String theory beyond the Planck scale

The high-energy behavior of string scattering amplitudes is studied to all orders in perturbation theory, with the aim of exploring the short-distance structure of string theory. It is shown that the sum over all Riemann surfaces is dominated by a saddle point. Consequently, the high-energy limit is universal and simple to calculate. In this limit, furthermore, the amplitudes fall off in a stringy way - much faster than that allowed by field theory. The dominant saddle points are identified as coming from world sheets which are ZG+1 symmetric algebraic curves, and their contribution to the scattering amplitude is evaluated. An interesting spacetime picture of the high-energy limit emerges. The issue of summing the perturbation expansion is addressed. (orig.)