SHEAR-FREE GRAVITATIONAL RADIATION

It is shown that, with the exception of the Schwarzschild metric, gravitational systems described by suitably regular vacuum solutions of Einstein's equations admitting a shear- and twist-free congruence of diverging null rays must be radiative. Robinson and Trautman have demonstrated the existence of type-II solutions of this kind, which describe radiating gravitational systems with bounded sources. However, attempts to display an explicit radiative solution through specilaization to conformally spherically symmetric, Kerr-Schild, conformally Kerr-Schild, and type-D space times lead to singular metrics. Finally, important physical properties of these systems, including energy, angular momentum, radiation flux, and trapped surfaces, are discussed. © 1969 The American Physical Society.