Sunday, 8 March 2015

gravity - Does rotation affect gravitational lines of force

An spherical electric charge has the same electric field lines whether spinning or not. The difference between those two cases is entirely in the magnetic field. Thus, one should expect as similar thing to happen for gravity.



The parametrized post-Newtonian formalism, weak-field GTR has the metric
mathrmds2=(1+2Phi),mathrmdt2+2mathcalAj,mathrmdt,mathrmdxj+(12Phi)deltaij,mathrmdxi,mathrmdxj


where PhiequivU is essentially the Newtonian gravitational potential, while mathcalAjequivtfrac74Vjtfrac14Wj in terms of the other PPN potentials. For the four-velocity Ualphaequivmathrmdxalpha/mathrmdtau=(U0,vecU), the geodesic equation for time-independent Phi and mathcalA becomes, to linear order in Phi,
fracmathrmdvecUmathrmdtau=U0(vecG+vecUtimesvecH)text,

where the gravitoelectric field is vecG=nablaPhi and the gravitomagnetic field is vecH=nablatimesvecmathcalA, here nabla being used in the ordinary sense of nablai=partiali, i.e. with respect to the Euclidean metric deltaij. If Phi and mathcalA are not time-independent, then vecG=nablaPhipartialtvecmathcalA, paralleling electromagnetism, but there will be an extra term in the analogue of Lorentz force that has no electromagnetic counterpart.



Much the same thing can be done in any stationary spacetime, including the rotating Kerr black hole. See also Costa and Natário (2014) for a much more general treatment of several gravito-electromagnetic analogies.




References:



  1. Costa, L. F. O, Natário, J. "Gravito-electromagnetic analogies". Gen. Rel. Grav. 46, 1792 (2014)[arXiv:1207.0465]

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