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Gravitomangetic B-field units

  1. Apr 20, 2009 #1
    Does the gravitomagnetic B-field (from the GEM equations) have units of frequency (1/s)? And if so, what exactly does the B field describe?

    I guess what I'm really trying to find out is whether the B field around say a rotating spherical object describes the rate of frame dragging, or does it describe how a particle near it will spin around its own axis, or something entirely different?

    And also why is it that because gravity is a spin-2 field, the gravitomagnetic charge is twice that of the gravitoelectric charge?


  2. jcsd
  3. Apr 20, 2009 #2


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    Hi stubby,

    can you give a reference to put this in context ? I don't know about GEM but in GR the field around a rotating body will induce rotation in test bodies. See Hamilton and Lyle arXiv:gr-qc/0411060v2.

  4. Apr 21, 2009 #3

    Jonathan Scott

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    There are various different conventions for the gravitomagnetic B-field, differing by factors of 2. However, they all effectively represent an angular velocity. Basically, if you are in such a field, you feel as if you are rotating, in the same way as if you're in the E-field component, you feel as if you are accelerating. The rotation can be detected for example by rotational frame-dragging of a gyroscope.

    The gravitomagnetic equations can't be matched up exactly with the electromagnetic equations; one way or another there's always a factor of 2 needed somewhere, and different sources use different conventions. (Last time I looked, the Wikipedia entry on the subject was not even self-consistent). This does seem to relate to the need to use a tensor rather than a vector as part of the description of the force between relatively moving objects. However, I don't personally know how this relates to "spin 2".

    For more details on GEM, I suggest you Google "gravitomagnetism" and have a look at the 2000 review paper by Mashhoon. There seems to be a more recent paper too, but I haven't looked at that.
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