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Saturday, December 02, 2006

Finiteness of String Theory and Mandelstam



It might be that the laws change absolutely with time; that gravity for instance varies with time and that this inverse square law has a strength which depends on how long it is since the beginning of time. In other words, it's possible that in the future we'll have more understanding of everything and physics may be completed by some kind of statement of how things started which are external to the laws of physics. Richard Feynman



I was lead into this subject of Quantum Gravity, by Lee Smolin's book called, "Three Roads to Quantum Gravity." As a lay person reading what our scientist's have to say, I have a vested interest in what can start one off and find, that changes are being made to the synopsis first written. Did I understand his position correctly from the very beginning? I'll have to go back over my notes.

But with this format now I have the opportunity to...ahem... get it..directly from the horses mouth(no disrespect intended and written based on knowing how to read horses). As I said, I tried early on to see how the situation of string theory could be refuted. I "instigated" as a comparative front for Lubos Motl and Peter Woit to speak from each of their positions. I had to disregard "the tones" set by either, as to the nature of whose what and how ignorant one might be, and comparatively, one might be to intelligent design? To get "some evidence" of why string theory might not be such a good idea?

Now I believe this is a more "civil situation" that such a format has been proposed and that Lee Smolin can speak directly. As well as, "further information" supplied to counter arguments to Lee's position.


A sphere with three handles (and three holes), i.e., a genus-3 torus.


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.

What’s certainly not clear (since you asked a very technical question, you will forgive me if my response is rather technical) is that, beyond 3 loops, the superstring measure over supermoduli space can be “pushed forward” to a measure over the moduli space of ordinary Riemann surfaces. It was a nontrivial (and, to many of us, somewhat surprising) result of d’Hoker and Phong that this does hold true at genus-2 and -3.


Just a reminder about my skills. While I do things like carpetry, plumbing, electrical, I do not call myself a Carpenter, a Plumber or a Electrician. Nor shall I ah-spire to be more then I'm not, as I am to old this time around.

Greg Kuperberg:
The string theorists are physicists and this is their intuition. Do you want physical intuition or not?

Okay, Smolin is also a physicist and his intuition is radically different from that of the strings theorists. So who is right?


Yet, least I not read these things, can I not decipher "the jest" while it not being to technical? Shall I call it a Physicists intuition or I will only call my intuition what it is?

Jacques Distler:
When most people (at least, most quantum field theorists) use the term “finiteness,” they are referring to UV finiteness.


While the things above talked about from Jacques are served by hindsight, "the jest" follows what comes after this point.

The Jest of the Problem?

My present research concerns the problem of topology changing in string theory. It is currently believed that one has to sum over all string backgrounds and all topologies in doing the functional integral. I suspect that certain singular string backgrounds may be equivalent to topology changes, and that it is consequently only necessary to sum over string backgrounds. As a start I am investigating topology changes in two-dimensional target spaces. I am also interested in Seiberg-Witten invariants. Although much has been learned, some basic questions remain, and I hope to be able at least to understand the simpler of these questionsStanley Mandelstam-Professor Emeritus Particle Theory


Gina has asked questions in context of "academic excellence" in relation to what is being seen in relation to string theory. Of course we thank Clifford for providing the format for that discussion.

The Trouble With Physics,” by Lee Smolin, Index page 382, Mandelstam, Stanley, and string theory finiteness, pages 117,187, 278-79, 280, 281, 367n14,15

For reference above.

Gina:
I raised 16 points that I felt Lee’s arguments were not correct or problematic. This is an academic discussion and not a public criticism, and I truly think that such critique can be useful, even if I am wrong on all the 16 points.

Three of my 16 points were on more technical issues, but I feel that I can understand Lee’s logical argument even without understanding the precise technical nature of “finiteness of string theory” (I do have a vague impression of what it is.) I think that my interpretation of this issue is reasonable and my critique stands.


I find this interesting based on what information has been selected to counter the arguments that Lee Smolin used to support his contentions about what is being defined in string theory.


Stanley Mandelstam Professor Emeritus Research: Particle Physics
My research concerns string theory. At present I am interested in finding an explicit expression for the n-loop superstring amplitude and proving that it is finite. My field of research is particle theory, more specifically string theory. I am also interested in the recent results of Seiberg and Witten in supersymmetric field theories.


So of course, here, I am drawn to the content of his book and what is the basis of his argument from those four pages. I hope my explanation so far summarizes adequately. For the lay person, this information is leading perspective as to the basis of the argument.

Lee Smolin:
Perturbative finiteness is a major element of the claim of string theory as a potential theory of nature. If it is not true then the case for string theory being a theory of nature would not be very strong.

-Perturbative finiteness has not been proven. There is evidence for it, but that evidence is partial. There is a complete proof only to genus two, which is the second non-trivial term in an infinite power series, each term of which has to be finite. The obstacles to a complete proof are technical and formidable; otherwise we would certainly have either a proof or a counterexample by now. There is some progress in an alternative formulation, which has not yet been shown to be equivalent to the standard definition of string theory.

-This is not an issue of theoretical physicists rigor vrs mathematical rigor. There is no proof at either level. There is an intuitive argument, but that is far from persuasive as the issue is what happens at the boundaries of super-moduli space where the assumption of that argument breaks down. In the formulation in which there is a genus two result it is not clear if there is an unambiguous definition of the higher order terms.

Is string theory in fact perturbatively finite? Many experts think so. I worry that if there were a clear way to a proof it would have been found and published, so I find it difficult to have a strong expectation, either way, on this issue.


It should be known here and here that all along I have been reacting to Lee Smolin's new book. The title itself should have given this away?

The explanation of scientific development in terms of paradigms was not only novel but radical too, insofar as it gives a naturalistic explanation of belief-change. Thomas Kuhn


So of course knowing the basis of my thought development is a "good idea" as the links show what spending our dollars can do, having bought what our good scientist Lee Smolin has written.

There is a little "tit for tat" going on right now, but I think the point has been made sufficiently clear as to where Gina's thoughts in regards to the points on Finiteness is being made beyond 2?

In these lectures, recent progress on multiloop superstring perturbation theory is reviewed. A construction from first principles is given for an unambiguous and slice-independent two-loop superstring measure on moduli space for even spin structure. A consistent choice of moduli, invariant under local worldsheet supersymmetry is made in terms of the super-period matrix. A variety of subtle new contributions arising from a careful gauge fixing procedure are taken into account.


Yes I think I have to wait now to see if the discussion can now move beyond the first three points raised? Hopefully Lee will respond soon?

How do you fight sociology

Because this by any of the leaders of string theory. it was left to someone like me, as a quasi "insider" who had the technical knowledge but not the sociological commitment, to take on that responsibility. And I had done so because of my own interest in string theory, which I was working on almost exclusively at the time. Nevertheless, some string theorists regarded the review as a hostile act.

The trouble with Physics, by Lee Smolin, Page 281


I have discovered one of Lee Smolin's objection to a string theorist. They are only craftsman, and not seers.

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