  There are at least two distinct general notions of time machines,  which we will call Wellsian and Thornian for short. In The Time Machine, H. G. Wells (1931) described what has  become science fiction’s paradigmatic conception of a time machine:  the intrepid operator fastens her seat belt, dials the target  date—past or future—into the counter, throws a lever, and sits back while time rewinds or fast forwards until the target date  is reached. We will not broach the issue of whether or not a Wellsian time machine can be implemented within a relativistic spacetime  framework. For, as will soon become clear, the time machines which  have recently come into prominence in the physics literature are of  an utterly different kind. This second kind of time machine was  originally proposed by Kip Thorne and his collaborators (see Morris  and Thorne 1988; Morris, Thorne, and Yurtsever 1988). These articles  considered the possibility that, without violating the laws of  general relativistic physics, an advanced civilization might  manipulate concentrations of matter-energy so as to produce CTCs  where none would have existed otherwise. In their example, the  production of “wormholes” was used to generate the  required spacetime structure. But this is only one of the ways in  which a time machine might operate, and in what follows any device  which affects the spacetime structure in such a way that CTCs result  will be dubbed a Thornian time machine. We will only be  concerned with this variety of time machine, leaving the Wellsian  variety to science fiction writers. This will disappoint the  aficionados of science fiction since Thornian time machines do not  have the magical ability to transport the would-be time traveler to  the past of the events that constitute the operation of the time  machine. For those more interested in science than in science  fiction, this loss is balanced by the gain in realism and the  connection to contemporary research in physics.