Foundations of Physics Vol. 38, pp. 959-968 (2008)
The original publication is available at www.springerlink.com
http://dx.doi.org/10.1007/s10701-008-9245-x
It has been shown that for the Reissner-Nordstrom solution to the vacuum Einstein field equations charge, like mass, has a unique space-time signature [Found. Phys. 38, 293-300 (2008)]. The presence of charge results in a negative curvature. This work, which includes a discussion of effective mass, is extended here to the Kerr-Newman solution.
Charge, geometry, and effective mass in the Kerr-Newman solution to the Einstein field equations