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Time Travel Theories

Time travel has been a popular topic for science fiction and fantasy throughout history.  The idea of time travel breaks two cardinal rules that we have come to know about our existence: The past cannot be changed, and the future is unknowable.  This essay will strive to give a technical description of the many theories about how time travel might "work" and the dynamics therein, as well as a description of the type of time travel present in different elements of popular culture.  Note that all these theories are independent of the method through which time travel is accomplished.


Time Travel Theories

There are many theories about how time travel might work, but four primary theories stand out.

1. Nonexistent Time - "Time" does not exist as an actual substance or quantity - it is an entirely man-made concept, therefore, time travel is impossible - there is nowhere (or nowhen) to travel to!

2. Viewable Time - Time exists, however, due to physical laws - modification of the past is impossible.  You can, however, view the past in great detail as if it were a movie.

3. Instant Time - Time travel is possible, but in an odd twist of fate, all time travel occurs exactly at the same time, and so the past and future cannot actually be changed.  Because of this, paradoxes are not possible, and all time travel that will occur, has already occurred.

4. Editable Time - Time travel is possible, but the events that occur are "sticky".  That is, minor changes won't set off any chain reactions, and if there aren't any major changes, then everything will continue to stay the way it was.  Or will be.  In addition, if one major event is changed, only the direct reactions of that event will tend to be affected, if something else is independent, it will stay the same.  This is the most popular time travel theory.

5. Resetable Time - When you travel back in time, you "reset" the future - all decisions and chance events reset.  Therefore, in this theory, even making no changes could produce a wildly different outcome than before.  Time travel is very dangerous in this scenario!

Theory Subsets

In scenarios 1, 2, and 3 time travel to change the past or future is impossible.  In scenarios 3 and 4 there are 2 subsets that explain in more detail how time travel works:

1. Sequential Time Travel - Time travel happens in sequence, which allows paradoxes, changing of the future, but not killing yourself.

2. Quantum realities - When you time travel, you create a new Quantum Reality, separate from the previous one, allowing unwary time travelers to become trapped in alternate timelines - even if they don't exist anymore.

If the theory is Editable Time or Resetable Time, then there are different treatments of temporal paradoxes, such as variations on the old "Grandfather Paradox", based on what subset is correct.


Subset A - Sequential Time Travel:

1. You travel back in time and accidentally kill yourself
2. Since you accidentally killed yourself no one grows up to travel back and accidentally kill you
3. Therefore, you don't exist in the future to go back in time and accidentally kill yourself
4. Your younger version therefore grows up
5. You then go back and accidentally kill yourself
6. Since you accidentally killed yourself no one grows up to travel back and accidentally kill you
7. etc.
It just keeps looping until...
Eventually. You go back in time to accidentally kill yourself but a capacitor blows on your time machine and you can't go back in time and the accident never happens - the timeline is stable!
All you would remember is a freak accident kept you from going back in time! (you don't remember any "looping") You would notice every time you tried to make a paradox a freak accident would stop you!

This phenomenon is called a "temporal loop" and the conclusion is called "chance compensation".  This will be a minor chance event at the time of time travel for Editable Time, and very well be a major event, such as never building the time machine, for Resetable Time.  Editable temporal loops are also great in number while Resetable time loops are very quick, and may not even fully loop once.


Subset B - Quantum Realities:

In this theory, each time you time-travel you create an alternate time line. In this case, if you traveled back in time and accidentally killed yourself, you would still exist but your younger version would not. You would have, in effect, "jumped" realities. If you went back to your time no one would know who you were because in their reality, you don't exist!


Timeline Stability

Finally, there is also the idea, primarily applying to Editable Time, of Timeline Stability, or Modification Delay. This means that there is a delay between the change in the timeline, and it's effect on the timeline as a whole, in addition to the time-traveler.  This is primarily used as a means for mistakes to be fixed before the changes percolate throughout the timeline.  Sometimes there is also Temporal Shielding that protects time travelers from changes in the timeline.


Time Travel in Popular Culture

Here is a list of the different theories and subsets with a corresponding list of examples of different movies, novels, webcomics, etc., that can be classified as one of the above possibilities.  The theory of Nonexistent Time is not listed as time travel is not possible in that scenario.