Figure 5: The physical setup for the scattering problem. A star of mass has undergone
spherically-symmetric collapse to form a black hole. The ingoing null worldtube lies outside
the collapsing matter. Inside (but outside the matter) there is a vacuum Schwarzschild metric.
Outside of , data for an ingoing pulse is specified on the initial outgoing null hypersurface .
As the pulse propagates to the black-hole event horizon , part of its energy is scattered to .
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