Quote from: Tahmoh;891187so although it seems like he outsmarted you when you used the returned to the same point he left idea, later in the campaign throw a few ripples at him and have the rest of the party go along with you as if they aren't ripples

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It's awesome idea. Totally borrowing it.

...and why didn't I think about it in the first place? :hmm:

Come to think about it, time might adjust to the changes... slowly. Piece by piece, one NPC at the time. Delayed ripple-effect. :hmm:

If the main world where the rest of the party is does not allow any reverse time travel at all, then I would tend to do think that hopping to other planes of existence and returning would also not allow that, so choices I would consider for when the described world-hopping PC returns would be:

1) The hopping PC returns as if his time in the other worlds had not happened... i.e. the same time he left, plus the time taken in the "highway" plane between worlds. Since he spent a year presumably aging in the other world, he's now a year older in the world he came from, than he would have been if he'd stayed home. This can be annoying because the PC who left gets to spend years doing stuff, while the PCs left behind can't really game anything out until that PC returns.

2) The subjective time the PC experienced, affects the time of return linearly. So the PC spent a year in the other world falling forward in time. The fact he went backwards 10 years doesn't affect his return time. So he returns a bit more than one year later in his original world - he's still exactly as old as he would have been. This is nice because it means the PCs left behind still get to do things.

3) Taking a page from modern physics, relative time dilation occurs, of some sort. This means the PC can travel at what seems to him faster than the speed of light, but the rest of the universe progresses faster than he does. This might impart a delay in the original world simply for moving to the other world and back. As for the reverse time travel in the other world, I don't know what that involves, but it might also have a time dilation side effect that affects the time the traveler will arrive if/when he returns to his original world. That is, it might be even more than 1:1, and/or might be based on the time traveled backwards. So it's like 2, but with additional multipliers and/or charges for the world travel and time travel. So the return time might be +1 year for the time hanging out in the other world, and maybe +10 (or +100, or + 1000), for the time traveled in reverse. So hopping worlds and traveling in time might impose a long delay on the time you can return to your original world. This is great for the GM not having to worry about meddling exploits, but can be "oh crap I'm screwed" for the PCs that do it. Unless an entire party does it, in which case the GM needs to decide if they want to advance time in their campaign world, or find new PCs to play with until those ones return.

(I've played in some interesting games where the party did fall forward a lot in time without aging, resulting in a new campaign world state. It's of course a huge world reboot for the GM.)

Of course there are reverse-time options too, though I would only do these if I were wanting to run a full time-travel intervention game (pretty unlikely). If the PC goes back in time 10 years in the other world, spends a year falling forward and then steps back, then it could also make sense that he returns 9 years before he left.

As explained in my previous post, I'd tend to rather have that create a new timeline/reality/universe where the old world is as it ways 9 years ago, but suddenly it has a new PC arriving from 10 years in the future on that old timeline. That PC would never appear again in the original universe, unless he somehow managed to find a way to target that thread of the universe. He'd need to be brilliant or find some sort of mega-competent time lord who was interested in helping him out.

But if as I said, I were really running a time-travel intervention game, then ok, he arrives 9 years before he left. My first question would be - what to do with the version of the PC that was already in the world 9 years ago. Three options come to mind immediately:
a) The younger PC vanishes from existence at the moment the older PC arrives. (easiest/avoids some problems)
b) The younger PC exists and is an NPC who will do what the PC did 9 years ago, up until he meets some new circumstances caused by time travel or it's side-effects or butterfly effects. Similar for the rest of the world, and the other PCs. This means the traveling PC's actions for the next 9 years need to not change much, or else the other PCs and past gameplay are going to be invalidated.
c) The younger PC exists and is an NPC, but suddenly has new agency, choices, luck, etc. The whole world starts doing more and more different things than it did originally. I like this, but it invalidates all future play and world events that were resolved in past gameplay in the original world. The other PCs left behind are invalid, unless they also somehow time travel to the same or an earlier time. So the other players may as well just be given new character sheets for their characters 9 years in the past, and/or be given the option to start new characters at that point in time.
d) The continuity of the time-traveling PC's history needs to be maintained, or else they will vanish in a puff of logic. That is, if the time-traveling PC does something that means he will not be able to time travel as he "did" 9 years in the future, he won't have done that, and so the timeline won't include him as a time traveler any more. Less minor side effects will also be carried forward. Unfortunately, this also applies to the world, events, and the other PCs... however, too much disruption will just cause the timeline to "correct" the PC's time travel, snapping back to the old timeline. For example, if the PC somehow causes something that would have the original party not end up at the time portal in exactly the same conditions that caused this time traveler to travel in time to cause the change, then that violates the logic of the timeline, so clearly that did not happen. That can either be enforced by having whatever the change is resist happening, or just by reverting to a timeline that makes sense. I.e. you can't use time travel to invalidate your own time travel as it happened for you in your past experience. However, it seems to me that that logic also invalidates time travel itself, which is one of the reasons I prefer to think interventionist time travel is nonsense anyway.
e) Intervention without logical continuity enforcement. The time-traveler can mess with themselves and their friends and the world and not vanish themselves in a puff of logic, despite some idea of this making sense with only one universe. Generally this involves a conceit and not thinking too much about it, and really having reality created by the continuity timeline created by gameplay and PC perspectives. I.e. each PC (or NPC the GM cares to pay attention to) has a valid experience and point in time and history that they can play with. The question becomes which point in time do you choose to play out next, and what side-effects, if any, do you have affect the other situations other characters are in, or not. I think this tends to be the mode that a lot of time travel stories/films operate in. The actual reality is the character's experience being paid attention to at the time, and things do or don't have consequences depending on what seems fun or interesting or clever or thematic. Of course, it introduces the conflict that if reverse time travel is possible and has consequences that get reconciled, then unless events wipe out everyone or all competition, then the future will be full of a near-infinite source of meddlers who might decide to travel back and change things, changing or invalidating who knows what. This is even harder to actually track than a universe with an infinite number of threads. It also seems infinitely pointless because a practically infinite number of time-traveling scoundrels, villains, heroes, police, spies, whatever can be expected to be competing to change everything until someone sooner or later just gets annoyed and decides to destroy it all to end the madness.

Oh, and I think time travel is one of the places where staying experiential is most valuable. I would definitely stick as much as possible to only answering questions about what each PC experiences directly, and zero questions about what is actually going on, or how time travel or the universe work. When players inevitably start speculating and trying to draw the GM into discussions such as the above, or even to seek out world experts on time travel and so on, I would suggest picking a random perspective and having a discussion, but having that have no relation to how the GM is actually managing time travel and universes and butterfly effects or whatever. Mag'myzak the Lord High Wizard of Time may grant them an audience and talk to them for hours or days about his understanding of time and causation, but Fildar the White Scholar and Steven Hawking and Einstein and the Pope and the GM on Thursday and the GM on Friday may also do so, and express completely different ideas.

Limiting the players' knowledge of how time travel and the game universe works, to actual experience and perceptions of their PCs, makes it more mysterious and immersive, and keeps the game being played rather than talked about, and gives the GM peace to figure out what to do and choose how to do it, etc.