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Why does gravity not create resistance?

  1. May 31, 2014 #1
    If a planet is traveling through space and distorting space, why does it not slow down? If a boat travels through water and distorts the water it will slow down.

    I guess a better way to put it is why does empty space not resist being bent?
     
  2. jcsd
  3. May 31, 2014 #2

    A.T.

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    There is energy dissipation in form of gravitational waves.
     
  4. Jun 9, 2014 #3
    You are talking about one of the theories driving observations as though it were part of the theory itself.

    This very lack of resistance (the whole Michelson and Morely thing) is among the initial facts that were taken as proof that it is in fact space itself and not some wave-bearing medium *in* that space being deformed by gravity.

    If there were a natural resistance to motion in space, we could attribute that to the inertia of a "wave-bearing medium" that set the speed of light according to principles similar to those governing waves in fluids. But then we could determine along which axis the Earth is moving fastest, by measuring the resistance to our motion in various directions. So we could preserve faith in an absolute frame of reference, relative to which the Earth must be moving.

    But there isn't. So we can't. That space does not resist being bent is a datum, or an axiom. It is what we observed, instead of what we theorized based on the earlier forms of the wave theory of light propagation requiring a medium.
     
    Last edited: Jun 9, 2014
  5. Jun 9, 2014 #4

    PeterDonis

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    The Michelson-Morley experiment did not measure "resistance of space to being bent". It measured the velocity of light in different directions.

    No, it isn't. See A. T.'s post. Spacetime (which is a better term than "space" in this connection) does "resist being bent" in the sense that orbiting objects like planets produce gravitational waves, which extract energy from them and thereby slow their orbital motions. This happens very, very slowly, so we can't directly measure it in the solar system, but we can and have measured it with binary pulsars.
     
  6. Jun 9, 2014 #5
    Pretty obviously *no one* conceived of the resistance of space to being bent until after someone had thought of space being bent. But any such thing would have *looked just like* the density of aether, the measuring of which is what the ultimate end-game of the Michelson-Morley programme was.

    Their purpose was to prove that light had a bearing fluid, and the physical properties of that bearer caused the constancy of the speed of light. They did more than one experiment. And i was referring to their entire design and concept and not to any specific one of those experiments.

    Things move forward, but the lack of a density to the aether *is* what made us consider that space might bend. We accepted the lack of any such force as a basis for new reasoning, and a datum that new theories would have to meet.

    We do not account in computations for the rigidity of space, as a direct force to overcome, and in general we assume that it has none.

    There may be new experimental data, but new discoveries do not change history.
     
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