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Gravity model

  1. Sep 15, 2007 #1
    As i know the model of how gravity works is. The fact that there is a web of space time that you place a ball (sun) into it pushes down onto it, then placing smaller balls (earth) into this dent and they will in and around it...

    What I do not understand is how in the world is this logically sound because for the ball to create a dent in the space time.. it would require gravity....
  2. jcsd
  3. Sep 15, 2007 #2


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    I imagine what you're looking for is that, in general relativity, massive objects curve space-time. However, I don't understand your question: a curve in spacetime is gravity.
  4. Sep 15, 2007 #3
    Well if you look at all the models of gravity they say "Gravity is like putting a ball on a stretched out net." but that act of it falling into the net and pushing down /is/ gravity which doesn't truly explain it to me
  5. Sep 15, 2007 #4
    I added some emphasis to your post. It's just an analogy to help you visualize the situation, add some physical picture to the mathematics.

    Are you asking how/why mass curves space-time?
  6. Sep 15, 2007 #5


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    I think he is referring to the fact that the explanation in terms of stretched nets seems to rely on gravity already working, so that the balls can make the dents.

    To some extent, such "explanation" is a petitio principii, but it is convenient because it's easy to imagine. The actual formal mechanism in general relativity does not use a "basic force" or "meta-gravity" to stretch the sheet.

    The point you need to take from it is that matter modifies what "straight line" means for small objects in their vicinity, in such a way that when they (using information from close around them) keep their best possible straight line behavior, they may be describing closed paths, when looked from afar.
  7. Sep 15, 2007 #6
    Not only do we fall short with a 3-D model (ball and rubber sheet) which should act as a 4-D system, but this also fails to incorporate the phenomenon of inseparable spacetime geometry imparting acceleration to a mass.

    The most effective model of spacetime I have seen is of continuously embedded Minkowski (special relativistic) light cones at angles corresponding to their local acceleration, in three dimensional space.
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