New paper

This paper follows up on our earlier work on the null energy condition (NEC). Imagine you've built a device which "warps" spacetime enough to create a wormhole or time machine. General theorems show that somewhere the matter in your device has to violate the NEC. In our earlier work we showed that any classical system which does so is unstable to small perturbations. In this paper, we show that semi-classicality of the device spacetime is a strong enough condition to require semi-classicality of the matter fields from which it is constructed. In other words, a device which warps space in a deterministic (nearly classical) manner is subject to the earlier results and is unstable.

We leave open the possibility of intrinsically quantum (or "fuzzy" devices) whose spacetime is strongly fluctuating. However, these might not be the most safe or realiable means of transportation! A wormhole or time machine cannot be both predictable and stable.


Semi-classical wormholes and time machines are unstable
Authors: Roman V. Buniy, Stephen D.H. Hsu

We show that Lorentzian (traversable) wormholes and time machines with semi-classical spacetimes are unstable due to their violation of the null energy condition (NEC). Semi-classicality of the energy-momentum tensor in a given quantum state (required for semi-classicality of the spacetime) implies localization of its wavefunction in phase space, leading to evolution according to the classical equations of motion. Previous results related to violation of the NEC then require that the configuration is unstable to small perturbations.

http://arxiv.org/abs/hep-th/0504003