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So I'm trying to see if this scenario would work for a piece I'm writing. I know timetravel can be quite specific and most people dont like it because its executed poorly.

To specify say the characters memories and knowledge are transported back into the mind/body of their past self some years back through the use of magic. Almost as if the future self is posseing his past self. They are allowed 24 hours to change whatever they want and then once the time is up the "future" timeline they came from would basically die as well as their future self. Thus leaving their past self to live out a new timeline that has been changed by what the future self did.

Does this makes sense? I'm basically trying to avoid branching timelines as well as the whole "if you do this then you're future self has no reason to go back" aspect that there is to time travel.

I'm also wanting to avoid having the "past" self have any knowledge of what was once an alternate future by no letting the future self leave any hints or clues behind as its against the rules.

TLDR: Character's mind "possesses" his past self through time travel. He changes an event in the 24 hours that he is allowed. Once the 24 hours is over, the possession ends and the future timeline he came from dies to make way for a new timeline in which his past self must live through. So it would basically as if Hitler is sitting in his bunker and makes a deal with the devil to send him back in time to the beginning of the war. The devil agrees but say only his memories will go back and he has 24 hours to make changes. He goes back and in the 24 hours proposes certain plans. The 24 hours is up and the future we know dies. The story then would follow the beginning of war Hitler who is working with plans that his fiture self started to help him win the war. Would this work?

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    $\begingroup$ Hello @TheInquisitor577. For future reference, questions like this are a challenge because, since time travel isn't real, there's nothing to say you're right or wrong. Please consider reading the wiki for the internal-consistency tag and editing your question to meet its expectations. In a nutshell, you set the rules for your world and create a specific circumstance that uses those rules and we judge whether or not you're consistently using the rules you've created (or if they're somehow deficient). Out of curiosity, though (*Continued*) $\endgroup$
    – JBH
    Commented Jul 30, 2023 at 18:20
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    $\begingroup$ ...If person A travels back to 1989, creating a new timeline (deleting any and all other timelines) then person B in that new timeline travels back to 1988 and does something that completely eliminates A's intent such that A's expected new timeline is entirely erased... is that a problem in your world's rules? BTW, your last paragraph demonstrates that the 24 hour rule is perhaps meaningless. Will A always go back and stay there? Or can A fail within the 24 hours and get yanked back to the original "present?" The 24 hour rule doesn't seem to have value. $\endgroup$
    – JBH
    Commented Jul 30, 2023 at 18:22
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    $\begingroup$ But there isn't a future self after the 24 hours, right? Maybe I don't understand. After the 24 hours is up, does A' personality revert to the original "past" personality? Or does the future A personality remain? If the former, then the rule makes sense. If the later, it doesn't. $\endgroup$
    – JBH
    Commented Jul 30, 2023 at 19:29
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    $\begingroup$ @JBH It would be the former! Sorry I realize now that my initial ask is still vague. I think this example would help the most though it is cliche and gross. So it would basically as if Hitler is sitting in his bunker and makes a deal with the devil to send him back in time to the beginning of the war. The devil agrees but say only his memories will go back and he has 24 hours to make changes. He goes back and in the 24 hours proposes certain plans. The 24 hours is up and the future we know dies. The story then would follow the Hitler who is working with plans that help him win the war. $\endgroup$ Commented Jul 30, 2023 at 19:32
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    $\begingroup$ @Tortliena Ah sorry! I've never posted here so I'm sorry if stuff is all wonky. I'll start doing that! $\endgroup$ Commented Jul 31, 2023 at 7:30

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In Time Travel, Consistency is More Important than Exact Rules

Because we do not have any time travel IRL, any possible explanation of how it works is equally plausible, especially magical time travel. The issue where most time-traveling fictional universes goes wrong is not in how they describe the causal relationship between past and future, but in their inability to be consistent with it. As long as your explanation can answer any given paradox using the same rules every time (which I believe your setup can), then it's as good of an explanation as any.

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    $\begingroup$ You know... amen, amen and amen. $\endgroup$
    – JBH
    Commented Jul 31, 2023 at 22:51
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Seems fine to me.

The only problem is that Hitler would not know why he made all those changes since it wasn't him that made them. He just had a 24 hour blackout where he was apparently quite busy, but remembers nothing.

Or if he remembered being possessed he might go crazier than he already was.

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If you go with the many worlds theory of time travel, the act of going back in time would result in the time traveller ending up in a branched-off universe in which history plays out differently to the way history turned out in their universe of origin.

This time travel could almost as easily be 'in-person' or 'mental piggybacking', with whatever rules you wanted.

In effect, in the origin timeline, the time-traveller may have ceased to exist as of the moment that they went back in time if they went physically, or the copy of their mind would have ceased to exist if sent back to piggyback on their past self. However, as you are following the post-time-travel branch, what happens in the pre-time-travel branch becomes irrelevant.

Because the post-time-travel universe becomes seperate to the pre-time-travel universe, paradoxes don't happen.

However, don't be too concerned about a situation in a single-world situation. By definition, time-travel to the past may result in looped causality, but you're only going to see and remember a self-consistent end-result.

Let's do a thought experiment:

Imagine that we have a time machine that is a wormhole that can take a small object back in time a few seconds and a few metres distant. The time machine's entry point is set up so that a billiard ball can roll into its mouth.

If the time machine is set up so that a researcher can roll the ball into the machine and see the ball emerge a few seconds before it enters, a ball will only exit the earlier-time mouth of the machine if the earlier ball will roll into the later-time mouth of the machine. Let's say we set up the experiment to catch the ball and roll it toward the later-time mouth repeatedly.

If we let the setup roll the ball into the earlier-time entry mouth of the time machine repeatedly, then try the following tests, we can predict the results:

  1. If we do not disturb or attempt to disturb the ball from going into the entry mouth, the ball emerges from the exit mouth at the earlier time.

  2. If we set up the experiment to try to disturb the ball on its way from the launcher to the entry mouth after a ball has exited the exit mouth, no ball will ever exit the exit mouth unless it fails to prevent the earlier ball from reaching the entrance mouth.

Result 2 happens because the time machine causes looped causality. The ball rolls into the entrance and emerges from the exit, preventing the ball from entering the entrance, preventing the ball from exiting from the exit, which allows the ball to enter the entrance... and so on until something happens to break the cycle. Something will happen to break the cycle, because each cycle will play out in its own independent piece of space-time, with quantum uncertainties ocurring differently each loop.

Depending upon how robust the experimental setup is, the event that causes the experiment to be consistent may vary from trivial to extreme... but from the researcher's point of view, something will happen if they try to cause a paradox.

  • If it is as simple as the experimenter grabbing the earlier ball once the later ball emerges, the experimenter will fail to prevent the earlier ball from reaching the time machine's entry mouth, or no ball will exit the exit mouth and no ball will subsequently enter the entry mouth. Why? Because of the time loop, the experimenter will only remember the result where the result is consistent.

  • The more the experimenter 'hardens' the experiment to try to cause a paradox, by building a setup that shouldn't allow for malfunctions, the more loops the paradox will cause until something breaks the loop. The more robust the experiment, the more extreme and unlikely the event which breaks it out of the loop must be. Maybe the ball spontaneously breaks. Maybe a passing car smashes through the wall and breaks the experimental apparatus. Maybe the building spontaneously collapses. Maybe some piece of the experimental apparatus fails... but whatever it is, it will happen, because the experimenter (and the universe) will only 'remember' the last iteration of the loop.

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  • $\begingroup$ This seems to just be a different fictional version of time travel, not a response to the one outlined in the question. I don't entirely buy into your reasoning, but am not sure discussing it in these comments fits the site format. $\endgroup$
    – IMSoP
    Commented Jul 31, 2023 at 6:41

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