  A method is given to compute the stress-energy tensor for a massless
minimally coupled scalar field in a spacetime where a black hole forms from the
collapse of a spherically symmetric null shell in four dimensions. Part of the
method involves matching the modes for the in vacuum state to a complete set of
modes in Schwarzschild spacetime. The other part involves subtracting from the
unrenormalized expression for the stress-energy tensor when the field is in the
in vacuum state, the corresponding expression when the field is in the Unruh
state and adding to this the renormalized stress-energy tensor for the field in
the Unruh state. The method is shown to work in the two-dimensional case where
the results are known.
