Abstract

The symmetry group of the asymptotic null boundary of asymptotically flat spacetimes is the infinite dimensional BMS group. It is expected that it will play an important role in any candidate holographic description of gravity. Recently, Strominger showed that the Ward identities for the BMS group are equivalent to Weinberg’s soft graviton theorem. A similar story holds for the Ward identities of suitably defined ‘large’ gauge transformations, these are related to soft gluons.

To go beyond these kinematical considerations I’ll introduce two theories which live on the null conformal boundary of Minkowski spacetime. One of them describes gravitational degrees of freedom, and the other Yang-Mills degrees of freedom. Correlators on the worldsheet of these theories reproduce the tree-level S-matrix for N=8 sugra and N=4 sYM respectively. These theories contain charges which implement the action of the symmetry algebra, their Ward identities on the correlators give the soft graviton and soft gluon theorem. There are also currents which implement proposed extensions of these symmetry algebras, these give the subleading contributions to the soft factors.