I am often surprised at the level of disdain and resentment with which string theory is viewed by non-string-theorists. I’m thinking not so much of people on the street, but of physicists, other scientists, and even other academics. As a physicist who is not personally identified as a string theorist, I get to hear all sorts of disparaging remarks about the field from experimental particle physicists, condensed matter physicists, astrophysicists, chemists, philosophers, and so on. I sometimes wonder whether most string theorists understand all the suspicion directed against them.
It shouldn’t be like this. String theory, with all of its difficulties, is by far the most promising route to one of the most long-lasting and ambitious goals of natural science: a complete understanding of the microscopic laws of nature. In particular, it is by far the most promising way to reconcile gravity and quantum mechanics, the most important unsolved problem in fundamental physics. At the moment, it’s a notably incomplete and frustrating theory, but not without genuinely astonishing successes to its credit.
The basic idea is incredibly simple: instead of imagining that elementary particles are really fundamentally pointlike, imagine that they are one-dimensional loops or line segments — strings. Now just take that idea and try to make it consistent with the rules of relativity and quantum mechanics. Once you set off down this road, you are are inevitably led to a remarkably rich structure: extra dimensions, gauge theories, supersymmetry, new extended objects, dualities, holography, and who knows what else. Most impressively of all, you are led to gravity: one of the modes of a vibrating string corresponds to a massless spin-two particle, whose properties turn out to be that of a graviton. It’s really this feature that separates string theory from any other route to quantum gravity. In other approaches, you generally start with some way of representing curved spacetime and try to quantize it, soon getting more or less stuck. In string theory, you just say the word “strings,” and gravity leaps out at you whether you like it or not.
So why wouldn’t anyone be happy about string theory? For one thing, we don’t understand the theory very well. It’s easy to say “replace particles with strings,” but quantum field theory isn’t really about “particles” — particles are just the observable momentum eigenstates in a perturbative regime, not the fundamental building blocks of the theory. At this point it’s a little unclear what the fundamental building blocks of string theory are; there are some reasonable proposals for complete non-perturbative definitions of the theory (matrix theory and AdS/CFT, for those in the know), but connecting these formulations to a more complete picture isn’t easy.
But most of the grumbles about string theory from other physicists aren’t about a complete non-perturbative definition of the theory — they are about the lack of connection to experiments. One often hears that string theory simply makes no predictions, but that’s clearly false. If you scatter two particles together, string theory unambiguously predicts that the cross-section should look stringy, not like that of fundamental point particles. [With caveats discussed in the comments.] The problem, of course, is that the difference between these two possibilities is only noticeable when the energy of the collision approaches the Planck scale (or really the string scale, likely to be similar) — fantastically far away from what we can actually reach in accelerators. So string theory makes predictions, it’s just that we are as yet unable to test them. In other words, string theory is either right or wrong, it’s our challenge to come up with clever ways to figure out which.
There is a matter of principle here that scientists, of all people, should understand. To wit, our current understanding of nature — based on classical general relativity and the quantum-mechanical Standard Model of particle physics — is simply incoherent. It just doesn’t make logical sense. It is very easy to ask questions to which we don’t know the answer: “What is the gravitational field of an electron?” For that matter, since the Sun is made of elementary particles, we can’t even sensibly talk about the Sun as simultaneously a source of gravity and as a source of light and heat. This is not acceptable. Our goal as scientists is to understand how the world works, and relying simultaneously on theories that are deeply incompatible with each other is nothing to be happy with. Even if it won’t help us make a better TV set or understand the mass of the proton, we need to have a coherent theory of quantum gravity.
Recently there has arisen another sense in which string theory purportedly makes no predictions, associated with the “landscape” of possible string vacuum states. Just as in quantum field theory, the observable spectrum of low-energy string excitations and their interactions (that is to say, particle physics) depends not only on the fundamental string physics, but on the specific vacuum state in which we find ourselves. String theory predicts more spatial dimensions than we directly observe, so one of the characteristics of our vacuum is the way in which the extra dimensions are hidden from our view. It now seems quite plausible that the number of possible ways for this to happen is enormous — perhaps 10500 or so. If true, this puts a damper on the hope that string theory would predict a unique vacuum state, and we could explain (for example) the ratio of the muon mass to the electron mass from first principles.
Well, too bad. It would have been great to make such predictions, but the inability to do so doesn’t render string theory non-scientific. The appropriate comparison for string theory is not to “the Standard Model of Particle Physics,” it’s to “quantum field theory.” Nobody complains that there are a huge number of possible quantum field theories, and we actually have to go out and measure the properties of actual particles rather than calculating them using pure thought. If string theory turns out to be the same way, that’s life.
My own view is that string theorists have been a victim of their own characteristically aggressive form of optimism. Not only, we are told, is string theory a consistent theory of quantum gravity, but it’s a theory of everything, gives us wonderful new insights into gauge theories, and possesses a mathematical beauty that is so compelling that the theory simply must be correct. These kinds of arguments just don’t carry that much weight with the non-converted. If I were in charge of the string theory public-relations machine, I would be emphasizing over and over again the basic feature that we’ve understood for a very long time: it’s the most promising way we know to quantize gravity. If there were multiple very successful ways to quantize gravity, it would be important to distinguish between them experimentally; but so long as the number of successful models is less than or equal to one, it makes perfect sense to make every effort to understand that model.
Which is not to say that we shouldn’t also pursue alternatives. I’m all in favor of supporting research on loop quantum gravity, dynamical triangulations, causal sets, and whatever else smart physicists might personally find promising. As long as we don’t know what the correct theory is, individuals need to use their own judgment about what clues to follow. String theory, starting as it usually does from talk about perturbative excitations propagating in a background spacetime, will not seem especially compelling to someone who thinks that background-independence is the most profound feature of gravity. It’s certainly good to support plucky Apples and Linuxes in the face of the Microsoft-esque dominance of the string theory approach; you simply can’t tell ahead of time when someone will hit on a brilliant new idea.
On the other hand, string theory has thus far been fantastically more fruitful than any other idea. When you get into string theory, one of the things that keeps you going is that you don’t get stuck — the rate of progress waxes and wanes, but the progress is very real. It didn’t have to be true that the five string theories studied in the 1980’s would turn out to all be part of one big theory, but they are. It didn’t have to work out that the entropy of a black hole calculated from semiclassical gravity ala Hawking would be equal to the entropy of a corresponding gas of strings and branes, but it is. It’s clues like these that keep the believers moving forward, hoping to understand both the inner workings of the theory and its ultimate connection with what we observe. We interested outsiders should be cheering them on.
“But this question about bounds you can get in the limit of small coupling doesn’t really have anything to do with my original objection to Sean’s “prediction”. If the real world is governed by a string theory, you have no particular reason to believe that the coupling is arbitrarily small. It’s some finite number, and for that finite number, non-perturbative effects may dominate.”
The coupling doesn’t have to be arbitrarily small for e^{-1/g} effects to be numerically utterly negligible compared to perturbative g^{2h} effects. Please entertain yourself by plugging in some sample values for g which are “small, but finite”.
There are, however, two things to be said about this.
1) Even if g is small, there are some effects to which the perturbative contributions vanish and the dominant contribution goes like e^{-1/g}. You can see discussions of some of those effects (and how they affect the vacuum structure of the theory) up-thread.
2) There are reasons to expect that, depending on what class of string backgrounds you are considering, the string coupling may not be small if that background is to describe the real world. There is, roughly-speaking, a map from the fundamental parameters (the string scale, the string coupling, and the compactification scale) to the observable 4D parameters (the 4D Planck mass, the GUT scale and the GUT gauge coupling).
Some view that as a problem. I view it as an opportunity.
Dear Sean, the problem with current unpleasant status of string theory into the community was built by own string theory community. Let me first remember to you some basic points —the list, of course, is not exhaustive—, which will help to you to rewrite your “cheers”.
– String theory did born like a failure to explain strong force and since it has been always a complete failure. Nothing predicted and all past claims shown to be false. String theory is a theory without laws or postulates because they are modified with time. Please, let me remember to you the history of dimensions: 4D -> 5D -> 26D -> 10D -> 11D -> 12D (some people is working in more than a time dimension) -> 4D (Segal has claimed that we may find a 4D version for solving compactification problems), etc. The claim of string theory is “open” is, of course, a complete nonsense when claim is properly interpreted on both epistemological and ontological terms.
– It is well known that string theorists have manipulated public opinion about string theory. In fact, no popular string theory writer has still convincingly explained to public that string theory failed like a TOE since is being substituted by still unknown M-theory. The popular dissemination of string theory to non-experts violates the most basic ethic guidelines.
– The arrogant attitude of many string theorists is also very well known. Please talk with some critics of them like Peter Woit or Glashow and learn the true sense of the word “pressure”.
– String theory is a mathematical goulash and a dishonest copy of formalisms developed by others. For example, after of decades of very wrong claims about the supposed TOE, now string theorist recognized that were wrong since usual quantization of string was not exact. Now they are launching the TFD version of string theory BUT TFD was previously developed outside of string theory. Again, non-string theorists were correct and “smart” string theorists (e.g. Witen Greene, Vafa, Schwartz, etc.) completely wrong. In fact, none string theorist did contributions to TFD. Even some string theorist has recently recognized that string theorists usually copy the work of others and after “rename” it like string theory.
– All past claims by string theorists were shown to be false, absolute all! In fact, the popular idea of that pointilike particles would be substituted by one-dimensional strings has been superseded by recent M(atrix) formulation by Banks and others, which is basically a quantum mechanics of pointlike particles (D0-branes). People again are ignorant of that, since that, like openly admitted by some theorists, the old name “string theory” is maintained by marketing purposes.
– It is also well knonw and denunciated in several occasions that string theorists have ignored other approaches to quantum gravity. It is very hard for a loop theorist to hear in a popular talk —given by a string theorist— that string theory is the only approach to quantum gravity. The only game into the city!
– It is also well known that young researches were forced to research into string theory because financial support of other theories was stopped in departments, funding agencies, and others due to aggressive string marketing activities. Many young physicists begin a PhD on string theory, discovered that string theory was a waste of time (real string theory is not the same that popularised version of string theory), and leaved the field. Some of them feel…
– Let me take a simple example from chemistry. According to “ignorant” and very arrogant people like Ed Witten, string theory is a promising TOE and reduces all of others sciences, e.g. chemistry. A moment, chemists know that is false, the reduction of chemistry to physics is a myth, as brilliantly explained in innumerable papers in Foundations of chemistry and others journals. Interestingly, 30 years ago some chemists were working in advanced formalisms for explaining behaviour that cannot be studied with usual methods. If you compare the very advanced theoretical work developed in the 60 and 70s with corresponding string theory status you found that string theory was wrong like a TOE even a joke. String theorists, arrogant as they are, ignored all of that and claimed that all of chemistry was an application of string theory. String theory was so advanced that no one other theory could provide to us an explanation of nature more profound, they said. Of course, chemists smiled, like they smile in the 20th century, when physicists (including Nobel laureates like Stark) attempt to convince to them that chemical bond was modelled by classical electrodynamics and that Lewis bond theory was, in simple words, nonsense. Now in the last part of 90, some string theorist discover that all past claims were wrong and are developing a new version of string theory called non-critical one. It is interesting that all past quantum methods and basic stuff is abandoned whereas work developed in the 60s by chemist Ilya Prigogine (see for example his Nobel lecture) used for a radical generalization of old string theory. But the ideas used NOW in string theory were developed in the 60s by other people! Prigogine and others were correct, string theorists again wrong. Interestingly, the ideas of the 60s have been updated in the 90s by the Prigogine and co-workers. Therefore, the current “radical” generalization of string theory by string theorists is, again, an outdated theory. This is real status of string theory; an authentic revolution if one read that masterful piece of marketing called the Elegant Universe (by Brian Greene) and focused to laymen, but claimed to be “very conservative” and outdated in the recent conference Quantum future by expertises that know stuff. Said I again once more? The first step for any serious theorists is to read previously published literature and then develop a better theory, but crackpots are specialist in ignoring the scientific method. If string theorists continue to develop a really outdated theory at one hand and arrogantly claim that are doing (they believe that in their infinite ignorance) the most important, the most powerful, the most fundamental theory at the other, then they would feel comfortable with the mocking of their colleagues. If Brian Greene, offensively claim in his talks that we may quantize everything, and Dyson convincingly reply him saying that Greene is providing no solid arguments in his belief, the problem is not with Dyson, the problem is with Brian Greene, that would first study serious stuff before doing irrelevant claims surrounded by a halo of pomposity.
– Etc.
Sincerely, I believe that non-string theories have been very generous with string theory community. String theorist would please to us our kindly attitude.
Once “refreshed” your memory, let me now comment some of your points.
The idea of that string theory is the most promising way to reconcile gravity and quantum mechanics, is one of well-established myths of literature. In an absolute sense, loop quantum gravity is so “successful” like the strings but in a relative sense (successes / total number of researchers), the loop approach has been around 10 times more satisfactory. Please, let me remember to you again that string theory has been substituted by M-theory.
It is false that “string theory” is based in one-dimensional loops. In fact, you appear to unknown the current joke on Internet that say that “string theory is now a theory without strings”. Yes, you obtain a remarkably rich structure, but just at mathematical level.
It is false that string theory predicts or explains gravity (this is another myth). “To predict” a massless spin-two particle is not the same that quantum gravity. In fact, causality is defined on a flat fixed metric with graviton modes arising in the perturbation, which violate GR basic idea of that full causality is defined in the full metric. This has been the main criticism of general relativists and loop theoreticians during decades. Now, string theorists are recognizing that great mistake (in the past they claimed that one would not take GR “too seriously”) and are unsatisfactorily searching for a background independent version of the old (outdated) string theory.
“In string theory, you just say the word “strings,” and gravity leaps out at you whether you like it or not.” This is not true, in fact one use previous ideas from GR, like to leave “freedom” to the metric into the string action. Somewhat like we need know previously that universe looks 4D and then introduce an arbitrary (that is by hand) compactification 10D -> 4D x 6D.
“At this point it’s a little unclear what the fundamental building blocks of string theory are” It is clear that string is NOT the fundamental entity into the non-perturbative regime.
“One often hears that string theory simply makes no predictions, but that’s clearly false. If you scatter two particles together, string theory unambiguously predicts that the cross-section should look stringy, not like that of fundamental point particles.”
Humm, even ignoring that prediction really mean, this is another myth. Let me simply quote to D. Friedan:
“Even if some particular macroscopic background spacetime is chosen arbitrarily, by hand or by ‘initial conditions,’ string theory still fails to be realistic at large distance. The large distance limit of string theory consists of the perturbative scattering amplitudes of the low energy string modes, which are particle-like. But the particle masses are exactly zero, and the low energy scattering amplitudes are exactly supersymmetric. String theory fails to provide any mechanism to generate the very small nonzero masses that are observed in nature, or to remove the exact spacetime supersymmetry, which is not observed in nature. More broadly, string theory is incapable of generating the variety of large characteristic spacetime distances seen in the real world. At best, for each macroscopic background spacetime in the manifold of possibilities, string theory gives large distance scattering amplitudes that form a caricature of the scattering amplitudes of the standard model of particle physics.”
The problem, of course, is not the popular statement of the difficulties for testing Planck-scale physics as you incorrectly argue; the problem is that nobody has obtained the successful standard model from string theory. A first step of any new theory is obtain that is already known before predict any new (including Planck-scale physics). So string theory is not compatible with available experimental data. “it’s just that we are as yet unable to test them.” As explained above, this is wrong.
“Recently there has arisen another sense in which string theory purportedly makes no predictions, associated with the ‘landscape’ of possible string vacuum states.”
“Well, too bad. It would have been great to make such predictions, but the inability to do so doesn’t render string theory non-scientific.”
No comment!
“… and possesses a mathematical beauty that is so compelling that the theory simply must be correct.”
The world is as it is, no that we like we want that it was. Mathematical beauty is a guide newer a justification. Moreover, string theory is rather ugly, at least for me. About his supposed “beauty”, sceptics suggest that string theorists try to colourfully camouflage the well-known theory’s flaws, like “a 50-year-old woman wearing way too much lipstick”.
“These kinds of arguments just don’t carry that much weight with the non-converted.”
Yes, I believe that “converted” is the correct word to use here. String community looks like a sect or, in the words of one of its most famous members, “a kind of a church”. It is not science.
“it’s the most promising way we know to quantize gravity. If there were multiple very successful ways to quantize gravity, it would be important to distinguish between them experimentally; but so long as the number of successful models is less than or equal to one, it makes perfect sense to make every effort to understand that model.”
This marvellous piece of promotion just emphasize the myth of string theory is the only game in the city. Please read literature in semi phenomenological approaches to quantum gravity and recent advances in other theories like LQG and predictions for the future LHC.
Yes, the comparison between Microsoft and Apple Linux is correct!! Microsoft is a layman-oriented business, whereas Apple or Linux are more specific but more serious. Moreover, it is well knonw that windows OS is a copy of graphical Apple OS and certain kernel properties of Linux/Unix. The success of Windows is in marketing and layman orientation, somewhat like string theory. It is interesting remark that when Microsoft presented the revolution of the trash icon, graphical copy and paste, multitasks, and others features in his first versions of windows, all of that was already known for decades for Mac users. Remember the famous Windows blue display. Yes, your comparison is really good!!
“It didn’t have to work out that the entropy of a black hole calculated from semiclassical gravity ala Hawking would be equal to the entropy of a corresponding gas of strings and branes, but it is.”
Another myth!! Loop quantum gravity obtains the entropy of Schwarzschild black holes. In “string” (really brane) theory, one traditionally has worked with BPS and idealized models of black holes. Strictly speaking, the “traditional” results in string theory do not concern, precisely, black holes, as they are found in a limit in which the gravitational constant is turned off. But they concern systems with the same quantum numbers as certain black holes. There is a kind of analogy instead of an identity with GR black holes.
The problem with string theory is stated by Philip Anderson:
“string theory is the first science in hundreds of years to be pursued in pre-Baconian fashion, without any adequate experimental guidance. It proposes that Nature is the way we would like it to be rather than the way we see it to be; and it is improbable that Nature thinks the same way we do.”
The results of string theory – impressive as they are, are not close to dispelling the scepticism.
Peter,
I think the analogy with QFT is good to reply to your point- QFT does not make general predictions (or rather it makes very mild ones), you have to choose a model to make predictions. In a certain class of string models there is a robust feature which can falsify them, albeit in a very science-fictional way (no such model-independent feature exists at accessible energies, the double edge sword of universality…). This feature does not exist in other corners (e.g models based on 11dim SUGRA), and for many corners we don’t know. To be precise I would call this feature a predicition of perturbative string theory. I agree with you that one ought to use precise language, though this is a non-technical forum.
Sure that assuming we are in the calculable regime has no apriori justification. At any point in time one has to choose between looking under the lamppost (where else would you look?), or developing the theory further. This is a personal choice, and it is good to have people doing both. The latter possibility is intimately related to further developing the language of QFT and quantum gravity (not sure there is a clear distinction), or as WL phrased it above, figuring out how things work.
best,
Moshe
A short question. If, as rumored, experiments show that gravity weakens at small distances, would this be a serious blow to string theory?
I think I finally understand Peter’s point about the non-perturbative regime, and it’s a fair criticism of my claim that string theory makes unambiguous predictions — it’s only true at weak coupling. (Of course, I’m not the one to be talking about these things, but I understand that the blogosphere is self-correcting.) In the uncompactified regime, the low-energy limit seems to contain five critical string theories at weak coupling, and at strong coupling looks like eleven-dimensional supergravity. It’s possible that our four-dimensional world is best described as a compatification of 11d supergravity rather than one of the string theories. In which case, I think it’s fair to say that you wouldn’t be able to see unambiguously “stringy” effects in perturbation theory. Of course, you might see them, and claim such an observation as evidence for weakly-coupled string theory, but the general framework becomes harder to falsify.
(You might wonder whether the same problem would exist for strongly-coupled field theories, where we’re not very good at calculating things. But strongly-coupled field theories tend to rearrange themselves into weakly-coupled field theories with a different set of low-energy degrees of freedom — e.g., QCD becomes a theory of pions. When strongly-coupled string theory rearranges itself into supergravity, the essential stringiness is obscured.)
I wrote a long set of comments on this issue over in my own blog, because I thought it would be rude to post a comment of that length. Given some of the things that have been posted since, I may have been wrong, but consider this a manual TrackBack:
http://www.steelypips.org/principles/2005_07_17_principlearchive.php#112205078094646246
(And, of course, the long URL got broken by the comment system… I hate computers.)
I disagree with the sentence “So string theory makes predictions, it’s just that we are as yet unable to test them”.
Quantum gravity effects are expected to be observable in the context of large extra dimensions, so in the past years many physicists studied their experimental signatures. Basically all results have been obtained from Einstein general relativity, while unfortunately nobody could tell what a “stringy” prediction really is.
(I think this can be considered as a fact. To be precise computations of stringy cross sections have been done in string models that at low energy have contain super-symmetries and no protons. Since LHC will collide non-supersymmetric protons, this kind of string results is far from relevant).
This lack of concrete results in the scenario where having a theory of quantum gravity would have been of practical relevance is probably another factor that contributed to deteriorate the way string theorists are perceived by high-energy physicists.
Sean, actually I’m not sure I agree with your final comment above. I think that 11 dimensional supergravity might be a nice *positive* example of the entire non-perturbative completion changing the theory from perturbative strings to something that does not look stringy. And it is not so different at the core from things that can happen in field theory properties you mentioned. It is positive because supergravity is still a recognisable way that the (*low energy*) strongly coupled physics has organised itself. So we have not lost control and all wierd hell has broken loose, as is the concern that Peter Woit was expressing about strong coupling.
Note that we do know of examples where strongly coupled field theories actually don’t just reaarange themselves into familiar-looking field theories. Take the example of several five dimensional theories that actually become six-dimensional at strong coupling, and for which we cannot write a proper Lagrangian because the natural degrees of freedom couple to a two-form potential. The theories are just wierd at first sight, and aren’t in your typical textbook, but we come to grips with them eventually and see that they still might be under control.
The moral of the story I think that that Peter is right that we should worry about what the final shape of the theory and dynamics are once we’ve allowed for fully non-perturbative physics to take over, but that is not reason to abandon the program altogether because it might not look too much like it does perturbatively. I need only point to QCD as a good example. The most common phase in which we find it is a strongly coupled mess where the basic degrees of freedom in terms of which we formulate the theory (quarks and gluons) are not at all apparent. Someone might say that it is *not* a good example becasue we can get access to at least some of the perturbative regime by doing high energy scattering. Good, and in return I would say that perhaps when we understand strings a lot better, we will find that there are regimes where the strongly coupled -maybe the most common- phase is not recognisably string theory and that there are no stringy features left to observe under most circumstances, but perhaps there will be extreme processes analogous to high energy scattering in QCD that we will conceive of (perhaps in early universe cosmology -goodness knows?) which do probe the weakly coupled regimes (perhaps via duality) where we see the “stringiness” directly in the physics. I just made that up, but I don’t see it as unlikely in principle.
I just think that it is *way* too early to say one way or the other. My feeling is that the theory is still in its infancy. It is unfortunate that the Stringevangelists have promised so much so soon. But their exuberance was/is understandable, if somewhat problematic for having produced a bit of a backlash. I still am excited about string theory and its related theories and endeavours. It is undeniably rich and promising. (And it is useful for other things besides just figuring out “theories of everything”.) We just should make sure to qualify our enthusiasm with caution about how long it might take to turn the framework into a predictive theory -or familiy of theories- that can then be confronted with Nature. Also, while I have a gut feeling that it is premature to look for stringiness in Nature in so much detail that will rule in or out the whole enterprise, it does not hurt to keep a dialogue going between string theory and phenomnology in case we get lucky. There’s a lot of intersting effort out there should *should* continue. Maybe some of the broad principles underlying a possible *class* of things that string theory is part of might be already there for the harvesting.
Best,
-cvj
Sean,
Some strongly coupled field theories rearrange themselves as string theories, and vice versa. It is therefore just inconsistent to talk about string theory and QFT as separate frameworks. Falsify the one and you falsified the other.
Rather, the stringy language seems like a more efficient way (perhaps not the final story) to organize the beast, at least sometimes. As a framework and a language, string theory is not some future promise, it is already a very successful enterprise.
But I’ll stop here, lest I find myself over-hyping…
best,
Moshe
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It is all very well but where is the promised answer to “What is the gravitational field of an electron?”
Very interesting comments, I think this blog is setting new standards for the level of discussion. All hail cosmicvariance.com!
After beating up on Sean about predictivity, I should point out that I agree with Clifford and others that coming up with real predictions that go beyond the standard model is probably too much to hope for right now. Another way to say it is that it’s pretty clear our tools now aren’t good enough, so we need better tools before we can build something better than the standard model. I’m all in favor of string theorists working on building better tools involving string theory, but feel the field needs to encourage people to also build new tools that don’t involve string theory.
I’m tempted to argue with Moshe about his claims that QFT and string theory are all the same thing, but maybe I should just agree with him. Maybe space will open up in theory groups for young theorists who want to do pure QFT once everybody agrees that QFT=string theory.
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Oh, Peter, I should say one thing on that point. Don’t forget that “QFT=StringTheory” is not another of those later annexations that our field organised in its often-claimed “assimilation” of all of physics. (We’ll get around to the rest of science eventually, he says provocatively.) At least for the case of gauge theories, (and in a superficially different sence from what Moshe and I were talking about above) this was something taught to us by ‘t Hooft in the early 70’s, right? And this is an example of what I was saying about strings being good for other things. It is a window into the nature of Quantum Field Theory, which we *definitely know* is a powerful tool for describing Nature. I’m looking forward to the day when we get really powerful insights or tools from string theory into calculating in useful dynamical regimes of field theory that we probe with real experiments. I would consider 30 or 40 years of effort in string theory worthwhile if we acheived just that alone.
Yes, there should be jobs for young people working on hard problems in field theory. Some of those should have powerful string theory technology in their toolboxes so as to serve their employers well.
-cvj
I also think that whatever happens, we need to be able to do String Theory. String Theory isn’t really a new theory – it’s just quantum mechanics (and a bit of supersymmetry). It seems to me that we really need to understand the ramifications of that theory before we can move on. The richness of String Theory is stunning giving its simple origins from quantising the classical string.
The idea of giving up that richness to study another area before we’ve fully explored the consequences of Strings seems insane to me. Before the String theory programme really got going nobody could have predicted the connections to fields like classical elliptic functions and modular forms, group theory via Monstrous Moonshine, the unification of the 5 string theories, mirror symmetries and so on (I’m a few years out of date myself). Vast amounts of fascinating mathematics have been pouring our from the work of String theorists. And yet these are all just consequences of quantizing a simple classical system with no extra hypotheses, no extra laws of physics. Until we understand these it can hardly be said that we understand quantum mechanics, and yet QM is the foundation of all modern theoretical physics. If we’re stil surprised by the consequences of quantising this simple little toy 1D system I don’t think we can’t be ready to move on at all.
Peter,
You made some excellent points, and I think constructive criticism is much needed in string theory, or in any other science for that matter. You’re doing a great job in that sense, and I certainly agree with your general point that one should avoid at all costs that a single hypothetical theory starts monopolizing all activity in a field.
However, I (of course 😉 don’t quite understand why you express so much undifferentiated disdain towards the “landscape studies” sector of our field. The working hypothesis for about anyone working in string theory is still that at least in principle it is a framework capable of describing the real world in a way compatible with all known principles of nature. How concrete this will ever become, or how useful insights from string theory (or any other theory) in predicting parameters of nature will turn out to be is still not known at this point. We certainly by far don’t know enough yet about the structure and dynamics of the theory to draw any definitive conclusions at this point.
Getting more insight in this obviously very important question is what has motivated many of us “landscape loonies” to start studying these things in a much more systematic way than people did before. The goal of this work, at least for me and the people I worked with, is not to try to keep on “pushing” string theory as the theory that will ultimately explain everything, but rather to find out, with what we know now, up to what extent one could actually expect string theory to be predictive in this setting. This is exactly the question you have been interested in for many years now, and if anything thus far, landscape studies have put a number of your objections on a firm footing — so you should be encouraging us rather then tell us to stop doing this stuff; we’re on your side! 😉 I personally would be quite happy if we could actually prove string theory has zero predictivity no matter how accurate measurements become, or if we could prove the other extreme, that no vacuum of string theory reproduces our universe. Either way, that would be progress.
But I want to stress that all this is still in its infancy, and that it is really premature to conclude, as you have suggested, that all hope is lost, and that therefore this line of research must be abandoned. Sure, the number of vacua is infinite, but we knew that all along — for example AdS_5 x S^5 with flux N gives you an infinite set of nonperturbatively defined vacua of string theory, since N can take any integral value. What we do not know at all is how many vacua resemble our own, and in particular not even how many lead to metastable de Sitter — from what we know now, it is still possible that there are in fact none of the latter! (There are a number of plausible constructions of metastable dS in string theory, but these are not nearly as firmly established as superymmetric AdS vacua.) Much more good work needs to be done to get a better grip on the set of vacua of string theory. I just hope young people entering the field now don’t get discouraged by the sometimes vitriolic comments towards those trying to think about the deep questions and ambitions of theoretical physics, and as a result would flee back in the safe confines of subjects that are as far from these questions as possible.
I agree with you and others in this discussion thread that it is probably premature to expect predictions beyond the standard model from any theory at this point. But landscape research has nevertheless caused what I would call a mini-revolution in our collective thinking about string theory: after the euphoria of the mid-nineties, it has brought new (and much needed) humbleness to the field, and it clearly re-exposed the holes in our knowledge that will have to be addressed to answer many of the fundamental questions that were the original motivation for the theory. This has lead to some revived interest in quantum cosmology and vacuum selection principles, topics which before were hardly addressed at all in the context of string theory. It also has led us to rethink the notion of naturalness and the rock solid expectation this used to lead to, of finding new physics just beyond the electroweak scale. And yes, it has re-stirred the debate about the extraordinary finetuning of the observable universe and the extent to which environmental selection principles may play a role in that — needless to say (I hope!), such selection principles can only account for a very small subset of parameters, assuming other parameters fixed, but it is a logical possibility, and logical possibilities should never be excluded in any science just on the basis of aesthetic prejudices (especially if the logical possibility in question is the only one that has met with some quantitative predictive success!).
In a way the greatest achievement thus far of this research is that the emergence of the landscape to the forefront of discussions has caused a new sense of crisis. Throughout history, crisis has proven to be necessary to be able to abandon old beliefs that obstruct progress; who wants radical new ideas if everybody is cozy with the old ones? Quantum mechanics would never have been born without the preceding period of utter confusion and crisis. So this “blow”, I honestly believe, is really the best thing that could happen to us at this point.
Finally, about the claims that people get depressed by all this — well, I don’t get that. Science always progresses in a rollercoaster way. Ideas get killed, enthusiasm waxes and wanes, promising approaches turn obsolete so formerly obscure ones can jump in the spotlight. It’s all progress. If it makes you feel sad, it means you either were holding on to a scientifically unhealthy illusion before, or you are in dire need of a vacation to realize again that there is more to life than physics.
Frederik (off to Mozambique 🙂
Peter,
I also vote for 3 cheers for Sean and the rest…
I am not sure I agree with the concise form you summarized my viewpoint. Let me just say that if you dream of a day where people study questions in field theory (say vacuum structure of SUSY gauge theories, anomalous dimensions of operators, helicity amplitudes,…), using both stringy techniques (SUSY, D-branes, topological strings etc.) and non-stringy techniques (matrix models, Bethe ansatz, twistor space, unitarity,…), you should consider yourself very lucky…
I also don’t agree with your definition of “real” prediction. In my mind a predicition is always done in the context of a specific model. I see that Frederic has joined in, so I’ll leave the discussion of such points in his capable hands (unless he is having way too much fun in Mozambique already).
best,
Moshe
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Congrats on an informative and balanced blog!
A short comment from yet another outsider. Feasible falsiability makes us trust theories and can debunk junk. It would remove (probably all) anthropic principles which have been critizised in all their applications. The landscape seems to need more selection principles anyway so it seems like a small sacrifice; again from the outside of course.
Hi Frederik,
Mozambique? wow. I’m writing from a lousy internet connection in Crescent City, California, on my way to Seattle. Much less exotic, and it’s difficult to write much this way. Do you still live in NY? If so we should get together sometime and discuss this over a few beers.
I understand your point that in some sense we’re on the same side. What you’re doing has gone a long way towards torpedoing long-held hopes for getting predictions out of string theory. If I thought you had a chance of conclusively showing that string theory was not falsifiable, and that this would cause string theorists to agree that it was a wrong idea (as an idea about unification), and do something else, I might even work on this stuff too. If that really is the direction you are going, more power to you. But when I last heard Michael Douglas speak, he seemed completely unwilling to even consider the idea that string theory might be wrong, much less say that this was something his research might be leading to. Maybe I misinterpreted him, or maybe he is changing his mind. But I found the experience of seeing him unable to come up with any plausible hope for predicting anything, and simultaneously still promoting string theory unification to be profoundly disturbing.
The problem is, I don’t think you can kill off string theory by studying the landscape, and the danger is that it is so huge that studying it could take up the whole careers of an exponentially large number of theorists. If the initial ideas about getting predictions out of statistics fail, people can always claim that finding the real M-theory will save things, perhaps by giving a framework in which there is a cosmological selection principle.
I realize that my comments about this have sometimes been vitriolic. But I really do see a significant number of string theorists reacting to the fatal problems of the theory by abandoning basic principles of science, instead of admitting failure. Ann Nelson asked why I was so tough on you landscapologists, that there were always lots of bad papers around. It’s true that there always have been and will be lots of people who write bad papers. But you and Michael Douglas are among the smartest people in the field. Seeing the best people in the business involved in a project that is being used by many people as a justification for giving up on science seems to me really worrying, thus the harsh commentary.
Have a good trip!
Moshe,
Sure, predictions are made in the context of a specific model, and these specific models are what you falsify if the predictions fail. So, you need to have a specific model or restricted class of models in mind when you say you have a theory that makes predictions. String theory involves such a huge class of models that in and of itself it doesn’t make predictions. You can imagine that there’s a small class of these consistent with some basic observational facts, then these would make predictions. People have been working hard on trying to do this for more than twenty years with zero success. I think there’s overwhelming evidence this won’t work. Every model I know of that has so far been investigated is already falsified by the fact that the model can’t reproduce the standard model. Maybe someday finally someone will come up with a string theory model that agrees with the Standard Model, but makes other predictions. Then you can say string theory predicts something, but not until then.
Dan,
You act as if particle theory can only investigate one thing at a time. While sociologically this sometimes seems to be true, there are several thousand particle theorists out there, and your argument implies that some should work on string theory, but there’s no reason all of them should.
Some comments of mine about the topics in this thread are here:
http://motls.blogspot.com/2005/07/strings-as-microsoft.html