Closed String Tachyon Condensation atc=1

The c=1 matrix model, with or without a type 0 hat, has an exact quantumsolution corresponding to closed string tachyon condensation along a nullsurface. The condensation occurs, and spacetime dissolves, at a finite retardedtime on I^+. The outgoing quantum state of tachyon fluctuations in thistime-dependent background is computed using both the collective field and exactfermion pictures. Perturbative particle production induced by the movingtachyon 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 theoutgoing state in perturbation theory. It is also shown that,non-perturbatively, information can leak across the tachyon wall, and tachyonscattering is not unitary. Exact unitarity remains intact only in the freefermion picture.Comment: Minor corrections; References added; 24 pages, 2 figures, harvma