Strings at Superplanckian Energies: In Search of the String Symmetry

The characteristic energy scale of superstring theory, which attempts to unify all the interactions of matter with gravity, is the Planck energy of 10²⁸ eV. Although this energy is 16 orders of magnitude higher than currently accessible energies, it is important to consider the nature of string physics in this region since it could shed light on the non-perturbative physics at the Planck scale, which determines the structure of the vacuum. In this paper I review some recent attempts to explore this domain. In particular, I discuss string scattering at very high energies, the indications of the existence of a large symmetry that is restored at short distances and the possible breakdown of our concepts of space-time at these energies.