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Questions about gravity

  1. Aug 18, 2013 #1
    Hi, I wonder whether physicists generally understand gravity as the petering out of energy imparted to matter by the big bang. Can we think of two objects separated by space as having some potential energy, which is expended as they move closer together? Can we consider that this potential energy was imparted to the objects by the big bang, which produced time, space, and matter? Would it be senseless, then, to talk about gravity in a universe that was not produced by a big bang?

    I guess I'm trying to think of objects in space as poised on the side of a hill, and rolling towards the bottom (the stable, zero-energy state). The question, then, is "How did the objects get up the hill?" I wonder whether the answer is the big bang. My explanation would go like this: The big bang created time, space, energy and matter, and also produced matter separated by space, which is an unstable state that is resolved (i.e., brought to a zero-energy state) by the action of gravity. Is that, more or less, how physicists think about gravity?

    Cheers, and I'd be happy to elaborate if any of these questions aren't clear.

  2. jcsd
  3. Aug 18, 2013 #2
    That is a very intuitive way to think about it, and I wouldn't be surprised if you would be in good company 50 years ago (your example would be a bit simplified, but not entirely unbelievable). However, the universe is expanding and the expansion seems to be accelerating. So your idea, while seemingly plausible, doesn't hold up.

    Be careful with counter-factual assumptions. Maybe it would be better to phrase this as
    "Could we construct a theoretical universe that has no beginning or end in which gravity behaves the way we see it behaving locally (in the solar system)?" I think the answer is yes. We could construct a steady state universe. I am not an expert in this area though.
    But of course, since everything was created by the Big Bang (or Horrendous Space Kablooie [Calvin,Hobbes;1992]) the total energy does come from it. The only significant physics error that I see in your thinking is that energy is not expended. The total energy is conserved, so any potential energy in a system can be regained if the conditions are correct.
  4. Aug 18, 2013 #3
    Hi DrewD. Thanks for your response.

    So, we've observed in the last 50 years that the universe is accelerating at an increasing rate, so it isn't valid to think of gravity as a simple "winding down" of energy imparted to matter by the big bang. It seems, I guess, that new energy is being created somehow with the expansion of space/time ("dark energy")? How does this affect our understanding of gravity?

    Right, so two objects at a distance of two meters will, by the force of gravity, reduce that distance to one meter. Potential energy will be lost (or converted to some other kind of virtual energy?). That potential energy could be restored by applying a force over time to one of the objects, to move it further away from the other object. I want to understand where the potential energy came from in the first place, to set those two objects at a distance of two meters. "The big bang" is a convenient answer, but maybe not the correct one.

    It was a bit over-reaching to say that gravity could only exist in a universe that was produced by a big bang, so I take that part back.

  5. Aug 18, 2013 #4


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    how do you figure that ?

    if the mass of an object stays constant, so does its gravity, its gravitation field doesn't decrease with time.

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