User:Foxwarrior/Time Travel
Time Travel[edit]
I felt like writing about various forms of it, out of annoyance.
Reasonable Time Travel[edit]
A reasonable time travel scheme has no assumptions about reality that require a skewed and anthropocentric universe. Although I suppose that D&D is a anthropocentric universe.
Determinism: Any changes you make to the past already happened. See this very high quality video for reference, as well as this Nature article. As a game mechanic, it's less than ideal, since the players can't do anything they didn't already do in the past. There is such a thing as a "paradox" in the Determinism system, but we already know that none of those occurred.
Timelines: Going back in time is functionally identical to rewinding the universe, inserting yourself into it, and then recording a new history. Whether or not the old timeline exists should probably not be relevant. This is the time travel scheme used by Stargate, Red Alert (except for "timelines merging!"), the Timeshift gameplay (the cutscenes are filled with completely unrelated technobabble, also about merging timelines, and sometimes about paradoxes which shouldn't be paradoxical, given the Timelines system), and A Sound of Thunder. If only the players get it and they share it together, it can work fine in D&D; antagonistic time travel wars are very inconvenient, since the person who goes further back can undermine all of the actions of the other people (or I suppose I should say "has already undermined all of the actions of the other people now"). There is no such thing as a "paradox" in the Timelines system: going back in time to kill your grandfather won't erase the you who went back to kill your grandfather from existence, it will just stop another you from being born.
Time Waves (Achron): Time waves travel from the past to the present, reach the present, and wrap around, propagating actions. If the past it wrapped around to was actually the starting point of the level, this would be functionally identical from the perspective of an inside character to Determinism. Since it doesn't go back that far, you can sometimes get permanent Chrono Clones and other odd things. Although it's wonky and particularly counterintuitive, it actually allows players to interact with each other while playing the game at different points in time, so it's probably the best system for antagonistic time travel.
Unreasonable Time Travel[edit]
The assumptions necessary for these time travel systems to make sense are grating.
Time Waves (A Sound of Thunder): When the consequences of your actions in the past affect the present in waves, spaced hours apart, that replace organisms living on the surface of the Earth in order of how "evolved" they are, you're misunderstanding basically every facet of modern science simultaneously. It might work in D&D though.
Determinism Demon: This functions like Timelines, except that when you try to change something, events happen to stop you from doing the thing you wanted to do. The Time Machine is a good example: the main character goes back in time to save his sweetheart, but she dies in a completely different way. Basically every episode of Doctor Who where the Doctor says something sad about Timey Wimey balls has a Determinism Demon, and it's a different one each time. Theoretically you could drastically change history by making sure the side effects of your failed actions are achieving your actual goals, but since the Determinism Demon in control of beating up the time traveler is almost always also the author of the time traveler, this does not happen. A DM who tries to implement Determinism but doesn't know how is likely to become a Determinism Demon. Since its underlying functionality is the same as Timelines, "paradoxes" can't actually happen, but the Determinism Demon is almost universally too stupid to realize that.
Present is Canon: Heroes used this intriguing method until the last season. Hiro Nakamura could go back in time and put messages into a sword, where they wouldn't be found until after the present time. The future was changed frequently, but nothing that we saw happen in the present was changed by interfering in the past. This might work excellently in D&D, provided that the players don't do anything in the past that would logically have to affect the present, like throwing the Lich BBEG into an Orb of Annihilation while he's still a baby. Since part of the universe is Deterministic and part of it is Timelines-based in some freaky combination, "paradoxes" are both relevant and difficult to eradicate (see: Lich baby->Orb). It doesn't work as a physical law, however, because even placing a small piece of paper inside a sword handle will affect the weight of the sword by an amount other than zero, which necessarily alters all of the present and future in a basically negligible but not nonexistent fashion.
Memory is Special: When you change the past, the old past isn't forgotten. Wheel of Time's balefire does this well. This isn't really time travel so much as an effect that tries to replicate it without getting all the details.
