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Gravity bends matter?

  1. Sep 26, 2003 #1
    I know gravity is said to warp spacetime, but does it also warp the matter in spactime? If it does, would this make the shape of an object completely dependent on referance frame?
     
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  3. Sep 26, 2003 #2

    chroot

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    Well, all matter is ultimately composed of a bunch of elementary particles. These elementary particles are considered to be point-like in all modern theories. Since there's no way to bend a point, gravity does not bend anything on the microscopic scale. On the macroscropic scale, however, gravity can certainly bend things. Observe electrical cables strung from pole to pole. (This is probably not the answer you were looking for!)

    - Warren
     
  4. Sep 26, 2003 #3
    Yeah, I was thinkin more along the lines of gravity waves possibly distorting form?
     
  5. Sep 26, 2003 #4

    selfAdjoint

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    Gravity waves do warp matter. This was the idea behind the first gravity wave detectors in the 1950s. They never actually did repeatably detect gravity waves. They were based on a very large long bar of metal. Passing gravity waves would distort it ever so slightly, and extremely sensitive detectors would sense that.
     
  6. Sep 26, 2003 #5
    OK, so does it warp in sense of chroot's example (power lines) which can classically be thought of as a force being applied to the line which bends it, OR does it warp matter more like you see in illustrations of warped space-time?
     
  7. Sep 27, 2003 #6
    Illustrations of warped spacetime can be somewhat misleading. After all, they are really only able to show an indentation in a two-dimensional plane, as opposed to the actual three-dimensional area.

    So, an example of gravity's warping matter could just be the fact that all planets are circular in shape. The gravitational pull (or, more specifically, the gravitational "bend") causes the constituent particles to all be attracted toward the center, and thus the "round" shape is the most energy-effecient configuration.

    Now, if you are asking about the effect of outside gravitation influence on the shape of macroscopic objects, then I really don't know how much effect they have, but they must have some, since (for example) the Moon is creating a bulge at the equator of Earth due to it's gravitation.
     
  8. Sep 27, 2003 #7

    pmb

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    "Warping" or "curving" refers to distance relations and curvature. The curvature of a surface can be altered by applying a force. That force can be gravity.

    Is that what you were refering to?

    Pete
     
  9. Sep 27, 2003 #8

    pmb

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    "Warping" or "curving" refers to distance relations and curvature. The curvature of a surface can be altered by applying a force. That force can be gravity.

    Is that what you were refering to?

    Pete
     
  10. Sep 27, 2003 #9

    selfAdjoint

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    Certainly gravity is a force which can move matter (e.g. planets) and can cause matter to assume various minimal energy positions (power lines). These properties of gravity are derived from three facts about it.

    - It is much weaker than the atomic forces and electromagnetism.
    - It is cumulative (no negative gravity to cancel the positive)
    - It is long range.

    From these facts we can see that it will be very small at the level of an individual atom, but over the length of a large piece of matter it will build up and cause the familiar effects of weight that we know. This would be warping matter.

    Gravity waves are different from straight gravity, just as radio waves are different from electric charge. In the case of the gravity wave detector bar, it would become shorter and thicker as the crest of a wave passed, and longer and thinner as the trough passed, and it would cycle through these changes as the train of waves went through it.

    The "warping" effect in GR does happen at the geometry level. If you were to take a small arrow (vector) and move it parallel to itself around a small circle in a gravity field, you would find it pointed in a different direction when you completed the circle than it did when you started. You could attribute this to a "force of gravity" acting on the arrow, or you could say that the effect was due to the geometry of spacetime. Saying the latter allows you to set up Einstein's equations and develop the General Theory of Relativity, which has passed so many tests successfully.
     
  11. Sep 27, 2003 #10
    You say the waves are different from straight gravity. How are they different? Is it just in the fact that the direction and magnitude of the force is changing?

    To some extent that answeres my original question. Now I'll ask what causes the deformation in the bar? Is it simply the force of gravity contracting and expanding the molecules of the bar, or is it an example of some sort of relativistic length contraction?

    Please bear with me! It might be that I'm just over complicating this whole thing in my head, but I'm not sure yet.
     
  12. Oct 3, 2003 #11
    My current interpretetion is that gravitational waves are every bit as much the matter as the the matter of reference. Since it would seem that gravitational waves can be distorted through motion - it could be inferred that matter does bend.

    I'm thinking out of the box of course.
     
  13. Oct 4, 2003 #12

    LURCH

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    A newer experiment is currently underway called the Laser Iterferometry Gravity-wave Observatory or LIGO. If I have understood your question correctly, you are asking if there is a qualitative difference between the way a bar bends when subjected to gravity waves and the way it would bend if subjected to, for example, a strong wind, right? If GR is correct, and I think that is extremely likely, then the answer is yes. As has been stated earlier, matter occupies space. And if gravity waves do exist, they should alter the position of matter not by moving the matter through space, but because the space in which the matter sits is moving.
     
  14. Oct 5, 2003 #13
    OK, so then it would seem to me that you really cant detect these waves with any conventional measurments (refering to the rod experiment) because these measurments are made in space and if that space is changing with the matter... you get my point. So, if I'm correct, how did they intend to detect these changes in space using a rod?
     
  15. Oct 5, 2003 #14

    selfAdjoint

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    The reason the bar experiment didn't work is not that it took place at Earth's surface but that gravity waves are too weak for the detectors of the time to detect. Even LIGO is classed as a daring experiment because the waves are still at the limit of the detector's ability. (The physicist who started all this was named Weber, and the experimental item was often called Weber's bar)
     
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