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What's the reason?

  1. Aug 3, 2005 #1
    THE THEORY OF GENERAL RELATIVITY states that the gravitation is due to the fact that the heavenly bodies create a sort of dent in the spacetime fabric.


    Let us for example take an elastic sheet and place metal ball on it. The ball will make a depression in the elastic sheet.

    But, the ball creates that dent because something (i.e. the gravity of the earth) is pulling it and forcing it to create such kind of dent.

    So, I repeat my question-
    What is the reason that the heavenly bodies (like stars, planets, etc) create such kind of a dent in the spacetime fabric?
  2. jcsd
  3. Aug 3, 2005 #2
    The ball will also create a dent if the fabric is accelerated - for example, consider a 1 meter square sheet of spacetime fabric in free space and a queball located at the center - now grab the four corners and accelerate the fabric - the inertia of the queball will deform the fabric. You don't need gravity to explain gravity. If you consider the Hubble sphere as having a radial expansion c, then the Hubble volume is accelerating - and the amount of acceleration is consistent with the gravitational constant - which incidentally has units of volumetric acceleration per unit mass.

    But one shouldn't take the analogy too far - the idea of a stretching spacetime fabric is metaphorical
    Last edited: Aug 3, 2005
  4. Aug 3, 2005 #3


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    Good question, I think we'd all like to know the answer to that. I doubt we ever will though. By definition: if a theory is fundamental, there is no further explanation possible. Only if relativity is ultimately shown to be part of an even more fundamental theory will it be superceded.

    Even with your description of the ball and the dent caused by gravity, nothing is answered - because you think it is a "reasonable" explanation not requiring further elucidation. But any theory of gravity comes back to exactly the same question: Why?
  5. Aug 3, 2005 #4


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    It is questions like this that lead to people advocating "intelligent design". That is the catchall answer to any question which science is unable to answer (at this moment).
  6. Aug 3, 2005 #5


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    See this thread generally, and posts #4 ff specifically.
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