Steinhardt on the String Theory Crash

The Edge web-site has something new up they call Einstein: An Edge Symposium (thanks to commenter Hendrik for pointing this out). It’s an exchange between Walter Isaacson, Paul Steinhardt and Brian Greene, nominally about Einstein, but ending up turning into a discussion of whether and how string theory has “crashed”.

Steinhardt forcefully makes the same point I’ve made ad nauseam here: the anthropic string theory landscape is not a valid scientific research program, but simply the kind of thing you end up with when a speculative idea fails.

In my view, and in the eyes of many others, fundamental theory has crashed at the moment. Instead of delivering what it was supposed to deliver—a simple explanation of why the masses of particles and their interactions are what they are—we get instead the idea that string theory allows googols of possibilities and there is no particular reason for the properties we actually observe. They have been selected by chance. In fact, most of the universe has different properties. So, the question is, is that a satisfactory explanation of the laws of physics? In my own view, if I had walked in the door with a theory not called string theory and said that it is consistent with the observed laws of nature, but, by the way, it also gives a googol other possibilities, I doubt that I would have been able to say another sentence. I wouldn’t have been taken seriously…

But what angers people is even the idea that you might accept that possibility—that the ultimate theory has this googol of possibilities for the laws of physics? That should not be accepted. That should be regarded as an out and out failure requiring some saving idea…

What I can’t accept is the current view which simply accepts the multiplicity. Not only is it a crash, but it’s a particularly nefarious kind of crash, because if you accept the idea of having a theory which allows an infinite number of possibilities (of which our observable universe is one), then there’s really no way within science of disproving this idea. Whether a new observation or experiment comes out one way or the other, you can always claim afterwards that we happen to live in a sector of the universe where that is so. In fact, this reasoning has already been applied recently as theorists tried to explain the unexpected discovery of dark energy. The problem is that you can never disprove such a theory … nor can you prove it.

Steinhardt dismisses attempts to hypothesize that maybe the landscape is somehow predictive as follows:

Do you mean as derived from string theory? I don’t believe that’s true. I don’t believe it’s possible…

Well, I believe that if you came to me with such a theory I could probably turn around within 24 hours and come up with an alternative theory in which property X wasn’t universal after all. In fact, you almost know that’s true from the conversation that’s been happening in the field already, where someone says, these properties are universal and these others are not. The next day, another theorist will write a paper saying, no, different properties are universal. There are simply no strong guidelines for deciding…

If a version of string theory with a googol-fold multiplicity of physical laws were to be disproved one day, I don’t think proponents would give up on string theory. I suspect a clever theorist would come up with a variation that would evade the conflict. In fact, this has already been our experience with multiverse theories to date. In practice, there are never enough experiments or observations, or enough mathematical constraints to rule out a multiverse of possibilities. By the same token, this means that there are no firm predictions that can definitively decide whether this multiplicity beyond our horizon is true or not.

After some prodding, Steinhardt makes clear that he is not claiming that string theory as whole has crashed, that it is just the landscape that is the crash. While insisting that people need to acknowledge that the landscape is simply a scientific failure, he holds out hope that some fix to string theory may still be found:

…it’s that point of view which is a crash, and needs a fix. I am not arguing that string theory should be abandoned. I think it holds too much promise. I am arguing that it is in trouble and needs new ideas to save it.

There’s also some discussion about what Einstein would have thought of string theory and the landscape, with Steinhardt of the opinion that Einstein would have liked string theory with its unification via geometry of extra dimensions, but that he would have rejected the landscape:

Einstein took gravity and turned it into wiggling jello-like space, and now string theory turns everything in the universe, all forces, all constituents into geometrical, vibrating, wiggling entities. String theory also uses the idea of higher dimensions, which is also something that Einstein found appealing.

What I was commenting on earlier was where the string program has gone recently, which I described as a crash. I can’t say for sure how Einstein would view it, but I strongly suspect he would reject the idea.

Three years ago I expressed the opinion that the promotion of the anthropic landscape would make Einstein gag, which so upset Joe Polchinski that he used this to argue that trackbacks to my blog should not be allowed on the arXiv (even though this was not about an arXiv paper, but a Scientific American article). At one point I regretted having used that expression, feeling it was somewhat over the top and inappropriate. In retrospect, seeing what has happened over the past three years, I’ve changed my mind. The kind of thing that would make Einstein gag has moved from popular science articles to regular appearance in the lectures and scientific articles of leading figures in particle physics. This would probably not just make him gag, but send him into a serious fit of depression.

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86 Responses to Steinhardt on the String Theory Crash

  1. Tom says:

    As an uninformed member of the peanut gallery, may I propose a new ground rule for this debate? Namely, that the dead be allowed to rest in peace and not forced to weigh in with their (imagined) opinions on the matter.

  2. chimpanzee says:

    “Every absurdity has a champion to defend it.”
    anti-religion website

    I had a conversation with Wolfgang Haken a few yrs ago

    [ UIUC mathematician (whose father was Max Planck’s PhD student) who proved the 4-color theorem with K. Appel, using computer ]

    who was working on cosmology in his retirement. He was very vocal about the “infinity of solutions” curse, & thought the scenario was hopeless! This, from someone who solved a famous combinatorial math problem.

    He mentioned something very interesting:

    “If you have not made a mistake, you have not been a great scientist!”

    He pointed out “Even David Hilbert was wrong!”. The moral of the story is “You have to try a lot of ideas” (shotgun approach, as per L. Pauling). This mirrors D. Gross statement “You have to try” (the W. Churchill quote).

    I think what P. Woit & L. Smolin are saying is that ST (over 20+ yrs) isn’t getting it done (“crash”), & it’s time to admit failure. Move on.

    “An honest man can make a mistake, but only fool persists in doing so”

    “No matter how far you have gone down the wrong path, turn back.”
    — Old Turkish proverb

  3. amused says:

    An interesting discussion, and this part seems potentially explosive!:

    GREENE: […] But just so I understand; you’re saying that this one particular way in which one may think about string theory—for which the endpoint is many many universes—is unacceptable.

    STEINHARDT: Right. I claim it needs to be fixed.

    GREENE: But you also agree—just so it’s clear—that’s not a crash in string theory per se; that’s a particular way of approaching the theory that you would not advocate because the endpoint would be unacceptable. You need to go further…

    STEINHARDT: That’s right; so it’s just what you were saying; some people say that is the endpoint, and I’m saying that’s not acceptable. If you believe that, it’s time to abandon it.

    GREENE: But it’s those people who’ve crashed.

    Wow, take that Susskind & Co!

  4. tytung says:

    Regarding the points raised by Steinhardt…I think it need to be made more clearly. An ultimate theory may allow certain physical facts as something that is selected by chance, and other universe, if allowed to exist, may take other possibilities. The candidates are the values of physical constants. I can’t even imagine that these values can be determined in some ultimate theory, free of any constants.

  5. mclaren says:

    Since the Uncertainty Principle made Einstein gag, this criterion should not be used as a measure of the measure of a physical theory.

  6. chris says:

    tytung,

    i think this is not the point. of course a theory with no constants to fix would be the ideal one. but take e.g. QCD. how many free parameters does it have? one plus the quark masses? how about the 3 in the SU(3) gauge group? why not 4? is that a free parameter?

    i think the trouble with the landscape is quite different. imagine, that QCD would not directly lead to the hadron spectrum or confinement (let’s assume for now, that they are sufficiently well established). let’s assume that we only have an effective theory of hadrons and we conjecture a fundamental theory behind it that first needs to break the ground state symmetry down to a subgroup in a very specific way. and let’s imagine that there are zillions of other plausible patterns that we are not able to exclude because of either internal consistency or experiment.

    now, even if the fundamental theory would have only one free parameter (or zero, if you want), the unsatisfactory feature is, that it leaves you with *no prediction*. only if you pile additional external assumptions upon the theory (how the symmetry is broken) can you arrive at a descriptive theory.

    the point with the landscape is exactly this. your fundamental theory may be nice (assuming it can ever be formulated correctly), but unless you pile ‘anthropic reasoning’ on top of it (=invoke god at this point) it does not predict anything. it just does not.

    it totally floors me to see how string theory can claim to have improved the state of affairs in quantum gravity. the original problem was lack of predictiveness. at this level, quantum gravity (the QFT with the einstein-hilbert action) is just doing fine! well, it is nonrenormalizable perturbatively (on first inspection), but what the heck. if you fix an infinite number of parameters, it is just fine. and even you can in principle fix them all with observations.
    even more so, there is strong evidence piling up that the QFT of gravity is nonperturbatively renormalizable. so it seems, that the original problem of nonpredictability will eventually turn out to be an intermediate technical difficulty.

    now compare that to the currently claimed nonpredictability of the antropic string landscape. the only reason i can imagine that this is not dead as a valid candidate for a physical theory is that too many people have spent too many years hunting the phantoms and won’t admit they went the wrong way. it is just absurd.

  7. milkshake says:

    Chance selection in the Ultimate theory: How many chance-selected parameters would you be willing to accept?

    “A multiverse encompassing every alternative compatible with our theory is possible according to the theory – so it could have been a combinatorial process that picked the winning numbers in our corner of the multiverse”

    This is just productive as saying: “Our creator is a mysterious dude who laid out the universe according to his subtle wisdom”.

    In both cases you can fit all the known facts in, with the help of scriptures and some scholastic reasoning.

  8. Thomas Larsson says:

    Mr Academic Failure Motl now decrees that the Albert Einstein Professor in Science at Princeton University is an incredibly stupid crackpot. It is really funny when AFM says that one of the leading theorists responsible for inflationary theory say “many breathtakingly dumb and bitter things about inflation” 🙂

  9. Thomas Love says:

    Peter said: “The kind of thing that would make Einstein gag has moved from popular science articles to regular appearance in the lectures and scientific articles of leading figures in particle physics. This would probably not just make him gag, but send him into a serious fit of depression. ”

    From what I’ve read about Einstein, he would neither gag nor become depressed, he’d be laughing his head off. As am I.

  10. King Ray says:

    To quote Einstein from his book The Meaning of Relativity:

    “More complex field theories have frequently been proposed. They may be classified according to the following characteristic features:

    (aa) Increase of the number of dimensions of the continuum. In this case one must explain why the continuum is apparently restricted to four dimensions.

    (bb) Introduction of fields of a different kind (e.g. a vector field) in addition to the displacement field and its correlated tensor field g_ik (or scriptg^ik).

    (cc) Introduction of field equations of higher order (of differentiation).

    In my view, such more complicated systems and their combinations should be considered only if there exist physical-empirical reasons to do so.”

    From this one might deduce that Einstein would not favor additional dimensions unless there was empirical evidence for them, which there is not. Also there does not seem to be a good explanation for why the continuum appears to be 4 dimensional insteand of some other number.

    Once you give yourself too much mathematical freedom, there are too many possible theories and you end up with a mess like the landscape.

    (I had to double the letters because it was turning the c in parentheses into a copyright symbol)

  11. King Ray says:

    I should also add that almost all of Einstein’s 30+ year search for a Unified Field Theory was confined to four dimensions. He rejected the Kaluza-Klein idea using higher dimensions.

  12. Thomas Love says:

    But the use of higher dimensions goes back even further.
    Robert S. Cohen wrote a fascinating introduction to the 1956 Dover reprint of Heinrich Hertz’ The Principles of Mechanics. :
    In the writings of Descartes, there is sketched just such an
    efficiently running world-machine, devised without forces or energies, built of rigidly connected space-time atomic entities. Although he was an advocate of the mathematization
    of physical reality, Descartes was driven to admit the inadequacy of ordinary space-time geometry to explain inertia and gravitation. These two properties of bodies, perhaps to be supplemented by other non-geometric properties in later investigations, seemed to require more than the pure mathematics of Euclidean geometry, but Descartes asserted that they should be analyzed by a higher geometry of many dimensions. “By dimension,
    I understand nothing else than the mode and aspect in respect
    of which a subject is considered to be measurable. Thus, it is not only length, breadth and depth which are dimensions; gravity is also a dimension, speed is a dimension of motion…
    and so on with innumerable other dimensions of this sort”
    (Rules for the Guidance of our Mental Powers, Rule xiv, tr. N. K. Smith).

    Thus, according to Descartes, we need a dimension for each physical quantiy we can measure!

    There iare certainly “physical-empirical reasons to do so”

  13. sophia says:

    Peter,

    Forgive me for even contributing here. I’m not a scientist, just an interested observer. But I do have a question that may be relevant.

    I followed the link and began to read. After a while I began laughing and then wincing and then, yes, gagging. Why? Well, stand back and survey the scenery: three intelligent men, two of them credentialed scientists, wondering what a dead man would think, because they have reached (sorry) a dead end. That’s not so pathetic it’s funny? To me it is.

    Don’t get me wrong: I revere Einstein as much as the next good American does, but dudes, he’s dead. Wondering what he would think is ludicrous. What is this, science or a seance?

    I gave up here:

    “I think most of us in the field absolutely will never have faith that this approach is right until we do make contact with data, but it would be great to have the insight of the master as to whether he feels that this smells right.”

    What does it matter what Einstein would think? What matters is what Brian Greene and Paul Steinhardt think!! What are THEIR insights?? And what is this business about “contact with the data”, as if “the data” are alien beings that we conjecture about but have never met??

    OK, that’s three questions.

  14. JPL says:

    Hi Sophia,

    Sorry to barge in but Einstein is quite alive, at least, a lot more so than the String Theory landscape! I am sure that this sounds silly to you, 3 grown (but living) men speculating about what a dead one would think, but in Physics this is hardly what it seems. Einstein is one of the few figures whose own sense of proportion and aesthetics has impacted the discipline enough that one can justifiably refer to it some 60 years after he passed and thus the argument makes sense and can be consistently addressed and narrowed as you see above.
    I would dare add that what Einstein would think matters much more than what Steinhardt, Greene or Isaacson think, otherwise they would not think to add that bit to their own opinions, now would they?

  15. Peter Woit says:

    Sophia,

    I agree that the “What would Einstein think?” question is kind of silly. He is rightly revered for his contributions to physics, but the greatest of these were from a very different time a century ago, when physics was facing very different challenges. This kind of question functions like “Would Jesus drive an SUV?” questions, as a rhetorical device for people to invoke what they see as high principles by embodying them in a prestigious figure.

    Of course, I’m personally convinced that Einstein would strongly dislike string theory, and I’ll construct a long, detailed argument about why this is true. Well, maybe not today…..

  16. Glenn says:

    In my more depressed moments, when pondering this topic, I wonder, “What if they’re right?”

    That is, what if the fundamental nature of the universe really is the unprovable ‘landscape’ or some other critter we can’t disprove?

    If that’s true, we’re doomed to an endless search for a ‘truth’ that lies outside the area we’re willing and able to search, like the drunk in the old joke who lost his keys in a dark alley, but is looking for them next to the street because the light’s better over here.

    I can only hope that “Science” isn’t about to hit then end of a centuries-long winning streak…

  17. King Ray says:

    Cheer up, we are just in a long period between paradigms, if you believe Kuhn’s The Structure of Scientific Revolutions.

    They say that a camel is a horse designed by a committee. I think string theory is physics designed by a committee. The efforts of a large group can never match the aesthetics of a single extraordinary genius like Einstein.

    It will take a new idea, probably by a single person, to make the breakthrough needed to have a successful theory of quantum gravity and a unified field theory.

  18. Changcho says:

    Paul Steinhardt said “…if I had walked in the door with a theory not called string theory and said that it is consistent with the observed laws of nature, but, by the way, it also gives a googol other possibilities, I doubt that I would have been able to say another sentence. I wouldn’t have been taken seriously…”

    Steinhardt is, of course, right. A ‘theory’ that gives you “a googol other possibilities” isn’t scientific, simply because a good science (at least good Physics) puts limits on what is and isn’t possible.

  19. anonymous says:

    “This would probably not just make him gag, but send him into a serious fit of depression.”

    But that’s not saying much; it was also true of Einstein and QM.

  20. anonymous said:

    “But that’s not saying much; it was also true of Einstein and QM.”

    As far as I know, Einstein never questioned the correctness and usefulness of quantum mechanics. He just thought that it is not the ultimate description of nature and that there should be a more fundamental underlying theory. That’s why he was reluctant to work on QM.

    String landscape is different: it does not explain or predict anything. So, it is questionable whether it belongs to science.

    Eugene.

  21. Belizean says:

    And we should not forget that virtually every competent theoretical physicist alive today understands general relativity better than Einstein ever did.

  22. King Ray says:

    Belizean said:

    “And we should not forget that virtually every competent theoretical physicist alive today understands general relativity better than Einstein ever did.”

    That’s an arrogant statement. I know of at least one very prominent string theorist who couldn’t even write down the formula for the Christoffel symbols in terms of the metric tensor when he was teaching a class on GR.

    I don’t think anyone today understands GR as well as Einstein did; he invented it, after all. It takes a lot more intelligence and insight to make a discovery than to understand it after the fact. Standing on the shoulders of giants doesn’t make you a giant.

  23. Thomas Love says:

    Thanks for the link to the Edge Symposium. For me, the most interesting statement was :

    ISAACSON: There’s a wonderful book that Einstein wrote called The Evolution of Physics with Leopold Infeld in 1938, which is not easy to find. I’ve gone over it again two or three times because I just love the way it was written. It was written to make money for both of them, because it’s the 30s, and Hitler, and refugees and stuff. It’s a popular book, but it has a deep philosophical argument, and the publisher is reissuing the book because I was pushing them to get it out there.

    The deep philosophical argument is that it will be a field theory approach that will work. It starts with Galileo; he talks about matter and particles, and just makes the argument that in the end it is all going to be reconciled through field theory. It’s about whether there is going to be a great distinction between a field theory and a theory of matter.

  24. Anders R says:

    i wonder if einstein would have gone into theoretical physics at all if he was a 16 year old student who was about to make a choice for college.

  25. Anders R says:

    in the year 2007 that is

  26. Mr. Mustela says:

    The fundamental issue seems to be that no progress has been made in the last 20-30 years on those fundamental questions. The anthropic principle succeeds as a theory by explaining that failure, if not much else. As such, it is probably a step in the right direction.

    The Woit-Smolin alternative rejects that theory, and instead blames part of the academic system for the failure. Personally, I believe that if there is no promising idea today (as Peter said a few posts ago) then it is unlikely to be the failure of the system. The system is bad, but not that bad. A good idea wouldn’t be ignored for that long. Stolen? yes, no problem. But not ignored.

    To me, a better approach would be to think of better explanations as to why we are making so little progress. If possible, ones with a backdoor to be able to validate them, of course.

    In this light, it seems to me that the true value of the anthropic principle has been overlooked. Sure, it is awful from the point of view of getting an explanation of concrete phenomena. But as an attempt to explain the limits of our current methods, it works better than not.

  27. Sophia,

    you should read “Conversations with Einstein’s brain” in “The mind’s I” by Daniel Dennett… Appropriate for the current conversation!

    Cheers,
    T.

  28. Coin says:

    The anthropic principle succeeds as a theory by explaining that failure, if not much else. As such, it is probably a step in the right direction.

    Seems to me like more of an excuse than an explanation, frankly…

    But as an attempt to explain the limits of our current methods, it works better than not.

    Well, for what reason must we conclude that our current methods have limits at all?

  29. tytung says:

    Chris,

    I was trying to say that one should distinguish those features of our world are to be explained in an ultimate theory from those that should not (because they can not).
    Regarding predictability, I think if it postulates some entity like string, then in principle the theory allows these to have observable effects, albeit one that is technically unavailable.

  30. chris says:

    “The fundamental issue seems to be that no progress has been made in the last 20-30 years on those fundamental questions”

    this is definitely false. see e.g. http://relativity.livingreviews.org/Articles/lrr-2006-5/
    or also see how far LQG has come. i think we are on the verge of proving the renormalizability of quantized GR. and it might not take a genius but hundred people working out the tedious details in a decade long struggle. better to abandon romantic views of heureka moments that come out of nowhere.

  31. Thomas Larsson says:

    One important thing that Steinhardt points out is that the Landscape is really at odds with observation. If the laws of physics fluctuate throughout the multiverse, there might be some fluctuations already in our observable universe. However, no such fluctuations are observed…

    Of course, this is not a hard argument. The fluctuations may take place on length-scales that are much larger than the size of the universe. Nonetheless, this fits well into the standard pattern: whenever some experimental signature is suggested by string theory (susy, extradimensions, laws-of-physics fluctuations, …), you can be sure that it is not observed. Maybe the Lord is trying to tell us something by that.

    OTOH, if string theory does not predict a Landscape, it is difficult to see how it can accomodate a nonzero cc 30 orders of magnitude below the Planck scale. So in that case string theory is probably ruled out by observation anyway.

  32. dan says:

    Dear Peter Woit,

    How do you feel about Briane Green’s contribution, for example,

    “…..When you look at the framework within which the standard model of particle physics sits, namely relativistic quantum field theory, you do find that there are a google, if not more, possible universes that that framework is capable of describing. The masses of the particles can be changed arbitrarily and the theory still makes sense, it’s internally consistent; you can change the strengths of the forces, the strengths of the coupling constants…..”

    “…..So how is that any worse than string theory?….”

    http://www.edge.org/3rd_culture/einstein07/einstein07_index.html

  33. sophia says:

    Peter,

    Sometimes I feel that no matter what I say on a comment, it’s misleading.

    I wasn’t 100% clear, so let me rephrase.

    I have no objections to anyone, in any discipline, getting iinspiration in a general way from the great practitioners of their field. So, in that limited sense, “what would Einstein think?” is no big deal. If that helps grease your mental wheels, go for it.

    But I do not think that’s what’s going on here. What is going here is bullshit, desperation and non-thought. It’s actively anti-science- not even non-science.

    The invocation of the numinous name of Einstein is the dead giveaway. I’ve noticed that with physicists there’s a tremendous amount of prophet-wannabeism, and there are two favorites whose names always seem to pop up when all else fails: Einstein and Feynman. Somehow, Dirac doesn’t do the trick….

    I don’t want to get off a nitpicky thing about Jesus, but since you mentioned him, I’ll respond to it, because it underscores further my problems with “what does the master think”-ism.

    Invoking Einstein’s name when you are out of ideas is much worse than asking whether Jesus would drive an SUV or vote Democratic, because no one knows exactly what Jesus said, there are canonical and non-canonical sources, and so on.

    But everyone knows what Einstein said. He left a recorded, written body of work that is unambiguous and free for all who care to, to access.

    And that’s the reason the business about making “contact with data” and having “the insight of the master as to whether he feels that this smells right” drove me so crazy.

    If one of your students said that to you, what would you say to him or her?

    If someone came to me and said that, I’d say, “Forget about channeling Einstein. Think your own thoughts. Come to your own conclusions. Make your own damn contact with the damn data. No contact? No data? Think some more.”

    Am I making any sense to you?

  34. Peter Woit says:

    sophia,

    I pretty much agree with you. I don’t think the “what would Einstein think?” question is an interesting or useful one.

    In my experience, it’s not actually a question physicists spend much time thinking about. It’s the sort of thing that comes up when someone is looking for a hook that will get the public’s interest. Basically Einstein gets dragged into any discussion of any kind about physics, no matter how inappropriate, because people figure that makes the discussion something the public can relate to. It’s not a good thing, but relatively harmless compared to lots of other things…

  35. Arun says:

    Peter,

    I think Einstein gets dragged into the whole thing because he is supposedly the exemplar of figuring out something about the world purely by power of thought. The implicit assumption in today’s research programs is that it is possible to perform similar feats and bootstrap ourselves from Standard Model QFT + GR to a way more comprehensive theory. Whether a given line of inquiry is fruitful is now a matter of taste, and Einstein becomes the arbiter of taste.

  36. Peter Woit says:

    dan,

    I really should write up a FAQ and include this issue of “QFT and string theory are on the same footing, lots of both..” Sure, by choosing arbitrary gauge groups, particle representations, etc, you can get an infinite number of QFTs. But the reason QFT makes predictions is that one of the simplest possible such choices works just about perfectly. If simple choices of QFT disagreed with experiment, and you had to go to extremely complicated sets of choices in order to avoid contradicting experiment, never actually getting to anything that you could test, then QFT would be in the same situation as string theory. It’s not as simple as just noting the “number of theories”.

  37. Anders R says:

    regarding what you’ve said sophia, i definately agree about the prophet wanabeeism, especially within theoretical physics. i think there seem to be large amounts of name dropping going on, especially among string theorists.

  38. Eric says:

    Simple choices of QFT do disagree with experiment. For example, wouldn’t it be simpler for the weak interactions to be described by SU(2) rather than SU(2)xU(1) as originally proposed by Glashow? However, this is not how nature is working. And isn’t this whole symmetry breaking business horribly complicated? Wouldn’t nature prefrer unboken symmetry since it is simpler? Or shouldn’t physics be described by SU(5) instead of SU(3) x SU(2) x U(1)? And why are there three generations when one would be much simpler, and for that matter why doesn’t the Standard Model predict the masses of each generation? QFT only makes predictions because there is experimental data available at the low energies at which it is valid. The problem with string theory is that it is a theory which is valid at very high energies, at which experimental data is sorely lacking.

  39. Arun says:

    The problem with string theory is that it is a theory which is valid at very high energies, at which experimental data is sorely lacking.

    What high energy experimental data are you looking for? Assume I can accelerate protons and anti-protons to Planck scale energies and collide them. What are the predictions?

  40. Peter Woit says:

    Eric,

    Last I heard, string theory was supposed to be valid at all energies…

    Very convenient to have your scientific research be on a theory which is not valid at any energy scale we’ll ever be able to reach.

    I don’t claim that the Standard model is the simplest possible choice among gauge theories, I do claim that it is one of a relatively small number of the simplest ones, so one can compare to experiment by looking at a small number of possibilities (not 10^500), which is exactly what people did in discovering the standard model during the 1960s and early 70s.

  41. ScienceLover says:

    [calling Einstein] is not a good thing, but relatively harmless compared to lots of other things…

    That’s why it is funny as if it were some sort of a sketch: 3 men are discussing theoretical physics. Then they get stuck. Silence. They look at each other and after a while one of them breaks the silence: “What would Einstein say?” The discussion recovers and although it was at a dead end one of them claims that Einstein would be certainly impressed by the great theory he and his colleagues worked out.

  42. Eric says:

    Peter,
    If you read my comment very, very closely you’ll see that I state that string theory is valid at high-energies as well as low energies where it should reduce down to conventional QFT. What string theory does is unify QFT with a perturbative formulation of quantum gravity. As such it is much more ambitious than QFT. However, the situation today is no different than if you had a formulation of QFT, but didn’t know anything about the weak or strong interactions. There is no possible way to deduce the standard model from QFT alone.

  43. Peter Woit says:

    Eric,

    No, you can’t deduce the standard model from pure thought about QFT alone. You have to learn how to calculate things in tractable QFTs, compare to results of experiment, identify the right QFT, then make predictions and test them.

    In string theory what you are doing is doing calculations in tractable string theories, comparing them to experimental results, seeing they don’t agree, trying for 25 years to find some way around this and failing, then, instead of giving up and trying something else, telling everyone that the problem is just that it’s “too early to tell”, you need to do more calculations (10^500), and then maybe it will work. And trying to also tell people that the problem is those damn low energies, that surely at unmeasurably high energies your calculations would agree with experiment.

  44. Eric says:

    Peter,
    You are presupposing that experimental data exist to guide you to identify the right QFT. The problem for the last 25 years has been a lack of any experimental data at energies a magnitude above the EW scale. Once this data is available, hopefully from LHC/ILC, this situation will change drastically. I sometimes wonder if this isn’t why you seem so excited anytime there is a rumour of a delay in starting up LHC. Such a delay gives you that much more time before things come crashing down.

  45. Peter Woit says:

    Eric,

    There’s plenty of experimental data out there to guide people trying to do unification, the problem is string theory can’t reproduce it.

    Funny how you think the electroweak scale is the problem, that string theory has something to say just above the electroweak scale, but not below it. That’s nonsense.

    I in no way think a delay in the LHC is a good thing. Quite the opposite. Any delay just delays the date at which the standard nonsense about finding evidence of string backgrounds in LHC data comes crashing down.

  46. Eric says:

    Peter,
    Yes, string theory can describe the current low-energy data below the weak scale and there is at least one string model which does so. The question of whether or not such models may uniquely predict the parameters of the standard model is presently unanswered. For this, the problem of moduli stabilization must be completely addressed and this is where a lot of the current work is focused. In this respect, string theory is currently no better than QFT.

  47. Anders R says:

    “You are presupposing that experimental data exist to guide you to identify the right QFT. The problem for the last 25 years has been a lack of any experimental data at energies a magnitude above the EW scale”

    maybe the experimental data is there, but the theoretical phycisists who are working on it right now just aren’t competent enough to work out what it is.

  48. Anders R says:

    sorry peter i didn’t see that you wrote almost the same thing

  49. David B. says:

    Dear Peter and Anders:

    Saying there is plenty of data out there to guide people trying to do unifcation is not accurate. The current situation is like trying to find New York City by the following algorithm:

    it’s not London, it’s not Paris, It’s not Geneva,…

    Any model of high energy physics will impact the low energy physics by the addition of non-renormalizable couplings. This is at best an indirect measurement. Most of these are beyond current experimental bounds (meaning experiments are consistent withthe standard model), and this fact only indicates that either the unification scale is too high to see, or the correct theory has a huge conspiracy to cancel all possible signatures in the low energy physics.

    As it stands, this does not give you any information on what the theory is, but it definitely gives you a lot of information on what it is not.

  50. Amos Elberg says:

    Einstein got “dragged” into the thing because the thing is a symposium about Einstein. An Einstein tribute. I think its entirely appropriate during such a symposium to ask other physicsts what Einistein would have thought. The ensuing discussion is pedagogically useful.

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