Three Kinds Of Timelines

The Shield of Time, 1987 A. D., pp. 26-33.

In this chapter, Manse Everard explains the need for the Time Patrol to Wanda Tamberly. As ever, I feel a sense of confusion. 

It is easy to think that there are just two logically possible kinds of timelines:

immutable, as in Poul Anderson's The Corridors Of Time and There Will Be Time.

mutable, as in the Time Patrol series.

However, analysis of Time Patrol texts shows that they refer to two kinds of timelines:

mutable;

immutable but discontinuous.

Confusion results when discussion oscillates between these two kinds of timelines.

"Mutable" means "changeable." Change involves a relationship between a state changed from and a state changed to. That is a relationship of before and after, in other words a temporal relationship. Any timeline is itself a single set of temporal relationship between spatially three dimensional states of the universe. We call these states "successive" precisely because they are related temporally, not spatially.

If one state of a timeline can change into another state of the timeline, then we must distinguish between temporal relationships within a timeline (these are between 3D states of the universe) and the temporal relationship between states of the timeline. In fact, instead of saying that one state of a timeline changes into another state of the timeline, it is simpler to say just that one timeline changes into or is replaced by another timeline. This difference is terminological. The two sets of temporal relationships (within a timeline and between timelines) are two temporal dimensions at right angles (ninety degrees) to each other as the three spatial dimensions are at right angles to each other.

On the mutable timeline model, a time traveller who "changes the past" arrives in the current timeline from the previous timeline. ("Current" and "previous" in the second temporal dimension.) On the immutable but discontinuous timeline model, there is only one timeline. A time traveller who "changes the past" arrives from nowhere and nowhen:

"'It's sort of like quantum mechanics, scaled up from the subatomic to the human level.'" (p. 30)

These are different concepts.