Circular Causality And Discontinuity

Poul Anderson, Time Patrol (New York, 2006).

In "Star Of The Sea," the Time Patrol must:

preserve a timeline in which, as described in Tacitus One, Veleda preaches war but later peace;

prevent a timeline in which, as described in Tacitus Two, she continues to preach war.

The Patrol are in the Tacitus One timeline, which is why they preserve it. Therefore, Tacitus Two is merely a potential although it is one that might be realized unless there are effective countermeasures.

This raises two questions:

(i) why did Veleda initially preach war?
(ii) how does the Patrol know of the potential timeline?

To answer (ii) first, the Patrol acquires a Tacitus Two manuscript. How this might happen in the Tacitus One timeline is discussed in the post, "Unstable Space-Time II."

(i) Veleda preached war against Rome in the name of the goddess because, in her teens, she was raped by Roman merchants but rescued by Janne Floris of the Patrol whom she mistook for the goddess. Janne was able to witness and interrupt the rape because she was remotely observing Veleda's birthplace for any odd events. She was doing this because she had already observed Veleda, later in life, preaching war in a way that could potentially lead to Tacitus Two. And Janne observed Veleda's later life because the Patrol had found the Tacitus Two manuscript.

Thus, the discovery of the Tacitus Two manuscript is a necessary part of the Tacitus One sequence of events. There are two time travel paradoxes here: not only circular causality but also causation from a prevented timeline.

To ensure that the German rebels make peace with Rome, Manson Everard of the Patrol acts as the initial go-between. I had thought that there must have been an original timeline in which Romans and rebels made peace without any help from the Patrol. However, both the Tacitus Two manuscript and Janne's intervention in Veleda's early life are necessary for Tacitus One. Thus, Patrol efforts to prevent Tacitus Two also seem to be necessary for Tacitus One. So Everard's involvement in the peace negotiations was there from the beginning.

If Veleda's companion, Heidhin, had not killed himself but instead had remained alive and continued to argue for war, carrying Veleda with him, then the outcome would have been Tacitus Two.