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Smolin's answer to Polchinski

  1. May 26, 2007 #1


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    The most recent activity in the Smolin/Polchinski discussion was 24 May (a couple of days ago) and again today. It's apt to get buried in the thread of comment at CV, so I will paste it here. Also it's easy to lose track of how many steps there've been in the discussion.

    1. Smolin wrote the book, TWP appeared September 2006
    2. Polchinski offered a defense of string in a December 2006 "guest" post at CV, rebutting statements made in TWP
    3. Smolin responded in April 2006, clarifying and emphasizing points of his message here
    http://www.thetroublewithphysics.com/Response to Polchinski.html
    and in the comments thread at CV.
    4. Polchinski countered with a second, May 2007, guest post at CV
    5. Smolin responded 24-26 May, in the comments to that post.
    These are comments #40, 43, 46, and 53 at the CV thread. Of these, the last three are discussion with other posters some of which (like #53) is pretty interesting in its own right and not directly part of the back-and-forth with Polchinski. The one directed to Polchinski is #40, which I will paste here:

    Lee Smolin on May 24th, 2007 at 2:06 pm

    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.


    Last edited: May 26, 2007
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  3. May 26, 2007 #2


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    another interesting comment on the CV thread

    This comment just went up at CV, it is #53
    It is pretty interesting, I think, and has some bearing on the discussion with J.P.
    There is also a long quote from Witten here:
    Lee Smolin on May 26th, 2007 at 10:28 am

    Dear Amos,

    Exactly who thought what and how many worried or were confident is an interesting question for an historian of science. However, I have a strong memory of worrying about this after conversations with string theorists and after reading Witten’s paper hep-th/0106109,written in 2001, roughly halfway through the period in question, 1998-2003 . I can recall lots of people I talked to were worried. In the same period I was giving talks and writing papers about the implications of positive lambda in LQG, partly because they are beautiful and partly to show that LQG could incorporate positive Lambda, while string theory could not. I always made that point in talks on the subject and was never challenged.

    But, to answer your question with an example, lets use Witten, whose writing is pretty unambiguous. Here is more of the paragraphs I quoted in TTWP:

    However, an important no go theorem [6,7] says that there is no classical compacti cation of ten- or eleven-dimensional supergravity to de Sitter space of any dimension. This means that there is no classical way to get de Sitter space from string theory or M-theory. By a “classical” compactification, I would mean a family of compactifications in which G (n2)=n becomes arbitrarily small and a supergravity or string theory description becomes arbitrarily good.
    The no go theorem means that this does not exist.

    In fact, classical or not, I don’t know any clear-cut way to get de Sitter space from string theory or M-theory. This last statement is not very surprising given the classical no go theorem. For, in view of the usual problems in stabilizing moduli, it is hard to get de Sitter space in a reliable fashion at the quantum level given that it does not arise classically. (For an analysis of a situation in which most moduli can be stabilized, leading in the large volume limit to a nonsupersymmetric vacuum with Lambda= 0, see [8].)

    The absence of a classical de Sitter limit suggests that the possible values of N in string/M-theory are sporadic, rather than arising from in nite families, and that there might be only finitely many choices. If the number of choices is finite, I would not personally expect it to be possible to get N > 10^10^100. But de Sitter space with such large N is needed to agree with the most obvious interpretation of recent astronomical data.

    Now it turns out that at least at the semiclassical approximation KKLT work, Witten’s expectations appear to have been at least partly wrong, because N >> 10^500 seems to exist and to be sufficient. Of course, we don’t know whether there is a real consistent perturbative string theory past the semiclassical level of that calculation, so he could turn out to be right in the end.

    I don’t know why so much is made of this, as to the extent to which this was a crisis for string theory, it was, at least at leading approximation solved by 2003, by the KKLT paper building on ideas and techniques Joe and others introduced. So this is a strory that comes out positively for string theory and the optimists among the string theorists. As I said before, I told the story to emphasize the implications of how the problem was so far solved.

    Lets put it another way. The positive lambda string theory landscape was found in 2003. It has seriously puzzling implications. Were there another way to get positive lambda in string theory the crisis of the landscape might be avoided. But there is none known. Therefore there was no string theory compatible with positive lambda before 2003. But lambda was measured to be positive in 1998. Therefore there should have been a crisis.

    The right thing to say,which is not what Joe is saying is, ‘we were very worried that the theory might be wrong, because it disagreed with data, and so we worked hard and solved the problem and saved the theory from otherwise having to be abandoned.” To the extent that string theorists want to claim there was no crisis in 1998-2003 they are implicitly claiming that disagreement with data is not a reason to suspect a theory is not true.
    Last edited: May 26, 2007
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