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Non geodisic motion

  1. Feb 6, 2007 #1
    What is non geodesic motion?
     
  2. jcsd
  3. Feb 6, 2007 #2

    HallsofIvy

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    Obvious definition- motion that does not follow a geodesic! In relativity, geodesics are the paths followed by objects with no "outside" forces acting on them (gravity not being considered an "outside" force here) so non-geodesic motion is motion with no outside forces.
     
  4. Feb 6, 2007 #3

    Chris Hillman

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    Just to elaborate quickly on what HallsofIvy said: in general relativity, the world line of a (nonspinning) test particle is a geodesic, i.e. a curve with zero path curvature. More generally, the path curvature at any event on some world line is just the magnitude of acceleration experienced by the corresponding particle. For example, a charged particle in an "electrovacuum solution" of the Einstein field equation (EFE) will experience a nonzero Lorentz force, whose magnitude (at each event on the world line of the particle) agrees with the path curvature.
     
  5. Feb 6, 2007 #4

    JesseM

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    I'm guessing you meant either "geodesic motion is motion with no outside forces" or else "non-geodesic motion is motion with outside forces"?
     
  6. Feb 6, 2007 #5

    Chris Hillman

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    Good catch, JesseM, I missed that! But I am sure HallsofIvy simply mistyped.
     
  7. Feb 6, 2007 #6
    Why spinning particles don't have a geodesic path?
     
  8. Feb 6, 2007 #7

    Chris Hillman

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    Spin-spin forces plus an essential PF link

    I didn't quite say that. However, in gtr, a spinning object immersed in a nonzero gravitational field (perhaps caused by a much larger nearby and spinning object) will in general experience a tiny "spin-spin" force and thus will be pushed off a geodesic path. However, these forces would be much too small to measure in any currently envisioned solar system experiments (at least, not any I know of). See for example "Dixon-Papapetrou equations" in Stephani, General relativity.

    By the way, I urge all members to carefully read https://www.physicsforums.com/showthread.php?t=5374 if they have not already done so, and indeed to bookmark it for future reference.
     
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