC, maybe it’s that…
Peter:
Going beyond the anthropic, and even the string issues, to the more important problem of the stagnation of particle physics in general, do you have any sketched-out notions on what particle physics – or better, that segment not wedded to theories/programs that are too adaptable to be tested – could do to be prepared to test things with the tests the LHC could do?
Even if the various measurements and experiments that can be performed with the LHC don’t effectively test, say, stringe/brane/M theory, is it possible for them to test other things? Advance particle physics and weed out theories the way the lack of proton decay did, for instance?
Is LQG, are twistor theories, is Lisi’s extension of standard physics, as untestable w/r/t the LHC as the current dominant line of string theory is?
Also, will some string theory lines probably be closed off by LHC-performed measurements and experiments?
[RE: “ANY experiment at low energies is ‘evidence’ for QFT.”]
Somebody,
Even as a framework, QFT has certain empirically founded underpinnings, does it not? (I have in mind special relativity.) What are those, and what evidence supports them?
Similarly, as a framework, what empirically founded underpinnings does string theory rely upon? Does it assume anything that QFT doesn’t assume? If so, what is the empirical support for those assumptions?
Peter, have you found anything interesting among the talks at
titled
“In Search for Variations of Fundamental Couplings and Mass Scales”
Somebody, you say:
“The real problem is that constructing models with the stringy paradigm is hard because we don’t understand the theory well enough”
How long should one wait for String Theory to reach maturity? Would reaching that stage guarantee that the astronomical number of vacua will suddenly disappear or that the theory will yield testable predictions?
“Similarly, as a framework, what empirically founded underpinnings does string theory rely upon? Does it assume anything that QFT doesn’t assume? If so, what is the empirical support for those
assumptions?”
Ok, I’ll try.
Perturbative string theory has something called conformal invariance on the worldsheet. The empirical evidence for this is gravity. The empirical basis for QFT are locality, unitarity and Lorentz invariance. Strings manage to find a way to tweak these, while NOT breaking them, so that we can have gravity as well. This is oft-repeated, but still extraordinary. The precise way in which we do the tweaking is what gives rise to the various kinds of matter fields, and this is where the arbitrariness that ultimately leads to things like the landscape comes in.
In principle, this is a model-building/technical control issue. But it is a pretty damn serious one, and probably something that will require us to understand the theory much better than we do now. Besides, the kind of understanding that we have of string theory at the moment allows us to build only certain kinds of models.
About other empirical expectations: string theory has a lot of GENERIC features we expect from a good theory of the world. It can easily give rise to things like multiple generations, non-abelain gauge symmetry, chiral fermions, etc. some of which were considered thorny problems before. Again, constructing PRECISELY our matter content has been a difficult problem, but progress has been ongoing.
But the most important reason for liking string theory is that it shows the features of quantum gravity that we would hope to see, in EVERY single instance that the theory is under control. Black hole entropy, gravity is holographic, resolution of singularities, resolution of information paradox – all these things have seen more or less concrete realizations in string theory. Black holes are where real progress is, according to me, but the string phenomenologists might disagree. 🙂
Notice that I haven’t said anything about gauge-gravity duality (AdS/CFT). Thats not because I don’t think it is important, but because I think it is THE most important thing. But at the same time, I don’t know of a good way to convince a layman (assuming you are one) of its importance. Because it is one of those cases where two vastly different mathematical structures in theoretical physics mysteriously give rise to the exact same physics. In some sense, it is a bit like saying that understanding quantum gravity is the same problem as understanding strongly coupled QCD. I am not sure how exciting that is for a non-string person, but it makes me wax lyrical about string theory. It relates black holes and gauge theories. Things like AdS/quark-gluon-plasma stuff is interesting not merely because we can make some semi-quantitative predictions, but also because it has opened up new ways of THINKING. You can find a bound for the viscosity to entropy ratio of condensed matter systems, by studying black holes – thats the kind of thing that gets my juices flowing. Notice that none of these things involve far-out mathematical masturbation, this is real physics – or if you want to say it that way, it is emprically based.
To me, the immediate reason for doing string theory is not falsifying it at LHC. Of course, to get funding you need to sell it to the powers-that-be, and LHC is perhaps all they care about. But really, LHC is just a miniscule blip in the energy-scales all the way up to Planck scale. Should we expect to see ANYTHING interesting at LHC? Perhaps. Perhaps not. Of course it would be nice to see some indication of string theory there because public relations is unfortunately important in science, but I personally think that people are being short-sighted when they hedge their bets on LHC when selling it to the piublic.
String theory is a large collection of promising ideas firmly rooted in the emprirical physics we know which seems to unify theoretical physics[1], and thats why it should be worked on. The spinoffs are in that sense are OBVIOUS, but they are not necessarily going to fit anybody’s schedule for making a collider.
[1] I don’t mean unification of forces necessarily, I mean convergence of the different ingredients that constitute theoretical physics.
‘Perturbative string theory has something called conformal invariance on the worldsheet. The empirical evidence for this is gravity. The empirical basis for QFT are locality, unitarity and Lorentz invariance. Strings manage to find a way to tweak these, while NOT breaking them, so that we can have gravity as well. This is oft-repeated, but still extraordinary. The precise way in which we do the tweaking is what gives rise to the various kinds of matter fields, and this is where the arbitrariness that ultimately leads to things like the landscape comes in. … It can easily give rise to things like multiple generations, non-abelain gauge symmetry, chiral fermions, etc. some of which were considered thorny problems before. Again, constructing PRECISELY our matter content has been a difficult problem, but progress has been ongoing. …
But the most important reason for liking string theory is that it shows the features of quantum gravity that we would hope to see, in EVERY single instance that the theory is under control. Black hole entropy, gravity is holographic, resolution of singularities, resolution of information paradox – all these things have seen more or less concrete realizations in string theory. Black holes are where real progress is, according to me, but the string phenomenologists might disagree. Notice that I haven’t said anything about gauge-gravity duality (AdS/CFT). Thats not because I don’t think it is important, … Because it is one of those cases where two vastly different mathematical structures in theoretical physics mysteriously give rise to the exact same physics. In some sense, it is a bit like saying that understanding quantum gravity is the same problem as understanding strongly coupled QCD. I am not sure how exciting that is for a non-string person, but it makes me wax lyrical about string theory. It relates black holes and gauge theories. …. You can find a bound for the viscosity to entropy ratio of condensed matter systems, by studying black holes – thats the kind of thing that gets my juices flowing. Notice that none of these things involve far-out mathematical masturbation, this is real physics – or if you want to say it that way, it is emprically based. … String theory is a large collection of promising ideas firmly rooted in the emprirical physics we know which seems to unify theoretical physics …’
No it’s not real physics because it’s not tied to empirical facts. It selects an arbitary number of spatial extra dimensions in order to force the theory to give the non-falsifiable agreement with existing speculations about gravity, black holes, etc. Gravity and black holes have been observed but spin-2 gravitons and the detailed properties of black holes aren’t empirically confirmed. Extra spatial dimensions and all the extra particles of supersymmetries like supergravity haven’t been observed. Planck scale unification is again a speculation, not an empirical observation. The entire success of string theory is consistency with speculations, not with nature. It’s built on speculations, not upon empirical facts. Further, it’s not even an ad hoc model that can replace the Standard Model, because you can’t use experimental data to identify the parameters of string theory, e.g., the moduli. It’s worse therefore than ad hoc models, it can’t incorporate let alone predict reality.
“How long should one wait for String Theory to reach maturity?”
I have no idea about this of course, but I think this question arises from the feeling that string theory is like a tunnel. You get into it on one side and the enterprise is “wasted” unless you get out the other end with predictions for electron masses or whatever. But the truth is that there is a lot of understanding+spin-offs that we have gleaned on the way, some of which I mentioned in my response to Chris W.
Another thing I have noticed is that people tend to conflate the fact that we have not managed to solve the problem FULLY, with the incorrect statement that we have not managed to solve it AT ALL. This arises because theoretical physics is such an esoteric enterprise and it is hard to give people a feel for the progress until you reach a concrete enough punctuation mark, like say, “predicting electron masses”. But there has been tremendous progress in constructing string vacua which was unimaginable even ten years ago.
I just think that string theory has some great tools and ideas and it would be ridiculous not to use them when thinking about fundamental problems. For instance, holography in gravity was a very interesting observation, but nothing really concrete, before Maldacena. When Maldacena came along, connected up gauge theories with strings&gravity and broke new territory, holography actually became a useful TOOL and not merely a curiosity. These sort of insights are certainly of relevance in our way forward. To brush them all off and just stick to what we knew before strings would be counter-productive – even if you are convinced that strings are ultimately NOT a central ingredient in the construction of the universe. In particular, string theory allows you to do quantum gravity thought experiments, especially in the context of black holes, which would have been impossible without it.
“Would reaching that stage guarantee that the astronomical number of vacua will suddenly disappear or that the theory will yield testable predictions?”
If we reach that stage (if pigs had wings), we would be able to construct MODELS that yield testable predictions. The existence or non-existence of the landscape is not what makes a theory non-predictive, despite popular impression. Most theories (general relativity, QFT) have landscapes. It is not their existence, but our inability to systematically construct them that is the problem.
Or at least this is one way (the boring, frontal, difficult way) in which a solution can be realised. But there might be new ideas that can come up which might mix things up. It is only because people kept plugging away back in the days of the heterotic string (even though we had no idea how to stabilize moduli) that we have understood things about black holes and ads/cft. Right now is CERTAINLY not a worse off position than many other periods in the strange history of string theory.
Anyway, thanks for those questions, it made me think about what is important and what is not in string theory myself.
Anon.:
The problems we are trying to solve, like “quantizing gravity” are already speculative by your standards. I agree that it is a reasonable stand to brush these questions off as “speculation”. But IF you consider them worthy of your time, then string theory is a game you can play. THAT was my claim. I am sure you will agree that it is a bit unreasonable to expect a non-speculative solution to a problem that you consider already speculative.
Incidentally, I never said a word about supersymmetry and Planck scale unification in my post because it was specifically a response to a question on the empirical basis of string theory. So I would appreciate it if you read my posts before taking off on rants, stringing cliches, .. etc. It was meant for the critics of string theory who actually have scientific reasons to dislike it, and not gut-reactions.
This has gotten pretty much completely off-topic, and nothing new or informative is being said. Enough.
Thanks, Somebody, for that response. It was helpful to see the main threads brought together that way. (And yes, I am a layman.)
my view of anthropic arguments such as the anthropic principle is that they are not scientific and have no place in physics.
it seems to me to be a priori that the existence of human beings has no role in how the universe works. the universe existed long before human beings.
that the constants of nature happen to be those that allow us to exist does not say anything, of course they are that way, otherwise we would not be observing them.
to invoke a premise that there is some grand design at work is not physics but rather religion.
real science produces testable refutable theories, the anthropic principle is not in that category.