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Could GR generalized to non-integer dimension?

  1. Apr 10, 2009 #1
    could GR generalized to non-integer dimension??

    let us suppose that the dimension of space time is NOT an integer then , could we generalize GR to obtain an expressions of Tensor, Covariant derivatives... in arbitrary dimensions ?? let us say 4.567898.. or similar, i mean GR in non integer dimensions of space time
  2. jcsd
  3. Apr 10, 2009 #2
    Re: could GR generalized to non-integer dimension??

    What is a "fraction of a dimension"?
  4. Apr 10, 2009 #3


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    Re: could GR generalized to non-integer dimension??

    Maybe zetafunction is talking about the concept of fractals having fractional dimension--there's a basic conceptual explanation here, and someone wrote up a good post here, the basic idea being:
    I'd doubt that the topological definition of dimension can be applied to differential geometry to give metric spaces with fractional dimension, which would mean you can't generalize GR to fractional dimensions, but who knows...
  5. Apr 10, 2009 #4


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    Re: could GR generalized to non-integer dimension??

    Apparently there are some speculative theories of quantum gravity in which spacetime has a fractal structure. On a macroscopic scale, everything is 4-dimensional as in classical relativity. The suggestion is that as you zoom in to quantum scales, the fractal dimension continuously reduces to a lower value, e.g. 2 in the limit. On Planck scales, spacetime becomes a self-similar fractal, it is suggested, so as you zoom in even further there is nothing more to see, just a copy of what you've already seen. Apparently a 2D version of quantum gravity is much easier than higher dimensions. Not that I understand quantum theory, or fractal theory, very well.

    Reference: Jurkiewicz, Loll and Ambjorn, "Using Causality to Solve the Puzzle of Quantum Spacetime", Scientific American, July 2008. On page 1, follow the link to "Zooming in on Spacetime".
  6. Apr 10, 2009 #5
    Re: could GR generalized to non-integer dimension??

    Fractals are not even manifolds, much less differentiable manifolds. The answer to your question is no, although may be one day someone will find a sweeping generalization of the manifold concept that includes fractals; such a generalization has not been found in mainstream math or physics to date.
  7. Apr 11, 2009 #6


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    Re: could GR generalized to non-integer dimension??

    You can complexify dimensions in GR so that the real parts of each dimension are fractional.
  8. Apr 11, 2009 #7
    Re: could GR generalized to non-integer dimension??

    I was also thinking along the lines of post #5....but as always approximations might provide an alternative depending on the scales of investigation....

    In addition another physical variation is the possibility dimensionality limitations suggested by string theory T duality..insensitivity to size R or 1/R...

    And even Planck scale minimums from quantum theory might thwart GR in any formulation to date....
    So there seem to be several different views of space/time that are not very smooth nor even classical in concept...more to come, I'm sure...
  9. Apr 12, 2009 #8
  10. Apr 12, 2009 #9
    Re: could GR generalized to non-integer dimension??

    Dimensional regularization is just a formal trick for taming integrals, it is not equivalent to a geometric consideration of non-integer dimensions: 4 - epsilon etc has symbolic (mathematical) meaning only, not physical meaning, except that the trick leads to correct answers when tested against experiment.
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