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kanon14
2003-Oct-13, 06:24 AM
you guys must have heard about the paradox. some suggested that when you travel back in time and changed the history, from then on you "created" a parallel universe.
my question: isn't that we cannot create/destroy matter/energy? how can someone "create" an entire universe?

kenneth rodman
2003-Oct-13, 07:35 AM
maybe the matter\energy has always existed and was just waiting to be shaped?

kanon14
2003-Oct-13, 08:18 AM
but how can one travels from one universe to another instantaneously?

Eroica
2003-Oct-13, 10:53 AM
I think the argument is that you can't actually travel back in time. So when someone appears to have gone back in time to our past, they have really left our universe and travelled to an already-existing parallel universe (which they can then proceed to mess up in any way they choose).

Of course, it's all just mathematical speculation. There is not the slightest bit of evidence that there are any other universes. Some people think that because their existence is compatible with relativity or quantum theory, they must exist. But it's just as likely that there are loopholes in these theories which will eventually be closed up by better theories.

kenneth rodman
2003-Oct-13, 10:56 AM
well, your not really traveling back in time but to an alternate universe where time is x amount of years behind the universe you left. If this is the case then your actions do not create a parallel universe, the parallel universe has always existed. Also neatly gets around the theory that the past cant be changed by a theoretical time traveler.

AstroSmurf
2003-Oct-13, 11:48 AM
My concern is not so much that you create an "alternate" universe, it's that you remove the matter in your own body from this universe, and transfer it to the other universe. This violates the law of conservation of matter/energy in both universes, separately. This is because that, from the viewpoint in one universe, the mass either vanishes or emerges without possible explanation, creating a violation of this law. The other universe is unobservable, and thus cannot influence how the process appears in this one.

The other main problem with time-travel is causality violations, and those will have to be taken into consideration whether you travel from one universe to another or from one point in time to another. If we're talking universe shifting, you can still create a causality violation by travelling from universe A to universe B, and then from B back to A in a different point in time. To prevent this would require some sort of trans-universal timescale, aka an absolute frame of reference - violation of relativity, anyone? :lol:

Time-travel really opens up a whole bag of snakes, doesn't it? ;)

Normandy6644
2003-Oct-13, 12:35 PM
My concern is not so much that you create an "alternate" universe, it's that you remove the matter in your own body from this universe, and transfer it to the other universe. This violates the law of conservation of matter/energy in both universes, separately. This is because that, from the viewpoint in one universe, the mass either vanishes or emerges without possible explanation, creating a violation of this law. The other universe is unobservable, and thus cannot influence how the process appears in this one.

The other main problem with time-travel is causality violations, and those will have to be taken into consideration whether you travel from one universe to another or from one point in time to another. If we're talking universe shifting, you can still create a causality violation by travelling from universe A to universe B, and then from B back to A in a different point in time. To prevent this would require some sort of trans-universal timescale, aka an absolute frame of reference - violation of relativity, anyone? :lol:

Time-travel really opens up a whole bag of snakes, doesn't it? ;)

Just out of speculation, when you "time travel" from this universe to another your mass is converted into energy, keeping the conservation laws. Don't ask me what this energy is, I would just imagine that it would have to be there.

The second part is a bit easier. If you travel from universe A into universe B, you cannot return to A. You can return to something like A, but since you time traveled out of A the universe you travel back to must be some universe C. Did that makes sense? #-o

AstroSmurf
2003-Oct-13, 01:05 PM
Um ... if the body mass of the traveller is converted into energy, I would highly recommend you do your experiments elsewhere. Such as, on another planet. :o

Anyway, the argument remains whether you're talking matter or energy. You made an attempt to explain the situation in the universe I'm travelling from, but how does it work in the other one? Where does the mass/energy come from?

Normandy6644
2003-Oct-13, 04:37 PM
It could be some kind of existing energy around for this exact purpose. It might be in a form that we are currently unaware of. I'll think about it some more and get back to you.

PS- I'm liking this conversation so far. Very thought provoking :D

Donnie B.
2003-Oct-13, 06:12 PM
There's another problem with time travel. It's not feasible to travel in only the time dimension.

Say you were able to travel to "yesterday" without changing your absolute position (assuming, for the moment, that there is such a thing). But yesterday, the Earth was not where it is right now... it was 1/365 of the way "back" along its orbit -- roughly 1.6 million miles away from where we are now. Similarly, the whole solar system is moving in its orbit around the center of the galaxy, the galaxy is falling toward the Great Attractor, and the universe is expanding...

An even more fundamental issue is that there is no absolute position in the first place. If you travel to yesterday, but want to end up on the Earth of yesterday, you have to travel to where the Earth was yesterday... but relative to what? The Sun? The distant galaxies? How can this even be determined?

You might argue that a time machine would somehow "stick" to the point on the Earth where it starts out. I can't imagine why it would, but let's say it did. So what exactly does it stick to? The continents slide around, faults open, volcanoes erupt, mountains erode. Go back far enough and your time machine could be inside solid rock or under the ocean.

So, to travel in time you must also travel in space. It wouldn't be much fun to head for, say, the year 1066 and pop out in a cold, airless void between the stars, or under a landslide that you didn't know had happened in the meantime!

Donnie B.
2003-Oct-13, 06:18 PM
By the way, the problem I described in the previous post can be circumvented if you stipulate that the time machine must exist and be operating for the entire time you travel across. If that's true, you'd simply pop out of the machine at the opposite end, where/whenever that is/was. The only drawback is that you can't go back any further than the day the time machine was first turned on. I suppose the same argument would hold for travel toward the future -- you could only go as far forward as the machine will still be functioning. {Yikes! There aren't enough tenses to describe these things!}

Normandy6644
2003-Oct-13, 06:20 PM
That's true, but I don't think anyone was really denying this as a principle for time travel. I would think time travel would be the manipulation of all four dimensions, rather than just time (as you said). Nonetheless, assuming we have the means to travel through time I would think the technology would allow for changes in position as well.

mike alexander
2003-Oct-13, 11:12 PM
I'm pretty sure the time/space popout problem was done in a Feghoot many years ago. Anybody remember?

kanon14
2003-Oct-14, 02:23 AM
WoW thx for all the replies :D I didn't expect I’ll get these many replies... time travel must be a very interesting topic ^^

Anyway, before answering to any specific reply, i would post my hypothesis on time travel. but I must first say that for my hypothesis to work I needa make a whole lot assumptions, which are likely to be wrong since I’m not so good at physics as you guys are. Keep in mind that I’m arguing the “logical possibility” here but not the “physical/technical possibility”. So here it goes:

First I have to assume multiverse exists. By that, I mean there exist many universes, some similar to ours, some very different, and some exactly alike. I guess that’s oversimplifying stuff here but you get the general idea. Further, I assume every universe is just a still frame, where all the objects within it are motionless. We perceive our environment as moving because our “senses”, or may I use the word “souls”, travel from one still frame to another. Moreover, our souls are traveling at the speed of light, so that there seems no time has passed when we travel from one universe to the next. It’s just like watching animated cartoon. We perceive the characters as moving objects only because we see the still frames in sequence which generates an illusion of moving objects. Also, it is only our souls that travel, but not our bodies. Our souls travel from one still body in universe A to another still body in universe B.

But how do our souls know which universe to go to? Would they end up in a very different universe then the previous one? Usually not. How they travel is governed by the rules of probability. Let’s say I have an exam at the end of the week. I’ve studied very hard, attended every class and did every assignment, then when I receive the exam question sheet, the probability of me answering the questions correctly is a lot greater than the probability of me sitting in the exam room totally clueless. But that doesn’t mean there is 100% of answering all questions correctly; there is just a greater probability. So when our souls travel from one to next, they would travel according to this rule of probability.

In such universe, the meaning of “time” would be the path connecting all the universes we’ve so far traveled. So time does not actually flow here; it’s just an illusion. Then what does it mean to travel back in time? There are two possibilities.

1) only soul travels. An example could be dreams. When we dream during sleep, our souls travel to universe which could be very different from the previous one, since dreaming could somehow shatter the effect of probability.
2) Both soul and body travel. We generally call this case “time travel”. But how do we get the body to travel at the speed of light? In theory, if there is a machine (let’s not discuss the technical possibility of such machine first) that can convert the body to pure energy, then send the energy and soul to another universe (let’s say the universe I’ve been 10 years ago) and the energy reforms into a body, then there I’ve traveled “back in time”. In this case, even if we did something which could “alter” the history, say killing my 10-years-old self, I’m not really changing the history, since once the other souls, say my mom’s, left that universe 10 years ago, she would have no knowledge about that universe whatsoever, and the changes made in that 10-years-ago universe would have no impact in the "current" one.

I’ll stop here cuz I dun wanna make this post too long and scare out some potential repliers. But before you criticize my hypothesis, make sure that I’m only arguing the “logical possibility” here. I’m not trying to setup a whole new theory in physics/cosmology, but rather I’m arguing the logics within it, such as “is there any contradiction?” or “would it contradict what we perceive everyday?” etc etc.

Looking forward to your replies 8)

Eroica
2003-Oct-14, 10:57 AM
Hmmm, kanon14, your theory of static multiverses is intriguing. I may have to steal it. I prefer the term consciousness, though. It sounds more scientific than soul.

Avatar28
2003-Oct-14, 04:07 PM
Hmmm, kanon14, your theory of static multiverses is intriguing. I may have to steal it. I prefer the term consciousness, though. It sounds more scientific than soul.

Actually, I read that exact theory in Discover a few years ago. Still an interesting idea though.

Amadeus
2003-Oct-14, 04:26 PM
The Largest time travel paradox I know off is the "grandfather paradox"
If you go back in time and shoot your grandfather you will not be born so there wouldn't be a you to go back in time to shoot your grandfather...... so your grandfather wont get shot and there WILL be a you to go back in time to shoot your grandfather......(brain hurts!) :cry:

I think the main answer to this is NOT to send homicdal maniancs back in time :lol:

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kanon14
2003-Oct-14, 09:08 PM
Hmmm, kanon14, your theory of static multiverses is intriguing. I may have to steal it. I prefer the term consciousness, though. It sounds more scientific than soul.

Actually, I read that exact theory in Discover a few years ago. Still an interesting idea though.
really??? do you remember which issue it is? i'd like to read mroe about it cuz i'm sure they would've said much better and much deeper than i did
and hopefully they can provide more scientific proof of such theory

kanon14
2003-Oct-14, 09:16 PM
Hmmm, kanon14, your theory of static multiverses is intriguing. I may have to steal it. I prefer the term consciousness, though. It sounds more scientific than soul.
yea "consciousness" sounds much more scientific. I just borrowed the term "soul" cuz it may be easier for people to refer to. also although there is no way to prove, people tend to believe soul is sth more "concrete" than "consciousness", yet i dunno what they mean of sth is concrete when u can't even perceive it whatsoever. nevertheless it's easier to conceive "souls" to travel in space than "consciousness"... dunno why tho

kenneth rodman
2003-Oct-15, 03:14 AM
The Largest time travel paradox I know off is the "grandfather paradox"
If you go back in time and shoot your grandfather you will not be born so there wouldn't be a you to go back in time to shoot your grandfather...... so your grandfather wont get shot and there WILL be a you to go back in time to shoot your grandfather......(brain hurts!)

I think the main answer to this is NOT to send homicdal maniancs back in time

well the multiuniverse hypothesis gets around that paradox because the grandfather your killing is the realative of a different you who may not be born in the universe you traveled to. doesnt matter you were born in the universe you came form.

and if indeed one did travel back in time in ones own universe then you would not succeed in killing your grandfather. Something would always happen to prevent you or you wouldnt be there trying in the first place.

I imagine zeus would have to turn you to stone if you were about to succeed......after all, its what he did when he saw the fox who always catches what it chases chasing the rabbit that always gets away from whats chasing it....otherwise poof universe explodes due to the paradox glitch...........

Normandy6644
2003-Oct-15, 03:25 PM
and if indeed one did travel back in time in ones own universe then you would not succeed in killing your grandfather. Something would always happen to prevent you or you wouldnt be there trying in the first place.


That's right, I remember reading that somewhere. Might have been Michael Shermer's book, but regardless I think that this is a common misconception of time travel. It also supports the parallel universe theory because let's say you were able to travel back in time to the very same universe. You would be unable to kill your grandfather (thus barring your existence) because you already do exist in this universe. If, however, you travelled back in time to another universe, you could kill your grandfather because he might not be your grandfather, and you wouldn't exist in that universe. In fact, if you do kill that person it is necessary that you don't exist in that universe so that the paradox doesn't take place.

Avatar28
2003-Oct-15, 05:23 PM
Hmmm, kanon14, your theory of static multiverses is intriguing. I may have to steal it. I prefer the term consciousness, though. It sounds more scientific than soul.

Actually, I read that exact theory in Discover a few years ago. Still an interesting idea though.
really??? do you remember which issue it is? i'd like to read mroe about it cuz i'm sure they would've said much better and much deeper than i did
and hopefully they can provide more scientific proof of such theory

I don't, sorry. But I clearly remember reading it and I thought it was really interesting.

Ah, wait. Here it is (http://www.discover.com/issues/dec-00/cover/). It was the December, 2000 cover story. It's only going to be available until Oct 31 though. Starting Nov1, only magazine subscribers can access the archives.

kanon14
2003-Oct-15, 06:26 PM
thx for the info Avatar28 8)
i'm actually doing a research on the paradox of time travel
i thought up this "static multiverse" awhile ago and thought it would be a very creative idea, but now it seems that it's sure interesting, but guess not a new idea

kanon14
2003-Oct-15, 10:07 PM
and if indeed one did travel back in time in ones own universe then you would not succeed in killing your grandfather. Something would always happen to prevent you or you wouldnt be there trying in the first place.

well i guess if one happened to travel back to ones own universe, there is still no paradox of killing his own grandpa. My theory states that the only causal connection between the past universes you've travelled is they direct which universe you're going next, hence probability. but once the cause has fulfilled its role, it is no longer necessary.

Say for the person X and his grandpa. the birth of X is directly connected to the life of X's grandpa. So for we to get the the universe where X is born, we must travel to where X's grandpa has married to X's grandma and gave birth to X's dad. But once X is born the causal connection of X's existence between X and X's grandpa vanished. In another word, once X's existence is "is", then the previous probability does not have any impact on his existence anymore. it's a simple knowledge of probability. Or think about it this way: while doing a multiple choice exam, you chose "B" and then handed in the exam. when u received your exam a later date, even if you change the answer to "C", it doesn't affect your mark anymore.

However, that doesn't mean the death of X's grandpa would have no effect on X's life. For those events that haven't occured, that is the universes X has not yet travelled, which may need his grandpa's existence to trigger the probability, X will not be able to travel to.

Does that make sense?

Eroica
2003-Oct-16, 07:50 AM
I can't say I agree with your reasoning. The whole point of travelling back to our past universe is that anything you do when you get there must already - here and now - be part of our past. For example, if someone in the year 3000 built a time machine and went back to 1936 and killed Hitler in Munich, our history books today, now, would read:

Adolf Hitler: (1889-1936) German political activist; murdered in Munich by strangely dressed individual.
The fact that they don't say that means that no one is ever going to do such a thing.

kanon14
2003-Oct-16, 08:04 AM
The fact that they don't say that means that no one is ever going to do such a thing.
Not necessarily. In the traditional sense of time travel, if you manage to travel back and kill Hitler, when you come back to 2003, you should find the history books write the way you described. However, if you go around and ask people, would they tell you the historians suddenly decided to change the history text today OR they would say the history has always been that along the way? I guess most people would agree it is the later case, which means that if you're not the person doing the time travel, you won't notice there's a change in history.

Zee'd
2003-Oct-16, 08:42 AM
and if indeed one did travel back in time in ones own universe then you would not succeed in killing your grandfather. Something would always happen to prevent you or you wouldnt be there trying in the first place.


That's right, I remember reading that somewhere. Might have been Michael Shermer's book, but regardless I think that this is a common misconception of time travel. It also supports the parallel universe theory because let's say you were able to travel back in time to the very same universe. You would be unable to kill your grandfather (thus barring your existence) because you already do exist in this universe. If, however, you travelled back in time to another universe, you could kill your grandfather because he might not be your grandfather, and you wouldn't exist in that universe. In fact, if you do kill that person it is necessary that you don't exist in that universe so that the paradox doesn't take place.
Let me try and give everyone a headache on my very first post :) . note that this is also pure theory (similar to the soul/consious multi-verse earlier on this thread)

assuming that there is only one universe, or you are trying to go back to the same universe you originated from. also assume that the above quotes are true.

Time travel will therefore not exist, because even if you do not have homodical thougts towards your grandfather, someone else might have, or will alter events in the past to "kill" you. (example: if you kill hilter (to quote someone), maybe your grandmother would have met someone else who otherwise would have been seperated in the great war. thus you will not be born).

Thus, this indirect "killing" will be a paradox, and the ultimate killer, the inventor of the machine will not exist...... huh ?

:)
guess this storngly supports the requirement of the multi-verse for time travel

kanon14
2003-Oct-16, 08:43 AM
I can't say I agree with your reasoning. The whole point of travelling back to our past universe is that anything you do when you get there must already - here and now - be part of our past.
The problem is how we see "time". Let's assume time exists and forget about the multivers with no time (I know this idea is hard to swallow). The fact that we assume time is only 1-d may be where the problem lies. Imagine time is actually 2-d. If it's hard to visualize, try the following way: darw X and Y coordinates on a piece of paper. label the X-axis "Personal Time" and the Y-axis External Time". the units of them are both "year". Then draw a line starting from (0,0) with a slope +1. That line represents person A's timeline.

Definition of PT (personal time): the time that a specific person experiences
Definition of ET (external time): the general time flow

Now consider A is 20 yrs old at the coordinate (20,20) and he travelled back to visit his 10yrs old self. We tend to think that A did travel back to his 10yrs old self at (10,10) but actually what happened was that A's now at (20,10). The reason is that the time machine could only let him move along the Y-axis (ET) but not the X-axis (PT).

Actually we have this idea since day one when we think of time travel. When A travels back in time to 10yrs ago, his body would not change back to his 10yrs old body, but it stays at the current 20 yrs old one. Let's say the time machine instanteously takes A back 10 yrs. A would experience as if no time has passed, hence PT = 0, but actually he has gone back to 1993 so ET = -10. Therefore, we get a 2-d time plane.

Lorentz transformation for time tells us that if there are 2 people, one at rest on Earth and the other flying through space at 0.5c, the time experienced by these 2 people is not the same. Then in this case where time is 2-d, there is also no possible way to "step into the same river".

kanon14
2003-Oct-16, 08:48 AM
Thus, this indirect "killing" will be a paradox, and the ultimate killer, the inventor of the machine will not exist...... huh ?

yea i agree with you that this indirect killing would generate a paradox according to our traditional sense of time. but i don't see how the inventor of the time machine will not exist? why?

btw, welcome to the board Zee'd =D>

Zee'd
2003-Oct-16, 09:18 AM
quoting myself:





and if indeed one did travel back in time in ones own universe then you would not succeed in killing your grandfather. Something would always happen to prevent you or you wouldnt be there trying in the first place.


That's right, I remember reading that somewhere. Might have been Michael Shermer's book, but regardless I think that this is a common misconception of time travel. It also supports the parallel universe theory because let's say you were able to travel back in time to the very same universe. You would be unable to kill your grandfather (thus barring your existence) because you already do exist in this universe. If, however, you travelled back in time to another universe, you could kill your grandfather because he might not be your grandfather, and you wouldn't exist in that universe. In fact, if you do kill that person it is necessary that you don't exist in that universe so that the paradox doesn't take place.
........
assuming that there is only one universe, or you are trying to go back to the same universe you originated from. also assume that the above quotes are true.
...........
those quotes are obtained from somewhere earlier in this thread, if you are wondering.

kanon14
2003-Oct-16, 10:12 AM
those quotes are obtained from somewhere earlier in this thread, if you are wondering.
yea i understand that. but i still dun get how would the inventor not existed due to the paradox. do u mean the reality simply doesn't allow such person exists to prevent the paradox?

Amadeus
2003-Oct-16, 12:33 PM
Ok... this is one to miss with your heads....
How do we know that the past is not being changed constantly?
We would have no memory of the "old" past... As for the whole going back and killing Hitler thing.... we asume that only the (good) nations will have this capability. So a future facist nation could be going back in time to save him each time. Or and this will realy mess with your head.... he never died and the burnt body that was found outside the bunker was a clone and he was brought to the future to rule this future facist state.... :o

Ok my brain is starting to leak out of my ears now....

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Grey
2003-Oct-16, 02:39 PM
Ok... this is one to miss with your heads....
How do we know that the past is not being changed constantly?
We would have no memory of the "old" past... As for the whole going back and killing Hitler thing.... we asume that only the (good) nations will have this capability. So a future facist nation could be going back in time to save him each time. Or and this will realy mess with your head.... he never died and the burnt body that was found outside the bunker was a clone and he was brought to the future to rule this future facist state.... :o

Ok my brain is starting to leak out of my ears now....

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brain no work beer without
I recall a short story where someone invents a time machine and goes back to kill Hitler. He discovers dozens of other time travellers (all in varying bad approximations of German clothing from something like that time period), all of whom have come back to do the same. Hitler is actually very perplexed that all these people want to kill him, so he's assembled them in an auditorium and is talking to them. He sounds very reasonable, and all the time travellers are thinking that maybe they were mistaken. The protagonist, however, remembers that Hitler was known for his charisma, refuses to be taken in, and shoots. In the ensuing confusion, we hear the other leaders saying that to keep the German economy and morale improving, they'll have no choice but to use the "double". One protests that the double is completely unstable and it's a bad idea, but in the end they agree that it's the only alternative. So the murder of a reasonable man was what actually propelled a madman to power. An interesting story about whether it's possible to change the past. :-?

Normandy6644
2003-Oct-16, 03:57 PM
Wow, that's awesome! I need to find that and read it.

informant
2003-Oct-16, 04:08 PM
Borrowing the analogy...

Suppose some evil villain invents a time machine and goes back to WWII to give Hitler the A-bomb. You know about his intentions, you have another time machine, and you intend to stop him. As you are getting the time machine ready for the jump, a thought comes to you: if the villain had succeeded, then you should remember the fact that the Nazis won WWII. But you don't. You go to a public library, and all history books agree that the Allies won WWII, and the Germans never had the A-bomb. So you conclude that the villain failed for some reason, and don't bother to try and stop him. Right?...

Or should your conclusion be that he hasn't succeeded yet, but may soon succeed, and still needs to be stopped?

These are two ways to conceive of time travel. The first one, AFAIK, was the one adopted by Heinlein in his time travel stories. There is only one past, which unfolds once and forever. Even if you can travel back in time, whatever you do there will just mesh with the past you were aware of before making the trip.

If time travel follows these rules, you could conclude that the villain had failed in helping Hitler, in my example above. With these rules, going back and killing your father would create a true paradox, since the timeline must be unique.

But other authors look at time travel differently. One example is the Back to the Future films. Marty MacFly effectively changed the past - and the present - when he travelled back to 1955. It became different than what he remembered; his trip changed people's lives, even though he's the only one who's aware of the change.

This second scenario can only make sense if time is in some way multidimensional. You have standard time, which everyone lives through, even Marty. In this "timeline", Biff Tannen is his father's boss, and Marty's mother never met her future son. But "later" - after all this has been experienced by everyone - Marty travels back in time, and changes the past. It's no longer true that his mother never met Marty when she was a teenager. This new past is just as valid as the previous one, but replaces it. There are multiple timelines, and changing the past means that you "tie" a timeline (1985-A) to another (1955-B) in a sort of loop. If you started to tell the story from beggining to end, you would go through a 1955 where there is no Marty MacFly, then a 1985 where he gets into the time machine, then you'd go through 1955 again, but this time Marty would be there, and then you'd get to 1985 again, but this time his parents would be different.

This second framework for time travel might not be affected by time travel paradoxes. Say that Marty accidentally ran over his father and killed him. If you follwed his life line, you'd find that he was born in timeline A, in which George MacFly was alive in 1985, then travelled to 1955 and killed George. This put him in a timeline B, where George dies in 1955, before Marty is born. But that would not necessarily mean that Marty could not exist, because his father was the George MacFly of timeline A (who was alive in 1985 and had a son), not the George MacFly of timeline B (who died childless in 1955), and both timelines are real. Both real, only at different... times! Or metatimes.

I'll stop now because my head is starting to hurt. :o

Amadeus
2003-Oct-16, 04:22 PM
To follow on.....
Perhaps the future has not changed because you will go back in time to stop him. But if you decide not to because you think he will fail because you hame no memory of him suceeding that action will change history.....

Time travel discusions do my head in... :o :o :o :o :o

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kanon14
2003-Oct-16, 09:55 PM
This second scenario can only make sense if time is in some sense multidimensional.

this is exattly what i was saying a few posts above. Time is 2-dimensional.


This second framework for time travel might not be affected by time travel paradoxes. Say that Marty accidentally ran over his father and killed him. If you follwed his life line, you'd find that he was born in timeline A, in which George MacFly was alive in 1985, then travelled to 1955 and killed George. This put him in a timeline B, where George dies in 1955, before Marty is born. But that would not necessarily mean that Marty could not exist, because his father was the George MacFly of timeline A (who was alive in 1985 and had a son), not the George MacFly of timeline B (who died childless in 1955), and both timelines are real. Both real, only at different... times! Or metatimes.

This is just like the multiverse idea, but put into phrases easier to understand. But it generates the same "problem": there is no way to return to your past. every time yoou travel back, you are actually entering another timeline instead of trvelling along your own one.

Stylesjl
2003-Oct-17, 12:06 PM
i was just thinking of that therodynamics thing


A matter swap between universes would fix it


Your body goes to other universe and another peice of matter (like air) would go to your universe

Therefore it doesn't disobey the laws of therodynamics

informant
2003-Oct-17, 03:18 PM
there is no way to return to your past. every time yoou travel back, you are actually entering another timeline instead of trvelling along your own one.
Unless if it's also possible to travel in metatime. In which case there will be more than two "time" dimensions.
P.S. This may get trickier with multiple time travellers.

kilopi
2003-Oct-17, 03:29 PM
In which case there will be more than two "time" dimensions.
Wouldn't just more than one (http://www.badastronomy.com/phpBB/viewtopic.php?p=6405&highlight=dimensions#6405) be enough? :)

informant
2003-Oct-17, 03:53 PM
You might find the story The Unicorn Girl (http://www.amazon.com/exec/obidos/tg/detail/-/158715420X/qid=1066405831/sr=1-2/ref=sr_1_2/102-2668383-8290512?v=glance&s=books), by Michael Kurland, interesting. Not that it's meant to be taken seriously, or anything.
I read The Number of the Beast years ago, and also thought that the two additional dimensions (tau, teh) were time dimensions. But I may not remember it well.

Grey
2003-Oct-17, 04:20 PM
Unless if it's also possible to travel in metatime. In which case there will be more than two "time" dimensions.
P.S. This may get trickier with multiple time travellers.
Actually, the physics of our universe would be noticeably different if there were more than one time dimension (well, it would be okay if they were "compactified" like the extra dimensions of string/M theory, but then you couldn't travel through them anyway, so it wouldn't matter). So postulating extra time dimensions to solve these paradoxes doesn't work. Of course, not to be a spoilsport, but remember that the easiest way to resolve time travel paradoxes is just to say that time travel isn't possible, which does seem like the most likely case. :(

mike alexander
2003-Oct-17, 04:45 PM
Science fiction has been playing with time travel for so long that there are categories. Some examples I think of off the top:

Not My Problem: Wells' 'The Time Machine'

Time Wars: Anderson's 'Time Patrol' stories, Leiber's 'The Big Time'

The Recursive Loop: Heinlein's 'By His Bootstraps'

Hiding in the Details: Anderson's 'There Will be Time'

Start From Scratch: Card's 'Pastwatch'

All Bets are Off: Gerrold's 'The Man Who Folded Himself'

Strange Combos: LeGuin's 'Lathe of Heaven'

Just to mention that I enjoyed all of the above.

kilopi
2003-Oct-17, 04:47 PM
Actually, the physics of our universe would be noticeably different if there were more than one time dimension
By different, do you mean the laws are different? That would seem to be obvious, no?

If you mean that there is no way to add another time dimension, rework the laws, and have them agree with experiment--then I'd like to see the proof of that! :)

Donnie B.
2003-Oct-17, 06:19 PM
I can't say I agree with your reasoning. The whole point of travelling back to our past universe is that anything you do when you get there must already - here and now - be part of our past. For example, if someone in the year 3000 built a time machine and went back to 1936 and killed Hitler in Munich, our history books today, now, would read:

Adolf Hitler: (1889-1936) German political activist; murdered in Munich by strangely dressed individual.
The fact that they don't say that means that no one is ever going to do such a thing.
What? Are you saying Hitler is dead?

That's odd... I distinctly remember that big 100th birthday bash they had for him in New Berlin in '89...

kanon14
2003-Oct-17, 06:27 PM
Actually, the physics of our universe would be noticeably different if there were more than one time dimension
By different, do you mean the laws are different? That would seem to be obvious, no?

If you mean that there is no way to add another time dimension, rework the laws, and have them agree with experiment--then I'd like to see the proof of that! :)

yea just because our current knowledge of physics/science suggests time is only 1-d does not make "time is 1-d" to be the only way to undertsand the universe. our understanding of physics can be fundamentally wrong before, so can it happen again.
every generation of people think that they've already solved every solvable question, but turn out that they were all deceived by variuos illusion of nature. so why can't we live in the time where the illusion is as big as before?

Also for time to be 1-d in our understand, doesn't special relativity suggests time could be 2-d since 2 people can experience different time. it is not possible to draw out the graphical explanation in a 1-d timeline, but it has to be a 2-d diagram

kanon14
2003-Oct-17, 06:33 PM
I can't say I agree with your reasoning. The whole point of travelling back to our past universe is that anything you do when you get there must already - here and now - be part of our past. For example, if someone in the year 3000 built a time machine and went back to 1936 and killed Hitler in Munich, our history books today, now, would read:

Adolf Hitler: (1889-1936) German political activist; murdered in Munich by strangely dressed individual.
The fact that they don't say that means that no one is ever going to do such a thing.
What? Are you saying Hitler is dead?

That's odd... I distinctly remember that big 100th birthday bash they had for him in New Berlin in '89...

:roll: can i get an autograph from him

kilopi
2003-Oct-17, 06:35 PM
Also for time to be 1-d in our understand, doesn't special relativity suggests time could be 2-d since 2 people can experience different time. it is not possible to draw out the graphical explanation in a 1-d timeline, but it has to be a 2-d diagram
No, that is not true. The 2-d diagrams have space as one of their dimension.

kanon14
2003-Oct-17, 06:46 PM
Also for time to be 1-d in our understand, doesn't special relativity suggests time could be 2-d since 2 people can experience different time. it is not possible to draw out the graphical explanation in a 1-d timeline, but it has to be a 2-d diagram
No, that is not true. The 2-d diagrams have space as one of their dimension.

i understand how the original diagram looks like, but i'm arguing the possibility of a 2-d time diagram to explain time which the sapce-time diagram can explain.

kilopi
2003-Oct-17, 06:54 PM
What I meant was, special relativity doesn't suggest that.

kanon14
2003-Oct-17, 09:15 PM
What I meant was, special relativity doesn't suggest that.
ohh... then what about the Lorentz transformation which tells two people can experience different time? how can that be explained in a 1-d time diagram??

kilopi
2003-Oct-17, 09:31 PM
What I meant was, special relativity doesn't suggest that.
ohh... then what about the Lorentz transformation which tells two people can experience different time? how can that be explained in a 1-d time diagram??
I'm not arguing against multiple time dimensions, just that special relativity implies them.

Space and time, as far as relativity is concerned, are intertwined. It's Spacetime. The difference between the two people is their relative velocity, which affects how they measure time. The coordinates transform. But I'm sure you knew that--why do you think you need two time dimensions to explain it?

kanon14
2003-Oct-17, 09:44 PM
nonono i'm not trying to use 2-d time to explain that, just that special relativity implies the possibility of a 2-d time. i'm using 2-d time to explain the paradox of time travel

kilopi
2003-Oct-17, 10:01 PM
just that special relativity implies the possibility of a 2-d timel
yeah that's what I disagree with. I don't believe that it implies it at all.

kanon14
2003-Oct-18, 06:45 AM
Special relativity implies length contraction and time dilation to objects moving at high speed. Like when a person is moving at high speed, his wristwatch ticks slower when compared to clocks sitting on earth.

umm... believe it or not that's as far as i can go right now lol. i admit i was wrong saying that special relativity implies a 2-d/multi-d time. i just got that idea intuitively. but i don't really buy a multidimensional time system anyway. i'm more in favor of the timeless world :wink:

Grey
2003-Oct-18, 05:10 PM
Actually, the physics of our universe would be noticeably different if there were more than one time dimension
By different, do you mean the laws are different? That would seem to be obvious, no?
Well, of course, but so would the physical effects we observe. As an analogy, the fact that gravity goes like the inverse square of distance is a natural result of there being three macroscopic space dimensions. If there were four, gravity would go like the inverse cube, and we can see clearly that this doesn't happen, so we can surmise that there isn't another macroscopic physical dimension. One of the issues with having multiple time dimensions is that causality gets really strange, since there's more than one path to a point in time. Two events could precede each other, resulting in the same kinds of causality paradoxes that we have with the time travel issue, but worse. Since we don't seem to get this kind of chaotic causality, it doesn't seem likely that there's more than one time dimension.


If you mean that there is no way to add another time dimension, rework the laws, and have them agree with experiment--then I'd like to see the proof of that! :)
Of course I can't prove that no theory ever with two time dimensions could describe the universe we see. But I can say that there is currently no such theory (unless you're hiding it in your pocket :D ). And the theories that do seem successful assume a single time dimension.


nonono i'm not trying to use 2-d time to explain that, just that special relativity implies the possibility of a 2-d time. i'm using 2-d time to explain the paradox of time travel
So, since this is the Against the Mainstream forum, consider me to be playing Devil's Advocate. Adding an extra time dimension to our current models of the universe (relativity and quantum mechanics) would add things that we don't observe (weird causality violations and other issues), so in fact special relativity specifically does not allow a second time dimension.

If we're only adding this extra dimension to try to explain something that we also don't observe (people travelling through time, and I'm still not quite sure how it resolves these issues), then perhaps it makes more sense to assume that people can't travle through time, and we get a nice self-consistant picture. Of course, that would mean that mucking around with the past remains forever in the realm of fiction, but then again, perhaps that would be for the best. :D

kilopi
2003-Oct-18, 06:12 PM
Of course I can't prove that no theory ever with two time dimensions could describe the universe we see. But I can say that there is currently no such theory (unless you're hiding it in your pocket :D ). And the theories that do seem successful assume a single time dimension.
Not in my pocket per se, but on another hard drive somewhere. Someone gave me the web address a couple years ago--sorry, I don't have it at hand. I'll see if I can find it.

Google didn't help me, but for what it's worth, the BA's friend Sten Odenwald has pretty much the same take on it (http://itss.raytheon.com/cafe/qadir/q1980.html) that you do.

PS: Found this short note (http://www.aip.org/enews/physnews/1997/split/pnu347-4.htm) on Vafa's F-theory (M-theory is for mother, F-theory is for father?), which seems to incorporate 10 spatial dimensions, and 2 time dimensions.

Grey
2003-Oct-19, 04:25 PM
Not in my pocket per se, but on another hard drive somewhere. Someone gave me the web address a couple years ago--sorry, I don't have it at hand. I'll see if I can find it.
Cool. That would be interesting to look at.


Google didn't help me, but for what it's worth, the BA's friend Sten Odenwald has pretty much the same take on it (http://itss.raytheon.com/cafe/qadir/q1980.html) that you do.

PS: Found this short note (http://www.aip.org/enews/physnews/1997/split/pnu347-4.htm) on Vafa's F-theory (M-theory is for mother, F-theory is for father?), which seems to incorporate 10 spatial dimensions, and 2 time dimensions.
Fascinating. Doing a few searches for some of his papers (http://xxx.lanl.gov/PS_cache/hep-th/pdf/9602/9602114.pdf) it looks like he is indeed compactifying one of the time dimensions. I that case, I would think that might solve some of the causality issues, as I pointed out originally (the same way you can save the inverse square law of gravity in a universe with more than three physical dimensions, as long as any extra dimensions are really small). However, I am by no means an expert on M-theory, so I don't completely understand all of the implications or effects. Do you know of a slightly less technical treatment of his work? While looking at the actual papers is always useful, given the complexity, I'd be interested to start with a high-level overview.

Of course, even if it turns out not to describe our universe, it's still pretty interesting to imagine what the universe would be like if there were extra time or space dimensions. It can help to understand the nature of the universe we do live in to think about what it could have been like, if something were different.

Added: here (http://arxiv.org/abs/hep-th/0008164)'s an interesting paper that gives a brief survey of some of the issues involved in having a second time dimension. It sounds like compactification does not solve the problems of a second time dimension as easily as it can deal with the extra space dimensions, but there are ways around them. Very interesting.