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Tyro question

  1. Apr 12, 2004 #1


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    in quantum gravity theories, can a quanta of gravity exist in isolation
    or would it need other interacting quanta to perpetuate?
  2. jcsd
  3. Apr 12, 2004 #2


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    this is a good question in that it
    could produce a shower of divergent opinion,
    almost a "full spectrum" of views

    I think in asking it you probably want to elicit a wide spectrum.
    I shall give one very focused view

    I dont think of gravity as a substance that you can have quanta of.
    I think of gravity as revealing geometry
    the shape of space

    I think that the correct name for "quantum gravity" is really
    Quantum Geometry. the quantum theory of how space is shaped
    (because the shape determines how matter flows around, how
    planets orbit etc)

    I think that one should ask not about quanta of gravity but about
    the quanta of geometry
    and these are things like quanta of area
    and quanta of volume

    distance and curvature might come in discrete amounts too
    anyway it is geometrical entities that should out to be quantized
    in a quantum theory of geometry

    I think that a "graviton"
    is a fictional entity that is convenient in analyzing the behavior of the
    gravitational field in certain restricted (approximately flat) situations.
    it is a ripple in the fabric of geometry
    but in highly curved dynamic situations analyzing in terms of "gravitons"
    wouldnt work very well---they dont really exist it is just handy sometimes
    to pretend they do.

    The real quanta of geometry are quanta of area and volume and suchlike geometrical things, and if you know the volumes of enough regions and the areas of enough surfaces you have a handle on the geometry of space.

    In quantum gravity one of the landmark results is that areas and volumes of things are in fact quantized and are somewhat like the energy levels of an atom. That's what I look for from a quantum theory of spacetime geometry,
    that is, a quantized version of GR.

    other people doubtless have different perspectives on this
    Last edited: Apr 13, 2004
  4. Apr 13, 2004 #3


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    i will be quite happy now, if i can relate how an inspiraling body
    gives of geometry, or how mass is converted to geometry, thanks
    for your patience.
  5. Apr 13, 2004 #4


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    right as usual, very hard-to-understand stuff here
    how does mass effect the curvature of space?
    when two neutron stars spiral in towards each other
    they radiate energy (carried away by ripples in the gravitational field)
    and the system loses mass
    as it loses energy
    how can this possibly be?

    there are equations to describe what happens
    but I have no inkling of the mechanisms that make those equations work
    and why they fit

    maybe someone else can add insight here
  6. Apr 13, 2004 #5


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    if i considered that the destruction of mass initiated a shock wave that perturbed the quanta of geometry akin to the pixels on an oscilloscope, and the continuum of quanta transfered this shock energy to new born mass perterbances of geometry would this be a conservation of mass, geometry?
    or am i thinking utter ***P
    Last edited: Apr 13, 2004
  7. Apr 14, 2004 #6


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    there is a group of people who use a solid state model (condensed matter model: analogous to crystal lattice model) to describe gravity

    there was one of them here a couple of months ago, I will try to find a link

    I imagine these people think like that (maybe similar to how you are thinking)
    and they relate gravity to how shocks and defects propagate thru a crystal lattice
    someday, if they havent already done it, they might make computer-generated animated films of these "solid-state" gravity ideas

    it's way out of my ken
    all I can do is try to remember who that guy was that came to PF
    and try to dig up some keywords for a search

    IIRC the guy was unhappy. from his point of view so much attention is given to string and LQG that there is not enough attention left over for
    "condensed matter" models of gravity

    one thing you can say for those people is they have some different ways of visualizing gravity

    incredibly, wolram, the lady and I went scubadiving today
    it had been several years since I had a tank on my back
    speaking of gravity, it is a very different experience in neutral

    the difference between up and down becomes more subtle

    one rolls over or drifts heels over head rather easily but it doesnt make
    much difference

    drawing a deep breath (of air from the tank) makes you more buoyant and you start to rise, letting out a breath (as a cloud of bubbles) makes you less buoyant and you stop rising and slowly settle back down

    I admit this is irrelevant, but it was a nice way to spend the afternoon
    you are venturing ahead of the crowd, if you want to speculate what makes gravity work
    talk about pushing to the ragged edge!
    What I see going on today is they're mainly just trying to get a satisfactory quantum description of GR----able to match the usual GR effects---no easy
    matter in itself

    I'd guess they wont try to imagine actual mechanisms until after GR is quantized.

    progress on gravity is like tunneling through a mountain of solid rock
    a few inches a day, and they forgot to bring the dynamite
  8. Apr 14, 2004 #7


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    a wonderful thing diving under water, its a different world, i could never
    do it, fly light aircraft yes, i intend to look up condensed matter theory
    and see if i can find a gem of wisdom, happy S,Dving....
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