Abstract: 

String theory offers a unique perspective on concrete questions about the nature of quantum gravity. I will review how such questions can be formulated in terms of the string theory S-matrix at high energies. Attempts to sharpen the S-matrix calculations reveal the need to revisit the very formulation of string perturbation theory in Lorentzian space-times. Mathematically, it relates to the open question of how to choose a compactification of the moduli space of punctured Riemann surfaces. I will review the recent progress in addressing such questions in the open and closed superstring sectors at one-loop level, resulting in concrete numerical predictions for high-energy string collisions.