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What mechanism causes the acceleration?

  1. Aug 14, 2013 #1
    If I were to hold a tennis ball out at arm's length, and release it, we all know it would fall due to gravitational attraction from the Earth's mass.

    In Newton's physics, gravity was a force that was created by and acted on all mass, causing two objects to accelerate toward one another, at a rate proportional to the sum of the masses and the inverse square of the distance between them (I think that's right). This is easily grasped even by schoolchildren.

    When Einstein came along, he removed the 'force' from gravity by describing it as a curvature in space-time, caused by the presence of the mass. Analogous to an acceleration, but not one, this concept is somewhat more difficult to wrap the brain around.

    My question is this. If gravity acts from curvature in space-time, what is the mechanism pushing the acceleration of the masses?

    The tennis ball in my hand, from the example, is at rest relative to the center of mass of the system. Why do the masses accelerate toward one another rather than remaining relatively motionless, when there is no energy being added to the system?

    Please don't say that holding the ball off of the ground adds 'negative energy' to the ball, and the mass loses that negative energy by falling. This is not satisfactory because the existence of 'negative energy' is questionable, and not really confirmed by any experiment that I know about, except in the case of considering gravity itself to be a form of negative energy.

    Please if I am off base with any of my information or assumptions, don't call me stupid. I'm here to get answers from those who know better than I do.
  2. jcsd
  3. Aug 14, 2013 #2

    I've read other posts about this same subject, but I didn't understand much of it, because of extensive use of words I don't know. Please say it in a way a child can read.
    In order to visualize curved space I had to imagine a Rubik's cube and then squish in the area where the center of mass was present.....yea.
    As a side note, I'd love to work for NASA...as a janitor, maybe?
  4. Aug 14, 2013 #3


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    Science Advisor

    Did you see this animation already?


    Free falling masses are not really accelerated, a free falling accelerometer measures 0 proper acceleration. They start moving in space, because their unaccelerated straight path in space time, deviates from the initially purely temporal direction, due to coordinate distortion (see video). This results in coordinate acceleration.
  5. Aug 14, 2013 #4


    Staff: Mentor

    It is important to remember that the curvature is in spacetime, not just space. The tennis ball may be at rest in space, but it is still moving through time. The curvature of spacetime essentially curves some of that motion through time into motion through space.
  6. Aug 14, 2013 #5
    Thanks for this very concise and easy to understand explanation. =)

    Wouldn't diverting the motion away from the time direction cause an 'acceleration' in time, in the direction of the past? the apple experiences less time passing as it is falling?
  7. Aug 14, 2013 #6


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    Science Advisor

    Yes, the faster something moves through space, the slower it ages.
  8. Aug 14, 2013 #7
    oh yea duh....that's universal...
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