I have a long-percolating post that I hope to finish soon (when everything else is finished!) on “Why String Theory Must Be Right.” Not because it actually must be right, of course; it’s an hypothesis that will ultimately have to be tested against data. But there are very good reasons to think that something like string theory is going to be part of the ultimate understanding of quantum gravity, and it would be nice if more people knew what those reasons were.
Of course, it would be even nicer if those reasons were explained (to interested non-physicists as well as other physicists who are not specialists) by string theorists themselves. Unfortunately, they’re not. Most string theorists (not all, obviously; there are laudable exceptions) seem to not deem it worth their time to make much of an effort to explain why this theory with no empirical support whatsoever is nevertheless so promising. (Which it is.) Meanwhile, people who think that string theory has hit a dead end and should admit defeat — who are a tiny minority of those who are well-informed about the subject — are getting their message out with devastating effectiveness.
The latest manifestation of this trend is this video dialogue on Bloggingheads.tv, featuring science writers John Horgan and George Johnson. (Via Not Even Wrong.) Horgan is explicitly anti-string theory, while Johnson is more willing to admit that it might be worthwhile, and he’s not really qualified to pass judgment. But you’ll hear things like “string theory is just not a serious enterprise,” and see it compared to pseudoscience, postmodernism, and theology. (Pick the boogeyman of your choice!)
One of their pieces of evidence for the decline of string theory is a recent public debate between Brian Greene and Lawrence Krauss about the status of string theory. They seemed to take the very existence of such a debate as evidence that string theory isn’t really science any more — as if serious scientific subjects were never to be debated in public. Peter Woit agrees that “things are not looking good for a physical theory when there start being public debates on the subject”; indeed, I’m just about ready to give up on evolution for just that reason.
In their rush to find evidence for the conclusion they want to reach, everyone seems to be ignoring the fact that having public debates is actually a good thing, whatever the state of health of a particular field might be. The existence of a public debate isn’t evidence that a field is in trouble; it’s evidence that there is an unresolved scientific question about which many people are interested, which is wonderful. Science writers, of all people, should understand this. It’s not our job as researchers to hide away from the rest of the world until we’re absolutely sure that we’ve figured it all out, and only then share what we’ve learned; science is a process, and it needn’t be an especially esoteric one. There’s nothing illegitimate or unsavory about allowing the hoi-polloi the occasional glimpse at how the sausage is made.
What is illegitimate is when the view thereby provided is highly distorted. I’ve long supported the rights of stringy skeptics to get their arguments out to a wide audience, even if I don’t agree with them myself. The correct response on the part of those of us who appreciate the promise of string theory is to come back with our (vastly superior, of course) counter-arguments. The free market of ideas, I’m sure you’ve heard it all before.
Come on, string theorists! Make some effort to explain to everyone why this set of lofty speculations is as promising as you know it to be. It won’t hurt too much, really.
Update: Just to clarify the background of the above-mentioned debate. The original idea did not come from Brian or Lawrence; it was organized (they’ve told me) by the Smithsonian to generate interest and excitement for the adventure of particle physics, especially in the DC area, and they agreed to participate to help achieve this laudable purpose. The fact, as mentioned on Bloggingheads, that the participants were joking and enjoying themselves is evidence that they are friends who respect each other and understand that they are ultimately on the same side; not evidence that string theory itself is a joke.
It would be a shame if leading scientists were discouraged from participating in such events out of fear that discussing controversies in public gave people the wrong impression about the health of their field.
Joe,
I think we can agree that if the LHC sees something new, people will, if they can’t get their model to fit the data, drop it and start working on something that maybe will fit the data. But what if there is not something new (i.e. just a SM Higgs), then what?
Sure, the best argument for string phenomenology is that by playing with these models people will learn things and come up with new possible BSM signatures for the experimentalists to look for. The best argument against it is that it may be covering only a wrong-headed piece of the space of possible new physics, experimentalists might pay too much attention to this, and make a huge mistake like design their triggers to look for the wrong thing.
I agree with your explanation of why people are doing these calculations, just disagree that this is a good thing. Having some new tools that allow you to push forward down a direction that hasn’t worked means it’s possible to do this, doesn’t mean it’s a good idea.
The question I always come back to is, what if the landscape is essentially correct? That is, what if string theory is the correct theory of the world, that it has zillions of metastable vacua, and that inflation creates zillions of “pocket universes” (our universe being one of them), each with its own metastable vacuum?
I’ve never heard an argument why this could not possibly be true. And if it is true, it simply does not matter that it cannot predict the details of our particular particle physics, just like it does not matter that the theory of planet formation cannot predict the details of our particular solar system.
In this case, the thing to do would be to strive to narrow down the possible string vacua that are compatible with the Standard Model (as amended, if necessary, by whatever is found at the LHC), and then to look for predictions common to this class. Unless and until this is done, it is simply not possible to make blanket statements as to what it will or will not be possible to predict.
“People can read it for themselves, but I think the Choi-Nilles review is clear in its claims, and they are explicitly not claiming what you are, that string theory leads to specific values for the gaugino mass ratios.”
Peter, could you please point to my comment where I claimed that “string theory leads to specific values for the gaugino mass ratios.” I can’t seem to find where I said that this prediction would be model independent.
Mark Srednicki:
I agree with your comment. It is basically in the same vein as my commentary with Jacques. Can a ground state be found that is in fact consistent with the SM and future observations from the LHC?? Nobody seems to know the answer but some researchers such as Jacques apparently are still very hopeful and I wish them the best. What will it take to have them accept the reality of a landscape, IF it exists?? Beats me.
I have thought about the same scenario as you Mark. It very well maybe that that is just the nature of the beast and we will just have to accept it. Zero predictability. This maybe the result of ANY TOE. Period. It can not be proved one way or the other.
c,
“I’m unaware of any construction or a set of constructions where one can get “just about anything”
for the ratios of the gaugino masses”
“Peter, have you tried to understand the state of SUSY phenomenology? Broadly speaking, there are a few basic ways of mediating SUSY-breaking to the Standard Model fields, and they tend to be fairly robust in terms of the outcome for basic quantities like the ratios of gaugino masses.”
OK, the last one was from “anon”, not you, and I guess I missed the weasel-word “tend”, and the fact that it allows for the fact that some models don’t give robust results for gaugino masses.
Not clear that it’s worth arguing about the definition of “just about anything”, seems to me an accurate description of what Choi-Nilles say you can get out of two string constructions.
For a while I actually did believe, based upon what you and anon were saying, that there actually were robust predictions of a sort about gaugino masses that I didn’t know about.
Cecil & Mark,
Excuse me, but harping on with questions like
“What if [insert weird, untestable idea] is essentially correct?”
is not the scientific method. Scientific method invariably consists in choosing the simplest possible explanation for the observational data.
Cecil, I think it is very unlikely that we will end up with *zero* predictability, though we may only be able to predict things that are too much of a technological challenge to measure (eg, superplanckian scattering cross sections). But we just won’t know until a lot more work is done.
There is also the possibility that there are still key missing ingredients that will radically change the picture. For example, Lee Smolin says that he suspects that the possibility that ours is one of a vast collection of universes with random laws “is not in fact a logical possibility in the correct theory of quantum cosmology” and that “by the time we understand quantum cosmology the notion of time will have been so altered as to make [this possibility] either nonsense or uninteresting” and that this could all happen within the framework of string theory. (Quotes extracted from various comments by Lee at http://asymptotia.com/2007/03/23/questions-and-answers-about-theories-of-everything ) This, or something like it, could well turn out to be the case. But we just won’t know until a lot more work is done.
Some people apparently think this work should not be done, because string theory has “failed”. I think that giving up on string theory at this point would be like giving up on the nebular theory of planet formation in the 19th century, because it could not predict the number and type of planets in our solar system.
Chris, if I knew of a simpler explanation than string theory for the observational data (Standard Model plus gravity), I would embrace it. But I don’t. (The various flavors of LQG do not qualify, in my view, because no one has shown that any of them has the required semiclassical limit, and my intuition [which could be completely wrong] is that none of them do.)
Another point to remember: the Standard Model is not pretty. It’s got three different gauge groups with three different gauge couplings, fermions in five different irreducible representations, Yukawa couplings that range over five-and-a-half orders of magnitude. ANY correct theory must reproduce this mess. It’s not going to be easy, or pretty, for ANY theory to do this. ANY framework will either have to explain why this mess is the unique ground state, OR invoke a landscape-style explanation.
Compare and contrast:
Sigh.
I really have been wasting my time here, haven’t I?
Oh well, maybe I’ll try again the next time I see the (even illusory) semblance of a physics argument again.
Peter, you said: “People can read it for themselves, but I think the Choi-Nilles review is clear in its claims, and they are explicitly not claiming what you are, that string theory leads to specific values for the gaugino mass ratios.”
How is my saying: “I’m unaware of any construction or a set of constructions where one can get “just about anything” for the ratios of the gaugino masses” equivalent to “string theory leads to specific values for the gaugino mass ratios”?.
It seems absurd to say that I have claimed “STRING THEORY leads to specific values for the gaugino mass ratios” when I was taking about the existing classes of models where the predictions for the gaugino masses were DIFFERENT. I went through the relevant papers trying to give you some very specific technical arguments to reinforce my claim that the two models you said can give “just about anything” can be quite predictive by explaining the ways to address the uncertainties described by Nilles and Choi.
“Not clear that it’s worth arguing about the definition of “just about anything”, seems to me an accurate description of what Choi-Nilles say you can get out of two string constructions.” Can you get M1
“Not clear that it’s worth arguing about the definition of “just about anything”, seems to me an accurate description of what Choi-Nilles say you can get out of two string constructions.” Can you get M1 smaller than M2 smaller than M3 in the G2 compactifications? No, you cannot because the anomaly mediated contribution to M1 is always smaller than the tree level value, even when the threshold effect to the anomaly mediated piece are taken into account.
Jacques,
Your tactic of taking what I write out of context, deleting parts of it, and changing the rest by adding emphasis that wasn’t there in the original, then pairing the result with some sneering comments on your part is as charming as ever.
Mark Srednicki, #302 and on. I can’t believe that I see it again. Here is what I commented on Peter’s blog just couple of weeks ago, referring to a comment made by somebody who claimed to be “a string theorist”:
“Your argument about planetary orbits is yet another propaganda trick pulled by AP desperados, that appears in the introduction part of all Arkani-Hamed talks (at least of those that I attended). Yes, we do not understand initial conditions for many motions in the Universe, and its not a big deal that we do not understand them for the solar system which is in our nearest proximity. However, you cannot compare this ignorance to our lack of understanding of say electron’s mass which is relevant not only in our proximity, but everywhere that we know in the Universe.”
Jean-Paul, the point (which I’m sure you understand) is that, in the scenario under discussion, our universe is just one tiny piece of a much larger “multiverse”. In other universes, the electron has a different mass, or doesn’t exist at all. This is exactly analogous to our living in a single solar system that is just one tiny piece of a galaxy, with each solar system having different properties.
Some may claim that the analogy is not precise, because there is (as far as we know now) no way to observe the other universes, whereas we can observe the other solar systems (to some limited extent). I don’t think this objection is very strong, however, as we can easily imagine intelligent life in (say) the atmosphere of a gas giant, with no hope of building technology that could see through their clouds. Yet any hopes they might have of explaining their world based on its (to them) unique properties would be hopeless, because their world was in fact produced by random initial conditions.
We might be in the same situation with regard to the particle physics we see in our universe. I personally think this is rather likely, precisely because the Standard Model is such a mess; it’s hard for me to see how it could emerge as the unique ground state of any “nice” theory, other than in a landscape scenario.
And, it would be nice if the critics of string theory would propose something else to work on. (I’m exempting Lee here, since he has his own well-articulated program.) Peter Woit has said that we should work on nontrivial representations of gauge groups, but it’s awfully hard to figure out what this would mean: in QED, the statement that the trivial representation is the correct one is equivalent to imposing Gauss’ Law. So a nontrivial representation would violate Gauss’ Law. Introducing states that violate Gauss’ Law just doesn’t seem helpful to me. And it certainly doesn’t seem to point towards any explanation of why the top quark is 5.5 orders of magnitude heavier than the up quark …
It’s easy to throw stones at houses one doesn’t like, much harder to design and build one yourself …
QJT.
The idea is simply that the algebraic structure underlying quantum gravity should be the combination of those underlying general relativity (background independence) and quantum mechanics (lowest-energy representations). As is well known already in 1D, interesting lowest-energy representations of the diffeomorphism algebra are necessarily anomalous (Virasoro algebra), and the higher-dimensional case is not different (multi-dimensional Virasoro algebra).
Dear all,
Overall, this seems to be a good discussion and it gives quite a good picture on the matter of how string theory will connect to empirical evidence. Two points which I was getting are:
1) While string theory as a whole does not give at present (and there is no guarantee it will give in the future) concrete predictions (e.g. Gaugino mass ratios; whatever they mean) specific fragments of string theory do.
Therefore, new empirical findings expected in the (5-10) years ahead may lead to developments within string theory. The role of such empirical evidence to establish the correctness of string theory seems rather questionable. So is the (related) issue if it will allow for concrete predictions from string theory on later empirical observations.
2) It is somewhat premature to discuss it since we do not have yet a single example of a model from string theory that agrees with the standard model. (When one such model will be found there will be automatically many such models.)
I hope I got the picture at least approximately right.
(And people also discussed several scenarios/prophecies on what LHC will find. I can very cautiously offer the following prophecy: If the LHC findings will be depressing we will be depressed while if the findings will be encouraging we will be cheered.)
Dear Peter,
You wrote : “…and if the model makes testable predictions that were checked, of course I would agree that I had been completely wrong, highly foolish, and shut up about this for good.”
I do not think we should regard this discussion as a “prophecy competition.” If you (or anybody else) make good (and novel) academic/scientific arguments “against” string theory then there is nothing to be ashamed of even if later the ideas turned out to be false. (And vice versa!).
Maybe, I have an over-romantic view about science but the way I see it the quality of the arguments themselves counts a lot and not just the bottom-line.
The subcritical free string has a conformal gauge anomaly, but can nevertheless be quantized with a ghost-free spectrum (no-ghost theorem). So not all anomalous gauge theories are inconsistent.
Of course, an anomalous gauge symmetry becomes a global symmetry on the quantum level, which acts on the Hilbert space instead of reducing it. So anomalies are a means to gauge symmetry breaking, possibly an alternative to the Higgs mechanism.
Mark,
I am no expert on String Theory, but one does not need to be an expert to realise that Occam’s Razor does not apply to the vast majority of the efforts in this direction. See, for example, comment #204 here. The extra dimensions seem to be what leads to all the problems, but there was no interest whatsoever when a former colleague of mine advanced the possibility of a 4D version. As far as I can see, the attitude of ST practitioners has never been scientific – it has always been, “Gee, Wow! Let’s see where this amazing idea leads!” Which brings me on to your comment: “it would be nice if the critics of string theory would propose something else to work on”. This has been happening all the time – my own modest efforts being part of this – it is just that no-one notices because of the noise generated by the excitable children who do String Theory.
Chris, you don’t seriously think that somebody who knows The TRUTH is interested in alternatives? 🙂
Thomas,
You’re right. All this scepticism … I feel terrible. Makes me feel a bit like Pontius Pilate – “What is truth?” Sorry I spoke. Maybe as I type this, the veil of the temple is being rent in twain – which I guess here in London would mean the Temple Underground Station – I’ll check the news when I finish work today.
Mark,
I am referring to your comments #308 and #315.
First and foremost, please note that I am no expert in theoretical physics. So, most of my comments will not be on your physics arguments (which I am not able to comment properly).
You are saying:
“And, it would be nice if the critics of string theory would propose something else to work on.”
This is a weak argument, for at least two reasons.
First, if I see a weakness in your theory, I can point it to you, and I am not required to provide an alternative theory.
But, second, and most important, building a ST-like theory is an immense task. As far as I understand, it has required 20+ years of work of top level physicist.
You cannot seriously ask for “something alternative”. Years of work by Witten and all the others string theorist(including, as I understand correctly, you) cannot be replaced by the work of one or two persons (unless they are true geniuses).
If the problem was simple, you and your collegues would have already solved it, and everyone would be happ about the solution.
I cannot judge your results. But you can. You have said “Chris, if I knew of a simpler explanation than string theory for the observational data (Standard Model plus gravity), I would embrace it”.
The point is, you cannot find what you are not searching for. Don’t you believe it could be a worthwile effort to try to define an alternative to String Theory?
At first I was going to try and respond to Mark Srednicki’s comments about me and about representations of gauge groups, but decided that this would take this thread far off-topic. But then I realized that Mark’s comment is perfectly on-topic. Sean described string theory advocates as losing the public debate, because they won’t “Make some effort to explain to everyone why this set of lofty speculations is as promising as you know it to be”. Instead, Mark seems to think that he”ll convince people of the promise of string theory by attacking Smolin and me, including behavior like publicly jeering that Smolin is a “crackpot” (at an event being videoed for public distribution).
Go right ahead and keep doing this if you want to keep damaging your own cause.
Hi Peter,
Mark and Lee had recently nice and very interesting physics exchanges on “asymptotia”. (They both made interesting points and I think they even agreed at the end on some issue.) It could have been nice to have a discussion on the usefullness of non-trivial representation of guage groups and their potential role. And how this can still be consistent with Gauss’ law. If you think it is off-topic you can simply ignore this remark. But bringing back this “crackpot” things seem counter-productive.
Anyway, I do not think that questioning the role of non-trivial representations of gauge groups can be regarded as a personal attack of any kind. (Or perhaps we should work for a universal agreement that the only personal attacks from now on will be representation-theoretic.)
Actually this point even looks like an interesting scientific issue.