Bondi-Sachs Formalism
  The Bondi-Sachs formalism of General Relativity is a metric-based treatment
of the Einstein equations in which the coordinates are adapted to the null
geodesics of the spacetime. It provided the first convincing evidence that
gravitational radiation is a nonlinear effect of general relativity and that
the emission of gravitational waves from an isolated system is accompanied by a
mass loss from the system. The asymptotic behaviour of the Bondi-Sachs metric
revealed the existence of the symmetry group at null infinity, the
Bondi-Metzner-Sachs group, which turned out to be larger than the Poincare
group.
