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Mass and attraction

  1. Dec 28, 2014 #1
    what happens if 2 massive objects are placed in an empty space.
    Does it attracts each other?
    Does any change happens in its position?
    If there is a force where did it come from?
     
  2. jcsd
  3. Dec 28, 2014 #2

    Vanadium 50

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  4. Dec 28, 2014 #3

    ChrisVer

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    typically they will attract each other after some time [if we consider gravitation propagating with the speed of light].
    Yes, there is change in position, coming from the geodesic equations (because of gravity acts as an inertial force)
    The force will come because the objects will in general choose to follow the least action principle. Because there is gravity, they will choose to follow the geodesics I mentioned above.
     
  5. Dec 28, 2014 #4

    PeterDonis

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    You can't actually do this, because it would violate conservation of energy. Massive objects can't simply appear out of nowhere. So the question as you ask it isn't really well-defined; there is no consistent physical model that corresponds to it.

    That said, there are plenty of other ways of posing scenarios that show that massive objects attract each other.

    It is true that gravity propagates at the speed of light (at least, it does according to GR, and our best current belief is that GR is correct in this respect). However, for the reasons given above, the simple thought experiment proposed by the OP can't be used to show the speed of propagation of gravity, because it's not a consistent scenario.
     
  6. Dec 28, 2014 #5
    Hi Peter,

    True - as a practical matter - it is not possible to suddenly create a mass from nowhere ...but theoretically it costs nothing to create a mass inasmuch as the creation process also creates its negative energy G field at the same time - this was among Feynman's first musings in his lectures on gravity... that as since been embellished upon by a number of writers who support the proposition of a zero energy universe
     
  7. Dec 28, 2014 #6

    Vanadium 50

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    I don't think the OP is insisting that these masses appear from nowhere. If I say "two apples are placed on a table", it doesn't mean they appeared out of nowhere.
     
  8. Dec 28, 2014 #7

    ChrisVer

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    well then the "space" should be replaced, because the space could be translated to universe which contains everything. :w
    It would be better to say "2 masses exist in an empty space", and not "2 masses are placed in an empty space" (placed from where?).
     
  9. Dec 28, 2014 #8
    They should gravitationally attract each other, albeit I read some insane peopleo0) suggested they could repel in a matter-antimatter case.
     
  10. Dec 28, 2014 #9

    PeterDonis

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    Feynman speculated about this, yes, but he was not talking about classical GR, he was talking about a possible theory of quantum gravity that hadn't even been developed yet. My statement was referring to classical GR.

    This has nothing to do with mass being created "out of nothing" along with a corresponding negative gravitational potential energy. It has to do with the fact that a closed universe model can be expressed mathematically in the same form as the equations for a body that is launched upward under Newtonian gravity and then freely falls. In this model, no matter or energy is created anywhere; the classical GR constraint for local energy conservation is obeyed. The "zero energy" just refers to the fact that you can choose the "zero point" of gravitational potential energy so that it always just cancels the total kinetic energy of the matter. This is a mathematical trick, not a physical statement.
     
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