Reconstructing Vaidya mass from a radial null geodesic
= Reconstructing Vaidya mass from a radial null geodesic
{title2=$M(v)=\tfrac12r_0(v)(1-2r_0^{\prime}(v))$}
An outgoing <radial null geodesic of the Vaidya metric> prescribed as $r=r_0(v)$ determines the mass along that curve by $M(v)=r_0(v)(1-2r'_0(v))/2$. This inverse relation comes from the radial <null condition>. It determines a function of advanced time wherever the prescribed curve is differentiable and has positive radius.