A Superluminal Subway: The Krasnikov Tube

The ``warp drive metric recently presented by Alcubierre has the problem nthat an observer at the center of the warp bubble is causally separated from nthe outer edge of the bubble wall. Hence such an observer can neither create a nwarp bubble on demand nor control one once it has been created. In addition, nsuch a bubble requires negative energy densities. One might hope tha..