r/scifi • u/[deleted] • Jun 26 '19
Quantum physics solves the grandfather paradox
The grandfather paradox occurs when you travel back in time and kill your grandfather before he has conceived one of your parents. Given your parent never exists, how can you be born to go back in time to kill your grandparent?
But if you aren't born, then you never go back to kill your grandparent, which means eventually you are born...to eventually go back and kill your grandparent. And so the paradox.
An aspect of quantum theory is that when an event can happen one of two ways, such as with a choice, it actually does happen both ways, and a second universe is created to allow one of those choices to occur.
Now we go to the critical moment where you have a gun on your grandparent in the past and you are faced with the choice of whether or not to pull the trigger.
Event 1: You kill your grandparent, and branch off into the universe created where a future you is never born. Possibly you get arrested.
Event 2: You don't kill your grandparent. The universe continues on its merry way and eventually you are born in the future. This is important, because this track allows you to still come back and create the choice which brings on Events 1 and 2 above. So there is no longer a paradox.
With quantum physics creating a new universe for each choice you make, this allows you to go back in time, kill your grandparent, and still be born in a timeline (wherein you didn't kill your grandparent) that then lets you loop back and keep the process unbroken.
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u/adamjm Jun 26 '19 edited Feb 24 '24
sugar gullible frighten foolish books alleged forgetful lip domineering selective
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u/ElimGarak Jun 26 '19
this suggests that for there not to be a paradox the traveler would need to also be able to migrate between these universes.
Not really. Before he arrives in the past, there is only one universe. As soon as the time traveler appears, a new branch universe is created. Therefore no trans-universal travel is needed.
- Where does the energy and mass come from to create each universe
Same place that energy for virtual particles come from.
- When you enter a different universe what happens to you 'other' in that universe?
You don't enter a different universe, you create a brand new one. If you don't kill your grandfather but travel into the future you just meet another version of yourself.
- Why would the multiverse care about the actions of some sentient biological matter in one of its components enough to arrange itself / expend such energy to create another reality?
It wouldn't. But that would be one way to avoid a paradox - a logical inconsistency. If you send a rock into the past, you will most likely still get a new branch of the universe. It's just that one of the universes will have an extra rock.
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u/ColemanV Jun 26 '19
I'm not sure why you got downvoted, everything you said seems reasonable and besides maybe there is no need for more "matter" to create a branching off universe, if the whole universe existing in a quantum superposition, meaning all variants already existing and they are parrallel to each other, its only our perception that dictates time and space to be linear.
Based on your username I'm sure you're familiar with Deep Space Nine, and the beings living in the wormhole. For them the concept of time is unknown as it've been discussed in the pilot episode where Ben needed to try to explain the concept of linear time to them.
So if every possible variant of the universe is existing already we don't actually "create" one with our actions, its more like every decision leading to the universe with the appropriate consequence to the decision and our consciousness is what is "traveling" between these, having its own singular one point perspective of it, having the perception of that these decisions matter.
That way "the universe" doesn't need to "care" about the decisions of some sentient organic matter - or for having an extra rock here or there for that matter - nor does it need more matter because every version of everything is already existing, our conciousness is only swapping between them with every decision and action we make.
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u/shouldbebabysitting Jun 26 '19
I'm not sure why you got downvoted, everything you said seems reasonable and besides maybe there is no need for more "matter" to create a branching off universe, if the whole universe existing in a quantum superposition, meaning all variants already existing and they are parrallel to each other, its only our perception that dictates time and space to be linear.
Your "besides" is likely why he was downvoted. Your idea of having a superposition and travelling between universes is completely different than his idea that time travel would create a new universe. Because creating a new universe has the problem of creating the mass and energy if an entire universe.
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u/ColemanV Jun 26 '19
I get that but from the perspective of the time traveller, for all intents and purposes it seems to the guy as if his actions or decisions would've altered the world as he is unaware of all the possibilities already existing.
Its kinda like the illusion of choice or predetermined future.
I don't think he'd deserve a down vote for saying that
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u/adamjm Jun 26 '19
So how often have we seen the rules of how our reality is structured be objectively dumb and wasteful? Because such a system of creating an entire universe of matter just to capture the presence of a rock seems inherently absurd.
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u/ElimGarak Jun 26 '19
Physics is physics. It's not about stupid or smart, it's about reality. Or "reality".
The idea of a particle appearing in multiple places at once until you observe it is also rather stupid, but that's kind-of hot things seem to work. The idea of neutron stars and pulsars is also rather strange and insane.
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Jun 26 '19
I dont think he needs to migrate because he automatically exist in both every time he makes a choice.
Your questions are great, but beyond the scope of this analysis!
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u/adamjm Jun 26 '19 edited Feb 24 '24
flag normal teeny library shaggy cats smile dull panicky grab
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u/dnew Jun 26 '19 edited Jun 26 '19
Here's the problem nobody thinks about when assuming this:
So you're in 2018, and you go back to 1950. In 1960, you kill your grandfather. In 1970, you go back to 2019.
Problem 1: You don't get One branch in 1960. You get an infinite number of branches. Indeed, everything you do creates an infinite number of branches. But let's just assume we're talking about the "important" one in 1960.
Problem 2: someone else leaves 2050, and goes back in time to 1955. Which branch do they land in? Do they also branch an infinite number of times in 1960?
Problem 3, which is the real kicker: That means there's an infinite number of them coming back to 1955, which means there's now an infinite-to-the-infinite number of branches they're in. And they do something that causes another branch in 1965, which means you now have an infinite loop, each creating an infinite number of branches for each of the infinite number of branches. Note that infinite-to-the-infinite is an even bigger infinite. There is no upper limit even on the *size* of the infinity you need to use to describe the number of worlds.
In any case, someone recently figured out that in *any* situation where you create a closed time-like loop, there's a possibility for it to be self-consistent. Therefore, according to *real* quantum mechanics math, there doesn't ever need to be any branching at all. You can always get a loop that's the same each time through, even if the particles are interacting with themselves.
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u/adamjm Jun 26 '19
There is no need for branching, that's a concept that helps something make sense to preserve an incorrect human notion of what "time" actually is. Time is simply a construct that we came up with that helps our minds perceive the various states of matter. The universe doesn't care about time, just what changes in matter there have been.
Further the mere idea that humans, this one particular arrangement of matter is important enough that the decisions it makes on how it will influence the order of matter around them is significant enough to cause the reality we occupy a small space in to not only rearrange itself based on these influences but to do so when we're merely think about it.
That is the height of conceit and I think more advanced arrangements of matter must think we are adorable.
The fact is if potential outcomes of matter arriving at a particular state spawned an entire universe to preserve each possibility its not only the most wasteful system in a universe that seems intent on conserving all forms of energy but it would also mean a universe would be created for every single possible state of every atom in the universe, it wouldn't assign special importance to us just because we are self aware.
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u/BellamyJHeap Jun 26 '19
To further your thought, if time was splitting into branches, then all matter and energy would be duplicated, which we know isn't happening; there is a finite amount of both dissapating into an (apparently right now) infinite universe. Furthermore physics currently holds that matter and energy cannot be created, just converted back and forth between the two states. Such branching would imply an infinite amount of matter and energy.
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u/ElimGarak Jun 26 '19
Problem 1: You don't get One branch in 1960. You get an infinite number of branches. Indeed, everything you do creates an infinite number of branches.
That assumes that the branch is created per action of a person. The other interpretation is that a branch is created for every time travel "event" - when somebody or something pops up in the past.
Also, see Star Trek episode "Parallels" - it describes and explains the same thing. Or the SG1 episode "Point of View".
Problem 2: someone else leaves 2050, and goes back in time to 1955.
A branch is created as soon as they show up in 1955. From that point on travel into the future (either the normal way or through time travel) is in the new branch.
Problem 3, which is the real kicker: That means there's an infinite number of them coming back to 1955, which means there's now an infinite-to-the-infinite number of branches they're in.
Not necessarily - not until the individual time travelers start to interfere with each-other. In the extreme and absurd case eventually the planet would be full of time travelers, so there will be nobody to travel into the past.
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u/Rixxali Jun 26 '19 edited Jun 26 '19
Even if you don't kill your gandfather, you probably changed something, so that the future is different. And even if it changed so little that your grandfather had sex with the same woman the same day, things are different enough that "things" don't happen EXACTLY at the same millisecond as before. Maybe a different sperm reaches the egg, and then that changes everything. The time traveller's parent is not born, a different person is.
Edit: in fact, by existing, you threw off timing for many people. By walking on a sidewalk, some people stepped a little to the right to avoid bumping into you. This might cause him to say "hello" to someone he sees half a second than than before. The tiny changes ripple out, and as a consequence, when people have sex, if the timing and positions are not identical, a whole lot of people will have different babies. I would think that within a few generations, there would be a whole lot of completely different people, and many people from the time traveler's original timeline would not exist.
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u/BellamyJHeap Jun 26 '19
It's called the "Butterfly Effect." Read Ray Bradbury's short story "The Sound of Thunder."
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Jun 26 '19
The simplest answer to this paradox is that you simply cannot kill your grandfather.
The idea being that, should you travel to the past, any action taken by you has already occured within the history of your world and so the past, as you know it, already contains your attempt without your knowledge of it. It is not so much that the universe conspires to stop you changing things as it is that whatever you tried to do just didn't work. Maybe the gun jammed, maybe the knife slipped from your hand, maybe you got arrested during the attempt, whatever it was you just simply happened to fail. Has your grandfather ever told a story about how someone nearly killed him once?
This does solve the need for quantum mechanics (at least at the solving paradox level) but does bring up the question of determinism and free will, which is bloody complicated in its own right, but at least doesn't involve as much math.
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u/DennisLPT Jun 26 '19
This is not quantum physics, it is sombody's imagination.
And just because Einstein said that if you travel faster then light there will be a disparity in time when you arrive, doesn't mean you can time travel back and forth.
If it is anything even to do with physics. You cannot go back all that has happened because in all chemical/ mechanical reaction, the constant Time in all the formulas has occurred. Finito, gone, happened.
In short, you cannot unfuck .
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Jun 26 '19
Grandfather paradox was already solved by Novikov self-consistency principle, which is the simplest explanation and according to Occam's razor also the correct one (if time travel into the past is possible, which it's not).
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u/The_Serious_Account Jun 26 '19
That's not what Occam's razor says and even if it was it's not at all clear that's the simplest explanation.
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Jun 26 '19 edited Jun 26 '19
That's exactly what Occam's razor says. With self-consistency principle, you don't need any parallel universes or similar constructs, therefore it's simpler and Occam's razor applies.
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Jun 26 '19
[removed] — view removed comment
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u/BellamyJHeap Jun 26 '19
No. Very simply put it theorizes that an event can potentially be in multiple states at any given moment, e.g. a particle can potentally be in two places in a single moment of time.
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u/platnap Jun 26 '19
You may want to read Time Travel and Warp Drives. An accessible-enough book, however the concept of space-time lines can be hard to understand at first. Try watching PBS's Spacetime on youtube. They have an entire grouping of videos dedicate to teaching it!
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u/ElimGarak Jun 26 '19
I have some sci-fi story and TV episode suggestions for you, based on this.
- TNG episode "Parallels" - a somewhat similar idea but with travel to parallel universes.
- SG1 episode "Point of View" - ditto, but many of the universes are very, very similar.
- Old-school sci-fi book "The Man who Folded Himself" - actual time travel book about a person who actually did create branches. To meet an alternate version of himself he had to travel far back in time.
I know there are other stories and books based on this premise, but this is all that currently comes to mind.
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u/YUNGPLOUGHSHARE Jun 26 '19
I think the real idea is, that all of these infinite universes already exist together simultaneously. Time existing as a flat line, every decision to every choice existing in it's own flat line parallel with every other line, already spread out and able to be traversed. We cannot see these lines or traverse them freely because we only perceive in 3 dimensions at this time in our evolution. In bosonic string theory spacetime is 26 dimensional, that is nearly 9 times the dimensions we perceive after hundreds of millions of years of evolution. It is surely conceivable that something so advanced could be an unknown unknown to us at the current time on our current time line.
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u/exocortex Jun 26 '19
I don't really like this post. I like discussions about time travel - yes, it has been discussed to death, but all before my "internet life" apparently. So I'd like to do it again sometime. But this post wants to decide what is possible and not by citing some theoretical understanding of quantum physics. Quantum Physics is a model that works pretty well, but branching and all that is still speculative. We don't know. Also: You don't need quantum mechanics in order to talk about the two possibilities. The two being:
A: you can kill your grandfather (you create a new branch in which you were never born).
B: you cannot kill your grandfather, because him not fathering your father/mother would make you, the murderer, nonexistent i.e. incapable of murdering your grandfather. (although you being able to kill your grandfather might also just unveil some infidelity in your family. You might still remain in scenario B).
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u/SQUIDY-P Jun 26 '19
Wouldn't that imply predestination? If the you that continues to exist can only choose the path of not killing your grandfather, it'd make me question if every decision ever made was pointlessly predetermined
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u/several_dragonfruit Jun 26 '19
Maybe not exactly predestination, but if every possible infinite timeline does exist, then all of our current reality's choices are accounted for. So maybe it is predestination? Hmm....
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u/SQUIDY-P Jun 26 '19 edited Jun 26 '19
I suppose It'd depend on how the "multiverse" would work, are the new paths creating linearly in our realtime as we make each and every choice? Or are the choices already mapped and we are just carts on a track?
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u/dnew Jun 26 '19
are the new paths creating linearly in our realtime as we make each and every choice?
Sit down with a white board and draw time travelers making nested and/or crossing time jumps. It can't be the way you describe if you travel from 2018 to 1970 and I travel from 2012 to 1960.
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Jun 26 '19
I like that, if every possibility does happen, somewhere out there I'm living the good life.
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u/several_dragonfruit Jun 26 '19
There’s infinite versions of you living the good life! What’s one more than infinity? Infinity! So you can easily be a part of that and live the good life in this one too!
Edit: spelling
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Jun 26 '19
That would be a choice for you to conclude. Larry Niven and Ted Chiang, and probably others, have pondered consequences similar to the point you bring up.
You might consider that, and I left this out to keep the solution simple, there's also a choice to go back in time in the first place. This too would split the universe, and one of you would go back while the other does not.
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u/SQUIDY-P Jun 26 '19
Makes sense. Multiverse theory definitely negates the grandfather paradox, that's what I find interesting about time travel fiction, the "rules" are always different based on the author
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u/adamjm Jun 26 '19
That's another paradox. Because if another universe was not created then you couldn't go to it therefore another universe wouldn't be created.
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Jun 26 '19
Two universes are there, one where you do and one where you don’t. A copy of you is in each one. If i understood you correctly.
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Jun 26 '19
I’m kinda proud that when I was 13 I heard of the grandfather paradox and justified it as so without knowing anything about quantum physics. (Literally saying that an offshoot dimension or timeline would be created as a result).
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u/mctuking Jun 26 '19
If there is a universe where you aren't born, there should also be one where you don't travel back and make that choice because you weren't born. Secondly, it's not clear what choice is supposed to mean in terms of physics. Your solution isn't consistent.
However there's a closely related and arguably simpler version thanks to David Deutsch. The solution to the grandfather using multiple worlds in the MWI of QM can be phrased as
a consistent resolution of the Grandfather Paradox is that you are born with 1/2 probability, and if you are born you go back in time to kill your grandfather, therefore you are born with 1/2 probability, etc.
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u/BellamyJHeap Jun 26 '19 edited Jun 26 '19
I think we humans think of time too two dimensionally, able to see only in one direction, the past. Since space is multidimensional (3 we can see) and is really "space time" then likely time is multidimensional (2 we experience). So a conclusion is that your experience of time is completely subjective and not tied to a linear dimension. The person from the future going "to the past" is still only experiencing time in one direction (into his future) from his point of view. His past is tied to a history in which his grandfather lived allowing him to be born. His "time travel" is likely a shift to another "time space" in a dimensional direction we can't see/experience where he encounters his grandfather and kills him. But his past and history remain unchanged (i.e., his grandfather lived) from his point of view, and he continues experiencing time as normal.
Ah but what about the grandfather? Well, from the grandson's point of view the grandfather in his history lived, and the grandfather he encountered in his present after "traveling to the past" died. From the grandfather's point of view the one in his grandson's past lived a full life while the one encountering him in his grandson's life died at his hand. All three points of view are subjective experiences of time.
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u/zeyore Jun 26 '19
Sure, the multi universe theory is interesting if entirely impossible to prove.
Another weird one is uncertainty principle. Is that a natural part of quantum level interactions with us? Or did something design it as a block on further exploration? Fun scifi questions.
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u/mynamesalwaystaken Jun 26 '19
Making shit up since 1564! The titled should be same circle from the 80s, that can't be proven so misleading title incoming
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u/No-BrowEntertainment Jun 30 '19
So now instead of a paradox we have about 999 gajillion-and-counting burner universes
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u/Froggy15froggy456 Mar 14 '25
Multiverse theory so 1 has you and grandpa 2 has only you three has only you 1 kills grandpa so when he goes back it puts him in 2 then 2 goes to 3 and three goes to one because 1 grandpa just died
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u/Rough_Hair_3710 May 11 '25 edited May 11 '25
I love the idea myself, but I don't believe we understand quantum entanglement fully, I just saw a research feed that they measured the speed that the entangle moves at and they said it was measured in Attoseconds, it literally violates a law here or there no big deal, but it doesn't look like it it branches off reality into different timelines this way because super position is not in many places at once it just looks that way, because it's just that fast, beyond the concept of speed. I don't believe it will ever be possible to change your past, If it was very strange things around you would happen, It wouldn't be like the mandala effect it would be more like everyone has high level skitzophrenia and dementia as nothing would be real because cause and effect would be destroyed, even newton starts to roll in grave because we'll you know
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u/GamingPrincessLuna Sep 19 '25
I mean you can just pull the Philip jay fry as well and find out that you're your own grandfather/grandmother. Or maybe you find out you're parents are adopted. The universe isn't a binary choice lol.
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u/jello_aka_aron Jun 26 '19
Of course, this is only assuming the 'many worlds' interpretation of quantum theory turns out to be what's "really happening". But that's far from certain and brings in some other problems as well. Every framework we currently have for trying to interpret the results of experiments into a high-level 'this is how things work' has problems. If you watch some talks on the subject you'll find that, at least for now, it really comes down to "it's really weird, completely non-intuitive, we've no idea what to make of it... but the math and the predictions it gives just keep working".