Paradox-free time travel is theoretically possible(npr.org)
npr.org
Paradox-free time travel is theoretically possible
https://www.npr.org/2020/09/27/917556254/paradox-free-time-travel-is-theoretically-possible-researchers-say
193 comments
Exactly. The article mentions the grandfather paradox, but doesn't really address it at all. Can't "self correct" shooting your past self in the head.
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.
The thing about this "self-correction" is that nobody, but nobody has a mechanism to explain this force. Not how it works or worse yet, how it will know how to self-correct. What particular force decides that the friction on some surface will briefly rise and cause the gun to jam? How does it know to do that?
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.
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.
I think the flaw here is assuming "knowing" rather than a mechanism that makes it impossible to express the paradox, or alternatively, impossible for you to find yourself in a world where an action that created a paradox happened.
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.
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.
That option implies time travel is not possible, since your mere existence at a past moment is already mutating it. Which is the opposite of what the article claims.
My description explicitly hypothesised a universe that is entirely immutable, and where the passage of time is purely "read only". Your presence at any given point in time then would never mutate anything ever, any more than watching a movie mutates the movie.
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.
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.
Why? If time is immutable (essentially a Block Universe where everything is predetermined) then you traveling in time doesn't mutate anything, just as you experiencing the current time isn't mutating anything.
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).
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).
Exactly.
E.g, a possible example immutable scenario that doesn't violate anything (with each number representing a discrete period of time)
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.
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.
I don’t think this really gets at the paradox. What if you shot yourself?
You didn't shoot yourself. We know this to be true.
Ok. I guess verb tense falls short in this case. :-)
It does, but that's also the point. The past is fixed in this model. If you go back in time, that's not new. You always did that. How the present looks now is a function of you having always done that. We know that you didn't shoot yourself, because you didn't.
I think free will is orthogonal to mutability/immutability of time.
Free will is a nonphysical illogical concept. It's impossible to define free will inside physics in a way that feels "free".
Your comment is orthogonal to my comment.
I guess the mechanism is just a constraint on the possible solutions for the scenario. As an analogy imagine holding a rope in your hand and wiggling it up and down, this way you can make waves of more or less any wavelength travel along the rope. Now fix the far end of the rope somewhere or close it into a loop and you will no longer be able to make waves of arbitrary wavelength but only integer or half-integer fractions of the rope length.
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.
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.
Like so many things in physics, a differential equation with constraints and initial conditions to satisfy.
I always thought the idea was some kind of many worlds style event where whatever universe you find yourself in after the split is guaranteed to be paradox free because something must have happened to have prevented it
Of course there's a mechanism to explain it: determinism.
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.
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.
This explains absolutely nothing. You time travel back and kill your former self. Paradox. How to resolve? Determinism.
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!
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!
You are still thinking as though someone can 'choose' to go back in time and kill themselves. Choice has no bearing on it. Either something happened, or it didn't.
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?
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?
[deleted]
> The thing about this "self-correction" is that nobody, but nobody has a mechanism to explain this force.
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.
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.
I like this explanation, but it essentially doesn't solve the paradox. It just shifts it to knot-stability.
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.
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.
> The question is: can I create a situation which can not lead to stability?
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.
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.
My favorite knot-stable time loop story is "By His Bootstraps", by Anson MacDonald.
> The thing about this "self-correction" is that nobody, but nobody has a mechanism to explain this force. Not how it works or worse yet, how it will know how to self-correct.
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.
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.
This. As far as we're aware, physics doesn't understand narrative, and doesn't see any difference between killing a person, moving a gun or even disturbing a speck of dust: all entail things being in the wrong place compared with the original timeline.
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.
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.
[deleted]
That force might not be necessary. There are at least two ways this would work:
* 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.
* 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.
If I open a box and find a bag of apples inside, how did the universe in the past "know" that the bag of apples needed to somehow end up in that box?
The thing is, the time traveler’s existence in the past is a contradiction, so if the universe prevents contradictions it must prevent backwards time travel absolutely.
Yeah I fail to see anything addressed at all.
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.
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.
"The past is obdurate."
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.
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.
There's also a James Franco mini series based on the book, which is alright albeit a bit slow moving.
I could see how a universe with such rules could exist. But is it our universe? In the universe you describe, wouldn't we already have seen time travellers from the future?
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.
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.
I have a short story for this!
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.
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.
That seems as though it could be solved by making very short trips back and allowing the local gravity to continually correct your velocity and positioning until you reach your destination.
yes, but you need a lot of energy to move some matter in a single jump, so more jumps mean exponentially more initial energy because of something akin to a "time-rocket-equation" (you need to bring more mass to get energy but that means more mass or spending more time in a place to re-accumulate it).
It's a fun idea though, thanks :)
It's a fun idea though, thanks :)
I can’t see how a universe like that could exist. Perhaps if some time-traveler was unlucky, their gun might jam, but I can’t figure out how you could successfully thwart a “well-prepared time traveler” who knows all the details of the situation they’re jumping into and has double-checked their gun after making the jump.
The logical extension of this would be, given how local interactions might affect events across the globe over time, that the entire past becomes immutable. That would reduce time travel to a read-only experience, so it wouldn't have any utility beyond time tourism.
Read only time travel would still be immensely useful as there are so many situations where having a clear answer to what happened is interesting - history, solving crimes, analyzing accidents (or bugs as we are on HN), re-measuring experiments, ...
It would be, but also immensely troubling, as explored in "The Dead Past", by Isaac Asimov.
First consider that a gun is just a dramatic way of illustrating the concept. If a butterfly travels back in time and flaps its wings it could cause a tornado on the other side of the world (butterfly effect).
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.
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.
Isn't the point that that guy thwarts himself.
So this is essentially magic.
Any self correcting mechanism implies magic.
How does 'universe' know what needs to be done to fix timeline?
Any self correcting mechanism implies magic.
How does 'universe' know what needs to be done to fix timeline?
How do particles in delayed-choice quantum eraser [0] experiments know whether or not their path will eventually be observed?
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
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
We don't know how it work.
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'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.
> But inserting as a cause magical forces that we cannot even begin to explain as means of correcting time discrepancies.
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.
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.
My apologies I don't think I phrased it clear enough.
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).
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).
> The delayed choice experiment is not a proof of concept of time travel.
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.
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.
I know nothing, so this instead is my theory:
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?
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?
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.
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.
I think it's still a lot easier to treat each going-back-to-the-past as a fork().
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.”
[0]: https://www.epsilontheory.com/gell-mann-amnesia/
“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/
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.
[deleted]
Pasting the abstract from the paper:
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
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
"Further studies will be necessary to find genuine physical scenarios realising the acausal processes we have discovered."
To bad no one went back in time to tell the authors whether they actually discovered anything or not.
To bad no one went back in time to tell the authors whether they actually discovered anything or not.
Maybe we just need to wait until someone builds the womhole or the time machine from Primer.
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.
Where did the story come from?
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?
Your story allows a “closed timelike curves” to exist, then it must be considered as a physical constant.
Where do c (the speed of light), G (Gravitational constant) or π comes from?
Where do c (the speed of light), G (Gravitational constant) or π comes from?
Can you elaborate?
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 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.
The film Predestination also deals with this paradox.
https://www.imdb.com/title/tt2397535/
https://www.imdb.com/title/tt2397535/
Have you ever read "All you zombies" by Heinlein?
https://gist.github.com/defunkt/759182/ad44c6135d168ae54503a...
See also "As Never Was", by P. Schuyler Miller.
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.
So yet another paradox (like so many in quantum mechanics) that's easily solved with Superdeterminism.
https://www.frontiersin.org/articles/10.3389/fphy.2020.00139...
https://www.frontiersin.org/articles/10.3389/fphy.2020.00139...
Third law of the thermodynamics has to be false for that to be possible. It is still a paradox.
Would you mind expanding on that?
If you send an apple back in time the conservation of energy is broken. The energy sum of everything increases in the past.
The conservation of energy only applies to a closed system. If we can "create" an apple at any given point in time, that only means that "the universe at a given point in time" isn't a closed system.
Only if you consider conserving the energy sum of all possible spatial locations to be the rule we need to keep.
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.
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.
That's an interesting point. I'd even go a step further and say that which of those two definitions is true determines whether time travel is possible. Put another way, conservation across space-time (instead of a single point in time) is necessary for time travel to exist, and the existence of time travel is the only reason conservation would be across space-time.
I'm not sure that time travel is the only reason - it might turn out to explain some other peculiarities in the universe (spooky action at a distance jumps into my mind as something that could potentially have an explanation linked to this).
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.
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.
1. Not necessarily: nobody said the time-travel couldn't pump energy at your destination to compensate for the energy the future is sending.
2. What does it have to do with the third law of thermodynamics (which is about the impossibility to reach a temperature of 0K)
2. What does it have to do with the third law of thermodynamics (which is about the impossibility to reach a temperature of 0K)
I know next to nothing about all this so excuse my ignorance -and limited English-, but I've always thought that if you send, say, an apple (as in GP's comment) to the past... wouldn't that apple's atoms already exist there in the past -in the same apple if it already existed (you are sending it just some minutes back) or elsewhere like in the tree, the tree's soil... animals that later died and nourished the soil/tree... in rocks... wherever- and so you are duplicating them and hence creating energy/mass and breaking the law of conservation?
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?
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?
> pump energy at your destination
What does that even mean? This is the kind of hand-waving the OP was pointing out.
He probably meant the first law.
What does that even mean? This is the kind of hand-waving the OP was pointing out.
He probably meant the first law.
> What does that even mean
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”
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”
So like the gate of truth from Fullmetal Alchemist?
I think I messed up the number.
Not if he has more information because he has seen the future - then he'll likely act different.
Indeed.
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.
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.
> "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.
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.
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.
> What if killing the toddler means you can't travel back to your understood future?
How would killing a toddler prevent me from time travelling back? Killing someone will not affect the functionality of my time machine.
How would killing a toddler prevent me from time travelling back? Killing someone will not affect the functionality of my time machine.
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.
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...
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.
Sounds like they're describing the Novikov self-consistency principle [1] in a roundabout way.
[1] https://en.wikipedia.org/wiki/Novikov_self-consistency_princ...
[1] https://en.wikipedia.org/wiki/Novikov_self-consistency_princ...
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.
[0] https://en.wikipedia.org/wiki/Behold_the_Man_(novel)
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)
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"
The difference is entropy/uncertainty. You can change anything whose effects are too small to measure or distinguish from equivalent energy states.
> 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?
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"
I haven't finished the book yet though :)
I haven't finished the book yet though :)
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.
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.
No, because time and universe is not discrete, there are no branching.
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
- 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.)
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.)
Thank you! That explanation actually makes the paper make a lot more sense.
Time travel violates the law of conservation of energy. By travelling back in time, you yourself are injecting more mass/energy into a system. So it's great as a science fiction trope, but if you're going to talk physics, that needs to be addressed.
I suppose it would be possible under certain assumptions- The main one being that both the past and present would need to be parts of the same system.
Take this with a grain of salt, as I'm vastly underqualified to even form an opinion on this subject.
Take this with a grain of salt, as I'm vastly underqualified to even form an opinion on this subject.
Iirc there is no requirement for mass/energy to be conserved, and some models of expansion require conservation to be violated.
Depends if the system is open or closed and if mass is converting to energy or visa versa. Also conditions near big bang are not representative.
More fundamentally, energy is a conserved quantity because the laws of physics are time invariant. See Noethers theorem and gauge symmetries.
that's wrong. Noether's theorem says that "energy is conserved" is equivalent to "the laws of physics are time invariant". If you assert that "energy is conserved" because "the laws are time invariant" that risks begging the question. At the very least you should be asking "ok, then, why are the laws of physics time invariant?"
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.
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.
Nope, talking about post BB conditions.
I don’t have the details but I’m pretty sure passage of time is due to a tendency for entropy to increase, and that if I go back in time a second by, say, walking backwards, it is no more a violation of the cons of energy as if I walk forward.
How do you know you aren't walking backwards back in time? To be complete you'll also need to be losing your new memories and becoming younger - it's experimentally undistinguishable.
I think the point about conservation of mass still applies. There are literally 2x yourself at a certain point in time, mass is not conserved.
Always conservation of mass is maintained by broadening the scope of the system under test. You can take matter out of a local system and make that local system less massive, but it’s not a violation of physics because the broader system maintains a steady state.
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?
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?
I dont think conservation of mass works like this between "disparate" points in the 4th dimension. It has to be continuosly connected. Otherwise things could appear out of thin air without causality. Reminds me of this software: https://4dtoys.com/ (4d is time in this case not another spacial dimension but the intuition about discontinuity still applies)
[deleted]
If you could just send information back in time that would not violate conservation of energy. But otherwise would have a similar effect as traveling back yourself. You simply tell your past what to do rather than going back and to do it "with your present yourself". This also avoids a lot of other paradoxes and we already know that the reverse (sending information to the future) is super simple.
it is possible to explain it assuming there are multiple timelines. overall multiverse has finite energy/mass.
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
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
Just consider the entire thing (spacetime) as one system.
I won't find my young self and give him all of the lotto numbers for every major drawing. Instead I would go claim it myself and enjoy life at that point. Younger me will eventually go in the same loop.
In Isaac Asimov's "End of Eternity" he posed a similar concept where contrary to the butterfly effect where minor changes ripple through time, time has a mechanism of self-preservation which is resistant to change. Asimov never ceases to amaze me with how prescient he was for his time.
Such is also the underlying premise in Poul Anderson's Time Patrol universe.
Just give it to me in popular movies: Back to the future style time travel can’t happen but Terminator style time travel can.
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)
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)
One of the short stories in Ted Chiang’s recent book Exhalation deals with this theme. It’s a pretty interesting and entertaining read.
I believe you're referring to The Merchant and The Alchemist's Gate. All of Exhalation is great, but I particularly enjoyed that one!
Yes, the chapter titled "The Merchant and the Alchemists Gate". In my opinion one of the more powerful shorts in the collection.
I've never seen the past or future. Only physical things still or moving, combining with other things or not ...
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
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
> So where was the 'instant of time' I travelled to stored so I could go there?
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...
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...
That article doesn't mention general theory or relativity, quoted discussions indicate that eternalism mostly relies on geometric expression of (plain) Minkowsky spacetime, which is interpreted literally as indication of existence of global time dimension. General theory or relativity is bad news for eternalism, because curved space is more obviously local and thus less compatible with eternalism which favors globality.
How do you reconcile this idea with special relativity?
Every single oscillating particle needs to travel more absolute distance when speed increases, so it will oscillate with lower frequency for the external observer, thus causing time dilation.
The Earth is not the center of the universe. Similarly, time-travel seems to be an idea still stuck in the Newtonian idea of absolute time. As SR demonstrated: There is no such thing.
“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...
“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...
In special theory of relativity time is local, so objects interact only with immediate moment - present, and not with past and future. You need to first somehow overcome locality to postulate global time, which you would be hard pressed to do.
By the speed of how these things move, interact or change state relative to each other.
Just some thoughts:
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.
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.
I'm so agnostic concerning the concept of time itself at the end of the day.
I feel that we're so busy struggling to find a way to make time travel work, that we forget that time itself isn't a mechanism of the universe, and that it's just a word we use to describe some sort of effect or bi-product that we're observing.
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.
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.
> Compare the concept of time travel to something like 'smell travel'. Is it really that different?
What do you mean by moving from one smell to another smell?
Time is a kind of position; smell isn't.
What do you mean by moving from one smell to another smell?
Time is a kind of position; smell isn't.
So do you think that special relativity is misleading, or, if not, how do you reconcile this point with special relativity?
I think you partly misunderstood his comment. Basically instead of time we can look at it as a change of states, just a linear process. There is no time. If you place something on a table (in a perfect environment with no decay) you can't tell how much time it sat there. It could have sat there for 1 hour or 1 million years. It just didn't move and no one tracked the "time". Time is just a concept in our minds to track changes in space (or state of space). Special relativity I think doesn't contradict this. It just says that the "speed" of the state changes (e.g. you go from A to B) is relative. But this speed can't go negative. There is no trace of previous events where you could go back to.
My thought is that in special relativity, there is no absolute present, only the present from different reference frames. Essentially, “now” isn’t an objective thing. “Happen simultaneously” is not an observer-independent equivalence relation, and so there is no observer-independent way of splitting things into “the present (which exists)” vs “the past” vs “the future”.
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.
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.
I’m not an expert, but why would the paradox they are talking about be a problem for the concept of time travel?
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.
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.
Are there ideas out there which "explain away" the problems arising from planetary/stellar movement?
My naive understanding of time travelling back a single day would put me in a spot where earth will be a day later.
My naive understanding of time travelling back a single day would put me in a spot where earth will be a day later.
And why stop this line of reasoning with our local star system? There’s no absolute reference frame.
This really got me thinking.
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.
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.
While also not quite what you asked for, Ted Chiang's short story "What's Expected of Us" deals with some of the emotional issues of determinism.
Oh, and his "The Merchant and the Alchemist's Gate" is a stylistically beautiful exploration of people intertwined through time travel.
Oh, and his "The Merchant and the Alchemist's Gate" is a stylistically beautiful exploration of people intertwined through time travel.
[1] Predestination is a good Time Travelling Film, similar to the plot you describe.
[1] https://www.imdb.com/title/tt2397535/
[1] https://www.imdb.com/title/tt2397535/
Probably not what you're asking for, but there's a classic Futurama episode that fits that description https://en.m.wikipedia.org/wiki/Roswell_That_Ends_Well
I would recommend Steins;Gate as a fun exploration of time travel [1].
[1] https://en.wikipedia.org/wiki/Steins;Gate_(TV_series)
[1] https://en.wikipedia.org/wiki/Steins;Gate_(TV_series)
It’s impossible to create a paradox, because the events that led to you being able to go back in time were created by whatever you did in the past in an attempt to change the future.
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.
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.
It's possible that reality is defined by different rules than you are reasoning with.
(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)
(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)
This is the "there's no such thing as free will" argument, which is even more implausible than time travel.
If cause comes before effect, then there is free will but there is a limited possibility of effects. If cause comes after effect, there isn’t free will but there is an unlimited possibility of effects.
I like William Gibsons version of interactions with the past, as featured in The Peripheral.
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.
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.
I don't understand how people view changing the past with time travel. If I go back in time I might experience a given point in time twice, but that point in time still only happens once. So if I go back to 1940 and kill my grandfather, then the first time 1940 happened he got shot. The second time...there is not second time. (Fortunately my parents were born in 1930s.)
Worth noting that you can’t conclude time travel is impossible or irrelevant just because other people have. There could exist time travelers who secretly have control either of your life or the world with advanced technology (potentially up to the level of mind control chips) who are keeping the evidence from getting out.
I always imagined that time travel into the past would involve some statistical uncertainty. If you got close to creating a paradox, you'd simply transition back. Disappear essentially.
Make a nice story - ghosts are time travelers, appearing and disappearing without making any permanent changes.
Make a nice story - ghosts are time travelers, appearing and disappearing without making any permanent changes.
I find every solution except "time travel always results in a parallel universe" to be too messy and weird. I also prefer that solution for faster than light travel, since that's also basically time travel.
Of course, this is entirely an aesthetic opinion and not scientific on my part.
Of course, this is entirely an aesthetic opinion and not scientific on my part.
Why can’t the linear flow of time and a singular instance of reality just be the narrow lense through which we view the universe?
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.
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.
Sometimes I came here to read good news and interesting article...
most of the time just clickbait...
Time travel stories always seem to be centered around what if fantasy.
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
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
You need to chart the future first, for even in moderately advanced countries you risk showing up during a riot or other agitations that could get you arrested and stranded, especially as our surveillance capabilities expand it will be hard to evade detection.
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.
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.
Assuming I have enough gas in my time machine, I could just jump back. Eventually in this dream, it would be known , say Paris 2500 , is a cool spot . So people would travel there and spend money ( barter) .
Would love to see this explored in a novel
Would love to see this explored in a novel
ah yes that's an option too, I now realize I'm conditioned from what early comics I read in thinking that when one time travels the time machine stays where it is, but that has not to be necessarily the case
I don't think you can travel back in time so it's moot as far as I'm concerned.
Why would it be impossible to travel back in time?
It leads me to think that no one that would fuck up the future will be able to do time travel in the first place. So it is not possible because anyone who dreams about going back in time wants to change something.
Doesn’t conservation of energy preclude backwards time travel? If not, would a hypothetical machine which sends an atom back in time be able to fill some previous universe with infinite numbers of the same atom?
Andrew Basiago claims to have time-travelled:
https://www.youtube.com/watch?v=Gr_qHl9dWRU
Pretty amazing, if its true.
https://www.youtube.com/watch?v=Gr_qHl9dWRU
Pretty amazing, if its true.
There might not be a paradox but there could be unintended consequences to time travel. A physical object can be degraded in space. Why not also degrade the time it exists in also.
what if he goes back in time to the moment before leaving and disintegrates the time machine?
if selfconsistency is right it makes me wonder if a time traveler can actually be interacting with the past around him
if selfconsistency is right it makes me wonder if a time traveler can actually be interacting with the past around him
In case anyone is interested, Sean Carroll did an amazing podcast episode on the mechanics of time travel in a scientifically accurate way [1]. He sometimes works as a scientific advisor on Hollywood movies and claims to be responsible (in part) for the line "You mean Back to the Future is bullshit?" in 'The Avengers: Endgame'. He also touches on the topic of killing Hitler as well as different tropes in time travel movies. He also points out one particular movie which he thinks did time travel logic best: Bill and Ted's excellent adventures.
[1] https://www.preposterousuniverse.com/podcast/2020/11/23/124-...
[1] https://www.preposterousuniverse.com/podcast/2020/11/23/124-...
This article claims to refute the butterfly effect with no support. I get that strange things happen in Physics. I would have liked more support.
When you take this theory to it's logical conclusion, you end up with some strange outcomes. The strangest is the idea of going back and killing baby Hitler. When you attempt to do that, the universe, in it's self righting ways makes the assassination attempt not work. Since Hitler is a pretty big enemy for a lot of people, many people over time try to go back and kill Hitler.
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.
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.
This is all just a bunch of nonsense, but I enjoy discussing it regardless.
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.
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.
Self-fulfilling prophecy, yes.
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.
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.
I find the concept absurd as well. If you do something like go back in time and murder Hitler, you're going to be impacting the actions of every single human being alive. Within a few generations there's going to be an entirely different set of people alive than the set of people alive in the universe in which Hitler was not murdered. This idea that you would still have to go back in time because the universe corrects itself makes no sense because there's just no way you would still exist. I'd be willing to entertain the idea that someone else would go back in time for the same reason, but it's all just wild speculation.
Pratchett: “Shoot the dictator and prevent the war? But the dictator is merely the tip of the whole festering boil of social pus from which dictators emerge; shoot him and there'll be another one along in a minute. Shoot him too? Why not shoot everyone and invade Poland?”
Perhaps it’s the paradox that makes the travel
impossible rather than the physics with the infinite loops settling/converging on the scenario where time travel is not discovered / possible.
This is assuming you will be successful in assassinating Hitler and that the timeline is mutable as opposed to the idea that time travel was always 'meant' to happen.
Somewhat portrayed in a "Love, Death & Robots" episode (Alternate Histories).
https://www.imdb.com/title/tt9788516/
> Want to see Hitler die in a variety of comically fantastic ways? Now you can. Welcome to Multiversity!
https://www.imdb.com/title/tt9788516/
> Want to see Hitler die in a variety of comically fantastic ways? Now you can. Welcome to Multiversity!
The time-travel in the third Harry Potter book seemed consistent and paradox-free. Seems like a good reference for this sort of discussion.
Why time travelers in sci-fi also move to the position and rotation of the Earth at the given moment? They should end floating in space.
Oh actually, I've found the way to convenience myself about that: The "reality" is, when somebody travels through time, most of them (god knows how many) got killed in the process, but the story will just pick another "instance" who has survived the journey from the pool of "Multiverse" and continues.
I watched this short movie yesterday night
"One-Minute Time Machine" https://www.youtube.com/watch?v=vBkBS4O3yvY
Only 5 minutes and it's kind of along this line of thought. I won't spoil the details.
Only 5 minutes and it's kind of along this line of thought. I won't spoil the details.
Genius!
Is there some sort of highly entropic and massive hash string being constantly printed, recorded, and widely distributed so that one could see if any changes happened in the past? Or rather, ensure, with a comparison, that the observer is still in the same timeline?
Is there any similar data that can be used? Perhaps global weather records?
Is there any similar data that can be used? Perhaps global weather records?
Relevant webcomic: https://www.smbc-comics.com/comic/2013-02-09
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.