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embargiel
25-Mar-2003, 02:43
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?

Sxotty
25-Mar-2003, 02:51
Well one view is that somehow you just could not do it, like if you tried it would never work out because something would always get in your way, but I don't really care to much because I don't have a time machine

yet. ;p

Tagrineth
25-Mar-2003, 03:01
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.

Crusher
25-Mar-2003, 03:09
I think the generally accepted hypothesis is that it is only possible to travel forwards in time, and then only in a relative sense, and not instantly.

pcchen
25-Mar-2003, 03:56

kyleb
25-Mar-2003, 07:06
I think the generally accepted hypothesis is that it is only possible to travel forwards in time, and then only in a relative sense, and not instantly.

sure enough.

BoardBonobo
25-Mar-2003, 10:39
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.

kyleb
25-Mar-2003, 15:43
i have yet to see one reasonably plasuable explanatin for traveling backwards in time, i think you are reading sci-fi BoardBonobo. :wink:

Althornin
25-Mar-2003, 15:48
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.

This is how i see it.
it also nicely resolves the whole idea of "time paradoxes" by making them impossible.

Fred
25-Mar-2003, 16:26
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.

25-Mar-2003, 16:54
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.

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... :wink:

Natoma
25-Mar-2003, 17:36
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.

embargiel
25-Mar-2003, 22:14
Originally posted by Sxotty
Well one view is that somehow you just could not do it, like if you tried it would never work out because something would always get in your way, but I don't really care to much because I don't have a time machine

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:

Originally posted by Tagrineth
Basically, time travel can work, in that any interventions... have already happened.

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...

LOL, I was just trying to make a discussion, if you thought that it was stupid than I'm sorry.

Originally posted by Natoma
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.

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.

Originally posted by Fred
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.

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?

26-Mar-2003, 03:50
I was just trying to make a discussion, if you thought that it was stupid than I'm sorry.

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! :D

jvd
26-Mar-2003, 05:02
Well. Anyone see the futurerama when they go back in time to right before roswell and it turns out that they are the aliens . Also fry gets his grandfather killed , sleeps with his grandmother and that is why he still exists . that was a funny eps

Vince
26-Mar-2003, 05:30
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.

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. :wink:

kyleb
26-Mar-2003, 07:50
i am pretty sure Godel's solutions as well as the NUT theory one were based on models of universes different than our own, so they are not really viable concepts.

Nathan
26-Mar-2003, 08:02
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...

Vince
26-Mar-2003, 09:11
i am pretty sure Godel's solutions as well as the NUT theory one were based on models of universes different than our own, so they are not really viable concepts.

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...

DemoCoder
26-Mar-2003, 10:06
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.

Crusher
26-Mar-2003, 11:09

So do Women and the use of imaginary numbers in calculations, but they're both commonly found... :wink:

DemoCoder
26-Mar-2003, 11:37
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.

Natoma
26-Mar-2003, 14:28
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.

*cough*

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.

:)

V3
26-Mar-2003, 18:38
The logical explanation is that person would vanish, wouldn&amp;#8217;t he? But if he vanished, then there&amp;#8217;s nobody who killed his mother, thus his mother was still alive cause the effects wouldn&amp;#8217;t be able to exist without the cause.

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 ?

Fred
26-Mar-2003, 19:05
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)

Tagrineth
26-Mar-2003, 20:02
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 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.

jvd
26-Mar-2003, 22:57
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 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.

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...

embargiel
26-Mar-2003, 23:15
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...

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.

Crusher
26-Mar-2003, 23:22
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.

Fred
27-Mar-2003, 00:21
'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)

embargiel
27-Mar-2003, 00:45
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.

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.

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.

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.

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.

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.

Tagrineth
27-Mar-2003, 15:23
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 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.

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...

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. :P Problem solved.

zidane1strife
27-Mar-2003, 15:53
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.

embargiel
30-Mar-2003, 00:53
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.

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]

30-Mar-2003, 17:46
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

Tahir2
30-Mar-2003, 17:51
Axiomatic by Greg Egan.

Neeyik
30-Mar-2003, 20:28
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...
The particles themselves are not teleported - only information about the quantum state is "teleported" across a distance. This (http://physicsweb.org/article/world/11/3/9/1) 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.

Fred
31-Mar-2003, 19:22
Feynman's absorber theory is no longer held as the penultimate say on the subject.

Quantum field theory has advanced sufficiently that we can abstract away some of the annoying parts of both Feynman and Schwingers theories.

zidane1strife
31-Mar-2003, 20:38
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...

Neeyik
31-Mar-2003, 23:11
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!

BoardBonobo
31-Mar-2003, 23:32
I watched "The One" @ the weekend. Jet Li busting some funky multiverse kung fu style paradox busters. He can't act but it was an interesting story line...

How could a paradox happen in time if we exist in an infinite continuum where every possible effect of every conceivable cause has already happened? Wouldn't the paradox have already happened and therefore, chucking aside free will etc.. ;), it was already determined that paradox would occur and that our time traveller would experience a mico second of horror before they realise their ultimate fate...forever.

K.I.L.E.R
01-Apr-2003, 03:09
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.

Hence dark matter. If you know what I mean? :)

PC-Engine
01-Apr-2003, 07:28
What would happen to your memory if you were to truly travel backwards in time?

zidane1strife
01-Apr-2003, 14:07
it's possible for one of these pairs to cross over the boundary and never be able to annihilate itself with the virtual partner.

Cross over the boundary? The event horizon? Is it created outside the event horizon, or does it actually cross over it... Or are you referring to some other boundary. If it's the event horizon only possible way to do that would be through teleportation, no?

Deepak
02-Apr-2003, 09:36
looks like time travelling is like OOPs (object orinted programming).... :lol: ....endless loops.. :D

Deepak
02-Apr-2003, 09:39
Recursion..... :lol:

Neeyik
02-Apr-2003, 10:57
Cross over the boundary? The event horizon? Is it created outside the event horizon, or does it actually cross over it... Or are you referring to some other boundary. If it's the event horizon only possible way to do that would be through teleportation, no?
Eh? Why are you on about teleportation still? The particle-antiparticle pair are created just "outside" of the absolute event horizon of the black hole - during the tiny amount of time they exist for, it is perfectly possible for them to circumscribe little paths in spacetime (just as long as they annihilate each other at the end of this). However, if one of the pair passes over the event horizon along this path it will not be able annihilate itself with the other particle.

Due to vacuum fluctuations, you would "see" a cloud of particles radiated from the event horizon - each particle being one half of the pair created; the other half having popped across the event horizon, never to return. Since this really would violate the conservation of mass/energy outside the uncertainty principle, the black hole itself loses the same mass that it radiates away. You can read more about the calculations and whatnot at this (http://library.thinkquest.org/C007571/english/advance/english.htm) site.

Bigus Dickus
02-Apr-2003, 17:28
My own personal thoughts echo those of David Deutsche in this matter. Time travel is impossible in a single universe, but allowed in the multiverse (but then it's not really time travel, just a form of travel having that appearance at a cursory glance).

zidane1strife
03-Apr-2003, 10:33
Oh I see. It's just that I'd heard something about an energy threshold or something that sometimes particles could surpass and beam over to the other side... I'd thought it was in a black hole related paper, but it appears I'm mistaken.

zidane1strife
03-Apr-2003, 13:59
Looked at the black hole site, and found this:
In fact we can't really prove it without a lot of quantum physics (It depends on the tunnel-effect).

That's what I was talking about when I said teleportation.

another site said something similar, to what I think, about it...

"Imagine throwing a handful of pebbles lightly at a window, and most of them bounced back, just as you expect, but a few of them disappeared on this side of the glass and reappeared on the other side of the window without breaking the glass. Welcome to the Tunnel effect in the surreal world of quantum physics where the pebbles are smaller than atoms.

In modern electronics tunnel diodes take an electron from here and put it there without allowing it to occupy the intervening space, or as the textbooks drily put it, "the penetration of the wave function into the classically forbidden region." Scanning tunneling electron microscopes work on this principle. An electric field applied to a metal tip so that the electrons in the tip have enough energy to reach a metal surface underneath for a short distance. However the electrons cannot exist in the vacuum between the tip and the surface. A small current results from the electrons tunneling out of the tip, teleporting from the tip to the surface. No electron can be detected between the tip and the surface.

The electron "pebble" on this side of the barrier has a tiny chance that it
could also be a metre to the left, ten centimetres upwards, or even on the
other side of the barrier. So when you send lots of low energy electrons or
photons against a barrier, a small fraction of them appear on the other
side.

This kind of disappearing and re-appearing act happens all the time within
individual atoms. Electrons orbit an atomic nucleus in shells of different
sizes and distances, sort of like a planet orbiting the sun, but where the
planet is smeared out into a ball around the sun. The electrons can move
nearer or closer or further away from the nucleus as they gain or lose
energy, say by absorbing or emitting light. However, the electrons are not
allowed to be between these shells. The space between orbits is a forbidden
zone. So how do electrons make the jump from an inner orbit to an outer
orbit if they're not allowed to travel the space in between - the
no-mans-land Pauli exculsion zone? The answer is that they use the tunnel
effect. They disappear from their old orbit and reappear in the new orbit,
without the bother of actually moving through the space in-between. It kind of puts old Captain Kirk to shame.

This is what I'd call REAL teleportation. Captain Kirk had the kind of
teleportation being researched in the labs of the University of Wales, IBM
laboratories, and many other institutions. Its a major factor in unbreakable
quantum cryptography. Briefly you convert the original Kirk into a coded
signal, killing him in the process. You then send this file by radio or by
post, or by wire to the receiver, where Kirk is resurrected - destroying his
file in the process. You'd never get me travelling on one of the damn
things! And of course, the maximum speed is lightspeed."

http://www.abc.net.au/science/morebigquestions/tunnel.htm

I, dunno if that is an appropiate definition, but that's what I was referring to.

EDITED

zidane1strife
03-Apr-2003, 14:18
As for my thoughts on time travel. I think ANYTHING IS POSSIBLE, there is nothing IMPOSSIBLE in this world.

A great man once said that the greatest science would be indistinguishable from magic... I say the apex of knowledge goes beyond magic and that which can be fanthomed by men... ALL laws can bend to the point where they are no more... when they are broken. Causality, the laws of physics, there is no boundary that cannot be undone.

embargiel
04-Apr-2003, 02:14
A small current results from the electrons tunneling out of the tip, teleporting from the tip to the surface. No electron can be detected between the tip and the surface.

Does tunnel effect also mean "worm hole"? When you can simply going from one point to another without doing it in a "traditional" way. When you can simply skip all the space that exist between that 2 different points simply by bending them together which makes the nearest distance of those points is not a straight line anymore?

Neeyik
04-Apr-2003, 10:51
Teleportation isn't the best word to use with the quantum tunnelling effect - the object isn't really "disappearing" from one place and then "reappearing" in another. Particles tunnel through boundaries because of the uncertainty principle; the size of the indeterminacy of the particle's position/wavelength happens to be in the same order as the size of the boundary. Therefore, when attempting to ascertain exactly where the particle is, there will be finite probability that it's on the other side of the boundary that you expect it to be. Other conditions will affect the wave function and thereby alter the probability as to where it will be.

The electrons in a STM tunnel across the gap between the examined surface and probe tip because (a) the indeterminacy in their positions covers the surface, gap and probe; (b) there is a large potential difference across the probe, so if the electrons do "appear" in the probe then they are accelerated rapidly away from the gap and therefore are unlikely to "appear" in the surface again. The overall sum of these probabilities means that you get a small current flow (no more than a nanoamp) between the surface and the probe.

This site (http://www.chembio.uoguelph.ca/educmat/chm729/STMpage/stmtutor.htm) is a nice little starter for STMs.

There's also the point that the distance covered by tunnelling is incredibly small - it's not like humans could ever make use of this!

Wormholes come about due to certain solutions of the equations in general relativity, rather than the quantum tunnelling effect. Those solutions give rise to what are called Schwarzschild black holes (no charge, no angular momentum), which Einstein and Rosen discovered some time later, that SBHs could also represent a tube that connects two regions of spacetime together (aka a wormhole). This idea also solves the "problem" of having a singularity with a black hole.

Anyway, forward on several decades, and you've got also sorts of top ideas about wormholes. Use exotic material (negative energy density matter) to prevent the wormhole entrance from collapsing (which it would do in mere fractions of a second). You've got Thorne wormholes, Hawking wormholes, Coleman wormholes; barking - the lot of them! :wink: As interesting as wormholes may be, no are going to be viable prospects for human teleportation.

K.I.L.E.R
04-Apr-2003, 11:32
You want to teleport?

Get a machine that will make an exact copy of you and your thoughts/memories etc... (assuming that the machines can do that, despite us being all chemical).

The machines make you into 1 particle, the particle is displaced for 10 years, the machine then makes you back into human form (you know, billlions or whatever particle being) and then you would have gone forward by 10 year. If you have aged or not is another issue. :)

You can't go back in time as time is a constant. Only goes forward, not backwards.

zidane1strife
04-Apr-2003, 14:00
Teleportation isn't the best word to use with the quantum tunnelling effect - the object isn't really "disappearing" from one place and then "reappearing" in another.

Well, it might not be teleportation, but it sure feels like it. Some researches(yes, I know you guys obviously know this already, but still...) have made photons quantum tunnel and arrive at their destination several fold the speed of light. Some have even sent mozart using such technics.

Neeyik
04-Apr-2003, 14:31
"Time" is definitely not a constant; any more than spacial dimensions are. The arrow of time (forwards or backwards, etc) makes no distinction in sub-atomic processes, such as particle interactions - there is a symmetrical solution to things like Maxwell's equations when having waves travelling forwards or backwards in time.

Entropy, though, could be used to define the arrow of time for the macroscopic world that we live in. Natural processes increase in entropy; start with something simple and ordered, finish with more complexity and less order. Since the whole universe seems to be increasing in entropy, we could use that to declare the arrow of time. However, it is perfectly possible (with an open system) to do the opposite of this by the input of energy - we, as humans, can decrease entropy for given systems. Using the entropy definition of the arrow of time, it seems feasible (if just not necessarily possible) for a system to go backwards in time.

JohnH
07-Apr-2003, 13:41
Anyone read "Time Ships" by Steven Baxter ?

John.

Crusher
07-Apr-2003, 17:16
First off, nobody ever said time was a constant. What I said was

One could almost argue that the 4th dimension is not a variable, but is constant across all universes.

and I said that in the context of traveling between parallel universes. My belief is that if multiple universes exist, then they exist at the same time. That is, I think that given a coordinate system describing an infinite set of space-time universes (x,y,z,t,d), where x,y, and z are the 3 dimensions of our universe, t is the time of our universe, and d is a 5th dimension that binds all sets of space-time to our own, then at any given moment, I believe t holds the same value in all space-time sets.

I think the only possible way a human being could feasibly visit a scenario where all values of (x,y,z) are identical to those in our universe at some previous time t_past = t_present - T, is to travel through the 5th dimension to a different universe, where everything has occurred exactly the same as it has in our own universe, except the Big Bang happened T later than it did in our own universe. Being that it is a completely different universe, the affects of your presence there does not alter this universe, thus allowing you to kill an identical version of your mother in that universe without erasing your existence or causing the entire system to collapse. However, your presence in that universe would coincide with your absence in this universe, and if you spent a day traveling to the parallel universe and back, then it would be a day after you left when you returned to this universe.

None of this can ever be proven, however, and it's just my opinion. If you want to believe that it's possible to travel backwards through the time dimension in our own universe, be my guest. I don't think that's possible, and even if it were, it's not a good idea to do it. You're better off taveling through the 5th dimension and altering someone else's universe where your actions don't affect events that have already taken place.