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An way to visualize dark matters effects on space-time?

  1. Nov 13, 2007 #1
    In an attempt to explain space-time curvature and gravity, the analogy of a flat rubber sheet is often used whereas a massive object, such as a bowling ball indents the sheet in the same way that a massive object such as a star creates curvature in space time.

    Would it be a fair analogy then to say that dark matter, with its repulsive qualities can affect the same rubber sheet, however instead of indenting it, it causes a rise on the sheet?

    Is that an accurate way to visualize how dark matter affects the curvature of space-time?

    Thanks,
    FRQ

    PS - Please keep the reply in layman's terms if at all possible.
     
  2. jcsd
  3. Nov 13, 2007 #2

    DaveC426913

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    Gold Member

    Dark matter does not act repulsively. Dark matter acts gravitationally exactly like matter i.e. attractively.

    What distinguishes dark matter from ordinary matter is that it is electromagnetically invisible - it interacts with neither atoms nor the EM spectrum.

    Dark energy acts repulsively.
     
    Last edited: Nov 13, 2007
  4. Nov 14, 2007 #3
    But it's a very bad analogy, and its a shame that so many popular science treatments of the subject use it.

    This shows why its such a bad analogy. The analogy tries to say that it is the intrinsic curvature of the rubber sheet that causes a deflection. But the intrinsic curvature is the same whether the rubber sheet is pushed downwards or upwards. So unless both have the same effect then the analogy is wrong. Of course your intuition tells you that they are different, but that is because you assume that there is a gravitational field external to the rubber sheet. So the analogy is using gravity to explain gravity.
     
  5. Nov 15, 2007 #4
    I thought the analogy was saying that mass dictates how spacetime curves, and the resultant curvature tells matter how to move within spacetime. No?

    Thanks,
    FRQ
     
  6. Nov 15, 2007 #5
    I did a google search to find out what was it was that was curved that "tells matter how to move". Sometimes it was space, sometimes it was spacetime.

    The trouble with the space version is that it doesn't accurately reflect what general relativity is about, which is the curvature of spacetime

    But then the trouble with the spacetime version is that things don't move within spacetime as this would imply some sort of time external to spacetime.

    Yes, matter has a geodesic path within spacetime, whose curvature is determined by the distribution of the matter. But in the case of the rubber sheet analogy the path would be the same whether the sheet went upwards or downwards - I think it would be deflected inwards in either case, so if the analogy suggests that it would be deflected outwards for a sheet pulled upwards then there's a problem with the analogy. The thing is that a ball bearing rolling on the sheet does not follow a geodesic of the surface.
     
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