Asymptotically stationary and static spacetimes and shear free null geodesic congruences

Abstract

In classical electromagnetic theory, one formally defines the complex dipole moment (the electric plus ‘i’ magnetic dipole) and then computes (and defines) the complex center of charge by transforming to a complex frame where the complex dipole moment vanishes. Analogously in asymptotically flat spacetimes, it has been shown that one can determine the complex center of mass by transforming the complex gravitational dipole (mass dipole plus ‘i’ angular momentum) (via an asymptotic tetrad transformation) to a frame where the complex dipole vanishes.We apply this procedure to such spacetimes which are asymptotically stationary or static, and observe that the calculations can be performed exactly, without any use of the approximation schemes which must be employed in general. In particular, we are able to exactly calculate complex center of mass and charge world-lines for such spacetimes, and—as a special case—when these two complex world-lines coincide, we recover the Dirac value of the gyromagnetic ratio.