I’m pretty sure that this is the most off-topic in this off-topic forum. Anyway, IMO there’re several probabilities about time order. The first one is the linear timestream, in this case, there’s only one timestream and the idea of “time traveling†in this scenario is turning back time. Every single disturbance that occurs as it shouldn’t be would cause a difference or the changing of history. Let’s make a scenario, a person made a time machine, he came back to the past (before he was born) and killed his own mother. The logical explanation is that person would vanish, wouldn’t he? But if he vanished, then there’s nobody who killed his mother, thus his mother was still alive cause the effects wouldn’t be able to exist without the cause. If the mother was never killed, the history would never change and he would give birth to an inventor of time machine and would be killed by him and we would be stuck in an infinite loop. Anybody has a thought? Is there any problem-solving for this paradox?
Timetravelling paradox
- Thread starter embargiel
- Start date
Well I think Anne McCaffrey put it well in the Dragonriders of Pern series.
Basically, time travel can work, in that any interventions... have already happened. And you can't avoid them. In other words, if you went back in time to kill someone, you will not be able to because it didn't happen. However, if, say, someone was killed mysteriously, and you went back to see who did it, and nobody did... then it could be that you did it, and when you went back, you decided to kill the person. Or something along those lines. So you killed them all along, you just didn't know it, until you went back in time and realised nobody was there to kill him, so you did kill him. Or something like that.
(the big one in the book series, was a huge number of people vanished one time way back in the past, and it turned out that one of the characters of the 'present' had gone back in time and warped them forward - which was why they vanished)
Yes, it's weird, but it kinda makes sense.
Basically, time travel can work, in that any interventions... have already happened. And you can't avoid them. In other words, if you went back in time to kill someone, you will not be able to because it didn't happen. However, if, say, someone was killed mysteriously, and you went back to see who did it, and nobody did... then it could be that you did it, and when you went back, you decided to kill the person. Or something along those lines. So you killed them all along, you just didn't know it, until you went back in time and realised nobody was there to kill him, so you did kill him. Or something like that.
(the big one in the book series, was a huge number of people vanished one time way back in the past, and it turned out that one of the characters of the 'present' had gone back in time and warped them forward - which was why they vanished)
Yes, it's weird, but it kinda makes sense.
I remember reading somewhere that the furthest point back in time you could reach is limited by the invention of time travel. So if someone invented the machine today @12pm then that is the limit to reverse excursions.
As for an infinite loop, I don't think that could happen. I believe that causality would splinter and the looping part would just cease to exist in any measurable time frame. The energy that made up everything in the blip would be re-absorbed into the multiverse.
And if it did happen then the effect would most likely have a local focus point. It wouldn't affect anything other than itself.
I would think that since the big bang that quite a few errant high energy particales have got themselves caught up in a temporal blip of some kind.
As for an infinite loop, I don't think that could happen. I believe that causality would splinter and the looping part would just cease to exist in any measurable time frame. The energy that made up everything in the blip would be re-absorbed into the multiverse.
And if it did happen then the effect would most likely have a local focus point. It wouldn't affect anything other than itself.
I would think that since the big bang that quite a few errant high energy particales have got themselves caught up in a temporal blip of some kind.
Hawking postulated the chronology protection conjecture. Its his belief (shared by most physicists), that any CTC (closed timelike curve, physics jargon for a path that travels back in time) will be impossible due to quantum gravity effects negating the appearence of such an object.
Its unproven of course, and the unfortunate thing is that in General Relativity (a classical theory) time travel is quite possible theoretically. Moreover, even in quantum field theory, tachyonic motion occurs, and it seems more and more likely that we cannot rule out such occurences (String theory and various QFTs thrive on Tachyons for instance). Fortunately we can in some instances reidentify the objects as propogating causally.
There's other interpretations such as the sum over histories approach (not shared by most physicists) that could in principle save causality.
Either way, its irratating to scientists that such eventualities seem to be allowed, and a large portion of work is underway to get rid of the problem. None of this probably will be final until a full quantum theory of gravity is found.
The subject of time is still hotly debated in science, as there are a number of different 't' variables that seem to be mutually distinct. For instance, there is the 't' associated with entropy. Upon time reversal, entropy however always increases (its an invariant).
There's the socalled Friedmann covariant time 't' that is identified with the start of the bigbang, and the propagation of the equations of state and motion of our universe.
There's quantum mechanical times 't', and then there's causality times 't' (associated with past, present, and future local events), etc etc
Most of these are consistent with one another, if you do a little mental and mathematical juggling, however there are subtelties that are still fiercely contested.
Its unproven of course, and the unfortunate thing is that in General Relativity (a classical theory) time travel is quite possible theoretically. Moreover, even in quantum field theory, tachyonic motion occurs, and it seems more and more likely that we cannot rule out such occurences (String theory and various QFTs thrive on Tachyons for instance). Fortunately we can in some instances reidentify the objects as propogating causally.
There's other interpretations such as the sum over histories approach (not shared by most physicists) that could in principle save causality.
Either way, its irratating to scientists that such eventualities seem to be allowed, and a large portion of work is underway to get rid of the problem. None of this probably will be final until a full quantum theory of gravity is found.
The subject of time is still hotly debated in science, as there are a number of different 't' variables that seem to be mutually distinct. For instance, there is the 't' associated with entropy. Upon time reversal, entropy however always increases (its an invariant).
There's the socalled Friedmann covariant time 't' that is identified with the start of the bigbang, and the propagation of the equations of state and motion of our universe.
There's quantum mechanical times 't', and then there's causality times 't' (associated with past, present, and future local events), etc etc
Most of these are consistent with one another, if you do a little mental and mathematical juggling, however there are subtelties that are still fiercely contested.
MrsSkywalker
Newcomer
Hm, intersting. I would think, however, that time would be relative to the individual. So as his life may be in a never ending loop, the rest of the world would move on without him. Like people walking by a perpetual motion machine. The motion never changes, back and forth, back and forth, but the world goes on it's way.
You would think, however, that someone smart enough to build a time machine would realize the stupidity of killing his own mother before he's born...
I believe that it is impossible to travel through time, as we're discussing it. As we move, we travel through 3-dimensional space, without even giving much thought to it (4 dimensional space when taking into account time).
Looking at it from that perspective, I'm of the belief that traveling through time is not actually traveling through time, but traveling through a higher dimension to a parallel earth that just so happens to coincide with the particular timeframe that you're trying to reach. As anyone who knows the theories surrounding multiple universes has learned, every particulate action to ever take place can have an infinite amount of possible outcomes.
So there is a universe out there that has progressed exactly as ours has, but is 200 years behind ours. Or another universe that is 1 million years behind ours. Or another universe that is 300 years ahead of ours.
Because of the fact that the instant your time machine reappears in the future or the past, you have changed the course of history, even infintessimally, you simply cannot be in your own universe anymore. I believe that you will have in fact entered into an existing universe, or will have created a completely new one.
The *only* way imo to then get back to your own universe would require you to find the particular vibrations that designate that that one is yours, think of a universal ID card, and return at the *exact* moment you left. Hell, it might be impossible to return exactly to your universe, but possibly one that is almost exactly the same, except that you reappeared in one universe, and not the other.
Who knows. The whole concept is one that is fun to think about, and it solves all of the problems associated with time travel. Though it requires you to accept the possibility of more than four dimensions, which some people have trouble with. hehe.
Looking at it from that perspective, I'm of the belief that traveling through time is not actually traveling through time, but traveling through a higher dimension to a parallel earth that just so happens to coincide with the particular timeframe that you're trying to reach. As anyone who knows the theories surrounding multiple universes has learned, every particulate action to ever take place can have an infinite amount of possible outcomes.
So there is a universe out there that has progressed exactly as ours has, but is 200 years behind ours. Or another universe that is 1 million years behind ours. Or another universe that is 300 years ahead of ours.
Because of the fact that the instant your time machine reappears in the future or the past, you have changed the course of history, even infintessimally, you simply cannot be in your own universe anymore. I believe that you will have in fact entered into an existing universe, or will have created a completely new one.
The *only* way imo to then get back to your own universe would require you to find the particular vibrations that designate that that one is yours, think of a universal ID card, and return at the *exact* moment you left. Hell, it might be impossible to return exactly to your universe, but possibly one that is almost exactly the same, except that you reappeared in one universe, and not the other.
Who knows. The whole concept is one that is fun to think about, and it solves all of the problems associated with time travel. Though it requires you to accept the possibility of more than four dimensions, which some people have trouble with. hehe.
it could be another possibility. In this case, the history would be just one, and the time travelling itself had been recorded in history. Even the twisted time couldn't change the history for it's been there. Example, your money just dissapear without any logical enlightenment, you wanted to go back to your past to know what is the reason behind that only to notice that you would take your own money and you'd be the reason behind that. This is what I mean:
LOL, I was just trying to make a discussion, if you thought that it was stupid than I'm sorry.
Or go to another universe which has a same state as ours, in this scenario, it could "seem" like going back to the past as the fact that you're only "switching" the time stream into another one which was totally the same with yours.
CTC is a timeline that will go back to the state it was before right. It's kinda like curve and at certain points and under specific circumstances, it would go back to the beggining of the loop, isn't it?
MrsSkywalker
Newcomer
Not at all! It's good to take a break from war discussions, and it was something interesting to think about today. I discussed this with my Dad, who is a big time sci-fi guy, and he scratched his head, said, "Huh." and then went to draw it all out on paper before concluding that the only one who could answer it would be the time machine builder if he had the foresight to leave a notebook of his life's happeneings where he would inevitably find it once he is again an adult. I'm glad he didn't try and support us as a sci-fi writer when we were growing up!
Fred said:
Then how do they explain the solutions Godel found? I can understand this used on NUTs, but is Godel still rulled out on practical means only? Or am I just missing this, which is a definite possibility.
Also, did you read Joao Magueijo's new book? From the little I've seen him speak on C-Span, it looks compelling. Or, perhaps thats just my subconscious want to see 'classical' science proven wrong?!? First c, next the particle.
Travelling back in time would cause a discontinuity. Our protagonist suddenly appear in his chosen time-destination. Maybe the "causality-link" between himself in the past and himself in the present would be severed by the discontinuity. He could then kill his mother without disappearing in a puff of logic; he would simply continue living.
I don't particularly like Anne McCaffery's idea. What would be stopping you from killing your mother? It would either have be to fate conspiring against you (which means that you effectively have no feel will, since your actions are predetermined) or a higher power stopping you (God).
I don't think discontinuous time travel is possible. Travelling at relativistic speeds is certainly reasonable way to travel through time - if you only want to go forwards...
I don't particularly like Anne McCaffery's idea. What would be stopping you from killing your mother? It would either have be to fate conspiring against you (which means that you effectively have no feel will, since your actions are predetermined) or a higher power stopping you (God).
I don't think discontinuous time travel is possible. Travelling at relativistic speeds is certainly reasonable way to travel through time - if you only want to go forwards...
kyleb said:
Yeah, exactly, thats why I asked if Godel's was found to be impossible for any reason other than purely practical means (eg. the fact that it must take place in a slowly rotating universe.) Because, it's consistent with all of the known field theories and is self-consistent AFAIK. Thus, it's a plausible outcome filled with crazy pathologies that violate Mach's principle and has CTCs and it only not possible for practical reasons, but is possible.
As for NUT, you can insert these types of singularities (with some work)into any type of expansing universe model or black-hole AFAIK.
But, you might be right...
David Deutsch wrote the definitive essay on this matter (The Fabric of Reality), that shows once and for all that the type of recursive time travel often imagined in science fiction simply isn't possible, but "sliders" type time travel (travel to the past is really travel to a parallel universe, but it won't effect the future, you won't meet yourself, etc) is still a logical possibility.
He does this quite ingenously, in a way that does not depend on the laws of physics, by drawing inspiration from computer science. He first posits a "universal reality renderer", like a universal Turing machine, a sort of uber-startrek-holodeck, that can simulate any laws of physics, including our own.
Then, using this virtual reality scenario, he posits the construction of a time travel device, that will attempt to simulate what it is like to time travel. That is, imagine you went onto the holodeck of the Enterprise, and asked it to simulate the experience of a time machine in your laboratory, that could send you five minutes into the past, in your laboratory.
While exploring the repercussions of this, he shows that to make the simulation behave as shown in sci-fi, that is, you can not only travel back into time, but you can interact with the past, and affect the future, you in essence, have to violate the Halting Theorem. You end up with a logical contradiction, and this logical contradiction is completely independent of the laws of physics, so one of your assumptions must be wrong.
It turns out, that the assumption which is wrong, is the single-universe theory. If you assume that time travel to the past, is really time travel to a parallel universe (e.g. Quantum Many Worlds interpretation), a "Sliders"-like time travel, then it is possible to work out logically, otherwise, it contradicts logic.
He does this quite ingenously, in a way that does not depend on the laws of physics, by drawing inspiration from computer science. He first posits a "universal reality renderer", like a universal Turing machine, a sort of uber-startrek-holodeck, that can simulate any laws of physics, including our own.
Then, using this virtual reality scenario, he posits the construction of a time travel device, that will attempt to simulate what it is like to time travel. That is, imagine you went onto the holodeck of the Enterprise, and asked it to simulate the experience of a time machine in your laboratory, that could send you five minutes into the past, in your laboratory.
While exploring the repercussions of this, he shows that to make the simulation behave as shown in sci-fi, that is, you can not only travel back into time, but you can interact with the past, and affect the future, you in essence, have to violate the Halting Theorem. You end up with a logical contradiction, and this logical contradiction is completely independent of the laws of physics, so one of your assumptions must be wrong.
It turns out, that the assumption which is wrong, is the single-universe theory. If you assume that time travel to the past, is really time travel to a parallel universe (e.g. Quantum Many Worlds interpretation), a "Sliders"-like time travel, then it is possible to work out logically, otherwise, it contradicts logic.
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