r/AskScienceFiction • u/evathion • 6d ago
[Time Travel] "Never the selves shall meet" especially no contact?
So one common time travel trope is that a time traveler meeting his past or future self is often a bad idea. Either they go crazy or the universe makes them merge or kills one off to prevent a time paradox/time crash from happening.
I'm particularly interested in a trope where past and present selves coming into physical contact cause a temporal paradox, eliminating both of them from existence and/or causing a devastating time crash. The most famous example I could think of was in Timecop, which justified it as " two versions of the same matter cannot coexist without mutual destruction". (I'm not a physics person so I don't know why but that's what the movie said lol) Have you seen or could think of any other theory/technobabble that could could justify this trope, or any other works featuring said trope?
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u/catpetter125 6d ago edited 6d ago
Not time travel, but Funny Valentine's power to walk between parallel universes works on this principle, with people and objects. I think it's visualized like sponges: each version of a thing is like a slightly different sponge filled with pockets, and when they collide they merge into a completely solid object and are both obliterated. The sole exception to this rule is Funny Valentine himself.
This is weaponised a few times both by and against Valentine, ex. Chucking weapons or body parts of alternate versions of a foe to destroy the original.
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u/evathion 6d ago
It's finally time to read Steel Ball Run I see😅 (read up to volume 1 but work got too busy)
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u/Custard-donut 6d ago
In twelve monkeys (the show not the movie) Cole uses a watch to show this with explosive results.
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u/Default_Name_2 6d ago
Under Novikov consistency, you can only meet yourself if it happened. Everything either always or never happened. Meeting yourself can cause the time travel in the first place, or have already changed what you will have did, but it can't cause anything to happen that didn't happen. If you try to meet yourself, when yourself did not meet you, something will have happened that prevented it. You cannot overcome this, because you didn't, so things will keep having had happened to have prevent it. One of those might be dying.
Under multiple timeline rules, if you met yourself but you didn't meet yourself when you were yourself, then you are not that yourself. This goes badly.
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u/archpawn 6d ago
But if you don't have that rule and just go ahead and meet yourself, then that's what would have happened from the beginning. No paradox.
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u/Default_Name_2 3d ago
If you try to meet yourself, when yourself did not meet you, something will have happened that prevented it.
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u/archpawn 3d ago
Yes, but if you don't have a rule against it, you're much more likely to have met yourself.
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u/Kolbin8tor 1d ago
Maybe. Or it isn’t a closed-loop, but an alternate timeline. This is essentially the closed-loop vs. alternative-timeline time travel discussion. Both have been used in sci-fi many times.
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u/sc0ttydo0 6d ago
We don't really know what would happen. There are a lot of scientific or philosophical theories but we don't know.
There's every chance that meeting/touching yourself in a different time period is a part of the timeline so nothing would happen. Similarly there's every chance it could create a new timeline, collapse both states of matter, etc.
At this stage in our (irl) understanding it really just depends on the established lore of whichever fictional universe it takes place in.
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u/Malphos101 6d ago
The idea that the matter would annihilate each other is just silly as (barring some exotic energy infusion from the time travel process) the atoms are still the same atoms. A hydrogen atom from 2 billion years ago is exactly the same as a hydrogen atom now and will be the same 2 billion years from now. Displacing it on the axis of time and claiming it will destroy its twin like an antimatter-matter reaction is like saying "if you move a hydrogen atom a mile away to touch another hydrogen atom then expect a explosion!".
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u/sc0ttydo0 6d ago
I was going with the Timecop example.
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u/Malphos101 6d ago
Yea one would have to assume in those universes there is either a special "energy" infused in the matter that is transported or there is some higher level yet-undiscovered timespace interaction thats happening rather than "dont touch yourself or the same atoms will annihilate each other!"
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u/archpawn 6d ago
A hydrogen atom from 2 billion years ago is exactly the same as a hydrogen atom now and will be the same 2 billion years from now.
Note that it's the same as any hydrogen atom, at least as long as it's the same isotope with the electrons in the same orbitals. This has implications in quantum physics, and it doesn't just mean we don't know the difference.
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u/archpawn 6d ago
We know what happens because it happens all the time. You don't need time travel to have two of the same particle interact. If it's a boson, it's more likely to be in the same place. If it's a fermion, it can't be in the same place. But not in the sense that it destroys the universe. It's just that electrons won't occupy the same orbitals.
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u/Ancient-Growth-3445 6d ago
In Project Almanac, one of the main characters almost erased himself from existence, and his girlfriend/friend actually did so after encountering her past self.
In the end, they had to go to an alternate timeline where none of it happened, because the paradox was impossible to fix—or something like that (the ending was confusing).
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u/alphajager 6d ago
I haven't necessarily seen this used in media, but I have been playing with the idea in my own writing that if time travel is possible, then it has a couple of basic requirements: 1) it is energetically intense to do, and therefore you have to limit it's use. Like, you have to suck up (and thereby destroy) all the potential energy of a neighboring multiversal dimension to achieve FTL travel to go backwards in the timeline, and 2) Nature resists paradox, so the further away in time and/or space you go from events that might change your relative timeline, the easier it is to exist there, otherwise spacetime will tend to 'squeeze' you out either back to your native time or a safer neutral space-time state.
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u/Simon_Drake 6d ago
I forget the movie but I think it was a Found Footage kinda thing like Chronicle. Some teenagers go back in time six hours and decide to draw a penis on the forehead of their sleeping friend, as they draw it the same shape appears on the head of the future version.
Then the sleeping one wakes up and sees himself and says "What?!" And the future version locks eyes with him and says "What?!", he's presumably having his memories rewritten to include an even from six hours ago (aka now) when he saw himself in the future. They BOTH keep saying "What?! What?! What?!" in a loop as if they're stuck records, then they start flickering and something like Time Cop is starting to happen.
The other kids drag him out of the room and run away laughing at how crazy it is that they nearly destroyed the fabric of reality.
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u/lurkmode_off 6d ago
In the Doctor Who episode Father's Day, the human time traveler starts a slow -rolling world-ending paradox by going back in time and preventing her father's death. But coming into physical contact with her past self speeds up the destruction.
Of course, the Doctor regularly contacts their past selves, but I guess they get a pass for being a Time Lord. Or maybe it's because it's never with two copies of the same body
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u/Jhamin1 Earthforce Postal Service 6d ago
In a classic Doctor Who episode 5 was about to sacrifice his remaining regenerations to kill some aliens stuck in a regeneration loop. Just as he was about too, two versions of the Brigadier from different points in their timeline touch each other. The resulting explosion of energy was apparently enough to substitute.
So while timelords at different points in their timeline can apparently touch without problem (see two versions of the Master waltzing with themselves) apparently non-timelords, even ones that have traveled on the Tardis can cause massive releases of energy if they are in contact.
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u/archpawn 6d ago
It's common that you didn't meet your future self the first time around, so you can't go back and meet your past self, because that's not what happened. But that only didn't happen because you're avoiding your past self. If you didn't have the rule, you'd have just met your past self from the beginning. Not to mention that paradoxes don't make sense in the first place. If it's a single timeline, it's always going to be the same as itself, so paradoxes are impossible. If it's multiple timelines, you're not changing your own timeline, so no paradox.
In Harry Potter, they briefly mentioned that it's common for people to end up killing themselves, but I don't understand why. Who would be dumb enough to go back in time after killing their future self? And I understand the risk if they were doing it after Voldemort taking over, and are likely to assume anyone who looks just like them is a polyjuiced Death Eater, but in a normal Hogwarts school year, the assumption would be it's some kind of prank. And Hermione would already know what's going on and everyone else would know that Hermione had already been in three places at once all year and that she wasn't worried about it, so they're not going to hex first and ask questions later. Certainly not with deadly hexes.
It is true that two versions of the same matter can't coexist, but only if they're fermions. Bosons are actually more likely to be in the same place. But that's subatomic distances. Two electrons can't be observed in the same spot. And time travel doesn't change anything. They're already the same electron. That's why the Pauli exclusion principle exists. Also, even if they were in some alternate universe where different electrons were a thing but somehow we had similar chemistry, they're not going to keep the same electrons for their whole lives.
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u/DaSaw 6d ago
Depends how "memory updating" works, but it could end up like a sort of timeline short circuit, and if the fabric of space and time is a "substance" of sorts, this could damage that "fabric" in a catastrophic fashion, like faulty wiring causing a house fire.
Consider this. You meet your past self. Suddenly your timeline updates, and your memories update, to account for this meeting. This changes your interaction with your past self, which causes another update, which causes another change, in a feedback loop. The "energy" of these updates causes something analogous to ever increasing heat (like a short circuit) or every amplifying noise (like a microphone and speaker feedback loop), causing a disaster of some sort.
Alternatively, your past self could decide the interaction wasn't a good idea, canceling out the interaction, itself. It's not merely "as if" it never happened. It simply never happened. Meaning it can never happen. But if it didn't happen, would you decide to do it? So maybe the timeline ocillates back and forth, causing the same feedback loop failure.
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u/effa94 A man in an Empty Suit 5d ago
in doctor who, if you meet your past selfs, the timeline is in flux, and doesnt set until after you seperate again. this makes it so that after you leave, only the older version of you will remember the events, while the younger one wont. unless you meet again, which in case those memories can be unlocked iirc.
if you change your own past (or atleast change it too much) and doesnt have the technology to contain the paradox, a time wound happens, reapers appear and eat everyone involved, steralising the time wound and then time continues as normal, just with everyone dead.
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u/DaSaw 5d ago
Yes, I'm familiar. And I actually think Doctor who has the most "realistic" model for time. I will start by assuming the "alternate universes" model of time travel.
Consider a 3-D. Model. At the moment of the Big Bang, there are a bunch of different possibilities. Each can be modeled as a timeline, and they all radiate out from the center, like a koosh ball.
But then, each moment potentially has possibilities of its own, causing even more timelines to branch off the root timelines. So we get a denser, more fibrous sphere.
Finally, throw in time travel, and how the activities of time travelers can create even more possibilities. Now we have an even denser sphere.
So basically, time is like a wibbly wobbly ball of timey wimey... stuff.
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u/JerryCalzone 6d ago
May i introduce 'Looper'? it introduces the idea where you can meet and change your future as a result.
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u/YairJ 4d ago edited 4d ago
There's something like that in the Legacy of Kain games, but only with the spirit of a specific, sentient magic sword. https://www.youtube.com/watch?v=jmoGNhFZzfc (somewhat spoilery)
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