<P><B>Alcubierre drive
<P>
<P>The Alcubierre drive, also called the warp drive, is a technique, originally proposed by the physicist Miguel Alcubierre, which consists of a toy model of a Lorentzian spacetime with properties somewhat reminiscent of the fictional "warp drive" from the science fiction series Star Trek. In the semipopular literature, this proposal has often been described as having the status of an exact solution of the Einstein field equation, but this characterization is wildly misleading. In fact, every possible spacetime is an exact solution given a certain configuration of energy. It has also been claimed that the Alcubierre drive is grounded in a well-established physical effect, the Casimir effect, which is currently understood in terms of quantum field theory, but this is also rather misleading. We will deal with the Alcubierre drive in more detail in chapter 6.
<P>
<P><B>EmDrive
<P>
<P>The EmDrive exploits an idea first suggested by Allen Cullen in the 1950's, an electrical engineer then at University College London, that involves forces created by bouncing microwaves between opposite walls of a cavity. The "trick" is to try to design a cavity in such a manner that forces on one side are greater than the other.
<P>
<P>The drive comprises a resonant cavity flooded with microwave radiation. The radiation exerts radiation pressure on the walls of the cavity, and normal Newtonian mechanics would indicate that, no matter what shape the cavity is, the forces exerted upon it from within must balance to zero. This problem is resolved, however, as relativistic effects cause a cavity shaped like a truncated cone to experience a larger force against the large end than the small end, due to the group velocity of the wave changing as the local diameter of the cavity varies. We won't deal in depth with the EmDrives in this volume, but refer you to the writings of Roger Shawyer for more information.