Strings and gravitation

Of the four fundamental forces of Nature gravity is the oldest one known to us. Yet it has proven to be the hardest to accommodate in a coherent framework including relativity and quantum mechanics. Superstring theory is an invention for theoretical physics. it is regarded as a promising candidate for a consistent quantum theory of gravity. The precise relation between string theories and gravity is however subtle and far from being understood at some deep, fundamental level. In this paper, the author reports on some progress recently made on the understanding of this relationship, both through general considerations and through the explicit computation of certain suitable processes. The paper consists of three parts: general considerations on the connection between classical (resp. quantum) strings on one hand and classical (resp. quantum) gravity on the other; a study of (gedanken) Planckian-energy string collisions and a comparison with General Relativity expectations; and speculations on the new Physics predicted by string theory around the Planck and/or the string scale