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Paradox-free time travel is theoretically possible

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Re: Paradox-free time travel is theoretically possible

#121

Post by ricardobeat »

vidarh wrote: post #93
at_a_remove wrote: post #69

Earlier quoted context omitted.

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,…

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 exampl…

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.

Re: Paradox-free time travel is theoretically possible

#123

Post by tremon »

rightbyte wrote: post #105
littlestymaar wrote: post #85

Earlier quoted context omitted.

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.

Re: Paradox-free time travel is theoretically possible

#124

Post by Contax »

littlestymaar wrote: post #111
rightbyte wrote: post #105

Earlier quoted context omitted.

If you send an apple back in time the conservation of energy is broken. The energy sum of everything increases in the past.

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)

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?

Re: Paradox-free time travel is theoretically possible

#125

Post by danbruc »

at_a_remove wrote: post #69
mpalmer wrote: post #27

Earlier quoted context omitted.

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,…

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.

Re: Paradox-free time travel is theoretically possible

#126

Post by wongarsu »

ricardobeat wrote: post #121
vidarh wrote: post #93

Earlier quoted context omitted.

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 exampl…

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.

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).

Re: Paradox-free time travel is theoretically possible

#127

Post by [deleted] »

notahacker wrote: post #87
at_a_remove wrote: post #69

Earlier quoted context omitted.

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,…

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 le…

[deleted]

Re: Paradox-free time travel is theoretically possible

#128

Post by littlestymaar »

ricardobeat wrote: post #119
littlestymaar wrote: post #111

Earlier quoted context omitted.

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)

> 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

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”

Re: Paradox-free time travel is theoretically possible

#129

Post by noxer »

jennyyang wrote: post #19

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.

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.

Re: Paradox-free time travel is theoretically possible

#130

Post by AnIdiotOnTheNet »

at_a_remove wrote: post #69
mpalmer wrote: post #27

Earlier quoted context omitted.

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,…

> 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.

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