Paradox-free time travel is theoretically possible
npr.org
npr.org
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.
This concept is narritively explored in Stephen King's "11/22/63" if anyone finds this interesting.
The novel is an entertaining read and the audiobook available on the Libby app (requires a local library card here in the US) is high quality.
The only explanation I can think of for why we haven't seen any, if we live in such a universe, is that we need to build an "arrivals hall" first, and that as soon we manage to do that, people from all over start popping in for a visit.
A gun merely jamming solves nothing.
Either the time traveler was there "the first time around" or he must be stopped from going back somehow. Because the first and the second time around must be exactly identical.
My solution to "where are all the time travelers" is that we are unable to correct for motion, we send people in the past but they end up million of kilometers away in space.
So the past is perfectly accessible, but we can't get to any useful spacetime coordinate.
It's a fun idea though, thanks :)
In all of the paradox-forbidding time travel scenarios, nobody has ever mentioned anything deeper than "Well, uh, reality just won't let that happen." If I right now posit that each time travel scenario spawns an intelligent angel whose sole job it is to dash about a closed timelike curve invisibly nudging away paradoxes through minuscule miracles of improbability, that's quite literally more than you'll see written about in these situations.
You might expect some similarities on a macro scale regardless: perhaps if you kill baby Hitler, Germany can still be expected to end up with a different right wing leader who invades Poland, perhaps even assisted by people who were prominent Nazis in our time, and maybe even loses to the same allies for the same reasons. But on a micro level the differences are huge: the dates will be different, people dying will be different, and the rest of the twentieth century will be different. A century of different atoms bumping into different atoms, different sperm are getting to the front of different queues, and different people being influenced into different decisions results in a different world. Our would-be time traveller isn't even born, never mind motivated to look for Ms Schiklgruber's illegitimate child.
One option: We tend to see time as mutable, but it's not clear that time needs to be mutable. Imagine time as a window into an immutable graph of deterministic state transitions, for example. You could come up with such a model where loops "back in time" were possible, but where the model simply can't express situations that leads to paradoxes, and where we by extension can't exist in a universe where people can ever take steps that would lead to a paradox.
Of course it (seems to) follow that you don't have any free will. But there's nothing in science or philosophy contradicting that (if anything we seem to find more and more evidence to support that idea).
E.g, a possible example immutable scenario that doesn't violate anything (with each number representing a discrete period of time)
1. An old man appears and says hi to me and disappear.
2. 20 years pass.
3. I obtain a time machine.
4. I travel back in time and return (in my absence from this time period, I'm at time step 1 saying hi to myself)
1a. Nothing happens.
1b.An old man appears and says hi to me and disappear.
2. 20 years pass.
3. I obtain a time machine.
4. I travel back in time and return (in my absence from this time period, I'm at time step 1b saying hi to myself).
And that removes e.g. the grandfather paradox, because a static, immutable representation of it that is consistent with our other rules (not having people spontaneously come into existence, for example) isn't possible.
1. Time machine backwards
2. Shoot themselves
3. Time machine forwards
And that’s how they would have experienced it. The whole premise seems to be that this is allowed:
1. Time machine backwards
2. (do something)
3. Time machine forwards
Of course step 2 always happened since it’s the past. And yet they shot themselves. But they couldn’t shoot themself and exist now as well. And that’s the paradox.
Two choices at this point:
1. You can’t really do anything in step 2. In which case time travel is impotent, and hence why even dignify calling it time travel at all?
2. You have some potency in step 2, but you can’t do things that lead to paradoxes. But then how would you handle those special cases?
Just stating that “the past is fixed” doesn’t solve the paradox, it restates one of the premises.
> The whole premise seems to be that this is allowed:
No. That was not the premise. The premise was an immutable representation of self-consistent state transitions, so what occurs in step 2 will always have occurred at that point in time, because otherwise the system isn't immutable. Step 1. just links step two into the causal chain of events in a way necessary to make the existence of 2. consistent.
Again: Think of a graph. An immutable graph. The flow of time in this model simply follows the graph. It doesn't do anything at all, it just replays the events.
The problem here is thinking about this as if there are choices being made as time passes.
> Just stating that “the past is fixed” doesn’t solve the paradox, it restates one of the premises.
It's not stating the past is fixed. It's stating everything is fixed.
And whether I “get it” or not is secondary to the point of whether you have truly managed to confirm or disconfirm the time travel paradox, which I still claim you have failed to do. It is simple to wave away a time travel where you do something compeletely benign, like frickin’ wave or greet your former self. Try dealing with something of consequence, like killing your former self (as in: the person in that point of time-space which carries the mutable signifier of “yourself” which is younger than yourself) or giving yourself a kilogram of gold from the future, which I think would violate the law of preservation of energy or whatever.
Is time travel backwarks possible or not? If it is, see the questions in my previous comment.
> No. That was not the premise.
And yet if time travel backwards is possible you simply cannot deny that one would experience (!) that time travel as steps 1–3. Temporal experience. Hah!
There’s absolutely nothing hard about the concept of time being immutable; that’s how children’s cartoon handle time travel, not as two timelines where the first one gets erased right after the second one “happens”. (Okay, okay, they probably use both of these tactics.) The true puzzle is time travel which introduce paradoxes, which your original comment says absolutely nothing about. But maybe you’ve answered that in another comment (I have only kept up with this subthread). If not, you should probably try to do that instead of dewelling on this minor philosophical insight of yours.
You can make up impossible scenarios without time travel too. Like someone that shoots themselves (lethally, thorough and immediate) and then a month later they go out to lunch and eat a ham sandwich.
The ability to write out a scenario doesn't mean the scenario is possible.
The theory is that "I go back in time and kill myself" is just as impossible as "I kill myself then a month later eat a ham sandwich". And for largely the same reason: both scenarios violate causality, so the universe can't be shaped that way.
A paradox is when two contradictory things are true at the same time.
The grandfather problem is a paradox if time travel is possible. If it isn’t possible then it is simply an impossibility.
> And for largely the same reason: both scenarios violate causality, so the universe can't be shaped that way.
So you agree that time travel is impossible. Great... we agree. However I was discussing this with someone who apparently thinks that it is concievable... I wasn’t merely making up impossible scenarios; I was demonstrating how their theory gives rise to paradoxical scenarios.
> The grandfather problem is a paradox if time travel is possible. If it isn’t possible then it is simply an impossibility.
It's only a paradox if you're making certain assumptions about how time travel works.
You need to admit that these are assumptions.
Consider the airplane on a treadmill paradox. There is a hidden assumption that it's possible to accelerate the treadmill in isolation to match the wheel speed, but the logic fails because the speed of the treadmill is entwined with the speed of the wheels on the plane.
> So you agree that time travel is impossible. Great... we agree.
No, that's not what I said.
> I was demonstrating how their theory gives rise to paradoxical scenarios.
It doesn't.
You're assuming you can choose your actions independently from the time travel. The theory says that every choice is entwined with the fabric of the universe, so that by the time you see that an instance of time travel exists, the critical non-paradox-causing choices are already set in stone.
No. A paradox is when two contradictory things appears to be true at the same time. It simply signifies that our model is flawed or incomplete.
The grandfather problem is a paradox if our model of time travel is one which is impossible. That is the point. It is simply a way of demonstrating why there needs to be constraints that makes our more naive notions of time travel impossible. As such it constrains the set of possible valid models to those in which the grandfather paradox can't be expressed within the constraints of the model.
> I wasn’t merely making up impossible scenarios; I was demonstrating how their theory gives rise to paradoxical scenarios.
You've done no such thing. The paradoxical scenario you imagine can't be expressed in the model I proposed, and so it solves the paradox.
1. You start your day normally
2. You meet yourself from the future and discuss some niceties.
3. Many years later, you enter a time machine, and you arrive back at time 2, where you have that conversation with yourself that you remember.
The past has already happened, so it can't change. So, if you haven't met a time traveler so far, that is proof that no one will ever time travel to the moment and place where you just were, even if time travel becomes possible in the future.
Now, you may ask, 'what happens if at step 3 I decide to shoot myself, or not have that conversation?'. Here the answer is simple : that question makes no sense. You have already done our not fine these things, there is no power of decision that could influence this. It's like asking "what if I now decide that yesterday I didn't go to the store". The fact that the event at time 2 happened is fixed both in the past and the future. It was in fact fixed at the time of the universe's creation.
Now, you may not accept determinism, or you may believe in free will, and there are good reasons for both (quantum indeterminism, subjective experience, etc.). In either case, I don't think you can believe in time travel, since all of the paradoxes reappear. But if you believe the past and the future are already set in stone, there is no paradox if the two cross each other in certain ways. But it doesn't make sense about what you 'will decide to do', because this is as fixed as what you ate yesterday.
Again, it's just like asking "what if I try to not go to the shop yesterday, what would happen to the bread I'm eating?". Can you decide today to not do something yesterday? No. In the same way, a time traveler can't "choose" to modify the past - it's something that's already happened.
Of course, this does imply that the future is also set in stone, so in fact you can't choose to go the shop or not tomorrow any more than you can choose to have done so yesterday. This is not a paradox, it's just counter to our subjective experience.
The way this particular idea of time travel works is like this:
1. A time traveler throws a grenade, killing Joan of Arc.
2. History happens.
3. The time traveler who killed Joan of Arc is born.
4. The time traveler goes back in time and kills Joan of Arc.
5. We hear of this event.
If we are looking at this when at time 5, do we find it surprising that it happened? Do we need any explanation? We can ask "what would have happened if the time traveler had instead killed Chairman Mao?", and we can get into a discussion on the possible future life of Joan of Arc etc. But since the event has already happened, we don't need a fundamental explanation for "why didn't it go differently", "what stopped the time traveler from doing differently" etc.
If you then accept that the future is just like the past - it is already decided - then the same banality happens if we learn of this event at time 3. We can ask "I wonder what would happen if this child wouldn't one day kill Joan of Arc", but it is simple musing about alternative histories, like asking "I wonder what would have happened if Hitler hadn't killed himself". The fact is that Hitler did kill himself, and that the child will kill Joan of Arc.
If you imagine the future as changeable, or if you imagine that beings can choose what they will do, then the paradox arises. But if you consider the future as set in stone, and if you consider that humans can't chose to do one way or another, than there is no paradox: history COULD have been different, but it just isn't. The same thing that stops you from changing the past also stops you from changing the future.
Note: I am not claiming that this view is "correct". Just that, if you accept it, than time travel really doesn't give rise to any paradox.
Just as much, if you have a time machine, and go to the past, you will do exactly what you had already done in the past. It is not something you have a choice on, it is simply a consequence of the universe so far. The same would be true if you went to the future - all your future actions were decided the moment you were born, the moment the Earth formed, the moment the universe was formed. There is no choice.
I myself can't entirely believe this notion of a fully deterministic universe. Especially one that doesn't even involve randomness (though randomness by itself wouldn't bring in any possibility of choice).
It is entirely irrelevant what it is. The point is that if the system is immutable it is impossible to represent a combination of state transitions that alters an existing state, as such the set of possible space-time combinations would in that case only include those where the time travel does not introduce a paradox that depends on change.
The same way the timeline does not include any states where I fold an elephant, because it's not possible to do so.
You have not presented any paradoxes that can work with an immutable space-time other than perhaps if said space-time introduces divergent timelines at any point of mutation, in which case the paradox can also easily be resolved.
> And yet if time travel backwards is possible you simply cannot deny that one would experience (!) that time travel as steps 1–3. Temporal experience. Hah!
You can experience it that way, but your "do something" step 2 would be deterministic and could not possibly include steps that introduced a paradox.
> There’s absolutely nothing hard about the concept of time being immutable;
You're still thinking in terms of a mutable space-time. I did not talk about time being immutable, but the entirety of space-time. Your "do something" step does nothing that changes anything in an immutable space-time, because everything is fixed from the start. Nothing moves. No action is taken. Everything is unalterable.
Time, and cause and effect, in that scenario is simply an illusion of consciousness at every point of a fixed 4d (or more) canvas that never changes in any way.
> The true puzzle is time travel which introduce paradoxes, which your original comment says absolutely nothing about.
My original comment gave immutability as an example of a possible solution. Time travel with paradoxes is not possible. That is the entire point of a paradox: The problem as stated reveals a problem with the presumed model. The solution is to identify and remove flaws in the model. Removing mutability is one such solution.
Alters? Aha! But you cannot alter that which is immutable! Seems that you still don’t get the concept.
See how silly it is to language police people on such subjets?
> would in that case [only include those] where the time travel does not introduce a paradox that depends on change.
Highlighted the problematic part with brackets. This is just hand-waving. “Only those”? Your theory would have to be able to explain concrete examples like the grandfather problem, or else it doesn’t rise to the level of being a theory that can explain the paradox.
> The same way the timeline does not include any states where I fold an elephant, because it's not possible to do so.
Irrelevant unless there is an actual elephant folding paradox that I’m missing. Impossible things can be explained; paradoxes cannot since they reveal a contradiction in the model.
> You have not presented any paradoxes that can work with an immutable space-time other than perhaps if said space-time introduces divergent timelines at any point of mutation, in which case the paradox can also easily be resolved.
This is the first time (?) you’ve mentioned divergent timelines or hinted at non-just-straight-line-time. And yes, that would probably be perfectly adequate, and is the best answer you’ve given thus far.
> You can experience it that way, but your "do something" step 2 would be deterministic and could not possibly include steps that introduced a paradox.
“Determinism” is either irrelevant or non-sufficient. “Could not possibly” is pure hand-waving as there is nothing which makes sure that you are able to do something benign (like greet your former self) but not do something like kill yourself.
> You're still thinking in terms of a mutable space-time. I did not talk about time being immutable,
Wow, I have to hand it to you. I did mention “space-time” once (although only once) and I utterly failed to be rigorous and spell out the whole package deal, namely space-time. What an utter embarrassment.
Not that it changes my point in any way.
> My original comment gave immutability as an example of a possible solution. Time travel with paradoxes is not possible.
You can’t resolve the Barber Paradox by simply saying that the Universe wouldn’t allow the barber to shave himself. That’s effectively what you’re doing here, only using vaguely hard science terms with a seasoning of “you still don’t get it”.
If that was what I was doing, yes. But it wasn't, and your example is nonsensical because you referenced a statement where I explicitly pointed out that you cannot alter that which is immutable.
> Highlighted the problematic part with brackets. This is just hand-waving. “Only those”? Your theory would have to be able to explain concrete examples like the grandfather problem, or else it doesn’t rise to the level of being a theory that can explain the paradox.
No, that is not a problematic part at all. The grandfather paradox is a statement of a problem. You can not solve it without specifying a set of alterations or constraints on the unconstrained model that creates the problem. That is the point: The existence of the paradox suggests the model it is posed within is not possible. The solution to a paradox is to resolve the flaw in the model, as you yourself point out next:
> Irrelevant unless there is an actual elephant folding paradox that I’m missing. Impossible things can be explained; paradoxes cannot since they reveal a contradiction in the model.
That the reveal a contradiction in the model by pointing out a means by which the understanding of a model would allow an impossible thing is in fact the point. The solution is to identify constraints that makes the impossible thing impossible to represent in a model. Just like we can point out ways in which folding an elephant is impossible.
> “Determinism” is either irrelevant or non-sufficient. “Could not possibly” is pure hand-waving as there is nothing which makes sure that you are able to do something benign (like greet your former self) but not do something like kill yourself.
Immutability means you can not DO anything at all. No change ever happens. The passage of time in such a model is an illusion. You're still hung up on treating this as a series of events that are happening rather than a replay of an immutable state from a set of such states that needs to be possible to represent.
What makes "sure that you are able to do something benign" is simply an inherent constraint of needing to be able to represent graph of the transitions in an immutable way consistent with what we otherwise think we know about cause and effect. There's no other mechanism needed. A whole lot of seemingly benign things would also be impossible to express because it would cause interactions we're unable to account for, but we would never see the effects of that because those things also could never be expressed and so has never happened. The same way we can't express the states required to allow me to fold an elephant within the constraints of the universe we inhabit.
> You can’t resolve the Barber Paradox by simply saying that the Universe wouldn’t allow the barber to shave himself. That’s effectively what you’re doing here, only using vaguely hard science terms with a seasoning of “you still don’t get it”.
The Barber Paradox is an ontological paradox. We can certainly turn it into a space-time paradox by positing that the events in it are actually played out, in which case it is simply a restatement of the grandfather paradox.
And resolving such a paradox by introducing a constraint that ensures the paradoxical state is impossible is exactly how to do it.
I've not done any handwaving at all - I've given a very simple hypothetical model, and explained how it resolves it by making the impossible states impossible to represent.
Let me try one more time. S(t) represents the state at a given time. Each state is immutable, has always existed and will always exist exactly as it is, entirely unchanging in every way. That makes the following set of state transition a representation of the hypothetical example:
S(1) -> S(2)
S(2) -> S(3)
S(3) -> S(4)
S(4) -> S(2)
S(4) -> S(5)
Now, to introduce the paradox as stated, S(2) must involve you being killed. But immutability requires that S(2) has and always will be the same. It does not change when the time travel "happens", so any effect of the time travel must be consistent with what happened before it, in both previous states (S(1) and S(4)) and the next state - S(3). The time travel alters perspective, it does not alter the model. So in drawing up what goes into each of these states, you have a choice: Either you're always dead when transitioning to S(3) or you were never dead when transitioning to S(3). So either no killing happens, or it always happened and always will. In the latter case, the killing can not happen by you travelling back from S(4) to S(2), because in that case you never existed at S(4) and never will exist at S(4) and so future you never existed at S(2) and never will. To allow the paradox to exist, you need to be able to exist at S(4) to travel back in time, and so the only valid set of state transitions that involves you travelling back are ones where you're still existing in S(3) (and so in S(4)).
The paradox is explicitly the notion that we lack a clear understanding of whether there is a rule that prevents us from introducing an instability. Immutability in this case then introduces a rule that makes instability impossible to represent because that instability requires mutability, and so makes it impossible to represent the paradoxical state in question, and so if the universe is immutable, the paradox is resolved.
You can continue to argue this isn't the case, but you would simply be wrong.
[Incidentally, another way of resolving this paradox is a model where the instability is perfectly fine - where the system is mutable and if instabilities like this are introduced, the system will just keep oscillating between states as a "wavefront" of state transitions keep rolling through. This would require a model with a "memory" of past states to allow transitions to get "undone", but it's perfectly possible to define such a model. The only thing that makes this a paradox is that it appears to be at odds with what we think we know about the world, and so it tells us that what we think we know about the world is wrong.]
You're saying there's a puzzle about something that the premise at hand simply says is impossible. You might as well imagine being able to create energy out of nothing and then say that's a puzzle in a universe in which we have conservation of energy. Just because you can imagine something impossible doesn't make it a paradox.
And I’ll reiterate: I accept all of these premises
No you don't. You keep saying "but what if you go back and change the past?"
EDIT: You can certainly imagine more "relaxed" models that would also work, where mutability is possible, but where constraints increasingly restrain which mutations are possible when looping back on itself, but an immutable model is the simplest to describe in order to illustrate that models that allows loops can be made self-consistent.
In case of the loop the two ends of the rope are now identified and therefore must have the some displacement at all times which you only get if the wavelength is an integer fraction of the rope length. I haven't thought about this in much detail but I would guess that if you wiggle the rope at one point the wave now goes both ways around the loop and interferes with itself. Similarly for the rope attached at the far end the wave will be reflected and interfere with itself. In the end the result of the interference is probably strong damping of all wavelengths that do not have a suitable wavelength.
On the next level down you could than analysis this in terms of the atoms and the electromagnetic forces between them which in combination give rise to the macroscopic behavior of the rope. And as I said, I guess time travel self correction would be essentially the same thing, some kind of interference effect disallowing certain solutions. Just like in the loop example where you can not wiggle the rope in a way that the two ends of the rope have different displacements, i.e. you can not go back in time to kill your grandfather and have this prevent you from doing so. There is a consistency constraint by the rope being a loop and the rope not allowing discontinuities in it.
How this would work out in a complex system that involves the real world and people trying to kill other people is probably really far beyond human ability to figure out. Maybe just one neuron in you brain would not fire and prevent you from pulling the trigger. Maybe aliens would capture you on the way to your grandfather. Maybe something would get into the way of the bullet and capture or deflect it. Impossible to say.
I have a hypothesis: it's about stability. First, let us discard the notion of time "happening" from a perspective of the universe as a whole. Now, let us posit that only universes with stable structures can exist the same way that quantum superpositions can only collapse into certain states based on conditions [0].
In a stable time loop, the 4-dimensional-string model of the universe contains a section that loops back on itself. Irrespective of how any 3 dimensional beings in the string might experience it, this never "happened", it is just a part of the string's structure, which is stable.
This seems strange from the perspective of a 3d observer within that universe because they believe they have free will [1], but it is simply how physics works. A universe where the time travel causes instability isn't destroyed or anything like that, it simply can't come into existence at all according to the laws of physics.
However, I will further posit that, from the perspective of the time-travelling observer, it is possible to change history even in a stable universe! Consider the following scenario: You travel back in time to kill your own grandfather and succeed. Somewhat surprisingly you realize you still exist, evade the authorities, invent a new identity, and raise a family. Later, one of your own grandchildren decides to enact the same experiment you did in the "previous" [2] timeline and travels back in time to kill his own grandfather (that's you, in case that wasn't clear), and does so before you have a chance to kill your grandfather. Unbeknownst to yourself, this is part of the "original" timeline which now proceeds as normal with you travelling back in time to conduct an experiment in which you kill your own grandfather. This knotted structure is also stable, as are indefinitely complex variations [3].
Of course, the much simpler solution is that no universes can exist wherein this kind of time travel is even a possibility in the first place, but where's the fun in that?
[0] consider the double-slit experiment, wherein a wave-like pattern is formed even if only one electron occupies the experiment at a time. The electron states destined to land in the gap regions can't exist in the first place.
[1] and they do, from their perspective. Philosophically this is an interesting argument, but it is irrelevant here.
[2] it might be more accurate to say "meta-concurrent"?
[3] this form of time travel is rare in fiction, but I have at least seen a knot-stable time loop represented in Dark.
The question is: can I create a situation which can not lead to stability? And if so, what happens? Will it simply not work? Or will every situation lead to the possibility of stability?
Say, I travel back in time at the moment the universe "ends", and from that point onward in the past I destroy all time machines with a certain technology. Then, no one can fix what I did, and no stable loop could exist.
Now it is the same paradox all over again.
I mean, my guess is that such a situation simply can not exist, therefore I can not experience it - I can not live in this universe where this is possible. So, I can only live in the universe where my technology somehow fails exactly when someone travels back in time to correct my loop.
From the perspective of the universe there is no "create" or "destroy" because time is not experienced, it is just one dimension of its existence. The concept of "create" only exists from the perspective of a 3 dimensional being experiencing the time dimension. We believe we can choose the future because we believe it hasn't happened yet, but that's an illusion of perspective. In a sense, this means that there is no free will, but that's a different argument.
So to answer the question: no, because the fundamental nature physics drops the probability that you would choose such an action to 0, the same way electrons have 0 probability of choosing paths that lead to the gap regions in the double slit experiment.
> And if so, what happens? Will it simply not work? Or will every situation lead to the possibility of stability?
From your perspective, every situation will lead to stability in the end, because the universe where it didn't has a probability of 0.
> Say, I travel back in time at the moment the universe "ends", and from that point onward in the past I destroy all time machines with a certain technology. Then, no one can fix what I did, and no stable loop could exist.
> I mean, my guess is that such a situation simply can not exist, therefore I can not experience it - I can not live in this universe where this is possible. So, I can only live in the universe where my technology somehow fails exactly when someone travels back in time to correct my loop.
Pretty much yes. Though universes in which the situation you describe is impossible to achieve regardless (either because time travel does not exist or because there's nothing you could do to prevent it from being invented again) are significantly more likely.
It's got nothing to do with 'knowing' anything, it's just that we experience life as a vivid playthrough of causes and effects that could never have started or ended any differently.
Now, you might not agree with that explanation, but it can't be precluded, and it solves all of your concerns above.
That’s just invalid.
“Knowing” here just referred to the magical force that will keep time travel suicide from happening. But without a magical force you’re even more stumped!
It is still consistent that a cause is needed for an effect, so an effect cannot eliminate a cause.
You're just describing something that can't/doesn't happen. There are plenty of rules that describe things that can't or don't happen in space.
You wouldn't have any concerns with me saying it's impossible for me to go to the bathroom in my own house and when I walk out, I am in your house.
How does the bathroom door magically know which house I am in?
You could think of it that the universe where we exist must have a stable/consistent state.
If we go back in time and change the state, then that universe can't (didn't) exist, so it can't be the universe that we are in. From our point of view in the present (before time-travelling), the state change was in the past, it will have already happened before we start time-travelling.
This doesn't technically preclude us from going back in time. It's more that the present state (at least as we observe it) is pre-determined, since we already exist in the present.
* Events after the time travel event in the past bring about the future from when the time traveler comes. It seems forced, I admit, to assume that it just works out. I can see it working if the time traveler does not exactly know what they did in the past, which could make the paradoxes more beningn.
* If you travel in time, the timeline gets forked at the moment where you intrude. This is what happens in Dragonball Z.
Just fluff words and old sci fi trope.
How is this presented as something published in a peer reviewed journal.
Is the article completely misrepresenting the research or is the research itself this level of nonsense.
I must be missing something here.
Any self correcting mechanism implies magic.
How does 'universe' know what needs to be done to fix timeline?
If we accept "magic" as an explanation, then it would appear based on experimental evidence that the universe is, in fact, magic.
[0] https://en.wikipedia.org/wiki/Delayed-choice_quantum_eraser
That's it. Unless someone comes up with testable idea.
But inserting as a cause magical forces that we cannot even begin to explain as means of correcting time discrepancies.
So whats the point concocting hypothesis that there is no issues with time paradox all based on unknown? Its a plot for a SciFi novel not a paper.
I don't want to come across as a party-pooper, but those are outrageous claims with no meat and no bones.
That is my point though: there is no need for magic for this mechanism. The double slit delayed choice quantum erasure experiment is analogous to time travel as there is no way the particles could "know" that their path would be observed at some point in the future, yet if they were, those particles mysteriously only took paths that created the gaps. Given that this is a real, testable phenomenon in the real testable universe, it would not require magic to explain that idea that only timelines in which a time traveler doesn't cause a paradox can exist. The time traveler, like the particles in the experiment, chooses paths as they see fit and it only later turns out that they necessarily were ones in which paradoxes were avoided.
The delayed choice experiment is not a proof of concept of time travel.
The effect of the experiment looks like a time travel. But we don't know and cannot really explain it.
Let's make similar conjecture.
Because entangled pair interacts with each other at distance instantly this is a proof that you can interact at light speed at arbitrary distances. So there must be some kind of dimension that they stay connected through. We should be able to use this dimension to wormhole travel across arbitrary distances.
Therefore 'entangled wormholes' are possible.
Don't get me wrong the delayed choice experiment is quite disturbing. It really looks like there is some kind of intelligence at play. But thats not a proof of god (if it was not so hard to explain this experiment to common no 'science interested' person it would be 100% used by all preachers as sign of god at work).
That isn't quite what I'm claiming. All I'm trying to say is that an effect that looks impossible without some kind of mind behind it (or magic) is a testable real thing. I don't think we are disagreeing here: there is no magic.
That we do not know if time travel is possible[0] isn't the point, the point is that if it were you wouldn't need magic to explain the self-correction effect. Like the apparent "magic" of the delayed choice experiment, it would just be a fundamental property of the universe wherein events occur such that they form a stable time loop without the requirement of a mind that has to work out how to fix it that way. Like the particles in the experiment, they don't "know" anything, it just always works out that way.
[0]I think the vast majority of physicists would admit to a strong suspicion that time travel is impossible, and in any model where time travel is possible it is regarded as a flaw.
The universe is a VM alike smalltalk.
What you have now, is the action of the chain of actions of the past.
The VM of now is done.
You travel back.
Arrive at the VM of the past, WITH THE STATE of the past.
You mutate state. It work as expected, you die.
But in the STATE of now, you are alive.
When the time traveler exit the VM, the VM go nowhere. It already happened now, but ONLY "remember" the changes of IT, not of the OTHER vm.
Assorted musings:
For a short time, the universe get splitted(?)
The STATE run on RAM, and that is why it not "push to the future" because is NOT persisted?
The universe is multi-core, and time travel is possible because unused computation time that is assigned to the past VM when suddenly it get awakened?
If the STATE is not persisted (to make all this work), how this make sense at all? Or it have snapshots (backup) and you load for a moment the past?
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.”
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.
2. What does it have to do with the third law of thermodynamics (which is about the impossibility to reach a temperature of 0K)
What does that even mean? This is the kind of hand-waving the OP was pointing out.
He probably meant the first law.
Any variation on this: “sending this item in the past, will grab the same amount of mass from the past, taken at random in a way that preserve the energy on both ends”
Either it's impossible to travel back in time in this universe or it is not an isolated system... or the law of conservation could somehow be broken... do my thoughts make any sense?
If instead the rule is that we need to conserve the energy sum of all possible locations in space-time, then no law is broken.
It also would seem to be impossible to calculate energy at a specific moment. Energy = mc^2, or expressed differently, e = m * d^2 * t^-2. If t = 0, then t^-2 is not calculable.
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.
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.frontiersin.org/articles/10.3389/fphy.2020.00139...
> "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.
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.
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.
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...
[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.
I haven't finished the book yet though :)
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?
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.)
We know that all of the laws of physics we observe at the local scale are time invariant, but that is not necessarily true for any law we haven't discovered yet. While on the local scale that feels so unlikely as for it to be basically considered "inviolable without extraordinary evidence", that is not necessarily the case in the community for laws at the cosmic scale.
So isn’t the simple layman’s answer that the “system” also spans the 4th dimension and you are just displacing mass around in a 4D space instead of the typical 3D space?
Take this with a grain of salt, as I'm vastly underqualified to even form an opinion on this subject.
when you move to a different timeline, you are moving mass from a timeline to another timeline, so total mass in multi verse remains the same
It doesn’t say what constitutes a “major” change and what is a minor one though. The only guidance we are given is that the original motivation for going back must be preserved but otherwise very large changes can happen (planet of the apes marky wahlberg version)
What we call 'the past' is a mental picture of where physical things used to be positioned and their condition. But everything in 'that now' has changed. Just as when we 'travel' in physical space, when we get 'back home' it has changed.
If I were to 'time travel' I'd be gone from now. But everyone else would go on living in now. So where was the 'instant of time' I travelled to stored so I could go there? And the infinite number of instants between it and now?
Fanciful idea, but no evidence ... none zero zip nada
“Einstein said, ‘Time has no independent existence apart from the order of events by which we measure it,’”
https://phys.org/news/2011-04-scientists-spacetime-dimension...
Not where, when. [1] The block universe ontology is pretty commonly offered as an interpretation of general relativity. People consider GR as pretty strong evidence for such an ontology. Personally I don't but I'm just saying your criticism isn't very good.
1. https://en.wikipedia.org/wiki/Eternalism_(philosophy_of_time...
Would you be aware of some force stopping you from performing an action that would change your motivation to time travel in the first place? For example, if you tried to walk up to yourself in the past, would it feel like some force field stopping you? Or will it be false sense of agency, where you suddenly make the decision not to do something? Or will some event happen to simply kill you? Or will you suddenly vanish from this timeline to live out a life you over-wise would have based on the consequences of your actions?
If we are a result of a series of tiny influences from our environment, will the entire past refuse to be changed? Or will you be able to live out a version of a timeline where a different set of tiny influences add up to the same future where you make the same actions? As a result, does that also limit what you are actually able to do in the past without changing so much that one of these alternative states is impossible?
If the past doesn't want to be changed, how about the future? Just because we perceive the clock running in one way, doesn't mean that it actually does. I would think that if the past must happen in some specific way, then the present will always occur and so will a specific future. Whilst we claim to experience free-will, we have so far not measured it. I guess this is really the idea that the quantum realm simply consists of hidden variables that are only perceived as 'random', rather than actually having any form of random element.
Here's a can of worms... Compare the concept of time travel to something like 'smell travel'. Is it really that different? We are conditioned to know what smell is because our body has a mechanism to "interpret" particles and send signals to our brain which we 'remember'... How do we know that Time is any different? Perhaps travelling back in time is a flawed concept in itself, in the same way that "travel back in hearing" or "travel back in smell" makes little sense at all because we're just describing an effect or mechanism of our biology, rather than an effect or mechanism of the universe.
If you want to refer only to that which you presently observe as “existing”, then there is only the boundary of your past light cone, but this would be a bit solipsistic imo, as any other person you observe, the boundary of their past light cone would mostly not be part of the boundary of yours (it would be in it, just not on the boundary of it), and so they would seem to see things that “don’t exist”. This doesn’t seem sensible to me.
I don’t see any good alternative to treating the past and future as being fully real.
What do you mean by moving from one smell to another smell?
Time is a kind of position; smell isn't.
To me, a believer in the multiverse, it seems that the obvious answer is that in our particular instance of the universe no one in our future has figured out time travel, or have not gone back in such a way to let us know about it. For whatever reason.
We pay too much attention to what we perceive to be the “arrow of time”, the direction. Forward, backward? What’s the difference really? Is it our illusion of “free will”, that somehow there is more to the atoms of our body than can be explained by the same laws of nature that governs the rest of the universe, that simply won’t accept that we might just be like billiard balls moving along predetermined paths (admittedly unbelievably complex ones) and if we accept that, then the direction of time doesn’t really matter at all.
My naive understanding of time travelling back a single day would put me in a spot where earth will be a day later.
Does anyone know of any great sci-fi stories centered around the lengths the universe goes to to prevent a curious time traveler from testing the grandfather paradox.
Oh, and his "The Merchant and the Alchemist's Gate" is a stylistically beautiful exploration of people intertwined through time travel.
If you did successfully change the past so that you never ended up going back, you would not live the life where you go back in time to make that change. But if you did find yourself even going back in time it means your efforts already failed anyway, so might as well not even bother.
(I don't think arbitrary travel backwards in time is possible in our universe; there could be very rare circumstances where it ends up occurring, but not where you can build a device that takes you anywhen)
Since no-one altered our past we can't be in a timeline where people from the future had influence on the outcome. Therefore any timelines after the point of contact run parallel to ours and we cannot change our own present by traveling back in time.
Interesting to imagine the different "solutions" to the paradox though.
Make a nice story - ghosts are time travelers, appearing and disappearing without making any permanent changes.
Of course, this is entirely an aesthetic opinion and not scientific on my part.
If reality is infinitely branching, then time travel is merely a way for the traveler to perceive and move between those infinite branches.
If you kill your grandfather in an alternate timeline your grandfather is still alive in the timeline you came from, hence no paradox. You (an alien in the new timeline) can then watch a world develop in that alternate reality where your grandfather died.
If time travel were in fact possible, this would be the only way my puny brain could conceive of it actually working.
If there’s only strictly one path that reality takes even from the perspective of an outside observer (God) then I can’t see how time travel could be possible in such a construct. If it is possible it seems you must implicitly admit that there is not in fact a singular thread of reality/time.
Just because we define some equations where t can flow in reverse and fit them into the standard model doesn’t seem to me to be persuasive evidence one way or another.
I think one of the movie adaptations of the time machine has the protagonist trying to save his wife.
I would like to explore what would happen if one went years into the future to party ? Would certain time periods become tourist traps
And, if we assume we live in a one timeline hypothesis, all that work is still at risk from other time travelers altering a midpoint between now and where you mapped.
Would love to see this explored in a novel
https://www.youtube.com/watch?v=Gr_qHl9dWRU
Pretty amazing, if its true.
if selfconsistency is right it makes me wonder if a time traveler can actually be interacting with the past around him
[1] https://www.preposterousuniverse.com/podcast/2020/11/23/124-...
Hitler, on the other hand is just living his life as an Austrian child and young adult suffering from constant attempts on his life from people who he has no way of knowing. He gets the idea that people are always out to get him (because they are) and becomes evil, and uses the assassination attempts to justify creating the Holocaust.
But the article reads like some sci-fi bs tbh. What's considered the 'same outcome'. If I kill baby Hitler, would the universe create another just like him? Or someone else will do same things? If someone else does them, surely it won't (for example) kill as much people. So someone dead would now be alive in the future (or vice versa). And now the outcome isn't the same yeah?
Sorry but this is bs.
Why bother going back so far? Let's think of what would happen if you went back 30 seconds. As soon as you arrive in the past, take a baseball bat to the time machine. What then?
And then let's take the article's scenario at face value. If you went back 5 minutes and somehow events always led back to you successfully using the time machine, your consciousness would be stuck in a permanent 5-minute loop. Maybe you could spice up those 5 minutes but you'd be right back where you started soon enough. Wouldn't the same be true if you went back 5 seconds? That would be hell.
But that's not the end of it. If you went back in time, why would we assume you'd displace to a new location? One of two things would need to happen: A) you'd need to teleport AND erase a blank area of space perfectly sized for your arrival, or B) you'd rewind like a tape playing backwards.
I like to think of the B scenario. If it were true we would never know it. The effects of the time machine would never be observable. We could have working time machines all around us right now for all we know. As I sit in this chair I could be at the destination of a trip through a time machine which I will one day invent. And when I finally enter the machine and flip the button, nothing will seem to happen. A pointless device.
https://www.imdb.com/title/tt9788516/
> Want to see Hitler die in a variety of comically fantastic ways? Now you can. Welcome to Multiversity!
Only 5 minutes and it's kind of along this line of thought. I won't spoil the details.
Is there any similar data that can be used? Perhaps global weather records?