Abstract

The "four-wave mixer", a laser technique for reversing the motion direction of light waves so that they can be turned around and returned to their point of origin was the subject of my last Alternate View column (ANALOG, June-1985). In this AV column I want to go one step further by examining a hypothetical kind of time-reversed light wave which should actually go backward in time. As we shall see, such backward waves could be used to send information from the present to the past, resulting in a sort of "poor man's" time travel. Recent science fiction novels like Jim Hogan's Thrice in Time and Greg Benford's Timescape have been based on backwards-in-time communication and the situations that might arise from it. In both books a world disaster is averted by sending a message to the past. In Timescape a side-effect of the signalling prevents Lee Harvey Oswald from assassinating President Kennedy, irreversibly altering the course of events after 1963.