The lack of any serious (Lubos doesn’t count) response to my book from string theorists since its appearance in the UK a month or so ago has begun to surprise me a bit. I was also suprised at how weak the defense of string theory was that the Wall Street Journal’s Sharon Begley put in her recent article. It seems that she did talk to some string theorists, but couldn’t get much usable from them (one supposedly told her that the best argument for string theory was the anthropic landscape). Peskin’s argument that string theory’s biggest success is its “explanation” of the number of generations was evidently the best he could come up with, although it’s obviously very weak.
I’ve heard that the WSJ has gotten some correspondence about this, with Jacques Distler writing in to complain about the article. In one letter from him (also signed by his two collaborators), he claims that Smolin is wrong to say that string theory is not falsifiable, since Distler has a recent paper called Falsifiying String Theory Through WW Scattering.
This paper was discussed extensively here. You can make up your own mind about it, but it’s undeniable that the calculations in the paper don’t involve string theory at all (Distler’s two co-authors are not string theorists). Pretty amazing trick to show that a theory is falsifiable without actually using the theory at all.
There are two obvious problems with the claim in the title of the paper, the first is that when one says a theory is falsifiable, one is talking about the characteristic predictions of the theory, and that’s not what the paper is about. The second is that “string theory” is an ill-defined term, and many versions of “string theory” don’t satisfy the assumptions of the paper (one of the co-authors admits this in the comment section). To fudge his way around this, in the letter to the WSJ, Distler refers to “the canonical definition of string theory” as opposed to “string theory”, although he provides no reference to what this is. Putting “canonical definition” and “string theory” into Google doesn’t turn up anything relevant.
It will be interesting to see if a referee can be found who will go along with allowing the “falsifiying string theory” claim. Most physicists, string theorist and not, that I’ve talked to about this think it’s way out of bounds. It seems to me pretty amazing that Distler would choose to take this case to the Wall Street Journal.
We’ll also see if the WSJ publishes the letter, if not I’ve asked one of the co-authors if they’ll let me publish it here.
I could certainly do a better job of defending string theory than the people Begley talked to. The strongest argument string theorists have is clearly the one that they have “the most promising approach to quantum gravity”. The problem with this is that there are plenty of people who disagree, especially those who do LQG. This is the reason that Distler has been on an anti-LQG campaign throughout the blogosphere recently. His latest posting is about this, with comment section featuring the always incoherent Lubos Motl, and the trademark Distlerian sarcastic sneering at people he disagrees with, e.g. the following comment on a paper by Thomas Thiemann:
I suppose that it is only Thomas’s natural modesty that prevented him from submitting this paper for the Clay prize.
In the first comment, someone quotes from my book something I have to say about the issue being discussed in this posting. For the record, I’m no expert on LQG, and can’t judge exactly how close they are to having a fully satisfactory quantum gravity theory, but my impression is that what they are doing is a more promising approach to quantum gravity than string theory, and the fact that they are convincing more people about this is what is getting Distler and others very worked up. I also don’t think either LQG or string theory has made any headway on the problems of the standard model, although several orders of magnitude more effort have gone into the string theory approach.
Since Distler has a whole posting and ongoing discussion in his comment section about this and I’m no expert, if you want to discuss LQG, its problems and prospects, or comparisons to string theory, please do it there, not here.
As expected, the first comment here was another sortie in the string theory/LQG warfare. I’ve deleted it, and will delete any others. The commenter did make the useful suggestion that an alternate location for people to discuss this is:
http://christinedantas.blogspot.com/2006/06/top-10-results-in-lqg-and-spin-foams.html
No, the comment was about mostly about you. I only mentioned the link and the frustrating fact that nobody has responded and referred them there.
OK Aaron,
I don’t want anyone to feel that what I’m doing is deleting criticism of me, so here’s the relevant part of your deleted comment:
“I’m beginning to think that string theorists can’t win. When we ignore LQG, we’re being insular and closed-minded. When we criticize LQG, it’s because “they are convincing more people” and that’s making us “very worked up.””
It was deleted purely because since it wasn’t explicitly aimed at me, I was sure others would take up the cudgels and respond and the battle woud soon be raging here.
If this criticism is aimed purely at me, I’ll just note that my comments about this have not been that string theorists “ignore LQG”, it’s that their response to it is nearly uniform hostility. Robert Dijkgraaf, normally a sensible sort, spent part of his summary talk at Strings 2006 making fun of it. David Gross at his recent public talk called it something he didn’t normally discuss in polite company.
Whether this hostility is reasonable or not depends on what you think of the achievements of LQG versus the achievements of string theory. Personally I don’t think it’s reasonable, but that discussion belongs elsewhere, not here right now.
Hey Peter,
The Distler et al. paper would in principle (in the absence of a light higgs) falsify any theory whose S matrix obeys Lorentz invariance, unitarity and analyticity for all scales. Given that (at least in its perturbative version) string theory was constructed to produce an S matrix obeying such requirements, a violation of the given bounds will imply that whatever it’s been called string theory so far can not be a valid UV completion of the standard model. To me, the fact that the paper does not explicitely reffers to what string theory is, or any GUT in particular, is more of a virtue than a weakness, and it is a very elegant way to overcome the decoupling problem (Distler quote as Georgi’s). There is a remaining question, namely, at which scale the new physics will enter and whether or not it will be Planck supressed. We are yet to await until LHC answers that question…
Regarding LQG, or other attemps such as discrete QG, it is possible that they might emerge as possible candidates since Lorentz, as well as unitarity violations (without so far drastic consequences) have been found in such scenarios.
Not that this proves much of anything, but of all the scientists I’ve talked to, no one’s made an impression on me like Thomas Thiemann. He’s so smart that it’s depressing.
On the subject of “sarcastic sneering”, it seems to me that this is a lot more common now than it once was, and the only explanation for this is that it is more effective now. That is, that people are more afraid now of being ridiculed as a fool.
I am reminded of the Kaczynski’s manifesto, where he said that the population was being excessively socialized, and that this was removing their ability to achieve satisfaction. (He found satisfaction by living off the woods and mailing bombs.)
In the logical world of physics and mathematics, sarcasm is an appeal to social pressure that has little obvious place. I’ve almost just written that it is too bad that there is not more Unabomber types in Physics.
Carl
Peter,
While not disputing at all the way you characterize the tone of the discussion at Distler’s blog, it is true that there is a serious point there. I’ll only say that it seems to me that one must read the papers for oneself, there is no other reliable way of knowing the state of LQG.
Garbage,
The paper is never going to falsify anything. Even the authors admit that their result is only relevant if something that they think is extremely unlikely happens: no light Higgs and you see a violation of their bounds. This is not going to happen, even they don’t think so. The result is of purely theoretical, not practical interest.
The claim of “we can falsify string theory” is there for a political reason, to allow one of its authors to attack people like Smolin (e.g. by writing letters to the Wall Street Journal), and any one else who dares point out that string theory is a vacuous idea since it predicts nothing and can’t be falsified.
You can’t get something for nothing. You can’t “falsify string theory” by writing a paper that has nothing to do with string theory. If the authors can show that their results apply to all the classes of models that people doing string phenomenology are currently using to try and make contact with experiment, that would be interesting. They need to actually discuss such classes of real, viable string models and write a paper that says something about string theory before they can claim to be able to falsify the idea.
If their bounds actually were by some miracle violated, what you would see happen is that most string theorists would argue that this meant that QFT was no longer viable and one had to do string theory.
Is there any class of string models (however unrealistic) that their dispersion relation bounds would not apply to?
If you know of one, you should write a paper about it. As far as I understand, these relations are generic to all (perturbative?) string theory backgrounds.
Anon,
Most supposedly realistic string theory models that I know about involve introducing branes, which are inherently non-perturbative objects. I just don’t see that their arguments apply to these models. If they have an argument that they do, they should make it.
In any case, it’s ridiculous to go around saying that you can “falsify string theory” when your argument only applies to perturbative string theory, and the standard ideology of the subject is that understanding non-perturbative string theory is what is going to save the subject from its well-known problems with making contact with reality.
Recall that Gross and many others continually tell us “we don’t really know what string theory is”, and that understanding what it is will involve getting rid of our conventional ideas about space (and maybe time). Once we do that, what happens to the author’s Lorentz invariance and analyticity assumptions?
Every string background (with some number of flat dimensions) that I know of has an S-matrix which satisfies the usual analyticity assumptions.
If you’re arguing that some future developments in nonperturbative string theory will produce backgrounds which violate this property, it’s hard to say with any certainty that you’re wrong. But there’s zero evidence that you’re right.
On the other hand, if you are arguing that there’s a known string background that violates this property, then you should just say what it is, and end the argument right there.
Robert Dijkgraaf, normally a sensible sort, spent part of his summary talk at Strings 2006 making fun of it.
I wasn’t in Beijing, but Dijkgraaf often gives humorous talks. I’d guess that the ‘hostility’ you’re referring to was simply a joke.
I find it depressing that you seem to ignore all the scientific criticisms and just focus on this ‘hostility’. Frankly, I’d wish you’d focus a lot less on personality issues and more on science. So, Lubos is an idiot, and you don’t like Jacques. That has absolutely no bearing on whether string theory is ‘not even wrong’ or not. Take this, for example:
The claim of “we can falsify string theory” is there for a political reason, to allow one of its authors to attack people like Smolin (e.g. by writing letters to the Wall Street Journal), and any one else who dares point out that string theory is a vacuous idea since it predicts nothing and can’t be falsified.
This is baseless speculation. You have no idea whether it’s true or not. So, why say it? I doubt it helps you convince other people.
Anon,
My point is that you don’t know what the underlying non-perturbative theory is that determines the background. In the full string theory these “backgrounds” are supposed to have some sort of dynamics. You’re working in an approximation where you ignore that and fix the background. In this approximation, if you make certain assumptions, like 4 flat dimensions, maybe you will get the properties at issue. But I just don’t see any argument that they are still there outside this approximation.
Again, every talk I’ve seen David Gross give in recent years he goes on about how really understanding what non-perturbative string theory is will require giving up our standard idea of what space and time is at short distances At short distances, Lorentz invariance is more or less precisely our standard idea of what space and time is. If Gross is right, where’s the argument that the analyticity and Lorentz invariance properties at issue will survive in a full theory without conventional short distance notions of space and time?
String theorists assume for no good reason that some unknown dynamics is spontaneously breaking the 10d Lorentz invariance of the perturbative superstring down to 4d Lorentz invariance, no? What’s the argument that this unknown mechanism that ruins things in 10d leaves them alone in 4d?
“You’re working in an approximation where you ignore that and fix the background.”
In asymptotically-flat spacetimes, different backgrounds are superselection sectors.
“Again, every talk I’ve seen David Gross give in recent years he goes on about how really understanding what non-perturbative string theory is will require giving up our standard idea of what space and time is at short distances…”
In other words, you don’t have an example of a string background whose S-matrix violates these analyticity assumptions, and you are wildly speculating that some future developments in nonperturbative string theory might produce one which does.
That’s a pretty thin basis on which to go on the attack.
Aaron,
Yes, Dijkgraaf’s comment was a joke, one about how incompetent the LQGers are. Supposedly the audience erupted in laughter at this.
I take one look at Jacques’s comment section on his posting, see him up to his normal sneering, other major participants Lubos and Lubos-wannabee Michael, and have a very hard time believing that these are people trying to have a serious scientific discussion. I’ve had the experience with all three of them of trying to have a serious scientific discussion about string theory, and finding it a waste of time since they weren’t interested, but were only interested in attacking me as a crackpot/incompetent/whatever.
Sorry, that’s a commentary on personalities and their hostile behavior, but that’s what I see.
As for the “string theory is falsifiable” issue, again, I’m calling it as I see it. The WSJ article contained the perfectly accurate statement by Lee that “string theory cannot be disproved”. Distler chose to write to them to complain about this, invoking a bogus argument about “canonically defined string theory” that is besides the point, and he should know it.
Anon,
You’re ignoring what I write and bringing in irrelevancies.
“In asymptotically-flat spacetimes, different backgrounds are superselection sectors”
Is it known that realistic backgrounds are all different superselection sectors of non-perturbative string theory? I know plenty of string theorists who don’t want that to be true and are operating under the assumption that it isn’t (e.g. Gross).
The rest of your comment just completely ignores my answers to your question. I don’t see the point of retyping it again.
I think you completely misunderstand Gross’s position, and am pretty certain he would disagree with you on the S-matrix in nonperturbative string theory.
If you want to cite him as an authority in your argument with Distler & co., you’ll have to do better (like maybe solicit a statement from him, supporting your position).
Sorry, that’s a commentary on personalities and their hostile behavior, but that’s what I see
If you don’t like the tone there, how about the criticism by people like Nicolai, Peeters & Zamaklar, Helling & Policastro, and others? It’s hardly fair of you to ignore it. Regardless, the personality of the person doing the criticism has absolutely no bearing on the legitimacy of the critique.
Question:
Is each of the 10^500 worlds of the anthropic principle a (theoretical) closed system? If so, I would say that the requirement for the existence of 10^500-1 universes completely immune to observation is a particularly troubling weakness of string theory.
(6 yr old Physics *AB* here, so please don’t get too technical…)
One obvious question is why did 2 coauthors agree to put their names on a paper with a title “Falsifying String Theory …” if the paper has nothing to do with ST?
Peter:
I have enjoyed reading your blog. I discovered it by googling for “critique of string theory”. I will certainly buy your book when it comes out. I think I understand your arguments against string/M theory. But to help clarify your position can you please respond to the following.
There seems to me to be three possibilities:
1. String/M theory is wrong. Our universe is not described by this theory.
2a.String/M theory is correct but our universe is not unique and that there is no vacuum selection principle. There are multiple universes and ours is just one manifestation of this theory.
2b. String/M theory is correct. Our universe is unique but we humans will forever lack the mental capacity to discover the vacuum selection principle.
2c. String/M theory is correct. Our universe is unique and we humans have the mental capacity to discover the vacuum selection principle but as of yet have not done so.
3. In the development of string/M theory a logic error or incorrect assumption was made and if corrected a valid theory will be discovered.
If 2 is correct then by definition the theory CANNOT be falsified. And the theory can make very general predications and be adjusted to agree with any observation. This seems to me to be your primary issue.
I am most interested in the third possibility. If you believe that string/M theory is “not even wrong” is it due to option 1 or 3? It seems to me that it maybe possible for some smart iconoclastic theorist to revisit the formulation of string/M theory and revise some of the assumptions used to develop the theory. Thus possibilly avoiding the vacuum selection problem. Reducing from “real” 10 or 11 spacetime dimensions to 4 forces one to invent or discover a vacuum selection principle. But suppose we are misled in to believing that compactification is even required?
Suppose that the universe really has a fourth spatial dimension of the same “size” as the other three that we experience. How would this fourth dimension make itself known to us? Either through our senses or instruments that we make to observe some measurement. But the measurement to be made is based on some theoretical prediction. Most arguments that reject more than three spatial dimensions assume that GR is valid in more than four spacetime dimensions. Suppose it is not valid then what? How would string/M theory look if compactification was not invoked? Just some thoughts.
Thanks for a great blog. I look forward to reading your book.
sunderpeeche,
The two coauthors are not string theorists.
Aaron,
I don’t know how many hundred times I have to say something like this. I’m a particle theorist, not a quantum gravity theorist. As a result, I’ve not spent time paying close attention to exactly what the state of the LQG program is and don’t intend to. Many other smart people are and I think that’s great, but life is short and one has to decide what to spend one’s time on. People with these interests should respectfully debate the technical problems involved, learn new things and try and make progress, or give up and do something else if the problems are insuperable. If I wanted to work in this area, I would encourage this discussion here and take part in it. I don’t at the moment, but want to think about and discuss other things. I have every intention of ignoring the detailed criticisms of LQG, just as I ignore the details of what people working in LQG have accomplished, and I think this is perfectly fair. If string theory’s only claim was that it was a theory of quantum gravity, I’d be ignoring that too, and so would most of the rest of the physics community.
Anon,
You continue to just ignore the points I make and questions I ask and raise irrelevancies. I was not invoking Gross as an authority for anything I had to say, just as the most well known example of someone who believes that string theory is someday actually going to explain the properties of the standard model. If string theorists have completely given up on this, they should say so, and let Mike Peskin know he should stop telling the media the reason to believe in string theory is that it is going to explain the number of generations.
Cecil,
My belief is simply 1. As an idea about unification, string theory is just wrong, and fundamentally so. It can’t be fixed by just changing one piece of it.
The physical effects of extra dimension depend on exactly how you introduce them. The study of these possibilities is what thousands of physicists have been working on for more than 20 years. None of the infinite variety of effects predicted by any of these possibilities have ever been observed, and even most string theorists don’t expect to be able to observe them in the forseeable future.
As a non-string theorist, I would not put my name on a ST paper. I would not put my name on any paper where I was not satisfied or could not validate the contents (I might compromise on the wording etc with coauthors, but not on the substance of the paper). Why would 2 non-string theorists agree to put their names on a paper if they could not establish the veracity of its contents?
sunderpeeche,
You’ll have to ask them. I assume that they found assurances from their co-author and other string theorists they talked to sufficient. It’s not exactly unusual though in science for some co-authors of a paper to not be experts on all aspects of the paper, and to rely on the expertise of others.
Peter, you say
“… The lack of any serious (Lubos … and the trademark Distlerian sarcastic sneering … doesn’t count) response to my book from string theorists since its appearance in the UK a month or so ago has begun to surprise me a bit. ….”.
Such silence sounds to me similar to the 1950s when Oppenheimer, the then-current Pope of Princeton who advocated the Copenhagen interpretation of QM, said about Bohm, who was working on an alternative interpretation:
“… if we cannot disprove Bohm, then we must agree to ignore him. …”.
The source of the quote was Max Dresden (in my opinion impeccably honest) and The Bohm biography Infinite Potential, by F. David Peat (Addison-Wesley 1997),page 133. Here are some relevant excerpts from that book:
“… Max Dresden … read Bohm’s papers … errors were difficult to detect … von Neumann’s “proof” … did not rule out the sort of theory that Bohm had proposed. … Oppenheimer [said]…
“We consider it juvenile deviationism … we don’t waste our time …” [by] actually read[ing] the paper …
Dresden … present[ed] Bohm’s work in a seminar to the Princeton Institute …
The reception he received came as considerable shock to Dresden. Reactions to the theory were based less on scientific grounds than on accusations that Bohm was a fellow traveler, a Trotskyite, and a traitor. It was suggested that Dresden himself was stupid to take Bohm’s ideas seriously. … all in all the overall reaction was that the scientific community should “pay no attention to Bohm’s work” … Abraham Pais also used the term “juvenile deviationism”. Another physicist said that Bohm was “a public nuisance” …”.
It seems that the silent treatment plus ad hominem attacks has been a tradition of the USA physics community for at least 50 years. Plu ca change …
Tony Smith
http://www.valdostamuseum.org/hamsmith/
sunderpeeche said:
My name is on papers I have yet to read fully. Many times (if I know the other authors personally and trust them) I have written my section and, based on a few meetings, have assumed the other authors would do a good job.
I am not in anyway endorsing my own actions, but I am making the point that not all scientists take as much care as you.
OK, a quick glance at this paper, where is the string theory? You might as well replace the words “string theory” by “homuncular biology”, “phlogiston dynamics”, or “epicyclic astrometry”.
Suppose that the universe really has a fourth spatial dimension of the same “size” as the other three that we experience. How would this fourth dimension make itself known to us?
You couldn’t tie your shoes.
-drl
“I’ve not spent time paying close attention to exactly what the state of the LQG program is and don’t intend to.” — PW
This is a little frusterating for those of us who would like to engage you in a discussion in the relative merits of string theory as a research program.
1) You say that you would like to see hep-th research substantially diversify.
2) Many of us point out that there is really no viable alternative to string theory as a theory of quantum gravity. If planck scale physics is your interest, string theory is the only game in town.
3) You object, saying that LQG is a neglected but nevertheless thriving alternative to string theory as a research program in QG.
4) We disagree, and try to explain why in scientific and technical terms
5) You shut down the discussion, explaining that you don’t care about QG anyway and you have no interest in becoming educated on the issue.
Unfortunately, QG is the primary achievement of string theory. If you don’t care about QG, its no wonder that string theory is not of interest to you—for physics at the TeV scale, everyone will agree that the explanatory power of string theory is weak at best. Fine, but if you don’t care about QG, you have no right to criticize people who do.
But I suspect that you do care about QG. Every hep theorist wants to know physics at the ultimate cutoff. In this case, if you want to be critical of string theory, you should have something to say about its obvious alternatives. Especially, you should be open to the possibility that the alternatives might not be any better, or even far worse (as is the case for LQG, in my opinion).
A very nice letter from Michael Peskin to the EPP2010 committee. In the end, one must just see what the LHC produces (and build the ILC!). It’s the only way to know just how wrong life, the universe and everything is.
www7.nationalacademies.org/ bpa/EPP2010_Feedback_Peskin.pdf
clickable link
http://www7.nationalacademies.org/bpa/EPP2010_Feedback_Peskin.pdf
“If Gross is right, where’s the argument that the analyticity and Lorentz invariance properties at issue will survive in a full theory without conventional short distance notions of space and time?”
Sure the scenario might no be that realistic, nevertheless, it opens a new windows worthwile exploring. But that is not the point here, the point is that, if the bounds are actually violated in the absence of a light Higgs (physically possible though unlikely) ST will have to start taking these ideas you point out a bit more seriously….LQG at al. have explored them enough to start to be a serious candidate if such case were to happen in nature.
I dont understand why is it so hard to admit that this sort of dispersion relations are sensitive to the UV in such a way that assumptions on the UV completion of the SM can be tested, included ST or whatever GUT you want.
Peter wrote:
“I’m a particle theorist, not a quantum gravity theorist. ”
Einstein’s basic idea was that the particles (he only spoke of the electron and proton) should arise naturally from the geometry of space-time. What is quantum gravity but a study of the excitations of space-time? Thus I would argue that ‘particle theory’ and ‘quantum gravity’ are synonyms.
Aaron Bergmann, the critique is unreasonable because some of these issues at least have been pointed out to Distler and he keeps making the same wrong or irrelevant points over and over again.
Most of the criticism can be reduced to “it’s unfamiliar”.
Other of the criticisms voiced (re: QCD) have actually been answered by Thiemann on the String Coffee Table more then a year ago. The attack on the mathematical quality and accuracy of the work of people on the level of Thiemann and Freidel discredits Distler further. It’s hard not to read this as malice.
Peter:
Thanks for the reply. But if you believe #1 in my list of possibilities, and since the theory has not been falsified what do you SPECIFICALLY believe that the theory assumes is not compatible with our universe. Is it the idea of strings per se or some addition assumptions that are invoked to eliminate some apparent mathematical problems.
Justin,
I don’t agree at all with your comments about the relative value of string theory and LQG as theories of quantum gravity, but that’s just not a point I want to spend my time arguing. Other people can do that much better than me. I’m not shutting down this discussion, I’m pointing people to the most appropriate places to carry it on, which are at the blogs of people working on LQG or actively interested in discussing it. Some string theorists refuse to believe me, but I honestly take great care to not go on about things I don’t understand. I just don’t have the time now to develop the kind of expertise required for me to feel comfortable engaging in the kind of arguments over LQG that people want to have.
The problem with string theory as a theory of particle physics is not that its explanatory power at the TeV scale is “weak at best”. Its explanatory power as regards particle physics is zero at any scale, up to and including the GUT scale and beyond. If string theorists will finally admit this, and stop continually promoting their subject as a unified theory that is going to explain the standard model, you’re going to hear a lot less from me about this. What I am upset about and trying to do something about is the disastrous effect string theory has had on the subject that matters most to me, particle theory. If you want to argue this point, I’m happy to do it here. If you want to argue about LQG, you should be doing it with an LQG expert.
Cecil,
I specifically believe that the idea that the universe is 10 or 11 dimensional, with the properties of particles dependent on how you get rid of the other 6 or 7 dimensions, is fundamentally wrong and can’t ever work.
Reply to ksh95 — in a large expt HEP collaboration, indeed one cannot demand editorial control over the whole paper. But (I presume) you could in principle validate the contents of the whole thing if you tried. Surely the two coauthors on Distler’s paper could verify if the contents of the paper actually had any ST? Or if the title was appropriate?
Thomas,
Unfortunately, Einstein was wrong about this. Particles are excitations of quantum fields. If someone has an idea of how to get these quantum fields out of the geometry of space-time, I’m all ears, but I haven’t seen a really promising one yet.
FH,
Maybe you should leave a comment over there, pointing out his errors and misunderstandings. If he’s wrong or off the mark, it should be easy to point out where he’s gone astray.
The subtext of Justin’s comment it is very clear: a theory of particle physics is not ultimate, so particle physics is not a worthy pursuit, only gravity is.
Garbage,
What I’m denying is that “string theory” is well enough understood in the UV for this kind of argument to falsify it. That’s all. The argument is “If string theory satisfies A, B, C then bounds”. If bounds are violated, this doesn’t falsify string theory.
I should perhaps explain my point of view about quantum gravity a bit more.
Sure, I’m interested in it, and try and follow what people are doing. But, unless somebody figures out a way to measure QG effects experimentally, it’s a problematic research area. I’ve written about this elsewhere. I believe that we’re not going to really understand QG until we understand in a unified way the dynamics of space-time geometry and the dynamics of the Standard model geometry. If and when we do that, we’ll have a QG which is linked into a structure which is testable. String theory purports to do this, but it has failed. Pretty much all the LQG stuff I have seen doesn’t even try and do this, except for some recent stuff, which to me looks highly preliminary, but if it starts looking more convincing, I’d definitely be spending time paying attention to it.
Aaron Bergmann, the critique is unreasonable because some of these issues at least have been pointed out to Distler and he keeps making the same wrong or irrelevant points over and over again.
What does Jacques have to do with any of this? As best I’ve been able to discern, the questions I asked over on Christine’s blog have not ever been responded to, except that Lee did say that he has wondered about the Immirizi parameter issue, too. A lot of physicists read this blogs if you haven’t noticed, and I can tell you that the silence in response to these issues plays into the bad reputation that LQG has with some people I’ve talked to.
Out of deference to Peter, I won’t say any more here, but if you claim the questions have been answered, can you please post some links over on Christine’s blog, and I can respond there?
Peter, “It’s a difference of opinion which makes horse races”.
I replied privately with details.
I did a little more googling, and I’m being a bit unfair to Lee. He did respond that, contra Carlip, he believed there was a unique quantization of quantum gravity in 2+1 dimensions (I believe Freidel implied the same) and encouraged me to do further research on the comparison of Carlip and LOST.
To D R Lunsford:
You responded to my completely reasonable question:
“Suppose that the universe really has a fourth spatial dimension of the same “size” as the other three that we experience. How would this fourth dimension make itself known to us?”
with:
“You couldn’t tie your shoes.”
I suppose that is to be interpreted as a rejection of a large fourth spatial dimension. But maybe you can tell me: How do you know? How would this fourth dimension reveal itself? In some braneworld models the higher dimensions are hidden apparently except through gravity. But I was even questioning this. How do we know that GR is valid in more than four spacetime dimensions?
“The argument is “If string theory satisfies A, B, C then bounds”. If bounds are violated, this doesn’t falsify string theory.”
??????
If string theory satisfies A,B,C and the bounds which follow from such assumptions are violated, the only logical conclusion is that A,B,C can not be part of the UV completion of the SM, and all the version of ST with A,B,C should be ruled out [There are thecnical issues as to whether higher order operators might spoil the bounds if the cutoff is somehow lower than naively expected]
If, as you point out, that means we should give up in Lorentz invariance, that is a huge step and the efforts to produce such type of constraints extremely important an worthwhile pursuing. Can you imagine to turn on the LHC and be able to probe Lorentz invariance!!
Again, it is unlikely there is a light higgs as it is that the bounds will be violated, it is also unclear whether there is Planck supression, this is also related to extra dimension and the actual scale of gravity. That doesnt mean, as a matter of principle, that all the forms of ST known so far to obey Lorentz invariance (analyticity and unitarity) will be potentially falsified!
Note: I’d say that unitarity might as well break down at scales where our notion of spacetime doesnt make any sense. I have no intuition for analyticity though… 🙂
Peter,
I found your book in the two main Cambridge (UK) bookshops (Heffers & Borders) today. FWIW, the Borders one had a handwritten staff recommendation (the only one in the science section). One thing though – I don’t think that the picture of you on the inside back cover is very suitable. You look too much like a student who has just committed a prank.
So for the US edition I propose the following: a picture of you holding a sword aloft with a suitably stern expression and hair and doctoral robes billowing out behind. Borrow a wind machine from the labs if necessary to achieve the latter effect.
Sunderpeeche said “… A very nice letter from Michael Peskin to the EPP2010 committee. In the end, one must just see what the LHC produces (and build the ILC!). …”.
Although I very strongly agree with Peskin’s overall recommendation that the LHC should be supported and the ILC built (particularly with respect to getting a fuller understanding of the Higgs mechanism), Peskin made a statement that I consider to be significantly historically inaccurate:
“… Super-symmetry was first discovered as a theoretical concept in superstring theory. …”.
IIRC, supersymmetry became of high interest to particle physics theorists with respect to supergravity theories, in which super-Lie algebras were used to evade the Coleman-Mandula theorem that obstructed unification of gravity with the Standard Model.
This is not just a minor historical nit-pick, in light of the tendency of partisans in the superstring/LQG war to act as though the only games in town for Quantum Gravity are superstrings and LQG,
despite the fact that
supergravity theories united Gravity and Particle Physics by using the MacDowell-Mansouri mechanism with respect to a Sp(2) = Spin(2,3) Lie algebra (as part of the relevant Lie superalgebra) to produce a gravity Lagrangian that looks a lot like Einstein-Hilbert.
Therefore, it seems to me that modification or generalization of supergravity should be at least an equal participant in the Quantum Gravity wars now dominated by superstring and LQG partisans.
In fact, since some M-theorists see N=8 supergravity as a possible limiting case of M-theory, it may be that supergravity could be advocated as part of the string theory agenda, but to do that right, they would have to show explicit equivalence between the direct superstring gravity formulation and the limiting case supergravity MacDowell-Mansouri gravity (i.e., to explicitly show in detail the superstring – M-theory – supergravity relationship). AFAIK, that has not yet been done.
My primary point in writing historical comments such as this and a few other recent ones on this blog and JoAnne’s entries about LHC data at Cosmic Variance is
that those who ignore history may be condemning themselves to repeat its mistakes.
Tony Smith
http://www.valdostamuseum.org/hamsmith/
PS – It seems to me that DRL’s statement that if we live in 4 large spatial dimensions we could not tie our shoes is clear, but another commentator seemed to have difficulty with it, so here is another (more or less equivalent) formulation:
If we lived in 4 large spatial dimensions there would be no way to distinguish left-handedness from right-handedness, thus invalidating a lot of particle physics experimental results (there was an old Outer Limits TV show about that point).