Time travel and parallel dimension stories sometimes speak of sideways time.  The term is used to describe different concepts under different theories of time.

It is most commonly heard in connection with parallel dimension theory, in which there are presumed to be a vast and possibly infinite number of parallel worlds alongside our own.  In some versions of the theory, these universes are all different, frequently very different, as their histories have diverged based on random variations in events over time; thus stories built on this concept are less about time travel and more about universes that are similar but distinct.  The 1970 Dr. Who episode Inferno with Jon Pertwee is such a story, as is the original Star Trek series episode Mirror, Mirror and its Deep Space Nine sequel Crossover, and all the episodes of the series Sliders.  In these contexts, sideways time means moving to a universe which has always existed as a distinct universe, that is similar to but divergent from a "home" universe.

In a purer version of parallel dimension theory, sideways time still indicates traveling to another universe, but that the other universe is identical to the "home" world.  In some versions of the theory, these universes are offset incrementally, such that moving sideways across them gives the appearance of moving forward and backward in time, but that anything the time traveler does in one universe will change it without affecting any other.  The problems with this concept are myriad, most notably that a single time travel event will disrupt the parallelism of an infinite number of universes, making all future sideways time travel unlike time travel because the parallel universes have all been put out of synchronization with each other.

The concept of sideways time is sometimes mentioned in connection with divergent dimension theory as a means of getting from the present universe to the lost original universe, although in most cases, such as in Back to the Future Part II, it is discounted as an impossibility.  The original universe still exists in divergent dimension theory, but its integrity (the prevention of paradox within it) demands that it be unreachable from any diverging universe.  Were it possible for someone in a divergent universe to return to his universe of origin at any point prior to his departure from that universe, he would have to create yet another divergent universe or all the advantages of the theory would be lost (paradox would not be prevented).

Illustrations of anomalies in replacement theory sometimes are perceived as suggesting the possibility of sideways travel to a previous version of history; however, under replacement theory the previous version of history is being erased as the new version is created, and thus there is never a corresponding "now" between the two histories.  Under fixed time theory the concept is irrelevant:  there may be parallel dimensions, but traveling to them is unlike time travel in any way and should not be confused with time travel.

The theory of two-dimensional time treats sideways time differently.  In this case, time exists in complete form from beginning to end (or from infinite past to infinite future), but this complete history of the world can be changed by movement akin to sideways time, typically by the movement of time travelers to points in their pasts.  The theory is that the arrival of a time traveler in his own past changes all of history from that moment forward, and so the universe shifts to a new timeline.  In such circumstances, the old timeline becomes inaccessible, but objects and persons already severed from their own origins by time travel may continue to exist in the new timeline.  Also of interest in this is the concept of supertime, a construction that attempts to use something like sideways time to allow change to propagate through the history of a new universe at a delayed rate.  Again under this theory the original universe might continue but is inaccessible; sideways time travel is in one direction only.

Our next article will examine the predestination paradox, also known as the causal loop or uncaused cause.