One thing that can not be avoided in time travel fiction is paradox. Will they or won’t they, did they or didn’t they? But what is it, how does it work and why is it so darn much fun? Well all of this and more will be answered below.
The basic idea of a time travel paradox involves using time travel to cause something to happen that is either impossible or didn’t happen before. Take the venerable “Grandfather Paradox” as the classic as well as a prime example. You, having invented some means for travelling into the past, do something that leads to your grandfather being killed. Since your grand dad died, your father was never born and with out a father you were never born. And that is just the beginning of the paradox, it only gets better from there. Because if you were never born then you couldn’t travel back and accidentally kill your grand father, so he wasn’t killed and you’re safe to invent time travel after all… So, are you domed to cycle through these possibilities for ever in a sort of closed ended time loop, or can this somehow be resolved? Have no fear intrepid time traveller, like vacuum the universe abhors paradox and will do what it can to fix it.
Of course, the simplest method of resolving the paradox would be to leave your grandfather dead. Since he can’t die by your hand a freak accident or a heart attack will do nicely. So no grandfather, no “you” to trouble the universe with these pesky temporal revisions. This requires a small tweak to the time-line so that your change can be caused by someone or something else. This “frees” the time-line from your interference, removes most of the paradox and has the bonus (from the universe’s point of view) of leaving the universe short one time traveller that can’t stop paradoxing up the place! Now I say it removes most of the paradox since there is still the small matter of a spontaneous change to the time-line. However, the total amount of paradox from this event is much smaller than continuing to allow a time traveller to disrupt things everywhere he goes! So the universe accepts the trade off and moves along it’s merry way back down the time-line.
Another commonly accepted method of resolving paradox is to simply define it as impossible and be done with it. In this case, you travel back in time but find it impossible to intentionally kill your grandfather let alone accidentally. One way this can be achieved is “random circumstance” getting in your way. So even if you try something drastic, like shooting him, the gun may jam just before your grandfather can be shot, or you trip and miss the shot. So no matter what you try there will always be something that stops you from causing that paradox. Another way to keep the paradox from occurring is based on your imperfect knowledge of the past. Say you travel back and kill the man you thought was your grandfather, but you don’t cease to exist. Then you go back to your time and find out that your grandmother re-married or had an affair so the man you killed was not really your biological grandfather, paradox resolved! That, of course, is also when you find out that there was always a mystery regarding that man’s death, a mystery that has now been solved. This is the simplest method and gets used in stories a lot, but generally I find that this is the least satisfying of methods of fixing paradox.
The final method of resolution is one of multiple universes. This version also maintains that paradox is impossible but in this case changes you make are not altering your time-line in your universe but, in fact, creating a whole new parallel universe! Now killing that man that looks like your grandfather is no (temporal) problem. Your real grandfather is safe back in your original universe, and the man you killed, though seemingly so familiar was really a stranger. Thus your own personal history is intact with no paradox. Of course, the police in that universe may take a dim view of killing one of their citizens, but that’s one of the risks of the intrepid time traveller.
There is, it seems, one other possibility and that is that your paradox is retained. Now this can have some strange consequences such as your continued existence though you were never born, in this time-line. Why would a universe that abhors paradox so much allow it to continue? There are two possible answers to that question and the first is that the universe doesn’t really care after all. This allows all sorts of time anomalies, paradoxical effects and so forth which can be fun with people and things changing and popping in and out all the time… Generally those worlds are unsatisfying as logic and continuity can be thrown away on a whim. The best reason, in my opinion, is related to the first method for resolving paradox which allows exchanging a large paradox in favour of a smaller one. Say for example, that somehow you were responsible for saving the entire universe at some point in your past. That might mean that allowing you to cease to exist from the time-line would also allow the universe itself to disappear. Now since this is the largest paradox possible then “merely” allowing your grandfather to be dead and yet allow you to continue is small in comparison! Hooray, you get to exist without have been born in this version of the time-line.
So there you go, various methods of resolving (or not) paradox. What do you all think? Does that wrap it all up or did I leave some thing out? Since this is such a complex and highly theoretical subject it boarders on the philosophical and that’s one of the reasons that I think I like it so much. That’s why I’m curious to find out what everyone else thinks!