7 ms·

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

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.

hderms 6y ago
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
deleted 6y ago
[deleted]
notahacker 6y ago
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.
- deleted 6y ago
  [deleted]
vidarh 6y ago
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.
- ricardobeat 6y ago
  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.
  - wongarsu 6y ago
    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).
    - yrimaxi 6y ago
      I think free will is orthogonal to mutability/immutability of time.
      - alisonkisk 6y ago
        Free will is a nonphysical illogical concept. It's impossible to define free will inside physics in a way that feels "free".
danbruc 6y ago
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.
- alisonkisk 6y ago
  Like so many things in physics, a differential equation with constraints and initial conditions to satisfy.
AnIdiotOnTheNet 6y ago
> 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.
- zwaps 6y ago
  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.
d1sxeyes 6y ago
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.
- yrimaxi 6y ago
  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!
  - d1sxeyes 6y ago
    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?
throwaway2245 6y ago
> 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.
- ku-man 6y ago
  If time travel was possible, this proposition is the most logical (you put it very well, by the way). But at the same time, it's almost useless for the time traveller. Imagine a man that suffered a terrible accident when we was 10 years old, and had a miserable life from that point on. When he is 60 time travel is possible, so he decides to travel 50 years back to prevent the accident. He succeeds, but this new timeline doesn't mean the previous timeline won't/didn't happen. This new him will have a better life but the other him will still be miserable.
EliRivers 6y ago
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?
samus 6y ago
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.

hderms 6y ago

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

deleted 6y ago

[deleted]

notahacker 6y ago

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.

deleted 6y ago
[deleted]

vidarh 6y ago

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.

ricardobeat 6y ago
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.
- wongarsu 6y ago
  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).
  - yrimaxi 6y ago
    I think free will is orthogonal to mutability/immutability of time.
    - alisonkisk 6y ago
      Free will is a nonphysical illogical concept. It's impossible to define free will inside physics in a way that feels "free".

danbruc 6y ago

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.

alisonkisk 6y ago
Like so many things in physics, a differential equation with constraints and initial conditions to satisfy.

AnIdiotOnTheNet 6y ago

> 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.

zwaps 6y ago
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.

d1sxeyes 6y ago

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.

yrimaxi 6y ago
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!
- d1sxeyes 6y ago
  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?

throwaway2245 6y ago

> 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.

ku-man 6y ago
If time travel was possible, this proposition is the most logical (you put it very well, by the way). But at the same time, it's almost useless for the time traveller. Imagine a man that suffered a terrible accident when we was 10 years old, and had a miserable life from that point on. When he is 60 time travel is possible, so he decides to travel 50 years back to prevent the accident. He succeeds, but this new timeline doesn't mean the previous timeline won't/didn't happen. This new him will have a better life but the other him will still be miserable.

EliRivers 6y ago

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?

samus 6y ago

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.