Abstract

This chapter explores the concept of time itself, as physicists understand it. Einstein's special theory of relativity requires worldlines of physical objects to be timelike; the field equations of his general theory of relativity predict that massive bodies such as stars and black holes distort space‐time and bend worldlines. Suppose space‐time becomes so distorted that some worldlines form closed loops. If one tried to follow such a closed timelike curve (or CTC) exactly, all the way around, one would bump into ones former selves and get pushed aside. But by following part of a CTC, we could return to the past and participate in events there. According to a recent calculation by J. Richard Gott of Princeton, a cosmic string passing rapidly by another would generate CTCs. The chapter concludes that if time travel is impossible, then the reason has yet to be discovered.