(I had hoped to simply continue the space ecology material as an uninterrupted series, but for whatever reason, I don’t have the energy for that, and I do have the energy for this. Probably has something to do with a book I read recently. That, and the fact that I’ve recently been re-watching one of my all-time favorite cartoons.)
Time travel is a difficult thing to do right in fiction. It’s a good thing, too-if we knew enough about it for us to know true consistent rules for it, we’d probably be leading pretty messed up lives.
That aside, time travel always presents the sticky question of paradoxes. How does it work?
Sticky Possibility #1: The timeline has always included the events caused by the time traveler.
While this version neatly excludes paradoxes, quite frankly, the philosophical implications of this are rather disheartening. Why? It takes the possibility of free will out of the equation (at least, it can be seen to). In the Beast Wars Transformers series, the character Dinobot had a huge psychological struggle over the question of whether his choices were really his if the timeline worked this way. (It being a cartoon, the time flow didn’t work that way, and things got serious.)
Sticky Possibility #2: The timeline isn’t stable, and changes wrought by a traveler have the possibility of causing severe, even irreversible changes to the timeline.
You’ve seen it in Back to the Future. You’ve seen it in tons of cartoon, comic books, etc. The question is always “if you can do this, what the heck?”
Back to the Future‘s version of this (and to some degree, the version of it used in the Beast Wars cartoon, incidentally-for which I forgive it, because the series would have lacked much of its urgency and tension with any other model) is patently absurd. If a time traveler is directly affected by the changes s/he enacts regardless of time period, then the paradox is self-correcting, and time travelers should merely be disintegrated with a grisly pop anytime they try to fool with things. More logical is the possibility that a time traveler becomes disconnected from the “native” timeline, and the timeline that s/he has arrived in is a new, different timeline.
There are two ways to interpret this, as well.
Messy Alternative #1: The “new” timeline replaces the “old” timeline, overwriting events from after the arrival of the traveler.
Lots of people have a major paradox problem with this one, and with good reason-where did the time traveler come from now?
Messy Alternative #2: The “new” timeline “forks away” from the “old” timeline, becoming a parallel world.
This can be seen, to some extent, in the Marvel comic book universe. Since there have long been references to parallel worlds of all kinds in Marvel stories, this is the neat answer in this situation: Choices make new timelines, and time travel is merely a type of choice as far as this system is concerned.
There are still questions; for instance, some philosopher disliked the idea of branching worlds because this philosopher (the name, if I ever knew it, escapes me) believed that God would only “follow” one branch. (And I have to ask, how does that follow? But that’s getting off the subject, so I won’t go there.) However, it neatly cuts out the paradox as well.
To avoid problems, I advise the following: Pick a method. Then, use that method. Do NOT pick a new one later. Do NOT change midstream. (Do NOT pass go, and do NOT collect $200.) Be consistent, or you’ll drive your readers and possibly yourself completely insane.
Oh, yes, and here’s a good reason why time travel won’t be mucking up the real world anytime soon: There are actually four theoretical time machines designed by physicists (the number may have changed up or down, depending on advancements in the field in the past decade) which could work, provided one has absurdly advanced technology and tremendous amounts of matter and energy at one’s disposal. (At the very least, you’d need an advanced spaceship just to use them, and you’d need a lot more than that in order to build them.) All of these time machines have one significant limitation: They cannot take you to a time before they were built.
Stephen Hawking once remarked that the clearest evidence that time travel is impossible is the decided lack of time traveling tourists; this might be another explanation.