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David Louapre, at Lyons, another one to watch

  1. Sep 12, 2003 #1

    marcus

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    http://www.ens-lyon.fr/~dlouapre/

    home page has a CV
    also a link to some funky music
    and a card for French "buzzword bingo"
    I am finding as I go along that the newest work in
    quantum gravity is being done by virtual children
    from my (perhaps too old) perspective

    edit: I see that Louapre was born in 1978
    The cry of "Bingo" in French is apparently "Foutaise" which sounds
    slightly racy to me

    even Rovelli, at marseilles, who has the written the book
    on current efforts to quantize GR, looks to be
    rather young from his photo----35 at most I'd guess---
    and he's one of the senior people

    Anyway Louapre has come up with interesting ideas
    vis-a-vis the "assymptotic 10j" obstacle which Baez
    identified one year ago in some papers he and some others wrote circa September 2002
    ( mentioned a few days ago in TWF #198).

    So I will try to get some links to a David Louapre paper or two.
    And then as Baez said there will be a conference on spin foams
    in Spring 2004 in marseilles and Louapre (does he even have his PhD yet?) is one of the organizers and we will see what he says
    there. But in the meantime I will post what I can find in this thread

    edit: Here is one from December of last year
    http://arxiv.org/abs/gr-qc/0212001
    "Diffeomorphisms and spin foam models"
     
    Last edited: Sep 12, 2003
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  3. Sep 12, 2003 #2

    selfAdjoint

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    Marcus, I didn't know you were old enough to consider 35 young! As you go along you will find that age becomes meaningless and the 21 year old may have more to say to you than your contemporaries.
     
  4. Sep 12, 2003 #3

    marcus

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    Whoah! You took me by surprise. I didnt know anyone would be
    reading this thread.

    I see where Louapre has an idea vis a vis the "assymptotic 10j"
    problem. But it is only partially there and should be completed in a followup paper I have not yet seen.

    http://arxiv.org/hep-th/0209134

    from the abstract:
    "...we compute the asymptotic expansion of the 10j symbol which is shown to be non-oscillating in agreement with a recent result of Baez et al. We discuss the physical origin of this behavior and a way to modify the Barrett-Crane model in order to cure this disease."
     
    Last edited: Sep 12, 2003
  5. Sep 12, 2003 #4

    marcus

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    A new PhD thesis from Marseilles

    this week a new PhD thesis was posted
    by Etera Livine (directed by Carlo Rovelli)
    http://arxiv.org/gr-qc/0309028

    "Boucles et Mousses de Spin en Gravite Quantique"

    Livine has already published a number of shorter papers with
    Robert Oeckl, Daniele Oriti, and possibly others---I forget.
    He sometimes goes by his middle name Richard.

    As the epigraph of his thesis, Livine quotes from the Babylonian Talmud:

    "It is never advisable for anyone to speculate on these 4 questions:

    What is above?
    What is below?
    What was there before the world?
    What will there be after it?

    It would have been better for him had he never been born."

    In fact I noticed Livine's research several months ago and have
    found several of his papers (co-authored with others) of some interest. So I found his thesis in the course of a regular check on
    what he and other young European researchers have been posting.
     
  6. Sep 12, 2003 #5

    marcus

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    the gnome hiding in the low energy limit

    I recently exchanged email with Carlo Rovelli and learned that he is in Rome (La Sapienza) this season, rather than at his homebase at the University of the Mediterranean (Marseilles).

    Who but Giovanni Amelino-Camelia is at La Sapienza?

    This summer, Giovanni A-C posted what I think is a major paper with Smolin. The paper is short, 19 pages, but could have considerable impact. I would guess that Rovelli wants to understand the conclusion and that it may influence a chapter of his draft book or some aspect of his research.

    http://arxiv.org/hep-th/0306134
    "Quantum Symmetry, the cosmological constant and Planck scale phenomenology"

    One often hears that some theories of gravity have difficulty assimilating the case with a positive cosmological constant.
    Perhaps this is because of some built-in Poincare invariance! Whether or not this is true, this is what I have seen mentioned from time to time, and it is suggestive of one of the results in this paper. The abstract says:

    "We present a simple algebraic argument for the conclusion that the low energy limit of a quantum theory of gravity must be a theory, not under the Poincare group, but under a deformation of it parametrized by a dimensional parameter proportional to the Planck mass...
    The argument makes use of the fact that the cosmological constant results in the symmetry algebra of quantum gravity being quantum deformed, as a consequence when the limit
    lPlanck2 Λ--> 0
    is taken one finds a deformed Poincare invariance...."

    Something interesting is going on here. Some months back in a PF thread I calculated this lPlanck2 Λ quantity---the Planck area multiplied by the cosmological constant--- and I recall it came out to be about 1.3E-123. That is very small-----the cosmological constant or "dark energy" density is believed by cosmologists to be non-zero and very important (73 percent of the energy in the universe) but also very small, so that its ratio to the corresponding planck quantity is
    10-123.
    Incredibly, this positive dark energy constant must have the effect of deforming Poincare invariance (by the argument of
    Giovanni A-C et al) so that when one looks for the low energy limit under a tree root or large leaf one finds not an ordinary gnome but a deformed ---perhaps hunchback----gnome. A good theory of gravity, therefore, should not have Poincare invariance at its low energy limit but rather should have deformed Poincare invariance.

    This paper is also interesting because it relies comparatively more on verbal reasoning and less on equations. What equations it has are on the simpler side----lists of commutation relations for
    a couple of κ-Poincare algebras. I must say it piqued my curiosity about this type of algebraic deformity.
     
    Last edited: Sep 12, 2003
  7. Sep 12, 2003 #6

    marcus

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    Stephon Alexander at SLAC Stanford is another one to watch

    I already posted about Stephon A, who is in the high energy physics theory group at SLAC. He recently tried his hand at Quantum Gravity and got a nice result. The link I gave earlier is

    http://arxiv.org/hep-th/0309045

    "Quantum Gravity and Inflation"

    Here is a sample from the paragraph of conclusions at the end of that 18-page paper:

    "The results of this paper represent a step towards a detailed study of the very early universe beyond the semiclassical approximation, in which quantum gravitational effects are treated in a non-perturbative and background independent manner. For each potential V(φ) and classical slow roll solution u(T) consistent with inflation, we have found a quantum state given by (56) which is an exact solution to the quantum equations of motion, but has a classical limit given by that classical solution. Furthermore we can construct normalizable states which are wavepackets around the initial conditions that generate that classical solution. Thus, inflation is here described in terms of exact quantum states..."
     
  8. Oct 28, 2003 #7
    That's funny because I have to prepare a CV and I tried to take a look at CV's of quantum gravity researchers by typing something like "CV quantum gravity" on google...and I just found this thread talking about my CV on my highly non-updated web page ! I should update that :-)

    David


     
  9. Oct 28, 2003 #8
    By the way I should say that what we proposed to cure the 10j problem is so far a very "ad hoc" modification and I don't realy believe in that anymore...but I've got new ideas on this issue..not yet enough to write a paper on that.

    David

     
  10. Oct 28, 2003 #9

    marcus

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    hello David, it is very nice to hear from you

    can you give us any idea of what Carlo Rovelli's talk
    this Friday about spin foams will cover---any new work
    that's likely to form the basis of the talk?

    I used the Foutaise card from your website to teach
    two French visitors how to play Foutaise, last month.
    They had never heard of this game but, having connection
    to industry, they understood it very well
     
  11. Oct 28, 2003 #10
    I'm not at the strings/loop meeting but I know that Carlo Rovelli's talk will actually be given by my boss Laurent Freidel. I don't know what's gonna be inside the talk because Laurent hadn't finished it when he left for Germany.

    However it's gonna be a review talk intended for non-expert audience (string people) so don't expect too much fresh news in it, i think...
     
  12. Oct 28, 2003 #11

    marcus

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    Thank's for the news David, I will be hoping to hear about
    Freidel's talk soon. (after Friday)

    best wishes,

    marcus
     
  13. Oct 28, 2003 #12

    selfAdjoint

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    BTW, I just heard from Thiemann, and he's going to give a talk at the meeting by video (he won't be there in body). Marcus we are on the edge of wonders!
     
  14. Oct 28, 2003 #13

    marcus

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    some changes for sure!

    I am happy and at the same time frustrated with the F/L
    paper, they say the heart is section 4
    I can sort of understand or feel I WILL be able to understand section 2, and section 3 is technical so that one only needs to be able to apply the key things from it to 4
    but when I look at 4 I am a bit humbled.
    As by the London times sunday cryptic crossword which articulate person should be able to solve but they normally cannot---it is mostly not arcane it is just hard chains of reasoning. besides today was hot

    but I do think that F/L paper "spin networks for non-compact..." is important, and likewise Livine's thesis, so if I could just understand them I would feel on the edge of something

    I'm glad you are taking a break and will see the fall colors in Wisconsin, Illinois and so on---hope you find trip satisfactory---I could use a break. I think id like to go sailing but I dont have a boat.

    keep me posted please if you learn more of the goings on
     
    Last edited: Oct 28, 2003
  15. Oct 29, 2003 #14

    selfAdjoint

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    Well, I've got a couple of days here yet. Why don't we try digging into that section 4 of F/L "Projective..." and share our thoughts here? I wanna see how they do Lorentz boosts!
     
  16. Oct 29, 2003 #15
    That's a well written paper. Here are some guidelines for section 4 :

    The key part to understand is 4.1.1
    It contains the definition

    4.1.2 is generalization to many Cartan and is formally the same if you understand Weyl integration formula.

    4.2 is generalisation, no too hard since (4.29) shows that the h petals case is not really more difficult than 2 petals case.

    4.3 mixes that with the flower construction, the key step being the invariance under the change of maximal tree which is proven by showing that two trees can always been related by a sequence of 'moves' and that the gauge fixing procedure gives the same results under the moves.

    If you get 4.1.1 the remaining part should follow, maybe you'll have to think a little more for 4.3 and the proof of invariance.

    David

     
  17. Oct 29, 2003 #16

    marcus

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    In case anyone is just joining us the comments here refer to
    http://arxiv.org/hep-th/0205268
    "Spin Networks for Non-Compact Groups". Section 4.1.1 (that is, 4.1 part 1) begins near bottom of page 13 and is called
    "1. Case of a unique Cartan subgroup H"

    Section 4.1.2 (that is, 4.1 part 2) begins on page 16 and is called
    "2. The case of many Cartan subgroups"

    In fact it was just at 4.1 part 1, which David calls the key part to understand (containing the definition in the simplest case) that I was having difficulty understanding earlier (essentially page 14). Just got back home and will make another attempt at it.
     
  18. Oct 30, 2003 #17

    marcus

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    I need some help understanding the case where the group is SL(2,C)

    then, referring to page 9, there is only one Cartan subgroup
    I am not familiar with Cartan subgroups but I tentatively imagine that in this case things might reduce to something quite understandable and elementary.

    Hopefully someone can help me out here. Perhaps if G is just SL(2,C) then, in the notation of the Freidel/Livine paper, the cartan subgroup H is, say, the diagonal matrices with determinant unity.

    Corresponding to that H, the Weyl group is N(H)/H where N(H) the normalizer of H

    and I think if H is diagonal matrices then the normalizer consists of H AND the counterdiagonals (the minor as opposed to major diagonal) always with det = 1.

    so then the Weyl group is looking to me like Z2, kind of like a boolean toggleswitch which decides to flip the main to the minor and the minor back to the main

    to my embarrassment I never did study about Cartan and Weyl subgroups, but in this one example they seem to be nice ideas

    if we can get this one case, then section 4 of that paper will boil down to section 4.1 alone and we can see how to construct the measure
     
    Last edited: Oct 30, 2003
  19. Oct 30, 2003 #18

    marcus

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    Since cartan and weyl groups are new to me i am flying blind, or relying on guesses---presumably all or partly wrong in this first attempt

    I would suppose that the cartan subgroup of SL(2,C) is NOT a normal subgroup
    and yet on page 9 there is an integral over G/H
    this G/H, I should imagine, is not a group(!) if H is not normal
    I should picture it as a bunch of cosets, or as a bunch of equivalence classes

    a kind of easy trick (equation 3.6) is used to define a measure on it

    so already I'm curious to know what G/H looks like
    perhaps it is non-compact!

    but to visualize G/H I must be more certain as to what the cartan subgroup H is. I was thinking (main) diagonal matrices with entries (z, 1/z) on diagonal where z is any complex number. But maybe this is not right?
     
  20. Oct 30, 2003 #19

    marcus

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    page 9 has a defintion we need later
    G1, it says, consists of elements of G which can be conjugated to a Cartan subgroup

    the simple case I have in mind is SL(2,C) where there is just one cartan subgroup---diagonal matrices I'm presuming for sake of argument. so G1 would be "diagonalizable" matrices

    it also says they're going to call those element "regular", so G1 is the regular elements of G

    it also says that G1 consists of elements x for which Adx is diagonalizable. I am not sure what it means for Ad x, the adjoint mapping, to be diagonalizable.

    it also says that G1 is almost the whole group G, that is its complement has haar measure zero.

    well, courage, things look to be in a pretty desperate pass, but dlouapre says this is a well-written paper (the F/L "Spin Networks for Non-Compact Groups") and it looks that way to several others including me, and if we just restrict attention to the group we need namely SL(2,C) maybe we can understand some of this paper and be better off for having tried.

    We have to know what G1 is because the very first thing they do in section 4.1.1, the heart of the paper, is define an injection G1 --> A2
     
  21. Oct 30, 2003 #20

    marcus

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    that reminds me, what is A2

    well on page 12 they define it as G2/Ad(G)

    and say it is a geometric quotient of same dimension as the group G (this is in the "rank one" case which I think we are in)
    and
    what G2 is might be understandable as the set of pairs in G x G for which a certain combination that one is going to need to diagonalize is in fact "regular" or "G1"
    basically G2 is equal to G x G except for measure zero and is a convenience set that has some nasty pairs removed so I will think of it as just G x G

    so this key thing A2 is essentially (G x G)/Ad(G)

    and this mapping they define on page 13 (in section 4.1.1 the only section I want to really understand for sure) is a map
    j : G1 --> GxG/Ad(G)

    this map j (which depends on choosing a section s(x) but then turns out the measure doesnt depend on the choice, the old story)
    this map j is how they define the friggin measure

    so if I can just understand this map j
    of equation (4.1) on page 13
     
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