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String Gravitons yield GR. NOT

  1. Sep 23, 2004 #1

    selfAdjoint

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    This paper does a lot of testing of different kinds, and concludes that the string theorists assertion that the graviton reproduces the physics of GR in flat spacetime is a myth.
     
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  3. Sep 23, 2004 #2
    Sorry for asking but what are the ramifications of this discovery for string theory.
     
  4. Sep 23, 2004 #3

    marcus

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    I printed it out and read all of it I could. It looks like a careful paper, not at all polemical.
    I'm tongue-tied. Nothing to say except the obvious---have to wait
    and see how things sort out. Paper of this class will get discussion by people of, say, Hermann Nicolai stature at some conference. We may not have to wait all that long. Things never as simple as they seem. Very exciting.

    Ganesh is the god with the elephant's trunk for a nose. He has many hands. He is the god of luck. Padmanabhan lives on Ganesh Street
     
  5. Sep 23, 2004 #4

    marcus

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    Trying to see who Thanu Padmanabhan is. here is a picture of him and an interview
    http://www.esi-topics.com/nhp/2004/january-04-ThanuPadmanabhan.html

    He had a hot paper, on tachyons and the expansion of the universe, that he published in 2002 and in January 2004 something called "Essential Science Indicators" picked up on that this paper was getting a lot of citations, so they interviewed him.

    He may have several highly-cited papers. I will check.

    He used to be at Cambridge and then at Tata Institute, now at IUCAA Pune (center for astron. and astroph.) He has published over 100 papers of which 9 were "hot" (over 50 citations). these are listed here
    http://www.slac.stanford.edu/spires...manabhan+and+topcite+50+&FORMAT=WWW&SEQUENCE=

    the hot papers go from present back to 1987 when he was at Cambridge
    the numbers of citations received are, most recent paper first:
    142, 155, 61, 88 120, 67, 61, 54, 57

    the most-cited of his papers is
    COSMOLOGICAL CONSTANT: THE WEIGHT OF THE VACUUM.
    By T. Padmanabhan (IUCAA, Pune),. Dec 2002. 112pp.
    http://arxiv.org/hep-th/0212290

    Padmanabhan has published string, or string-inspired, papers for example
    he got 120 citations on his
    ACCELERATED EXPANSION OF THE UNIVERSE DRIVEN BY TACHYONIC MATTER.
    By T. Padmanabhan (IUCAA, Pune),. IUCAA-16-2002, Apr 2002. 4pp.
    http://arxiv.org/hep-th/0204150

    Thanu Padmanabhan has never written any LQG papers---not a supporter of some rival to string--- may be assumed disinterested, or so I reckon.

    Here is his full list of published papers---183 papers going back to 1981.
    http://www.slac.stanford.edu/spires/find/hep/www?rawcmd=FIND+author+padmanabhan&SKIP=150
     
    Last edited: Sep 23, 2004
  6. Sep 24, 2004 #5

    selfAdjoint

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    Toward the end of the paper Padmanabhan says that the graviton produces a lagrangian with four extra terms besides the Einstein ones. It might be interesting to see if this theory can be compared with GR by experiment or astronomical observations.
     
  7. Sep 24, 2004 #6
    What's the Palatini form? It's some differential form appearing in the Palatini formulation of GR?
     
  8. Sep 24, 2004 #7

    selfAdjoint

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    It's a form of 1/R gravity (I believe that's a class of extra term versions of GR proposed to account for things like dark energy).

    See for example, this.
     
  9. Sep 25, 2004 #8
    I too was rather shook when I spotted this paper on ArXiv and read it.
    I just want to make two preliminary comments:
    First off, over on SPS Lubos has a post entitled "Diff Invariance ..."
    in which he says he disagrees with this paper.

    Second, in his second paragraph, Padmanabhan (P) makes a distinction between
    the Gupta Feynman Deser approach, and the closely related
    Weinberg Boulware Deser Grischuk Wald approach,
    which reaches similar conclusions. (See P's footnote 6.)
    P says his criticism is only aimed at the first approach, not the second.
    Also in his acknowledgements on page 15, P says he corresponded with Deser
    and they reached agreement on some but not all points.

    I may or may not have further substantive comments after I have read and thought some more,
    but my first take is if this paper truly upsets these long accepted results, it is revolutionary.
    Even if it only opens a small loophole, it is important. But at the moment I am still sceptical.

    Best, Jim Graber
     
  10. Sep 29, 2004 #9
    Padmanabhan finds so to say the "correct rule" (Eqn (15)) to define the transition from the flat metric to a curvilinear one (in order to be able to use functional derivation) which leads to the correct definition of the energy-momentum tensor of a spin 2 field. Correct means here that when one couples it to itself things lead to GR.
    But he gets this rule by linearizing GR and then reading off the tensor which couples to the perturbation metric and identifying it then with the energy-momentum tensor of a spin 2 field.This procedure, of course, requires the knowledge of GR!
    But this is similar to the things that Deser, Feynman,... did when they put general covariance in to get it out as Padmanabhan complains.
    Ok, he seemingly got the "correct" definition of the energy-momentum tensor for a spin 2 field but what is of even greater importance is to DERIVE its form from fundamental principles, I think.
     
  11. Sep 29, 2004 #10

    selfAdjoint

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    He is criticising there work, so he is free to use the strategies that they do. You seem to miss the point that he is not trying to derive the energy momentum tensor but attacking the "common wisdom" that says you can do this is flat space with no problems.
     
  12. Sep 29, 2004 #11
    Yes, I agree that he is free to use their strategies and I didn't want to complain too much about that.
    I just wanted to state that now that we apparently know a (the?) correct form of the energy-momentum tensor further investigations about it's (the definitions) origin are needed.
    Another thing that came into my mind was if his S_ab is the only one which yields GR for as there a many other choices possible (cf. eqn (10)).

    Do you mean by, "...you can do this (?) in flat space...", finding a procedure of developing GR from a spin 2 field by selfcoupling?
     
  13. Sep 29, 2004 #12

    selfAdjoint

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    Yes that is what I meant. The fact that the tensor S_ab works, and is not T_ab, shows that the attempts made were futile.
     
  14. Nov 2, 2004 #13

    marcus

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    Thanu Padmanabhan has just posted a very good perspective on
    cosmology and dark energy
    astro-ph/0411044

    I am getting a more rounded-out picture of him. My respect is growing.
     
  15. Nov 2, 2004 #14

    selfAdjoint

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    Very impressive, and his cites to his work with colleagues look good too. He should only learn to spell Boojum.
     
  16. Nov 2, 2004 #15

    marcus

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    You are right. He should correct the spelling.
    I have taken the liberty and arranged for him to be
    informed of this forthwith.
     
  17. Nov 11, 2004 #16
    Question: this paper should indeed be revolutionary like a member pointed out, but has it actually been seriously accepted by other string theorists and physicists?? Does it really provide definite proof that string theory does not yield gravitons? Why haven't there been any other papers reacting against this one? Just some questions.
     
  18. Nov 11, 2004 #17

    marcus

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    hi Curious, I can't answer the main thrust of your questions
    but I will try to add to the discussion by a little reinterpretation

    I think string theory will (if and when it finally gets developed as a complete, testable theory) yield gravitons.

    I dont think there is any question about the string approach being able to crank out gravitons. Padma doesnt challenge this.

    what padma's paper brings into question is whether yielding gravitons is the same as yielding gravity----the gravity we know and love from Gen Rel.

    It could be that gravitons dont really exist in nature and that they are just a (not completely satisfactory) man-made mathematical approximation.

    It could be that gravitons are a mathematical tool which sometimes, in some situations, is appropriate to use in describing how the gravitational field works.

    what I have said here is only the beginning---something we should recognize at the outset. Padma goes much farther than this and indicates that gravitons (a la string) actually give a faulty answer about gravity.

    I dont think this is settled.

    I think that until it is settled, String people should not say that their theories include gravity. this is presumptious, or wishful thinking.
    but maybe it will be settled in the future---and then they can say either that it does or that it does not. To say it does, now, would seem to me to be jumping the gun.

    There will surely be more papers pursuing this line of investigation that Padma initiated. But it is deep technical stuff and I would not expect followups to be real soon. A couple of months is IMO still a little short time to expect String theorists to respond to the questions raised.

    Just to have the link handy
    http://arxiv.org/abs/gr-qc/0409089
    From Gravitons to Gravity: Myths and Reality

    (as you can see it was just September it came out, not time yet for string folk to respond. I hope they do, as I expect you too)

    BTW has Padma corrected the spelling of Boojum in his latest preprint?
    http://arxiv.org/abs/astro-ph/0411044
    Dark Energy: the Cosmological Challenge of the Millennium
     
    Last edited: Nov 11, 2004
  19. Nov 11, 2004 #18
    OK, thanks again marcus for your insight!! I see what is being discussed, and although I don't have a lot to add, I believe that this will as you say be a very technical issue and will take quite a while to be settled.
     
  20. Nov 29, 2004 #19

    marcus

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    multiple-graviton states constructed in LQG

    Stringy theories are not the only approaches to Quantum Gravity in which one can find gravitons, of course. The emphasis in LQG is on describing gravity through the quantum geometry of space, not by means of gravitons running around on a fixed rigid space. But one CAN construct gravitons in LQG.

    This was recently done, for example, in a paper by Florian Conrady, who is at the Albert Einstein Institute in Golm (part of the Max Planck Institute for Gravitation Physics outside Berlin)

    Free Vacuum for Loop Quantum Gravity
    http://arxiv.org/gr-qc/0409036

    "We linearize extended ADM-gravity around the flat torus, and use the associated Fock vacuum to construct a state that could play the role of a free vacuum in loop quantum gravity. The state we obtain is an element of the gauge-invariant kinematic Hilbert space and restricted to a cutoff graph, as a natural consequence of the momentum cutoff of the original Fock state. It has the form of a Gaussian superposition of spin networks. We show that the peak of the Gaussian lies at weave-like states and derive a relation between the coloring of the weaves and the cutoff scale. Our analysis indicates that the peak weaves become independent of the cutoff length when the latter is much smaller than the Planck length. By the same method, we also construct multiple-graviton states. We discuss the possible use of these states for deriving a perturbation series in loop quantum gravity."

    by free vacuum is meant perfectly flat, not curved by matter, and static. this the kind of idealization used in perturbative approaches to QG---often e.g. in string contexts. So it can only be an idealized approximation.
    However the perturbation series approximation is a useful tool in many
    kinds of field theory---basic to QED for instance. At least it gets you started calculating!

    So I guess it was thought that, although LQG is mainly a NONperturbative approach that does not need a static flat vacuum on which to calculate perturbations, and does not need gravitons as a tool in that kind of analysis, nevertheless the OPTION of resorting to perturbative methods could be handy.

    Hence Conrady's recent paper.
    In an earlier paper along these lines, Conrady co-authored with Rovelli, who is evidently interested in this himself and keeping abreast of it.
    Minkowski Vacuum in Background Independent Quantum Gravity
    http://arxiv.org/gr-qc/0307118

    In effect, one can construct gravitons, and they may eventually come in handy as analytical constructs, but if one judges from the paper by Thanu Padmanabhan which this thread is about, then merely to find gravitons in a theory is not, by itself, ENOUGH so that one can say the theory contains gravity!

    For this, see
    T. Padmanabhan
    From Gravitons to Gravity: Myths and Reality
    http://arxiv.org/abs/gr-qc/0409089
     
    Last edited: Nov 29, 2004
  21. Nov 29, 2004 #20

    jeff

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    I'm sorry marcus, but as everyone here knows, you've stated quite plainly numerous times that you don't believe in the idea of gravitons. Of course you said this because you thought that lqg - your religion - is inconsistent with the idea of the graviton so that this idea must be unholy.

    You've changed your tune now because yesterday I pointed out that carlo rovelli states in the introductory chapter of his new book that...

    "What we need is not just a technique for computing, say, graviton-graviton scattering amplitudes (although we certainly want to be able to do so, eventually)"

    Of course I've tried to explain this to you many times.
     
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