Two cheers for string theory

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.

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105 Responses to Two cheers for string theory

  1. There is a problem of principle with the idea of the string: Where is length? In QFT, length is in the background spacetime, in String theory, it is both in the background and in the particle. For some years strings theoretists promised background free theories, so this problems was to be removed in the future. I do not know if the future is already here. It should, if one thing that Superstrings are already more than thirty years old as a theory.

  2. WL says:

    > 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.”

    That’s exactly how I view it – a framework generalizing QFT and
    including gravity. We had overwhelmingly strong clues since 20
    years that string theory, in its current background-dependent
    formulation, would not predict a unique vacuum state but rather the
    opposite of it, but that has never bothered me working on it since
    then.

    A more modest goal, namely trying to better understand the *principles
    according to which things work* (gauge theories, black holes,
    D-branes, say) leaves more than plenty of things one can sensibly
    do, even without making predictions that would be experimentally
    testable today. There are so many highly non-trivial phenomena,
    which often can be tested via “theoretical experiments”, that it
    would be foolish not to investigate them and see what one can learn.

  3. Clifford says:

    Wow. I go for a long working lunch, and come back to find that Sean has written a nice long post on strings that I’d promised TM in the comments of another post I’d get around to writing one day. Thanks Sean, this is great! I can go back to lunch and continue thinking about my string theory problem. -cvj

  4. Robert says:

    Ouch, calling string theory the Microsoft of quantum gravity hurts!

  5. Robert says:

    I am not sure that string theory is to be compared with QFT. In the end, it’s really just one theory (or very few) and it is the state rather than the theory that varies. However, this is dangerous to say if all we know to do is pertubation theory around states as the other states might be very far away (dynamically).

    Here, I expanded about this a bit. In short: If there were only one state in string theory, that would be quite scary, as it would (if strings are realized by Nature) predict everything. Everything, including everything you see right now. And smell right now. And (maybe) think right now.

    It there were only one state in Maxwell theory, you could (in princliple) compute what your radio would be playing in an hour.

  6. Dan says:

    Nice job, Sean. Very fair, balanced and informed description of what’s going on. You are so many orders of magnitude more intelligent and reasonable when you talk about physics, rather than politics. Stick to the former please.

  7. Cameron says:

    Sean,

    As a lay-person in the physics realm, could you explain what guage theories are?

  8. Levi says:

    I think you promised a post giving your view of string theory way back in the Preposterous Universe days. Glad to see it here. I love the new blog, even the posts about dimwitted movies (e.g. Mr and Mrs Smith) Only, where are the poems? Where are the pictures? But I do love the new blog, and all of your accomplices.

  9. Sean says:

    Cameron– Gauge theories are basically generalizations of electromagnetism. In E+M, the behavior of both the electric and magnetic fields can be described in terms of a single four-dimensional vector, the “vector potential.” But there is a symmetry: you can shift the vector potential by the derivative of a scalar field, and the corresponding values of the electric and magnetic fields don’t change. That’s gauge symmetry. It can be generalized to more complicated kinds of symmetries, e.g. SU(2) etc. The Standard Model makes heavy use of gauge theories, to describe the strong and weak forces as well as electromagnetism.

  10. Geoff says:

    Ignore Dan. Not only are people who try to tell others what to write about annoying, your politics posts provide a nice counterpoint to the physics. It creates a richer blog. Dan can go get his own site if he doesn’t like what’s written here.

  11. Dan says:

    Stop whining, Geoff. The comment section is for comments on what’s been posted, you know. I like it here, even if I don’t agree with the politics. It’s you snotty elitists left-wing liberals who can’t stand other people’s opinions. I’m quite all right with the diversity. I still like to argue, debate and comment.

  12. Kuas says:

    I fell asleep somewhere in the second or third paragraph of this post. Anyhow, I guess publicly going to bat for string theory is not a bad move when you find yourself on the job market.

  13. I watch string theory from the sidelines as a journalist and read about if often. There are some aspects that are often a oversold and the scientist should probably frame their comments a little differently.

    As you point out, it’s not a theory of everything. The theory of everything tag sounds like it’s taking away free will, and a lot of people react negatively to that.

    Also, since it’s not testable with today’s accelerators, I always wonder if it should be the string hypothesis. I understand the math is well worked out and waiting for confirmation. Does anybody know when quantum mechanics and general relativity went from hypothesis to theory?

    The conflict between quantum mechanics and general relativity is always mentioned, but the details usually aren’t, or summarized glibly as fuzzy and jumpy quantum/versus flexible spacetime. The light flashed over my head this year when I finally read someone explain that general relativity’s bendable spacetime conflicts with the rigid spacetime of quantum mechanics, where things don’t bend.

    Your new blog is interesting so far. I can’t say I ever like the “Continuing reading post…” option. The full version is only text and it’s not like the whole post slows down loading. And it’s easy to spin the mouse wheel to the next post.

  14. Bob McNees says:

    “I am not sure that string theory is to be compared with QFT”

    Maybe not with if you’re talking about states, but I think it’s a very good comparison to make when people raise the issue of a landscape as signalling some sort of fatal flaw in string theory (This is a point I recently raised on Peter Woit’s blog, maybe foolishly). Our understanding of string theory is far from complete. Holding up a tentative observation as evidence that string theory is wrong or, worse, unscientfic is…silly.

    As Sean mentions you can start writing down field theories and never finish. Imagine that you were to write down all field theories and then try to find a “realistic” theory that describes whatever experiments you are interested in. One way of narrowing down the field is to do experiments and tune the parameters of your generic theory, but it’s not until we understand certain selection principles at work in the space of field theories that we can really get anywhere. Gauge invariance allows us to reduce an infinite number of ostensibly different theories into a single theory (imagine writing down all of the gauge fixed versions of a theory with a continuous gauge fixing parameter). Or suppose, after making some obscene number of measurements, we still find ourselves with an infinite number of field theories consistent with our observations. It’s not until we incorporate ideas like the renormalization group and effective field theory that we really understand what it means to distinguish two theories at the scales set by our experiments. Ideas like this completely change our original goal, which was to identify the unique field theory (out of an infinite number) that describes the results of our experiment. Instead, we make due with the fact that our tools (measurements up to some scale) for picking out a particular theory will never completely pin things down (above that scale). We’ve revised our question by understanding effective field theory.

    If we didn’t understand all of the principles at work in QFT we would be up the creek trying to answer that original question. We wouldn’t even understand why we weren’t making progress on a question that turned out to be naive.
    That wouldn’t represent a failing of QFT, just our understanding of it.
    Asking “which of these field theories is the right one?” seemed like a reasonable question at first, but as we learned more about QFT it needed to be refined. Understanding a theory means, among other things, knowing what kind of questions you can ask and how to ask them.

    With respect to the landscape, string theory is in a similar situation. We have ideas about how to formulate string theory, but certainly not with the degree of generality that we would like. We know what the proper observables are in some situations, but not others. In some ways our understanding is comparable to the hypothetical field theorist in the previous example: we’ve learned a tremendous amount but we know there are important principles that we still don’t understand. The landscape may be an important question for string theory, or it may just be an artifact of our own ignorance.

    Based on the successful aspects of string theory that Sean describes, I think it’s reasonable to suspect that string theory is full of powerful and sophisticated mechanisms that will either eliminate the landscape or change the question. That’s my guess. Can I prove it? Not yet, but I’ll work on trying to. Of course, maybe I’m wrong and string theory contains no means of singling out a unique vacuum: i.e. there really is a landscape. What Sean says still applies. String theory isn’t automatically rendered unscientific, we’ve just learned that it doesn’t have anything to say about one of our questions.

  15. Peter Woit says:

    Hi Sean,

    As you might guess, I strongly disagree with some of your claims about string theory, more specifically:

    ‘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.”

    Well, what exactly is the prediction? Let’s say that tomorrow someone figures out how to make an accelerator that collides electron-positron pairs at 10 times the Planck energy, with as much luminosity as you want. What will the detectors see? You say the cross section won’t be point-like, but will be “stringy”, what exactly does that mean? First of all there’s the problem that you don’t know what the string scale is. It’s a parameter times the Planck scale, but this parameter could be anything. So, whatever your “stringy” effects are, you don’t even know at which energy scale they’ll show up.

    OK, this is just one parameter, and if one could make predictions that just depend on one parameter that would be great. You’re implicitly assuming that this parameter is small, so small that perturbative string theory gives a series that, while divergent, is a useful asymptotic expansion, i.e. cutting it off after the first term or two gives a close approximation to the true result. Why do you believe this is true? You’re assuming that ignoring all non-perturbative effects is a good approximation? Why? Presumably by “stringy” you mean that one will see a spectrum given by the vibrational modes of a string. How do you know that, before you get to the first vibrational mode above the vacuum state you won’t start producing black holes (or “branes” of some other kind)? Sure there are characteristic signatures of perturbative string theory, but my claim is that there aren’t any for non-perturbative string theory and that’s the real thing.

    The existence of an infinite number of very different vacuum states around which you can build perturbative string theory expansions makes the issue more pointed. If you believe that perturbative calculations are valid and so the theory can make predictions, you also have to believe in the infinite number of vacuum states, which makes the theory radically non-predictive at energies we can ever hope to measure. Most string theorists I know would prefer to believe that perturbation theory is no good for determining the vacuum state, that non-perturbative effects will pick out a vacuum that looks like our world. But you can’t have it both ways: you can either believe that perturbation theory is a good approximation (although there’s no good reason for this, the question is undecidable until you have a non-perturbative theory), in which case there’s a “stringy” spectrum, but no hope of predicting anything at accessible energies, or you can believe that non-perturbative effects are important, in which case you can hope for low-energy predictions, but there is no characteristic prediction for what happens at the string scale.

    So, again, you’re claiming that string theory makes predictions that we could check if we had a high enough energy accelerator. What are they, and if you’re using perturbative string theory, how are you dealing with the fact that by itself perturbative string theory is an inconsistent theory?

    I’ll comment on some of the other points in your post separately.

  16. Peter Woit says:

    Bob McNees is ignoring the long response I wrote to him on my blog explaining the problem with his analogy about QFT and string theory. You can read it there, but the fundamental point is simple. QFTs make an infinity of well-defined predictions about low-energy physics. You can go out and compare them to the real world. You then see that one of the simplest QFTs agrees precisely with everything experimentalists have seen. Experimentalists do more experiments and get results agreeing with what this QFT predicts. This is a beautiful example of the scientific method. The utter lack of any connection between string theory and experiment makes it something completely different, and raises real issues of whether it is a science at all, especially if anything like the landscape exists.

  17. Peter Woit says:

    About some of Sean’s other points:

    1. Why there is some “disdain and resentment” about string theory among other physicists.

    First of all, until recently there was very little of this. I’ve now spent twenty years complaining to physicists of all different kinds about string theory. Until the last couple years, the reaction I got from most string theorists was that I was an idiot, too stupid to understand the theory. The reaction from non-string theorists was that they weren’t so sure themselves about string theory, but many string theorists were geniuses and I should be careful about criticizing my betters. I made attempts to publish some rather moderate criticisms of string theory, and found these thwarted, generally by non-string theorists who were pretty convinced that only a crackpot would be claiming that string theory was completely on the wrong track.

    Things have changed a lot during the past few years. First of all, because of the internet, venues have appeared in which the problems with string theory can be laid out extensively and in public. Secondly, despite what Sean says, progress on string theory has virtually come to a halt, and many of the people doing it have become discouraged or even left the field. Finally, the apparent existence of the landscape has led to a large number of prominent string theorists engaging in what is obviously pseudo-science. The words many physicists, string theorists and non-string theorists, use to describe what is going on are not printable in a family-type blog like this one.

    Fundamentally, more and more people in the physics community are beginning to feel that they’ve been had. The theory groups at Harvard, Princeton, Stanford, etc., etc. are completely dominated by string theorists. String theorists have collected every reward academia has to offer (except the Nobel Prize) for work that Lawrence Krauss accurately describes as a “colossal failure”. This is driving a lot of the “disdain and resentment”. The incredible degree of over-hyping of the theory that has gone on is part of this. When you spend twenty years going on and on about how beautiful and wonderful your theory is, and then it becomes clear it doesn’t work, you shouldn’t be surprised that some people don’t think very highly of you.

    2. There’s a lot to be said about the generalized claims for successes of the theory that Sean makes, but to do this properly is a long story. I’ve done it elsewhere, and since I’m about to leave on a trip, I don’t have time to do this anyway. I hope a useful discussion will ensue here, I’ll try and write more later on my own blog and stop making long comments here.

  18. Joe Bolte says:

    Dr. Carroll,

    You wrote: “If you scatter two particles together, string theory unambiguously predicts that the cross-section should look stringy, not like that of fundamental point particles.”

    Does this mean that you can use string theory to write an expression for the differential corss-sectoin, d-sigma/d-omega, which will differ from the one predicteed by the Standard Model? If so, please either tell me what this expression is, or direct me to a paper where one is derived.

    If no one can write such an equation, in what sense does “string theory unambiguously predict that the cross-section should look stringy”?

    Best Regards,
    Joe

  19. SM says:

    “If string theory turns out to be the same way, that’s life.”

    Not sure if that was an intentional pun..but it’s hilarious 🙂 If anthropic reasoning does turn out to be the crucial factor in selecting a string vacuum (the physicist in me does kind of recoil from this kind of approach, I’ll admit it) then perhaps biology really will turn out to be the science of the 21st century!

  20. Peter Erwin says:

    Hi,

    Interesting post (and interesting blog!). I have a couple of brief comments from an outsider’s perspective (I’m an astronomer).

    I think part of the grumbling you’re concerned about stems from what some people see as the overselling of string theory — claims that it would solve all sorts of problems which it now looks like, perhaps, it won’t. It’s interesting that you seem rather sanguine about string theory maybe not being able to provide unique values of e.g. fundamental particle masses, etc., since that was one of the claims that people were making, at least at the popular level: that string theory could explain all the arbitrary values that had to be measured and put into the Standard Model by hand.

    Genuine predictions about Planck-scale effects would certainly be a nice start, but I’d argue that an unspoken part of the idea that “a scientific theory makes testable predictions” is that these really should be feasible predictions. Few astronomers would take (very) seriously an astrophysical theory whose tests required, say, 10-million-kilometer-diameter telescopes, or observations over a period of several thousand years.

    Brief reply to Philip Downey’s question: The first “experimental test” of GR was the solar eclipse observations of 1919, which is pretty good for a theory published in its complete form in 1915. The test could have been done with earlier eclipses, but World War I got in the way. (I think Einstein’s first prediction of a measurable gravitational lensing effect was published around 1912 or even a little earlier, and there was apparently an attempt to check this with a 1913 eclipse — but bad weather made it impossible). This is what got Einstein his international fame, though I don’t know how long it was before GR was generally accepted by the majority of the physics community.

  21. Bob McNees says:

    Hi Peter,

    Please don’t take me not posting on your blog yet the wrong way. I will get around to it.

    I don’t think you’re responding to the argument I’m making. When you say that QFTs make lots of predictions about low energy physics you’re really talking about effective field theories. If you give me enough data I can reconcile it with an effective field theory. Furthermore, that effective field theory will provide me with a sophisticated framework for predicting the outcomes of other experiments. Inherent in its definition are the criteria for what I can and cannot predict.

    This is very different than what I mean when I ask you to pick out a quantum field theory using some finite set of low-energy measurements. Forget all the things you learned that led you to appreciate the notion of effective field theories. Write out the most general lagrangian you can. Put everything in it. Include all the interactions consistent with the most basic principles of field theory. Don’t exploit any internal symmetries to simplify things, and don’t forget all the interactions which, if you knew about renormalization, you would classify as unspeakably irrelevant. Now set up some experiment that you think should be described by quantum field theory. Make all the measurements you want but keep them below a certain scale, and tell me what QFT you think describes that.
    Whatever set of field theory parameters you give me, I know that there is going to be some very irrelevant (and probably very fine tuned) interaction that I can
    add that won’t be ruled out by your data. But I certainly don’t claim that QFT doesn’t explain your experiments. I just refine my original question and learn to ask about effective field theories.

    I know this example sounds contrived to you. We know a lot about QFT. Given an effective field theory we understand when it should be valid, and we know that my example of adding an ultra-fine-tuned irrelevant interaction isn’t relevant to the experiments you are doing (no pun intended). But there are a lot of deep things we must understand about QFT before we can think in those terms. They all represent clever and hard-fought answers to difficult problems. The scientific method isn’t just the part you mention (doing more experiments and getting more agreements). It also involves figuring out what the theory can and cannot tell you, why that is so, and how you go about asking the meaningful questions.

    Answering those last few questions is what allows effective field theories to make the predictions you refer to. I think we all appreciate how non-trivial this is. What would you have said to early field theorists when their calculations came up full of infinities? Would you have told them to give up? It seemed that their theories were making predictions which could not be meaningfully compared with experiment. Lots of people gave up on it for just this reason. A few people stuck with it, and it payed off. By your standards how could they ever have made progress? Nothing you’ve said about QFT is meaningful without their resolution to this problem.

    I don’t think string theory gets a free pass on most of the questions you raise. I think that some of them represent good, genuine criticisms that should interest all of us. But why immediately hold string theory, a young field, to the same standards you apply to a mature subject like quantum field theory? We understand much more about QFT than we do about string theory. There are lots of problems in QFT where we understand the resolution. Previously, one might have thought that those problems rendered QFT incapable of making predictions about the real world. Its *only* because we solved these problems that we understood precisely what predictability means in QFT. So, my comparison is not between string theory and effective field theory. Rather, I’m comparing the current state of string theory with QFT before all of the developments that proved to us that it provided a consistent, predictable framework. That seems like the more relevant comparison at this point.

  22. Bob McNees says:

    When Sean mentions string theory predictions for scattering, I assume he’s referring to the different high-energy behavior one would expect. Field theory amplitudes are hard (power law) at high energies. String theory amplitudes are soft. If you ever do a high energy scattering experiment and see a soft amplitude, you have strings.

  23. Haelfix says:

    I said this elsewhere, but in QFT there is a distinct notion of minimality. The correct effective field theory for our world is almost the simplest one you can write down.

    ST currently lacks this, but thats not to say such notions as the previous commentator wrote won’t appear (renormalization group flow, etc)

    The problem as I (naive physicist with little string theory background) see it is cosmological in origin. We observe that we live in one vacuum, and it seems therefore that we must demand the meta stable desitter vacua to not decay or tunnel to some other spot on the landscape, with error bars to at least the order of the age of the universe (observed as uor laws of physics seem to not have changed). This will require enormous, perhaps infinite amounts of fine tuning, not just in our vacua but in all the other infinitely close landscape modes.

    The anthropic principle (or principle of arrogance) does away with that, it just says it is that way because thats what we observe. However if such notions appeal to you, I claim its simpler to just say the world is the standard model with semiclassical nonrenormalizable gravity. Both have infinite amounts of fine tuning, and the latter is simpler.

  24. Ann Nelson says:

    Hi Sean

    I am glad you posted this. As a phenomenologist/model builder who got her PhD in 1984 and found precious few jobs available which were not reserved for string theorists, I shared the disdain for and dismay about string theory that you are trying to counter. In the 80’s, many string theorists were clearly out of their minds–for instance I heard one famous theorist exult “now we don’t need experiment anymore”, and another made a bet that the elementary particle Yukawa couplings to the Higgs would be computed within 10 years.

    The second string revolution changed my mind. I still think most string theorists are way too arrogant and unscientific in their unshakable beliefs. Also the sociology of the field has many pathologies. That said, the field and its practitioners have redeemed themselves by discovering deeper and more interesting insights into Quantum Field Theory then ever would have been found by phenomenologists or by traditional formal quantum field theorists. Spin-offs of string theory including D-branes, AdS/CFT, orbifolds, dualities, supersymmetry, and new dimensions have all found their way into testable particle physics phenomenological model building, and particle theory would be would be completely moribund without them. Also some interesting experiments have been done in response to some of the string-inspired models, such as tests of the gravitational inverse square law at the sub-centimeter level, that would not otherwise have been conceived, and which very well might find something revolutionary. Other approaches to quantum gravity dont offer many such spin-offs, as far as I can tell, and neither do other formal approaches to quantum field theory.
    Finally, as to whether string theory and its predictive power should be thought of in an analogous way as quantum field theory—ADS/CFT has taught us that string theory IS quantum field theory and vice versa. For instance QCD, which is certainly a quantum field theory, can very likely also be formulated as a string theory and the ADS/QCD approach shows that the latter formulation can give interesting insights into phenomenona such as vector meson dominance and confinement.

    I certainly wouldn’t defend every paper in the field, including some of the ones Peter Woit uses to disparage the whole enterprise. But I now would hate to see the pendulum swing so far that string theorists have as hard a time getting jobs as phenomenologists did in the 80’s.

    Ann

  25. Scott says:

    So I have a question about the claim that ST predicts gravity that “you just say the word “strings,” and gravity leaps out at you whether you like it or not.” Does it simply predict the existance of a 2 spin, massless, chargless, ect particle or does it also predict that all particles emit and absorb this graviton based on their energy and momentum as expected? Also does the existance of a particle that is also predicted by all the failed quantizations of gravity really a good thing for a theory to predict?