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tl;dr: What adjustments to a "rewind" power allow multiple equally powerful characters?

Background

There are many different models of time travel. Personally I find it easiest to tell stories using the "fixed history" model, where the past is immutable because it has already happened: time travel can cause or provide information about past events, but it cannot change any aspect of history (see 12 Monkeys or the time-turner from HP).

However, it has consistently come up in feedback that people have two problems with this model.

  1. They do not understand it. ("You can't change anything." "Yeah, but what what if you did this...")
  2. They feel that it is too restrictive. ("What's the point if you can't change anything?")

So I've decided to write my next story with a model that allows the past to be changed.

One-person Model

The model I have in mind is based on a "rewind" mechanic. This has been used successfully before: see, for example, Next1 or Life is Strange.

In this model, the main character has the ability to rewind time, usually by a limited amount. This power manifests itself to the outside world as precognition.

My internal model of how this works is something like a git repo.

  • The state of the universe is continuously stored as time progresses forward.
  • At some point, the main character chooses to jump backward to an earlier moment. This can be handled in two ways:
    • The character somehow chooses a specific moment, and jumps immediately to that point.
    • Upon triggering their power, the character experiences time in reverse (at normal or accelerated speed). They cannot change anything at this point, only choose how far they go back.
  • The entire universe undergoes a rollback and is returned to its previous state, with one exception:
    • The main character's current mental state is copied into their past body, so that they retain the knowledge they gained in their (current) future.
    • Unlike Life is Strange, you cannot hold onto anything that you pick up, and you resume in the location you were at the time you return to.
  • Time progresses forward normally again.

There are a few things that I like about this model:

  • There is a cost to the rewind. If you rewind one minute sixty times, you'll have wasted an hour of your time: which could be boring at best or mentally exhausting at worst. It probably has a poor effect on your sleep cycle/circadian rhythm as well.
  • Making money is nontrivial. If your rewind limit is two minutes (as in Next) you cannot cheat the lottery, which usually must be purchased fifteen minutes or more before the numbers or drawn (I assume a similar rules exist for horse racing and the like). You can't cheat games where the randomness comes after your bet, like craps, roulette, and slot machines (although you can increase your odds by playing multiple times, with the aforementioned time cost). You can cheat games where the randomness happens before your bet, like blackjack and poker; but they are not high-payout games, and a string of wins looks suspicious.
  • An outside observer sees no discontinuities. You don't blink out or teleport. The only indication that something has happened is that you are disoriented briefly while your brain readjusts to the sudden change in sensory input. (Although maybe for jumps which "push the limit" concussion-like effects, coma, seizure, etc. may be possible.)
  • You can do lots of impressive things in the short-term (like dodge bullets), but can't preexamine the long-term effects of your actions; so most storytelling devices still apply.

However, I don't like the part in the 'rules' that states "main character." The idea that this mechanism is specific to one particular individual is unattractive to me.

Extending the Model

Braid introduces a new feature into the mix: some people in the universe are not affected by your power. In terms of the model above, their mental state is also coped into the past.2 I don't want to use this exact behavior for two reasons:

  1. The rewind ability is still asymmetric, since the rewind still falls under sole control of the main character.
  2. Anybody with this 'persistence' property would experience a rewind whenever you initiated one, even if they were on the other side of the planet and never met the 'rewinder.' This would be highly annoying at least!

This is the crux of my question: how can I modify the rules to allow a second character with the rewind power?

Here are my requirements:

  1. All empowered characters must obey the same set of rules regarding the rewind power.
  2. Characters must be able to tell when another is using the rewind power against them and counter somehow. ("Going around again? Nice try, but you can't use that trick on me!")
  3. However, characters must not be forced to relive a moment more than once if they choose not to exercise their power. (Bonus points: two characters can remain unaware of each other's nature or even existence until they interact.)
  4. Unpowered outside observers must not observe any glitches. If you use a branching-timeline model this includes observers in all timelines.
  5. If you use a branching-timeline model, at the end of any series of rewinds by both parties there must be only one timeline remaining.
    • For example: interpreting the original model as a branching-timeline one, another timeline branches off the current on every rewind. All the "stub" timelines end when the main character chooses to jump. Time only continues forward in the new branch.
  6. A hard time limit on rewinds is not required.

1 From my understanding, Cage is supposed to be seeing the future, not experiencing and rewinding it; but the story plays the same either way and matches what I'm thinking.

2 The actual game mechanic is that they continue to propagate forward in time as you rewind. This works great for the game, but doesn't make physical sense in the real world.

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    $\begingroup$ Check out the videogame Achron; it's a multiplayer RTS game where both players can rewind the clock and even fight at different times during the game (ie; I am controlling my army at 20:00 into the game while at the same time, my opponent is redoing a battle at 17:30) and it has a number of constraints to make this work AND keep it competetive. $\endgroup$ – Erik Apr 20 '15 at 8:06
  • $\begingroup$ Also I am not sure how your listed rules allow you to dodge bullets, considering your power stops working if you die, and you need to know that you'll be hit before you can choose to rewind power. You might, at best, be able to dodge grazing wounds, but anything that sends you into shock or instantly kills you cannot be dodged it seems. $\endgroup$ – Erik Apr 20 '15 at 8:07
  • $\begingroup$ Did you watch the anime film, The Girl Who Leapt Through Time? $\endgroup$ – Fhnuzoag Apr 20 '15 at 12:36
  • $\begingroup$ @Erik You're right that dodging bullets is a little bit of a stretch, but I imagine that a) rewinding can be trained into an unconscious reflex by skilled practitioners, and b) most bullet wounds are not instantly fatal. Maybe the Big Bad (with 20+ years of time-travel combat experience) could dodge shots fired by the Hero (who has never held a gun before), but would not be able to avoid someone with self-defense training who is firing to kill. $\endgroup$ – 2012rcampion Apr 20 '15 at 12:56
  • $\begingroup$ @Fhnuzoag I did some time ago, I'll have to refresh my memory (it should still be in my library somewhere...). $\endgroup$ – 2012rcampion Apr 20 '15 at 12:56
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  1. Characters must be able to tell when another is using the rewind power against them and counter somehow. ("Going around again? Nice try, but you can't use that trick on me!")

  2. However, characters must not be forced to relive a moment more than once if they choose not to exercise their power. (Bonus points: two characters can remain unaware of each other's nature or even existence until they interact.)

I think the best way to accomplish goals two and three together is to have passive rewinders only aware of the future on a subconscious level:

Those with the ability who are brought along during someone elses rewind can only notice on a conscious level when they see a divergence with their own eyes.

Simply put, if you have the power and a rewinder brings you back in time, you can't tell. That is, until you witness a divergence in the timeline, at this point the rewinder will see a person or object split, a ghostly form carrying on with their original behaviour, while the corporeal version doing their new behaviour. Upon witnessing this, the rewinder will then know that someone else is using their ability to alter the past in a way which will effect them.

This way, it's very difficult for those with powers to fight each other, as if you were about to lose and rewind, your opponent will see the ghostly version of himself beat you and do the same thing.

Advanced rewinders could also use this as an advantage against newer rewinders by continually rewinding the last minute and filling the room with ghost fighters as a distraction.

However, this doesn't completely make rewinders immune to each other, as a seriously clever puppet master type could theoretically defeat another rewinder if they can come up with a convoluted plot which changes the past of the rewinder without directly effecting anyone that the rewinder sees (e.g. the puppetmaster messes with the phone lines at the local exchange so when the rewinder gets called in the morning, the lines switch with a sex chat line, which distracts the rewinder enough to make him late enough that he doesn't have time to foil the puppetmaster's plans, nor will he have a chance to run into his ghost future self who did foil the plot).

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    $\begingroup$ Although I think my story's implementation will be a little different, the key idea--travellers notice a rewind iff their perceptions of the two timelines differ-- does everything I want. $\endgroup$ – 2012rcampion Apr 20 '15 at 13:29
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As far as I can see, your model is flexible enough to allow for this already. Since you're employing the use of a fairly large amount of handwavium, you can simply employ some more and just say that more than one person with this power is possible. There are a few major ways I can think of about how it would affect other rewinders:

  • Only the rewinder performing the rewind is copied. So, only the person who rewound time remembers that they did it; nobody else, including other rewinders, notices any change.
    The problem with this is that it doesn't fit fit one of the rules you mentioned: that other rewinders must be able to detect a rewind.
  • All rewinders are copied. Whenever a rewind happens, all rewinders have their mental state copied back with them. This enables detection of a rewind but not prevention.
  • All rewinders must agree. Whenever someone tries to rewind time, every other rewinder must allow it before it happens. Perhaps this is mentally controlled: their brain is set up to automatically allow rewinds, but they are capable of stoping a rewind if they really want to.
    This method can be combined with number 2 to enable detection and prevention.

I think number three is the most viable option here.

As a bonus point, rewinding could leave signals. Whenever someone rewinds, their aura or rewindium cloud or whatever changes. A bit like someone releases a lot of rewindium dust into the air in the location of the rewind. The dust decays after a while, but the rewinder would be detectable by other rewinders who can sense the dust.

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    $\begingroup$ This covers everything that I want, except that (2) and (3) cause everyone who is empowered to experience every rewind. This means that the Big Bad will know as soon as my character discovers his/her power. (Although it would provide a good reason for the Big Bad to hunt down the main character... "Every time you rewind? I rewind too! Do you have any idea what that's like?") $\endgroup$ – 2012rcampion Apr 18 '15 at 18:45
  • $\begingroup$ @2012rcampion I imagine that your protagonist would get equally annoyed when someone else rewinds. TimB's answer contains a good idea of how to do it. $\endgroup$ – ArtOfCode Apr 18 '15 at 19:27
  • $\begingroup$ zeeman - if you want to add an idea, either comment it or add a new answer. Edits that conflict with the post authors intent are likely to be rejected. $\endgroup$ – ArtOfCode Apr 20 '15 at 7:48
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I like ArtOfCode's answer but wanted to add one more touch:

When a rewinder uses their power they start going back in time. As they do so a "bubble" spreads out from them at the speed of sound (this will take several hours to cross the entire planet). I picked the speed of sound just because that's how long it will take most of the changes you make to ripple out. Speed of light is another obvious candidate but would be too fast. Just picking an arbitrary speed would also work but it feels better if there is some significance to the speed.

As the bubble reaches any rewinder they get pulled into the rewind, they can either actively help it, allow it or resist it at that point.

This means that small and localised rewinds, for example going back a few seconds, are normally hard to stop. As you go back further and further in time though you need more and more rewinders to either allow it or help push it through since the bubble grows larger and larger including more and more of them.

Two rewinders in hand to hand combat would not be able to use the power much at all, two in a gunfight may be able to rewind a fraction of a second here or there.

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  • $\begingroup$ If you attempt to rewind, and the bubble reaches me (also being a rewinder), do I force the bubble to recede once I have declared (mentally, I suppose) that I do not agree to the rewind? Is it then a battle of will, ie. that the stronger mind (or minds, in the event that many rewinders join either side) will overcome the weaker and force the bubble in their favour? $\endgroup$ – blaizor Apr 18 '15 at 10:58
  • $\begingroup$ @blaizor That's down to what is needed for the story. Some rewinders may be stronger or weaker than others. Anyone using the idea also need to decide whether entering the bubble allows you to prevent any rewind that's already happened or just stop it rewinding any further. $\endgroup$ – Tim B Apr 18 '15 at 11:00
  • $\begingroup$ If you rewind to a point before the bubble reaches you will the bubble also be affected and recede? $\endgroup$ – AndyD273 Apr 18 '15 at 13:55
  • $\begingroup$ Unfortunately this still has the same problem as ArtOfCode's answer, which is that all empowered minds are affected by the rewind bubble. It also introduces an arbitrary speed (speed of sound where? Dry air at sea level? Local speed of sound?) and a lot of new problems, like what happens when two separate bubbles, rewinding different amounts of time and to different points in the past, intersect? If I can't fully understand the system I can't expect my readers to get it. $\endgroup$ – 2012rcampion Apr 18 '15 at 18:51
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I think that your analogy of a git repository is quite handy for settling what would happen. Version control systems are designed for multiple users. The version of events that sticks depends on a last mover advantage.

Say we have two combatants, Tom and Ann. Let's consider their first meeting. Neither knows that the other is unstuck in time. Ann makes the first move, she attacks Tom as she would anyone. Tom zips back in time to anticipate Ann's attack. Ann notices Tom come unfocused just before she had planned to attack.She then knows that Tom possesses the ability to shift back, and can spring back herself far enough to be out of sight of Tom. From there she can adjust her strategy to take Tom out before he even knew there was about to be a fight.

Now, suppose that both combatants are already aware of each other. All other things equal, the fight turns into an exhausting game of nim. Whether the jumps are hard limited or limited by stamina, the outcome is the same. Both Ann and Tom would be running through cycles of attack-observe-rewind as quickly as possible, in an attempt to place their opponent in a position of no escape. Each would also be attempting to wind back farther than the other, to wipe out their gained knowledge. Eventually, either one combatant passes out from too many loops and lack of sleep, or they manage to land a knockout blow before the other can react. In the case of a faint, the winner would be fresh as a daisy, having basically done nothing to defeat their foe, and in the case of a perfect strike, the winner would be terribly mentally drained.

This ties things together nicely, without the need for complicated energy waves or other distracting fluff, and it operates on a single, continuous timeline. The only rule needed is that no two travelers can shift back exactly simultaneously.

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  • $\begingroup$ This doesn't quite answer how the mechanic actually works... but it is an excellent answer to the next question I was going to post ('hand-to-hand combat with time travel'). I'll let you know when that gets posted so you can copy this answer there! $\endgroup$ – 2012rcampion Apr 20 '15 at 12:59
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    $\begingroup$ Fair enough. The purpose of the answer was basically to say that the system of time travel given didn't need any augmentation to be interesting and balanced. $\endgroup$ – Emmett R. Apr 20 '15 at 14:09
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How about that:

When someone rewinds time, minds of all other time-benders are attempted to be copied into their past bodies too, but there is a choice of which version of mind to use.
By default human subconscious declines the copy attempt and keeps using the old mind. On any time rewind all time-benders just get some weird feeling - like deja vu, maybe. Some of them, probably, know that it means time rewind, and some don't even know that.
But if a future mind shows strong negative emotions, like it is in a danger - then the subconscious accepts the changes, so the time-bender is aware of the rewinded timeline.

It makes more sense if time-bending is evolutionary, people with this trait are very likely to avoid life-threatening situations, but they do not waste their mental resources on numerous rewinds of all others.

What it gives:
- Characters are able to tell when another is using the rewind power, but usually can just ignore it.
- If before the rewind they knew they are confronted - enemy's rewind lets them know exactly what would happen.
- They are likely to be unaware of each other's power; they can even be unaware of their own power. But if confronted, they not only get to know it, but they know it in advance.
- If you would need some stronger time-benders, you can give them more control of a process. Like, some meditation or medication, which allows to always accept a new mind (exhausting, but probably a good choice for a desperate villain), or even to make a conscious choice between the old and the new minds (but probably it should take some time).

Drawbacks:
- Obviously, if some time-bender is scared, when, accidentally, someone on the other side of earth rewinds the time, he would be forced to relieve that rewind. On the other hand, he have just effortlessly gained some knowledge about the future danger, so probably he won't mind it too much?
Also, it probably is the only way to find out that you're a time-bender.
- You could mess with any untrained time-bender as much as you'd like as long as he thinks he's safe.

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One way to do this is by saying when a rewinder is using their power, they create a bubble of control that is only detectable by other rewinders, and is responsible for the persistence effect. Everything inside and outside the bubble is turned back, but if another rewinder is close enough to be inside the bubble they are aware of it happening and the persistence effect includes them.

This way, if I'm a rewinder and another on the other side off the planet rewinds time I'd never know as I'd be outside of the bubble and would be rewound along with everyone else, but if someone near by was trying to mess with me I'd know what was happening and be able to counter it, maybe stop the rewind from happening.

A different method is to say that as a rewinder if time is rewound by anyone anywhere I'd keep my memories of the other timeline. That would let me know if there were others using their power, and if my timeline was under attack. This can be paired with the control bubble to good effect.

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I've thought long and hard about this question, and its requirements. I want to lead off with a couple of observations I've made.

As far as the branching-timelines needing to end; this is really a non-requirement. The story will be told from the vantage point of a given person or group of persons. Prior to any interaction between two rewinders, there will have been a near infinite number of divergent timelines created by rewinders around the world, doing things as simple as un-dropping a contact lens. Since the entirety of the universe is rewound, leaving nothing changed otherwise, there is no reason for any special treatment - we need only be able to observe the interactions involving the people in the story, all of whom exist in every timeline where they didn't die yet. Sure, this leaves a complicated multiverse, but no one exists that can actually see across the boundaries - so it doesn't matter. What's more, if you change your mind later, you can write in someone who can pass through the boundaries. Less is more, here.

As far as outside observers not being able to notice, you've essentially already written this mechanism as well as you need to. If you actually make the rewinder experience the rewind in real time or some (maybe selectable) multiple of real time, preventing them from manipulating anything during the rewind, there's nothing to notice. An adept rewinder could get skilled at watching his environment during the rewind and more or less seamlessly reenter the "new" situation.

Now we need to tackle the requirements that really don't mesh well; it is desirable that no one can be forced to relive a situation, but we'd like for the rewinders to know or reasonably suspect when they've been rewound, and be able to do something about it. For them not to relive a situation, they must not perceive the situation recurring, but to detect a rewind, there has to be something that is perceptible to them. But everything has to rewind, and everyone has to follow the same rules.

As such, I propose the following;

  1. Rewinders experience a profound sense of deja vu when in the vicinity of another rewinder.
  2. This feeling waxes and wanes based on proximity.
  3. Physical contact with another rewinder provides a sense of sureness that this other person is a rewinder.
  4. Any two rewinders activated in the same timeline cancel each other out.
  5. As a logical consequence of (4), all rewinders are familiar with lurching to a halt mid-rewind and generally consider it a loss of focus on their part.
  6. The cancellation halt is somewhat disorienting, and at your option somewhat painful.

Using these mechanisms in plot, it is now possible for a scooby gang of friendlies to find each other out, and gives the resident big bad a method of hunting down the people who keep interrupting his attempts to return to childhood with his newfound power and knowledge - perhaps something he has done several times without pain until Our Hero discovered his abilities, which disrupt his.

As far as handling the timeline during a cancellation event, lets work on an example. Allen and Barry are rewinders on opposite sides of the world. Lets say our story primarily involves the life of Allen, and how Barry deals with Allen. Allen starts a rewind at 1:47 pm that halts unexpectedly at 1:43 pm, the exact moment that Barry had started to go back a year or so. Allowing branch timelines means some things have happened here. In one, Barry never lurched - he went on back happy as a clam, but we don't care about that one, or the Allen in that particular timeline. We watch the one where Allen goes about trying again so he can try a different pickup line on that girl, and hopefully undo that part where he lurched and fell on his face.

Note that the only thing that matters is when Allen rewinds across the start of another rewind - in his timeline, when he starts, there may or may not be any other rewinds in progress. If there are, he'll misfire, to his surprise. When he does, he'll try again - and maybe, misfire again. It will be like two people calling each other and getting a busy signal - eventually, Allen's rewind will start first and he'll be able to continue back in his frame of reference, until he crosses the start of someone else rewinding, which could be days in his timeline.

Now let's put Allen and Barry in the same room, and make one try to kill the other. Barry shoots Allen in the chest - Allen rewinds to make his way back out the door he came in and manages to rewind himself all the way to the point where (he thinks) Barry became an adversary. Things move on for Allen until Barry becomes aware again and makes another attempt, only this time Barry intentionally did six two-second rewinds right before the attempt on Allen's life. Timelines are thrown left and right, but Allen is the only one we care about, and Barry would continue in both timelines created, and each time there would be a rewind event to cancel. Following Allen-prime's frame of reference, he gets shot and attempts to rewind, lurching both of them still in the same room and cancelling that rewind. Allen runs for his life, and in a panic tries to rewind - he gets to relive the run for his life, the moment of the cancellation, and then he gets to the next to last rewind Barry did before he rounded the corner. They both lurch, but now, Allen-prime has been shot and has lurched twice, but Barry has only lurched once, still fully intends to kill Allen, and has a full clip.

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  • $\begingroup$ I don't know what you mean by "in progress". His experience of 8:47 will be one timeline or another and that moment may have been rewound back over any number of times. A bscktrack is never "in progress" but you can have a region where the backtrack exists. There are braching points, but the time beyond the "old" branch before the person decided to initiate is only special in that the person on the new branch has memory of it. $\endgroup$ – JDługosz Apr 20 '15 at 8:26
  • $\begingroup$ @JD, in my theory of events a rewind causes a split, one side of which necessarily contains Allen-prime. Any rewind not containing the frame of reference we care about doesn't have a rewind we care about. the theory is as complete as I can get it, and is still very easy to handwave by simply ignoring the logical consequences. $\endgroup$ – Sean Boddy Apr 20 '15 at 8:46
  • $\begingroup$ The reason I have the "no persistent branches" requirement is twofold: 1) to make the story easy to tell, since we can traverse the multiverse in bounded time; 2) I want to avoid the ontological problem of having one 'true' timeline with a 'good' outcome, and uncountably many with possibly 'bad' outcomes. Also, your synchronous write-lock method only solves the two time-travellers problem by only allowing one to travel at a time. I want asynchronous access to the timestream. $\endgroup$ – 2012rcampion Apr 20 '15 at 13:17
  • $\begingroup$ Also, I'm a little confused about your first worked example. How can Allen ever "start first" if Barry has already made his jump? $\endgroup$ – 2012rcampion Apr 20 '15 at 13:21
  • $\begingroup$ @2012rcampion, in this, after the cancellation, the rewind isn't there any more. Happy rewinding Barry isn't in this timeline anymore - after the split, Allen-primes Barry will get shut down by Allen-prime, after which they both try again. I developed the idea after deciding that having no one notice was really important. Expanding rewind waves creates a differential that would be noticed, I think. Last mover advantage doesn't work - it didn't happen from first mover perspective in his timeline. Anyway you go you need to pick a reference, or else have causality issues in your writing. $\endgroup$ – Sean Boddy Apr 20 '15 at 14:26

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