Rapidity Gaps in Deep Inelastic Scattering
  A simple semiquantitative picture of diffractive electroproduction is described. Although the diffractive component of $F_2$ is approximately independent of $Q^2$ and $W^2$, this mechanism is "soft," i.e. it depends upon large-distance physics and is not readily describable within perturbative QCD. 
J. D. Bjorken (SLAC & Oxford University)