In the worlds of time travel stories there are a few major theories which are used to make sense of how time would work if anyone, or anything, could experience it out of sequence--most particularly, if it were possible to travel to the past, what would happen?  Could you kill your own grandfather?  Could you become your own grandfather?  Answers to questions like these are based on a theory of time.

Anyone who has seen a few time travel movies should realize that they do not always follow the same rules.  In general, most time travel stories wind up breaking even their own rules at some point.  However, theoreticians have attempted to define how time works, and how that would impact what happens if someone were to travel to the past; sometimes even characters within the stories have such debates.  There are three major theories, with a few variations.

In nearly all cases, the very concept that it is possible to travel in time demands that the past and the future in some sense exist; that is, traveling to the past is like traveling to another place.  It is as if all of history were a book already written, or a movie already completed, and we are in a particular part of that story; but the story behind us is still there, a few pages or frames back, and the story ahead of us is already awaiting our arrival.  This concept that all of time exists as if it were space is fundamental for any notion that someone could travel to it.  However, the sense in which it exists may differ according to the theory of time, and in some theories the future does not exist, in some cases such that even if you came from it you cannot return to it.

Perhaps the leading theory is called Fixed Time Theory.  According to this conception, the past is already established, and nothing in it can be changed.  Starkly distinct from it is Parallel Dimension Theory, which maintains that a traveler going to "the past" lands in a different universe.  In pure parallel dimension theory, this universe has always existed alongside our own, and has had a history identical to our own until the time traveler disrupts it.  There is a variant version, Divergent Dimension Theory, in which the new universe is created by the arrival of the time traveler, whose presence changes history for that new universe but has no impact on the world from which he departed.  The third major theory, followed to some degree by many time travel stories, is Replacement Theory, which holds that a traveler to the past will change history, erasing what was and creating a new timeline.  Variants of this theory mostly diverge based on their rules regarding what happens if history is changed in certain ways.  These major theories will be examined in more detail individually in future articles.

There are other theories, and partial theories, attempting to resolve what might happen if someone were to travel through time.  Efforts to provide a framework for popular time travel stories have led to the investigation of the possibility of Two-Dimensional Time, in which history exists from beginning to end, but time travel causes it to change "all at once", creating an entirely new independent history of the world in which some elements from the previous history might be retained despite having no source or cause in the new world.  The concept of Supertime has been proposed as a means of rectifying changes in history that break causality.  These theories attempt to create the possibility that a time traveler could unmake the history from which he came and yet still exist in what he perceives as the past of the new history he has made.  Concepts such as these sometimes seem to be behind particular time travel stories, such as Poul Anderson's Time Patrol series, but it is difficult to derive a consistent theory of time from most such stories.

There are also stories which challenge the notion that the past is still there.  Stephen King's The Langoliers appears to be suggesting that people live only in the present, racing forward from second to second to keep ahead of the creatures who devour the world of a few minutes ago.  Under such a concept of time, time travel would be impossible, and anyone who succeeded in stepping out of the present moment would find nothing, no one, and possibly nowhere.

Next time the Fixed Time Theory will be examined in more detail.