  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 $0=\int d\sigma\bigl(\delta X(\sigma)\bigr)^2 =
  \sum_{\ell=-\infty}^\infty \delta x^\mu_{-\ell}\delta x_{\mu \ell}.$ Some
possible consequences are discussed.
