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[Gravity in general relativity] I dont understand this help!

  1. Dec 4, 2013 #1
    Please view the following...


    This is the way scientists try to explain the warping of space to explain gravity effects between to objects.

    The very demonstration requires gravity to work! Why does one object track along the displaced track to begin with? Because gravity is drawing it downhill into the dimple. The dimple does cause a curved track but something else is causing the object to move within the influence of the dimple....gravity.

    Let's say we place two earth sized planets in space at, say, 300000 km apart. No movement relative to each other. Space time is distorted but so what. What causes one object to begin to be moved thru the curved distortion?

    I can visualize how the 'track' of an already moving object will be curved by distortions as it travels. But I don't understand what would cause a 'relatively' unmoving object to 'want' to begin to move toward another?

    With the video, earth's gravity is pulling down on the balls causing them to accelerate downward into the fabric depression caused by the other object. In other words, gravity itself is necessary to explain gravity. This kind of visualization only makes me more confused.

    Perhaps someone can help.

  2. jcsd
  3. Dec 4, 2013 #2


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    Science Advisor

    You have to include the time dimension to describe that:


    Yes, it's a very bad way to explain General Relativity.
    Last edited: Dec 4, 2013
  4. Dec 4, 2013 #3
    Gravity warps space-time, not just space. You're always moving through space-time even when you're at rest because you're going to the future which means that you folow a line in space-time even when you're at rest that line is called the world-line. The warping of space-time forces the object that was initially at rest to start falling towards the earth because that's the way it's world-line goes through the warped space-time. You have to keep in mind that relativity is a four-dimensional geometric theory.
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