Derivation of the Null Energy Condition
  We derive the null energy condition, understood as a constraint on the
Einstein-frame Ricci tensor, from worldsheet string theory. For a closed
bosonic string propagating in a curved geometry, the spacetime interpretation
of the Virasoro constraint condition is precisely the null energy condition, to
leading nontrivial order in the alpha-prime expansion. Thus the deepest origin
of the null energy condition lies in worldsheet diffeomorphism invariance.
