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Smolin letter to a string believer

  1. Aug 15, 2005 #1


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    Lee Smolin has done a fair amount of string research over the years and written a bunch of string papers. His most recent contribution is
    http://arxiv.org/hep-th/0503140 [Broken]
    A quantization of topological M theory
    I think this paper could be important and by itself could justify considering Smolin a string theorist even without his other string papers. Incidentally it has been cited by two other string theorists recently. Other Smolin work has also been cited by Dijkgraaf, Gukov, Neitzke, Vafa. In the 1990s Smolin was one of those who developed the idea of quantum gravity as a constrained topological field theory, an idea which (whether or not he is personally acknowledged for it) is now proving influential.
    But Smolin is not a string theory believer. Apparently this has the curious effect of excluding him from "the string community".

    In nonperturbative QG research the situation is somewhat different and faith seems less of a prerequisite for belonging to the community of scholars. One can contribute to the development of several QG theories without being a believer in any particular one.

    Anyway Smolin has noticed this distinction. some people do their work, or at least talk about it, as believers, and others don't. And he has written this letter on "cosmicvariance".
    --------quote smolin------

    Lee Smolin on Aug 15th, 2005 at 9:01 am
    Dear Clifford,

    Thanks for your piece on the landscape, which I agree is in a certain sense a very sensible view. At the same time, your stance is deeply puzzling to me. I’d like to take this opportunity to explain why, because I believe this is the core of the disagreement between those who consider themselves “string theorists” and those who like myself, remain outside the “string community”, in spite of doing some technical reserach on string theory.

    You seem to reason from an unstated premise, which is that, whatever happens, string theory will turn out to be relevent for the description of nature. Even in your closing, when you contemplate different possibilities, including that string theory is just “The Next Theory” you don’t mention the possibility that string theory will just be not relevent for nature. This is also evident in the reasoning in your paragraph: “The idea began to arise that maybe not everything about our universe is fundamentally computable in string theory… I would go as far as to say that it is perfectly fine for us to accept that this might true about string theory while still remaining extremely enthusiastic about it, given its remarkable properties.” You don’t consider the possibility that nothing will be computable in string theory because it is not the right theory.

    There are two logically possible styles of reasoning about string theory.

    Method A: ASSUME 1) that there is a real non-perturbative theory behind all the approximate caclulations and 2) that it is relevent for nature. Then interpret various results, having to do with dualities, the landscape etc given these asumptions.

    Method B: Look for evidence that the two assumptions of method A are true.

    One evaluates results very differently, depending on whether one uses method A or method B. There is nothing wrong with using Method A from time to time, so long as the assumptions are made explicit, and the risks that are thereby taken on explicitly acknowledged. One can learn things that will turn out be true about the theory, if 1) is true, or about nature, if 2) is true. But one cannot do science only or even mostly by Method A, no matter how promsing an idea may seem. What I find disturbing in your essay, and in many conversations with string theorists is that they reason by Method A but they do not state expliclty their assumptions. This puts me often in the uncomfortable situation, when discussing with a string theorist, of having to add, “but there is one more possibility, the theory might be wrong.”

    Many of us who seem fated to remain outside the “string community” are there becase we approach string theory by method B. We may, as I do, work sometimes on technical problems in string theory, motivated by our hope that evidence be uncovered that will show us whether assumptionss 1) and 2) are true or not. This leads to a different evaluations of results. For example, from the point of view of Method B work aimed to demonstrate the assumptions, such as attempts to prove conjectures like finiteness or the different dualities, is more highly valued than it seems to be by people whose work seems to grounded on the assumption that those conjectures are true.

    I should emphasize that we are not being unfair here. Most of those who work on other approaches to quantum gravity and particle phsyics approach our own theories through method B. If you come to the loops05 meeting--and you are very sincerely invited--you will find that we are at least as hard on our own approaches as we are on string theory. Observing both communities, what I see is an overemphasis on self-criticism in the non-string communities, and too much reasoninng with method A in the string community.

    Nowhere is the difference stronger than in the evaluation of the landscape results. From the point of view of method A, we are just following the theory to see where it leads. Since we assume beforehand that the theory is right, this is a worthy project.

    But from the point of view of method B, the failure to come up with any method to make falsifiable predictions, coupled with the failure to find a fundamental, fully non-perturbative formulation of string theory, both after many years of work by many smart people, count as evidence against asssumptions 1) and 2).

    I myself am drawn to the ideas of string theory, and I would be happy if they turn out to be true. But I believe an objective scientist must appraoch an untested theory by Method B rather than by Method A. The reason is that reasoning by Method A can lead to a situation where a large group of people come to irrationally believe in the existence of a theory they can neither construct nor test.

    Another way to say this is that it is more scientific to work on problems, presented by nature, rather than theories. If we commit ourselves too strongly to theories before they are confirmed by having survived many attempts to falsify them, we risk wasting lots of time and careers on ideas that turn out, beautiful as they are, to be false.

    Another consequence of Method A seems to be a lack of interest in other directions. Someone, perhaps Moshe, said on a blog recently that if there were good results on quantum gravity people would get excited and work on them. If by “people” was meant “string theorists” this is just not the case. There have been a continuous stream of significant, non-trivial results on several background independent approaches to quantum gravity over the last 20 years and the community of people who works on such approaches is growing fast. But we see very few string theorists taking an active interest in any of these approaches. If you think I exaggerate the significance of the results, come to loops05, or look at recent papers by the speakers there. Or just talk to someone in the field.

    The problem is that if you reason from Method A, you are bound to over-evaluate results in string theory, and under-evaluate results in alternative approaches, because you are already committed to one view being right.

    Perhaps you think I am being unfair in characterizing your reasoning in terms of methd A. So let me pose a question, “What would make you give up string theory? Is there a theoretical result, an experimental discovery, or the lack of such, that woud make you put your considerable talents in other directions?”

    Lest you think this is unfair, I know the answer for myself, for each of the several theories I work on, and can happily answer the same question, if needed.


    ---end quote---

    I would like to take a closer look at this letter of Smolin and some of the reaction to it on Cosmic Variance blog.
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  2. jcsd
  3. Aug 15, 2005 #2
    Thank you for this. Please post Clifford's reply if you eventually come across it.
  4. Aug 16, 2005 #3


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    According to my grapevine, Smolins
    http://arxiv.org/hep-th/0503140 [Broken]
    A quantization of topological M theory
    will be discussed by more than one guest speaker.
    Last edited by a moderator: May 2, 2017
  5. Aug 16, 2005 #4


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    thanks so much for passing along this bit of news, Chronos.
    you are referring to some conference (conferences have invited speakers plus other stuff sometimes like posters and parallel sessions) but I cant tell which conference you mean.

    there is Loops 05 this October
    there is Strings 05 that is over now, it was middle July.
    there are many other conferences and workshops, too many to mention.

    It would be very sensible to expect that paper of Smolin hep-th/0503140
    to be discussed at Loops 05 in October of this year (one of the invited speakers Robbert Dijkgraaf will almost certainly refer to it since he works in topological string theory)

    is by any chance the conference you are talking about Loops 05?

    (or, what would be incredible news, too much to hope for really, are you really talking about NEXT YEAR'S STRING conference? if your grapevine has advanced word about String 06, which I dont know even where it will be held, then you belong to a very advanced grapevine indeed!)
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  6. Aug 16, 2005 #5


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    I find it fascinating that Smolin has written a number of string papers and made what seems like a substantial contribution. Ironically, his may eventually be found to outweigh the contributions of some in "the string community" country club now carping at him.

    It's interesting to scan Smolin's string papers:
    I will pick some out of this list
    Please let me know if I have included any that are not concerned with stringy matters.

    1. hep-th/0507235:

    Title: The case for background independence
    Authors: Lee Smolin
    Comments: Latex, 46 pages, no figures

    2. hep-th/0503140:

    Title: A quantization of topological M theory
    Authors: Lee Smolin
    Comments: 20 pages, Latex

    10. hep-th/0401087:

    Title: String theories with deformed energy momentum relations, and a possible non-tachyonic bosonic string
    Authors: Joao Magueijo, Lee Smolin
    Journal-ref: Phys.Rev. D71 (2005) 026010

    18. hep-th/0212043:

    Title: BRST quantization of Matrix Chern-Simons Theory
    Authors: Etera R. Livine, Lee Smolin
    Comments: Latex, 8 pages

    20. hep-th/0201031:

    Title: Matrix models as non-local hidden variables theories
    Authors: Lee Smolin
    Comments: 25 pages, latex, no figures
    Subj-class: High Energy Physics - Theory; Statistical Mechanics

    22. hep-th/0106097:

    Title: A thermal instability for positive brane cosmological constant in the Randall-Sundrum cosmologies
    Authors: Stephon Alexander, Yi Ling, Lee Smolin
    Comments: 14 pages. The discussion on the relation between temperature and effective 4d cosmological constant is changed. References are added
    Journal-ref: Phys.Rev. D65 (2002) 083503

    23. hep-th/0106073:

    Title: The Maldacena Conjecture and Rehren Duality
    Authors: Matthias Arnsdorf, Lee Smolin
    Comments: 14 pages, 1 figure

    24. hep-th/0104050:

    Title: The exceptional Jordan algebra and the matrix string
    Authors: Lee Smolin
    Comments: LaTex 15 pages, no figures
    Subj-class: High Energy Physics - Theory; Quantum Algebra

    25. hep-th/0009018:

    Title: Holographic Formulation of Quantum Supergravity
    Authors: Yi Ling, Lee Smolin
    Comments: 30 pages, no figure
    Journal-ref: Phys.Rev. D63 (2001) 064010

    26. hep-th/0006137:

    Title: The cubic matrix model and a duality between strings and loops
    Authors: Lee Smolin
    Comments: Latex, 32 pages, 7 figures
    Subj-class: High Energy Physics - Theory; Quantum Algebra

    27. hep-th/0003285:

    Title: Eleven dimensional supergravity as a constrained topological field theory
    Authors: Yi Ling, Lee Smolin
    Comments: 15 pages+7, Appendix added
    Journal-ref: Nucl.Phys. B601 (2001) 191-208

    28. hep-th/0003056:

    Title: The strong and weak holographic principles
    Authors: Lee Smolin
    Comments: Latex, 53 pages, no figures
    Journal-ref: Nucl.Phys. B601 (2001) 209-247

    29. hep-th/0002009:

    Title: M theory as a matrix extension of Chern-Simons theory
    Authors: Lee Smolin
    Comments: Latex, 17 pages, no figures
    Journal-ref: Nucl.Phys. B591 (2000) 227-242

    30. hep-th/9910146:

    Title: Holography in a quantum spacetime
    Authors: Fotini Markopoulou, Lee Smolin
    Comments: 16 pages, LaTeX

    31. hep-th/9904016:

    Title: Supersymmetric Spin Networks and Quantum Supergravity
    Authors: Yi Ling, Lee Smolin
    Comments: 21 pages, LaTex, 22 figures, typos corrected and references completed
    Journal-ref: Phys.Rev. D61 (2000) 044008

    32. hep-th/9903166:

    Title: A candidate for a background independent formulation of M theory
    Authors: Lee Smolin
    Comments: Latex 46 pages, 21 figures, new results included which lead to a modification of the statement of the basic conjecture. Presentation improved
    Journal-ref: Phys.Rev. D62 (2000) 086001

    34. hep-th/9808192:

    Title: Towards a background independent approach to M theory
    Authors: Lee Smolin
    Comments: 21 pages, LATEX, no figures. Contribution to a special issue of Chaos, Solitons and Fractals on " Superstrings, M,F,S....Theory"

    35. hep-th/9808191:

    Title: A holographic formulation of quantum general relativity
    Authors: Lee Smolin
    Comments: The Lorentzian boundary conditions have been extended to a much more general class, more details given of quantization in the Lorentzian case
    Journal-ref: Phys.Rev. D61 (2000) 084007

    36. hep-th/9801022:

    Title: Strings as perturbations of evolving spin-networks
    Authors: Lee Smolin
    Comments: Latex, 18 pages, no figures
    Journal-ref: Nucl.Phys.Proc.Suppl. 88 (2000) 103-113

    37. hep-th/9712148:

    Title: Nonperturbative dynamics for abstract (p,q) string networks
    Authors: Fotini Markopoulou, Lee Smolin
    Comments: Latex, 12 pages, epsfig, 7 figures, minor changes
    Journal-ref: Phys.Rev. D58 (1998) 084033

    39. hep-th/9710191:

    Title: Covariant quantization of membrane dynamics
    Authors: Lee Smolin
    Comments: Latex, 21 pages, no figures
    Journal-ref: Phys.Rev. D57 (1998) 6216-6223

    40. hep-th/9703174:

    Title: Chern-Simons theory in 11 dimensions as a non-perturbative phase of M theory
    Authors: Lee Smolin
    Comments: 18 pages, Latex no figures, improved treatment of reduced sector

    44. gr-qc/9609031:

    Title: Three Dimensional Strings as Collective Coordinates of Four Dimensional Non-Perturbative Quantum Gravity
    Author: Lee Smolin
    Comments: 32 pages

    45. gr-qc/9505028:

    Title: Linking Topological Quantum Field Theory and Nonperturbative Quantum Gravity
    Author: Lee Smolin
    Comments: TEX File, Minor Changes Made, 59 pages
    Journal-ref: J.Math.Phys. 36 (1995) 6417-6455

    47. gr-qc/9405015:

    Title: The Chern-Simons Invariant as the Natural Time Variable for Classical and Quantum Cosmology
    Authors: Lee Smolin, Chopin Soo
    Comments: 32 pages, gr-qc/9405015, CGPG-94/4-1 (revised and extended, Oct. 94)
    Journal-ref: Nucl.Phys. B449 (1995) 289-316


    This list might still need winnowing to remove papers with no bearing on string theory, but you get the idea. Notice in the letter Smolin invites question about what would falsify string approach to QG or cause him to discard it as a research venture. Obviously he continues, for the time being, to pursue string AMONG OTHER qg OPTIONS. And he says he would be HAPPY to say, for each of the several theories he works on, what experimental or theoretical result would make him satisfied to discard that particular line of research. If I found myself at the same lunch counter with Smolin I might get my courage up and ask him what particular discoveries would rule string out for him.

    Actually (but this is off the subject a little) I think Lolls Triangulation approach has more promise of yielding a fully nonperturbative quantum theory of gravity----more than string anyway---because it constructs an entirely new kind of 4D quantum spacetime continuum.
    (mathematically very different at short scale from a 4D differentiable manifold). String spacetimes are stock differentiable manifolds, even with geometries fixed in advance. So Loll CDT breaks new ground compared to string, and has better prospects in my view. So, since Smolin ALSO works in CDT, I might ask him what theoretical or experimental discovery would convince him to give up THAT approach. The answer could be interesting.
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  7. Aug 16, 2005 #6


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    Here are some reactions. The names don't matter. I will just call them "second string theorist" and "third string theorist". To my ear THIRD sounded more collegial, and less sarcastic, than SECOND. Anyway here is a sample of the reaction to Smolin (whom I consider to be "first string theorist" and has already spoken)

    ----quote SECOND string theorist---
    Hi Lee,

    Thanks for your comments. And thanks for your valuable and insightful psychoanalysis of me and “the string community”.

    As a member of the scientific community, I tend to use the customs of my people: We confront scientific ideas ... I thought it was clear to everyone reading this article (and the one it is the sequel to) that this is one such scientific idea. Perhaps I was wrong.

    In my mentioning one approach to a problem, I did not realize that I have to stop and mention all the other approaches all the time. I just took it for granted that the intelligent reader (having seen the word “research” and other such phrases) can conclude that we don’t know the answers yet, and so we cannot know if we’re looking in the right place.

    I’m so sorry if I was not clear.

    Let me say it clearly for you and those people out there who you think might be confused:

    “Research” is here taken to mean that we don’t yet know the answers. By examining a set of ideas to see if they contain the answers, we are allowing for the possibility that those ideas do not contain the answers.

    I do apologize to anyone out there who was misled by my articles into thinking that the act of my describing a particular area of research implied that we already knew that it was the answer - before having completed the research.

    I thank you, Lee.


    Hi again Lee,

    I forgot one thing. You’re reminding us in your comments in several places that we are not as honest and hard on ourselves as those excellent people in your community. You mention “observing” our community.

    Where exactly do you make these observations? The last time I saw you at the kind of workshop where the really hard work on string theory is taking place -where the kind of self-questioning and examination of the whole endeavour is put into practice- was very very many years ago in the middle to late 90’s in Santa Barbara.

    Now, I’m not asking for an attendance sheet from you. Let me be the first to say that I’ve not been to all the workshops. Not even most of them. But then, neither have you. So are you sure that you should be making such pronouncements about the motivations, character, and scientific honesty of an entire community of people based on your “observations”? If so, your clarity of vision is ….. remarkable.


    ---end quote---

    ---quote THIRD string theorist---
    Lee was, most recently, at Strings 2005, and he does hang out with his stringy colleagues at Perimeter.

    I don’t know whether that counts for you, but it does for me.

    I also don’t know what the contours of the ultimate theory of quantum gravity will look like. But I do think that it is very likely that, whatever it turns out to be, it will have classical solutions about-which it is well-approximated by a perturbative string theory. (Which is not, necessarily, to say that our world is likely to be well-approximated by a perturbative string theory.)

    I could explain why I think that, but, instead, I will throw out what I see as the possible alternatives:

    1) quantum gravity is never perturbative, about any of its vacua
    2) it does possess perturbative vacua, but the short-distance behaviour is totally-different (non-stringy).

    Discuss and enjoy …
    ---end quote---

    ---SECOND string theorist again---


    Strings meetings are not the best measures of the activity that goes on in the field. They are reports on results. There’s never much time to have open discussions about what we’re doing, where we’re going, etc. This year’s strings did (admirably and unusually) try to have an official discussion, and that is good. But panel discussions are not what I’m talking about, I’m talking about workshops and smaller meetings, where the real work gets done.

    So if it “counts” for you, that’s fine. But I don’t think that going to a few meetings, (big or small) and having some colleauges that do string theory occupy the same building qualifies one to make broad comments about the scientific integrity and motivations of an entire scientific field. Not even most of the “insiders” of the string community would do that!

    ---end quote---

    I have several reactions to these voices, which represent for me a kind random sample of people doing string research. The identities are not important to the conclusions I want to draw. I see that one of them ("Third") sounds openminded and generous spirited. He wants to include Lee as one of "us" and he says Lee's participation (which he does not detail except in a light sketchy way) "COUNTS" for him.

    It would seem that for Third the "string community" is not seen as an exclusive country club closed except to right-thinking boosters who talk string happy talk and attend the proper workshops. The "community" seems to have room for others: for doubters perhaps, for loners pursuing the long-shot, and for those who offer constructive criticism of a prevalent attitude. Here I don't suggest that Smolin fits any of those descriptions but merely illustrate that Third's view of the "community" seems comparatively roomy.

    I don't want to imply that the actual people ARE the way they sound here.
    Third, of whom I get a good impression from the quote, might be different in real life. Second might really be a sweet guy, and so forth.
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  8. Aug 17, 2005 #7


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    Loops 05, marcus. Rumor has it should be very interesting. I don't know much about strings 06, but I do know Smolins BI paper has created a stir in the string community. Witten, for one, has even drawn a bit of fire from the right wing for commenting favorably upon it.
  9. Aug 22, 2005 #8


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    Smolin continues

    Posts #90 and #95 on the CV "Landscape" thread

    ------------SMOLIN REJOINS THE THREAD, POST #90-------------
    Lee Smolin on Aug 21st, 2005 at 5:30 pm
    Hi, if this thread is still alive, I have some comments on earlier posts:

    To Aaron on background independence:

    “Background independence and string theory are not in any stark disagreement.” If by this you mean that no one has proved there is no background independent formulation of string theory I agree. But this is very weak. If one exists, we should find it.

    ” I can’t help but think that to say that any quantum theory of gravity must be background independent smacks of hubris. Nature is going to work how it’s going to work. We certainly don’t get any say in the matter.”

    Would you say the same thing about the principle of inertia? That principle is extremely well tested, but it is still possible that the quantum theory of gravity will reverse it, and return to the Aristotelian idea of a preferred state of rest. But to take the principle of inertia as something to be preserved in the next theory is hardly hubris, it is good sense to keep well tested principles unless there is a strong reason to abandon them.

    But background independence has exactly the same status. It is extremely well tested at the classical level, because general relativity is background independent and its predictions are well confirmed. Were there not exactly two degrees of freedom, which is a consequence of diffeo invariance, the precise match between theory and experiment for the binary pulsar data would fail. Thus, it is only good sense to expect that this well tested principle be preserved in the next theory.

    To Sean on finiteness. I’d be curious what is your understanding of the status of perturbative finiteness. Your comment seems to imply that there is very strong evidence, just short of a rigorous proof. I wish this were the case, but this is not my understanding of the situation. Perhaps you can correct me, if so I would be very grateful. My understanding is that even at a theoretical physics level of rigor all claims fall short of a complete demonstration. Either they rely on additional assumptions or do not rule out all possible sources of divergences, as in Mandelstam and Berkovitz or they go only to genus two, as in d”Hoker and Phong. Thus, this is not a typical sistuation where we theoretical phyicists have a convincing argument, awaiting only translation into a rigorous theorem. As Feynman told me in 1976 (with regard to confinement in QCD), “If lots of really smart people have tried to prove something and failed, perhaps its because it is actually not true.”

    Do I personally think supersymmetric string theory is perturbatively finite? I certainly hope so, otherwise I for one have wasted a lot of time on an inconsistent theory. At the same time if after twenty years of work by really smart people, the experts on the technical issues involved have not found a complete demonstration, I don’t think we should be complacent. I would instead think we should present perturbative finiteness as a conjecture that there is some evidence for, but which remains open despite efforts to prove it. Perhaps if we emphasize that this important question is unsolved, this will motivate someone to do the hard work needed to complete the proof or find a counterexample.

    [several replies skipped]
    -----------SMOLIN CONTINUES. POST #95---------
    Lee Smolin on Aug 21st, 2005 at 10:12 pm
    Hi Aaron,

    You cannot seriously mean that “It is simply not the case, however, that it is even possible to do an experimental test of it.” There is a long list of confirmed predictions of GR, some to very high precision. These strongly constrain the parameters that measure possible deviations from GR-the post Newtonian parameters.

    I also can’t imagine you mean what you said: “GR has been extremely well confirmed in one particular background. There haven’t been any experiments in a different background that I know about….”

    The point of GR is that the full metric is dynamical. This means that the metric emerges as a solution of the field equations. So there is no metric that is put in to define the field equations, as there is for field theories on fixed backgrounds, such as Maxwell theory or free string theory. The statement that GR is background independent is, so far as the metric is concerned, equivalant to the statement that the metric is fully dynamical. A theory of a linearized tensor field on a fixed background is unable to reproduce all the predictions that GR successfully makes.

    So I just don’t know what you could mean by “GR has been extremely well confirmed in one background.” This is like saying, there is a particular Maxwell field that describes the E and B fields in the universe, which happens to be a solution to the Maxwell equations, but I don’t take that as a confirmation of the truth of the Maxwell equations.

    There is one solution that describes our world but it is not a background. That one solution is incredibly complex, as the observations of lensing and the precision solar system tests all show in detail, the geometry changes dynamically, partly to reflect the distribution of matter. It has no symmetries and structure on a huge range of scales. The solution that describes our world is one of a continuous infinity of such complex generic solutions. It has no other special status. There is no plausible explanation for this than that the metric of the world is a solution to the Einstein equations, at least within a certain regime.

    What I am arguing for is only that the correct quantum theory of gravity preserve the key feature of the well tested classical theory of gravity, which is that the spacetime metric is fully dynamical. This rules out theories like perturbative string theories or linearized spin two fields whose equations of motion require the prior specification of a fixed background metric.

    -------------JAQUES REPLY, POST #96----------
    Jacques Distler on Aug 21st, 2005 at 11:24 pm
    This rules out theories like perturbative string theories or linearized spin two fields whose equations of motion require the prior specification of a fixed background metric.

    That perturbation theory requires a choice of background metric (about which to perturb) is a tautology. It is equally true of perturbation theory applied to GR.

    Any consistent interacting theory of a massless spin-2 field must have (the full nonlinear) diffeomorphism invariance. Perturbative string theory is diffeomorphism invariant. It also has the aforementioned (and much-maligned) split into a background metric plus perturbation.

    It is hard to imagine a theory that has both full diffeomorphism invariance and a perturbative split into background metric plus perturbation, which is not, in fact, background-independent. If you can construct an example, that would be interesting.

    There are other arguments for background-independence in string theory; I’m sure I don’t have to repeat them.

    What we don’t have, in perturbative string theory, is manifest background-independence.

    I think we can all agree that manifest background-independence would be a nice thing to have. But, in a pinch, non-manifest background-independence will do.

    Personally, I’m not sure that the concept of “background independence” (as you are using the term) is even relevant to the problem of finding the fundamental formulation of string theory. “Background independence” merely requires a formulation in terms of the (full) dynamical spacetime metric. But, in string theory, there are solutions which admit multiple interpretations in terms of different spacetimes, with different spacetime metrics and different spacetime topologies.

    So, when one says, “… depends only on the (full) dynamical spacetime metric,” the relevant question is, “Which metric is that?”

    ---------------SMOLIN RESPONSE, POST #101-----------------

    Lee Smolin on Aug 22nd, 2005 at 9:06 am
    Dear Aaron and Jacques,

    I agree that there is evidence for the existence of a background independent string theory, but this is not the same thing as to construct it. I also agree that the most convincing evidence is from the fact that a necessary condition for the conformal anomaly to vanish is that the target space satisfies the vacuum Einstein equations, to leading order and the quantum effects renormalize the metric. As to what evidence makes me think that string theory doesn’t have a metric that is fully dynamical, one is the fact that we only know the precise amplitudes for supersymmetric strings to propagate and interact in backgrounds with timelike or null killing fields, which is to say we only know that perturbative superstring theory exists, in detail, for metrics that are non-dynamical.

    It may be that this limitation can be transcended. But it is good to be precise about what has been shown and what is still conjectured.

    I’m heartened that we all agree that there is enough probability that a background independent formulation of string theory exists that it is worth while looking for it. So lets move on to discuss what are good strategies to look for such a theory. I know of three,

    1) Construct an 11 dimensional Chern-Simons like theory. (Banados et al, Horava)

    2) Construct a background independent matrix theory, and derive the existing background dependent matrix formulations of string and M theory by expanding around solutions of it (hep-th/0002009,0006137,0104050, 0212043).

    3) Construct a background independent quantization of 11d supergravity (hep-th/0003285, with Yi Ling).

    I’d be curious as to your view of these approaches and what other ideas you have about how to go about finding the background independent formulation of string theory.
    --------END QUOTE--------

    For convenience, i have expanded the above arxiv references into links:

    2) Construct a background independent matrix theory, and derive the existing background dependent matrix formulations of string and M theory by expanding around solutions of it
    http://arxiv.org/hep-th/0002009 [Broken]
    http://arxiv.org/hep-th/0006137 [Broken]
    http://arxiv.org/hep-th/0104050 [Broken]
    http://arxiv.org/hep-th/0212043 [Broken]

    3) Construct a background independent quantization of 11d supergravity http://arxiv.org/hep-th/0003285 [Broken] with Yi Ling.
    Last edited by a moderator: May 2, 2017
  10. Aug 22, 2005 #9


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    Yep, it seems that Lee is looking for public discussion with people who is expected to understand string theory. In a first reply it seems that Aaron is refusing confrontation, or at least refusing to follow topic as Lee suggests it should be followed. Lets see Jacques.
  11. Aug 22, 2005 #10


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    Arivero, I was hoping that Jacques would engage more but stuff just ricochets off him. Or how do you see it? But Moshe has started to discuss with Smolin and he seems more interested in engagement of ideas.
    We are getting this realtime. Smolin just posted this about 15 minutes ago:

    --------SMOLIN POST #109-----
    Lee Smolin on Aug 22nd, 2005 at 6:49 pm
    Hi Moshe and Aaron,

    First, I agree that the focus on asymptotics is at least partly because it allows an easier class of observables to be studied. I also agree with Banks that different choices of asymptotic conditions define different classical phase spaces and hence distinct Hilbert spaces.

    In classical theories, whether GR, supergravity or topological field theories this is completely clear, and can be understood in detail. The reason is that the choice of asymptotic conditions break the diffeomorphism and gauge invariance on the boundary, and this gives rise to easy versions of local physical observables defined on the boundary. Furthermore, whenever there are boundary or asymptotic conditions, boundary conditions have to be imposed and boundary terms have to be added to the action, otherwise the action is not functionally differentiable and the field equations are not defined. These boundary conditions and boundary terms then become part of the kinematical specification of the phase space. There is no phase space that unifies them. If one follows standard quantization rules they lead to distinct quantum theories.

    There is also a subtle interplay between what is gauge and what are physical symmetries, whose details depend on the boundary terms and conditions. This leads to a clear separation between constraints that generate gauge invariances and observables such as energy, charge and momentum, that are defined by surface integrals at the boundary. There is more to say, but the best way to learn this stuff is to work carefully through the case of GR with different boundary terms and asymptotic conditions to see how it goes. One good, careful source is Ashtekar’s World Scientific book on the new variables. (btw the calculations are much simpler in Ashtekar variables than in ADM variables.)

    The analysis leads to an understanding that the formulations with boundary or asymptotic conditions are truncations of the full theory in which the added conditions reflect the presence of external sources and currents. These can be interpreted as coming from situations we observers set up, when we wish to isolate and study a part of the universe. If we then extend this wisdom from the classical theory to our conjectured background independent quantum theory we would conclude that the basic formulation will be the one without asymptotic or boundary conditions. Once this is in closed form, it is likely that models of isolated systems can be constructed, as in the classical theory, by imposing boundary or asymptotic conditions.

    Hence, I believe we only have to make one theory, and it is one without boundary or asymptotic conditions. If we can do this, we should be able to invent appropriate truncations to represent isolated systems, when needed.


    --------END QUOTE-----
  12. Aug 22, 2005 #11
    Is he just saying that background independence gives rise to a finite universe that can be observed and that is preferable ?
  13. Aug 22, 2005 #12


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    Reading prep for this thread

    to follow the discussion in this thread I guess the main homework prep would be to read the first 3 pages of the introduction section of
    http://arxiv.org/hep-th/0507235 [Broken]
    The Case for Background Independence

    these are pages 3, 4, and 5

    (pages 1 and 2 are just the abstract summary and table of contents, they can be skipped)
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  14. Aug 22, 2005 #13
    thanks Marcus

    having read that i think i can safely answer my own question which is...yes

    however it is not their discussion i was intent on following. The intent was to intitate our own discussion here.

    I think he made a good enough case for background independence just to fit the observables in 3d+time but he couldn't rule out the possibility of a background against which time and space is played out in
  15. Aug 22, 2005 #14


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    If you want a different discussion, please make a separate thread, Sam. My aim in this thread is focused on what Smolin is writing and I don't want to clutter it with side conversations like this.
  16. Aug 22, 2005 #15
    I do want the same discussion. You don't want to discuss what he is writing, just quote it ?

    Why then not just post the link to the source and join in the discussion there otherwise i don't see the point in posting up the transcript if not to inspire discussion in this thread ?

    kinda voyeuristiclly creepy

    "I think he made a good enough case for background independence just to fit the observables in 3d+time but he couldn't rule out the possibility of a background against which time and space is played out in"

    do you agree ?...discuss
  17. Aug 23, 2005 #16


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    See Marcus's post #10 on this thread. He does want to discuss, he just wants to hold down the empty speculation and BS. Those are not appropriate on this subforum. RTF paper and keep your discussion on topic and cogent.
  18. Aug 23, 2005 #17
    I did RTF paper and this is what i got from it...

    Smolin is basically saying deal with quantum gravity using what is here in our 3+1d universe in terms that fit what can be observed and tested without positing extra dimensions or conditions but allow for hidden variables that may crop up like non locality which means sorting out this time business for all of the theories as that is where they all break down

    he stops just short of positing extra dimensions to account for non locality in any hidden variable theory but it seems implied though not in a stringy compacted sense

    The paper raises more questions than it answers
  19. Aug 27, 2005 #18


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    I believe it is time to continue the story with another installment.

    To recap about Smolin, he is an interesting figure partly because by his research output he qualifies as a string theorist, with a track record for getting onto ideas early and having a nose for fruitful lines of investigation in several fields. Another asset he has is an outsider viewpoint, something that can be important at certain junctures.

    Recently he has posted this preprint: hep-th 0507235 "The Case for..." which could turn out to be influential. perhaps in combination with hep-th 0503140.

    The main thesis of "The Case for...", italicized on page 5, is actually a piece of advice to other string theorists, and can be freely paraphrased as
    try making it more background independent and see what happens.

    The context for this thread is that the stringtheoretical research program is in trouble, especially where there has been concentrated over-investment in that one approach instead of a diversified strategy. The crisis (which may well be resolved and prove only temporary) is shown by declining objective signs of vitality and by expressions of worry and defensiveness. We have other threads where this can be discussed and I shall not discuss it here. This thread is intended for readers who are already aware of the troubled string situation and interested in Smolin's ideas on what to do about it.

    Recently, on the CV blog, Smolin ADDED to his message to string theorists and contributed another piece of advice. This can be paraphrased as follows:
    "Don't assume string theory will turn out to describe nature." He finds that some, perhaps more than just a few, theorists have gotten into the mental habit of assuming that string theory is right and will eventually evolve into a successful, predictive, testable theory---empirical based. This assumption can distort one's thinking, like putting blinders on, he suggests. He particularly pointed out how it can make researchers less alert to developments on the margin, neighboring quantum gravity fields, essentially a case of complacency dulling periferal vision.

    One can like something a lot (as Smolin string) and find the ideas deeply appealing, yet still remain skeptical. (Of course this part of the message is not unique to Smolin but is a time-honored part of good science.)

    OK so these ideas come up in the little multipart "essay" or "letter" on CV blog. But they are scattered around in a huge number of other posts and by now almost impossible to find. You can't link to individual comments.
    So I have gathered them together, and I guess it is time to add another chapter.

    If you have physics questions of a general nature, feel free to start a separate thread---this will then not distract from our focus on this Smolin "essay".
  20. Aug 27, 2005 #19


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    the next part is written in an oblique style, I take it this is because the author is making a gigantic effort not to ruffle anyone's feathers and not to appear to blow his own horn, these being things YOU CAN DO IN A BOOK but are risky to do on someone else's blog

    interesting to notice how media can affect style

    quite a lot of water under the bridge since the last installment, and the conversation moved over to another thread.

    courteous restraint so predominates as to make the piece seem written in a kind of "code". I guess I will try, in a little while, to decode some. but first I will just copy in the text:

    ------quote Smolin on CV blog------
    ... I am happy also to acknowledge that the main points you have been arguing for are correct: there are critics of string theory who are not as informed as they should be, and they do under estimate the breadth and variety of views and approaches within the string community.

    But I would ask to impose just a bit more on your hospitality here to say something I think is terribly important, that exchanges on this and other blogs have illustrated. Let us call the range of views that can be heard from people who live in the string community R. My observation is that, however wide this is, it is a proper subset of another set P, which is the possible range of views that could reasonably be taken on various issues, which are as well supported by the evidence from calculations and from nature as are the views in R. I will call the views in P but outside R, O. I also observe that among those who are considered outsiders by string theorists, there are some who are in fact familiar with many of the technical details, and who could, and in some cases do, contribute research to string theory. Let me call such people the “competent outsiders”. Now here is what I think is so important:

    -One draws different conclusions about the possible futures for string theory, given views in O than in R. Thus, it matters a lot for physics, which views are correct. The importance of the competent outsiders is that they sometimes hold views in O.

    -Occasionally a view is moved from O to R. One illustration of this was the significance of 11 dimensional supergravity and the 11d supermembrane for string theory, which was moved from O to R in 1995.
    Another example is the view that the connection between string theory and nature will involve a landscape of equally possible theories rather than a unique, single theory, which was moved from O to R in the last few years. In these and other cases there were competent outsiders who had been insisting on the importance of these views, that were not listened to by insiders.

    -These examples illustrate the importance of O, as it can have a big effect on the direction of research when a view is moved from O to R.

    -Nevertheless, at any one time, insiders are often not aware of the views currently in O or, if they are, they do not accord them much interest. Even when a view is moved from O to R, some insiders think of it as a new invention and do not appreciate that the view has been held for some time by competent outsiders.

    -There remain views in O that may still turn out to be important. Among these I would put the fact that there are a range of possible versions of the AdS/CFT conjecture allowed by the evidence and the view that string theory cannot succeed unless a truly background independent formulation is found.

    So, while I agree that the breadth of views in R, held by insiders is sometimes unappreciated, I hope you can appreciate that some outsiders have a valid point when they remark that the range of views allowed by the evidence is wider than that usually heard within the string community.

    These stories raise several questions for me, that I am thinking about as I try to write a kind of intellectual history of the subject. First, wouldn’t it have been better if the views I mentioned that moved from O to R had been all the time included within the range of views discussed and considered by insiders? Does this imply that progress would be faster if the range of views considered seriously by string theorists were broadened?

    There are also some general questions about how science works. Is the situation I’ve described common, or is it special to string theory? Is R always narrower than P, and why? Is there commonly a class of competent outsiders? Related to this, why is R at any one time narrower than P? And why are insiders sometimes not aware of, or dismissive of views in O? Are strong divisions between research programs, such as we see between the different approaches to quantum gravity, generally good or bad for the progress of science?

    ---end quote---
    Last edited: Aug 27, 2005
  21. Aug 27, 2005 #20


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    people with a knowledge of the classics may detect echos of the Gilbert and Sullivan number, "If I were fortune, which I'm not," from the Mikado.

    We must ever strive to be worthy of the ancients.

    Even though obliquely worded, the piece may still have been a satisfaction to write, just to get it down in pixels. I will put a key here, for convenience:

    R: range of viewpoints inside a loyal support group or research "community".

    P: possible range of intelligent informed views-----P contains R

    O: competent outside viewpoints --- R is properly contained in P and O = P \ R

    Notice that Smolin indicates that one of his projects is some INTELLECTUAL HISTORY writing, maybe a book or a longer essay like his "Scientific Alternatives to the Anthropic Principle". I would guess that the topic is essentially "the String Adventure of recent decades and what we can learn from it about conducting science"

    he has been well positioned to observe, chronicle, and draw conclusions, so it might be an interesting book
    Last edited: Aug 27, 2005
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