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Internal struggles of ideas in QG

  1. Dec 9, 2007 #1


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    Internal struggles of ideas in non-string QG

    One should not exaggerate the internal struggles which are going on or ready to happen. But we might take note of some of the internal conflicts. Issues that will probably get worked out in the next few months----whether or not they are openly recognized as contradictions.

    I think it's a healthy sign. Also a sign of convergence of various approaches, because as people converge on a goal they naturally get closer together and begin to step on each other's toes. Differences that weren't so obvious before become more important.

    So I will list a few POSSIBLE tensions to be resolved, and in some cases these may have already been taken care of, or not actually be real----they could simply appear to me to be contradictions because I don't understand well enough.

    1. Does the LQG black hole bounce, or not? (Vandersloot and Boehmer mu-bar versus Gambini and Pullin).
    Gambini et al just last week posted http://arxiv.org/abs/0712.0817
    Vandersloot et al posted this in September http://arxiv.org/abs/0709.2129

    Gambini and Pullin have been major figures in QG for over 10 years, written many well-cited papers. Vandersloot and Boehmer are postdocs in the UK. Personally I have high regard for both sides. In neither case do they find a singularity. One says bounce (a new semiclassical spacetime region expands out the BH bottom, the other side says no bounce, at least if you use one of their two models. Apparently a new stable phase of very high density can condense at the pit of the hole. In neither case is there singularity, but two very different results. So it will be interesting to see how this is resolved.

    2. How do fermions arise in the E8 picture? (Smolin versus Lisi)
    Just this past week Smolin came out with a paper that was partly about E8, in which he seemed to assimilate and value part of what Lisi has done with E8, but suggested that the fermions could arise by a different mechanism. http://arxiv.org/abs/0712.0977
    Most likely a case where each person's proposal is worth developing further. So you proceed along both paths and don't worry about which is right until much later. But nominally at least there is a contradiction: two contradictory ways that the fermions could arise in that context.

    3. A divergence in the Asymptotic Safety programme. (Saueressig versus Reuter)
    Just this past week Frank Saueressig a postdoc at Utrecht came out with a Flow and Fixed Point paper that diverges significantly from the work of Martin Reuter. Saueressig has been a CO-AUTHOR with Reuter since 2001 of some 4 or 5 papers including some instrumental in establishing Reuter's A.S. approach. The two of them recently co-authored a pedagogical survey paper where they combined introductory overview talks they had each given in separate places a few months back. Reuter is the senior author in A.S. but Saueressig has been a key helper. Saueressig (working with a Renate Loll PhD student) tries Renormalization Flow stuff on a different action from what he and Reuter have (technically a different truncation) and he gets some favorable new results but also some contradictory signals, which he offers an explanation for. This means a possible new direction in A.S. and a tension that may have to be resolved.

    On the whole I guess this is very positive and the way things in an active research area ought to proceed. It is not supposed to run smooth all the time. There are supposed to be jolts now and then. So it's all right.

    4. Alarms and excursions about the new spinfoam model. This is an obvious case where a major advance was sensed and many people rushed together and collided. At a certain moment it was realized in what way the old Barrett-Crane spinfoam formula was not quite right. (Credit probably goes to Rovelli and friends for persistently probing weak points in the course of trying to make the model reproduce Newton law and semiclassical propagator in a flat case. They kept trying to make it work and thereby found out what needed to be changed.) Suddenly several different teams proposed improvements of the B-C formula. There was a brief scuffle. This must be normal. Maybe it is even necessary.

    5. There could be other differences to be resolved or ones that came up earlier this year---if anybody thinks of one please add it to our list!
    Last edited: Dec 10, 2007
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  3. Dec 10, 2007 #2


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    6. Canonical or covariant? (When I say covariant, I mean Lorentz covariant, where gravity is treated as just another field in flat spacetime.)
  4. Dec 10, 2007 #3
    How to test 1-5 (6)? (once -- if -- they converge?)
  5. Dec 10, 2007 #4


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    Hi Christine, I edited the first post to make clear that I mean internal dissention amongst NON-STRING quantum gravity researchers.
    You rightly point out that there is this overall problem of testing which, I think, everybody agrees on.

    everybody recognizes the need to derive testable predictions. So this is not a case of contending ideas within the community. And I don't think you presented it as such---you just mentioned a recognized overall problem.

    In this thread the aim is to identify cases where there are conflicting ideas within the non-string community. In each case we should identify two individuals or two lines of research that are in some sense on a collision course---or potentially in contention.

    So these are not questions that an OUTSIDER might ask about the whole field, but only issues about which there is internal struggle.

    For me, the most striking issue right now is the one raised (in such a low-key way that you might not even hear!) by Saueressig. Look at the bottom of page 45 in his recent paper with Pedro Machado.

    First he observes that the IR attractor fixed point which they discovered in the Renormalization Flow (with a logarithmic truncation) PREDICTS A SMALL POSITIVE COSMOLOGICAL CONSTANT. This is very interesting.
    Then he points out that, for various possible reasons which he discusses, their analysis does not always recover the UV fixed point basic to treating gravity by Asymptotic Safety.

    It is a 55 page paper. I am impressed by the quality. Renate Loll (who is Machado's PhD advisor) is in the acknowledgements as well as Martin Reuter and Roberto Percacci. I'm inclined to take seriously whatever ideas it raises. That is why I think of "Saueressig versus Reuter" as topping the internal conflicts, even though it is perhaps not so well known and I listed it as #3.

    EDIT: I went back and highlighted #3 in the original post.
    Last edited: Dec 10, 2007
  6. Dec 10, 2007 #5
    In Wikipedia’s LQG page, under LQG & Particle Physics, Bilson-Thompson’s theory is contrasted to Xiao-Gang Wen & Michael Levin’s theory. Is this still a valid comparison?
    Last edited: Dec 10, 2007
  7. Dec 11, 2007 #6
    Ah, ok, I see your point now. Thanks.
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