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Europe's most influential string theorist has an overview piece in Nature

  1. Oct 19, 2007 #1


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    The current (18 October) issue of Nature
    in the "News and Views" section has this piece by Hermann Nicolai

    String theory: Back to basics p797
    Hermann Nicolai
    Summary: "Long touted as a theory of everything, it seems that string theory may at last succeed as a theory of something very specific — the interactions of particles under the strong nuclear force."

    Hermann Nicolai strikes me as a modest person who would find it awkward to think of himself as Europe's most influential string theorist. :smile: But I think that is simply how it is. He is the Director of the Unified Theories wing of the Albert Einstein Institute at MPI Potsdam.
    He and Roy Maartens are chief co-editors of the journal GRG. He partnered with Abhay Ashtekar in organizing the October 2004 Strings Meet Loops conference at the Potsdam Max Planck Institute (AEI).
    He is a key person on the directorate of the European Science Foundation Quantum Geometry/Gravity support network. And I'm sure I'm just scratching the surface.

    The fate of a lot of research depends on his judgment---so his vision of how things are going is part of the fundamental physics research climate.

    I remember Urs Schreiber reporting from some conference in Germany in 2004 and it was clear that for him (the smartest postdoc string theorist in the String Coffetable lineup) his conversation with Nicolai was the most important chat he had at the conference.

    If anybody can think of a Euro string-person of equal leadership standing, or wants to correct what I said, please say. Tell me somebody else. This is just my impression from observing the scene for a few years.

    I can't read Nicolai's opinion/perspective piece in this week's Nature, because i haven't a subscription. I'd be glad to see exerpts from it, if anybody has a subscription and wants to share. It could very well not be earthshaking at all, just a measured balanced mainstream Euro-string viewpoint. I don't care if there is something new in it or not--I'm interested to hear anything overview H.N. says.
    Last edited: Oct 19, 2007
  2. jcsd
  3. Oct 19, 2007 #2
    What you're referring to is known as the AdS/QCD correspondence. The idea is to find a dual description of QCD. Just another great reason to pursue string theory. It’s tentacles are everywhere.
  4. Oct 19, 2007 #3


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    well, the problem is that it sounds as "lets use general relativity to explain particle theory". I can understand that a mathematical theory can be applied in different fields, and I can understand that a physical framework as Lagrangian Mechanics or Fluid Dinamics or, even, Quantum Field Theory, can be applied in different problems. But this upgrade of string theory towars a basic mathphys framework, it is strange. See, nobody says "Lagrange Mechanics is the true theory of planetary trajectories". They say "we can use Langrange Mechanics to describe planetary trajectories".

    The point is that string theory is very unique: 10 dim, some aspects of a unique M-theory, etc... it is very surprising to have a theory claimed to be so rigid and so flexible at the same time.
  5. Oct 19, 2007 #4
    That's what's so great about it.
  6. Oct 19, 2007 #5


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    Has anybody read the paper?
  7. Oct 19, 2007 #6


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    Really? Most newgroups are filled of posts with the same claim.
  8. Oct 19, 2007 #7
    If you accept that a purely gravitational theory can have a holographic description in terms of a pure Yang-Mills theory, the possibility of an AdS/QCD correspondence shouldn't be too shocking.

    Not “flexible”, “powerful”: A theory of everything virtually by definition should be both rigid and comprehensive.
    Last edited by a moderator: Oct 19, 2007
  9. Oct 20, 2007 #8


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    Marcus, I read the paid content, and it does not go beyond the abstract: it is basically a reference list of recent results.

    And if you accept that Hooke law relates distance to force, it is not shocking to notice that it generates elliptical orbits. But it fails to meet Kepler laws.

    String theory uses the mathematics of fields and forces, with an extra emphasis on unique or peculiar structures, so it keeps finding some results relevant to fields and forces and peculiar structures. It is not shocking, it is so elementary that it is almost irrelevant. Following along the lines of Hooke laws, if you expand any classical potential and it happens that the linear term can be ignored, then you have the quadratic term and it produces Hooke law and then the harmonic oscillator. But we do not start crying the harmonic oscilator is a theory of everything.
  10. Oct 20, 2007 #9
    This is nonsense. Hooke's and Keplers laws are really just two different applications of Newtonian physics to two completely different physical systems: There is no Hooke/Kepler duality. The gauge/gravity correspondence says that a purely gravitational system can have a precise holographic dual in the form of a system without gravity. They are thus different descriptions of the same system.
    Last edited by a moderator: Oct 20, 2007
  11. Oct 20, 2007 #10


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    That was the idea.
  12. Oct 20, 2007 #11
    Some excerpts follow.

  13. Oct 20, 2007 #12


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    Thank you Christine, that was kind of you!
    With someone like Nicolai, I really want to see exactly what he says and what he does NOT say---get a sense of the wind and current (as he may feel them).
  14. Oct 20, 2007 #13
    What I meant was - and I think you knew this - that your analogy was nonsense. My point still stands.
  15. Oct 20, 2007 #14
    It's great to see you're changing your tune about string theory. Am I misinterpreting your remarks? Perhaps for you, the royal road to strings as a theory of gravity is through what it says about the other interactions. Does lqg say anything interesting about QCD?
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  16. Oct 20, 2007 #15


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    I'm actually a fan of Nicolai's, for reasons I won't enumerate at the moment. I am going to do some manicure on the exerpt that Christine gave us, so as to make it more readable.

    ==exerpts from 18 October issue of Nature==

    Whether string theory can live up to its claim of being a ‘theory of everything’, and whether it will ever produce a falsifiable prediction as such, remain hotly debated questions. Meanwhile, developments in a quieter side-alley[1–8] indicate that the theory might be about to deliver something of its original promise: helping us to understand the physics of interactions mediated by the strong nuclear force. String theory was born in the 1960s, (...)

    But initial attempts to describe the forces between the quarks, and why they form the bound states they do, failed miserably. So particle physicists started casting around for other ways of attacking the problem. In 1968, the Italian theoretician Gabriele Veneziano made a brilliant guess [9] and wrote down a concrete mathematical expression, the Veneziano amplitude, that explained some important features of high-energy scattering. But his formula could not be understood in terms of point-like particles; instead, it required the existence of extended objects — strings. (...)

    The arrival in the early 1970s of quantum chromodynamics (QCD), the quantum-field theory of the strong interaction, dealt the final blow to these early attempts to understand nuclear physics in terms of string theory. But, unfortunately, QCD is incredibly complex. (...) In this ‘perturbative’ regime, we understand (at least in principle) how to work with QCD. But for the strong coupling that occurs over larger distances, one has to resort to computer-simulation techniques, known as lattice QCD. (...)

    The new approach that revives the link to string theory first suggested itself in 1998, when Juan Martín Maldacena conjectured[12] a link between a close relative of QCD and a ‘superstring’ living in a ten-dimensional curved space-time. (...) The Maldacena conjecture raised a lot of interest, but seemed for a long time to be quantitatively unverifiable. (...)

    Help came from an entirely unexpected direction. Following a prescient observation[13], the spectrum of the N = 4 theory has been found[1,2] to be equivalently described by a quantum-mechanical spin chain of a type discovered by Hans Bethe in 1931 when modelling certain metallic systems. (...) Indeed, even though the mathematical description of the duality on the string-theory side is completely different from that on the condensed-matter side, a very similar, exactly solvable structure has been identified here as well[3–5]. Puzzling out the details of the exact solution is currently an active field of research. (...)

    Just recently, Beisert, Eden and Staudacher[8] have extracted the analogue of this observable on the field-theory side, and have been able to write down an equation valid at any strength of the coupling. Since then, work has established that their ‘BES equation’ does indeed seem, for the first time, to offer a means of reformulating theories such as QCD as string theories. Much still needs to be learned from this one exactly solvable case. There is justifiable hope that this solution will teach us how to go back to the physically relevant case of QCD and finally arrive at the long-sought dual description by a string theory. It may even take us closer to realizing the quantum-field theorist’s ultimate dream, unfulfilled for more than 50 years: completely understanding an interacting relativistic quantum-field theory in the four space-time dimensions that we are familiar with. Progress towards this goal can be judged independently of loftier attempts to use strings in the construction of a theory of everything.

    Hermann Nicolai is at the Max-Planck-Institut für Gravitationsphysik (Albert-Einstein-Institut), Mühlenberg 1, D-14476 Potsdam, Germany.

    1. Minahan, J. A. & Zarembo, K. J. High Energy Phys. 0303, 013 (2003).

    2. Beisert, N., Kristjansen, C. & Staudacher, M. Nucl. Phys. B 664, 131–184 (2003).

    3. Bena, I., Polchinski, J. & Roiban, R. Phys. Rev. D 69, 046002 (2004).

    4. Kazakov, V. A., Marshakov, A., Minahan, J. A. & Zarembo, K. J. High Energy Phys. 0405, 024 (2004).

    5. Arutyunov, G., Frolov, S. & Staudacher, M. J. High Energy Phys. 0410, 016 (2004).

    6. Gubser, S. S., Klebanov, I. R. & Polyakov, A. M. Nucl. Phys. B 636, 99–114 (2002).

    7. Frolov, S. & Tseytlin, A. A. J. High Energy Phys. 0206, 007 (2002).

    8. Beisert, N., Eden, B. & Staudacher, M. J. Stat. Mech. P01021 (2007).

    9. Veneziano, G. Nuovo Cimento 57A, 190 (1968).

    10. Ramond, P. Phys. Rev. D 3, 2415–2418 (1971).

    11. Neveu, A. & Schwarz, J. H. Nucl. Phys. B 31, 86–112 (1971).

    12. Maldacena, J. M. Adv. Theor. Math. Phys. 2, 231–252 (1998).

    13. Lipatov, L. N. preprint available at www.arxiv.org/abs/hep-th/9311037 (1993).

    14. Zaanen, J. Nature 448, 1000–1001 (2007).

    NATURE|Vol 449|18 October 2007NEWS & VIEWS

    Most of Nicolai's references are to papers already several years old. There is one 2007 paper that plays a pivotal role in what he has to say, the BES. I will put it here for convenience of anyone who wants to check it out as well:

    Transcendentality and Crossing
    Niklas Beisert, Burkhard Eden, Matthias Staudacher
    31 pages
    (Submitted on 23 Oct 2006 (v1), last revised 14 Nov 2006 (this version, v2))

    "We discuss possible phase factors for the S-matrix of planar N=4 gauge theory, leading to modifications at four-loop order as compared to an earlier proposal. While these result in a four-loop breakdown of perturbative BMN-scaling, Kotikov-Lipatov transcendentality in the universal scaling function for large-spin twist operators may be preserved. One particularly natural choice, unique up to one constant, modifies the overall contribution of all terms containing odd zeta functions in the earlier proposed scaling function based on a trivial phase. Excitingly, we present evidence that this choice is non-perturbatively related to a recently conjectured crossing-symmetric phase factor for perturbative string theory on AdS5xS5 once the constant is fixed to a particular value. Our proposal, if true, might therefore resolve the long-standing AdS/CFT discrepancies between gauge and string theory."

    Notice where the guys are from:

    Niklas Beisert and Matthias Staudacher
    Max-Planck-Institut für Gravitationsphysik
    Potsdam, Germany

    Burkhard Eden
    Institute for Theoretical Physics and Spinoza Institute
    Utrecht University
    Utrecht, The Netherlands

    In Nicolai's perspective, it happens that research from Potsdam (AEI) and Utrecht plays an important role. I find it an interesting coincidence that Potsdam and Utrecht are like Perimeter Institute in being strong in non-string QG as well as string research. They are places where string and non-string QG researchers work in neighboring offices, chat in the coffeeroom and can easily attend each other's seminars. One group is not frozen out by the other. Grad students have a choice. That is not how it typically is in the US. Another thing I notice is the relative modesty of the claims that Nicolai starts with. I may be talking here about a kind of external style that connects with an internal way of thinking. Here's an illustration:

    String theory: Back to basics
    Long touted as a theory of everything, it seems that string theory may at last succeed as a theory of something very specific — the interactions of particles under the strong nuclear force.

    Whether string theory can live up to its claim of being a ‘theory of everything’, and whether it will ever produce a falsifiable prediction as such, remain hotly debated questions. Meanwhile, developments in a quieter side-alley...
    Last edited: Oct 21, 2007
  17. Oct 20, 2007 #16


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    Yes I know, but had you resisted the pun, in my position?
  18. Oct 20, 2007 #17


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    In any case, there is not such a thing as gauge/gravity correspondence in a general setting; it is only that you can hope to recover N=4 sQCD or N=2 or some similar beast, and probably it is because it already shares a lot of mathematical structure with string theory.

    Still, suppose it holds. Would you like to claim that N=4sQCD is the right theory of quantum gravity? Duality is a double edged sword.
  19. Oct 20, 2007 #18
    I too doubt what nicolai is doing will turn out to have anything to do directly with gravity. But who knows? It's just an interesting application of some of the ideas that came out of string theory research.
    Last edited by a moderator: Oct 21, 2007
  20. Oct 21, 2007 #19


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    Well, large N QCD came along string theory, not out of string theory research.

    Perhaps most of the theological problem come from the continuous interaction between a theory of extended relativistic objects (call it Brane theory) and a theory of quantized relativistic superstrings (string theory). The later is a very exclusive theory, with objects in 10 dimensions and with only a few uncompactifyed models. The later is not exclusive, it includes non quantized objects (eg 1-branes in 3,4,6 dimensions) and even can be extended to be understood as an approximation.
  21. Oct 21, 2007 #20
    Yes. But what did come out of string theory was a way to deal with general supersymmetric SU(N) gauge theories for large N in terms of low energy supergravity.

    I'm unsure what you mean here. It sounds as if you think that there are different aspects of string theory which are difficult to reconcile with each other. If this is the case, let me just mention one fact about the relation between strings and D-branes which is that D-brane states can be described mathematically in terms of open string states. More specifically, these two descriptions are T-dual to each other. In fact, this was how D-branes were discovered in the first place.
    Last edited by a moderator: Oct 21, 2007
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