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.
Hi Peter,
It’s not clear that trying to reproduce the precise structure of the Standard Model is a useful way to go. There is lots of evidence that string theory can give models with the qualitative matter structure of the SM: chiral fermions, multiple generations etc, and there are a host of explicit models to this end. As a gauge group, SU(3) x SU(2) x U(1) seems born to come from an intersecting brane model.
However it also seems that, supposing a string compactification does describe the real world, that the question of computing e.g. the Yukawa couplings will be isomorphic in difficulty to the problem of explicitly solving the Dirac equation on a full-blown Calabi-Yau and computing the resulting wavefunction overlap. This is a bloody hard problem, and maybe there’s a clever solution, but it’s not obvious a frontal assault on it is the best way to make progress.
A nice thing about string theory is that it can relate physics which from a low-energy EFT point of view is very different. This is why I would say it is more useful to look at the inter-relations that arise in different compactifications rather than trying to ask `can string theory predict the electron mass? yes or no?’
Best wishes
Joe Conlon
Jacques,
You are engaging in your standard tactic of deleting the part of what I write which contains my argument, then proudly saying that you have shown that I have no argument. I’m not going to bother with addressing the long list of out-of-context quotes with added formatting that you took the trouble to construct, the statements in them are accurate if sometimes requiring more detail to make clear exactly what the claim is.
Here’s the argument again:
1. Simple string compactifications that you can make real predictions with give wrong physics (e.g. no supersymmetry breaking, extra long range forces).
2. In order to avoid this, string theorists are now studying complicated, ugly constructions. They are unable to extract predictions from this class of constructions and there are good arguments (e.g. Denef-Douglas) that extracting predictions is impossible due to the huge number and complexity of the constructions. This framework is not capable of any kind of conventional, testable, scientific prediction about particle physics: e.g. telling us explicitly what deviation from the SM will be seen at the LHC or higher energy accelerator, or explaining the value of any of the numbers that characterize the SM.
You deleted this above, I’m including it again as part of this point:
“Very specifically I do claim that I have seen no argument from anyone working in this area that these compactifications cannot lead to values similar to the SM ones for the data that characterize it (number of generations, gauge group and couplings, fermion representations and mass terms)”
3. This is a classic failure mode of a wrong speculative idea. Simple versions of the idea disagree with experiment, you make things more and more complicated in order to avoid this, never actually having something predictive that can be successfully confronted with experiment. When it is clear this has happened, you are supposed to give up. It is clear that this is what has happened in this case.
That’s precisely what I mean when I say the landscape is unpredictive, and it is so because it is ugly and complicated. I don’t always repeat all those words. If you want to argue with this, argue with points 1, 2 and 3 in their full form. Don’t construct some stupid straw man argument. I’m not an idiot making a stupid argument which is obviously wrong, no matter how much you would like to believe this.
Joe,
I’m not asking for a computation of the electron mass. I’m asking for a computation of something, either something about the SM, or something that deviates from the SM and can be checked experimentally. This is just the standard request made of any scientific idea that purports to improve on the SM.
You say you want to look at “inter-relations that arise in different compactifications”. Depends what these are. If they’re convincingly testable, e.g. “all particles will come with partners of the same mass and quantum numbers”, great, they’ll be real predictions. If they’re vague and make all sorts of assumptions which may or not be true, they’re not going to convince anyone the model reflects reality.
By the way, I just don’t at all agree with “SU(3) x SU(2) x U(1) seems born to come from an intersecting brane model”…
String theory, just like the Standard Model coupled to gravity, has lots of vacua that don’t look like the real world.
Who cares?
As a particle theorist, you want to focus on those vacua that do look like the real world, and see whether you can extract predictions (about particle physics at accessible energies) from them.
String theorists are studying vacua in which the moduli are stabilized. I could care less whether you think those vacua are “complicated, ugly constructions.”
They are what they are, and you can keep your æsthetic judgments to yourself.
Denef and Douglas are discussing a problem quite different from the one under discussion here. Please don’t drag them into this, unless you are prepared to explain why you think their results are relevant to the problem at hand.
In particular, as a particle physicist (i.e, for the purpose of extracting predictions for accelerator experiments), I have no interest in figuring out how to tune the fluxes on cycles, disjoint from the locus where the Standard Model degrees of freedom are supported, so as to achieve an acceptable value for the Cosmological Constant.
As a blanket statement, this is false.
Since we have not yet found vacua that bear more than a passing resemblance to the Standard Model, no one can claim to have extracted such predictions.
But there’s no a-priori reason why the relevant calculations should be un-doable.
(Don’t bother invoking Denef-Douglas, as their results are not relevant to the calculations which need to be done here.)
There are too many negatives in that sentence. Could you please restate it in a fashion that I might have a fighting chance of understanding it?
I just did. And I’ll ask you to extend the same courtesy to my arguments.
So you don’t disagree with the proposition that, when we finally do find the set of vacua compatible with the Standard Model, they will, very likely, give rise to only a sparse set of points in the MSSM (or some other, similar extension of the SM) parameter space?
Because that proposition, if correct means that string theory is highly predictive.
I don’t know that it is, in fact correct. The only conclusive way to settle the matter would be to present the relevant vacua. However, for the reasons I have explained, I believe the proposition is highly plausible, far more plausible than the contrary proposition, that string theory is entirely unpredictive about LHC-scale physics.
Jacques,
Not a chance, sorry. Simple, beautiful theories are predictive because a lot of non-trivial facts about the world follow from simple assumptions. Complicated, ugly theories are unpredictive because the input is as big as the output. The ugliness is part and parcel of what is wrong.
No, these constructions combine particle physics and gravity and predict the CC that you will observe. This has to come out right. If you write down a compactification and claim that it is a unified model of particle physics and gravity, compute the CC and it is 120 orders of magnitude off, you have the wrong compactification. You have to find one with the right value. The Denef-Douglas argument is that these are likely to exist, but you can never actually find them.
Their results are relevant. It’s also true that, as Joe points out, doing something like accurately computing Yukawas is not something anyone has been able to do for a realistic model, or is likely to be able to do soon. What has happened, again, is that people are studying models so complicated that they can’t confront them with experiment. This is not because of convincing evidence that the vacuum state is complicated. It is what happens when you pursue a wrong idea, adding complexities to avoid wrong predictions, continuing to the point where you lose the ability to compute (although are still able to say “maybe with a few decades of effort we can do this complicated calculation..”).
I have no idea what the set of all points of all possible EFTs derivable from string theory constructions and compatible with observed standard model and cosmological parameters looks like, and I don’t think anyone ever will, because of Denef-Douglas and other inherent computational difficulties with their origin in the huge complexity of these models. This set may be sparse or dense but we’ll never know. The set-up of the problem is inherently so complicated that you can’t ever identify this set. People keep coming up with new constructions that just make the problem worse and worse. The point of what I quoted that you didn’t understand is that it removes one reason for doing all this: one might argue that there are generic things about these models that imply that you can’t get something like the SM. There’s no evidence for this, thus little reason to pursue that hope of falsifying string theory in that manner.
This is why string theory is unpredictive, because you can’t ever identify this set, not because I know for a fact that it is dense in the neighborhood of the SM parameters.
“No, these constructions combine particle physics and gravity and predict the CC that you will observe. This has to come out right. If you write down a compactification and claim that it is a unified model of particle physics and gravity, compute the CC and it is 120 orders of magnitude off, you have the wrong compactification. You have to find one with the right value. The Denef-Douglas argument is that these are likely to exist, but you can never actually find them”.
If we find a set of vacua where we predict the MSSM particle spectrum such that the terms in the soft lagrangian are independent of the value of the cosmological constant (there are explicit constructions where this is the case), we can just forget about the cosmological constant. I don’t understand this obsession with the CC problem. As long as we can derive the spectrum of superpartners or at least find some nontrivial relations between them, that is a real prediction. I can see that the CC problem is relevant for cosmology but as far as particle physics is concerned, it’s not that relevant.
If string theory predicts the ratios between, say, the gaugino masses and they are confirmed by the LHC, that’s it! In this case I don’t care if the CC is tuned or whatever mechanism makes it small, it’s just a completely decoupled problem.
c,
So, does this mean you have given up on string theory as a unified theory of particle physics and gravity, deciding to just pursue it as a way of doing particle physics, ignoring gravity?
Funny, but there seem to be a lot of string theorists instead doing the opposite, deciding string theory is only useful for investigating quantum gravity, can’t tell us about particle physics.
“If string theory predicts the ratios between, say, the gaugino masses and they are confirmed by the LHC, that’s it!”
This is what I have in mind when I say that much string theory research is now based on pure wishful thinking…
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.
Many people have been trying to tell you that in the string theory constructions people have been building in recent years, the cosmological constant problem is essentially decoupled from the low-energy phenomenology. With given low-energy phenomenology you can find the CC you want, without disrupting the MSSM-ish sector you are studying.
Jacques has been repeating this, and yet you don’t seem to want to acknowledge it.
If the LHC sees superpartners, it will probably give us some strong clues about how SUSY-breaking is mediated, which in turn could give us strong clues about the sorts of string constructions we might be dealing with. It’s not “anything goes.”
For that matter, even in field theory, it’s not true that there’s a wide variety of stuff we could observe at the TeV scale. You like to complain about how no one is doing anything predictive, but if you paid attention you might notice that precision constraints (from LEP, the Tevatron, B factories, etc.) are so strong that almost any model anyone ever tries to write down is either ruled out or must be very finely-tuned.
Model-building — in effective field theory or in string theory — is not easy. Most of us could name at most a handful of reasonable possibilities for TeV-scale physics (and a handful of other unreasonable possiblities). And yet you seem to think there are zillions of possibilities, all of which can be realized in string theory. That’s hardly the case.
“Many people have been trying to tell you that in the string theory constructions people have been building in recent years, the cosmological constant problem is essentially decoupled from the low-energy phenomenology. With given low-energy phenomenology you can find the CC you want, without disrupting the MSSM-ish sector you are studying.”
Exactly!
Thank you anon!
“Funny, but there seem to be a lot of string theorists instead doing the opposite, deciding string theory is only useful for investigating quantum gravity, can’t tell us about particle physics.”
Who said this?
anon.,
I am aware of how tight the experimental constraints are that rule out most beyond standard model models or force them to be highly tuned. But the point is that these are EXPERIMENTAL constraints, not constraints coming from string theory (or even SUSY with various mechanisms for mechanisms for SUSY-breaking). While one can write down all sorts of string theory models that give just about anything at the TeV scale, virtually all of them are already ruled by experiment. Sure the CC constraint on viable unified string models is in practice far less relevant than the tight experimental constraints that pick out the part of the landscape not contradicting experiment.
But this is an example of the kind of phenomenon that I’m trying to point out. As the experimental constraints get tighter and rule out various simple possibilities (e.g. mSUGRA), people are forced into more complicated things like non-minimal SUSY models. This is the sign of an idea that isn’t working: you’re forced into models with more and more parameters since your simpler ones with less parameters were falsified. The situation is nowhere near as bad for the MSSM as for string theory. A hundred or so parameters appearing in a simple way in a Lagrangian is something you can compute with, unlike the case of the string landscape, where you can happily throw in all sorts of very different things (brane worlds, anyone?) much more complicated that SUSY field theory.
Personally I take these tight experimental constraints as strong evidence we’re not going to see SUSY at the LHC. The all too depressing possibility is a 160 GeV (or whatever the number is that makes the theory consistent at very high energies) Higgs. The exciting possibility is something we haven’t thought of. The possibility you mention, some form of SUSY, which by incredibly bad luck was such that we can’t see indirect evidence of it now, seems to me quite unlikely. Anyway, we’ll find out within the next few years.
And, you know, even if you think I’m completely ignorant, it still might be a good idea to start off your comment with something more polite than obnoxious insulting rhetorical devices like
“have you tried to understand the state of SUSY phenomenology?”
c,
“Who said this?”
Well, just starting to reread through the comments here I got as far as #20 before finding
“I agree that string theory seems unlikely to tell us anything about electroweak scale particle physics(except perhaps through gauge/gravitational duality as applied to QCD). But it isn’t really fair to claim that it is thus dead in water. String theory excited many theorists by providing a quantum gravity candidate and, in the end, exploring that regime will be what determines if it sinks or swims.”
Not for these purposes it doesn’t.
The physics measured in accelerators is completely insensitive to gravity. The values of the cosmological constant and Newton’s constant are irrelevant to LHC physics.
As I patiently explained, there is a huge degeneracy of vacua whose particle physics (at energies probeable at the LHC and beyond) which differ in the values of the cosmological constant and Newton’s constant, but whose particle physics is absolutely identical.
For the purposes of making particle physics predictions, it doesn’t matter which of this exponentially large number of vacua you calculate in. You will get the same answers for all of them.
Figuring out which of those vacua has the right value of Λ and G_N is a computationally hard problem, as argued by Denef and Douglas. However, the exponentially large number of such vacua virtually guarantees the existence of a solution. Computing the Standard Model Yukawa couplings may be a technical challenge, but it is not computationally hard (in the sense of Denef-Douglas).
And you’re not interested in finding out either.
Which is fine by me.
What I take umbrage at is when
1) You categorically declare (for æsthetic reasons, since you, apparently don’t have a physics argument) that, because of the Landscape, string theory is inherently unpredictive about particle physics at accessible energies.
2) You excoriate an entire community of physicists for not abandoning this obviously “failed research program.”
I don’t want to minimize the difficulty of the problem. It will require the combined efforts of many very smart people to solve it.
But whatever may be “obvious” to you, you have failed to make any sort of a physics argument supporting that contention.
I think you owe the community an apology.
Whoops! Sorry. One of the sentences above got spooged (curses cut ‘n paste!). It should have read:
“While one can write down all sorts of string theory models that give just about anything at the TeV scale, virtually all of them are already ruled by experiment.”
The standard N=1 D=4 SUGRA prescription through which one obtains the MSSM soft breaking terms from string compactifications results in some very nontrivial relations between them.
One such example is the ratios of the gaugino masses. 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. This is completely independent of the experimental constraints from LEP, Tevatron, etc.
“Well, just starting to reread through the comments here I got as far as #20 before finding
“I agree that string theory seems unlikely to tell us anything about electroweak scale particle physics(except perhaps through gauge/gravitational duality as applied to QCD). But it isn’t really fair to claim that it is thus dead in water. String theory excited many theorists by providing a quantum gravity candidate and, in the end, exploring that regime will be what determines if it sinks or swims.” ”
I don’t know if Josh is a string phenomenology expert but this statement does not reflect the current state of the field, that’s for sure. I know some string theory people who work on things like black hole entropy, ADS/CFT and are unaware of the recent progress in string phenomenology.
I personally would not rely so much on the statements made by some other people and instead go and read some relevant papers myself.
Jacques,
Sorry, but I’m just not completely convinced that string theory unifies gravity and the other known forces, but that then things are completely decoupled in the way you claim. I don’t doubt that in many of the string theory models people work with this is true, but there are others in which I don’t see it. What about those extra dimensions and black holes that we keep hearing might be showing up at the LHC? Also, presumably the same Denef-Douglas problem applies to anything that is fine-tuned, and if I believe Arkani-Hamed, the electroweak breaking scale may also be fine-tuned for our existence. Is the electroweak breaking scale also completely decoupled from TeV scale particle physics predictions? Maybe you’re right and this is not a problem, it’s a bit late at night for me to completely think this through.
In any case, the Denef-Douglas problem, and other semi-theological problems of principle aren’t the main point that I keep repeating, and that you keep refusing to acknowledge or quote. I’ll happily stipulate that you’re right and Denef-Douglas is not a problem. You quoted my
“I have no idea what the set of all points of all possible EFTs derivable from string theory constructions and compatible with observed standard model and cosmological parameters looks like”
then deleted the rest of the sentence, so that you could go into your typical sneering personal attack mode:
the rest of the sentence wasn’t about how “I don’t want to find out”, it went
“and I don’t think anyone ever will, because of Denef-Douglas and other inherent computational difficulties with their origin in the huge complexity of these models. This set may be sparse or dense but we’ll never know. The set-up of the problem is inherently so complicated that you can’t ever identify this set. People keep coming up with new constructions that just make the problem worse and worse.”
This is the argument you absolutely refuse to ever acknowledge the existence of. I’ve made it at least twice more here, you continue to completely ignore it and claim I don’t have an argument. Here it is again:
“3. This is a classic failure mode of a wrong speculative idea. Simple versions of the idea disagree with experiment, you make things more and more complicated in order to avoid this, never actually having something predictive that can be successfully confronted with experiment. When it is clear this has happened, you are supposed to give up. It is clear that this is what has happened in this case.”
and
“Simple, beautiful theories are predictive because a lot of non-trivial facts about the world follow from simple assumptions. Complicated, ugly theories are unpredictive because the input is as big as the output. The ugliness is part and parcel of what is wrong.”
and
“It’s also true that, as Joe points out, doing something like accurately computing Yukawas is not something anyone has been able to do for a realistic model, or is likely to be able to do soon. What has happened, again, is that people are studying models so complicated that they can’t confront them with experiment. This is not because of convincing evidence that the vacuum state is complicated. It is what happens when you pursue a wrong idea, adding complexities to avoid wrong predictions, continuing to the point where you lose the ability to compute (although are still able to say “maybe with a few decades of effort we can do this complicated calculation..”).”
There’s not much point in reformulating this a fourth time. I think this is a fair characterization of the current state of the study of these backgrounds. They are just complicated enough to evade making predictions (simpler, predictive ones give wrong physics), and this ensures that people can’t calculate things reliably with them and get predictions out of them. There is no reason to believe that if you can do these calculations you would get predictions, much less ones that agree with experiment. This situation has every hallmark of a failed research program and I’m not going to apologize for pointing this out.
c, anon., Jacques,
There are three of you, but only one of me. And I’ve just spent the entire evening thinking about what you have written and trying to carefully responding to all three of youl, so haven’t finished preparing a talk for Saturday. I’ll be on a plane most of tomorrow, busy at the event Saturday, on a plane Sunday, teaching Monday.
It has been an educational argument, sometimes I do learn things, even if I’m not convinced by all of your claims. Since you all seem to consistently claim there are robust predictions about gaugino mass ratios, and I’ll freely admit that I am no expert on all details of possible supersymmetry breaking schemes, I’ll leave you with a question I’m curious about: What value of a gaugino mass ratio would falsify string theory. .1?, .01?
Will try and return to this when I can….
What value of a gaugino mass ratio would falsify string theory. .1?, .01?
Read the “Gaugino Code” paper by Nilles and Choi.
I’m not sure how to parse this sentence. Do you doubt that LHC physics is insensitive to the values of G_N and Λ? Surely not. Do you disagree with my argument that, the renormalizable couplings of the Standard Model are insensitive to the distribution of fluxes on cycles disjoint from the locus on which the Standard Model degrees of freedom are localized? Something else?
What about them?
Nonrenormalizable couplings, usually suppressed by inverse powers of M_{pl}, are unsuppressed in that limit. That’s just a symptom of the fact that 4D effective field theory breaks down, not at 10^{16} GeV, but at 10 TeV.
But, hey, if the large extra dimension scenarios are correct, we won’t be quibbling about computing Yukawa couplings. We’ll be directly probing stringy physics at the LHC.
The Denef-Douglas problem arises because all n fluxes contribute (in some complicated way) to Λ and G_N. If there are other couplings that are similiarly affected by all n fluxes, I suppose they, too, will be cmputationally difficult to calculate.
I don’t know of an explicit string theory realization of Arkani-Hamed et al’s idea. So I can’t say whether the Higgs mass falls into that cagegory.
Good question, though!
You complained that I failed to quote:
Since I’d already explained why Denef-Douglas is irrelevant to the problem at hand, I didn’t see the point of quoting your repeated invocation of it.
As to your other, unspecified, “inherent computational difficulties,” if you care to spell them out explicitly, I’ll be happy to try to respond. But I’ve learned not to make any assumptions about what you’re talking about.
So, if you want me to respond, you’ll have to spell out your argument.
That’s not a physics argument. It’s an æsthetic judgement on your part (and a severe mischaracterization of the history of moduli stabilization).
I’m really uninterested in your æsthetic judgements.
In any case, they certainly don’t justify your conclusion that String Theory is inherently unpredictive.
There are no realistic models, at present. There are semi-realistic models. But no one is willing to invest the energy to work out all the details of a merely semi-realistic model (i.e., one that we know doesn’t describe the real world).
(There are, in addition, some computations that people still don’t know how to do. They’re the subject of ongoing research. Somehow, though, I doubt that you intend your entire argument to hinge on those research efforts coming to nought.)
c, some phenomenological models make specific predictions for the two ratios among gaugino masses (and combining them one can fit whatever will be measured), but I am not aware of any stringy prediction for gaugino masses.
Jacques, the anthropic principle appeared when somebody noticed that the small electron, up and down Yukawas lie within the relatively narrow range that allows “life” (see astro-ph/9909295 for a review). So, it seems likely that anthropic selection plays some role even for Yukawas. I understand that computing Yukawas is difficult, but maybe it is possible to estimate if the number of different string predictions for Yukawas is or is not hopelessly large?
Well, that was the substance of my argument to Peter.
The distribution of fluxes on cycles disjoint from the locus on which the Standard Model degrees of freedom are supported doesn’t affect the Yukawa coupling of the SM. So the number of models with distinct particle physics is R^{n’}, instead of R^n, where typically n’≪ n.
Moreover, for the purpose of making predictions (as opposed to retrodictions), one could take the some of the values of known and well-measured SM couplings as given, and throw away all of the vacua which disagree with those.
That should leave one with a small, manageable number of vacua, for which one wants to compute everything.
Hi Peter,
One of the inter-relations I was thinking about was the relationship of gaugino masses, which various people have emphasised above. This is both reasonably easy to compute from the high-scale theory and reasonably insensitive to much of the model-building uncertainties that exist. Unlike the Yukawa couplings, it is also theoretically quite a clean observable. No gaugino mass ratio measured at the LHC is going to falsify `string theory’; it is going to falsify certain string phenomenology models of moduli stabilisation and susy breaking.
One can also look at other relationships, such as the ratios of moduli and gravitino masses, the expected scale of the axionic decay constant, etc.
Here is also a potential LHC measurement that would, I think, cut a massive big swathe through all the IIB flux models that people (including myself) study: a 1600GeV gluino with 400GeV first generation squarks.
Best wishes
Joe
Jacques,
Thank you for your response in comments #233 and #242. I found it to be a pretty clear statement of a defensible point of view. I agree, anthropic arguments are very uninteresting. Although I personally disagree with the entire anthropic way of thinking for fundamental theories, I mentioned it as a way of phrasing a selection criterion for candidate vacua; you could substitute “compatible with life” with another selection method that is more physical. In any case I think you answered the bulk of the question in which it was embedded.
The perspective about decoupling gravitational considerations from those of particle physics in vacuum selection is interesting, something I was unaware of. The exchange between you and Peter was also informative.
I won’t press you to answer the rest of my fourth question, regarding resource reallocation to alternative approaches to a more fundamental theory, although I would still be interested if you decide to do so…
In comment #254 it was my perception, but just a perception, that you discounted aesthetic judgements too much. I can only speculate that aesthetics were an important factor in your original choice to work in string theory. In my understanding of history, aesthetic considerations have been a valuable guide in theory building, although the earlier days of quantum mechanics would probably be an important exception. One could of course argue that “nature is what it is” and “nature doesn’t care about our sense of aesthetics,” but theories are necessarily human constructions and hence are strongly affected by our sense of what “feels” right.
Here is also a potential LHC measurement that would, I think, cut a massive big swathe through all the IIB flux models that people (including myself) study: a 1600GeV gluino with 400GeV first generation squarks
What about the more likely scenario that the LHC finds neither gluinos, squarks nor any other sparticles at all? Would that be a problem for string theory?
Wild (but highly amusing) rant by mclaren deleted, as it was twenty times longer than necessary. More importantly, I’m actually learning things from the string phenomenology discussion, occasional personal attacks notwithstanding, so I’d rather not derail that.
Jacques:
I cannot follow your detailed mathematical arguments with Peter, so if I can I would like you to answer the following questions from a very interested retired engineer.
1) How many metasable vacua are consistent with the SM? 10^10?, 10^20?, 10^100???
2) If you pick such a vacuum and make a “prediction” about some other paramater value and then experiments disagree with the prediction, then do you select some other vacuum that is now consistent with the new parameter value of the SM?
3) If the prediction is incorrect do you then predict a value for a new parameter?
4) Even if the prediction is correct in the first instance how can you be sure that the selected vacuum is the correct vacuum? Make anorther prediction about some other paramter value?
String theory may be the correct theory. But the process of determining the truth seems unending if my understanding of what your suggesting is correct.
Can you respond in a manner that a non-professional can understand. Thanks.