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Gravitomagnetism equations-wikipedia wrong?

  1. Nov 19, 2008 #1
    gravitomagnetism equations--wikipedia wrong?

    Hi, I read the wikipedia article from:
    http://en.wikipedia.org/wiki/Gravitoelectromagnetism

    (scroll down to the "Maxwell's eq" for gravity)

    where it says
    [tex]\nabla \times \mathbf{B}=-4\pi G\mathbf{J}[/tex]
    (neglect the other terms)

    however, when I compare that to Sean Carroll's book of GR (spacetime and geometry),
    on page 282, (7.31) it reads something like
    [tex] \nabla^2 \mathbf{\omega}=-16 \pi G T_{0j}=-16 \pi G \mathbf{J}[/tex]
    Carroll defines,
    [tex]\mathbf{B}=\nabla \times \mathbf{\omega}[/tex]

    so in effect, we have something like
    [tex] \nabla \times \mathbf{B}=-16 \pi G \mathbf{J}[/tex]

    while a factor of two is reasonable since the the force law in Carroll's book is without the 2 in front of B, the additional factor of 4 is just weird... I could not get around that at all. Is wikipedia wrong or am I missing something?
     
    Last edited: Nov 19, 2008
  2. jcsd
  3. Nov 19, 2008 #2

    Jonathan Scott

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    Re: gravitomagnetism equations--wikipedia wrong?

    I also noticed the same oddity recently. Even with their factor of 2 on Bg, something seems to be wrong there. When I tried myself to make gravity look like Maxwell's equations long ago, just assuming an overall factor of 2 was not enough to make it work, and I'm sure there were additional factors of 2 or 4 which appeared in the equations.

    My suspicion is that the equations quoted in the Wikipedia case have ignored the curvature of space and are therefore already missing some factors of 2 compared with a more accurate version of those equations. I'll see if I can find time to check it out more carefully later.
     
  4. Nov 19, 2008 #3

    Jonathan Scott

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    Re: gravitomagnetism equations--wikipedia wrong?

    OK, I found a useful refresher in arXiv:gr-qc/0311030, "Gravitoelectromagnetism: A Brief Review" by Bahram Mashhoon, referenced in the Wikipedia article, and I've also compared it with chapter 6 of Ciufolini & Wheeler "Gravitation and Inertia". (I don't have the Carroll book here). There definitely seems to be factor of 2 anomaly in one part of the Wikipedia entry compared with the other two. There are also some sign inconsistencies, but I think this is simply because the convention for the direction of the ordinary gravitational field varies between the sources.

    The gravitomagnetic fields is called B in Mashhoon's paper, Bg in the Wikipedia entry and H in Ciufolini & Wheeler. It appears that the definitions of these versions of the field are all different, giving H = -2B = 4Bg.

    The "Maxwell's equations" equivalent match between Mashhoon and the Wikipedia entry seems to be correct at least as far as factors of two are concerned (although the sign convention for E is reversed).

    The amount of gravitomagnetic field due to a rotating body also matches, in that the factor applied to J/r3 is 2 for H, -1 for B and 1/2 for Bg.

    However, the Wikipedia entry expression in terms of Bg for the "Lorentz force law" seems to be out by a factor of 2 compared with the other two sources. The cross-product factor in Ciufolini & Wheeler is H and in Mashhoon it is -2B, which means that in the Wikipedia entry it should be 4Bg.

    If the quantity which Carroll is using for the field is equivalent to H in Ciufolini and Wheeler, the equation would be consistent with the Wikipedia entry, as this would be equal to 4Bg. The Wikipedia entry is therefore slightly in error in a second way, in that some of the literature uses a field which is four times rather than twice Bg.
     
  5. Nov 19, 2008 #4
    Re: gravitomagnetism equations--wikipedia wrong?

    Thanks for the reply. that clarifies a lot!!! so the mistaken part is the wikipedia force law... someone seriously should change it and perhaps mention the different "B" fields occuring in the literature. I am not that good at writing explanations so maybe someone else can take on this task? :biggrin: (plus.. I'm feeling lazy right now)
     
  6. Nov 20, 2008 #5

    Jonathan Scott

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    Re: gravitomagnetism equations--wikipedia wrong?

    I already added a note to the talk page about the apparent error in the force law, to make sure it is at least recorded in some way.
     
  7. Nov 20, 2008 #6

    cristo

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    Re: gravitomagnetism equations--wikipedia wrong?

    There's a lesson here: don't trust wikipedia for advanced topic like this. Instead, why not use a review paper, like the one Jonathon links to?
     
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