Guest Post: Joe Polchinski on Science or Sociology?

Science or Sociology?
Joseph Polchinski, 5/20/07

This is a continuation of the on-line discussion between Lee Smolin and myself, which began with my review of his book and has now continued with his response. A copy of this exchange (without the associated comment threads) is here.

Dear Lee,

Thank you for your recent response to my review. It will certainly be helpful in clarifying the issues. Let me start with your wish that I do more to address the broader issues in your book. When I accepted the offer to review these two books, I made two resolutions. The first was to stick to the physics, because this is our ultimate goal, and because it is an area where I can contribute expertise. Also, keeping my first resolution would help me to keep the second, which was to stay positive. I am happy that my review has been well-received. Your response raises some issues of physics, and these are the most interesting things to discuss, but I will also address some of the broader issues you raise, including the process of physics, ethics, and the question in the title. Let me emphasize that I have no desire to criticize you personally, but in order to present my point of view I must take serious issue both with your facts and with the way that they are presented.

Regarding your points:

The fictitious prediction of a non-positive cosmological constant. This is a key point in your book, and the explanation that you now give makes no logical sense. In your book you say (A) “… it [a non-positive cosmological constant] was widely understood to be a consequence of string theory.” You now justify this by the argument that a non-positive cosmological constant is a consequence of unbroken supersymmetry (true), so A would follow from (B) Unbroken supersymmetry was widely understood to be a consequence of string theory. But even if this were true, it would not support your story about the observation of the dark energy leading to a “genuine crisis, … a clear disagreement between observation and a prediction of string theory.” There would already have been a crisis, since supersymmetry must obviously be broken in nature; seeing the dark energy would not add to this. But in fact the true situation, as you can find in my book or in many review articles, was closer to the opposite of B than to B: (B’) Supersymmetry is broken in almost all Calabi-Yau vacua of heterotic string theory. We have no controlled examples because at least one modulus rolls off, usually to a regime where we cannot calculate. The solution to this problem may have to wait until we have a non-perturbative formulation of gravity, or even a solution to the cosmological constant problem.

In your response you largely raise issues surrounding B’, including the Witten quote, but I want to return to what you have actually written in your book. It is a compelling story, which leads into your discussion of “a group of experts doing what they can to save a cherished theory in the face of data that seem to contradict it.” It surely made a big impression on every reader; it was mentioned in several blogs, and in Peter Shor’s Amazon review. And it never happened. It is an example of something that that happens all too often in your book: you have a story that you believe, or want to believe, and you ignore the facts.

You go on to challenge the ethics of string theorists in regard to how they presented the issue of moduli stabilization in their talks and papers. I am quite sure that in every colloquium that I gave I said something that could be summarized as “We do not understand the vacuum in string theory. The cosmological constant problem is telling us that there is something that we do not understand about our own vacuum. And, we do not know the underlying principle of string theory. These various problems may be related.” The cosmological constant and the nature of string theory seemed much more critical than the moduli stabilization problem, and these are certainly what I and most other string theorists emphasized.

This scientific judgment has largely been borne out in time. In 1995-98 these incredible new nonperturbative tools were developed, and over the next few years many string theorists worked on the problem of applying them to less and less supersymmetric situations, culminating in the construction of stabilized vacua. Obviously many questions remain, and these are widely and openly debated. It seems like a successful scientific process: people knew what the important problems were, worked in various directions (a fair number did work on moduli stabilization over the years), and when the right tools became available the problem was solved. As you point out, the stabilization problem is nearly one hundred years old, and now string theorists (primarily the younger generation, I might add) have solved it. You are portraying a crisis where there is actually a major success, and you are creating an ethical issue where there is none.

AdS/CFT duality. You raise the issue of the existence of the gauge theory. There are two points here. First, Wilson’s construction of quantum field theory has been used successfully for 40 years. It is used in a controlled way by condensed matter physicists, lattice gauge theorists, constructive quantum field theorists, and many others. To argue that a technique that is so well understood does not apply to the case at hand, the scientific ethic requires that you do more than just say Not proven! Sociology! as you have done. You need to give an argument, ideally pointing to a calculation that one could do, or at least discuss, in which one would get the wrong answer.

I have given a specific argument why we are well within the domain of applicability of Wilson: there are 1+1 and 2+1 dimensional versions of AdS/CFT, which are also constructions of quantum gravity, and for which the gauge theory is super-renormalizable (and there are no chiral fermions): the counterterms needed to reach the supersymmetric continuum limit can be calculated in closed form – thus there is an algorithmic definition of the gauge theory side of the duality. You could perhaps argue that there will be a breaking of supersymmetry that will survive in the continuum limit, and we could sit down and do the calculation. But I know what this answer is, because I have done this kind of calculation many times (it is basically just dimensional analysis). Similar calculations, for rotational invariance and chiral symmetry, are routine in lattice gauge theory.

As a further ethical point, in your book you state that it is astounding that Gary Horowitz and I ignore the question of the existence of the gauge theory, and you then use this to make a point about groupthink (this is in your chapter on sociology). While you were writing your book, you and I discussed the above points in detail, so you knew that we had not ignored the issue but had thought about it deeply. You do not even acknowledge the existence of a scientific counterargument to your statement, and in saying that Gary and I ignore the issue you are omitting facts that are known to you in order to turn an issue of science into one of sociology. Again you impose your own beliefs on the facts; thus I am reluctant to accept as accurate the various statements that you attribute elsewhere to anonymous string theorists and others.

You raise again the issue of a weak form of Maldacena duality. As you know, it is very difficult to find a sensible weak form that is consistent with all the evidence and yet not the strong form. In my review I have gone through your book and papers and identified three proposals, and explained why each is wrong. Again, you have not acknowledged the existence of scientific counterarguments, but have just reasserted your original point. If your arguments had been made in a serious way, I would expect that you would have given some deep thought to them and be ready to defend them.

There are some interesting points, one of which I will turn to next.

The role of rigor and calculation. Here we disagree. Let me give some arguments in support of my point of view. A nice example is provided by your paper `The Maldacena conjecture and Rehren duality’ with Arnsdorf, hep-th/0106073.

You argue that strong forms of the Maldacena duality are ruled out because Rehren duality implies that the bulk causal structure is always the fixed causal structure of AdS_5, and so there cannot be gravitational bending of light. But this would in turn imply that there cannot be refraction in the CFT, because the causal structure in the bulk projects to the boundary: null geodesics that travel from boundary to boundary, through the AdS_5 bulk, connect points that lie on null boundary geodesics. Now, the gauge theory certainly does have refraction: there are interactions, so in any state of finite density the speed of propagation will be less than 1. (Since Rehren duality does not refer to the value of the coupling, this argument would hold even at weak coupling, where the refraction can be calculated explicitly.)

You have emphasized that Rehren duality is rigorous, so apparently the problem is that you have assumed that it implies more than it does. Generally, rigorous results have very specific assumptions and very precise consequences. In physics, which is a process of discovery, this can make them worse than useless, since one tends to assume that their assumptions, and their implications, are broader than they actually are. Further, as this example shows, a chain of reasoning is only as strong as its weakest step. Rigor generally makes the strongest steps stronger still – to prove something it is necessary to understand the physics very well first – and so it is often not the critical point where the most effort should be applied. Your paper illustrates another problem with rigor: it is hard to get it right. If one makes one error the whole thing breaks, whereas a good physical argument is more robust. Thus, your paper gives the appearance of rigor, yet reaches a conclusion that is physically nonsensical.

This question of calculation deserves further discussion, and your paper with Arnsdorf makes for an interesting case study, in comparison with mine with Susskind and Toumbas, hep-th/9903228. (I apologize for picking so much on this one paper, but it really does address many of the points at issue, and it is central to the discussion of AdS/CFT in your various reviews.) You argue that there are two difficulties with AdS/CFT: that strong forms of it are inconsistent with the bending of light by gravitational fields, and that the evidence supports a weaker relation that you call conformal induction. We also present two apparent paradoxes: that the duality seems to require acausal behavior, and negative energy densities, in the CFT. The papers differ in that yours contains a handful of very short equations, while ours contains several detailed calculations. What we do is to translate our argument from the imprecise language of words to the precise language of equations.

We then find that the amount of negative energy that must be `borrowed’ is exactly consistent with earlier bounds of Ford and Roman, gr-qc/9901074, and that a simple quantum mechanical model shows that an apparent acausality in the classical variables is in fact fully causal when one looks at the full quantum state. Along the way we learn something interesting about how AdS/CFT works.

This process of translation of an idea from words to calculation will be familiar to any theoretical physicist. It is often the hardest part of a problem, and the point where the greatest creativity enters. Many word-ideas die quickly at this point, or are transmuted or sharpened. Had you applied it to your word-ideas, you would probably have quickly recognized their falsehood. Further, over-reliance on the imprecise language of words is surely correlated with the tendency to confuse scientific arguments with sociological ones.

Finally, I have recently attended a number of talks by leading workers in LQG, at a KITP workshop and the April APS meeting. I am quite certain that the standard of rigor was not higher than in string theory or other areas of physics. In fact, there were quite a number of uncontrolled approximations. This is not necessarily bad – I will also use such approximations when this is all that is available – but it is not rigor. So your insistence on rigor does not actually describe how science is done even in your own field.


Background independence. I think people are a bit tired of the who-is-more-background-independent argument, since it seems to come down to definitions. Let me put things in physical terms. As you say, suppose that the strong form of Maldacena duality is true. This would mean that we can consider a box as large as we want – a light-year, 106 light-years, with an arbitrarily small negative cosmological constant, and AdS/CFT provides a complete construction of quantum gravity within that space. This would include: the formation and decay of (nonsupersymmetric) black holes; graviton scattering at hyper-Planckian energies; physically continuous transitions from one topology, through a quantum state with no geometric interpretation, to a different topology; states where a submanifold of spacetime has a noncommutative geometry; states with a variety of apparent geometric singularities, where the physics is nonsingular. All of these, and many others with a variety of geometries and topologies (you can put a lot in an AdS box), and arbitrary quantum superpositions of them, can be identified in the gauge theory, and so are described algorithmically by the duality. It may not include spaces with interesting cosmologies, or with an effective positive cosmological constant. You call this a very weak and limited form of background independence.

Even here you are blowing things out of proportion: your reply refers five times to the “global symmetry algebra,” but almost immediately after the original work of Maldacena, the duality was extended to systems with reduced symmetry, or none. Your own PI colleagues, Alex Buchel and Rob Myers, have made important contributions to this subject, and I note also the series of papers by Hertog and Horowitz on strongly time-dependent boundary conditions.

A second physics point concerns the constraints. It is not that I am ignorant of the conventional wisdom here, I am challenging it. You believe that the large Hilbert space in which the constraints act is necessary in order to describe all possible backgrounds of quantum gravity. No, only the much smaller set of states that satisfy the constraints is needed. The larger space may play a useful auxiliary role, but it is not physical: the universe cannot be in such a state, and observables must keep the system within the physical subspace. So what are these larger spaces for? One thing we have learned, from emergent gauge theory, is that they are not necessary: one can start from a system with no constraints, only physical variables, and the constraints are needed only to describe the classical limit efficiently. We have learned a similar lesson from dualities such as AdS/CFT: these larger spaces are very different in different classical limits, they are not intrinsic to the quantum theory. Thus, all this focus on constraints is putting effort into something that is unphysical and actually intrinsic to a certain classical limit.

Cosmology. I have agreed that we may be far from sharp prediction. However, string theory has played a valuable role in suggesting new ideas. Moreover, the variety of kinds of models being explored phenomenologically is large; it is clear that some of these arise easily in the landscape (e.g. a pure cosmological constant), while others may be rigorously excluded (the constraints of Arkani-Hamed et al, and others).

Regarding the atomic analogy, the long period that I was referring to was the hundred years between the first scientific argument for atoms (Dalton) and the confirmation (Brownian motion). I agree, however, that one should not get too caught up in analogies. No analogy is perfect: in the 19th century there was a wealth of unexplained phenomena from the natural world, while our current era is historically exceptional in that phenomena beyond the Standard Model are so few.

RHIC. You say that quantum gravity is not being used here. But the QCD plasma entropy is being related to the Bekenstein-Hawking entropy, which depends on hbar, and the ideal viscosity (a concept discovered through AdS/CFT, which is now a standard idea in heavy ion physics) is quantum mechanical.

Moreover, I am puzzled by your repeated statement that the evidence supports AdS/CFT describing only classical supergravity. The gauge theory is fully quantum mechanical, so if it contains classical gravity why is this not exactly what we are all looking for: a theory that unites Einstein’s theory and quantum mechanics? This should be of great interest to anyone working on quantum gravity – how does the gauge theory manage to do this? So we look closer, and we find that it’s… string theory! It is clear why you have trouble with this: according to your book, gravity is a theory of principle, which must be understood by seers, while gauge theory is a constructive theory, which can be built by craftsmen. AdS/CFT would then imply that craftsmen are dual to seers.

But seriously, duality does erase distinctions that we make with our classical experiences and vocabularies, because one quantum theory has many classical limits. Thus, quantum mechanics first erased the distinction between particles and waves. QFT dualities erased the distinction between quanta and solitons, which once seemed absolute. Maldacena duality erases at least much of the distinction between gauge theory and gravity. Unexpected perhaps, but those who ignore this lesson are likely to end up as backward-seers rather than forward-seers.

Other physics. Sean has suggested that I comment on the understanding of string theory in time-dependent backgrounds. Here I will give my own way of thinking about this, which is rather particle-physicsy; other string theorists might emphasize different things. If you have the flat spacetime S-matrix, you actually know a lot about curved spacetime, since you can form a very complicated geometry by throwing together a lot of gravitons in a coherent state. From a particle physics perspective, where the goal is to measure the underlying Lagrangian, this is enough: the S-matrix encodes all local physics in curved spacetime. Further, with this effective Lagrangian one can study processes in a fully curved spacetime, as long as the curvature stays below the string scale. One can then list things that are not covered by this: first, cosmological questions like initial conditions and spacetime singularities, and these are indeed open questions and the subject of active research; second, the possibility of an intrinsic non-locality in physics, so that local measurements do not capture everything. The second possibility has been widely discussed: the black hole information paradox gives a strong indication that such nonlocality exists; the black hole complementarity principle, and the holographic principle, are general statements of the nature of the nonlocality; and, the BFSS matrix theory and AdS/CFT duality are very concrete realizations of locality emerging from a nonlocal starting point. Certainly deep questions about the nature of time remain, and I expect that the solutions will build on our current understanding of the holographic principle.

On the UV finiteness of string perturbation theory, the one-line physics proof is that the regions of world-sheet moduli space that would correspond to UV divergences in field theory actually turn out to describe IR physics. The decomposition of moduli space that Zwiebach uses to formulate closed-string field theory is probably the best for seeing this. The IR divergences are described by low energy effective field theory, so the finiteness problem is reduced to the already-solved problem of IR divergences in quantum field theory. This may seem awfully simple, but I have done enough calculational checks of different parts of it to take it seriously.

Ethics and sociology. Coming back to ethics, the principal scientific ethic is that scientists take responsibility for what they say: When a statement is made, to what extent has it been thought through, and appropriate checks and counterarguments considered (and, yes, the appropriate calculations done)? To what extent are known difficulties acknowledged? When a new counterargument is given, is it addressed, and the original assertion modified if necessary? Are facts presented in a clear and direct manner? This is howscientists judge one another. It is clear why this is necessary: science works by the parallel activity of many minds, and it is necessary that information be exchanged in as accurate a way as possible. Given the above discussion, I find your claim to the ethical high ground to be ironic.

Regarding group-think: you interpret the reaction of string theorists to your book as more evidence for your point of view. Rather, I think that much of this is a natural reaction to what many see as a distorted presentation of the facts. Regarding the personal insults, I think that you set the tone here with characterizations such as that quoted in the New Yorker, so it seems like posturing for you to claim the high road when a few string theorists respond in kind. However, I hope that those contributing to this discussion will try to keep to the same reasoned attitude that I have tried for.

Overwhelmingly the concentration on string theory is a scientific judgement, made by a very diverse group of theorists. Look at any of the several dozen most well-known string theorists: my own scientific experiences and tastes, both inside and outside string theory, are very different from any of theirs, just as they are from each other. I think of myself as a theoretical physicist first, and cross over the boundaries between string theory and several other fields depending on what looks important and interesting, as do many others. String theorists can be rather focussed, but they are not as closed to new ideas as you portray. For example, such ideas as holography and eternal inflation were developed outside of string theory, and might have become `alternative ideas.’ Instead they were recognized as likely parts of the big picture.

There is a reasonable concern that younger string theorists, educated in string theory rather than in other fields, might find it harder to cross these lines. Indeed, during the first and second string revolutions, there was inevitably more concentration, as these new ideas opened up a whole range of new concepts and methods. It is a very positive development that new connections between string theory and other areas have developed – heavy ion physics, low energy hadronic physics, LHC physics, cosmology, mathematics, general relativity, and many areas of quantum field theory – and that many young people are taking advantage of the opportunity to cross these lines, and in both directions. This broadening of perspective should be, and I think is, strongly encouraged.

Coming back to the question in the title, I have agreed that sociological effects exist; they must, since science is a human activity. However, when I read your book, knowing the facts, the case actually seems quite weak. To make the case for a strong sociological effect, at each turn you are forced to stretch the facts beyond recognition. On the other hand, when you discuss the science, your overemphasis on the usefulness and applicability of rigor ignores the kind of physical reasoning that physicists actually use in practice with great success, so your are leaving out at least 95% of what makes physics really work.

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109 Responses to Guest Post: Joe Polchinski on Science or Sociology?

  1. Blake Stacey says:

    That’s an awful lot to read. Could you maybe summarize it in lolcat form? Say, “Im in ur moduli space, describin ur IR divurgensez.”

    :-)

  2. tyler says:

    If there is a more interesting discussion underway anywhere on the planet, I’m certainly not privy to it. I am very pleased that this debate is public and appreciate the effort being made on both sides to keep things civil.

    It seems to me that the non-scientific parts of this debate, relating to sociology or epistemology, are reaching the end of their useful lifecycle, becoming (as such things tend to do) an essentially semantic debate with quickly diminishing returns. The specific scientific points, however, are becoming more interesting with each iteration of the argument.

    Thanks to you and Dr. Smolin for your efforts and to the Cosmic Variance owners for all their work.

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  4. none-of-the-above says:

    Professor Polchinski
    Thank you for taking the time to work through, and address scientifically, the arguments and allegations in Smolin’s book. I am sure that this is not what you would prefer to be doing with your time, but leaving these claims unaddressed would result in public misunderstanding of the status of modern theoretical physics, and risk long term damage to the field.
    Again, thank you for your time and effort.

  5. Mike says:

    Thanks, Professor Polchinski.
    I think, everybody with in interest in the status of modern theoretical physics (certainly me) really appreciates the public sientific debate on Smolin’s book.

  6. Nigel says:

    The Trouble with Physics does list some trivial issues with the claims of string theory, so I’m not surprised to see Professor Polchinski, author of a major work on string, take issue with these points.

    I would like to just ask: “what, if anything, would make string theorists finally concede that completely non-string alternative ideas should be objectively compared to string theory?”

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  8. none-of-the-above says:

    Nigel on May 21st, 2007 at 5:35 pm
    asks:

    “I would like to just ask: “what, if anything, would make string theorists finally concede that completely non-string alternative ideas should be objectively compared to string theory?” ”

    I am not a string theorist so I can’t speak for “string theorists”. However, as a working particle theorist I can say that it has *always* been standard practice that *all* alternative ideas should be compared not only to string theory but also to each other, and more importantly to whatever data we can bring to bear on them. I have personally worked on models of technicolour, compositeness, supersymmetry, susy-GUTS, non-commutative geometry (a la Connes, and a la deformation quantization), and string theory: I’ve also worked on the physics of black holes and on cosmology but that’s not relevant here [incidentally most working particle theorists that I know have a range of research interests at least as large as mine] . Some of these classes of models have been slain by data, others have hit a wall in terms of our ability to develop and understand them, and some have survived and thrived. That’s science! Physics is always bustling with novel theories, and ideas for their confrontation with experiment. All theories must show their (calculational) goods, and live up to the scrutiny of experiment and the comparison to other theoretical alternatives. So what you’re asking about is the normal practice of physics and goes on all the time.

  9. Illirikim says:

    I think that that was by far the most effective rebuttal of Prof Smolin’s book I have seen, and it goes some way towards cancelling the effects of the writings of you-know-who.

    The weakest point, however, is the discussion of the cosmological constant. I think that there is no denying that [a] the discovery of a positive CC was not expected by string theorists, and came to many of them as a very unpleasant shock, that [b] had the CC turned out to be negative, the temptation to claim this as a triumph for string theory would have been irresistible, and [c] to claim that the moduli stabilization problem has been “solved” is a bit over the top.

  10. mollishka says:

    I would like to just ask: “what, if anything, would make string theorists finally concede that completely non-string alternative ideas should be objectively compared to string theory?”

    I’d like to step back and ask: what non-string alternative ideas are there that are being seriously considered to explain what the Standard Model doesn’t explain (which, I assume, is primarily having gravity finally get along with everybody else)?

    And what is the moduli stabilization problem? (There’s no relevant wikipedia article! I’m lost!)

  11. Sean says:

    mollishka, when you compactify all those extra dimensions, there are various parameters that tell you the size and shape of the compactification. These show up as scalar fields in the four-dimensional world; the earliest example was good old Kaluza-Klein theory, where the radius of the compact circle leads to a scalar field now often called the “radion” or “dilaton.” More generally, the scalars are known as “moduli.”

    But these moduli usually have potentials, and they usually tend to not have minima, or at least not in a perturbative regime where you can calculate anything. Instead, they get pushed off to infinity, which is inconsistent with the world we observe. So we want to stabilize the moduli, which turns out not to be so easy. Various new ideas (of which Joe was one of the pioneers) involving combinations of warping, fluxes, branes, and instantons seem to show that it can be done. (Perhaps an expert can be more explicit than that.)

  12. Garbage says:

    “Various new ideas (of which Joe was one of the pioneers) involving combinations of warping, fluxes, branes, and instantons seem to show that it can be done. (Perhaps an expert can be more explicit than that.)”

    It seems highly fine tunned to me. Fluxes, branes, warping and instantons? it might as well be a bunch of angels. I am sure there is a vacuum where some little turtles do the job 😉

    I dunno, it looks like without experimental guidance (like Joe points out we live, unfortunately/fortunately, in a highly successful period with lilttle new physics out there to crank some calculations for) this whole discussion is just pure sociology. Of course we can appeal to the rigor of mathematics, but at the end of the day we want to understand nature not just play around with symbols, even though we all admit is fun. Polchinski is right in clarifying missleading argumentations, and after all this is about Smolin’s book and his claims and not the failure of String Theory (ST) to provide any experimental confirmation. I agree all what is has been developed under the ST tag is pretty amazing and worthwhile exploring given its potential usefulness in other areas of physics. The ADS/CFT correspondency is remarkable, even though from an EFT point of view isnt that incredible. Perhaps the best achievement is to relate a non-gravitational theory to a theory with gravity, that is cool. The other way around is also quite interesting, even though alcohol is quite different than water, we all kinda know this already, dont we? 😉

  13. OM says:

    Noboru Nakanishi and Izumi Ojima have written a sceintific masterpiece called “Covariant Operator Formalism of Gauge Theories and Quantum Gravity”. It is therefore most interesting to read what these authors have to say about the superstring paradigm and related issues. Here’s what I found (highly recommended, deep and spiced with a lot of humor…)

    <em> http://www.math.columbia.edu/~woit/nakanishi1.pdf </em>
    <em> http://www.math.columbia.edu/~woit/nakanishi2.pdf </em>
    <em> http://www.math.columbia.edu/~woit/nakanishi3.pdf </em>
    <em> http://arxiv.org/abs/hep-th/0610090 </em>

  14. Joe Polchinski says:

    Thanks everyone for your kind words. For now I just want to reiterate in response to Illirikim:

    The weakest point, however, is the discussion of the cosmological constant. I think that there is no denying that [a] the discovery of a positive CC was not expected by string theorists, and came to many of them as a very unpleasant shock, that [b] had the CC turned out to be negative, the temptation to claim this as a triumph for string theory would have been irresistible, and [c] to claim that the moduli stabilization problem has been “solved” is a bit over the top.

    I stand by my statement that Smolin’s story of the `prediction’ is fiction. I can tell you what my reaction was to the cc, because it was vivid: `Oh s**t, Weinberg got it right…’ There was no `Oh s**t, string theory got it wrong,’ because there was no such prediction. Had the cc been negative, there certainly would have been attempts to connect it with AdS/CFT – if the spatial curvature also turned out to be negative, AdS/CFT might have been a nonperturbative construction of our universe. But this would probably not be consistent with inflation, which requires a large positive cc early on.

    On the stabilization (which Sean has well-described) I have agreed that there are open questions, and yes our current picture may not be entirely right. But it’s really an issue of how well we can calculate: if one keeps only the first approximation to the potential, it usually has some scaling property that makes it monotonic (no minima) so you have to work harder to get a controlled approximation.

  15. Seebee says:

    Regarding the comment of Garbage: stabilizing moduli by adding non-perturbative effects in by hand does seem that way, but there do exist instances in which the moduli are stabilized completely by fluxes (and perhaps warping). Such instances lend themselves to a dynamical means of moduli stabilization. This area of string theory is trying to make experimental contact through cosmology, namely, the cosmic microwave background. In light of these ideas, it seems that to proclaim string theory a failure is a bit premature since to say so would suppose that it is a complete theory. I don’t think that anyone would affix their name to such a claim.

  16. a says:

    The content of a discussion that starts with “let’s stick to physics” should be “my theory predicts A, your theory predicts B, let’see who is right by doing this experiment”

  17. invcit says:

    Dear Prof. Polchinski,

    Thank you for a very informative post.
    Proponents of LQG, such as Smolin, often make the claim that a fundamental theory ought to also give a satisfactory interpretation of QM; they seem to suspect that the two problems, quantizing gravity and interpreting QM, somehow are related physically. I was curious whether you saw it as a valid critique of string theory that no framework for understanding QM at some deeper level (whatever that would mean) has emerged from it. Do you think that it points to something missing from the theory, or is it just not an issue?

  18. Nigel says:

    a: that may be how science should proceed in an ideal world. The mainstream hype of string theory is not based on a comparison of string theory results to those of loop quantum gravity or other alternatives.

    This is not a new problem. The mainstream theory is always defended first and foremost by elbowing contenders out of the way by pretending that they don’t really exist, aren’t serious, or should be seriously considered only after they have had as much time invested in them as the mainstream theory (which is a catch-22 situation, due to the lack of funding and lack of promotion of the alternatives).

    Most people ignore alternatives because they don’t have the glamour of the mainstream.

  19. Maths PhD Student says:

    In response to Nigel: Yes, there does seem to be a nasty tendency amongst some researchers to label other theories as vacuous or worse simply because they are not the pet theories of the most famous people in a particular field. Of course, most people don’t operate this way– but there is always a small and vocal minority of people in any area who will speak out against unconventional ideas. The truth is, at least in mathematics, it is not really a matter of who is “right” or “wrong”; it is what turns out to be elegant and interesting. Sadly, it does often seem that elegant and interesting ideas get dismissed out of hand as “crackpot ideas” before they have a chance to be properly developed- this seems unfortunate, but only to be expected– because, after all, there is also a great deal of chaff along with these few gems, and who really wants to sort through every half-baked scheme?

    At the end of the day, most things with interesting structure and connections to other areas will find some application, somewhere– it just might not be the application for which the construction was originally intended. Research is like that– you might start by trying to use something to prove or establish something else and end up in a completely unexpected place. String theory might turn out to be a good example of such. Or, it could be an idea before its time; perhaps string theory contains seeds of a deeper truth within it but some “missing link” needs to be inserted between it and “classical physics”. I guess only time will tell what happens to all of these things.

  20. Thomas Larsson says:

    I would like to rephrase the point raised by invcit #17. Since no approach to QG can make any reliable calculation about anything testable, one should at least demand that QG resolves the conceptual problems, e.g. the problem of time. Here is my favorite version:

    * In GR, time is what a clock measures.

    * In QM, time is a c-number parameter rather than an observable; according to Pauli’s theorem, a self-adjoint time operator is incompatible with a Hamiltonian bounded from below.

    There is clearly a tension between these statements, since reading a physical clock is a physical experiment which must be described by an observable. It seems to me that the successful theory of QG must resolve this paradox, and that neither ST nor LQG is doing a particularly job at this. What hope is there to unify QM and GR, if you can not even unify the notions of time in the two theories?

  21. Illirikim says:

    Prof P said: “I stand by my statement that Smolin’s story of the `prediction’ is fiction.”

    I think I can agree with that, while conceding that S is right about one thing: the positive CC came as a complete surprise, and it was a great pity that our theory of everything failed to give even a single clue that something so basic and momentous was afoot. It wasn’t a failed prediction, but it certainly didn’t look good.

    I can tell you what my reaction was to the cc, because it was vivid: `Oh s**t, Weinberg got it right…’ There was no `Oh s**t, string theory got it wrong,’

    Yes, I agree. With the last sentence, not with Weinberg getting anything right…..

  22. einstein says:

    Not quite Bohr vs. Einstein.

    But then again, they are baby boomers, and like to pretend.

  23. none-of-the-above says:

    mollishka on May 21st, 2007 at 8:45 pm
    asks:

    “I’d like to step back and ask: what non-string alternative ideas are there that are being seriously considered to explain what the Standard Model doesn’t explain (which, I assume, is primarily having gravity finally get along with everybody else)?”

    I have two comments in response to your question:

    -First: At any energy that we have direct experimental or observational information about [up to the energy scale of inflation], gravity gets along with everybody else just fine. In fact, at laboratory energies quantum gravity is the most precisely calculable piece of the standard model. This is because it can be treated by Wilson’s methods of effective field theory; for a general review of the ideas of effective field theory see the first half of the review:

    – Effective field theory and the Fermi surface. Joseph Polchinski (Santa Barbara, KITP & Texas U.) . NSF-ITP-92-132, UTTG-20-92, Jun 1992. 40pp. Lectures presented at TASI 92, Boulder, CO, Jun 3-28, 1992. Published in Boulder TASI 92:0235-276 (QCD161:T45:1992) e-Print: hep-th/9210046

    For the explicit discussion of the effective field theory treatment of quantum gravity see the reviews:

    – Introduction to the effective field theory description of gravity. John F. Donoghue (Massachusetts U., Amherst) . UMHEP-424, Jun 1995. 26pp. Talk given at Advanced School on Effective Theories, Almunecar, Spain, 25 Jun – 1 Jul 1995. e-Print: gr-qc/9512024

    – Quantum gravity in everyday life: General relativity as an effective field theory. C.P. Burgess (McGill U.) . Nov 2003. 57pp. Published in Living Rev.Rel.7:5,2004. e-Print: gr-qc/0311082

    What string theory does is to provide a consistent quantum description of gravity at arbitrarily large energies [the Planck scale and beyond], where the effective field theory methods would no longer be applicable. It should be noted that there are also aspects of the particle physics of the standard model which break down at very large energy [Landau poles in scalar self- interactions, fermion mass Yukawas, and the U(1) gauge couplings], so both Einstein gravity and the standard model of particle physics eventually have to be replaced with a more encompassing quantum theory at very large energies [beyond the range of (at least present) experiment]. Any proposed alternative to string theory would have to provide an extension that encompassed all of this physics.

    Second: Within the standard model of particle physics there is a lot that is not understood. We have no understanding of why the quarks and leptons get the precise masses that they do; we have no prediction for the mass of the Higgs particle; we have no prediction of the values of the gauge coupling constants; we have no idea why the vacuum energy is so ridiculously small [cosmological constant problem]; we have no idea why the electroweak scale is so much smaller than the Planck scale [hierarchy problem]; we have no idea why there is no observable CP violation in the strong interactions; we have no idea why neutrinos have the masses they do; we have no idea why the mixing matrices in weak decays are what they are, both in the lepton sector and in the quark sector; we have no idea what composes the cosmological dark matter; we have no idea how the asymmetry between matter and antimatter, which we see in the universe today, was generated after inflation;…. There is a lot that we don’t understand; some of these questions may be answered in a “bottom up” manner (hence all the activity in the kinds of models listed in my post above), but others may require a unified theory like string theory to give a satisfactory answer.

  24. Blake Stacey says:

    After snaring the first comment spot with a silly not-quite-a-joke, I realized that Prof. Polchinski’s essay makes an observation which is relevant outside the field of quantum gravity research and, indeed, outside what we typically consider “theoretical physics” proper. I refer to the following:

    This process of translation of an idea from words to calculation will be familiar to any theoretical physicist. It is often the hardest part of a problem, and the point where the greatest creativity enters. Many word-ideas die quickly at this point, or are transmuted or sharpened. Had you applied it to your word-ideas, you would probably have quickly recognized their falsehood. Further, over-reliance on the imprecise language of words is surely correlated with the tendency to confuse scientific arguments with sociological ones.

    Isn’t this exactly the process which popularizations (vulgarisations if you want to be Gallic) typically fail to address?

  25. dark-matter says:

    A lot of talk. So so much talk. 500+ comments on the ‘string theory losing public debate” thread. But nothing new from 3 years ago on the string side, and the money keeps coming. The phenomenology people look at this and moves on. The math people can be forgiven to think string theorists have taken over their work. The cosmologists may want to increase the factual foundation of their field by not mentioning ST too much. The astrophysicists take a look and say “better to build real machines and observe”. The loop people don’t care and never did care. And the interested public, reflecting the recent talk by David Gross in Israel, take a look at that cartoon depicting the hamburger joint with the poster “Tonight. Is string theory bullshit?”, smiles and moves on.

  26. none-of-the-above says:

    dark-matter on May 22nd, 2007 at 10:58 am
    writes:

    “The loop people don’t care and never did care.”

    Actually, this thread exists because one of the loop people appears to have somewhat of an obsession with string theory, and to be upset that most string theorists are sufficiently excited by progress in their own approach so as to feel no need to work on others. I would qualify the degree of interest that would motivate someone to take a large chunk of a year out of their own research program, just to write a non-technical polemic attacking the research program of others, as being, at best, not conducive to their own research.

    There’s lots of research done that doesn’t particularly excite me. To take a random example, in my opinion large extra dimension models are problematic, for a variety of reasons. But I don’t write a book attacking them, and the people that work on them, rather I just work on theories that seem to me more likely to be right. I found some of the recent [last 10 years] conferences, which were largely dominated by LED talks, to be a waste of time, but that bandwagon just distracted people from working on the physics that I think is good, and which I then got to harvest without hordes of other people trying to do the same thing. Generally speaking if someone wants to write a book or paper on their own research, that I find interesting and often worth my time to read. If someone is writing a book just to attack the work or character of others, that I find to be a complete waste of time, and my advice would be to instead put that time and effort into improving their own research program.

    P.S. you also write “and the money keeps coming”. Actually, numerically, string theory is a relatively small perturbation on particle physics. Any reasonable attempt to count active string theorists would probably get less than 2000 world-wide. This is certainly less than the number of active particle phenomenologists, probably less than the number of active astroparticle theorists, probably a factor of 3-5 less than the number of active theoretical astrophysicists [I don’t have any feeling for the number of active observational astrophysicists / astonomers], and it doesn’t even add up to the number of scientists on ONE of the LHC experiments [I think that CMS now has over 2,500 people on their author list; the 4 LHC experiments between them count something like 7000 scientists]. This is as it should be: the experiments that will carry our understanding forward need, and deserve, the largest slice of support, and in a healthy scientific ecosystem theory that connects directly to experiment has to be encouraged. Rather than making snide comments about support for other types of research that you personally don’t like, perhaps a more constructive approach would be to make the case for increased research support for the full range of fundamental physics, including the type of research that you undertake.

  27. B says:

    #17 just a historical side remark reg Kaluza-Klein: as far as I recall Klein did not have the radion because he simply set the g_55 entry to be equal one. Kaluza had, but he was working only in a linear approximation and he did not talk about compactification – instead he had what he called the ‘cylinder condition’ which kind of artificially set derivatives wrt to extra coordinates to zero (that is, he dropped all excitations).

  28. tyler says:

    “And the interested public, reflecting the recent talk by David Gross in Israel, take a look at that cartoon depicting the hamburger joint with the poster “Tonight. Is string theory bullshit?”, smiles and moves on.”

    Nah. At least not the interested public I know. Some writers and bloggers may say that, but the reality I see is that the interested segment of the public is not particularly vested in any given outcome but is increasingly interested in the debate itself. See, none of our funding depends on the outcome….;o)

    In fact among the laypeople I know who think about such things I don’t know a single person who holds anything like a “belief” regarding string theory: that it is true, or is false, or anything. We are interested, probably more so now than ever, but unconvinced either way. It’s a very interesting “maybe;” the change in the debate is that now there are competing perspectives that are better know to us.

    This situation can’t go on forever of course. At some point people will tire of a endless debate without at least some experimental resolution, and that point will come much sooner if the debate takes on a flamey edge with explicit discussions of who deserves tenure and funding and who does not based on which camp they are in – something I know is important, and real, and relevant. But the one thing I can guarantee you with absolute certainty is that the taxpaying public does not want to see inside your sausage factory. This is not a winnable debate for either “side” if it is held in public – the debate itself is a losing effort.

    I say this out of deep respect for all of you and the work you do.

    If you all can keep the debate on topic (the science itself), and at a ~50% comprehensibility level for those of us without the math (which you’re all doing great at BTW, it’s fine for individual posts to go much lower that that), I am completely certain that this debate will energize rather than sap the public’s interest in the frontiers of physics.

    Of course, the “winner” can only be determined by predictions agreeing, or not, with experimental outcome. The semantic debate is fascinating but ultimately of limited relevance in the long run…though it gives us something to do until the LHC comes online.

    (great /. link today on the many petabytes of expected LHC data and the issues that raises, BTW, very interesting stuff)

  29. Plato says:

    Lee Smolin:

    What we are dealing with is a sociological phenomenon in the world of academic science. I do think that the ethics of science have been to some degree corrupted by the kind of groupthink explored in chapter 16, but not solely by the string-theory community. For one thing, it is the academic community writ large that makes the rules. In a court of law, a good lawyer will do anything within the law to advance the cause of his clients. We should expect that the leaders of a scientific field will likewise do everything within the unwritten rules of academia to advance their research program.

    A couple of things caught my attention in Joe’s response to Lee.

    One in regards to the view of mathematics as the basis, beyond what ideas are first presented.

    I couldn’t help but think that any ideas expressed in science here, having a mathematical basis, would set the course for “new ideas to come forth” and create a new basis of thinking, now having this new view of the world. It would have ignited new mathematical directions with which to develope those new ideas.

    If one had said certain limitation in regards to Mandelstam’s Finiteness, then the limits of that discussion would have been a point made by Joe in regards to the distance with which this mathematics had been taken. Lee ideas would have been shown to limit what he had to say in face of what Jacques imparted to him.

    Jacques Distler :

    This is false. The proof of finiteness, to all orders, is in quite solid shape. Explicit formulæ are currently known only up to 3-loop order, and the methods used to write down those formulæ clearly don’t generalize beyond 3 loops.

    This has brought us to the difference in opinion of the mathematical basis and hence the proof that Lee’s coments in this regard are limited in regards to the expression of his ideas?

  30. B says:

    sorry, #17 in #27 should have read #11.

  31. Diogenes says:

    Interesting press release from the Perimeter Institute today; Howard Burton, their director who hired Smolin as their first staff scientist, is out. Here it is:

    WATERLOO, ON, May 22, 2007 – It was announced today that Howard Burton, Executive Director of Canada’s Perimeter Institute for Theoretical Physics (PI), will be departing the organization in the near future. The search for an interim leader and long term successor will be conducted through collaboration by the Board of Directors, the Institute’s Faculty, and the preeminent Scientific Advisory Committee….

    {full announcement at:
    http://www.perimeterinstitute.ca/News/In_The_Media/News_Releases/
    just look for the first item on the page: “PI’s Executive Director to Seek New Challenges “)

  32. Gina says:

    Joe Polchinski’s review of Peter’s and Lee’s books was a highlight of the debate. The positive spirit and cheerful love of physics was everywhere present in what Joe wrote then.

    The new reply by Joe is very interesting, and raises a lot of points for discussion, but it does not raise to the the level of his original review. Joe “took off the gloves” this time, made all sort of rhetoric punches, and it was both prettier and more convincing, when he had the gloves on.

    Here are some points based on some of Joe’s “great lines for discussion” :

    1. Joe wrote: A chain of reasoning is only as strong as its weakest step

    This statement whether applied to evaluate a whole scientific theory or a specific argument is an extremely interesting issue in philosophy of science. It arouse in various scientific controversies from archeology to biology.

    While it is formally true that the strength of a scientific argument or a scientific theory is as strong as the strength of its weakest link (or step), in my opinion this is not really true. It is perhaps fair to say that the strength of an argument (or a theory) is not weaker than its weakest link but there are cases it can be stronger. There are cases the strong links give extra support to the weaker links.

    Certainly Joe’s line is not so positive when it comes to string theory. Some of the links (or steps) are not only weak but yet non-existing.

    2. Maldacena again.

    I share the opinion that there is no way one can attack string theory as a whole based on Maldacena’s conjectures which are a great triumph for string theory. (And I criticized Lee’s approach on this point.) But I do not see anything wrong with a research program aimed to find weak points in or counter arguments to the strong Maldacena’s conjectures. Lee raised the possibility that the validity of the weak form of Maldacena conjecture accounts for strong symmetry that is not present in the general case. In his book Lee raises this possibility (along with some other legitimate concerns) as an idea and not as a proved fact. This is fine. Joe is critical about several of Lee’s ideas concerning the strong Maldacena’s conjecture and that is fine as well. Joe even note that some of Lee’s coauthors have papers where they present evidence which goes against Lee’s approach, this is very fine.

    If indeed Joe’s shot down three specific suggestions that Lee had made, this is quite interesting but Joe’s critique of Smoling’s research efforts regarding this matter as a priori misguided is not clear.

    3. Joe wrote about rigor: In physics, which is a process of discovery, this can make them [rigorous result] worse than useless,

    Joe’s anti-rigor argument comes across as a little strange. It is certainly correct that string theory is not required to have stronger levels of rigor than that of highly successful physics theories and methods on which it is based. There is always the hope that the advances on the physics side (and sting theory itself) will help putting these methods and theories on more rigorous foundations and there is always the concern that the non-rigorous nature of the methods will cause them to collapse for such a far far reaching extension of their original usage as string theory is.

    If there are scientific reasons to reject the argument based on “Rerehn duality” that is fine, but what does it have to do with rigor in general? It is justified to claim that there are good reasons for non-rigorous nature of string theory as it is fine to point out that there are good reasons for the difficulty to make predictions in sting theory. But, in my opinion, it does not come across very well when (even justified) weaknesses are portrayed as advantages.

    4. Joe wrote: This process of translation of an idea from words to calculation will be familiar to any theoretical physicist. It is often the hardest part of a problem, and the point where the greatest creativity enters. Many word-ideas die quickly at this point, or are transmuted or sharpened.

    This is a great description! wow!!!

    (It also relates nicely to Lee’s point regarding the different approaches in physics of 1960-80 and earlier physics.)

    5. Three points that I’d love to hear more about:

    Joe’s explanations on duality, on the UV finiteness of string perturbation theory and, his point regarding the larger and smaller spaces concerning the constraints, are extremely interesting (but rather condense and cryptic) and it will be great hearing further explanations on these issues.

    6. Anecdotes and quotes:

    Lee anecdotes and quotes (one even from a Cosmic Variance’s blogger,) regarding string theory and string theorists gave very very little support to his overall argument. And by the same token also Joe’s anecdote concerning his discussions with Lee that are not documented in Lee’s book are not really damaging to Lee’s argument. All these matters are very very very side issues.

    7. In summary, overall, the scientific debate/discussion regarding string theory and related wider matters is quite good (and it is becoming overall better) and interesting. Peter and Lee deserve a credit for igniting this discussion. I also join the others thanking Joe.

    One strange aspect of this discussion is that we hardly see any retractions. In “normal” scientific discussions (like between scientific collaborators) a statements like “I was wrong on this point”, “this was a silly idea, let me try something else” can account for 80 percents of the discussion. Here, we do not see anything of that kind, even regarding small side-issues.

  33. none-of-the-above says:

    Gina on May 23rd, 2007 at 3:40 pm
    writes:

    “Three points that I’d love to hear more about: Joe’s explanations on duality, on the UV finiteness of string perturbation theory, and…”

    The first two are textbook material. Dualities are well treated in Johnson’s book on D-Branes. The fact that the prospective uv divergences in string theory are really ir effects is scattered in various texts [I assume that it’s in Polchinski’s but I don’t have my copies at hand to check].

    Beyond that I disagree that “the scientific debate/discussion regarding string theory and related wider matters is quite good”. Basically I disagree that this discussion, as presently conducted, is really even scientific; as Polchinski noted:
    “In my review I have gone through your book and papers and identified three proposals, and explained why each is wrong. Again, you have not acknowledged the existence of scientific counterarguments, but have just reasserted your original point. If your arguments had been made in a serious way, I would expect that you would have given some deep thought to them and be ready to defend them.”

    That’s not science. And it’s sad that a scientist as distinguished as Joe Polchinski has to take time away from his own research to defend his field against distortion and misrepresentation in the popular press.

  34. Joe Polchinski says:

    Thanks again for the comments and support.

    invcit #17

    This is an interesting question, to which there is no definite answer. On the one hand, since it was possible to quantize the other three interactions without changing the interpretation of QM, it is not obvious that one should not be able to do the same for gravity. If we restrict to `laboratory’ experiments with gravity (even building black holes in the lab), there is no sharp paradox that would require us to modify QM. QM makes us queasy, but if it gives consistent predictions for all processes we may just have to live with that. Things are much less clear when you get to cosmology. Chaotic inflation, for example, does seem to lead to paradoxes, which might be the clue to a deeper understanding of QM.

    Tyler #28 and Gina #32,

    I am glad that you are enjoying the physics, and I agree that this is the fun part. But Smolin’s book was presented as a discussion of the sociology of science with string theory as a test case, and many people have taken his arguments seriously. Thus I must also respond to this. In particular, this point about the existence of the gauge theory: The Wilsonian understanding of quantum field theory is one of the centerpieces of theoretical physics, and has been the area of some of my best work: the reformulation of renormalization theory, and the effective field theory analysis of Fermi liquid theory and BCS superconductivity (both outside of string theory, by the way). Regardless of who said what to whom, for Smolin to simply dismiss this subject as sociology and groupthink is outrageous.

    Gina – I don’t want to beat this subject of rigor into the ground, but your commentary about my statement `A chain of reasoning is only as strong as its weakest step’ is on the mark. A good physical argument can indeed strengthen the links around it; a rigorous argument is generally very rigid in scope and cannot do this. Also, rigorous arguments are generally constructed as chains, so the weaknesses add in series. Physical arguments are in general webs, with a complex interweaving of arguments – certainly the argument for Maldacena duality is of this type: for it not to be true, one would have to tear many strands.

  35. Hendrik says:

    Prof. Polchinski, I have to take issue with your statements regarding mathematical rigor:

    Generally, rigorous results have very specific assumptions and very precise consequences.

    Yes, the concepts must be defined clearly, the logic unbroken. I would have thought that since that made the results reliable, it is something to be desired, not spurned.

    In physics, which is a process of discovery, this can make them worse than useless,…

    Loose thinking with ill-defined concepts is fine during discovery, but once your physical mechanism is discovered (and you can only claim discovery after experiment confirmed your predictions), you need to analyze it to properly understand it. That means that you have to get your logic straight, i.e. make it rigorous. How else can you clear the fog?

    ..since one tends to assume that their assumptions, and their implications, are broader than they actually are.

    Loose thinking and bad applications are not the fault of rigorous results;- “it is a bad carpenter who blames his tools.”

    … a chain of reasoning is only as strong as its weakest step. Rigor generally makes the strongest steps stronger still – to prove something it is necessary to understand the physics very well first – and so it is often not the critical point where the most effort should be applied.

    A rigorous reconstruction of a piece of physics clarifies and polishes the physics. Moreover, it can often reveal aspects of the problem which were hidden. I cannot see how you can fully understand a piece of physics without having made it rigorous. So it is not possible to “understand the physics very well” without this. Of course it can be done badly, e.g. by choosing irrelevant features of the physics to build on.

    ..whereas a good physical argument is more robust.

    ..in the same way that a fog is robust;- it can accommodate anything. In fact, ultimately a rigorous mathematical result is far more enduring. It is harder to establish, but you can build on it wherever its initial assumptions apply.

    Post #34:
    A good physical argument can indeed strengthen the links around it; a rigorous argument is generally very rigid in scope and cannot do this.

    How can an ill-defined version of an argument be better than a logically clear one?

    Also, rigorous arguments are generally constructed as chains, so the weaknesses add in series.

    So once it is established, it can be relied on, and truth carried along the chain from the start to the end.

    Physical arguments are in general webs, with a complex interweaving of arguments – certainly the argument for Maldacena duality is of this type: for it not to be true, one would have to tear many strands.

    Where most of the arguments are ill-defined and made up of analogies, and cannot give a true understanding of their subject.

    It is a pity that in your remarks you see this conflict between logical rigour and physics theorising. As I indicated above, I do see a place for the nonrigorous approach of physics in the process of discovery. However proper understanding can only come from clarifying the concepts and their relations in a rigorous way, in fact, this can lead to subsequent discoveries.

    Since string theory presently seems to be speculative physics (the well-known lack of experimental confirmation), one would have expected that you should try to make its internal logical structure as solid as possible. Unfortunately, it seems the handful of papers produced by mathematical physicists on string theory topics are mostly ignored (e.g. Dimock, Wiesbrock etc.), and in the case of Rehren seems to be controversial. I think rigorous approaches can also offer alternative approaches to circumvent some problems of string theory, but these are lost on the community.

  36. NN says:

    This process of translation of an idea from words to calculation will be familiar to any theoretical physicist. It is often the hardest part of a problem, and the point where the greatest creativity enters. Many word-ideas die quickly at this point, or are transmuted or sharpened.

    What a gem!

    A good physical argument can indeed strengthen the links around it; a rigorous argument is generally very rigid in scope and cannot do this. Also, rigorous arguments are generally constructed as chains, so the weaknesses add in series. Physical arguments are in general webs, with a complex interweaving of arguments – certainly the argument for Maldacena duality is of this type: for it not to be true, one would have to tear many strands.

    Beautiful!

    I am most grateful to Prof. Polchinski for sharing his thoughts. When commonly held wrong perceptions are not debunked in such a clear, dispassionate manner, it is taken to be true by many outsiders (like myself).

    It is clear that string theory is a remarkably deep field (the deepest there is, in my opinion), and many should continue pursuing it.

    It seems to be much less convincing (if at all) in its original purpose of understanding/explaining particle physics. Of course, it is true it is superior to the effective field theories of particle physics since it UV completes it.

    What I mean is that it does not offer any specific guidance on particle physics models (sure there are brane-world models, KK models etc, but a lot of it following from QFT itself). For example, as far as I know, it does not restrict the possible 4D QFTs (beyond what is prohibited by QFT itself). Actually, I suppose, string theory does assert that the 4D effective field theories cannot be compactified from a, say, 320-dim QFT. So only certain KK towers are allowed. Is that correct? Still would leave a lot of possibilities, but is at least a restriction…

    Considering the large hierarchy, it seems unlikely (to me) that it ever will (e.g., chemistry from the standard model).

    PS:

    AdS/CFT would then imply that craftsmen are dual to seers.

    Wicked :)

  37. Pingback: String Theory: Not Dead Yet | Cosmic Variance

  38. dark-matter says:

    Regardless of who said what to whom, for Smolin to simply dismiss this subject as sociology and groupthink is outrageous.

    Write your own book. Explain ST and the landscape to the informed public and other experts, list its great accomplishments, delineate its predictions, show why it is solid science and has a great future, write a chapter for the younger generation of physicists, present data on its return-on-investment, and with the assistance of sociologists, present arguments and research results to support claims of no groupthink the past decade or so. Find publishers willing to take the substantial financial risks to publish it.

  39. invcit says:

    Dear Prof. Polchinski,

    Thank you for your comments.
    Could you please expand a little on what you meant by that chaotic inflation seems to lead to paradoxes that could be relevant for the interpretation of QM or point to a paper that discusses this? It sounds very interesting.
    Thank you!

  40. Lee Smolin says:

    Dear All,

    This has come at the worst possible time for me to reflect and reply, so I will not be able to reply quickly and in detail. I thank Joe for the response, there are many points where I would like in time to comment, and others on which we simply have differing scientific judgement. There is nothing wrong with having differing scientific judgements, nor with debating why we take different points of view about open questions. So I thank Joe for taking the time to reply.

    It is distressing to however read comments such as the following, “for Smolin to simply dismiss this subject as sociology and groupthink is outrageous.” Anyone who read the book would know that that is not at all what I did. The first 3/4 of the book are straight science and history of science, and the analysis of the strengths and weaknesses of string theory is done there completely on scientific grounds. And there are lots of places where I acknowledge the interest and importance of substantial results about string theory. The assessment of string theory and various claims about it given there is mixed, and balanced. Many successes are mentioned, as are several problems. It is a complex picture, with strong pros and cons, and the open questions are genuinely puzzling. That is why it was worth writing a book, to sort out what to make of it. I am extremely tired of comments which ignore the complexity of the subject and imply also that I ignored it. The useful discussion only starts when someone acknowledges that there are strong reasons for interest in string theory AND also strong reasons to be skeptical that it is the theory of nature.

    The discussion of sociology is only in the last of four parts of the book. And there was nothing there that was at all new to people who study the sociology of academics, or experts in general. I am surprised that anyone finds what I wrote surprising.

    Joe mentions some standards: “To what extent are known difficulties acknowledged? When a new counterargument is given, is it addressed, and the original assertion modified if necessary? Are facts presented in a clear and direct manner?” I agree these are important standards and I believe I have satisfied them. That is the reason why the assessment of string theory in the book is mixed and balenced. The whole book is the result of such a process, carried out over twenty years of work on and study of the subject, with many discussions with string theorist. Indeed, the book does not contain every argument I made or even published about string theory, precisely because my arguments have been altered by progress in the field as well as improvements in my understanding.

    For example Joe mentions here and before my paper with Matthias Arnsdorf on Rehren’s version of AdS/CFT, hep-th/0106073. I would be happy to discuss this, but please first notice that I do not mention the argument of the paper in the book. This is because we realized since posting the paper that there is an important difference between Rehren’s version and Maldecena’s version of AdS/CFT that makes a comparison between them less useful. This has to do with whether the special conformal transformations have anomalies or not. Rehren’s construction is rigorous but because special conformal transformations remain non-anomolous it does not apply to the context of Maldacena’s conjecture.

    One way to acknowledge difficiculties in arguments is not to trouble people with them again. So I wish you had noticed that the argument of that paper was not part of the book, and not made an issue of it.

    Further, in a few cases, such as the study of heavy ion collisions with AdS/CFT I have acknowledged that important things have happened since the book was finished.

    At the same time, it is also necessary that I discuss the extent to which these new results change the overall assessment of the promise of string theory to resolve the 5 major problems I gave in the book. And, for reasons I explained earlier they do not very much. This is because having a phenomenological model of QCD at high temperatures is not one of the five big problems that my book is about. I only discuss string theory there as a candidate for an answer to those questions, and if it happens that some aspects of string theory can help with another question that’s great-we are in the midst here of a very interesting workshop about that. But it does not obviously change the assessment of string theory in relation to the 5 big questions.

    My book was concerned only with our progress towards answering those 5 big questions. Most of phsyics is outside of that. If part of string theory is relevent to heavy ion collisions, wonderful, well worth working on. But I have heard no logical argument that this increases the likelihood that it is the fundamental theory of nature. Newtonian physics has many applications but it is not the theory of nature.

    Out of everything else, let me just quickly respond to one assertion:

    “I have recently attended a number of talks by leading workers in LQG, at a KITP workshop and the April APS meeting. I am quite certain that the standard of rigor was not higher than in string theory or other areas of physics. In fact, there were quite a number of uncontrolled approximations. This is not necessarily bad – I will also use such approximations when this is all that is available – but it is not rigor.”

    This does not acknowledge that in any subject the level of rigor is mixed. If you hear a talk by me you get a presentation of work with much less rigor than in a talk by Thomas Thiemann. In string theory as in other subjects there is a range of rigour.

    I made no claim that the subject of LQG has a uniformly higher level of rigor that string theory. I did claim that there are a collection of rigorous results proved by mathemtical physicists, and that they include existence and uniqueness theorems which anchor the foundations of the subject. This is important, first because it does put the subject of LQG on rigorous foundations, second because it refutes the impression that it is useless to require the presence rigorous results in justifying an approach to quantum gtravity.

    Thanks,

    Lee

  41. o says:

    Dear Lee,
    Will you address the first point of Joe’s response about the “fictitious prediction of a non-positive cosmological constant”?

    Thank you!

  42. Gina says:

    Dear non-of-the-above,

    Thank you for your comments on physics. You did not agree that the discussion is scientific. I see matter differently and let me try to explain my point of view. (Perhaps I should have referred to the discussion as “academic” rather than “scientific”.) Anyway, while the discussion has some components which are neither scientific nor academic it does have some scientific components which are, in my view, of value.

    You referred specifically to what Joe’s wrote

    “In my review I have gone through your book and papers and identified three proposals, and explained why each is wrong. Again, you have not acknowledged the existence of scientific counterarguments, but have just reasserted your original point. If your arguments had been made in a serious way, I would expect that you would have given some deep thought to them and be ready to defend them,”

    and concluded “That’s not science”. I beg to disagree especially on the example you have raised.

    Joe indeed described in his original book review (not in the main body of the text but in footnote [3]) his critique on two papers by Lee Smolin (one with Arnsdorf). Joe’s critique is quite concise and is given in plain English (with heavy physics accent but without any calculations.)

    That’s no science, you said; on the contrary, dear non-of-the-above: A critique offered by a well-known physicist on two papers written by other well-known physicists is a very nice and welcomed scientific contribution. (Even if not representing a cutting edge science.) These weblog discussions are quite a good place for such a critique to be made. (It is quite unlikely that Joe would have bothered to write a paper about his critique). It is especially important if Joe’s short remarks indeed definitely show that these (rather long and extensive) papers or some of their central ideas are flawed.

    Two additional points

    1) As it turned out, Lee also regards by now the argument in the paper with Arnsdorf as less convincing, and was not referring to it in his book. (Probably it could have been useful for Lee to acknowledge the weaknesses that he realized after publication in a new archive version of the paper or something of that kind and not just wait to a weblog discussion.) I do not know if Lee agrees to Joe’s specific points also regarding another large overview paper of him on quantum gravity.

    2) It is sort of interesting that Lee’s paper attracting such a scientific response (and in this case a critique which Lee may even agree with,) was a consequence not of the papers themselves but rather of Lee’s book and “anti-string” campaign.

  43. Lee Smolin says:

    Dear o,

    I will address it, but please keep in mind that I am in blog mode, I don’t have time as I usually do to check sources. Here is what seems to be not in dispute:

    -There was a problem in the period 1998-2003, there was an observed positive vacuum energy and none of the known solutions or vacua of string theory had that property.

    -Some people perservered trying to solve it.

    -It was solved in 2003 with the KKLT and following papers (in the context of a certain semiclassical approximation.)

    -A consequence was that there were 10^500 distinct vacua that solved the problem, to the same approximation.

    I don’t see how it can be in dispute that there was a problem in the first place, otherwise the KKLT paper would not have had the significance it did.

    What is the dispute about? Joe and I have different reminiscences as to how optimistic or pessimistic the people we read and talked with were about it. That is natural, it is usually the case with key unsolved problems. I can call on lots of evidence that some people saw this as a serious problem, even a crisis, Joe can call on lots of evidence that others were nontheless optimistic. There is no contradiction there, this kind of diverse opinion is, as I argue in the book, not only endemic in science but healthy and necessary for the progress of science.

    It is in fact a bit annoying that Joe claims I have my facts wrong when what is at issue is that different people had different judgements at the time.

    Joe if I recall correctly, in his first review argued that the problem was not believed to be serious in his community. He then argued it could not have been taken seriously because any solution would involve two further unsolved problems: supersymmetry breaking and moduli stabilization and people were working on them. Indeed, there were recent ideas about them using branes and dualities and these were being explored. In fact, this is how KKLT and others eventually solved the problem.

    Since the problem was solved, Joe’s optimism turned out to be more reliable than my pessimism in that period. That is not in dispute, any one who thinks it is misunderstands the role this episode plays in my book.

    I am interested in the story because it involves a repetition of a logic that we first encounter in Part I when I tell the story of the first higher dimensional unifications. The logic is that higher dimensional unifications lead to two big issues: instability of compactifications and freedom to choose many compactification topology and parameters. Recall that I emphasize that Einstein basically understood these were problems in the 1920s and the result at that time was his loss of interest in Kalaza-Klein theories.

    My point is that the same problem reappears in string theory, but in spades. The meaning of KKLT is that the cost of solving one of these problems (stablization) is to make the other problem (freedom and lack of predictibility) much worse.

    So the crisis in predictability that led Susskind et al to revive my proposal of the landscape has its routes in the basic issues of higher dimensional unifications- instability and increase in parameters-identified first by Einstein in the 1920s and confirmed in dramatic form by KKLT in 2003.

    The question is then what is the cost of this trade off. The conclusion I draw from this is that the cost of higher dimensional unifications is too high, because it results in a lack of predictability.

    Indeed, I should emphasize that the problem is never completely solved. The reason is that the kind of SUSY breaking involved raises the vacuum energy rather than lowers it, as in many other examples of symmetry breaking. Thus, solutions with positive vacuum energy with broken SUSY are generically unstable.

    So KKLT solutions with positive vacuum energy are all unstable, the issue is only for how long they remain excited. Applied to nature, this is either a bold or a foolish conjecture, time will tell. But it is not surprising that many worried that it could be pulled off.

    I hope this goes some way to addressing the issue.

    Thanks,

    Lee

  44. Doug says:

    RE: Polchinski [and Bousso]
    a – considering systems of planets within solar systems and these within galaxies, their concept of nested bubbles is very possibly correct.
    b – their concept of lowest energy levels may have to be modified to equilibrium energy levels.

    RE: Smolin
    c – LQG may be similar to loop planetary gravity which has only an axial perspective. If there are three spatial dimensions then there should also be sagittal and coronal perspectives tending to be sinusoidal because of helicity.
    d – spinfoam may have to be modified to twistfoam, adapting Penrose terminology,

  45. none-of-the-above says:

    Gina on May 24th, 2007 at 3:56 pm
    wrote:

    “You referred specifically to what Joe’s wrote

    “In my review I have gone through your book and papers and identified three proposals, and explained why each is wrong. Again, you have not acknowledged the existence of scientific counterarguments, but have just reasserted your original point. If your arguments had been made in a serious way, I would expect that you would have given some deep thought to them and be ready to defend them,”

    and concluded “That’s not science”. I beg to disagree especially on the example you have raised.”

    Dear Gina
    I believe that you have misinterpreted my point; perhaps I did not express myself clearly, and I’m sorry if my post was confusingr. My comment “That’s not science” was not directed at Polchinski’s post; even a popular explanation from Polchinski is science of a high standard. What I was referring to as not being science was that Smolin had not “acknowledged the existence of scientific counterarguments, but have just reasserted your original point.” When someone has refuted your arguments you don’t just ignore the refutation because you don’t like the conclusion. That’s not science.

    P.S. You also make the following comment:
    “2) It is sort of interesting that Lee’s paper attracting such a scientific response (and in this case a critique which Lee may even agree with,) was a consequence not of the papers themselves but rather of Lee’s book and “anti-string” campaign.”
    You need to put this in context. Every morning every working physicist in this field logs onto the “Los Alamos” archives [http://www.arxiv.org/], to get access to all the new papers appearing in the last 24 hours. On an average morning I read the titles and abstracts to the new papers in: hep-ph (particle phenomenology; average 20 papers), hep-th (formal particle theory; average 20 papers), gr-qc (gravitation and cosmology; average 10 papers), and astro-ph (astrophysics; average 40 papers). So on an average morning I read the titles and abstracts to somewhere between 50-100 papers. Of that group there are typically 2-4 papers which I download to read in their entirety (some I skim, others I will work through line by line over several days). Also, of that group there are typically at least 5-10 of them, on any given day, that after reading the abstract I think to myself “that doesn’t sound right”. I don’t download those papers to actually work through them to find out why they’re wrong; it’s a waste of time to read wrong papers, and with the volume of literature one needs to deal with every day time is very valuable. Wrong, or irrelevant, papers, don’t get extensively discussed and refuted, people just ignore them and read the good papers, of which there are already enough to eat up huge chunks of your time. So it’s not surprising that the paper of Arnsdorf and Smolin was so thoroughly ignored; that’s the way active workers in the field pass judgement. But when the approach to AdS-CFT duality proposed in that paper becomes the centrepiece of a popular book attacking string theory and seeking to undermine support for the field, then for the health of the field leading experts in the subject have to explicitly address why this work is wrong. This is a waste of Polchinski’s time; he is one of the world’s leading theorists, and one of the people whose papers I will automatically download and read if I see one on the archive in the morning. I would rather have Polchinski writing papers on his own original work, than wasting his time debunking dubious papers from lesser physicists. But by taking their attacks public in popular books Woit and Smolin have made such a non-response dangerous to public support for the field. That is why someone of Polchinski’s stature had to reply, and why in my first post above [Number 4] I wrote to thank Polchinski for his time and effort in doing this.

  46. Lee Smolin says:

    dear none-of-the-above,

    First, I did think about and respond carefully to Joe’s arguments. It is just not true that, I “have not acknowledged the existence of scientific counterarguments, but have just reasserted [my] original point.” Read what was actually written, please.

    For example, regarding the full quote in context, “In my review I have gone through your book and papers and identified three proposals,…” this is refering to proposals for alternative forms of AdS/CFT duality. Here we have apparently been talking past each other. My point is that the strong form cannot be regarded as proven or even well formulated, because it posits an isomorphism between two mathematical objects that are not yet precisely defined, and which have not been shown to exist. These are facts, I don’t see how anyone could disagree with them. There is no precise non-perturbative definition so far of either side of the duality. Therefor there can be no proof of the conjecture.

    Joe says that it is hard to find a weaker conjecture that accounts for all the evidence. Even if that were the case it doesn’t change the first point.

    I do suspect that there is a weaker conjecture that is consistent with all evidence, let me come back to that sometime when I have time to think it over carefully. Its a scientific question, more a subject for a paper than a blog entry.

    There is much else that Joe says that distorts the facts as I understand it, for example, “You raise the issue of the existence of the gauge theory…. First, Wilson’s construction of quantum field theory has been used successfully for 40 years. It is used in a controlled way by condensed matter physicists, lattice gauge theorists, constructive quantum field theorists, and many others. To argue that a technique that is so well understood does not apply to the case at hand, the scientific ethic requires that you do more than just say Not proven! Sociology! as you have done. You need to give an argument, ideally pointing to a calculation that one could do, or at least discuss, in which one would get the wrong answer.”

    The issue is where is the burden of proof and what Joe says here is shocking. IN physics or math the burden of proof is always on someone proposing a strategy to show that it works in detail.

    In fact serious attempts have been made to construct a lattice gauge theory with extended supersymmetry in 4d and all have so far failed. it is true that some progress has been made in lower dimensions, but in d=4 I have been told by experts that the issue is at best open and unresolved, after significant effort.

    The alternative is to try to use a lattice theory without supersymmetry and tune the theory so that it approaches a fixed point with superconformal symmetry. This is a strategy, which might or might not work.

    My assersion is only to point out that there is no non-perturbative construction or definition of N=4 SYM. Joe seems to say that the burden on proof is on me to show why there can be no such construction. I am sorry but the burden of proof is the opposite, it is one thing to argue for a plausible strategy to do something, but it is quite another to have followed that strategy to a definite result. Many well motivated strategies fail in mathematics and physics, so the burden of proof is on the person proposing it to actually carry it out. After all, if it were straightforward, than given all the thousands of papers written assuming the truth of this conjecture, someone would have done it.

    Now, back to Rehren one more time: regarding, “But when the approach to AdS-CFT duality proposed in that paper (i.e. Rehren) becomes the centrepiece of a popular book attacking string theory and seeking to undermine support for the field, then for the health of the field leading experts in the subject have to explicitly address why this work is wrong..”

    Where is this fantasy coming from that Rehren or my paper with Arndorf has anything to do with my book, let alone “becomes the centrepiece” of it. Did you both not read my book and not read what I wrote above?

    That paper is not mentioned anywhere in the book, for the reasons discussed. It is true that I chose not to discuss Joe’s discussion of that paper in his footnote, but for lack of time and space I indicated everything could not be addressed. Since that paper was not a part of the argument in the book Joe was critiquing I chose to ignore that. Now I have dealt with it.

  47. Amos Elberg says:

    Lee, this is from pages 153-54 of your book:

    “String theory could not explain why the cosmological constant was zero, but at least it explained why it was not a positive number. One of the few things we could conclude from the string theories then known was that the cosmological constant could only be zero or negative. I don’t know of any particular string theoriest who predicted that [it] could not be a positive number, but it was widely understood to be a consequence of string theory. The reasons are too technical to do justice to them here.

    . . .

    You can imagine the surprise, then, in 1998, when the observations . . . began to show that the . . . cosmological constant had to be a positive number. This was a genuine crisis, because there appeared to be a clear disagreement between observation and a prediction of string theory. Indeed, there were theorems indicating that universes with positive cosmological constant . . . could not be solutions of string theory.”

    Those are strong statements, and that argument is plainly the most persuasive thing in the book.

    You’re claiming the existence of both a) a consensus about a particular conclusion of string theory; b) a consensus that string theory was in trouble because of the positive cosmological constant. These are claims that should be verifiable, if not with certainty, then at least to the extent that you have a basis for them. Surely there must be SOME string theorist willing to say there was a consensus that the positive constant observation was at least a threat to the theory if not a crisis. I am not a defender of string theory, but I think the burden of proof is on you with this one, and it hasn’t been met yet.

  48. Gina says:

    Dear non-of-the-above,

    “What I was referring to as not being science was that Smolin had not “acknowledged the existence of scientific counterarguments, but have just reasserted your original point.” When someone has refuted your arguments you don’t just ignore the refutation because you don’t like the conclusion. That’s not science.”

    I understood what you meant. My comment was that Joe’s comment was a scientific remark exclusively made for these blog discussions, and these blog discussions are tailored for such remarks. I did not think it is significant that Lee did not respond to this particular point, and by now he did respond. My comment 2) was just an observation, not a critique of any kind, and putting aside these particular papers by Lee, I think your detailed description makes sense.

  49. Thomas D says:

    Has ‘dark-matter’ not read (for two examples) Brian Greene’s or Lisa Randall’s books?
    They are examples of publishers making a darn good deal out of positive and inspiring presentations of string theory.

    For you to then demand that Joe (or anyone else!) should write and publish a popular-level book to justify any statement they make about the state of the field is pathologically unreasonable and absurd. The fact that a publisher can make money out of a popularizing book has essentially no correlation with the correctness or usefulness of the statements made in it!

    Curiously enough there isn’t a single book which succeeds in popularizing LQG or its relatives. But I think this is not necessarily a bad thing, and can even be justified by the fact that it is harder to get a first intuitive physical picture of LQG than it is of string theory.

  50. nigel says:

    …Curiously enough there isn’t a single book which succeeds in popularizing LQG or its relatives. But I think this is not necessarily a bad thing, and can even be justified by the fact that it is harder to get a first intuitive physical picture of LQG than it is of string theory. – Thomas D

    This is the key to the problem: should mainstream hype be attacked or should rival theories be hyped like string theory? Professor Smolin has previously tried to popularise alternatives in books like Three Roads to Quantum Gravity, but such books fail (like academic books on loop quantum gravity, such as Rovelli’s), to really combat string theory.

    In his lecture series (available at Perimeter Institute website), Introduction to Quantum Gravity, Prof. Smolin does make the point very clearly that loop quantum gravity is an effective model for interactions between gauge bosons that carry energy and thus are a source of gravitation (i.e. gravitons carry a quantum gravity charge) as well as mediating gravitational interactions. Normally, the fact that gravitons should carry a gravitational charge makes their interactions crazy at high energy.

    The key fact is that you can get background independent (metric-less) general relativity from quantum field theory by summing interaction graphs in a Penrose spin network. That alone is impressive thing about LQG. What is interesting is that this result of LQG should apply to other things too; any field with gauge bosons which carry a charge. In the standard model, electromagnetism is mediated by uncharged photons, and this is supposed to be reason why electromagnetism is renormalizable. However, it might be the case that electromagnetism and gravity are related in a different way. For example, the U(1) part of the standard model might not be correct. It’s inelegant to explain with extra polarizations how an electric field can be either positive or negative if the gauge boson responsible is neutral. A simpler way would be to replace the standard model’s U(1) by a second SU(2), but this time with entirely massless gauge bosons: one positively charged (giving rise to positive electric fields), one negative (giving negative electric fields) and one which is neutral (giving rise to gravity). This scheme incorporates gravity into the standard model and makes predictions. Another option might be to modify the Higgs mechanism so that just a single SU(2) group produces both short range massive W+/- and W gauge bosons for weak interactions, and mass-less versions of those which mediate electromagnetism and gravity, respectively.