Some ideas are:

  • You branch off into another timeline and your actions make no difference to the previous timeline
  • You’ve already taken said actions but just didn’t know about it so nothing changes
  • Actions taken can have an effect (so you could suddenly erase yourself if you killed your parents)
  • Only “nexus” or fixed events really matter, the timeline will sort itself out for minor changes
  • something else entirely
  • snooggums@lemmy.worldEnglish
    391·
    9 days ago

    You’ve already taken said actions but just didn’t know about it so nothing changes

    12 Monkeys did this one perfectly.

    You can’t change things because if you undid the thing, then there wouldn’t be a reason to undo the thing. If you go back in time, you are just going to do what you already did because that is in the past.

    • Wwwbdd@lemmy.world
      10·
      9 days ago

      I’d totally forgotten about 12 monkeys. I had that VHS of this when I was 11 or 12 years old, I probably watched it 30 times and I never fully understood it. 25 years later I think it’s time for me to rewatch this

    • hisao@ani.socialEnglish
      64·
      9 days ago

      If you go back in time, you are just going to do what you already did because that is in the past.

      Only if the Universe is deterministic. If not, random rolls having different outcomes may completely change the course of events and decisions made by people.

      Edit: I see I’m being downvoted, so I’ll explain further, if the Universe is deterministic means everything will be the same any time you relive the same time segment, if not, it means even the weather can be different due to aggregation of butterfly effect of different random outcomes in the Universe, and weather being different is already big enough change to be able to influence decisions and course of events. And I’m not meaning weather in the exact same spot you time-traveled to. Even if you restored the exact same state of Universe at some snapshot, if the Universe isn’t deterministic, various random events happening after that point in time can have different outcomes which will aggregate and lead to even more different outcomes in future. Weather might be different the next day and because of that you decided to hide from rain in cafe and met someone there which can completely change your life.

      • AbouBenAdhem@lemmy.worldEnglish
        3·
        9 days ago

        You’re assuming that time travel is equivalent to “rewinding” the intervening time span as if it had never occurred—in which case, yes, nondeterministic events are likely to happen differently.

        But that’s not the case if time travel is a closed time-like loop (which is implicit in the “immutable-past” of OP’s second scenario). In that case everything happens only once, so it makes no difference whether or not the universe is strictly deterministic.

      • bamboo@lemmy.blahaj.zoneEnglish
        3·
        9 days ago

        Maybe this is the same as what you’re saying but my issue with the idea that “You’ve already taken said actions but just didn’t know about it so nothing changes” is that it means time travelers don’t have any free will once they go back in time. If that’s the case, then it bring up existential concerns and that might extend to non backwards time travelers (i.e. us)

        • AbouBenAdhem@lemmy.worldEnglish
          1·
          9 days ago

          Let’s suppose a time travel event occurs in which an agent with free will travels to their own causal past, and let’s suppose this creates a parallel timeline which can differ from the first (leading to a new version of the agent which creates a third timeline, and so on).

          We can consider this time-travel event as a function in which one timeline maps to a successor timeline — or in general, the event is an iterative map from the space of possible timelines to itself. If this map meets a few general criteria, we can apply the fixed point theorem and conclude that, after enough iterations, the process will converge to some fixed point that maps to itself (that is, the agent causes the past of their own timeline, even though they have free will). This timeline maps to itself—but it is also mapped to by an infinite succession of timelines in which the agent is free to alter their successor timeline, converging on one in which their choices cause no further alteration.

          At that point, we can dispense with the assumption that time travel creates parallel timelines, and assume instead that the fixed-point, self-causing timeline is the only real one.

      • snooggums@lemmy.worldEnglish
        1·
        9 days ago

        Nothing is truly random, including the weather. It is extremely complex and difficult to predict, but once it happens that is what happened. As long as dice fall with the exact same speed and hit the same surface in the same spot at the same angle it will always end up with the same result. The randomness of dice comes from how the very small differences influence the outcome.

        Going back in time with the knowledge of what happened the first time means that either you will choose the same thing because something led to that original choice or something will keep you from interfering. Free will exists because we don’t literally know the exact outcome of our actions or the things outside of our control in advance.