this post was submitted on 11 Jul 2025
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52
Scifi question about time travel: (lemmy.world)
submitted 5 days ago by bizarroland@lemmy.world to c/nostupidquestions@lemmy.world
46 comments fedilink hide all child comments
 

If you had a machine that created a window through which you could see the future, and in the future you wrote down the winning lottery numbers and relayed that information to your present self before that lottery number was drawn.

However, in your present selfs excitement, you turn off the machine before your future self wrote the winning lottery numbers into it for your past self.

What would happen?

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[–] Zonetrooper@lemmy.world 32 points 5 days ago (3 children)

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:

  • The closed loop theory: To maintain the loop, you will in the future build a time machine which will allow you to activate the machine in the past, maintaining the loop. Past you may even be unaware it was activated from the future.
  • The Parallel Universe theory: When future-you sent information into the past, they did not send it into their own past but rather into a universe in which you do not send the information back in the first place.
  • The Timelike Curve theory: Because there is no common reference frame for "time", each quanta of "you" is experiencing a different reference frame. The historic light cone of your future self sending the information back exists, and if you could follow those photons backwards you would find him doing this. But future you, in your frame of reference, will never see the machine activate.
  • The Emergent Time theory: Time is not a linear path, but a function of entropy. By inverting entropy, you have caused a reconfiguration of the universe into a version in which the machine is inactive.
[–] CallMeButtLove@lemmy.world 2 points 4 days ago* (last edited 4 days ago) (1 children)

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.

[–] Zonetrooper@lemmy.world 2 points 3 days ago* (last edited 3 days ago) (1 children)

Disclaimer, I am not a physicist, just a guy with interest in sci-fi, science, and too much free time.

is their any theory centered around our frame of reference having a past but not a future?

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.

[–] CallMeButtLove@lemmy.world 2 points 3 days ago

That was fascinating to read, thank you!

[–] TranquilTurbulence@lemmy.zip 7 points 5 days ago (1 children)

I think the idea of parallel universes solves time travel paradoxes in a pretty clean way.

[–] ada@piefed.blahaj.zone 8 points 5 days ago (2 children)

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.

[–] TranquilTurbulence@lemmy.zip 3 points 5 days ago (1 children)

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.

[–] ada@piefed.blahaj.zone 1 points 5 days ago (1 children)

Apologies, I copied and pasted the answer below from another reply I made elsewhere in this thread

==

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.

[–] BrinkBreaker@lemmy.dbzer0.com 1 points 4 days ago* (last edited 4 days ago) (1 children)

I think the disconnect here is between objective and subjective meaning. In an infinite multiverse, 'reality' isn’t a singular objective truth—it’s a collection of subjective experiences. But that doesn’t erase meaning; it just means meaning is something we assign, not something inherent.

You’re right that if every possible outcome exists, no single timeline is 'objectively' special. But in fiction (and arguably in reality), what matters is the perspective we focus on. A story isn’t weakened by the existence of other timelines—it’s strengthened by the fact that, out of infinite possibilities, this particular one is being told. The act of choosing a narrative is what gives it weight.

It’s the difference between nihilism (‘nothing matters, so why care?’) and absurdism (‘nothing matters* inherently, so we get to decide what does’). A multiverse doesn’t have to make things meaningless—it can highlight how rare and significant certain choices are, precisely because most versions of a person might not make them (e.g., Invincible).

I get the sense you’re resistant to this because it feels like it undermines objective meaning. But what if meaning was never objective to begin with?

[–] ada@piefed.blahaj.zone 1 points 4 days ago* (last edited 4 days ago)

Think of it this way.

"I went back in time to save my family" in an infinite timelines story means that going back in time spawns in infinite number of worlds that didn't exist before, in which the family doesn't make it, and an infinite number in which they do. And not a single one of those families is the "real" family of the person who went back in time.

The fact that the author choose to focus on one perspective in which it seems like the time travel has made a difference, doesn't change the fact that it didn't make a difference, and the family they were trying to save is gone. The infinite copies weren't "saved" from anything, because there are infinite versions that weren't.

The only way to tell a meaningful story in that situation is to create situation where the actions of jumping back in time alter the future of the person jumping back in time. And that means you either suck up the paradoxes, or you write a clever story in which the paradoxes are neatly accounted for before they ever occur (or you write a closed loop story)

Edit - Or you could tell a non infinite loop story, where a single universe is spawned by the act of jumping back in time. That still won't save the "real" versions of the family you jumped back to save, they're still gone, but at least it creates only a single version of them that the character can save.

[–] davidgro@lemmy.world 1 points 5 days ago (1 children)

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.)

[–] ada@piefed.blahaj.zone 0 points 5 days ago (2 children)

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.

[–] Zwuzelmaus@feddit.org 1 points 4 days ago (1 children)

But in a story, there is no "true" timeline, or a more "real" timeline.

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.

[–] ada@piefed.blahaj.zone 1 points 4 days ago (1 children)

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!

[–] FooBarrington@lemmy.world 1 points 4 days ago (1 children)

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.

[–] ada@piefed.blahaj.zone 1 points 4 days ago (1 children)

Invincible can't move between the infinite timelines though, and no storyline is hanging off of the important changes he makes those timelines by travelling through time/dimensions. He's not "saving" anyone by jumping through to another universe

[–] FooBarrington@lemmy.world 1 points 4 days ago

Yes, that's the restraint the writers are showing. That's my point: the issue isn't the multiverse aspect itself, it's the replacability brought on by unrestrained multiverse implementations.

[–] davidgro@lemmy.world 1 points 5 days ago (1 children)

I'd say that the one that's written is the 'true' timeline in the story the same way that the reality we experience is the only one that matters.

[–] ada@piefed.blahaj.zone 1 points 5 days ago (1 children)

The reality I experience is the only one that matters to me. To an outside observer, all of them are as equally real and there is no true timeline.

In a story, there is no real, there is only outside observers...

[–] Grail@aussie.zone 2 points 4 days ago

Well, "no objective reality" is a lot more accurate to the truth of the world than any alternative. It might not be as narratively satisfying as a story where objective truths exist, but I suspect the human desire for objective truth is a cultural value that would be in our best interest to shed.

[–] ExLisper@lemmy.curiana.net 4 points 5 days ago (1 children)

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?

[–] FooBarrington@lemmy.world 3 points 5 days ago (2 children)

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.

[–] CallMeButtLove@lemmy.world 1 points 4 days ago* (last edited 4 days ago) (1 children)

I've read your message and the OPs like 5 times and I still have no idea what is being described... I might be stupid.

His future self showed his past self the lottery numbers through the open window

Got it. We're good so far.

but he closes the window, so his future self can't show them to his past self.

This is what I'm stuck on. So he didn't actually? I get the irony of saying a paradox doesn't make sense but I'm not even following the thought experiment. His future self opens a window and says "Hey, get some paper and a pen, I've got some winning lottery numbers for you!" and his past self goes "Oh boy!" and then immediately CLICK (closes the portal) before ever being shown the numbers.

Could it be restated to say he gets the numbers from his future self but then 30 years later just forgets to do the same thing for his past self?

[–] FooBarrington@lemmy.world 1 points 4 days ago

His future self opens a window and says “Hey, get some paper and a pen, I’ve got some winning lottery numbers for you!” and his past self goes “Oh boy!” and then immediately CLICK (closes the portal) before ever being shown the numbers.

Here's your issue - it's not the future self opening the window, it's his past self. The future self can only speak through the window, but he can't open it. So since the "current" self closes the window, the future self won't be able to speak through it.

[–] kuberoot@discuss.tchncs.de 1 points 4 days ago

I would assume they sent them after the number was drawn, to before the number was drawn, which means the future self doesn't need their own message to learn the numbers.

[–] Couldbealeotard@lemmy.world 16 points 5 days ago (2 children)

I don't think you've thought through your question. The machine was open in both time periods. The machine was turned off after the information was given. There's nothing more to happen.

I get what you're trying to say, but it doesn't really apply to the scenario you are describing.

I saw a better version of what you are attempting to present: a scientist has a button that transports an object back in time by 5 seconds. An apple is tested successfully, it appears 5 seconds before he pushes the button and when he pushes the button the original disappears. They try it with a live mouse but something goes wrong; when it appears it's horribly disfigured and this scares the scientist who pulls away from pushing the button. The mouse has already appeared but he will not press the button. Now what happens?

[–] dickalan@lemmy.world 2 points 4 days ago (1 children)

Looks like somebody has read the long jaunt by Stephen King

[–] Couldbealeotard@lemmy.world 1 points 3 days ago (1 children)

Actually that was from Das Jesus Video (aka The Hunt for the Hidden Relic)

[–] dickalan@lemmy.world 1 points 3 days ago

That’s cool, if you find that interesting you should also check out that short story by Stephen King, I love a good horror story butI’m not easily scared but this one is actually scary

[–] Zwuzelmaus@feddit.org 2 points 4 days ago

Now what happens?

The disfigured mouse must come from "somewhere", or rather from sometime.

The logical explanation is: later he has developed his button further, for a longer time jump (and maybe for some other improvement that gave him better confidence) and then he tried it "again".

[–] philpo@feddit.org 4 points 4 days ago

Dude, just read the fucking manual, it's on page 95 right under "What to do when a Gloridian chokes you through the time window?" And just before "Things you should report to the Time Cops"

[–] LovableSidekick@lemmy.world 2 points 4 days ago

I would just turn the machine back on and wait longer.

[–] Zwuzelmaus@feddit.org 9 points 5 days ago

It is no paradoxon.

Only your story is a bit unclear at the point where it matters. Let me ask a question to clarify = to destroy what appears a paradoxon, but isn't.

Question:
Who controls the transmission of the information - the past self or the future self?

If the past self controls the transmission, then he receives no information. Case closed. It does not matter what the future self is doing.

If the future self controls the transmission, then he knows when the past self turns off the device. He can send it just a minute further into the past, before the past self turns off the device, and so the past self receives it "in time".

[–] davidgro@lemmy.world 7 points 5 days ago

What would happen is entirely your responsibility as the author of the scenario.

Some options may be more "realistic" than others, but since the existence of a working time machine is already beyond what seems to be feasible physics (requiring ridiculous amounts and density of negative energy for example, where not even any has been shown to be possible to make) the scenario becomes soft sci-fi, or in other words magic, and that means it's up to the writer to make up the rules.

Here is a post I found with many of the options you can choose from.

[–] Nibodhika@lemmy.world 5 points 5 days ago (1 children)

First of all this is not a paradox, unless you're not explaining something, there are two yous past and future, if past self turns off the machine before seeing the numbers nothing happened, if he turns it off afterwards the information has already been transferred so nothing happens either.

I have a feeling you might have recently watched Primer and are thinking of a similar working tome machine, where the machine needs to be powered on from past until future. But if this situation happened in Primer it wouldn't be a problem either because you're not in the box after you leave it. It's a bit weird, but if you imagine time as horizontal lines, the box allows you to travel diagonally, so you only exist inside the box in that timeline at the moment of exiting, before that you were in a different timeline, so if you exit the box, wait a while and turn it off you're only preventing yourself from using the box again. In fact that's one of the big reveals of the movie, except it's said in passing by mentioning that the boxes are multi-use.

[–] dickalan@lemmy.world 1 points 4 days ago (1 children)

I have watched this movie many times and I just bought it on Apple TV because it was five dollars, I can’t wait to not understand again

[–] ExLisper@lemmy.curiana.net 1 points 4 days ago (1 children)

Have you seen Timecrimes?

[–] dickalan@lemmy.world 1 points 4 days ago (1 children)

No. Thank you for the recommendation. I will be watching it soon.

[–] ExLisper@lemmy.curiana.net 1 points 4 days ago

It's the only time travel movie that makes any sense.

[–] Grail@aussie.zone 2 points 4 days ago

It's called a looped timeline pair. Timeline 1 creates timeline 2, and timeline 2 creates timeline 1. Both timelines exist on an alternating basis. It's one of the results of the bootstrap paradox. When people think of the bootstrap paradox, they usually think of single looped timelines. Those appear in media all the time. Harry Potter, Gargoyles, etc. But an unstable set of timelines can also stabilise across two, three, or more timelines instead of just one.

Out of the infinite versions of you in the multiverse, half win the lottery and half don't. Which one your consciousness inhabits is pure luck.

[–] fittedsyllabi@lemmy.world 4 points 5 days ago (1 children)

You’d still be poor.

[–] TribblesBestFriend@startrek.website 3 points 5 days ago* (last edited 5 days ago) (1 children)

Time to watch Déjà Vu I guess

[–] bizarroland@lemmy.world 2 points 5 days ago (1 children)

I was thinking about primer when I thought of this question.

[–] kambusha@sh.itjust.works 1 points 5 days ago

Meticulous, yes. Methodical. Educated. They were these things. Nothing extreme. Like anyone, they varied. There were days of mistakes and laziness and infighting. And there were days, good days, when by anyone's judgment, they would have to be considered clever. No one would say that what they were doing was complicated. It wouldn't even be considered new. Except maybe in the geological sense. They took from their surroundings what was needed, and made of it something more.

[–] chemical_cutthroat@lemmy.world 2 points 5 days ago* (last edited 5 days ago)

You can kinda see this play out in the short story by Ted Chiang called The Merchant and the Alchemist's Gate.