Null energy condition

New paper! A bit too technical to discuss here. But for Sci-Fi fans, the following tidbit: wormholes and time machines generally require violation of the null energy condition. Our paper shows, among other things, that none of the particles or fields of the type generally considered in particle physics will allow construction of a (stable) wormhole or time machine.

For those familiar with the Einstein equations, they are of the form:

{Curvature tensor R, fn of spacetime geometry} = {matter energy-momentum tensor T}

The set of possible spacetime geometries (could there be wormholes? loops in time?) satisfying the Einstein equations is unconstrained without some information about the right hand side of the equation. Traditionally in general relativity, one assumes that the matter fields satisfy certain energy conditions, for example the null energy condition, which requires that the tensor T, when contracted with any null (lightlike) four-vector n, is positive: Tnn > 0. We show that when this condition is violated, the system is necessarily unstable to small perturbations.


Instabilities and the null energy condition
Authors: Roman V. Buniy, Stephen D.H. Hsu

We show that violation of the null energy condition implies instability in a broad class of models, including all gauge theories with scalar and fermionic matter as well as any perfect fluid. When applied to the dark energy, our results imply that $w = p / \rho$ is unlikely to be less than -1.

http://arxiv.org/abs/hep-th/0502203