O.K., so I'm a sucker for a good time travel story; even a bad time travel story will hold my interest and give me something to think about.  So I've thought about a lot of those stories, seen a lot of the "temporal paradoxes" they've gone out of their way to create, and found the standard theories of time travel inadequate.  But I went beyond that:  I developed a set of concepts and terminology which would explain what would happen were someone able to travel through time--not what would happen to them, nor even (although it certainly is covered) what would happen to history, but what would happen to time itself if someone were to travel "out of sequence" through the time line.  I applied those ideas to the stories, especially the movies (also the TV shows, but their name is legion, and most are too complex and contrived and at some level foolish to contemplate).  With the publication of Multiverser, I had the opportunity to include those ideas as a way for referees to manage time travel.  After all, a game which allows that anything you can imagine is possible must allow time travel; and in allowing time travel--whether by technology, psionics, magic, or some innate body skill--the game must be prepared to unravel the consequences.  Here are the  tools which solve time travel problems, and the reasoning behind them.  But to understand them, you must follow the reasoning of sequential time.

This is an important point oft overlooked by many.  All of our experience with time suggests that it is sequential:  things which happen "before" must occur prior to things which happen "after".  Our concepts of "causality" are very much wrapped up in time:  An object falls from a great height; it strikes the ground with great force; striking the ground, the object impresses the ground as it shatters into fragments; the fragments mix with the disrupted earth as they scatter in a ring around the point of impact; the debris falls to the ground, settling into the rim of a crater.  Those events must occur in that order:  the crater cannot exist prior to the events which form it.

Yet those of us with intellect and education have been taught to think of time more spatially, as a dimension.  We are given to think that the past and the future and the present all co-exist in some sense which is atemporal; we cannot easily conceive this atemporal concept, yet there is a sense in which it must be so for time travel to work.  After all, if you are to leave 2000 and go to 1990, then 1990 must still be there (or then) when you arrive, or you have gone nowhere (or nowhen); it is much like trying to visit the Temple in Jerusalem:  you can go where it was, but if it isn't there, where are you?  (Additionally, time must be multi-dimensional, or at least capable of separate independent lines, so that your time machine can function in a sequential manner--turning itself on and then off--out of sequence with the rest of the world.)  So it would appear that time is spatial, that all times exist "now", in some atemporal sense.

This has led some to conclude that temporal events are fixed--if the past, present, and future all exist now as they ever were and ever will be, then nothing can be changed.  It has also led to the development of the absurd paradoxes--the man who becomes his own grandfather, the object which moves in a loop from the future to the past to the future, the fugitive who prevents his own birth.  These stories are based on the assumption that time is static, that whatever the past becomes is what it always was, that things are unalterable and therefore intrinsically absurd.  Whether the point of it is that the past is immutable, or that time travel is impossible, or some other notion, the idea of a single time line which is dependent on itself in non-sequential ways has become a standard in time travel stories.

I disagree with that view.  It flies in the face of causality, and against even the most conservative understanding of free will.  My theory maintains that a person can move through time, make changes in the past, and do any of the things which are said to cause those paradoxes, and that there are definite causal lines and clearly defined timelines which explain them.  In fairness, much of my theory is reflected in some of the movies on this web site; on the other hand, the number of mistakes found in these films suggests that this has not been fully considered.  Please permit me to attempt to outline the basics of my conception of time, and how time travel affects it.

I will begin by saying something I repeat often:  moving into the future has no effect on time or history, unless it is accompanied by a trip into the past.  Going three years into the future is no different than moving to Boston for three years, except that you aren't any older:  you are gone, and then you come back.  During those three years you did not age, but you weren't here, and you weren't anywhere else.  As far as anyone else is concerned, as far as history is concerned, as far as time itself is concerned, you went away for a while, and then you came back.

On the other hand, any time you move into the past, you change history.  Many people make the mistake of thinking that they can travel into the past without changing it, merely because they are careful not to change anything "important"--an extremely anthropocentric notion which believes that not changing recorded human history is the same thing as not changing the past.  To quote from Multiverser, "The character who makes every effort not to change the past can only prevent himself from making severe changes.  His very presence displaces molecules about him, alters total mass and gravitic forces.  His movement creates kinetic ripples throughout the universe.  He sweats, and leaves moisture, oils, and salts in the past which he robbed from the future; he drinks or eats, and removes moisture and matter from the past to return to the future.  He is seen, and becomes part of the memories of those who saw him; some may change their own actions based on the memory of the one they saw.  As soon as a character materially or visibly enters the past, he has altered the future."

That's an important point:  you can't go back in time without changing time.  It's a bit like the Heisenberg Uncertainty Principle:  the act of inserting yourself into the past makes the past different.

This is where my explanation of time begins.  We will postulate Traveler, an individual who develops a means of moving through time.  Let us suppose that he is born in the first year of the next millennium--2001.  In the year 2020 he begins work on the time machine, and in the year 2030 he completes it.  He decides to go back in time to the year 2000, the last year of the twentieth century, the year before he was born.  We will call that target moment "Point A", the earliest point in time which is affected by the trip.  We will call the moment from which he left "Point B".  All of the events which occur between point A and point B make up the history of the "A-B Timeline".

I would maintain that the history of the A-B timeline is necessary, and therefore exists.  Time advances sequentially from the year 2000 through the birth of Traveler and the creation of the time machine up to the moment of point B, the time Traveler moves into the past.  From a temporal viewpoint, at the instant he leaves the future he arrives in the past.  Therefore, at the instant Traveler leaves 2030, time ends for that history.  Recognize this:  when Traveler leaves 2030, he immediately arrives in 2000.  When he arrives in 2000, he changes history; therefore there can be no more future from that history after 2030, point B.

So what happens when Traveler leaves 2030, and goes to 2000?  Time itself snaps back to 2000, and history is repeated with whatever variations are caused by Traveler's presence in 2000.  The way we describe it, the instant Traveler reaches point A, he destroys point A, replacing it with point C; point C is exactly the same as point A temporally and in every other way, except that thirty-year-old Traveler is now here.  Let's take a look at the temporal anomalies chart; that might help you understand this much.  At the moment, we want to consider how time moves from A to B to C:

As you can see, time always moves from the past to point A and beyond to point B; at point B, when Traveler leaves time and heads to point A, time ends; but Traveler cannot arrive at point A, because he could not have been there in the original time line--he won't be born until next year, and he remembers the events between his birth and his departure, all now in the future.  But that time line has been altered.  The events which occurred at point A have been altered--however insignificantly--to create point C.

Time will now continue to point D.  Point D occurs at the same point in time as point B; but of course, the history which leads to point D is different in at least the fact that Traveler appeared full grown before his birth.  However, the events which occur at point D and the history which leads to it are vital in understanding what will happen next.

But first, let us understand this snap-back process.  For Traveler, he has come from a future which has a history in which he did not appear; that future is his personal past.  However, for everyone he meets, everyone who sees him, everyone in the year 2000 at point C, that history is still future; it does not exist, and in that exact form it never will exist.  The history which will exist is the one which includes the appearance of Traveler in 2000.  So time, as a video tape, has advanced until the counter reaches 2030, but is then rewound to 2000, and re-recorded so that the previous recording is erased.  It may be that the new version is nearly indistinguishable; it may be that it is completely different.  None of the characters know what the show was before it was erased except Traveler (who pushed the rewind button); nor do they know what it is now becoming.

On to the events at point D.

Let us suppose that for his entire life Traveler felt that he was born too late, that he would have liked to see the previous century.  He created the time machine and went back to view the celebrations of the end of the millennium.  Arriving in 2000, in discovers that there are no such celebrations, because the idiots who lived at that time all thought that the second millennium ended the year before.  Disappointed, he takes a brief look around, and returns to 2030 to live the rest of his life.  He has created the C-D timeline; however, all of the important events of that timeline will be preserved:  he will be born, grow to maturity, create the time machine, and go back to 2000, where he will be disappointed, look around, and return to the future.  Since the history of the C-D timeline preserves itself--that is, everything that was caused by the A-B timeline will be caused by the C-D timeline--the future may now occur based on that history.  This creates an "N-jump":  the past leads to the A-B timeline, which ends at point B by creating the C-D timeline, which continues into the future, as in the diagram:

Let's propose a different scenario.  Arriving in 2000, Traveler is disappointed.  He realizes that he's wasted thirty years of his life pursuing a meaningless dream.  He decides to save himself.  Settling into a quiet life, he looks for himself as a boy.  In 2010, he finds himself as a nine-year old, and tells himself to forget the past and focus on the future.  Whatever he says, the young Traveler is much impressed by this, and as he reaches maturity he turns his attention to something else--medicine, psychiatry, nuclear physics--and never builds the time machine.  Thus when the year 2030 comes, Traveler will not go back to the year 2000.  Since he has not returned to the year 2000, he has not altered the original history--he has erased the altered history.  Thus time is forced to snap back again, this time to point A, to repeat the A-B timeline.  But we know that in the A-B timeline, Traveler becomes interested in time travel, and goes back in time to create the C-D timeline; and we know that when he does so, he meets his younger self and discourages him from doing this, restoring the A-B timeline.  Thus we are caught in a perpetually recurring altering time line, what we call an "Infinity loop" or "Bow tie loop":

Let's create another scenario.  Not disappointed with the past, Traveler stays.  He doesn't seek out his younger self, but is naturally drawn to the place where he grew up.  He takes a job as a science teacher, inspiring young minds for the next decade or two.  One of the youngsters he influences is himself.  Perhaps he knows it is himself; perhaps it is just another face in the school attending a lecture.  But ideas, questions, concepts pass between them, from the elder to the younger.  The child grows with a greater understanding of that which he will do; he builds the time machine with greater understanding of the mechanics of time.  He reaches 2030, and he goes back in time to 2000 on schedule.  But he is not the same person who went back the first time.  His understanding of time is different; his approach to physics is different.  He might stay and become that teacher, influence that boy which was him; but the influence is different.  This history is different because Traveler himself is a different person.  Looked at from another angle, the Traveler who leaves point D in search of point C cannot find it, because the Traveler who arrived at point C was not the same man.  This Traveler departing from point D will arrive at point E, creating the E-F timeline.  And since he is different, he will have a different influence on himself, creating the G-H timeline.  We have created the "Sawtooth snap":

A sawtooth snap might repeat any number of times.  It could terminate after the E-F timeline, or it could repeat many times.  Theoretically, it itself could become an infinite trap, repeating perpetually in ever-changing unresolved timelines.  It could resolve itself such that the last created timeline causes itself, the best possible outcome, the "N-jump termination".  Or it could fall into a loop in which the last timeline prevents the journey causing the restoration of the original timeline (or indeed, in which the last timeline causes any of the previous timelines to recur), an "infinity loop termination", trapping time perpetually in repeated multiple alternating timelines.  (I want to thank Timothy Fox, apparently thinking along similar lines, for suggesting the phrase "Cycling Causality" for this.  Although I tend to use "sawtooth snap" in these pages, I do think that "cycling causality" is an especially good term for a sawtooth snap with an infinity loop termination.)

That's the basics of the concept.  There's more to it in Multiverser, including explanations of how these ideas resolve the most common paradoxes--but we have examples of that right here on the web site.  All of these anomalies are seen in the movies on this site, and many facets of these are explored.