Timetravelling paradox
- Thread starter embargiel
- Start date
Well, atleast imaginary numbers do not contradict logic. 
If we try and predict, via physics, the result of sci-fi style time travel, and find a contradiction, that's one thing. It means our theories are missing something.
However, if we try to figure out the result of sci-fi style time travel, from first logical principles, and we arrive at a contradiction *under any assumed set of physical laws*, that is more telling.
The first situation is like finding that basic arithmetic is either incomplete or inconsistent.
The second situation is like finding that all sufficiently non-trivial mathematical systems are either incomplete or inconsistent.
That is, time travel is the Godel statement for physics, or if you prefer, the Halting Theorem. It introduces the ability to create logical contradictions from extremely simple assumptions, which means, either it is not possible (given the assumptions for how it is supposed to work), OR, the physical world permits logical impossibilities to exist.
However, since the real existence logical impossibilities would be in contradiction to our observations, it's more likely that one of our assumptions is false, and that assumption is that time travel allows travel to the past within a single universe.
The only way to escape is to consider time travel to simply be a way of traveling to another universe in the multiverse.
If we try and predict, via physics, the result of sci-fi style time travel, and find a contradiction, that's one thing. It means our theories are missing something.
However, if we try to figure out the result of sci-fi style time travel, from first logical principles, and we arrive at a contradiction *under any assumed set of physical laws*, that is more telling.
The first situation is like finding that basic arithmetic is either incomplete or inconsistent.
The second situation is like finding that all sufficiently non-trivial mathematical systems are either incomplete or inconsistent.
That is, time travel is the Godel statement for physics, or if you prefer, the Halting Theorem. It introduces the ability to create logical contradictions from extremely simple assumptions, which means, either it is not possible (given the assumptions for how it is supposed to work), OR, the physical world permits logical impossibilities to exist.
However, since the real existence logical impossibilities would be in contradiction to our observations, it's more likely that one of our assumptions is false, and that assumption is that time travel allows travel to the past within a single universe.
The only way to escape is to consider time travel to simply be a way of traveling to another universe in the multiverse.
If time is just linear in a universe, going back in time would vanish that person during the time travel. That person wouldn't be able to exist, because in that time frame he hasn't come into existance yet.
He will probably return in his time machine unconcious and doesn't remember anything and he just wasted some times, time travelling.
I wonder, if you can time travel, how long will it takes to travel back to 1980 ?
Godels model is unphysical, its a dust universe with negative stress energy content (im simplifying, technically you end up with killing vectors that violate local energymomentum conservation) .
Gott however modeled a geometry where you have 2 cosmic strings interacting, that allows CTCs to be formed. Its a highly implausible geometry, as the energy would rip the known universe asunder, but its still interesting that GR naively allows these solutions apriori.
The problem with traveling back in time is bad, even if humans cannot, is that all one requires is for one energetic particle to travel through the CTC. That is enough to disturb the initial state, and basically destroy causality. History no longer makes sense, and concepts like entropy basically are no longer identifiable.
Its actually worse than that at the level of the mathematical theory. If the CTC is in contact with the rest of the metric, you can devise open sets that are no longer causally Hausdorff. Essentially, the problem is not even at the metric level, but instead appears as a topological problem since you have open sets that violates first assumptions. The physical theory basically breaks down essentially, ie its nonsense.
As far as Magueiro (sp), we had a discussion about him in one of our classes. He makes some simple mistakes in his book, however certain of his works (like introducing a fundamental length scale which might imply a timevarying speed of light) are plausible.
There might be forthcoming experiments that can test this, and the idea has some notable implications for String Theory and other quantum theories of Gravity (loop Quantum gravity, etc)
Gott however modeled a geometry where you have 2 cosmic strings interacting, that allows CTCs to be formed. Its a highly implausible geometry, as the energy would rip the known universe asunder, but its still interesting that GR naively allows these solutions apriori.
The problem with traveling back in time is bad, even if humans cannot, is that all one requires is for one energetic particle to travel through the CTC. That is enough to disturb the initial state, and basically destroy causality. History no longer makes sense, and concepts like entropy basically are no longer identifiable.
Its actually worse than that at the level of the mathematical theory. If the CTC is in contact with the rest of the metric, you can devise open sets that are no longer causally Hausdorff. Essentially, the problem is not even at the metric level, but instead appears as a topological problem since you have open sets that violates first assumptions. The physical theory basically breaks down essentially, ie its nonsense.
As far as Magueiro (sp), we had a discussion about him in one of our classes. He makes some simple mistakes in his book, however certain of his works (like introducing a fundamental length scale which might imply a timevarying speed of light) are plausible.
There might be forthcoming experiments that can test this, and the idea has some notable implications for String Theory and other quantum theories of Gravity (loop Quantum gravity, etc)
Nathan said:
I like this idea. So the guy goes back in time, kills his parents before he's born... but he's still there because he's disjointed from his own existance. Now, say he goes back to the future - technically he was never born, so his future would've evolved without him, thus he would still be there, but nobody would have any idea who he was, all his things like birth certificates and social security numbers no longer exist... etc.
Tagrineth said:
Well if i was to go back in time and kill my parents . I would not exist in the future. If i don't exist in the future i couldn't have traveled back in time. If i couldn't have traveled back in time i couldnt have killed my parents. Which means i would be alive in the future and able to build the time machine...
jvd said:
That's it. If you were to go back in time and killed your parents then your parents'd not be alive recently since you'd have killed them, but the question is they are still alive, if you were about to kill your parents then how could they still alive. Maybe time wouldn't allow us to change the history.
You simply cannot travel back in time. The only direction you can travel in the 4th dimension of a given universe is forwards, and that is acomlished at a fixed rate for everything. Relativity is the only reason it appears that you can do it faster than you normally would.
Even if it were possible to travel to an alternate universe that was, for all intents and purposes, identical to our universe at some previous point in our time, you would still not be traveling through time. Instead of traveling through the 4th dimension (time), you would be traveling through a 5th dimension, which tied together all the infinite sets of the first 4 dimensions, and links our space-time with the alternate space-time. In fact, the 4th dimension would remain unchanged, which is why you would complete the journey only having aged as much as you would have without making the trip, according to your relative time frame. One could almost argue that the 4th dimension is not a variable, but is constant across all universes.
Even if it were possible to travel to an alternate universe that was, for all intents and purposes, identical to our universe at some previous point in our time, you would still not be traveling through time. Instead of traveling through the 4th dimension (time), you would be traveling through a 5th dimension, which tied together all the infinite sets of the first 4 dimensions, and links our space-time with the alternate space-time. In fact, the 4th dimension would remain unchanged, which is why you would complete the journey only having aged as much as you would have without making the trip, according to your relative time frame. One could almost argue that the 4th dimension is not a variable, but is constant across all universes.
'One could almost argue that the 4th dimension is not a variable, but is constant across all universes.'
Classically, the 4th dimension is time, and it is a variable just like the three spatial ones. It however has a bit of a different behaviour according to special relativity, b/c it really is almost like a fourth spatial dimension.
It has a geometrical signature that is the opposite of the three spatial dimensions, (contrast this with what Newton thought.. 3 dimensions of space that (evolve) independantly of time)
Depending on what a oberservers relative velocity is, the length scale which we attribute to a time variable will CHANGE.
In general relativity, where gravity becomes important, the time variables characteristic unit of length will change depending on all sorts of different variables including the other spatial dimensions.
This is remarkable if you think about it. The clocks we use to measure things actually will give different results depending on what environment we are in. (if you compare to another observers clock)
Classically, the 4th dimension is time, and it is a variable just like the three spatial ones. It however has a bit of a different behaviour according to special relativity, b/c it really is almost like a fourth spatial dimension.
It has a geometrical signature that is the opposite of the three spatial dimensions, (contrast this with what Newton thought.. 3 dimensions of space that (evolve) independantly of time)
Depending on what a oberservers relative velocity is, the length scale which we attribute to a time variable will CHANGE.
In general relativity, where gravity becomes important, the time variables characteristic unit of length will change depending on all sorts of different variables including the other spatial dimensions.
This is remarkable if you think about it. The clocks we use to measure things actually will give different results depending on what environment we are in. (if you compare to another observers clock)
Tagrineth said:
So you would assume that time would allow the existance without any history, wouldn't you? Because he had been there, it doesn't matter on what happens, he would still be there, but he does change the history that he does.
Natoma said:
Or there's no way to come back to his own universe (though it's possible to go to another one which is similar), because as the time passes, the "vibrations" that construct the specific universe would have been changed also since he had been away for more than the smallest single unit of time which demands a changing.
DemoCoder said:
Don't you think that this is just a scenario or more likely an emulation? We wouldn't know what is gonna happen if that really applied, would we? If it violates the law of physics, it doesn't make it less valid in reality perception but in physics perception instead, that it does.
jvd said:
That's the point - you've been disconnected from your own causality due to your time travel. So after you've killed your parents and stopped the cause of your existance, you're still there because you've split off yourself, so to speak. If you go back to the future where you should've stayed, you warp back in the time machine you still have (or whatever) and though it was never invented, it would still exist because it, too, was pulled out of its own causality, again due to time travel.
Problem solved.zidane1strife
Banned
Hmmm, couldn't(ok, this is very very unlikely, nigh impossible.) a bunch of particles instantly come into existence in the present, with the exact patterns as those of someone who existed(will exist... but for an outside observer...) in the future... Thus bringing, in a way, someone from the future to the past?
EDITEDii
I mean from what I've heard, particles come in and out of existence all the time, they can also teleport from one region to another... and although nigh-impossible, a planet or something else can theoretically(although practically impossible.) come forth out of thin air.
EDITEDii
I mean from what I've heard, particles come in and out of existence all the time, they can also teleport from one region to another... and although nigh-impossible, a planet or something else can theoretically(although practically impossible.) come forth out of thin air.
Tagrineth said:
What do you mean, if something had been pulled from its causality, then how the heck could it exist? And as the fact that the time machine would still be there but never invented, don't you think that the sentence itself seems irrelevant? From your point, it's kinda like that some matters would never exist but still exist???? [/quote]
I think traveling back in time is very possible. But there are certain limits to what you can do with it.
If you build a time machine and 5 years later you decide to go back to that point where you built it, just you being there would alter that universe and the universe at that time would split off from the original. Now anything you change won't matter, you have already created a different universe. But let's say you lived in this new place for 10 years but you're unhappy with the changes that you made and you wish to return to your original time. You can't just go back because there is really only one universe to you. You would have to travel back to the exact point where you entered the new universe and return through the gateway (or portal or whatever) that is still open. To the people on the other side waiting for you, you would have left then instantly came back, only 10 years older.
Maybe you would have to setup seperate portals ahead of time (or behind) tuned to different "frequencies" kind of like an in-out door to get it to work without telefragging yourself.
I haven't read any books on it, mainly because no one knows for sure what would happen. But I think it's is a good possibility
If you build a time machine and 5 years later you decide to go back to that point where you built it, just you being there would alter that universe and the universe at that time would split off from the original. Now anything you change won't matter, you have already created a different universe. But let's say you lived in this new place for 10 years but you're unhappy with the changes that you made and you wish to return to your original time. You can't just go back because there is really only one universe to you. You would have to travel back to the exact point where you entered the new universe and return through the gateway (or portal or whatever) that is still open. To the people on the other side waiting for you, you would have left then instantly came back, only 10 years older.
Maybe you would have to setup seperate portals ahead of time (or behind) tuned to different "frequencies" kind of like an in-out door to get it to work without telefragging yourself.
I haven't read any books on it, mainly because no one knows for sure what would happen. But I think it's is a good possibility
The particles themselves are not teleported - only information about the quantum state is "teleported" across a distance. This article is a good little primer about it.
Feynman's absorber theory (with Wheeler) has electromagnetic waves travelling "forwards" and "backwards" in time - it's rather fundamental to the theory. Basically, accelerating charges emit waves backwards/forwards in time; other charged particles interact with the waves, causing oscillations which produce em waves forwards/backwards in time too. He developed the theory to explain the rather thorny problem of self-interaction. Anyway, from my own vague memories of Feynman's sum of histories approach, I seem to recall some people using it to figure out the probabilities of the paths taken by objects travelling through time tunnels - if I remember correctly, the self-inconsistent solutions were very low prob, whereas the self-consistent ones (ie. you don't go and kill mummy just on the point of your conception) are high prob.
Although most working theories currently employed in physics hold CPT conservation as fundamental, a few weak force interactions (the decay of the neutral kaon is the only one I can think of right now) do not conserve. Of course, going from simple fundamental particle interactions running "backwards" in time to something like a complex organism going back to watch dinosaurs get flattened by an asteroid is another thing altogether.
zidane1strife
Banned
Hmmm, I thought particles could actually teleport... I've heard that a beam of photons has been teleported from a lab to another one, and that they'll try to do the same with an atom later, although that will prove difficult. I thought teleportation was the reason that a black hole fades away after a while(long time if it's big, short time if it's very very tiny.).
Anyways I see another way to travel to the past... How about the person exits the universe, and from the outside go into the point in time he wishes to go and alters it.
PS I'll read the article now...
Anyways I see another way to travel to the past... How about the person exits the universe, and from the outside go into the point in time he wishes to go and alters it.
PS I'll read the article now...
Fred - it's been 14 years since I did any major studies with quantum physics (the only thing I do now is the basics, including Feynman's sum over histories, for 16-18 year old students), so I'm not surprised field theory has tidied up a lot of nasties! 8)
zidane1strife - you're think of Hawking Radiation with black holes; it's not a teleporting thing. Being it late now, my best explanation would be along the lines of pairs of virtual particle-antiparticles create and annihilate themselves all the time in a vacuum. Near the event horizon of a black hole, it's possible for one of these pairs to cross over the boundary and never be able to annihilate itself with the virtual partner. The other particle effectively becomes "real" and to balance the books of mass "just appearing", the black hole itself loses mass. I'm sure Fred can give a better explanation that this!
zidane1strife - you're think of Hawking Radiation with black holes; it's not a teleporting thing. Being it late now, my best explanation would be along the lines of pairs of virtual particle-antiparticles create and annihilate themselves all the time in a vacuum. Near the event horizon of a black hole, it's possible for one of these pairs to cross over the boundary and never be able to annihilate itself with the virtual partner. The other particle effectively becomes "real" and to balance the books of mass "just appearing", the black hole itself loses mass. I'm sure Fred can give a better explanation that this!
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