Summarizing And Synthesizing

When Poul Anderson's Time Patrol Academy instructors and cadets discuss the theory of time travel, they summarize and synthesize the thinking of many previous sf authors, particularly HG Wells, who introduced temporal vehicles, and L. Sprague de Camp, who suggested that the course of events might resist change except at certain nexus points.

Why kill your grandfather? Why not just prevent your parents from meeting? That way, you do not have to kill anyone but you nevertheless arrive in a world where you were never born. A space traveler from Earth can arrive on Mars where he was never born. A time traveler from timeline 1 can arrive in timeline 2 where he was never born or someone in a time machine might appear from nowhere. Logic dictates only that a given individual cannot both exist and not exist in a single timeline, not that every event has a cause. Indeed, apparently, quantum events can be uncaused.

Poul Anderson is to be commended both for the lengthy examination of a mutable timeline in his Time Patrol series and for the circular causality paradoxes in the immutable timelines of The Corridors Of Time and There Will Be Time. He has to be unique among writers on time travel for the way that he develops both premises in such detail.