What We Want From Time Travel

I might well not finish reading A. Bertram Chandler's The Coils of Time. I cannot get interested in the conflict between the Committee and the Council in the alternative timeline. But it has made me think about what I do want in genuine time travel fiction. There might be three kinds of preferred stories.

(I) Stories set in a single immutable timeline where the characters act freely and might seem to have "changed the past" but in fact turn out just to have caused it. Need I give examples? Relevant works by Poul Anderson and others have been listed and discussed on this blog before.

(II) I feel that stories about "changing the past" are not really about time travel. As soon as he either deliberately or accidentally "changes the past," the time traveller is no longer in the past of the timeline that he had started out from. The equivalent in space travel fiction would be an astronaut landing on the surface of an astronomically accurate Mars, then immediately transitioning to an alternative Mars with a breathable atmosphere, like ERB's Barsoom or Lewis' Malacandra. However, Poul Anderson's Time Patrol series takes the "changing the past" idea seriously. Guion tells Wanda Tamberly that sometimes the world-lines are disturbed but that the cause of the disturbance might not be in our "yet." The Patrol can only trace the disturbance back toward its source - which is a quantum fluctuation possibly taking the form of a temporal vehicle that appears from nowhere and nowhen as if arriving from a prevented future... Alternative timelines have to be considered and somehow taken into account even if they are also and at the same time regarded as entirely non-existent. Thus, a mutable timeline introduces an element of mystery that the author has to be able to convey to his audience. Guion does this. (But I cannot help continuing to argue the case. If Timeline A simply does not exist and has never existed, then it has not changed into Timeline B. All that does exist and has ever existed is a single discontinuous timeline.)

To be continued.