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Is there an absolute motionless?

  1. Jan 12, 2012 #1
    So I do not have a clue whether this question has any merit or not. (which is why I came here) I have heard that masses create ripples in space-time (gravity waves) as they move. I have drawn a couple of logical conclusions that I would appreciate if you guys would verify or discredit.
    Assumption one: To create a wave, energy must be expelled.
    Conclusion A: In the process of producing ripples in space time, the object in question would lose some of its momentum.
    Conclusion B: If movement through space is resisted, then there exists an absolute rest and therefore a reference to compare all velocities.
    Once again this is me just brainstorming. These conclusions kind of fly in the face of Newton's first law of motion so if someone can point out where I went wrong that would be great.
    Thanks.
     
  2. jcsd
  3. Jan 13, 2012 #2

    Drakkith

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    As far as I thought, this only happens between two massive objects in a very close orbit, such as binary stars. If it only results by the interaction of objects, then I suppose it does happen when something is moving through space, but maybe the effect is so small it is almost completely negligible?
     
  4. Jan 13, 2012 #3
    According to General Relativity there is an absolute rest - any object that is not experiencing an acceleration. Note the equivalence principle of acceleration and gravity and realize that you sitting at the computer are not considered to be at rest, yet an object in free-fall is. To put it another way - an object can be considered to be at absolute rest when it traces a straight path through a spacetime diagram, the x,y coordinates representing spacial movement and the z coordinate representing time.
     
  5. Jan 13, 2012 #4

    DaveC426913

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    What??? No.

    It would be in inertial motion. But it would not be at absolute rest. The very core of relativity is there there is no such thing as absolute. That's the relative in relativity.

    What you have described is relative rest. You are at rest if you choose the right frame of reference.
     
  6. Jan 13, 2012 #5

    DaveC426913

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    Where does this conclusion come from?
     
  7. Jan 13, 2012 #6

    Janus

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    It isn't movement that causes gravity waves, but acceleration or change in velocity.
    True
    [/quote] Again, the object would have to being undergoing acceleration to produce Gravity waves, mere motion will not do so. One piece of indirect evidence for the existence of gravity waves involves close orbiting binary pairs. As they orbit, they emit gravity waves these waves carry energy away form the pair causing their orbits to contract.
     
  8. Jan 13, 2012 #7
    It's a great question and shows that you are thinking about things.

    Masses emit gravitational radiation when they accelerate. If they are non-accelerating, they don't do anything. If you have a non-accelerating body it can't do anything otherwise we'd have the problem that you mentioned.

    It's even deeper than that. There are some symmetry principles that are in major theories of physics, and one thing that you can do is to start off with the symmetry principle and come up with a theory. In the case of general relativity, one basic symmetry principle is that there is no preferred reference frame, and from that you can figure out that gravitation radiation doesn't happen unless something accelerates.

    Also there are some deep mathematical theorems that show that some conservation rules come from symmetry. Saying that "there are no preferred reference frames" and "momentum is conserved" turn out to be the same statement.

    The other thing is that theoretical physicists *love* this sort of logic. If I ask you how much gravitational radiation a non-accelerating object emits, you could go through twenty pages of nasty math. Or you could come up with a two sentence argument (zero because if it did there would be a preferred reference frame) which is deeper and more insightful.
     
  9. Jan 13, 2012 #8

    PAllen

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    Technically, uniform acceleration of a body in isolation will not produce gravitational radiation. For an isolated body, you would need change in acceleration. In general, you need a change in quadrupole moment (in contrast to EM, where you have dipole radiation). In contrast to the isolated body, two massive objects falling straight towards each other (let alone orbiting as in the famous pulsar) will radiate. One may argue here you have two objects with no proper acceleration radiating.
     
  10. Jan 14, 2012 #9
    You are, obviously, correct. I got my terms mixed up.
     
  11. Jan 14, 2012 #10

    Chronos

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    Yes, Pallen, but, acceleration is, by definition, a second order effect.
     
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