MEANS OF TIME TRAVEL
Just because it's impossible doesn't mean you can't do it. Four common methods:
- Faster-than-light travel. If you travel close to the speed of light (theoretically possible), you actually travel into the future. If you could travel faster than the speed of light, you would go back in time. You can't, of course, but this is fiction. See also rules #3, 4, and 5 for space travel.
- Dial-a-time. You've seen Bill and Ted's Excellent Adventure, yes? Keanu Reeves' finest hour (if ever there was one). Their time machine was the soft sci-fi standard: don't explain how it works, just punch in a time and go. See also: Back to the Future.
- Wormholes. This is probably the most scientifically feasible method. If wormholes can be used to leap through space, then it should work for time too.
- In the minds of others. Like Quantum Leap, you don't go back in time yourself, but your mind does, implanting itself in the minds of others. You might be a watcher or you can take over that person's personality for a time and change things through them.
RESOLVING THE PARADOX
Most time travel stories must, at some point, deal with The Paradox. That is, they must answer the question: what happens to the present if you change something in the past? The impossibility of time travel means nobody knows, so you have a lot of freedom here. Beware, though, some of these devices are hard for a reader to wrap their head around.
- Time fork. If you change the past, then you actually create a fork in time. There's the "old" present that you came from, and the "new" present created by the events you changed. If you take your time machine back to the present, it will always be the "new" present, unless you can undo the changes you made.
- The Butterfly Effect. Like the time fork, except that any change--even your very presence or the butterfly you just swatted away--will have drastic effects on the future. This makes it highly unlikely that you can undo said changes.
- No change until you return. Say you kill your great-great-grandfather. In this scenario, you will continue to exist until you try to go back to the present, at which point you (and all descendants of your g.g.g-father) disappear. It doesn't make much sense, but it means you have a chance to undo things.
- Change occurs gradually. Like Back to the Future, your changes to the past become a ticking clock. If you stop your parents from falling in love, it's only a matter of time before you cease to exist.
- Change occurs immediately. If you kill your ancestor, you cease to exist there and then. Of course that's the true paradox: if you never existed, how did you kill your ancestor? Wouldn't that undo everything? Sometimes it does, sometimes it doesn't. This is where stories get REALLY complicated.
- Events cannot be changed. The opposite of the paradox. Any attempts you make to change the past will either (A) be thwarted (e.g. the gun jams, your ancestor trips and dodges the bullet, your ancestor is saved by a medical miracle after you leave the scene, etc.) or (B) prove to have been a part of the timeline all along (e.g. he never was your ancestor, but his death is what brought your real ancestors together).
The biggest problem with time travel is how powerful it is. If you can go back in time and change any mistake before it happens, it immediately raises the question, "Why don't you just...?" Like, "Why don't you just go back in time to before you made the machine and stop everything from happening?" This is another place where time travel gets all headache-y, and where you need to be the most careful. Some ideas:
- The machine is broken. So you can't go back and forth until it's fixed. Of course, once you fix it, you could just go back and undo everything, but if everything is right again, maybe you don't want to.
- It's against the rules. Time travel is essentially magic: you make up the rules, then stick with them. If there's a plot hole, make up a rule to patch it up, but make sure that new rule is consistent with everything else that happens. Maybe time travel is uncontrollable (as in Quantum Leap, or anything with wormholes), or you can get somewhen in a broad sense (say, a certain year), but not close enough to fix details (i.e. the exact place and time where you would have opportunity to fix everything). Maybe you can't change the past. Maybe you can only go one direction (forward or backward, not both) or you can only jump a specified amount of time (like in 5-year increments).
- It makes things worse. In an attempt to subvert the plot hole, you do go back in time to fix it, but your old self doesn't listen, or someone worse comes back and fixes the machine after you broke it, or you killed a butterfly and spaces monkeys take over the planet in ten years. Whatever.
WHAT'S BEEN DONE
The short version of what's been done in time travel fiction is: EVERYTHING. Nothing's original, we talked about that. If you want to see for yourself what's been done, take a week off of work and read these.
However, anything can be done well again. Mix it in new ways and make it your own. Just don't make the mistake of thinking you're the first person to come up with the idea of time tourism, time police, fixing the future, stopping someone from wrecking the past, beings that move through time, a modern-day teenager stumbling upon a trip to that period in history he can never seem to understand in school (God bless you, Keanu)...
It's all been done, but you can do it again and better. Just don't be boring, and you'll be fine.