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String irrelevant to quantizing General Relativity (quantum spacetime geometry)

  1. Jun 27, 2003 #1


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    Want you should know this is my individual view---I dont pretend to speak for the "high energy researchers" of the "physics community"

    LQG continues the on-going attempt to quantize the Einstein equation and General Relativity. It is the current stage in an effort that goes back to J.A.Wheeler's 1962 "Geometrodynamics" and earlier to work of Dirac.
    The point is that Einstein made the metric of spacetime dynamic, evolving governed by an equation. Gravity IS geometry. As early as the 1940s physicists have seen dynmamic spacetime geometry as something to quantize.

    String has no relevance to this effort---it is not even on the radar---not a quantum theory of spacetime geometry.
    It starts instead with the usual flat, static, unexpanding Minkowski space that conventional particle theorists use. It tries to treat gravity as a force like electrostatic attraction. This is Newton's approach and it is inherently inaccurate. A useful approximation in some situations but not the fundamentally correct way to go about things.

    The current stage of the longterm effort to quantize the geometric model of gravity is on a roll. Run of successes lately. Rovelli working on a graduate textbook. Among researchers, Bojowald for one has been bringing out roughly a paper every 60 days, either solo or coauthored. New people are getting into the action. Just from my readings of 2002 and 2003 papers, and their references, I note that:

    1. Thiemann has found that LQG gets rid of divergence in matter fields without renormalizations. the infinities of Quantum Field Theory automatically vanish when it is done in the geometrical context of LQG.

    2. Bojowald has found that Loop Quantum Cosmology does not suffer from a big bang singularity. The geometrical and the matter fields extend back to time zero and thru time zero wthout blowing up. A definite improvement over non-quantized GR.

    3. Corichi has has offered an argument that LQG PREDICTS a result about the vibration modes of a black hole. This is a new and intriguing idea and will probably be worked on.

    4. Bojowald has found that LQ Cosmology predicts Inflation. The timing and extent depend on the choice of a spin parameter. Inflation emerges directly from the theory without postulating an "inflaton" particle or field. Ordinary matter fields will do and are compatible with LQ Cosmology.

    These are some of many developments that have come to my attention recently----2,3,4 refer to 2003 and 2002 papers. Thiemann's result (1) just came up as reference in a 2003 paper. Matter in LQG is increasingly being explored and included as the field matures.

    In conferences on Gravity these days, when I see their schedules on the web, string theorists are mostly absent and LQG theorists are very active. In 2002, IIRC there was the 16th International conf on Gen Rel and it was maybe 1 stringer to 20 loopers and assorted others.
    This very month June 2003 there was another conference---on quantum gravity----only one stringer gave a paper and there were far more papers by LQG people, plus some LIGO gravity wave people, some astronomers, some spin foamers etc etc.

    LQG looks like it is emerging as the dominant research area in quantum gravity.

    As can happen during scientific model shifts, the name is changing.

    Ashtekar who is the nearest thing to being the father of the field
    likes to call it "Quantum Geometry"
    Thiemann, another central figure, calls it "Modern Canonical Quantum Gravity"-----alternatively one says "Background Independent Quantum Gravity."

    The identification with loops is less important (they enter in 1986 with the new Ashtekar variables) and what matters is
    that it is the modern (i.e. post 1986) continuation of the longterm (since before 1962) effort to quantize General Relativity

    There has been a bunch of portentous bunkum spouted about geometrical Quantum Gravity "severing all connection with the real world" or "burning its bridges to the low energy limit etc etc".

    But this is ridiculous in light of the facts---the theory continues to make interesting predictions about the real world which agree with semiclassical or classical calculation (the black hole vibration mode thing was spectacular---it got a number 4ln(3) out of the blue)----and to remove classical singularities and divergences (as quantizing the hydrogen atom did almost a century ago).

    Very active field, and accomplishing just what one expects a program of quantizing something to accomplish.

    Robust field too, original Sixties approach ran into roadblocks and then Ashtekar's breakthru in 1986 showed a way around that. Very rapid multipronged advance now---and of course could well encounter FUTURE obstacles (with experimental checks beginning) but then will very likely find ways to cope with them as well.
    Last edited: Jun 29, 2003
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  3. Jun 27, 2003 #2


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    Cosmology source

    since (loop) Quantum Cosmology is starting to get into predicting stuff like accelerated expansion of the universe, and inflation, and resolving the time-zero singularity I had better post the best Cosmology source I know which is Charley Lineweaver's.

    It just came on line May 2003


    It is mirrored at the Caltech Level 5 site.


    It is pedagogical---a tutorial like Ned Wright's except more up to date in some places, like about accelerated expansion. New diagrams.

    Printed out, Lineweaver's tutorial is 34 pages. I'd say it is well worth printing out.
  4. Jun 27, 2003 #3


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    Loop Quantum Cosmology survey---June 2003

    The best introductory overview to Loop Quantum Cosmology that I know is one that came out this month---June 2003

    arXiv: gr-qc/0306008

    It is by Martin Bojowald and Hugo Morales-Tecotl.

    The second coauthor is at Iztapalapa University, Mexico. Apparently they had a quantum cosmology seminar there.
    It is 42 pages.

    (Loop) Quantum Geometry is sketched out from first principles the authors do not assume the reader knows it already. Could serve as a review for some.

    the elimination of the big bang singularity by quantum gravity is discussed

    also the quantum gravity mechanism for inflation during a brief initial period
  5. Jun 29, 2003 #4
    I agree with Jeff. As I have stated in various other threads/polls about the 'final' or 'ultimate' or whatever theory, it is going to encompass a synthesis of both LQG and M-Theory(String Theories).
  6. Jun 29, 2003 #5
    Great synthesis often occur

    when people discover that two "different" theories have the same underlying structure.
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