The Positive Mass Theorem near null infinity
  In this short paper, we review recent progress on the positive mass theorem for spacelike hypersurfaces which approach to null infinity in asymptotically flat spacetimes. We use it to prove, if the functions $c(u, \theta, \psi)$, $d(u, \theta, \psi)$ vanish at certain retarded time in vacuum Bondi's radiating spacetimes, then the Bondi mass is nonnegative up to that time. 
Xiao Zhang