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Ball goes up again due to curvature of spacetime

  1. Dec 12, 2014 #1
    A ball thrown up comes down again because the ball follows the curvature of spacetime. So is it possible that the ball comes down and before hitting the surface, goes up again due to curvature of the spacetime,which now point upward.
  2. jcsd
  3. Dec 12, 2014 #2


    Staff: Mentor

    Why would the curvature point upward?
  4. Dec 12, 2014 #3


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    Staff: Mentor

    Sure, if you can arrange to curve spacetime in such a way that the curvature carries the path of the ball away from the surface of the earth. You could do that by arranging to fix a mass much larger than that of the earth directly overhead. You don't even need any general relativity to do this, ordinary classical Newtonian gravity will describe this (completely impractical and unrealistic) setup just fine.

    But if the earth is the only large mass in the area, the curvature will be such that the ball's path heads towards the center of the earth. Mass and energy are what curves spacetime, so you won't get a different curvature unless you introduce another large mass into the picture.
  5. Dec 12, 2014 #4
    Like in the picture, if the toy car is driven upward, when it goes to the other side, it comes down. Likewise if the line in the picture is high enough, when it can go up and come down, couldn't while it is coming down, couldn't it rise up again.

    Also why is that the ball thrown vertical, always remain overhead us and do not go ahead from us in the curve path.
  6. Dec 12, 2014 #5


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    Science Advisor

    Nope. The path always tends towards the wider part. Get a vase, some adhesive tape, a try it out. Stick the tape as straight as possible around the vase, without tearing or folding it.

    To come back it would have to fall through the Earth to the other side, where the curvature is the other way around, like shown here:

    This diagram only has the vertical space dimension in it, so it can only show a vertical trajectory. The other dimension is time.
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