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Loopy quantum gravity!

  1. Jun 24, 2003 #1
    I've posted a thread related to a simplified system of dimensional units using quantum mechanical phase, a length, possibly a time and plane angle;

    You don't need all that junk.

    and another about why gravity may occur in the world:

    Why does Gravity occur?

    Let's see what happens when we "marry" the two sets of ideas.

    The second post says that Gravity happens because nature tries to maintain a quantity called the Volumetric Rate of Expansion, constant. In ordinary units it has the dimensions l3/mt but if we suppose that gravity couples to a more basic quantity than inertia, namely quantum mechanical frequency, then in the new units the rate of expansion is just l3/Ω, where Omega is quantum mechanical phase.

    This just means that whenever quantum mechanical phase increments by an amount, say 2[pi]radians the volume of the Universe increases by approx. the cube of one classical electron radius.

    What do you loop quantum gravity people think about this?
     
    Last edited: Jun 25, 2003
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  3. Jun 24, 2003 #2

    marcus

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    Hello Tyger, you clearly have a theory here about GRAVITY as distinct from geometry. Because my interest is so much towards quantum geometry I cannot find much to say.

    I am most interested in theories that suggest something about how the curvature of space evolves---how geometry itself arises.
    these are the "background independent" models of geometry that dont begin by assuming a static spatial framework for things to happen in.

    String theory has never seemed very interesting or promising to me for this very reason. It begins by a committment to a static, un-dynamic geometry: standard Minkowski space!!!. It cant seem to get started without it. So I dont think it has much chance of being valid. Nature is not flat unexpanding space of special relativity in which light always goes in straight lines. Naive to expect that kind of approach to work.

    I think it is pretty clear that "gravity is geometry" and that a quantum theory of gravity must be a quantum geometry and must start with no prejudiced preconceived notion of the shape of the universe. Particularly true for *quantum cosmology*!!!

    Ashtekar calls "Loop Quantum Gravity" by the more accurate name of Quantum Geometry---it seems more descriptive to me.

    Right now the field is hot and has been having a run of surprising results. Quantizing area/volume. Predicting BH entropy and/or vibration frequency. Eliminating matter field divergences----getting rid of the need for QFT renormalizations. And most recently overcoming the Big Bang singularity----loop quantum cosmology goes all the way back to time zero without blowing up and has spacetime dissolve into something else for negative time.

    Loop quantum geometry is not merely offering an explanation of the "force" of gravity or some schematic way of relating the "force" of gravity to some other forces---in the fixed arena of a static space. It gives equations for the evolution of curved and expanding space and it predicts interesting numbers----and it is on a roll right now.

    So while its predictions may be tested experimentally and eventually proven incorrect and some refinement or alternative to it (like spin foams in some as yet unformulated version) prove superior, it is currently the most interesting background independet model of quantum geometry in my view. By far.
    Cant think of any alternative. No real competition as far as I know.

    I take a skeptical view of the truth of theories that are largely untested and still under development. so I am not a "believer" of any description. So I cant speak for the "loop" people whose opinion you asked.

    But I can react as someone who expects a background-free quantum geometry of SOME type to be the next major development in physics.

    Such a thing must be developed, I believe, before there can be any valid successor to the "Standard Model" because today's particle fields are only defined in a flat preconceived space. So we have no acceptable theories of particles or forces----only something very ad hoc.

    So imagining a valid "TOE" defined in unvarying flat space is premature and naive.

    Your theory has some appeal though----at least it works without postulating invisible extra dimensions conveniently rolled up so one doesnt trip on them. You could learn something by playing aorund with it.

    I should tell you that the planck unit of energy density (same as pressure, same as momentum flux) is guess what

    c7/G2hbar

    seems to be energy density (and its look-alike pressure) that
    determines curvature. looking at the dimensions may suggest how this works to someone, if not to me. You have been doing a lot of dimensional reasoning I think----where your current model of gravity came from.
     
  4. Jun 25, 2003 #3
    The said faction of space that supports the unit, will either be in expansion or contraction, the Varying Volume/Length evolves as a two-way traffic system.

    Alpha moves to Omega as Omega moves to Alpha? :wink:
     
  5. Jun 25, 2003 #4
    The Big Bang scenario

    says that matter existed before space, as we know it, appeared in the Universe. If that is valid then the correct description should start with matter and with the passage of time have empty space (the ubiquitious vacuum) appear at a later time, expanding all the while toward its current state.

    Of course it's possible that some parts of the scenario are wrong. We don't really know how matter arrived in the world, or whether it is being made right now by some process.

    But that's one of the big problems with Cosmology, devising all the possible scenarios and then finding some kind of experimental evidence to select among them.
     
    Last edited: Jun 26, 2003
  6. Jun 25, 2003 #5

    marcus

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    Re: The Big Bang scenario

    You might find this interesting. There has been a successful quantizing of the big bang which gets thru it without the model blowing up
    so there is evolution of the universe before time zero

    see last paragraph on page 25 of
    arxiv/gr-qc/0202077
    "in which the universe bounces off in order to enter an expanding branch. ....whether it expands forever or recollapses in order to start a new such process depends..."

    and pictures of a wave function passing the time-zero point (where the classical singularity was) on page 16 of
    arxiv/gr-qc/0303073

    these are recent papers (this year and last year) by Martin
    Bojowald who is a postdoc of Astekar and getting results very fast at present. he sort of leads the pack in loop quantum cosmology.

    i find myself slowly stopping to believe in the singularity at time-zero that we have always been told about (because classical GR predicts it)

    Bojowald's models include matter----matter and geometry are coupled throughout, so one does not preceed the other.

    in another paper he has found a quantum geometrical mechanism for inflation---accelerated exapansion early on. he is going thru a very productive phase apparently with guidance from Ashtekar---on the order of about ten innovative papers last year
     
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