The light cone in string theory

The causal boundary of string propagation -- defined as the hypersurface in loop space bordering the timelike(spacelike) domains in which two successive measurements of the string field do(do not) interfere with one another -- is argued to be $0=int dsigmaigl(delta X(sigma)igr)^2 = sum_{ell=-infty}^infty delta x^mu_{-ell}delta x_{mu ell}.$ Some possible consequences are discussed.