Novikov self-consistency principle

Stated simply, the Novikov consistency principle says that if an event exists that could give rise to a paradox, then the probability of that event happening is zero. Rather than consider the usual models for such a paradox, such as the grandfather paradox in which a time-traveller kills his own grandfather before he ever meets his grandmother, Novikov used a mechanistic model which was more amenable to mathematics; a billiard ball being fired into a wormhole in such a way that it would go back in time and collide with its earlier self, thereby knocking it off course and preventing it from entering the wormhole in the first place.

Novikov found that there were many trajectories that could result from the same initial conditions. For example, the billiard ball could knock itself only slightly astray, resulting in it going into the past slightly off course, which winds up causing it to knock its past self only slightly astray; this "sequence" of events is completely consistent and does not result in a paradox. Novikov found that the probability of such consistent events was nonzero, and the probability of inconsistent events was zero, so no matter what a time traveller might try to do he will always end up accomplishing consistent non-paradoxical actions.

The Novikov consistency principle assumes certain conditions about what sort of time travel is possible. Specifically, it assumes counterfactual definiteness which is the assertion that there is only one timeline and that multiple alternate timelines do not exist or are not accessible.

external ref: http://www.iap.fr/eas/EAS18/time18/ontime