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Where Does Gravity Get Its Energy From?

  1. Nov 18, 2013 #1
    Hi.
    As I understand gravity is a curvature of space-time. So as I throw an apple into the air I am giving it kinetic energy to travel "up" this curvature. This then is made into potential energy as the apple decelerates.

    Now lets imagine 2 objects in deep space, far from anything else. For fun lets say they are the same mass as our sun. From a distance of, say 2 light-years, I would think that these objects would have no potential energy towards each other. If these objects get close enough to each other, they will both accelerate towards one another. To move an object of that mass would require a lot of energy, yet they both accelerate? Where did they get the energy to do that?
     
  2. jcsd
  3. Nov 19, 2013 #2

    Drakkith

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    The two objects do indeed have potential energy at 2 light-years and will accelerate towards each other from that distance. Remember that gravity has an infinite range.
     
  4. Nov 19, 2013 #3

    russ_watters

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    They got that energy when the universe was created.
     
  5. Nov 19, 2013 #4
    If I could make a ball simply appear in mid-air, the potential energy would have, indeed, come from nowhere. The same happens in your example. You're just putting two suns somewhere.

    And you answered your own question right there. Gravity is a curvature in space time. So basically no energy is needed I guess.
     
  6. Nov 24, 2013 #5
    Interesting question.

    Just a note. Nobody knows what gravity is. It is represented or evidenced by a curvature in space time rather than 'is' a curvature.

    Gravity is still an unknown force and thus why it is the Holy Grail of physics to come up with a theory that explains both General Relativity and the quantum world. String theorists are still hard at it.
     
  7. Nov 25, 2013 #6

    Drakkith

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    I could argue that no one knows what anything is, but that doesn't really get us anywhere. Gravity and the other fundamental forces of nature are described by very accurate theories. Claiming that we don't know what those forces really are is missing the point of science.
     
  8. Nov 25, 2013 #7
    Not at all. Describing the effect of a force is not the same as understanding what it is. Newtonian physics can describe many phenomenon without smidgeon of knowledge of particle physics. It is still science. Science is a methodology of gaining knowledge and not an end game.
     
  9. Nov 25, 2013 #8

    Chronos

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    We don't know what mass is either, we can only describe it in terms of the effect it has on a measurement device. The same is true of forces. Science is ultimately a prediction of measurement outcomes.
     
  10. Nov 25, 2013 #9

    Drakkith

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    Maybe not, but you can't understand what it is without describing the effects.
     
  11. Nov 26, 2013 #10
    Many outside of the sciences don't understand that very essential point. Science is a tool to explain. The particular details of science are constantly being superseded.
     
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