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Gravity's reaction force in the relativist model?

  1. Feb 5, 2015 #1
    this is related to the following thread: https://www.physicsforums.com/threads/gravitys-reaction-force.28395/
    (but it doesn't seem to accept more replies so i posted here)

    my cuestions are:
    - ¿if gravity depends on mass, how come we define a black hole as an object so massive that photons (mass zero), cannot escape it?

    - ¿if we consider it a bending of space-time then where does the reaction force come from
    (does it still exist in the relativist model) ?

    i mean, i can see this picture:
    and imagine the moon might feel like a force F is dragging it, but is there still a F'=-F from the moon to the earth and how where does it came from?

    i apologise for my bad english (it's my second language) and limited knowledge regarding the topic (I'm just an engineering student and we barely touched the topic in class)
  2. jcsd
  3. Feb 5, 2015 #2

    Simon Bridge

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    Welcome to PF;

    1. because gravity does not only depend on mass - it depends on the energy-density. Mass happens to be highly concentrated energy.

    Note: a black hole is more properly described as a volume of space within which the escape velocity exceeds the speed of light in a vacuum.

    Black holes belong to general relativity though - so you need the space-time curvature model.

    2. the reaction force also comes from curvature (not bending, that implies there is something for it to bend in) of space time.
    In relativity there is no absolute reference frame so it is equally valid to say that one or the other or both objects move towards the other or each other.

    3. that picture shows the "rubber sheet" model, and it is somewhat misleading. You can safely ignore this analogy when it comes to drawing conclusions.
    The analogy ignores that one of the dimensions involved is time (hard to draw) so both objects try to go in a straight line in space-time, which, because of the geometry, is a curved line towards each other in space.

    Also see:

    ... and a more rigorous introduction to how it all works:
    http://preposterousuniverse.com/grnotes/grtinypdf.pdf [Broken]
    Last edited by a moderator: May 7, 2017
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