On the physical meaning of the Robinson–Trautman–Maxwell fields

Abstract

We study the Robinson–Trautman–Maxwell fields in two closely related coordinate systems, the original Robinson–Trautman (RT) coordinates (in a more general context, often referred to as NU coordinates) and Bondi coordinates. In particular, we identify one of the RT variables as a velocity and then from the Bondi energy–momentum 4-vector, we find kinematic expressions for the mass and momentum in terms of this velocity. From these kinematic expressions and the energy–momentum loss equation we obtain surprising equations of motion for ‘the centre of mass’ of the source where the motion takes place in the four-dimensional Poincare translation sub-group of the BMS group.