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Simple Question on General Relativity

  1. Aug 19, 2004 #1
    Ok, I have studied somewhat General Relativity yet it hasn't fully answered a question that keeps popping up in my head. Though it is probably easy to answer, yet hasn't been comprehended by me. The question is on Space curve caused by dense objects in space. I will ask as simple as possible and hope you all can answer it for me..

    What governs the direction mass and density sink into space?

    I may be off track completely and not getting general relativity, but this question is bothering me and if I'm not getting it I would like someone to explain the answer so I can understand lol ty
  2. jcsd
  3. Aug 19, 2004 #2
    the centre of mass does.
  4. Aug 19, 2004 #3


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    At every point, the stress-energy tensor (which includes mass density) contributes to the curvature of space at that point. Normal mass and energy always curves space positively.

    - Warren
  5. Aug 20, 2004 #4
    What does this mean, i.e. what does ..direction of mass and density... I never heard of mass/density having a direction.
    Huh? I don't understand what you mean by this. E.g. An open universe is a universe with negative spatial curvature. And that's with normal matter.

  6. Aug 20, 2004 #5


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    Apparently the original poster is thinking of the "rubbersheet" analogy in which a marble "sinks into" the rubbersheet, causing a warp that would cause another marble to circle around it.

    That is only an analogy. The "sink" would be in a "direction" not defined in the original space.
  7. Aug 20, 2004 #6


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    To expand on what Halls said, the rubber sheet has a two-dimensional surface, and gets curved in a third direction. Bringing the analogy to reality requires adding one dimension, so the three-dimensionsl surface we call the universe curves in a fourth direction. This is a direction in which we cannot move or look (or point, or even think).
  8. Aug 21, 2004 #7


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    I'm guessing that you are imagining normal 3-d space being embedded in a higher dimensional space.

    The answer is that the approach used doesn't care about any particular embedding. Geometry is studied entirely from the inside of our 3 dimensional space and 1 dimensional time. SInce any extra dimensions beyond these are not observable, we don't need to theorize anything about them. Mathemeticans call this studying "intrinsic curvature".

    The way this is done is by studying distances, and how they add. It's quite similar to the way that people navigate on the ocean using 2 coordinates, turning what would be a three-dimensional problem into a 2-dimensional version of the same problem, an approach that even a hypothetical 2-dimensional being could manage.
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