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Trying to understand Quantum Gravity

  1. Jun 14, 2004 #1
    There’s a consensus among many physicists that the universe did not emerge from nothing, if fact stating that nothing simply cannot exist.

    However while reading Carlo Rovelli book Quantum Gravity, the following on page 25 states:
    So if we continue with this interpretation of fields building fields, do we have a picture of the universe which is built from nothing? Or something entirely different?

    Hope I'm on the right track here. :frown:
  2. jcsd
  3. Jun 14, 2004 #2


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    I guess i would caution that how we humans contrive to model the universe is not necessarily the same as how the thing itself came into existence

    rovelli, if i understand him, is here not saying anythingabout how It was created

    he is talking about how we model it

    In Newton's time it was believed that you could not construct a satisfactory model without first assuming a background of absolute space

    certain areas of theoretical physics still require a fixed background
    e.g. conventional quantum field theory QFT is constructed on an artificial stage called "Minkowski space". this is a rigid spacetime with no curvature and no spatial expansion
    so it in this respect it is unlike nature
    however the theory leads to good approximations

    Many people therefore imagine that it is impossible to define a field without first setting out a rigid absolute spacetime framework on which to define it.

    However rovelli and others have described a proceedure for defining the gravitational field without prior committment to a fixed geometry
    this field then IS the geometry of spacetime and upon that field other fields can be defined.

    so the models of all fields/particles can be bootstrapped into existence
    without resorting to a prior static framework

    the model is dynamic from the ground up

    this is the whale on which the other animals are sitting


    notice however that this is talking about the human process of constructing models with which we desire to make checkable predictions

    God, as one laughingly says, may have used some absolute space in which to create the world and all its fields, or he may not have. Rovelli's parable
    IMHO IMHO IMHO is not supposed to suggest how the world came into being but how models of it are constructed. I fervently believe that the jury is still out about how the world came into being and that what one should be desiring is merely to have a model of quantum spacetime with matter fields in it that will actually predict numbers that one can check by observation. we humble vertebrates who still resemble our fish ancestors do not even have this
  4. Jun 14, 2004 #3


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    Vast, I forgot to tell you, it is really exciting to encounter
    someone else who is reading rovelli's book Quantum Gravity.

    in this book quantum states of the gravitational field
    eventually get identified with colored networks

    immense networks with jillions of edges, and the coloring
    of the edges and vertices can represent matter fields and particles

    and the model has "diffeomorphism invariance" that is you dont
    break it if you morph it around---it doesnt have a stiff preordained shape.
    so its a audacious new vision of things
    (which in the largescale limit is supposed to look like the familiar world, it is only if you take a microscopic look up close that the networkiness appears)
    actually einstein's 1915 GR already has diffeo morphability and backgroundlessness---GR has no fixed geometry the geometry comes out as a solution to the equations. So what rovelli and the others are trying to do is get a quantum theory that has the same freedom and morphability as the original 1915 theory of gravity.
    this would, I guess, allow these same qualities to spread to the rest of physics
    what is now QFT (the fields of matter and its forces) would be redefined
    in a background-free and morphable manner----on a whale instead of an island

    if it succeeds it will bring about a major change in physics
    this is my take on it anyway

    I hope you enjoy the book whether or not you agree with this personal view of it!
  5. Jun 14, 2004 #4
    So far I have been enjoying the book, and your personal view is appreciated!
    Spin networks and loops is still all fairly new and over my head, but already it is quiet exciting.

    My original assumption was that what was being described had implications to an actual explanation of how the universe is built. I didn’t read it that way at first but later on considered if it did actually represent the way the universe comes into existence.

    I have to say that using the gravitational field as an actual canvas does make sense.

    On another note how would this stand up to space being infinite? It seems to contradict space being infinite in extent, because as they say, if on the submicroscopic scale we see that there is a mesh of loops, wouldn’t that imply that space was finite?
  6. Jun 14, 2004 #5


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    I see. It is a problem! it does seem as if the LQG quantum states of the gravitational field (being networks) are intrinsically finite

    And in contrast, the simplest most troublefree flat classical cosmology model has space be infinite

    right now i cant think how that might be resolved
    maybe if the Loop gravity program succeeds it will narrow the cosmology menu down to finite space models, but it might not. Perhaps there is another way
    it could get sorted out

    quantum models describe the state of an observers knowledge, and I guess one's information is always finite even if the universe is not

    and if there is no bound on the number N of nodes in the network,
    then although each pure state consists of a finite network with some number of nodes, yet a mixed state which is an average or sum of many pure states could have an arbitrarily large indefinite number of nodes and
    it begins to look as if the u could be infinite afterall. I am afraid I am not thinking very precisely about this. Unless someone else responds I will try to get back to this question. it seems like a good question.
  7. Jun 18, 2004 #6
    are you reading the draft pdf version of this book? it doesnt seem to have been published yet...
  8. Jun 18, 2004 #7


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    publication is not expected until later this year
    Cambridge Press says September, Rovelli says October

    it is easy to download the draft pdf version
    which is what everybody at PF I've talked to who
    has read in the book has done.

  9. Jun 19, 2004 #8
    "Nothing" is "not anything". One cannot build with or from "not anything".
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