Towards an S-matrix description of gravitational collapse

Extending our previous results on trans-Planckian (Gs >> h) scattering of light particles in quantum string-gravity we present a calculation of the corresponding S-matrix from the region of large impact parameters (b >> G root s > lambda(s)) down to the regime where classical gravitational collapse is expected to occur. By solving the semiclassical equations of a previously introduced effective-action approximation, we find that the perturbative expansion around the leading eikonal result diverges at a critical value b = b(c) = O(G root s), signalling the onset of a new (black-hole related?) regime. We then discuss the main features of our explicitly unitary S-matrix - and of the associated effective metric down to (and in the vicinity of) b = b c, and present some ideas and results on its extension all the way to the b -> 0 region. We find that for b < b(c) the physical field solutions are complex-valued and the S-matrix shows additional absorption, related to a new production mechanism. The field solutions themselves are, surprisingly, everywhere regular, suggesting a quantum-tunneling - rather than a singular-geometry - situation.