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Rotating body and relavity

  1. Oct 31, 2008 #1
    Will there be any relativistic effects on a rotating body which rotates at velocity close to c...
  2. jcsd
  3. Oct 31, 2008 #2


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    Staff Emeritus
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    There's always Lorentz contraction and time dilation when high speeds are involved, and if the object is massive (any actual physical object is), there will also be general relativistic effects: curvature, frame dragging, etc.

    The most interesting special relativistic effect is that there's always a forceful stretching of the material when the angular velocity is changed.

  4. Nov 1, 2008 #3
    Does a rotating body have magnetic field? if so, will the magnetic field also gets contracted?
  5. Nov 1, 2008 #4
  6. Nov 1, 2008 #5
    We know neutron stars spin rapidly - so what of the objects we define as black holes?

    What happens when the centrifugal force & gravitational force reach equilibrium? - this would imply that singularities will not form, only that a star might only collapse to a given size.

    Thoughts, please.
  7. Nov 1, 2008 #6


    Staff: Mentor

    I think the singularity of a Kerr black hole is supposed to be a ring rather than a point, but it is still supposed to be a singularity (0 thickness).
  8. Nov 1, 2008 #7
    Yes, you're correct - why stop at rings when there's spheres, hypertoroids and other higher dimensional geometric singularities?

  9. Nov 1, 2008 #8


    Staff: Mentor

    Because they are not solutions to the EFE.
  10. Nov 1, 2008 #9
    Why limit oneself to Einsteins field equations?

  11. Nov 1, 2008 #10


    Staff: Mentor

    Because you are talking about black holes.
  12. Nov 3, 2008 #11
    Does a rotating body have magnetic field? if so, will the magnetic field also gets contracted?
  13. Nov 3, 2008 #12
    Since an electromagnetic field can be thought of as consisting photon quanta and photons are curved by gravity, gravitational forces DO curve electromagnetic fields. But gravity is I believe on the order of 10^80 times weaker than EM forces, so it's a negligible effect under most circumstances.
  14. Nov 3, 2008 #13
    It's possible, nobody knows for sure...The mechanism of collapse is still a matter of debate:
    From Peter Bergmann's THE RIDDLE OF GRAVITATION, PG 133:

    And he discusses interesting theoretical calculations conducted by Oppenheimer and Einstein...neitherof which was able to resolve the issue.
  15. Nov 3, 2008 #14

    Wiki sez:

    I can't think of a a reason for a non relativistic rotating solid body to otherwise have a magnetic field, but I would not rule it out. It's easier to visualize a viscous body perhaps having dipole's formed at higher rotational velocities.

    Also keep in mind that electric field in one reference frame is a magnetic field in another; so a static charge on a rotating body will have a perceived magnetic field component.
    Last edited: Nov 3, 2008
  16. Nov 3, 2008 #15
    But i asked in SR perspective not GR..rotating body experiences length contraction and so will the magnetic field also gets contracted from SR perspective?
  17. Nov 3, 2008 #16
    I'm pretty sure,Yes.

    But the exact mechanism is not clear to me.

    In THE RIDDLE OF GRAVITATION Peter Bergmann says:

    Maybe it can be thought of " as r changes due to relativistic effects, the particle separation changes and so the field must change"
  18. Nov 4, 2008 #17
    Is rotation motion an inertial motion? Is rotational motion equivalent to object at rest and object at constant velocity?
  19. Nov 4, 2008 #18

    Jonathan Scott

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    No, rotation is related to acceleration so a rotating frame of reference is not inertial. Mathematically, rotation is like an imaginary acceleration, or equivalently acceleration is like an imaginary rotation.

    Rotation is related to acceleration in the same way that magnetic fields are related to electric fields.

    The gravitational field of a moving or rotating body includes a "gravitomagnetic" part which has the effect of making a test object experience an apparently rotating frame of reference, which is known as "frame dragging". Gravity Probe B has been attempting to measure this tiny effect as caused by the Earth's rotation.
  20. Nov 4, 2008 #19
    I agree with Jonathan's post.

    How do you know rotational motion is NOT inertial?? because you feel a FORCE!!!!

    Constant velocity is unaccelerated motion, that is constant speed, in a straight line. You can think of rotational motion as constant speed (magnitude) with acceleration via a change in direction. Or to say the same thing another way, the radial acceleration radially exerts a force acting perpendicular to the direction of motion (tangential speed along the circumference).
    Last edited: Nov 4, 2008
  21. Nov 4, 2008 #20
    I had not thought of this before: If a rotating body did always have a magnetic field, we could rotate a piece of wood, for example, and produce a magnetic field to generate power....
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