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Can Dimension and Time be quantized?

  1. Mar 26, 2005 #1
    Can Dimension and Time be quantized? For example, distance and time is a whole number multiple a smallest unit.

    p/s: I don't have any facts to support my question, so this is possibly one of my illogical thought :smile:
     
  2. jcsd
  3. Mar 26, 2005 #2
    Some approaches to quantum gravity, such as loop quantum gravity, postulate that spacetime itself is quantized, this tentative theory being background independent (string theories for example are background dependent, that is say nothing about the structure of spacetime itself other that it is continuous). Sure the work is still in progress and some still argue that LQG is a dead end, yet I see no good reason for such a strong claim. We can never be sure of what the future has in store for us. The most prominent figure pursuing this research programm is Lee Smolin, if you are interested you can find a review of the actual 'state of affairs' on the field at:

    http://arxiv.org/PS_cache/hep-th/pdf/0303/0303185.pdf

    and Carlo Rovelli's account at:

    http://relativity.livingreviews.org/Articles/lrr-1998-1/
     
    Last edited: Mar 26, 2005
  4. Mar 26, 2005 #3
    That's the area of Loop Quantum Gravity : the quantization of space time. i wrote a nice introduction on LQG here : https://www.physicsforums.com/showthread.php?t=54167

    I suggest you read and try to understand it before starting on the Smolin-paper. You should also check out the website of Carlo Rovelli. Just google for his name.

    marlon
     
    Last edited: Mar 26, 2005
  5. Mar 26, 2005 #4
    I have previously wondered how string theory can predict gravitons but treat space as continuous. If gravity is the curvature of spacetime and it comes in little lumps called "gravitons", how do we get a continuous spacetime from that model?
     
  6. Mar 26, 2005 #5
    You are mixing this here : LQG is fundamentally different from String Theory. In LQG you hold on to general relativity and try to rewrite QFT so that space time becomes quantized. In String Theory you hold on to QFT and you rewrite general relativity. It is up to you what the best choice is... :wink:

    Gravitons arise in QFT actually. You know that particles are the excitations (vibrations) of fields (which are continuous). If you take a massive spin 2 field and you distrube it by a massive object (a source and a sink, if you will) this field will start to vibrate and therefore yield a spin two particle called the graviton. The disturbance is expressed by the stress energy tensor from general relativity.

    The reason that this field must be spin two has to do with the fact that it has 5 degrees of freedom. The stress energy tensor is a symmetric Lorentz tensor with ten degrees of freedom but this field theory will eliminate 5 degrees of freedom so that only five remain...


    regards
    marlon
     
  7. Mar 26, 2005 #6

    Hurkyl

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    I would like to point out something that is often misconstrued: LQG does not involve a quantized space-time. TMK, no serious theory does -- what LQG actually quantizes is area and volume.
     
  8. Mar 26, 2005 #7
    Yes, it does...though we need to be clear on what is meant here :

    I refer to page 31 of Rovelli's book : http://www.cpt.univ-mrs.fr/~rovelli/book.pdf

    Quantization of space-time means that starting from the volume-operator you can prove that it's spectrum is discrete, expressing these little grains of space time. Space time (the overall manifold that is) itself is no continuum anymore.

    Besides, the most common misconception of LQG is the fact that many think this theory is an attempt for a TOE, which is NOT the case.

    marlon
     
  9. Mar 26, 2005 #8

    Hurkyl

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    All right then. I was suffering from the popular image of "quantized space-time" meaning discrete points. (as the OP thought)
     
  10. Mar 26, 2005 #9
    Last edited: Mar 26, 2005
  11. Mar 29, 2005 #10
    In really simple terms then, does this mean that if LQG is correct then gravitons are not likely to exist, but if string theory is correct then gravity can not always be treated as the curvature of space-time?
     
  12. Mar 29, 2005 #11
    No, gravity can always be treated as curvature of spacetime. Both string theory and LQG do not state the general relativity is wrong. It is NOT.

    regards
    marlon
     
  13. Mar 31, 2005 #12
    I am still stuck on how gravity is caused by virtual particles _and_ the curvature of space-time. I don't think my popular science book on string theory explained this.
     
  14. Mar 31, 2005 #13
    Well, first of all you need to understand very well what virtual particles are. I have explained this many times so i refer to the 'virtual particles'-entry in my journal...

    Also, the curvature of space time is NOT related to virtual particles because the latter arise from QFT and the former arises from general relativity. Both theories are fundamentally different in nature and even very much contradictory, though BOTH correct in their physical regime

    regards
    marlon
     
  15. Mar 31, 2005 #14

    selfAdjoint

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    I think marlon overspoke a bit. Particle physicsts assert that GR is "correct" up to some limit, that it is an "effective theory", not necessarily a finally true description of the world. And some physicists have certainly denied the truth of gravity from curved spacetime, for instance Nobel winner Steven Weinberg, who called it a metaphor that was out of date. String theory produces a graviton picture against a background that may be curved, but is not dynamic or responsible for gravity.
     
  16. Mar 31, 2005 #15
    Sorry, i disagree,

    I specifically talked about physical regime.

    I find this hard to believe though i am not denying that Weinberg actually said that (i don't know).

    What exactly does he means by these words.

    I mean, curved spacetime produces correct experimental data so let us not start 'semi-philosophical' discussions as to whether general relativity is right or wrong. We have seen these discussions many times in the QM-section and they never brought anything worthy. Why ? Well, because QM is NOT wrong. Same thing here...

    regards
    marlon
     
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