> Researchers ran the numbers and determined that even if you made a change in the past, the timeline would essentially self-correct, ensuring that whatever happened to send you back in time would still happen.
I'm sure these researchers are pretty smart, but nothing in this article goes beyond the level of a grade-school debate. Maybe there's something enlightening in the paper. But if there was, you'd think it would make its way into the article.
Think what they're getting at is that it won't happen that way. Gun will jam, you die of a brain aneurysm. We don't exist in a universe where impossible things happen, so we can't bring them about. But yes, the article doesn't really sink its teeth into it.
It also pre-supposes that you aimed to kill your grandfather. Perhaps you accidently did this. The reason for you going back in time still exists.
However, I believe the worst that could come of this would be to split the timeline in the case that string theory and multiverses exist where the timeline we return to is one that is different than the one we came from.
What if we really are living in a simulation?
Could it be possible that any moment we arrive in the past would possibly have a random outcome on the future? What if every choice from the moment you arrive to the moment you left could have a random effect, and thus the world you arrive back at if it is a closed loop could be different as night and day, simply because all events that happened after your arrival were erased and all choices were given a second chance to arrange themselves like coin tosses mixed with end-user free agency.
If that were the case, how would you prove that it's a randomized closed loop vs string theory since to you the time-traveler, everything has changed which the easiest reason would be you're in a different timeline. It never dawned on you that events along the timeline could be shuffled at anytime and are inconsequential to how the universe works because we're actually in a simulation.
Given that this is NPR, a reference to Gell-Mann amnesia[0] seems appropriate:
“Briefly stated, the Gell-Mann Amnesia effect is as follows. You open the newspaper to an article on some subject you know well. In Murray’s case, physics. In mine, show business. You read the article and see the journalist has absolutely no understanding of either the facts or the issues. Often, the article is so wrong it actually presents the story backward—reversing cause and effect. I call these the “wet streets cause rain” stories. Paper’s full of them.
In any case, you read with exasperation or amusement the multiple errors in a story, and then turn the page to national or international affairs, and read as if the rest of the newspaper was somehow more accurate about Palestine than the baloney you just read. You turn the page, and forget what you know.”
I didn’t know this had a name, but it’s long been a cause of amusement (or despair!) for me. I too have noticed that any time I read an article on a subject on which I have even a passable level of expertise, I find it chock-full of errors.
I take it with a grain of quantum physics. When traveling back in time you are entangling the state of the moment of time travel with a past state of the universe, and the universe must collapse into a steady state with a single history... The history in which the time travel did nothing.
The theory of general relativity predicts the existence of closed time-like curves (CTCs), which theoretically would allow an observer to travel back in time and interact with their past self. This raises the question of whether this could create a grandfather paradox, in which the observer interacts in such a way to prevent their own time travel. Previous research has proposed a framework for deterministic, reversible, dynamics compatible with non-trivial time travel, where observers in distinct regions of spacetime can perform arbitrary local operations with no contradiction arising. However, only scenarios with up to three regions have been fully characterised, revealing only one type of process where the observers can verify to both be in the past and future of each other. Here we extend this characterisation to an arbitrary number of regions and find that there exist several inequivalent processes that can only arise due to non-trivial time travel. This supports the view that complex dynamics is possible in the presence of CTCs, compatible with free choice of local operations and free of inconsistencies.
FWIW, the last sentence of the paper itself (prior to the conclusion) reads: It is an open question how generic is this situation and what are the spacetimes and physical systems for which nontrivial, self-consistent time travel is possible.
As usual, non-scientific reporting overplays the ramifications.
The time-travel paradox arises only if you presuppose the existence of free will: if we don't, and just consider human beings as physical objects acting deterministically, then there can be no paradox: the time traveler will simply do what was already had been done in his past.
Ah yes, can you change the past or was there a time traveler version of you running around failing to change things the whole time? See season 5 of Lost for endless discussion on this point. You also can avoid paradoxes if it turns out that time travel just puts you on an alternate timeline.
There's a different paradox with this version of time travel though. Suppose I'm a kid and future-Me comes back and tells me a story. "you will write down this story and publish it and it will be famous." I do as he asks and he's absolutely right, leading me to eventually return the favor to my past self in a time loop.
It may be that, just as it's impossible for you to be killed by yourself from the future in this type of universe, it's also impossible to create information out of nothing, so the specific scenario you described would just be impossible.
This could be a kind of conservation law - imagine that instead of a story you had a golden coin, that you receive from your future self, keep, and give back to your past self after you travel, you've added some mass to the universe, which would be impossible because of conservation of energy.
This explains absolutely nothing. “Will simply do what was already done in his past” is the non-sequitur: if they shoot themselves in the past then that was what was done in the past. Sounds paradoxical? Because it is, and determinism can’t stop a human from shooting a gun any more than free will can because this has got nothing to do with volition.
What people mean by "time travel" isnt time-reversal. It's taking information from the future and injecting it into the past by moving the atoms of your now-body into the past.
This isn't "time travel" in any sense that physics uses the term. And is certainly impossible on very many grounds.
Physics has nothing to say on "time travel" in the popular imagination.
> In other words, a time traveler could make changes, but the original outcome would still find a way to happen — maybe not the same way it happened in the first timeline but close enough so that the time traveler would still exist and would still be motivated to go back in time.
> "No matter what you did, the salient events would just recalibrate around you," Tobar said.
I'd love to see how this works when you go back in time and kill your toddler self.
> "The universe is deterministic and attempts to alter Past Event X are destined to be the forces which bring Past Event X into being,"
But WHICH events? If things change around you when you change the past, then by definition some events will have happened, and others not (otherwise NOTHING would change, including your arrival in the past, and therefore time travel cannot occur). After all, a single breath you took at a specific point in time is an event. Waking up 2 seconds later is an event. Using your left foot first as you walked out of the house that day instead of the right is an event. So who decides which events are "important" enough that "forces bring Past Event X into being"? You'd need some kind of omniscient, omnipotent intelligence in order to accomplish that.
The very logic behind this idea is a contradiction.
Nothing changes in the past, including your arrival, because your arrival would have always happened. The past stays consistent because it was already determined to have happened.
How this would work going back in time to kill your toddler self is that, well, you don't. Gun jams, you have an aneurysm before you can, or maybe you killed the wrong toddler and there was just always a toddler killed in the past.
The fact that you are alive to go back in time means you were not killed in toddler form.
>I'd love to see how this works when you go back in time and kill your toddler self.
Logically, this would be consistent, as you are no longer present in the future, because you have travelled to the past. What if killing the toddler means you can't travel back to your understood future?
Your 'gotcha' doesn't really disprove anything, beyond raising another question which we can't conceivably answer.
I watch a lot of both American-made movies and TV shows as well as a lot of Japanese anime.
One thing I find curious is how different the models for time travel usually are.
In American media, it seems more often than not, time travel is modelled as if the past and the future are simultaneously existing parallel worlds, where the future is affected by the past in "real-time", e.g. Back to the Future's Marty gradually fading as the chances of his birth diminish, or Timeless's people in the future "watching" people arrive in the past and making sure their own team leaves "in time" to catch them.
These models don't really offer a solution to the grandfather paradox.
In anime however, time travel is near universally modelled with timelines, where time travel essentially creates a new parallel world each time. If you travel back in time and kill your grandfather, you simply create a timeline in which you were not born, but can continue to exist, because you are from a timeline where you were born.
I'm by no means suggesting that either model is unique to America/Japan (Rick and Morty for example uses the branching timeline model), I just find it interesting how they differ.
I think it depends on the show in anime. SPOILERS FOLLOW. Erased is not a parallel world, but someone trying to change their past. Madoka finds Homura repeatedly doing whatever is possible to save Madoka. Steins:Gate is timelines and parallel worlds (think of how many times Okabe again and again tries to save Mayuri). Girl who Leapt Through Time is the Groundhog Day story, same day over and again; bit like Tatami Galaxy. No idea where something like Haruhi Suzumiya might fall where time is played with in many different ways.
Meanwhile Evangelion (the later films) and Attack on Titan both seem to be veering towards some kind of inability to stop the same grand narrative cycle repeating again and again...
It also depends on the show in American media. Star Trek, for example, has nearly every version of time travel at some point or another - except the "simultaneous worlds" version described in GP with Back to the Future and Timeless.
Are they, though? I thought the Novikov conjecture postulated that time-travel would change nothing at all: if you travelled back to the past, you had "always" travelled back to the past, and anything you did there was already part of history before you started on your journey (aargh, time-travel tenses are hard). Michael Moorcock's Behold the Man[0] would be a good fictional example.
This, on the other hand, suggests you can change the fine detail of history (e.g. the identity of patient zero) but not its broad strokes (the occurrence of a global pandemic).
At least, that's what I gather from the article: I haven't read the paper, and suspect it would be a waste of time as my maths and physics are both rudimentary.
If there are such physical laws to allow details to change while broad strokes must happen, I'd be curious what exactly differentiates a detail from a broad stroke. But all the same, this idea imagines a more timey-wimey view of history that doesn't have just one state. This is an idea explored heavily in Neal Stephenson's "The Rise and Fall of D.O.D.O"
> I thought the Novikov conjecture postulated that time-travel would change nothing at all
Not exactly, from the wiki article:
> The simplest way to impose the principle of self-consistency in quantum mechanics (in a classical space-time) is by a sum-over-histories formulation in which one includes all those, and only those, histories that are self-consistent.
So it is possible to change the past, but only in ways that are self-consistent and don't give rise to paradoxes.
Regarding the ramifications for free will:
> Novikov supports this point of view with another argument: physics already restricts your free will every day. You may will yourself to fly or to walk through a concrete wall, but gravity and condensed-matter physics dictate that you cannot. Why, Novikov asks, is the consistency restriction placed on a time traveler any different?
Im reading "The Time Patrol" by Paul Anderson. It describes that even if an individual is killed, it's unlikely to matter much because a species is more like a collective DNA wise. They even go hunting in the past, have possible children there, etc. It's only in very specific cases that the time patrol has to step in to make sure that "X" specific significant event happens the way it's "Supposed to"
This suggests concepts like fate and the universe having a will and intent. It seems absurd. Wouldn’t a branching creating a new universe with its own timeline make more sense? Each unit of influence you exert as a time traveler branches a new universe in which you continue to exist and which has a completely new and separate timeline without paradoxes.
Neither possibility really makes more "sense" than the other. And it's effectively impossible to prove experimentally -- in a "many branching worlds" universe, you could never conclusively prove or disprove its existence, even if you experimentally demonstrated this kind of "self correcting" time travel, as you may have simply created a branching universe where it appears to you that you did so.
It's metaphysical philosophizing as far as I'm concerned -- interesting, but not terribly applicable, and heavily influenced by one's own beliefs and preexisting notions about the world.
The article makes it seem like "the timeline corrects itself." What the scientific paper actually says (as best I can understand it) is that, if the universe is entirely deterministic, the conditions necessary for time travel can only occur when the time traveling wouldn't cause a paradox. As the authors note, this isn't super relevant to our universe because:
- the conditions necessary for time travel probably don't exist
- the universe probably isn't totally deterministic
> What the scientific paper actually says (as best I can understand it) is that, if the universe is entirely deterministic, the conditions necessary for time travel can only occur when the time traveling wouldn't cause a paradox.
It's actually even more general than that: under non-deterministic laws (such as the way most people currently understand quantum mechanics), if time travel is possible at all, the probability of it will only be nonzero for time travel scenarios that do not cause a paradox. (I don't think this is discussed in the paper referenced in the article, but it is discussed in some of the other papers that that paper references--IIRC the Novikov paper is one of them.)
Readit News
I'm sure these researchers are pretty smart, but nothing in this article goes beyond the level of a grade-school debate. Maybe there's something enlightening in the paper. But if there was, you'd think it would make its way into the article.
However, I believe the worst that could come of this would be to split the timeline in the case that string theory and multiverses exist where the timeline we return to is one that is different than the one we came from.
What if we really are living in a simulation?
Could it be possible that any moment we arrive in the past would possibly have a random outcome on the future? What if every choice from the moment you arrive to the moment you left could have a random effect, and thus the world you arrive back at if it is a closed loop could be different as night and day, simply because all events that happened after your arrival were erased and all choices were given a second chance to arrange themselves like coin tosses mixed with end-user free agency.
If that were the case, how would you prove that it's a randomized closed loop vs string theory since to you the time-traveler, everything has changed which the easiest reason would be you're in a different timeline. It never dawned on you that events along the timeline could be shuffled at anytime and are inconsequential to how the universe works because we're actually in a simulation.
“Briefly stated, the Gell-Mann Amnesia effect is as follows. You open the newspaper to an article on some subject you know well. In Murray’s case, physics. In mine, show business. You read the article and see the journalist has absolutely no understanding of either the facts or the issues. Often, the article is so wrong it actually presents the story backward—reversing cause and effect. I call these the “wet streets cause rain” stories. Paper’s full of them.
In any case, you read with exasperation or amusement the multiple errors in a story, and then turn the page to national or international affairs, and read as if the rest of the newspaper was somehow more accurate about Palestine than the baloney you just read. You turn the page, and forget what you know.”
[0]: https://www.epsilontheory.com/gell-mann-amnesia/
Deleted Comment
The theory of general relativity predicts the existence of closed time-like curves (CTCs), which theoretically would allow an observer to travel back in time and interact with their past self. This raises the question of whether this could create a grandfather paradox, in which the observer interacts in such a way to prevent their own time travel. Previous research has proposed a framework for deterministic, reversible, dynamics compatible with non-trivial time travel, where observers in distinct regions of spacetime can perform arbitrary local operations with no contradiction arising. However, only scenarios with up to three regions have been fully characterised, revealing only one type of process where the observers can verify to both be in the past and future of each other. Here we extend this characterisation to an arbitrary number of regions and find that there exist several inequivalent processes that can only arise due to non-trivial time travel. This supports the view that complex dynamics is possible in the presence of CTCs, compatible with free choice of local operations and free of inconsistencies.
FWIW, the last sentence of the paper itself (prior to the conclusion) reads: It is an open question how generic is this situation and what are the spacetimes and physical systems for which nontrivial, self-consistent time travel is possible.
As usual, non-scientific reporting overplays the ramifications.
Link to paper: https://iopscience.iop.org/article/10.1088/1361-6382/aba4bc
To bad no one went back in time to tell the authors whether they actually discovered anything or not.
There's a different paradox with this version of time travel though. Suppose I'm a kid and future-Me comes back and tells me a story. "you will write down this story and publish it and it will be famous." I do as he asks and he's absolutely right, leading me to eventually return the favor to my past self in a time loop.
Where did the story come from?
Where do c (the speed of light), G (Gravitational constant) or π comes from?
This could be a kind of conservation law - imagine that instead of a story you had a golden coin, that you receive from your future self, keep, and give back to your past self after you travel, you've added some mass to the universe, which would be impossible because of conservation of energy.
https://www.imdb.com/title/tt2397535/
https://www.frontiersin.org/articles/10.3389/fphy.2020.00139...
What people mean by "time travel" isnt time-reversal. It's taking information from the future and injecting it into the past by moving the atoms of your now-body into the past.
This isn't "time travel" in any sense that physics uses the term. And is certainly impossible on very many grounds.
Physics has nothing to say on "time travel" in the popular imagination.
> "No matter what you did, the salient events would just recalibrate around you," Tobar said.
I'd love to see how this works when you go back in time and kill your toddler self.
> "The universe is deterministic and attempts to alter Past Event X are destined to be the forces which bring Past Event X into being,"
But WHICH events? If things change around you when you change the past, then by definition some events will have happened, and others not (otherwise NOTHING would change, including your arrival in the past, and therefore time travel cannot occur). After all, a single breath you took at a specific point in time is an event. Waking up 2 seconds later is an event. Using your left foot first as you walked out of the house that day instead of the right is an event. So who decides which events are "important" enough that "forces bring Past Event X into being"? You'd need some kind of omniscient, omnipotent intelligence in order to accomplish that.
The very logic behind this idea is a contradiction.
How this would work going back in time to kill your toddler self is that, well, you don't. Gun jams, you have an aneurysm before you can, or maybe you killed the wrong toddler and there was just always a toddler killed in the past.
The fact that you are alive to go back in time means you were not killed in toddler form.
Logically, this would be consistent, as you are no longer present in the future, because you have travelled to the past. What if killing the toddler means you can't travel back to your understood future?
Your 'gotcha' doesn't really disprove anything, beyond raising another question which we can't conceivably answer.
How would killing a toddler prevent me from time travelling back? Killing someone will not affect the functionality of my time machine.
One thing I find curious is how different the models for time travel usually are.
In American media, it seems more often than not, time travel is modelled as if the past and the future are simultaneously existing parallel worlds, where the future is affected by the past in "real-time", e.g. Back to the Future's Marty gradually fading as the chances of his birth diminish, or Timeless's people in the future "watching" people arrive in the past and making sure their own team leaves "in time" to catch them.
These models don't really offer a solution to the grandfather paradox.
In anime however, time travel is near universally modelled with timelines, where time travel essentially creates a new parallel world each time. If you travel back in time and kill your grandfather, you simply create a timeline in which you were not born, but can continue to exist, because you are from a timeline where you were born.
I'm by no means suggesting that either model is unique to America/Japan (Rick and Morty for example uses the branching timeline model), I just find it interesting how they differ.
Meanwhile Evangelion (the later films) and Attack on Titan both seem to be veering towards some kind of inability to stop the same grand narrative cycle repeating again and again...
Dead Comment
[1] https://en.wikipedia.org/wiki/Novikov_self-consistency_princ...
This, on the other hand, suggests you can change the fine detail of history (e.g. the identity of patient zero) but not its broad strokes (the occurrence of a global pandemic).
At least, that's what I gather from the article: I haven't read the paper, and suspect it would be a waste of time as my maths and physics are both rudimentary.
[0] https://en.wikipedia.org/wiki/Behold_the_Man_(novel)
Not exactly, from the wiki article:
> The simplest way to impose the principle of self-consistency in quantum mechanics (in a classical space-time) is by a sum-over-histories formulation in which one includes all those, and only those, histories that are self-consistent.
So it is possible to change the past, but only in ways that are self-consistent and don't give rise to paradoxes.
Regarding the ramifications for free will:
> Novikov supports this point of view with another argument: physics already restricts your free will every day. You may will yourself to fly or to walk through a concrete wall, but gravity and condensed-matter physics dictate that you cannot. Why, Novikov asks, is the consistency restriction placed on a time traveler any different?
I haven't finished the book yet though :)
It's metaphysical philosophizing as far as I'm concerned -- interesting, but not terribly applicable, and heavily influenced by one's own beliefs and preexisting notions about the world.
- the conditions necessary for time travel probably don't exist
- the universe probably isn't totally deterministic
It's actually even more general than that: under non-deterministic laws (such as the way most people currently understand quantum mechanics), if time travel is possible at all, the probability of it will only be nonzero for time travel scenarios that do not cause a paradox. (I don't think this is discussed in the paper referenced in the article, but it is discussed in some of the other papers that that paper references--IIRC the Novikov paper is one of them.)