High Energy String Theory and the Celestial Sphere

We elaborate on a string world-sheet connection to flat space-time holography. More specifically, in the high energy (zero tension) limit of tree-level string scattering in flat backgrounds the underlying string world-sheets can be related to the celestial sphere, with the saddle points of the high energy string description representing points on the celestial sphere. We show that this picture continues to hold at all subleading orders in the quantum fluctuations around this classical configuration. As a consequence there is a dual description of the high energy limit of string theory as the large energy expansion on the celestial sphere organized by (light) higher spin modes. This approach points to an intrinsic construction of celestial conformal field theory (CFT) by relating it to a (free) world-sheet CFT of string theory. We also elaborate on the high energy representations of tree-level open and closed string amplitudes and work out their subleading corrections. Their number theoretic properties and relevance as amplitudes of tensionless strings are discussed.