I am writing a time travel novel. A very complicated subject, with a considerable literary history! In an effort to organize my unlucky characters’ thinking on it, I began to draft a Taxonomy, illustrated by examples that with luck we will all know. Is some major category missing? Can you name other movie or book examples? Let me know!
1. Time travel can change the past
A. Changes appear instantly; old time line is immediately overwritten
(A1) BackTo The Future – Marty feels changes physically (disappearing hand) as the possibility of his non-existence becomes more likely, but his memories are never altered in any way. We don’t get to see what would have happened if he HAD disappeared. Would his actions in keeping his parents from conceiving him have been undone, creating a paradox or would this new timeline retain his (residual-alt-timeline) actions up to his disappearance?
A2) Looper – Time traveler feels the change physically AND mentally as, or right after it happens, but the current timeline retains his (residual-alt-timeline) actions up to that point. Characters around the time traveler see AND remember the changes to the traveler. (Someone could have witnessed Old Seth becoming disfigured just as we did.) For a full-of-spoilers but highly detailed analysis of how the time travel works in this movie, go here. You see why writing a time travel novel is turning my hair white?
B. Changes don’t appear until you get back to the/your present (A Sound of Thunder by Ray Bradbury, in which an accidental change in the past makes the present a world of intelligent reptiles). Old time line is replaced with new.
B1) The time traveler loses his old memories and only remembers the new altered reality. (As I recall this is how it worked in Time After Time by Jack Finney.)
B2) New memories coexisting with old memories. I think this is what The Butterfly Effect movie was like, right?
C. Changes appear but retain a residual effect (you can kill your grandfather, then disappear, but grandpa remains dead)
C1) There will be a duplicate of the time traveler if time traveler tries to go back to the future because it won’t be the same future reality.
C2) When returning to the future, duplicate disappears and time traveler is the only version but the future is still foreign to the traveler. (Timecop?)
C3) It is branching; every change you make creates a new different future. In theory therefore you could go and shop around all the different futures, looking for the one you like best. (There have been novels with thise theme, can’t recall one.)
2. Time Travel cannot change the past
A. Purely tourism; all you can take is photos. C.S. Lewis describes a story which he did not name, in which when the time traveler goes back raindrops penetrate like bullets, since nothing, not even the path of a raindrop, can be altered in the past.
B. Minor souvenirs can be taken back.
3. Time travel sometimes changes the past
A. Only big alterations have an impact (shooting Hitler has effect, shooting his janitor, no)
B. Even tiny alterations have an impact (Butterfly effect) and so you must be very careful what you do to the past, lest you destroy your present. (This is the angle I used in Revise The World.)
4. Time travel is an endless and intrinsic loop where effects create the causes that leads to the effects.
A) Cause – effect cycle has no origin.
A1) Harry Potter and the Prizoner of Azkhaban. There are several delicious loopy moments in this but I’ll pick one. Harry fights the dementors but is overcome by their numbers, Harry thinks his father comes to fights them off. A few hours later Harry travels back in time to arrive at the same moment when he see himself fighting the dementors in vein. Waits for his father to arrive and help the past Harry than realises it was himself that helped fight them off. An intrinsic loop where effect creates the cause creating the same effect.
B) Time travel object appear to have no origin.
B1) ABC’s Lost series. The Compass paradox. Richard gives John a compass in the 2000s. John travels back in time to 1950s. Gives Richard the compass. Richard waits half a century, giving the compass back to john. Compass has no origin of production but is ageing with every loop.
4. Time travel is one-way
A) Psychic time travel one-way. A good example would be Replay by Ken Grimwood.
B) Physical time travel one-way. I can’t name a specific work, somebody help me here. But I recall generally works in which unlucky time travelers are stranded in the far past, cooking trilobites over a campfire. And, of course! Titus Oates makes a one-way trip to the futre in Revise.