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
Probably the branch off one.
Though, speaking of time travel, I really don’t understand/like the whole Harry Potter dementor (however it’s spelt) lake scene in the movie where future Harry saves past Harry. How does that work? Wouldn’t in an initial timeline Harry have to somehow save himself before he could travel back in time to save his past self? The way I see it, it just looks like an infinite cycle of Harry saving his past self with no origin point.
Either 1 or 3. I tend to lean towards 3
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!
"Default behaviors is undo, no redo. You control undo the undo. Emacs.
I recommend undotree, which is also a non-destructive undo, but for some cases makes it easier to reach those points.
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.
Oh, I’ll have to watch that
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.
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.
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” scenario in OP’s point 2). 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
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.
I like the one where the motion of the universe is not accounted for, so the travelers drop into empty space. But someone figures out how to use that to travel through space.
Time Wars are fun though. Each prime timeline moving others toward them.
Be a potentially energy-efficient way to exit a gravity well in a spacecraft if you could exploit that and it doesn’t require too much energy. Instead of launching a spacecraft, just send it back in time to when a point was no longer in that well.
EDIT: if the above conditions hold (it’s possible and requires less energy than launch), you also have an infinite-energy-production machine, because you can obtain more potential energy than you are expending energy to time-travel.
If you screw up the calculation, your time machine can also end up deep under the mantle of the Earth. That would be a pretty spicy way to travel.
Are you asking which system I think is the most plausible, or which is the most desirable?
Plausibility:
Well, I’d guess that time travel probably isn’t possible, and if it is, it’s probably under extremely limited conditions that render it impractical for viable exploitation. But if you’re operating under the assumption that it is, I’d say the “your actions do not affect this timeline” or similar type.
Why?
We have had no record of time travel or seen phenomena likely resulting from it. If at time T, time travel is discovered, it seems unlikely that someone after that time wouldn’t have come back in time and done something that we’d have noticed.
And it’s not just us. If self-timeline-affecting time travel is possible, then you consider all the possible civilizations out there in the universe who might discover it at some point in time and want to take advantage of it. Yet we’ve seen nothing from them. It’s the Fermi Paradox on steroids. The Fermi Paradox asks why intelligent aliens, half of whom statistically probably evolved before us and should have colonized the universe if they’re out there, aren’t visible to us. The time to travel over even huge distances, though it is large, is small compared to the time required to evolve a spacefaring civilization. But in the presence of self-timeline-affecting time travel, then even the evolutionary time becomes a non-factor, since civilizations from the future could also show up, and roll back in time with their advanced technology and make use of it from then. The question is no longer just “where is everyone”, but the even harder to explain “where is everyone from all time?”
Okay, that’s the plausibility question. How about the desirability one, which system I’d like to exist?
Hmm. I guess I’d give the same answer, the “no affecting your own timeline” form. I think that if you could affect your own timeline, that probably some kind of incident in the future – only takes one – would be likely to have mucked up things sufficiently to wipe out civilization, and we probably wouldn’t be around to even be pondering the matter.
Primer because Primer. (Video warning and some spoilers for a bunch of different films.)
I don’t know if I would subscribe to it, but it is one of the more interesting ideas for time travel.
Primer spoilers, kinda, xkcd style:
Title
Probably something like attractor field theory from Steins;Gate. In my view it’s basically timelines with a bit of topological though thrown on it to combine closely related timelines into bundles, similar to some algebraic topology concepts I guess.
The one where you can only jump forward, not backward. It avoids the common paradoxes.
we already live in that one
See? Problem solved.
I have a unified theory that includes bits of everything.
- it’s possible to communicate in the same timeline between the future and past by using gravity (think something like Interstellar)
- By using gravity as an nondestructive line of communication, it’s possible to change events in your current existence without a causal shift or split.
- by sending actual matter into a timeline either ahead or behind where it’s supposed to exist it causes a causal shift or split in the timeline. this means you will never be able to see the future and can only contaminate the past enough to damage your present causing an inverted negative effect to your going back in time anyway.
- nexus events can only exist when it has a high gravitational marker attached to it. eg: a star will always go supernova, when doesn’t matter as much as the fact that it’s unstoppable because the gravitational function it applies on a universal scale across all timelines, known and unknown. think of it like flashing a flashlight into a room filled with mirrors. each mirror is the physical plane of existence for the timeline (the beam of light). the beam will hit the first plane and then bounce off all the others in the order in which the photons scatter. if you could slow down and witness the photons, they wouldn’t all hit at the same time nor at the same strength.
- It’s only possible to communicate across time using gravity and only if someone has picked up the “receiver” in the past. meaning if we’re not listening for the call “now”, we will never receive the call from “tomorrow”. I think this type of communication is completely within the reach of our technology today and quite possibly is being used today without world knowledge.
Whatever Primer did, cuz that movie scratches my brain real good.
Theory: time is immutable, and the universe exists as a single timeline that repeats itself through a high-dimensional recursive or map-reduce. By “time travel,” you are essentially moving yourself across loops and jumping to a different iteration of this universe.
The past, present, and future do not exist as separate states.
Imagine a vast array of all possible states of matter in the universe. Imagine reality has a finite spacial resolution. With a series of numbers, or even a single very large number, you could provide a unique identifier for every possible arrangement of matter in the universe. The positions of every star and galaxy. The detailed interactions of every quark. Imagine a list or array that would have a number of entries equal to some indecent multiple of “ten to the ten to the ten…” Imagine all these possible states, every possible configuration the matter of the universe could occupy.
Then realize…All of these possible states exist at once. They are all as real as any other. There is no preferred state. They all exist in some vast “10 to the ten to the ten” dimensional spacetime. What we perceive as the flow of time is simply us moving from one of these states to another. But our consciousness cannot move arbitrarily between states. There are elaborate rules on which states you will be able to observe dependent upon the states you previously observed. We call these rules the laws of physics.
So when you travel through time, you are simply altering your path on this vast multiverse of possible realities. There is no “real” reality. They are all real. Every possible configuration of the matter and energies of the universe physically exist concurrently.
There are no timelines to split or erase, because there are no timelines. There are just conscious minds moving through a near-infinite array of possible “nows.” And all of the nows exist simultaneously. There is no real one. From the perspective of a “time traveler,” it will seem like they changed “the future.” But the truth is the very idea of a past, present, and future as distinct entities is madness. We’re just consciousness drifting through the continuum, from one of the near-infinite nows to another.
That’s a really long way to say “the first one”.
I think it’s the 2nd option
It’s just the first one, only specifically a version where all timelines exist and you simply navigate them. I can see how it might feel like the second one because the timelines already exist, but from one’s subjective viewpoint it’s #1.
I think the first one leaves open what you do, as alternate actions lead to an alternate timeline. The second is more “read-only”, similar to what OP laid out.
The only difference between the first option and the response is that the response posits that all possible timelines exist in advance and rather than generating a new timeline with your decisions, you simply navigate to the one that represents them. It’s a distinction without meaning, especially because the first option doesn’t strictly specify whether the timelines existed in advance or not. It simply says “you branch off into another timeline” with no requirement that it be one generated as a result of your actions.
The second option is called “closed loop” or Novikov self-consistency and specifically requires that the outcomes of your choices align with the past already as defined, simply in ways you did not know. It’s what they use in 12 Monkeys and the 3rd Harry Potter book, and it limits free choice, unlike the first option and what the above poster’s response stated.
I think what you’re doing is combining closed-loop and multiverse theories to say that the multiverse theory IS closed-loop simply because the multiverses existed in advance, whereas closed-loop is intrinsically single universe/timeline.
I appreciate those detailed insights, I see where I misunderstood. I need to reread the 3rd HP novel with my son now :)
Stories involving 2 are often the most fun, as well as 4 if they aren’t lazy with the timeline corrections
1 feels the simplest and I would prefer it. With 3, unless the technology is limited to a few people, it’s going to get messy
Infinite branches.
It feels more intuitive, and doesn’t involve any strange problems. It implies that the multiverse has infinite possibilities, they are all realized somewhere, and a time machine allows you to jump between them.
Multiverse theory doesn’t really allow jumping like that though, each term in the wave function is independent and that’s kinda the point.
At best a time machine can just allow for further splitting of those terms, but that doesn’t actually mean anything special because we can do that without time travel by just measuring particle spins.
Sounds like a very different kind of multiverse than what I was thinking of. If that word is already taken, I should probably call this thing with a different name.
What you described to me sounds exactly like what I’m referring to, so I’m not sure tbh. I’m referring to the many worlds interpretation of quantum mechanics.
I just looked it up, and it seems like my intuition aligns pretty well with MWI.
Anyway, the idea is that as you jump 100 years back, you enter a special timeline that isn’t exactly like the one you left. It’s mostly the same, but a time traveler visited at some point, which could radically alter the future of that timeline.
So if you become your own grandmother, there’s no real paradox, since you and your grandchild are not the same person. You are from different timelines, you are different branches of the same tree. Time traveling doesn’t cause branching, since every branch already exists.
A time machine can’t travel entirely freely to any branch, but since there are infinitely many branches you get the illusion of complete freedom. You can not jump to a timeline that doesn’t already have you jumping in it.
That makes sense, and I think it literally just is the Deustchian model at least according to this source. It’s been a long time since I read the original paper so I guess I misremembered it as a slightly different thing, though further reading suggests this might be a flawed explanation of his own theory, so I’m just thoroughly confused now haha.
I can think of a number of problems with how it would work, depending on the way you set it up it could result in something like “wormholes” to the future just randomly opening up constantly and everywhere just due to the way probability works. There are certainly a lot of interesting mathematical phenomena that arise from time travel like that.
I like the persistent present. We simply live with the paradoxes.
“Remember when Hitler was assassinated in 1919, 1933, 1936, and 1939, then off’d himself in his bunker in '45?”
“An unidentified man in SS uniform reportedly tried to kill Hitler during a rally at the Berlin Sportpalast.” 1939
This was definitely the time traveller.
LOL. I knew there were a lot, never looked it up! The problem is, in the time travelling paradigm, those would fit in the self-mending timeline end of the theory. This version would simply have many dead hitler. Like schroedinger’s cat or photons or whatever.
