Chapter 5, Protecting the Past Summary and Analysis

Hawking thinks it is hard to speculate on time travel. All discussions begin with General Relativity. Time travel might be possible with wormholes, tubes of spacetime that connect different regions of space and time. Wormholes could break the light barrier because no thing would go faster than light, though moving along spacetime would appear that way from some perspectives. However, time travel generates the grandfather paradox: what if you went back and killed your grandfather before he was conceived.

Avoiding philosophy, the main question is whether the laws of physics allow spacetime to warp to such a degree to allow a spaceship to return to its own past. There are three types of answers to the question. General Relativity seems to rule it out. On "semiclassical" theory, matter behaves according to quantum theory, but spacetime...