Radiation and Boundary Conditions in the Theory of Gravitation
  The Sommerfeld boundary conditions, applied to an asymptotically weak gravitational field, are shown to imply that the 1/r part of the curvature tensor of a space-time, satisfying the Einstein equations, is of type null in the Petrov classification and that there is then a flux of energy carried away by the outgoing gravitational wave. 
Andrzej Trautman
gr-qc hep-th math-ph math.MP