The large-N limit of superconformal field theories and supergravity

We show that the large-N limits of certain conformal field theories in vari ous dimensions include in their Hilbert space a sector describing supergrav ity on the product of anti-de Sitter spacetimes, spheres, and other compact manifolds. This is shown by taking some branes in the full M/string theory and then taking a low-energy limit where the field theory on the brane dec ouples from the bulk. We observe that, in this limit, we can still trust th e near-horizon geometry for large N. The enhanced supersymmetries of the ne ar-horizon geometry correspond to the extra supersymmetry generators presen t in the superconformal group (as opposed to just the super-Poincare group) . The 't Hooft limit of 3 + 1 N = 4 super-Yang-Mills at the conformal point is shown to contain strings: they are IIB strings. We conjecture that comp actifications of M/string theory on various anti-de Sitter spacetimes is du al to various conformal field theories. This leads to a new proposal for a definition of M-theory which could be extended to include five noncompact d imensions.