Time Machines

Abstract

In this chapter we start with the introduction, in Sect. 1, of the concept of appearing time machine and give a simple example thereof. In the following section, a few less trivial examples are considered. The first of them is the Misner spacetime. It is one of the oldest and presumably the most important time machines: indeed, being just a flat cylinder it is in a sense the time machine in its pure form. And anyway, with its rich and counter-intuitive structure the Misner space deserves studying at least as a wonderful source of counter-examples [133]. Its simplest generalizations to the non-flat case are considered too. Then, in Sect. 3, we briefly discuss the process of evolution of a wormhole into a time machine, a widely known and popular process. Its importance lies in the fact that it is one of the most ‘realistic’ scenarios of how the universe might lose its global hyperbolicity. In Sect. 4 we introduce the notions of compactly generated and compactly determined Cauchy horizons. All time machines with such horizons are shown to have some important common properties. However, none of those properties are compulsory for a general time machine. We show that by example in Sect. 5.