Negative Energy Densities and the Limit of Classical Space-Time
  Although negative energy densities are predicted by relativistic quantum field theories, I present an argument that an "operational" positivity still holds: the energy in a region, plus the energy of an isolated device which traps or measures that energy, must be positive. If we assume Einstein's field equation, this means the local geometry of a negative energy-density region cannot be measured by the trajectories of test particles.   So far, all attempts to design thought-experiments to verify a classical local geometry in the negative energy-density region have failed. It seems we must impute a quantum character to such a space-time regime. 
Adam D. Helfer