Wormholes in spacetime and their use for interstellar travel: A tool for teaching general relativity, Michael S. Morris and Kip S. Thorne, Am. J. Phys. 56(5), May 1988
Rapid interstellar travel by means of spacetime wormholes is described in a way that is useful for teaching elementary general relativity. The description touches base with Carl Sagan's novel Contact, which, unlike most science fiction novels, treats such time travel in a manner that accords with the best 1986 knowledge of the laws of physics, Many objections are given against the use of black holes or Schwarzschild wormholes for rapid interstellar travel. A new class of solutions of the Einstein field equations is presented, which describe wormholes that, in principle, could be traversed by human beings. It is essential in these solutions that the wormhole possess a throat at which there is no horizon; and this property, together with the Einstein field equations, places an extreme constraint on the material that generates the wormhole's spacetime curvature: In the wormholes throat that material must possess a radial tension t0 with the enormous magnitude t0 ~ (pressure at the center of the most massive neutron stars) X (20 km)2 /(circumference of throat)2. Moreover, this tension must exceed the material's density of mass-energy, r0c2. No known material has this t0 > r0c2 property, and such material would violate all the "energy conditions" that underlie some cherished theorems in general relativity. However, it is not possible today to rule out firmly the existence of such material; and quantum field theory gives some tantalizing hints that such material might, in fact, be possible.