“Gravitational stress,” the Qax murmured in his ear. “This wormhole is a throat in space and time, Parz: a region of stress, immensely high curvature. The throat is lined with exotic matter throughout its length; we are traversing a tube of vacuum that runs along the axis, away from the exotic matter. The minimum width of the throat is about a mile. Our velocity is three miles per second—”
-Timelike Infinity, Ch. 8