A predestination paradox is a temporal anomaly which is said to be possible under fixed time theory.  It is also known as a causal loop or uncaused cause.

Fixed time maintains that the past and the future all exist already in static form, and that time is not more than our perception or experience of events within it.  Thus for something to move from the future to the past is not different in kind from something moving from the living room to the bedroom; it is simply in a different position within four-dimensional space, and it happens that the value of the dimension we call time has moved in the direction in which it does not usually move.  Since this is possible, it would also be possible for that object to initiate a chain of events which ultimately is the cause of its own motion from the future to the past.

The billiard ball example is most commonly used to illustrate this.  According to this example, a billiard ball goes into a pocket that is actually a wormhole transporting it to a moment in the past.  It emerges from the wormhole and collides with itself, driving itself into the pocket through which it has just traveled.  It is argued that since it is possible for the billiard ball to be knocked into the pocket by this collision, it is perfectly reasonable for this to be and always have been the history of the universe, that a billiard ball entered a wormhole because when it earlier emerged from that wormhole it collided with itself and knocked itself into that wormhole.

Opponents of the theory call this the uncaused cause, stating that this is something that can only happen if it happens, and in that case it cannot happen.  Other theories of time resolve these events very differently.

Under parallel and divergent dimension theories, the billiard ball which enters the wormhole emerges in another universe.  It then can collide with its temporal duplicate--the version of the billiard ball that is native to that universe--but that this collision will almost certainly prevent the ball from entering the wormhole.  There are thereafter two billiard balls in that universe and none in the originating universe.

Replacement theory offers a more complicated solution to the problem.  It maintains that there must have been a now lost original cause:  something else knocked the billiard ball into the wormhole, and then as it emerged from the wormhole it interfered with the original collision and replaced it with a new collision by which the billiard ball now knocks itself into the wormhole.  Assuming the trajectory is consistent, the billiard ball can become the cause of its own trajectory.

This is technically the umbrella paradox for one version of the two anomalies called the Grandfather Paradox, in which (in this case) a man becomes his own grandfather, or an object becomes the origin of its own existence.  Although that is properly a subset of this anomaly, it will be discussed in a future article.  Next time we will describe and distinguish the two types of grandfather paradoxes.