  The leading classical low-energy effective actions for two-dimensional string
theories have solutions describing the gravitational collapse of shells of
matter into a black hole. It is shown that string loop corrections can be made
arbitrarily small up to the horizon, but $\alpha'$ corrections cannot. The
matrix model is used to show that typical collapsing shells do not form black
holes in the full string theory. Rather, they backscatter out to infinity just
before the horizon forms. The matrix model is also used to show that the
naively expected particle production induced by the collapsing shell vanishes
to leading order. This agrees with the string theory computation. From the
point of view of the effective low energy field theory this result is
surprising and involves a delicate cancellation between various terms.
