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Birth of Spacetime in Quantum Gravity

  1. May 20, 2005 #1
    The question I am going to ask here lies on a thin line between quantum gravity research and philosophy, but I'd thought I'd post it in this forum here to get insights in line with the research on quantum gravity.

    Basically, in quantum gravity approaches, LQG is background-independent but string theory is background-dependent, so what implications do these two different approaches have for the birth of spacetime? Do they point to space and time always existing (as suggested in the Pre-Big Bang scenario of Veneziano et al.) or that spacetime (at least the 4D spactime of our Universe) arose during the Big Bang (as suggested by GR)? Could it be that the notion of space and time as can be measured is different from a more 'philosophical' notion of space and time as infinite and eternal concepts?

    I'd appreciate anybody's insights.
  2. jcsd
  3. May 20, 2005 #2


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    Look at Figure 1, on page 5, of http://arxiv.org/hep-th/0411152 [Broken]

    Our species of animal that evolved on this little planet doesnt know everything. We can't expect our "natural philosophy" or our math/science to tell us what spacetime is and where it came from. We are at a particular moment of history when we are finding out a little bit more. this figure 1, which doesnt look so impressive, is representative of the best our species can do at this time.

    It is produced by the CDT computer model at Utrecht, in holland.
    CDT has a transfer matrix or, if you like, a hamiltonian. It is a quantum theory of spacetime that seems so far to approximate General Relativity OK at large scale. It


    when you look at a mathematical object you have to ask what the words mean in that particular context. Words like "background independent", or "dimension", or "spectral" can mean different things in context.

    Most physics is built on a smooth manifold with metric (the distance function determining the geometry is chosen prior).

    Gen Rel and LQG throw away the prior-choice metric, but they keep the smooth continuum. So some, but not all, of the background they get rid of. They are both built on a smooth manifold----a continuum with fixed integer dimension and infinitely differentiable coordinate functions---only it starts out shapeless. that is good is a step in the Background Independence direction.

    CDT gets rid of MORE. It throws out the coordinates and the differentiability and the smoothness.

    the CDT spacetime is the limit of finer and finer piecewise flat assemblages of little pyramids or simplexes. IT IS NOT ITSELF MADE OF SIMPLEXES, but is a limit of this kind of mosaic of smaller and smaller little flat building blocks.
    and it is not just ONE but is a quantum or uncertain cloud of such limits of assemblages of little simplex blocks.

    it is extremely unsmooth but, surprisingly enough, it looks like smooth 4D spacetime at macro scale!

    Some people in Utrecht found how to simulate a process by which this spacetime comes into being or evolves from one possible shape of universe to another----they found how to simulate it in a computer.

    many things could, and did, go wrong. Last year (after about 10 years trying) they found the right recipe and got it to work.

    this is where our type animal, where humans, are at this time. Look at it. it is not all that impressive.

    they had to make the universe be closed and eventually collapse back in a crunch, but that is just a technicality----they had to make it use a finite number of building blocks because their computer was finite. with a bigger computer they could make it live longer and expand more.
    Last edited by a moderator: May 2, 2017
  4. May 20, 2005 #3


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    LQG points to the big bang having been preceded by a contraction phase.
    This has been shown at first in a highly simplified (isotropic homgeneous) model and then gradually, in a dozen or so papers, under more general assumptions. It still has not been shown to be a consequence of the full LQG theory nor in the most general case. that is work in progress.

    the results so far are very suggestive that there is no bigbang singularity in the LQG model and our expanding universe was preceded by a gravitational collapse---some kind of crunch or black hole.

    Also LQG is in the process of eliminating the black hole singularity.

    LQG is in process of getting a better understanding of what goes on inside what used to be called a singularity (a place where the classical theory breaks down)

    You asked about a BIRTH of spacetime. I didnt hear of any birth. I only heard that the old idea of the big bang singularity as the beginning of everything is wrong. It is based on 1915 mathematics. the breakdown of a classical theory now being repaired and improved. So time to forget about that picture of the Beginning.

    IMHO at this point the interesting question is WHAT IS spacetime and how does matter bend it? Find out what it is first, I would say, then come back later and ask how it began.

    CDT is new compared with LQG. CDT only goes back to 1998, while
    Loop goes back to around 1990. String and "branes" are older still, and may be a case of arrested development as far as physics goes.
    But in spite of being new, CDT is impressive. Anyway. look at that Figure 1 for an idea of where CDT is at the moment.
    Last edited: May 21, 2005
  5. May 21, 2005 #4
    Thank you for the interesting links. I had heard about CDT before but was not familiar with the approach they take to spacetime. The issue that I have difficulty understanding is how space, which seems to be such an abstract entity, can actually be torn apart (such as in string theory). I have seen diagrams of Universes shaped in the form of the letter 'C', where a rupture in space on the bottom and top part of this C-shaped Universe could create a wormhole (i..e. a new spacetime between the two areas). However, what was in between those two areas before? It must be space! What else can be used to describe it? Perhaps it's simply empty spacetime. I know this has a philosophical aspect to it, but I think it's important to mesh the advances and insights of modern physics with philosophy, in order to create a coherent model of the Universe in ontological terms.
  6. May 21, 2005 #5
    I don't think you can separate the two questions. If you are looking for a definition of what spacetime is, you'll probably have to know how it develops, which includes how it develops from nothing, or a singularity. Certainly if you know how it develops from nothing, then you have a definition in all circumstances.

    Perhaps it helps to explain quantum gravity in terms of quantum geometry. What does quantum geometry mean? Is it the superposition of every possible metric? Or does it go beyond that to include every possible dimension too? What does CDT say about this?
  7. May 21, 2005 #6


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    A very human view, that can not progress , without input, at this time
    theories are being tested, but it is the lack of hard data that inhibits
    progress, this is the time for re examinig the data we have, adding to it
    and emulsifying these theories, even allowing fringe data into the pot.
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  8. May 21, 2005 #7


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    I most sincerely disagree. Theories may flop all over the lot, but let us always strive to keep our DATA as pure as ever we can. This is the time to prune fringe data and demand that the theories try to match our very best experimental and observational results.
  9. May 21, 2005 #8


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    This is the blinkered horse view, one only looks at the data one wants to,
    if progress is to be made, "something that is lacking", in any theory of
    gravity, then "all", data should be looked at, "nothing" should be discarded
    until its irrelevance is beyond doubt.
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