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Basic papers in CDT gravity

  1. Nov 24, 2004 #1

    marcus

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    Ambjorn Jurkiewicz Loll seem to have settled on what to call their new approach to Quantum Gravity.
    In their most recent paper they consistently call the approach CDT, "Causal Dynamical Triangulations".

    What I have to say first off about this is controversial and I might even have to take it back. I do think that Loop Gravity embodies a lot of valid physical intuition and has made impressive advances recently---removing important singularities, clarifying the origin of inflation, and so on (these are outlined in several excellent survey papers that have appeared recently)---but I think that Loop Gravity may actually be more complicated than a basic computable quantum gravity model needs to be.

    The CDT approach has WORKED only just this year. It is comparatively elemental. The mathematics is comparatively concrete and rudimentary and it leads directly to computer models of dynamically changing space
    (spacetime "path integrals" analgous to Feynm. path integrals for particle).
    It has taken some 20 years to make this simple approach to gravity and quantum spacetime geometry work. It uses very basic, non-abstract, math-objects as tools, which makes it "clean" of unnecessary extra assumptions, and exceptionally free of fancy math. But basic-elementary does not seem to have meant that it was EASY for Ambjorn Jurkiewicz Loll to get it to work or that it is going to be easy to understand at first sight how.

    My thought about the relation to Loop is that LQG has been a trailblazing thing and good platform for people to get intuition from and see results, but that CDT is more fundamental, assumes less structure, and there is probably a subterranean connection. LQG results may turn out to be derivable in the more spare context of CDT. Maybe even some equivalence theorems between the approaches. (as there was earlier linking different approaches to Quantum Electrodynamics)

    Anyway, however it turns out---and everything is so preliminary one can only guess---it makes sense to identify the basic CDT papers. I'm offering this as a kind of minimal set. the three I have printed out and keep handy:

    http://arxiv.org/hep-th/0105267
    http://arxiv.org/hep-th/0404156
    http://arxiv.org/hep-th/0411152

    I will try to discuss just these three papers in this thread, and welcome any help anyone wants to give that focuses on these 3 basic CDT papers.
     
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  3. Nov 24, 2004 #2

    marcus

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    I will put in the names of the papers. You can see how they are working towards calling it Causal Dyn. Triang.

    Causal is a shorter word for "Lorentzian" because in Minkowski space there is a special set of timelike directions so one can be definite about the flow of causality. Causal means you use Minkowski 4D (special rel) space instead of Euclidean 4D where all coordinates are inherently the same. It sounds technical as hell. But Causal is at least a comprehensible 2 sylable word, and "Lorentzian" is, in the lng run, more obscure sounding and technical-arcane.

    We had some earlier discussion about two of these papers at PF here:
    https://www.physicsforums.com/showthread.php?t=52958
    I didnt do the greatest job explaining, in that thread, so I want to make a fresh start. Maybe get some help, if anyone else is interested in them

    ANIMATIONS
    2D animations at Jan Ambjorn's homepage
    http://www.nbi.dk/~ambjorn/lqg2/
    (click on the still to get the animation to load, takes about a minute to start)

    Ambjorn's homepage
    http://www.nbi.dk/~ambjorn/

    What Matt Visser (Univ. Washington) had to say about the Ambjorn/Loll work in 2002
    http://www.phys.lsu.edu/mog/mog19/node12.html

    Discussion on SPR (Baez, Larsson) of "Emergence of a 4D world..."
    copied to a PF thread that JB started:
    https://www.physicsforums.com/showthread.php?p=213245#post213245
     
    Last edited: Nov 24, 2004
  4. Nov 24, 2004 #3

    Chronos

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    It got my attention, seeing how I'm prejudiced toward any approach that does not appeal to a host of extrasensory dimensions for validation. I suppose I could learn to live with one, maybe even two of them, but I wouldn't be thrilled. It makes me feel like nature has something to hide if we have been willfully denied sensory access to certain aspects of reality.
     
  5. Nov 24, 2004 #4

    marcus

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    I expect they should have chucked it out when they found it wouldn't work in the usual 4 dimensions. :smile:

    As you say CDT's attractive because doesn't need a lot of extra machinery and assumptions, like dimensions.

    But as with combinatorial things generally, it does require chugging thru arithmetic---hopefully one can have the computer programmed to do that.

    It is puzzling to me why it took so long to get to work. ambjorn, loll, and others have been working on dynamical triangulations (maybe not causal)
    since 1993, over 10 years. It seems that in 2001 they still had it only in IIRC 2D (1 space + 1 time) and were looking into 3D. they seem to have proceded methodically one step at a time.

    In this latest paper, they recover a semiclassical quantum cosmology equation going back AFAIK to Hawking's quantum gravity group in the 1980s. On page 11 they say
    "We have thus calculated the wave function of the universe from first principles up to prefactors and corrections to the semiclassical approximation."

    And they cite the Hawking Hartle article Wave function of the universe published 1983 Physical Review Series D. Sounds like a Hawking title doesnt it? I have not seen the Hawking Hartle article, which AFAIK is not online. But it sounds like Hawking Hartle postulated a wavefunction which they thought might govern the scale factor (or some finite set of degrees of freedom describing the universe) and that now in 2004 these people get this wavefunction or something like it, to appear of its own accord out of their model.

    Yeah, it is their equation (22), and the wave function is a function of a, the scale factor or "size of universe". It is hard to fully believe this is happening. If they really do this, it's good.
     
    Last edited: Nov 24, 2004
  6. Nov 24, 2004 #5

    marcus

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    BTW Chronos, this from Woit's blog today

    "The NSF budget for research and related activities is being cut by .7% from its FY 2004 level, the first such cut in many years. The other main part of the NSF budget, that devoted to education, is being cut even more. A few years ago Congress passed a bill that was supposed to double the NSF budget over several years, but that bill is now very much no longer operative. It's not clear yet how physics and math specifically fare under this new budget, presumably we'll find out in the next few days.


    The bulk of particle physics funding comes from the DOE Office of Science, and there the budget situation is brighter, with an increase of 2.8% for FY 2005. Again, the details of exactly what is being funded and what isn't should soon be available."

    http://www.math.columbia.edu/~woit/blog/

    A glimmer of understanding! Its own special source of funds within DOE, as long as major advisors to DOE on particle physics remain convinced it is the "one best hope" of progressing beyond the standard model.

    Those allocating funds are not likely to support anything that doesnt appear to be particle physics. so at least in the United States, the other approaches to quantum gravity lack access to DOE and must apply to the leaner funds of NSF.

    If the situation Woit describes is true, then it could explain why the US establishment has been slow to fund promising non-string approaches to quantum gravity. And also might explain some of the rhetoric one hears (e.g. from Steven Weinberg) about the "one best hope" or "our only hope".
    It helps keep control of the fat preserve inside Department of Energy
    (for which other approaches seem not
    "particly" or maybe the word is "particular" enough to compete)
     
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