Casimir Energies and General Relativity Energy Conditions
  Quantum systems often contain negative energy densities. In general relativity, negative energies lead to time advancement, rather than the usual time delay. As a result, some Casimir systems appear to violate energy conditions that would protect against exotic phenomena such as closed timelike curves and superluminal travel. However, when one examines a variety of Casimir systems using self-consistent approximations in quantum field theory, one finds that a particular energy condition is still obeyed, which rules out exotic phenomena. I will discuss the methods and results of these calculations in detail and speculate on their potential implications in general relativity. 
N. Graham