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What internally consistent time travel rules would make for an interesting world to model and explore in a game or a book?

I exclude the "no free will" setup (where everyone is pre-destined to behave in a way consistent with past events if they travel in time); it seems quite boring.

I also exclude the extreme case of the "parallel universe" setup, where any possible event happens in some universe, and so the time traveler simply shifts between parallel universes depending on what he does. This setup results in no special importance attached to any particular timeline, which makes time travel have no impact on the world (except as a personal experience).

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    $\begingroup$ You might find this article about the different possible models of time travel useful. $\endgroup$
    – 2012rcampion
    Mar 22, 2015 at 23:11
  • $\begingroup$ @2012rcampion - Very interesting reading. Thanks! $\endgroup$
    – Bobson
    Mar 23, 2015 at 20:19
  • $\begingroup$ Here's another article you might like. $\endgroup$
    – AdamHovorka
    Mar 24, 2015 at 14:27
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    $\begingroup$ This is really broad. There are boatloads of possible rules that are at least debatably interesting, and even more variations of "magic" to enforce consistency. $\endgroup$
    – Telastyn
    Mar 24, 2015 at 14:29

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If you want internal consistency, you're stuck with some varient on 'you can't change (what you know about) the past', I'm afraid - what you're calling the 'no free will' version. Anything else allows paradoxes. Fortunately, that still leaves rather more wiggle room than you're giving it credit for.

The technical version of the rules for internally consistent time-travel is the Novikov self-consistency principle - it's a theory that was developed when a professional physicist started wondering about exactly that problem. It can be difficult to get your head around, but worth the effort if you're serious about trying to handle time travel logically. There are a lot of subtleties involved, and working out what isn't ruled out is just as important as knowing what is.

In layman's terms, the principle essentially boils down to 'you can't cause a paradox'. In particular, nothing you do while time-travelling can prevent you from going back in time in the first place, nor can it change what you 'know' at the time you do travel back in time. That doesn't mean that the real explanation for what you 'know' is the same as what you thought was the case when you went back in time, though. SF has given some good examples of this in his answer.

Setting the story in the present and having time-travellers come back from the future allows you to make much better use of the options that are available - you can do things like have a character escape from a locked cell by having themselves appear outside the door with a key, having come back from the future to let themselves out.

For a very well-executed version of this type of time-travel, you can check out Continuum.

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  • $\begingroup$ Given that many actions one can think of committing would violate Novikov's self-consistency principle, it seems this principle prevents people from acting on their ideas in some (many!) cases. It's hard for me to imagine how one can have "partial" free will, so I'd conclude that this principle can only work if people have no free will. Perhaps some limited parallel reality allows for less restrictive solutions? $\endgroup$
    – max
    Mar 23, 2015 at 12:32
  • $\begingroup$ Just to provide an example of the self-consistency principle: In one book, the time traveler knows that his wife and child are going to be in a certain area. His agent gives him a call to say "A bomb went off there". He explicitly does not say what happened to the family or other casualties. The main character then goes back in time to ensure that the manner in which the bomb went off ensured that no one was hurt (with intervention, it becomes a controlled demolition after everyone is cleared away). $\endgroup$
    – Bobson
    Mar 23, 2015 at 17:45
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    $\begingroup$ @max It doesn't really invalidate free will, but you have to run your analysis from the perspective of the time you're travelling to, not the time you're travelling from. A time-traveller who arrives in the present from the future is free to try do anything he likes, but there are some things that he can't succeed in doing. Not because they're impossible, but because he already knows that they'll fail, because he's already seen the results. (I did say that it takes a while to wrap your head around.) $\endgroup$
    – Toby Y.
    Mar 25, 2015 at 11:01
  • $\begingroup$ @TobyY. well, some things just can't fail. Like suppose he knows he got a newspaper delivered to him daily in 2020, and it never had anything handwritten on the front page. How hard can it be to stop by the house when the paper is dropped off on any one morning in 2020, and write on the front page "I'M FROM THE FUTURE SUCKER", just to test if he has free will. There's no way this can fail if he does have free will. And there's millions of things of that nature. $\endgroup$
    – max
    Mar 28, 2015 at 12:01
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    $\begingroup$ @max "There's no way this can fail if he has free will". What if his pen doesn't work? Or he's run over by a bus on the way to the house? That kind of thing happens to people every day, and we don't assume that it means they have no free will. (It might mean that he suffers a series of increasingly freakish coincedences, but 'got incredibly unlucky' still isn't the same thing as 'no free will') $\endgroup$
    – Toby Y.
    Apr 3, 2015 at 13:45
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"Future cannot be changed, but the past can, as long as it leads to the same conclusion."

The moment of start of the travel is a fixed point. But the past wasn't at all how you remembered it.

What can change is the knowledge of the time travellers, and events can be put in motion, that will not bear fruit at the moment of start of the travel, but will extend past it in a way that would not have occurred had the time travel not happened.

The movie "Primer" had something along these lines:

One of the protagonists knocks out his past self and stashes "himself" in the attic, then proceeds to go through the day, planting ideas in others' heads, subtly affecting the world, following the "rails" into which the universe ! forces him and when the natural time flow catches up with the hour he had departed, at the end of the "trip" the world appears just the same as when he had departed, but the events that he knows had occurred are not at all as he had remembered them from the first time around.

The time traveller is bound by invisible "rails of fate", physically unable to stray far from them; but there are freedoms he can take, do things in different ways; often significantly different, which will lead to that one fixed point in time to be "as remembered" but the results vastly different.

Say, in the future the authorities discover a nuclear bomb planted in the middle of the city. Disarming it is very difficult, probably the effort would fail. They find traces of the bomber, a notable terrorist who had arrived at the city two days ago.

You travel two days back into the past. You can't just kill the terrorist and deliver the bomb to the authorities, changing the future. But you can knock the terrorist out, sabotage the bomb, then plant it yourself along with evidence that it was the terrorist, and when the countdown ends, the bomb won't explode.

(you may resist the "rails of fate" and can change the future, but it would be a very unpleasant experience, literally pushing against moving walls that try to guide you like a puppet.)

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  • $\begingroup$ I was under the impression that Primer used the "parallel universe" or alternate-timeline setup. But, that's assuming that I understood the movie, which is questionable. $\endgroup$
    – KSmarts
    Mar 23, 2015 at 21:18
  • $\begingroup$ @KSmarts: Yes, but if you pay a close attention, unlike in the "explaining schematics" where they generate a new timeline for every jump backwards, there's only two extra timelines created by changing important events from the past; all the rest is just the characters "reliving" the same three timelines without altering them. And the start of travel point is always changed in only very subtle ways - knowledge of the traveler, the past events. Sure the future diverges from what it would be without the alteration, but the points are pretty much fixed. Think "same location, different momentum." $\endgroup$
    – SF.
    Mar 24, 2015 at 7:43
  • $\begingroup$ You forgot about the bit in Primer where someone else got into their time machine and they shut it off with him "inside" it causing a paradox (the guy would fall unconscious every time one of the protagonists got within 200 feet of him). $\endgroup$
    – Draco18s no longer trusts SE
    Feb 8, 2016 at 17:56
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Throwing it a completely different direction, check out the videogame Achron.

It's a very unique kind of RTS game in that it allows you to alter the future and past of the game you are playing. This means if you are losing a fight, you can go back in time and issue different orders to prevent the fight from happening. You can also transport units back in time and even units fight alongside future copies of themselves and more crazy crap.

It manages to do this without causing crazyness by having a time-boundary after which the universe becomes permanent with no more mingling allowed. If a paradox occurs then both versions of the paradox exist in waves; ie one moment you killed your own grandfather and one moment you aren't born, until you cross the time-boundary at which point whichever version happened to exist at that exact moment becomes the permanent situation.

It also has some rules about how the further you go, the more expensive mingling with the past/future becomes and you have a limit of how many orders you can give, managed by your available chrono-energy.

Considering that this is a working game, you can probably scavenge a lot of their ideas for how to avoid problematic situations in time-travel.

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  • $\begingroup$ I knew about this game, but I never considered applying its mechanic to story-based time travel. +1 for thinking outside the box. $\endgroup$
    – Bobson
    Mar 23, 2015 at 17:48
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Time flows asymmetrically. Why should time travel be symmetrical?

One way to make a logically consistent form of time travel, is to simply say that traveling in time only goes in one direction. Either you can only go further into the future, or you can only go into the past.

If you can only progress into the future, it entirely eliminates any chance of paradox, questions about matter/energy conservation, and just about every other question related to time travel. It also can be used to explain why there have been no time travelers yet - since no one in the past has a time machine, no one can get here. It's simple, easy to understand, and logically consistent. However, it's not very useful.

If you can only return to the past, then it no longer matters what your future was. You can rewrite it freely, and it won't have any affect on you or the universe. Shoot your grandfather? No problem, because you're already standing there with a smoking gun in your hand. The "you" there popped into existence at a specific point, and the fact that you happen to have the genetics and memories of someone who isn't born yet isn't relevant. It means that (should you live long enough in this new timeline), you won't be able to see yourself born, because the man who would have been your grandfather is dead. Likewise, any other changes you cause simply change what will happen as time continues on its normal track.

In this scenario, the only logical inconsistencies come when someone or something materializes out of thin air, without an apparent causal history. But once they exist, the universe treats them the same as anything else. Big picture, this does violate the conservation of mass, but not on a scale that is likely to be relevant.

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It seems a lot of this depends on reconciling the time traveller's "history" (sequence of events) and along with everyone else's concept of past. A simple answer is to just let the time traveler's concept of what the future is be invalid or violated, making so people can go back in time and alter things. If they go back to the future, it will be different. This is, on the surface of it, most similar to the time travel seen in the Terminator franchise.

Basically, it amounts to the time traveller having their knowledge intact, but be able to travel back in time. The universe does not care that the matter of the time traveller was not there before or care about history as the time traveler sees it, and simply lets stuff happen as normal in past time. The future (as seen from the traveller in some past time) is no more real or special than anyone else's idea of the future, except that the traveler already knows the outcome of certain events unless the traveler alters them somehow.

If the time traveller goes to their original time, things will very likely not be the same. Indeed, going back in time may even prevent the time-travel technology existing at all. Since the universe does not need to preserve the time-traveler's causality, the past changes and the future becomes something else. The universe just doesn't care about any particular future, and there is no dimension hopping, just re-shaping of futures.

This has the unfortunate effect of time travelers essentially giving up on everything and everyone in their time unless specific things are/are not done. Since the traveler may or may not know the specific things, they may or may not cause the future to change. That's just how this time-travel setup works.

To sum up this time travel:

Causality does not hold for the time traveler. They are meatbags at a certain point in space-time. Their future, which they have already experienced, is nothing more than a false memory easily abolished by their current actions.

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  • $\begingroup$ +1. I had a very similar thought, although it was different enough that I posted my own answer. How does this differ from the "parallel universe" setup, though? $\endgroup$
    – Bobson
    Mar 23, 2015 at 20:15
  • $\begingroup$ @Bobson There are no parallel universes. The future, at whatever point you'e at, does not exist yet, and therefore can be changed. $\endgroup$
    – PipperChip
    Mar 23, 2015 at 23:07
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    $\begingroup$ The problem of this is, aside of the "soliphism problem" as JDługosz calls it that arises if there is more than one traveller (if you change the past completely, will later time traveller arivals from already overwritten timelines still take place?) that you are now destined to prevent your own departure (or existence). Because if you don't a slightly different Version of you will travel (perhaps at a slightly different moment) into the past, meeting up with a growing group of slightly different "yous". $\endgroup$
    – Hothie
    Feb 8, 2016 at 17:33
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Check out *The Man Who Folded Himself" by David Gerrold.

However, any model of history "changing" in-place as it were, rather than creating different time lines, runs into the soliphism problem. If there is more than one traveller, which one overwites events containing the other? You have a "real present" that follows the traveller (the "write head" of events) as well as a meta-time in which changes to the history takes place.

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    $\begingroup$ To explain a bit further: You travel back in time, killing your grandfather, therefore in the new timeline you will never be born. You are a relict with "false" memories that comes into existence from nothing at the point in time you travelled to. Now another time traveler travels back in time from a time after you appeared from nothing, to a time before you did, changing the timeline and the past of the time you appear. Will you still appear as a traveller from a future that even in the "last" timeline did not exisit any more? Will you wonder about the changed past?Or will you be overwritten? $\endgroup$
    – Hothie
    Feb 8, 2016 at 17:28
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One solution would be a time portal that works the following:

You have to activate the portal. Than you have to wait. Any time later you can step into the portal, leaving it at the time it was activated (similiar to the movie primer), but you deactivate it by doing this (the tunnel collapses), so you break all bridges behind you. This places hard limits on where (when) you can go. But it avoids that a slightly different "you" does the journey again later and stack up with a group of slightly different "Yous". What you could do is to activate the portal again as soon as you left it, but this means the next "you" can only come back a few seconds later. If there is more than one traveller I suggest that time always flows to the next departure, than time is totally rewinded back to the point of the arrival and continues. So if Alice activates a portal at 2050, Bob at 2060, than Bob steps into it 2070 (travelling back to 2060) then there is a new timeline with two 10 year different Bobs (and only one active portal owned by Alice) and all the changes that timetravelbob introduces. If in this timeline Alice steps into the portal 2080, she travels to 2050, creating a new timeline in which perhaps (and perhaps not, depending on what she changes) bob will activate his portal 2060 and step into it 2070 travelling back to (the already changed by alice) 2060 and do whatever he want. The resulting timeline will be consistent and has two Alice and two Bobs and no active portal (unless someone else has another or one of the both activated theirs after their Arrival).

I'm not sure if "primer" does it exactly that way. It's to long ago that I watched it. But this is what I refined to circumvent all possible paradoxes and other Problems.

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