4w ago

If time travel were to exist, which theory would you subscribe to?

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

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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.
- 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
  
  Rewatched it recently, it held up really well!
- Logically speaking it's the only way time travel can be done, and for bonus points physics wouldn't have a problem with it.
  Any Back to the Future shenanigans is just creating alternate realities, which may or may not instantly destroy the original.
- 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.
  
  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)
  
  I think from a physics standpoint, strict free will is already an illusion and the only useful definitions of free will basically boil down to "choices can be made", perhaps as far as "Slight differences in initial conditions can lead to different choices" (but somehow excluding random processes). That kind of definition doesn't even require consciousness, and is compatible with a deterministic universe like ours seems mostly to be. Would also be compatible with the time traveler unwittingly doing everything as must happen, but still via individual choices.
  
  Choice is one of the slight differences that can lead to different outcomes. A rock falling down a hill will always fall downhill because of gravity. An animal can choose to slow itself or even work against gravity to move uphill. Instead of gravity, there are a ton of prior experiences that will influence that choice, but choice is still a distinct part of the process.
  
  Exactly. That's why I think the only useful definitions of free will are those that are weak enough to distinguish between the animal and the rock in a situation like that.
  
  Are you saying that even without time travel, free will is an illusion? Surely there has to be a time travel scenario, like going back 1 second in time and shaking hands, where all information is known to both travelers, and the future self would know what was done previously, and can choose to take a different action.
  
  I can think of a couple ways around that, the easiest is that I actually think time travel is impossible. (Like this for example)
  If it's not impossible, then single-timeline travel probably is, and all (backwards) travel would start a new timeline.
  Short of that, maybe something ridiculous would have happened when the traveler "first" went back, like one of them tripping or whatever, and the handshake they agreed to try didn't go as planned, and then "still" didn't the traveler's second time. Basically this.
  
  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.
  
  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.
  
  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.
  
  Nothing is truly random
  Modern physics says otherwise. Einstein also thought exactly like that with his "hidden variables" theory which was later disproven.
  Edit: I was interested to read some relevant discussions and here's some links with quotes
  https://philosophy.stackexchange.com/questions/29364/does-true-randomness-actually-exist
  https://physics.stackexchange.com/questions/651011/is-there-quantum-randomness-that-significantly-affects-our-macro-world
  It seems very likely that every deviation from perfect homogeneity and isotropy in the universe is due to amplified quantum fluctuations. (That's true in inflationary cosmology, and I'd expect it to be true in practically any alternative to it.) For example, the shape of Earth's land masses was probably determined by quantum fluctuations, and has had an enormous influence on human history.
  Quantum fluctuations is basically true randomness on quantum level.
  The randomness is largely canceled out, except in the case of unstable systems which magnify the effects of any perturbations, no matter how small.
  https://en.wikipedia.org/wiki/Macroscopic_quantum_phenomena
- "Default behaviors is undo, no redo. You control undo the undo. Emacs."
  2023 Emacs Enthusiast
  
  I recommend undotree, which is also a non-destructive undo, but for some cases makes it easier to reach those points.
- Oh, I’ll have to watch that

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