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Scifi question about time travel:
(lemmy.world)
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this post was submitted on 11 Jul 2025
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This is fundamentally a variation on the question of a Temporal Paradox, also known as a Grandfather Paradox ("You go back in time and kill your grandfather. What happens?"). Although no killing happens in this variation, the basic idea is the same: Information is transmitted to the past from the future, but results in a situation where it cannot be transmitted in the first place.
Accordingly, there are several hypotheses to cover this. This isn't even all of them:
I think the idea of parallel universes solves time travel paradoxes in a pretty clean way.
Except for the fact it makes every decision, every moment of tension and every event that occurs irrelevant, because an infinite number of universe exist in which the events occurred and in which they didn't occur.
Even knowing that everything happens every way in some other branch of the wave function (other universes) doesn't really affect our own little section of it. There's no communications or travel, so other universes if they exist have the same meaning to us as if they don't. Except in time travel stories like this.
Besides, the same "irrelevance" of decisions and events comes free with even one single universe given that it's deterministic - as physics seems to be. (Yeah there's quantum randomness, but random doesn't help either)
That said I still believe in free will and the importance of decisions. I just think it has to be defined so weakly that it still works in a deterministic universe. (So I have free will, but so do dice and pocket calculators.)
I'm not talking about about the possibility of real infinite dimensions. I'm talking about sci fi, and stories, which is the context of the OPs question.
In a "real" scenario, the experience that matters is the one I'm having, not the one other versions of me might be having.
But in a story, there is no "true" timeline, or a more "real" timeline. They're all being retold to us indirectly, and the choice of the version of the person retelling those experiences is arbitrary by the author. It doesn't matter what perspective the author chooses, because every other outcome also happened, the author just didn't tell us those stories.
That would be the most boring story ever.
It becomes interesting at that point where one (or some) of the possibilities get a special meaning "above" all the others.
That's exactly my point! In an infinite timelines story, there is nothing that has special meaning over the others, making it boring, because it's all irrelevant!
I get what you mean, but I have to disagree a bit. The slice of the multiverse we're looking at is special because we're looking at it. It only makes it irrelevant if the slices are treated as fully replaceable.
Take for example Invincible. The comics & series focus on a young superhero who could have become incredibly evil, but didn't. The multiverse is used to highlight this: it shows alternative versions of him that did become evil, and it even says that most alternative versions did so. This makes the version of him we focus on that much more special, and allows for interesting character progression through being confronted with his fears.
But it only works because of the restraint of the writers, never showing us another good version of Invincible, only focusing on evil alternatives.
I don't see that as a problem. Every possibility co-exists, and every reality is equally real. Every moment and decision forks the universe in infinite ways, but you get to choose the one where you go.
You can save a drowning person, or let them die, but in the big picture, it won't matter. That person will drown infinitely many ways anyway, but there are also infinitely many universes where they get saved. Don't worry about the big picture. What matters, is how you act and how the world acts on you in this universe.
Apologies, I copied and pasted the answer below from another reply I made elsewhere in this thread
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I'm not talking about about the possibility of real infinite dimensions. I'm talking about sci fi, and stories, which is the context of the OPs question.
In a "real" scenario, the experience that matters is the one I'm having, not the one other versions of me might be having.
But in a story, there is no "true" timeline, or a more "real" timeline. They're all being retold to us indirectly, and the choice of the version of the person retelling those experiences is arbitrary by the author. It doesn't matter what perspective the author chooses, because every other outcome also happened, the author just didn't tell us those stories.
I don't get it. Where's the paradox here? He gets to see the future but turns off the machine before getting any information from it so nothing changes. What I'm missing?
His future self showed his past self the lottery numbers through the open window, but he closes the window, so his future self can't show them to his past self.
I'm more of an Emergent Time/Timeline Curve theory guy. The others are cool for sci-fi and stuff but I just can't conceive of that being how it works.
Closed Loop Theory seems like too cheeky of an explanation. It's basically a bait and switch. Like: "What if you did thing? But then DIDN'T do thing!" with the answer being "actually you did but just later". To be fair though isn't the theory really just saying the universe will correct itself somehow?
Also since you seem knowledgeable on this, something I've always wondered about: is their any theory centered around our frame of reference having a past but not a future? As in we're blazing the trail forward like an ice breaker ship for everyone else to follow? There's probably a million fundamental laws of the universe that makes that impossible.
Sorry, I'm not very smart, but I do kinda love this stuff.
Disclaimer, I am not a physicist, just a guy with interest in sci-fi, science, and too much free time.
So, the answer is, yes, this is actually kind of a common theory on how time actually works. Maybe.
This has to do with physics, and the fact that no two observers have the same perfect frame of reference. For most of us humans, our frames of reference are close enough to be identical on a day-to-day basis. It's even close enough for (most) science. But it's not true on a perfect level. For instance, special relativity says that time passes differently for objects in motion; GPS satellites have to correct for the fact that their onboard clocks are experience "slower" time than us observers on Earth. Even astronauts "lose" about ~1/100th of a second for every year spent on the ISS.
What's this got to do with the future not existing, though?
So we know no two observers have a perfectly identical frame of reference - there is no objective "truth" of when something occurred. Cool. Now what? Well, what we can talk about is historic light cones. Because the speed of light in a vacuum is a universal constant, we can determine how far from you a photon departing your actions would travel. Places that photon would reach at any given point in time following your action are said to be within your historic light cone, and in common parlance, the past. The boundary of how far that photon is reaching at any given moment is, from your frame of reference, "the present". But since nothing can exceed the speed of light, it is impossible for an observer to view beyond the present, into the future.
The catch, of course, is reference frames. You used a plural - "our frame of reference", "we're blazing a trail forward" - but the reality is that each of us has a minutely different reference frame and is blazing a minutely different trail. Again, for almost any day-to-day purposes this is irrelevant... but there are certain scientific experiments which exploit or even rely on this absence of reference frame.
Cool, what about time travel again?
In my first comment above, I mentioned something called closed timelike curves. Those are an actual thing: By severely bending spacetime, you can theoretically cause a photon to "curve" around and end up at the same point in time it was produced, now in its subjective past, while mathematically not violating quantum physics.
This is where things get kind of freaky and headachy; if a photon can be sent into its subjective past, doesn't that imply a future now existing, in which that photon will be generated? The answer is, not in the frame of reference of that particular photon. A historic light cone of that photon being generated, now in that photon's future, still exists; but that photon is now generating a new, detached lightcone...
Like I said, headachy. I also have to emphasize that while the math holds up, there's ample reason to believe CTCs don't exist, chief among them that our mathematical understanding of quantum physics may still be imperfect.
tl;dr: Yes, absence of reference frames means that each distinct observer is blazing their own trail, which spreads into the "past" at the speed of light. The future, exceeding the speed of light, is unobservable. This framework does provide a mathematical concept of how you could send something into your subjective past, but such a means is still theoretical at best.
That was fascinating to read, thank you!