Most of us have probably read the classic story–or seen the movie Back to the Future–in which a character travels into the past, and does something to interfere with his/her own time stream, and then (perhaps slowly for dramatic effect) fades into nonexistence. This begs the question of what changed the time stream in the first place, since the character who changed it apparently never existed.
This seems to be a paradox, which is by definition a self-contradictory statement or condition. Writing about future time travel is much less likely to create a paradox, but it’s still possible. Traveling back in time, however, is fraught with danger–at least, in the traditional way of looking at things.
Different writers have taken different ways of looking at time travel to avoid paradox. Some have held that the future is immutable. Go ahead and travel back and change the future. You can travel back, but only because you already did, so it doesn’t matter what you try, because you already tried it, and the present time from which you are traveling is the result of the changes you already made. This explanation has the benefit of avoiding all paradox, but denies human agency (otherwise known as free will.) You can’t change anything from before, no matter what you do.
Others have taken a parallel universe approach. You can go back in time and change things, but you just shift over into a parallel universe when you do, because there are an infinite number of parallel universes. I actually have a problem with this one, because I think the parallel universe concept is largely misunderstood. More about that in another post, but I think it’s cheating to say you’re in a parallel universe without destroying the universe you came from. I don’t think that is mathematically possible, but I don’t have time to go into it here.
In Devil’s Paradise, I’ve taken a couple of approaches to time travel which have probably been done in all the thousands of time travel stories I haven’t read, but I haven’t seen them in the ones I have. One concept is that a field is created within a machine, allowing objects (including persons) to pass through the field toward the past or the future, coming out of the machine at the far end of the stream. But the machine that holds the field must exist at the other end, so it is impossible to travel back in time before the machine was created. No time traveling cars or Wells’ time machine. It does limit the dramatic possibilities, but it seems more plausible–assuming time travel is possible at all.
The other concept is simpler, but perhaps is less intuitive. The future does not exist at all. It never does. Once you travel back in the past, where you came from no longer exists. You are a traveler from a universe that may exist someday, but does not now, and may not ever, if you change anything.
Let’s take the classical idea of going back in time and shooting your father before you were conceived–or even killing your own younger self. There’s a paradox, if ever there was one, right? Nope. When you were at that moment in time in your own history, no creep from the future showed up to kill you. Ergo, that person you kill is not yourself–not even your earlier self. I don’t mean a case of mistaken identity, either. Because the future does not exist, there is no connection between the future and the present, as there is between the past and the present. So the younger you is actually a different person. You don’t fade away, you don’t pop out of existence, but perhaps you do get arrested and convicted for murder.
Turn it a different way. What if you didn’t kill your younger self, but simply became buddies, and gave some guidance about some of the poorer decisions you made before? You would change as the different choices are made, right? No. Your younger self would benefit, but you would not change. (I could see a plot wherein the older Riley becomes jealous of the younger Riley who benefits from the older Riley’s advice, and is–in the opinion of the older Riley–insufficiently appreciative. Lots of conflict possibilities there.)
It would seem, in the above example, that there is a paradox in two Rileys occupying the same time stream, but again not. The future does not exist, and they are not the same person.
There are some very interesting fictional possibilities from this last idea. Want an identical twin? Travel back in time a minute. Then stop your earlier self from going into the time machine again. Want an army of super soldiers? Get your perfect soldier, and send him back an hour. Then do it again, with both of them. Then again, with four. Wash, rinse, repeat.
It is not related to this concept, but in Devil’s Paradise, I put in some serious obstacles to use of the time machine, to prevent this kind of thing–which is a little too close (literally) to deus ex machina. It takes an immense amount of power to generate the time field, and traveling through the field is a huge strain on the human nervous system.
I’ll have to deal with the mathematical impossibility of parallel universes in another post.
Have a great day!