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SR/GR Space-Time

  1. Mar 5, 2008 #1
    Hello everyone,
    I am a self taught (dare I say Physicist?). I have been struggling with a specific concept of GR/SR. This concept is "Gravitation" and it's explanation. As I understand it, classic Newtonian gravity in which mass attracts to other mass is not technically correct. There are "geodesics" in space-time that objects of mass follow. I have two questions in reference to this concept.
    1) Why does space-time even bend for massive objects? In Illustration 1a I have provided my conceptualization of the "Rubber sheet" of space time.
    [View 1a] GR-Spacetime-nobend.jpg

    This is what I would initially view the rubber sheet as. Now the trouble I am having is why would the sheet bend? What compels the massive objects to bend this sheet?

    2)In the event that space-time does bend (perhaps as volume displacement?) why would a lesser object (B) be compelled to move toward the more massive object (A) given a scenario in which Object B has no momentum (velocity)?

    [View 2a] GR-Spacetime.jpg
    As I understand it, in a zero-G environment neither object would move regardless of the curve in the rubber sheet. What makes space-time different?

    I look forward to your replies as these are questions I have never had properly satisfied.
  2. jcsd
  3. Mar 5, 2008 #2


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    Science Advisor

    Curved spacetime is just a model to describe how massive objects affect their surroundings. The why-question cannot be answered without creating the next why-question.

    Very well observed :smile:. The very popular marbles-rolling-on-a-rubber-sheet-analogy is misleading, because the rubber sheet represents curved space not curved spacetime. To understand the difference look here:

    And then check out the links I gave in this post:

    And then ask further questions, if any.
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