Causality Vortex

Manson Everard of the Time Patrol must intervene in the Battle of Rignano, 1137, in order to prevent Lorenzo de Conti from killing King Roger of Sicily. One of Everard's fellow Patrolmen, present with him on the battlefield, says:

"'We are the reserves, here on the ground, if things go badly.' He left unspoken that in that case the causality vortex would probably have grown irredeemably great."
-Poul Anderson, The Shield Of Time (New York, 1991), p. 341.

What is a causality vortex? The Patrol guards a single sequence of causally related events. It is possible, though difficult, for a time traveler to deflect that sequence. If one event is changed, others change to compensate so that the overall course of history remains the same. As long as Roger loses but survives this battle, the details of the skirmishes during the battle do not matter.

However, some events are pivotal. If a pivotal event has a different outcome, then the entire subsequent course of history may also differ. Lorenzo de Conti's world-line interacts with the world-lines of so many influential figures that a small change in one of his actions can change history. Because of a random fluctuation in space-time energy, a timeline in which de Conti was merely present, no doubt fighting well, at the Battle of Rignano becomes instead a timeline in which he attacks and kills Roger, thus upsetting the balance of the medieval church-state conflict. Only if this conflict remains indecisive, each side wearing the other out, will democracy and science emerge in later centuries. An unopposed Church or Empire will prevent both.

If Everard prevents de Conti from killing Roger, then, it is hoped, the preferred timeline will be restored. But, if he fails, and maybe even dies in battle, then the course of events will rapidly become so different that it may be impossible for the few surviving Patrol members to rectify it. Going back yet again with even fewer resources to try to save both Roger and Everard might even make things worse. If the intervention is noticed and regarded as miraculous, thus changing history even further, then the Patrol's cause will certainly be lost.

I think that "a causal vortex" means a course of events immediately following an altered pivotal event in which causes and effects so rapidly diverge from the original timeline that the changes to history multiply instead of damping out. In that case, the usual inertia of events is overthrown and it becomes impossible for time travelers to counteract the changes. They are caught in a vortex.