  The c=1 matrix model, with or without a type 0 hat, has an exact quantum
solution corresponding to closed string tachyon condensation along a null
surface. The condensation occurs, and spacetime dissolves, at a finite retarded
time on I^+. The outgoing quantum state of tachyon fluctuations in this
time-dependent background is computed using both the collective field and exact
fermion pictures. Perturbative particle production induced by the moving
tachyon wall is shown to be similar to that induced by a soft moving mirror.
Hence, despite the fact that I^+ for the tachyon is geodesicaly incomplete,
quantum correlations in the incoming state are unitarily transmitted to the
outgoing state in perturbation theory. It is also shown that,
non-perturbatively, information can leak across the tachyon wall, and tachyon
scattering is not unitary. Exact unitarity remains intact only in the free
fermion picture.
