Every one of us has probably pondered this question but do we really know what we are talking about if we say time travel ?
Suppose I invented a time machine and I went back to the time when I was making the device and destroyed all the plans and blueprints to make it. What would happen to me ?
When we ask such questions we are actually wondering whether or not the universe is deterministic and if there is only one universe or several. You see there are many ways out of this apparent paradox :
1. one might argue it would be somehow prevented that I destroyed the plans because otherwise I could never have traveled back to the past.
2. the whole thing is simply impossible because there is no way time travel is possible, this might involve some kind of presently unknown law like the one we know that prevents us to go faster then the speed of light
3. I do go back and destroy the plans but by doing this I destroy my future and therefor am stuck in the past
4. I don't really go back to 'my past' but to the past of a universe in a parallel dimension, anything I do there has no effect what so ever at my own universe therefore making time travelling more like dimension travelling.

One final remark before I let you all lose on this subject : if time travel is possible then where are all the travellers of the future ? Are they prevented to interact because of some mysterious mechanism like in explanation 1 ? Have they perhaps (like our famous aliens who controll the ufo's ;) ) some sort of non intervention pact ? Aren't they there because it is impossible to begin with ?

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"If I have been able to see further, it was only because I stood on the shoulders of giants."
Isaac Newton

Plato,

You didn't expect me to stay away from this one, did you? :D

Ok, I know how valuable my opinion is in these matters, but whadaheck -- I'll put it out for the fresh people to see. I strongly feel that time has an ill-gotten reputation as a dimension. In science, it is a mathematical abstraction that has managed to turn into a runaway process. In folk terminology, the illusory coordinates of the future assured by inductive prediction in the context of thermodynamic evolution of the universe combined with the memory-derived coordinates of the past generate an appearance of a continuum, or a flow along some landscape of time. In either case, I do not think it is any more correct to think of time as a dimension, than it was to think of the sky as a crystal sphere.

Time is only useful when applied to describing the evolution of matter and energy. Without matter-energy, and I suspect something even more fundamental that also involves space, the concept of time is meaningless. In relativity, we learn that time is a relative measurement of rate; i.e. I compare the rate at which events occur in my reference frame to the rate at which they occur in another frame. I measure time by making use of an oscillatory process that changes states at a standard rate. Relative rate of events goes to 0 when the observed frame approaches lightspeed -- and little wonder, since everything's so busy flying forward at lightspeed, there is very little latitude left for motions orthogonal to the dominant velocity. So it seems that time is simply a characterization of rate at which events occur -- which is itself defined in terms of lightspeed and possibly a couple of other fundamental constants. Now, ask yourself -- does it make any sense to talk about traveling backward or forward in <u>rate</u>? Granted, you could possibly speed the rate up or slow it down, but this gives you nothing in terms of visiting the past or discontinuously jumping into the future. In fact, I don't even think the past and the future can be construed as existing (or as destinations, in other words); the former is merely a memory trace within the present, and the latter is merely an inductive expectation of the present.

Anyway, it's early and I can't sleep, and I probably am not writing very coherent prose -- so I'll just stop here before I loose not only my audience but even myself in my own jumble of thoughts. Hope this did make some sense though (I'll be kicking myself later if it didn't :))

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I am; therefore I think.

Plato, what you have described is the grandfather paradox.

"A man goes back in time and kills his grandfather before his grandfather had any children"

Therefore his father was never born, therefore he was never born.

Here is another possiblity:
You are suddenly blown apart and all your atoms go back to where they would have had you not been born.

Oh yeah, ever watched the movie "The Terminator".
The grandfather paradox in reverse.

On the lighter side, snipped from your post

“1. one might argue it would be somehow prevented that I destroyed the plans because otherwise I could never have traveled back to the past.”

But you can’t travel back to a time before the device is invented because the invention requires a receiving chamber. The best you could do is travel back to a time just before you left and prevent yourself from leaving, do you remember anyone trying to prevent you from leaving just before you left? Did that person look anything like you?

"2. the whole thing is simply impossible because there is no way time travel is possible, this might involve some kind of presently unknown law like the one we know that prevents us to go faster then the speed of light"

Time travel could be possible; there is nothing so far that prevents it theoretically, only practically.

"3. I do go back and destroy the plans but by doing this I destroy my future and therefor am stuck in the past"

see response number 1, it can’t be done

"4. I don't really go back to 'my past' but to the past of a universe in a parallel dimension, anything I do there has no effect what so ever at my own universe therefore making time travelling more like dimension travelling."

Cool!


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bobbapink
department of redundancy department

Boris,
I'm not sure what you are trying to say. I understood it to mean that time is only a local event reference which is only relevent to the event you are concerned with, and therefore there is no interconection between each events time reference, other then an implied sequential contiunation......(now I'm not making sense to myself, but you'd be used to that by now :) ).
Ok what if we called it "event hopping" instead of time travel. Also there is a strong argument for negative time in a wormhole system.

BTW I'm posting this message on the 13/4/2000 9:30 AM. YOUR FUTURE (unless your from NZ then its from your past). So don't worry guys tomorrow will come I'm already here to tell you it does :)


Bobbapink,

if you needed a receiving chamber to travel through time or hop from event to event then you can't go into the future either, you'd be stuck between the present and the event that created the time machine.

The Gradfather paradox,

IMO the paradox doesn't really exist. It can only exist if time moves in the forward direction only. i.e. if time travel were possible then you can no longer say that killing your father before you are born would not allow you to be born. If time was to move forward only then this would be true but if you CAN manipulate the direction then the sequence of events that brought you to the point where you COULD kill your father would still hold and the only thing that you would achieve by killing him would be to not have a father. In summary: Time travel or event hopping does not have the effect of erasing events, all that happens is that history no longer follows a sequential path in the forward direction. Although the events that lead you to YOUR present are sequential the sequence is no longer synchronised with a forward time reference. Man I hope this makes sense to someone.

Illustration:

Event 1: (1948) My father is born
Event 2: (1971) My Parents get married
Event 3: (1974) I am born
Event 1156: (2000) I invent and build a time machine
Event 1157: (2001) I "hop" back to 1970
Event 1158: (1970) I kill my father
Event 1159: (1970) I return to 2001
Event 1160: (2001) I hate myself for killing my father to prove a point


sequence of events still holds but the event sequence is no longer chronological due to time reference manipulation. History would hop from time reference to time reference however going back in time creates a new event which is independent of the original event within that time reference and hence does not alter cause-n-effect as we know it.

[This message has been edited by Rambler (edited April 12, 2000).]

Bad luck rambler, it was my past too.
You forgot that other aussies would have it in there past (or current time depending on how you look at it) :).

I like ramblers idea.
I was toying with that kind of idea.

The fact is we won't know until we've done it.
And even then we probably won't find out due to the person being rocketed off to another dimension or something.

Rambler,

What I was trying to say is that the "flow" of time is only an illusion. The past doesn't exist, and neither does the future. There is only the present, which undergoes sequential changes. (is that more comprehensible? :))

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I am; therefore I think.

Mmm, a lot of different opinions with one thing in common : if we want to address the possibility of travelling in time, we need to establish the nature of time.

Boris,

my man, I was sincerely hoping that you would bite into the bate ;)
I see that the nature of time with you is still firmly founded in Kantian theory even though you try to present your case from a relativistic standpoint


In relativity, we learn that time is a relative measurement of rate; i.e. I compare the rate at which events occur in my reference frame to the rate at which they occur in another frame. I measure time by making use of an oscillatory process that changes states at a standard rate.

This is very nice but the only thing you have done here is used an other word for the same thing, it makes as much sense to talk about time as it does about rate. Why however can there be a thing as different rates for different observers if a rate is nothing more then a mathematical parameter that we use to describe periodical processes ? Why does general relativity describes the universe in four dimensions and does such a good job ? Is it only for the sake of argument because the equations are more beautiful ?
When you say that time, space, matter and energy are very closely intertwined I couldn't agree more with you. But when you claim that time is nothing more then an illusion you are making the same mistake that Kant made. Think of it this way, if time is not a dimension then why can't I see Napoleon losing his battle when I stroll along the fields of Waterloo (which is only 10 km from where I live). I claim it is not there because it is physically in an other place with other coordinates : Waterloo 1815 in stead of Waterloo 2000.
However relativity also shows us that time isn't just a dimension as the other ones, the three space dimensions make up a Euclidian space but if you add the time dimension, you get a Minkovsky space. This is not just a mathematical trick but a very profound way of how time and space are related.

Krusher,

I know it was the gransfather paradox but I just wanted to give it a new look (like it?)
I don't like the blown apart bit though ;)

bobbapink,
(any relation with bobbafet ?)
why couldn't I build a timemachine without a receiving chamber ? Do you know something that I don't regarding the basic principles of making such a device ?

Rambler,


Time travel or event hopping does not have the effect of erasing events, all that happens is that history no longer follows a sequential path in the forward direction. Although the events that lead you to YOUR present are sequential the sequence is no longer synchronised with a forward time reference. Man I hope this makes sense to someone.

You are going back to an even older theory about time then Boris does, does the name Parmenides rings a bell ?
Of course you do realise that cause and effect in such a universe as you describe it doesn't exist at all. It is a pure coincidence that the event that follows on me pressing the light button is an event where the light in the chamber is switched on. This endangers our beliefs that the universe is knowledgable and would make all the assumptions that science made about the world invalid. Are you willing to take this step ?



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"If I have been able to see further, it was only because I stood on the shoulders of giants."
Isaac Newton

Plato,

How delightful, another debate on the nature of reality! Kinda brings back the good old times, don't it? :)

You are right, I merely used another word for "time" -- and I did it intentionally. The problem with "time" is that it is associated with the concepts of "past", "present", and "future". Rate has no such problems, and it conveys much more clearly what I mean when I talk about time.

Why would there be such things as different rates? Well, why are there such things as different gear settings in a car's gear box? It's the same box, with all the gears made of the same stuff, and fundamentally there is nothing about the box that relates to speed; yet the different gear ratios translate into different rotational rates of the axle. Same with event rates. Events are merely results of interactions between matter-energy, spacetime, and whatnot. The original substrate has nothing to do with rate -- or, if you like, we can postulate that for the substrate everything always happens at a fixed rate. What we observe as events are superficial phenomena on top of the fundamental substrate. What we observe as differential event rates in different reference frames are merely a manifestation of the difference between the reference frames -- namely, that they are traveling at a certain rate with respect to us, or that they are in a strong gravitational field, etc. Basically, think about it this way: if the speed of light is the limit, then if something is moving away from you at 0.9c, then they can only move laterally at a maximum speed of (1-0.81c^2)^0.5. This would slow down the rate of any events in a travelling frame relative to the frame of the observer. In fact, I wonder, if the summation is carried out properly over all directions of motion, whether such an effect alone will not be enough to derive relativistic time dilation.



Why does general relativity describes the universe in four dimensions and does such a good job ? Is it only for the sake of argument because the equations are more beautiful ?


Well, beauty is in the eye of the beholder. In my view, it is actually more beautiful to view time as a mathematical artefact that derives from the way we process our experiences, and not as one of the fundamental pillars of existence -- because the fewer fundamentals there are, the more elegant the model. This does not mean that the math has no connection to reality -- because it derives from reality -- but I don't think the math literally stands for what is happening; I think math is merely a linguistic representation that abstracts away from reality and cuts corners on realism.



Think of it this way, if time is not a dimension then why can't I see Napoleon losing his battle when I stroll along the fields of Waterloo (which is only 10 km from where I live). I claim it is not there because it is physically in an other place with other coordinates : Waterloo 1815 in stead of Waterloo 2000.


Well, that's one way to answer the question. Another way is to say that the past doesn't exist; the only thing that exists is the present. There is only one copy of the universe, and it undergoes fluid change. Once upon a time, the configuration of the universe was such that Napoleon was indeed in the process of losing his battle -- but the configuration has continued to change, so that now it is something altogether different (and there is no more Napoleon.) The reason you don't see Napoleon at Waterloo is because you are looking at the current configuration of the universe, which no longer includes Napoleon at Waterloo or any other location on Earth.



However relativity also shows us that time isn't just a dimension as the other ones, the three space dimensions make up a Euclidian space but if you add the time dimension, you get a Minkovsky space. This is not just a mathematical trick but a very profound way of how time and space are related.


Well, I'm not saying that rate and space are not related; in fact I tend to hope that rate and even space itself are artifacts of something yet more fundamental -- similar to how temperature is an artifact of inertia.

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I am; therefore I think.

Boris,

my pleasure indeed ! :)

So you claim that time or rate is a non existing dimension that only manifests itself because we see the three "real" dimensions changing and retain a memory of how it used to be and compare this mental image with the current situation.
The funny thing is I can make the same argument with any other dimension.
Actually our eyes only enable us to see two dimensions because the images we see are projections on our retina. I now claim that only these two dimensions and time are the only real existing dimesions because as time goes on, I turn my head and therefore have the illusion of seeing a third dimension while in fact time simply changes the world for me according to this third dimension. Therefore I say that we are actually two dimensional creatures who think we live in a three dimensional world because time creates this illusion...
The knife cuts both ways



Basically, think about it this way: if the speed of light is the limit, then if something is moving away from you at 0.9c, then they can only move laterally at a maximum speed of (1-0.81c^2)^0.5. This would slow down the rate of any events in a travelling frame relative to the frame of the observer. In fact, I wonder, if the summation is carried out properly over all directions of motion, whether such an effect alone will not be enough to derive relativistic time dilation.


I think what you mean is (1-0.81)^0.5 c as maximum lateral speed with c outside but I get the point, you basically have a triagle of the speed vectors with c being the hypothenusa, 0.9 c the realtive speed and 0.43 c the lateral speed. If you want all the speeds then (suppose the triagle was drawn from the origin) simply let the hypothenusa vector rotate around the origin then make an other circle around the 0.9 point with radius c, then all the points from the first circle who lie within the second one are allowed speeds, each speed vector is drawn from the 0.9 point to the first circle.
However I don't think with this only you will find the Lorentz equations because you are only talking about speeds, you will need one more thing since Lorentz gives the equations for time and space separatly.



similar to how temperature is an artifact of inertia.


:confused: You have me lost there, I thought temperature was kinetic energy but what has kinetic energy got to do with inertia ?

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"If I have been able to see further, it was only because I stood on the shoulders of giants."
Isaac Newton

Plato,

Well, the 2D picture doesn't exactly wash, since the third dimension is measured rather convincingly both through perspective and through stereoscopy (which we 2-eyed creatures are experts at.)



However I don't think with this only you will find the Lorentz equations because you are only talking about speeds, you will need one more thing since Lorentz gives the equations for time and space separatly.


Yes, you're right -- the space component is missing from the picture (but you do have to wonder how velocities get affected strictly along the line of travel, and whether the "space contraction" cannot be derived from the extreme difficulty of forward-directed interactions. But even if this is not correct (probably not, as it is too simple and someone would have figured it out by now) -- it conveys my idea. The idea is that time is not a coordinate and not a dimension -- but rather a local property of rate.



You have me lost there, I thought temperature was kinetic energy but what has kinetic energy got to do with inertia ?


:) Well, think about it: what is kinetic energy? Remember Newton's first law -- objects at rest remain at rest (which is just another description of inertia.) Once you get an object going, it acquires "kinetic energy" -- but what is that energy other than resistance against being stopped? In fact, the direct proportionality of kinetic energy to inertial mass paints it squarely as a manifestation of inertia.

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I am; therefore I think.

Boris,



Well, the 2D picture doesn't exactly wash, since the third dimension is measured rather convincingly both through perspective and through stereoscopy (which we 2-eyed creatures are experts at.)


That is exactly right and the same solution works for time for what is stereoscopic vision else but combining two images taken from the same view on the same time but from two different spacepoints. If we apply this and take two different images at two different times on the same spacial point then we have an impression of the fourth dimension : time.



Once you get an object going, it acquires "kinetic energy" -- but what is that energy other than resistance against being stopped? In fact, the direct proportionality of kinetic energy to inertial mass paints it squarely as a manifestation of inertia.


Very clever, I like it ! Doesn't this imply however that heavy mass = inertial mass ? Actually this brings us back to one of our previous discussions about what inertial mass actually is but let's stick with time for a while longer. (pun unintended ;) )

Let me argue from the stance of a philosopher here. At first glance both views, look at time as a simple variable or as time as a dimension, are equivalent. However if you claim that only the present really exists then the past really has no value anymore. This means that we might as well forget everything that happened in the past, this puts your graduation form highschool at the same level as the extermination of the Jews in Europe, both don't exist and are completely erased. Does it make any sense to contemplete history ? Something that really doesn't exist ?
What I'm saying is that looking at time as a physical dimension makes that things that happened to make us the way we are, are so much more real. This is of course not a valid proof but since the two views are on equal footing I prefer the latter one.

[This message has been edited by Plato (edited April 13, 2000).]

Plato,

And a "very clever" back to you about the dual-measurement pickup! Though you'd have to concede that perspective can be measured with just one eye, and besides, we can do things like radar or sonar ranging. No, the third dimension is just as fundamental as the other two; I don't think there is a way to make it any less fundamental. In fact, if you persist with examining the precise point at which measurement takes place, then you will have to conclude that there are no dimensions at all! After all, even a single eye is an array of photoreceptors arranged in roughly 2D. If you then consider just one photoreceptor, you can only measure points -- zero-dimensional objects. The very fact that you can arrange your sensors in 2 (or even 3) dimensions is a direct indication that those 3 dimensions are fundamental dimensions that matter can traverse or occupy. Time, however, does not fit under such classification.

To make any kind of a time measurement, you necessarily have to rely on memory of some sort. And memory is merely a stored state contained in the present configuration of the universe. You cannot define (nor indeed conceive of) time without usage of memory. Hence, my conclusion follows that time is but a cognitive illusion generated through memory.

You're right that my stance clashes with many folk notions. However, and at a risk of self-aggrandisement, I would remind you that the Copernican solar system, the Newtonian revolving spherical Earth, the Einsteinian relativity of space and time, and many other things used to equally clash with folk notions. So I'd argue that what used to make sense in the past provides a poor measuring stick against what actually might be real. But history is not useless or non-existent even in my framework; it is encoded in the current state of the universe, since the "present" is a direct sequitur of the "past". Indeed, history is part of our knowledge, and knowledge is primarily useful as a starting point of induction. Hence, it still makes sense to try and learn from "past mistakes", so that "history repeating itself" can be avoided.

As to the gravitational/inertial mass thing -- it's darn amazing that the two are related linearly, is it not? One just has to wonder what the constant of proportionality is, and more importantly where it comes from...

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I am; therefore I think.

Plato,
The point that I was trying to make is that IF time travel were possible then the sequence of events is important not the time at which they happen, and that if YOU went back in time and killed your father I don't see how the event of you being born would be erased. I described this point to my wife last night (something on TV prompted the conversation). I gave her the example in events like I did above. She asked OK, what does your mum think happened to your father. Got me thinking, and the answer is she would not know me or my father after the event that kills my father. So cause and effect as I see it is very real. Isn't the whole mechanism for cause and effect simply the sequence of events that lead up to the effect. So going back to the example I am not saying that time travel in this model would not have consequences but I don't see how the grandfather paradox can exist if time were able to be manipulated. I guess this model is hard to comprehend because our understanding of time is sequential. If you say well the paradox must exist because if you look at the history involved your father never had a chance to make you. Well I say you haven't gone back far enough in history (history being the sequence of events rather then the date in which they happened). For the time traveler obviously time would still be going forward all the way through it.


Parmenides: Never heard the name before, sorry

I would realy like to hear the errors in this thinking. I don't want to slow down your discussion but I'd really be keen to know why cause and effect doesn't exist in this kind of model.

Boris,

Ok let's look at a proposed time machine and look for its validity.

There is a thing, for the moment just in theory, that is called a worm hole. In order to make one we need something very exotic like negative mass (this could already be a weakness since no negative mass has ever been made until this point). It actually consists of a hole in the time-space continuum like a black hole but unlike a black hole it does not end in a singularity but in an other worm hole mouth sort of speak. (This is all old news for you I know but I just state it any way for other readers ;) ) Both mouths look like spheres so don't think on hole in the normal sense here because we are taking it one dimension higher.
What we have in effect are two openings who are connected with each other via the worm hole. This is for example a way to go faster then light, the worm holes may be lightyears away from each other but the time to get from one mouth to the other is simply going in and out again, this might take just a few seconds. They might however also work as a timemachine, in order to do this is you take one mouth and drag it along with you at a relativistic speed (assuming you have a device to take you to these speeds of course) say 0.99 c. You fly a few years on this speed and come back to the other mouth that you left behind.
The mouth you took with you will now have a connection with the mouth you left behind as it was several year ago, this due to the timedilatation that occurred between the two mouths.
This whole process doesn't make any sense if the past doesn't exist as a physical place. So or the whole theory of worm holes is bull and with it general relativity since that is the underlying theory, or you have to admit (which I know is very hard :D ) that you have it wrong on this one...
(I just know what your answer will be :D )

Rambler,



I would realy like to hear the errors in this thinking. I don't want to slow down your discussion but I'd really be keen to know why cause and effect doesn't exist in this kind of model.


The way I thought cause and effect worked was : Action A happens at time t_a and is the cause of effect B which happens at time t_b (with t_a < t_b). If you now go back in time and prevent A from happening then in your model effect B will just happen without any cause. What is to stop from cars just blinking into existince and running you over next time you cross the street ? Do you dare come out of your house again ? Of course that might also just vanish as if it has never been there or be replaced by a tobacco factory (I'm just raving here ;) )

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"If I have been able to see further, it was only because I stood on the shoulders of giants."
Isaac Newton

Plato,

I must admit that although I had heard of purported time machines using GR, I never looked into the details. However, with respect to the particular description you provide, I don't think it works at all.



They might however also work as a timemachine, in order to do this is you take one mouth and drag it along with you at a relativistic speed (assuming you have a device to take you to these speeds of course) say 0.99 c. You fly a few years on this speed and come back to the other mouth that you left behind.
The mouth you took with you will now have a connection with the mouth you left behind as it was several year ago, this due to the timedilatation that occurred between the two mouths.


This sounds like a form of the twin paradox. In this construction, the mouth you are dragging with you experiences a slowdown in time, while the other (stationary) mouth ages at normal rate. When you subsequently pass through the wormhole, you emerge into the "future" within the stationary frame (where the rate of events was higher than at the moving end.) So there's no timetravel -- this is equivalent to just traveling away at high speed, and then coming back (the classical twin paradox setup.) In fact, there's some interesting dynamics in a wormhole system where one mouth is moving -- since the mouths evolve under differential rates. I would have to wonder if such a system can in fact be at all sustainable.

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I am; therefore I think.

Hi guys.

Boris,
The way you describe cause and effect sill holds. Time (not date) as observed by the time traveler would still progress for them in a linear direction and therefore cause and effect would still hold true for the traveler......I have to give this some more thought. For time to work in the way you describe the traveler would only be able to go back in time as far as they were born, and as they went further back (still only as far as their birth, and still no paradox cause you can only kill your father AFTER you are born) they would get younger. The way I'm thinking about it suggests that the traveler still ages however he/she is able to hop from event to event and to any date.

Wormholes:
I have just recently read an article claiming a russian relativistic physics guru has just mathematicaly discovered a wormhole big enough to allow intergalactic travel. His wormhole PRODUCES the exotic matter needed for stability out of NOTHING via the manner in which it bends space-time. I will post the link as soon as I find it.

Rambler,



For time to work in the way you describe the traveler would only be able to go back in time as far as they were born...


Actually, if time worked in the way I describe, no kind of time travel would be possible at all. In my construction, time == rate, and it is not something along which you can travel.

P.S. regarding the wormhole article, here's one: <A HREF="http://www.newscientist.com/news/news_223420.html">http://www.newscientist.com/news/news_223420.html</A>. In case it gets outdated, the paper it refers to can be found at: http://xxx.lanl.gov/ (archive gr-qc, abstract 0003092). And despite reading the paper, I still don't understand how the author reconciles two starkly contradictory measurements of time conducted across the wormhole (one via regular spacetime, and another through the wormhole) -- see Remark 1 on pg 5 of the paper, and the preceding discussion on pg 4.

Plato, is this the type of time machine you were trying to describe?

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I am; therefore I think.

[This message has been edited by Boris (edited April 17, 2000).]

Boris,

I think Rambler ment to write 'Plato' in stead of 'Boris', hence the confusion...



the mouth you are dragging with you experiences a slowdown in time, while the other (stationary) mouth ages at normal rate. When you subsequently pass through the wormhole, you emerge into the "future" within the stationary frame (where the rate of events was higher than at the moving end.) So there's no timetravel


Let's call the dragged one D and the stationary one S. If you follow the procedure as I said and bring D back to S then going into D will make you emerge in the past of S while going into S will make you emerge in the future of D ! This scenario brings back the paradoxes I have described in my opening message. Here I would like to refer to solotion 4, the alternate dimension.
What we would have is that the wormholes wouldn't really give us acces to our own past or future but to a possible past or future that exists in a different or parallel universe if you will.

Where do these parallel universes come from ? They are created the moment that a possible paradox arises. This however creates nightmares of infinite universes and infinite Plato's who are tying to convince an infinite amount of Boris's that timetravel is possible at least some form of it. However Feynman comes to the rescue here ! If I remember his theory correctly he states that a particle actually follows all possible paths AT THE SAME TIME but only one is observed because all the other paths cancel each other out because for each path with a certian wavefuntion there exists an other path of which the wavefunction is 90° out of fase and thus is destructive for the other. This is why the infinite parallel universes ultimatly are just one universe, the one we see. However in making a wormhole we could very well disrupt this cancellation and make one of the infinite possibilities real.
So in a way I must concur with you, Boris in that no true timetravel is possible. But I'm convinced you are not at all satisfied with the way I come to accept your stand ;)

Rambler,

You describe cause and effect as a local event but actually it is a global one since no violation is tolerated for the universe as a whole if our understanding that we gained through studying is correct.
About the russian guru (you are not talking about Boris are you :D :D ) I would very much like to read more about it, you got my curiosity.

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"If I have been able to see further, it was only because I stood on the shoulders of giants."
Isaac Newton

I would like to refer to two different links in regard to my previous statements about wormholes and their use as possible time machines : http://www.dd.chalmers.se/~f93jojo/sidan2.htm
gives an argument pro, and claims that possible paradoxes do not arise because as the past, the future is fixed also ! It eliminates all free will and makes the universe a very dull place indeed (for me any way) http://www.aleph.se/Trans/Tech/Space-Time/wormholes.html#4
however holds up a different view as stated by Matt Visser that as wormholes with different propertimes get closer, a stream of virtual particles destroys the two mouths. I haven't found anything that supports my previous explanation but I really don't mind that much because I actually dispise the notion of parallel Plato's mailing to the exosci website. It raises a whole bunch of existential problems and makes more problems then it solves...

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"If I have been able to see further, it was only because I stood on the shoulders of giants."
Isaac Newton

Plato,

Don't take this as if I was denying outcomes of GR; rather, I'm trying to understand exactly what's going on with this "time machine". If you really do understand the derivation, then perhaps you could explain it to me -- because I'm seeing a slew of incongruities here.

First, let's look at what happens in the classical twin paradox. You start out with two twins; one stays on Earth, and another zooms at a relativistic speed to some distant star and back. When the twins meet again, they discover that the one who stayed behind has lived through more years of time than the one that travelled. This is perfectly understandable, and I have no problem with it.

Now, let's look at what happens when the two "twins" are the mouths of a wormhole. The mouth that moves now both experiences time dilation and doesn't (at least as I understand it) -- which is a contradiction! If you merely observe the moving mouth from the reference frame of the stationary mouth, then it is time-dilated. However, if you observe the moving mouth through the stationary mouth, its time flow is exactly the same as in the stationary frame (since the "distance" through the wormhole between the mouths doesn't change.) But only one of the above can be right!! So what the heck is going on here? :confused:

Next, using your own terminology, suppose that at the start of the experiment the time at both D and S is synchronized. Then we move D at relativistic speed for a while, so that in the end the time at S is T, and the time at D is T-deltaT (where deltaT is a positive large number, not an infinitesimal amount). Now, an object from D goes through the wormhole and emerges at S. The question is: why should it emerge at S at time T-dT??? In my view, it should emerge at T -- that is, in S's "present". Similarly, an object entering at S will emerge at D in D's time T-dT; suppose the object then spends some time T' at D, and returns to S. Then the object should emerge at S at time T+T' -- shouldn't it??? So where's time travel here? Sure, if an object originally from S emerges at D, it's own clock will be ahead of D's clock, and if an object originally from D emerges at S, its clock will be behind S's clock. However, just because the clock of the object is different from the clock of the reference frame into which it emerges, that doesn't mean that the object has travelled into <u>its own</u> past or future; all it means is that the object either experienced time dilation and finds itself in a frame that didn't, or the object didn't experience time dilation and the destination frame did. There's no "time travel" here -- or am I missing something?

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I am; therefore I think.

Hi Guys,

Boris the last post from me: I meant to address it to Plato, sorry about the confusion.

Wormholes,
There is a fellow (I have all these links and names somewhere, but I'm doing this from my office at work and don't have any of this stuff here), anyway the paper I read by him (sometime ago now) suggests that you don't need relativistic speeds to achieve time travel through a wormhole. Basically what he said was that all you need is to have one mouth move. Further it would be impossible to hold one end perfectly still and therefore traveling through the wormhole would always result in time travel (always to the past). The reason why time travel would always go backwards is because relative to the traveler only the exit would move no matter which mouth you chose to enter......If I have time I'm gonna dig this stuff up and post some links. I have also very recenlty read something about a successfull quantum scale teleportation. It was a vague article but I believe someone has successfully achieved it. The next step is to move on to atoms rather then quarks, have you guys heard about this. I wonder if its a spin off from wormhole theory.


Beam me up Scotty

I found something on my work PC. I must have saved this text a while back I only just read it, here's a little bit of it.
(I AM NOT THE AUTHOR OF THE FOLLOWING TEXT)

"In his memoirs, Einstein wrote that he was disturbed that his equations contained solutions that allowed for time travel. But he finally concluded: the universe does not rotate, it expands (i.e., as in the Big Bang theory) and hence Goedel's solution could be thrown out for "physical reasons." (Apparently, if the Big bang was rotating, then time travel would be possible throughout the universe!)

Then in 1963, Roy Kerr, a New Zealand mathematician, found a solution of Einstein's equations for a rotating black hole, which had bizarre properties. The black hole would not collapse to a point (as previously thought) but into a spinning ring (of neutrons.) The ring would be circulating so rapidly that centrifugal force would keep the ring from collapsing under gravity. The ring, in turn, would act like the Looking Glass of Alice. Anyone walking through the ring would not die, but could pass through the ring into an alternate universe.

Since then, hundreds of other "wormhole" solutions have been found to Einstein's equations. These wormholes connect not only two regions of space (hence the name) but also two regions of time as well. In principle, they can be used as time machines.

Recently, attempts to add the quantum theory to gravity (and hence create a "theory of everything") have given us some insight into the paradox problem. In the quantum theory, we can have multiple states of any object. For example, an electron can exist simultaneously in different orbits (a fact which is responsible for giving us the laws of chemistry.) Similarly, Schrodinger's famous cat can exist simultaneously in two possible states: dead and alive. So by going back in time and altering the past, we merely create a parallel universe. We are changing someone ELSE's past by saving, say, Abraham Lincoln from being assassinated at the Ford Theater, but our Lincoln is still dead. But does this mean that we are able to jump into H.G. Wells' machine, spin the dial, and soar several hundred thousand years into England's future?

No. There are a number of difficult hurdles to overcome. First, the main problem is one of energy. In the same way that a car needs gasoline, a time machine needs to have fabulous amounts of energy. One either has to harness the power of a star, or to find something called "exotic" matter (which falls up, rather than down) or find a source of negative energy. (Physicists once thought that negative energy was impossible. But tiny amounts of negative energy have been experimentally verified for something called the Casimir effect, i.e., the energy created by two parallel plates.) All of these are exceedingly difficult to obtain in large quantities, at least for several more centuries!

Then there is the problem of stability. The Kerr black hole, for example, may be unstable if one falls through it. Similarly, quantum effects may build up and destroy the wormhole before you enter it. Unfortunately, our mathematics is not powerful enough to answer the question of stability because you need a "theory of everything" which combines both quantum forces and gravity. At present, superstring theory is the leading candidate for such a theory (in fact, it is the ONLY candidate; it really has no rivals at all.) But superstring theory, which happens to be my specialty, is still too difficult to solve completely. The theory is well-defined, but no one on earth is smart enough to solve it.

Interestingly enough, Stephen Hawking once opposed the idea of time travel. He even claimed he had "empirical" evidence against it. If time travel existed, he said, then we would have been visited by tourists from the future, ergo: time travel is not possible. Because of the enormous amount of work done by theoretical physicists within the last 5 years or so, Hawking has since changed his mind, and now believes that time travel is possible (although not necessarily practical.) Furthermore, perhaps, we are simply not very interesting to these tourists from the future. Anyone who can harness the power of a star would consider us to be very primitive. Imagine your friends coming across an ant hill. Would they bend down to the ants and give them trinkets, books, medicine, and power? Or would some of your friends have the strange urge to step on a few of them?

In conclusion, don't turn someone away who knocks at your door one day and claims to be your future great-great-great-grandchild. They may be right."




[This message has been edited by Rambler (edited April 18, 2000).]

Boris,
If Hawkin can change his mind, maybe you will too. :)

Boris,

I will try and explain but I'm afraid much of your misunderstanding finds it root in the blunt denial of time as an existing dimension.

Let's take the two twins that you understand and place each twin next to a mouth. Suppose one of the twins takes a spaceship and drags a mouth with it. The spaceship flies at relativistic speeds and after 1 subjective year it arrives at Proxima Centauri. This place is 4 lightyears away thus the time passed in the stationary frame is 5 years (4 years plus 1 subjective year). However as you say through the wormhole there has been no timedelation so if the twin at Proxima Centauri goes through the wormhole, he sees his brother again and they are still the same age. This means he has gone back four years in the past, this is the four years it normally takes light to travel from alfa centary to here !
His brother will be very surprised to see him because they can still see his spaceship through their telescope, and it hasn't even crossed a fifth of the distance to alfa centauri...
However there is no real time travel here because both ends of the wormhole are separated by a to large gap to have any danger of the usual paradoxes. The problem begins if (after a consideral amount of refuelling ;) ) the twin at alfa centauri decides to turn his ship around and head back to earth. He does this again at the same speed and in an other subjective year he is home again ! Upon arrival he finds his brother again, now ten years older (as the normal twin paradox) while he is only two years older. When he enters his mouth (D), for the same reason as the first time he will again emerge from S and find his brother to be the same age as him. He now invites his brother to come with him trough the wormhole and go eight years ahead in the future. They emerge through D and find the future counterpart of the brother... They have gone through a timemachine and here come the paradoxes !

Rambler,

Could you please explain how the Casimir effect (which showed the existence of a zero-point energy as far as I knew) shows that negative energy or mass exists ? Could you perhaps give me a link to some kind of paper who explains it ?
By the way are you citing someone when you say you are an expert in stringtheorie or are you really an expert ? If you are, I love to have a discussion about it... :)

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"If I have been able to see further, it was only because I stood on the shoulders of giants."
Isaac Newton

[This message has been edited by Plato (edited April 18, 2000).]

Plato,

Bear with me...



However as you say through the wormhole there has been no timedelation so if the twin at Proxima Centauri goes through the wormhole, he sees his brother again and they are still the same age. This means he has gone back four years in the past, this is the four years it normally takes light to travel from alfa centary to here!


But the twin doesn't have to go through the wormhole for them to see each other. The stationary twin S can simultaneously observe D through normal space with one eye, and through the wormhole with another eye. And now the question becomes, which one of the two simultaneous and yet contradictory measurements is correct, and in fact is either of them correct? (neither of them is any more "special" than the other -- at least as far as GR is concerned.) This goes to the very core of relativity, which is based on measurement as a means of constructing reality -- here, we have two simultaneous and yet contradictory measurements; twin D simultaneously does and does not undergo time dilation. I mean, if this is not a mathematical contradiction, then I don't know what is! I can't understand why the contradiction is deemed acceptable; in fact, if I were the one trying to formulate GR and came up with this solution in the course of my work, I would probably go back to the drawing board because such a contradiction is a surefire indication that the theory is inconsistent and incorrect!



His brother will be very surprised to see him because they can still see his spaceship through their telescope, and it hasn't even crossed a fifth of the distance to alfa centauri...


That's not correct. Even if D travels at 1.0c, by the time it gets to Vega, from Earth we shall observe it to be half-way there. If D travels slower than 1.0c, then from Earth it will appear to be even closer to Vega at the time it actually finishes the trip (e.g. if D travels at 0.33c, by the time it gets to Vega, we will on Earth receive the light it emitted when it was 1- 0.33/(1+0.33) = 0.75 percent of the way there.)



When he enters his mouth (D), for the same reason as the first time he will again emerge from S and find his brother to be the same age as him.


Ah, but therein lies the rub. Why should this particular phenomenon occur here? What prevents twin D from going through the wormhole, and finding twin S to still be 8 years older? In other words, why does the observation through the wormhole suddenly take precedence over observation through normal space, whereas they are supposed to be equipotent? Is the observation through the wormhole only an illusion, and the observation through normal space the true representation of time flow at D? Or, conversely, what if the movement of space (which is essentially what happens when you drag a wormhole mouth around) carries D with it in a way akin to a warp bubble, and the "dilation" observed through normal space is a mirage? As far as I can see, only one of the two can be true at a time; both cannot be correct, or else we have a contradiction!

As for exotic matter and construction of wormholes, I believe I already provided the reference to the article that Rambler seemed to had referred to. Here it is again, in case you missed it: http://www.newscientist.com/news/news.jsp?id=ns223420 It concerns some work done by a Russian theorist that shows existence of traversable wormholes that automatically create their own exotic matter due to the way they are constructed.

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I am; therefore I think.

[This message has been edited by Boris (edited April 18, 2000).]

Boris,

I see your problem, I really do, but again I tell you it arises because you don't want to see time as truly existing.



The stationary twin S can simultaneously observe D through normal space with one eye, and through the wormhole with another eye. And now the question becomes, which one of the two simultaneous and yet contradictory measurements is correct, and in fact is either of them correct? (neither of them is any more "special" than the other -- at least as far as GR is concerned.)

They only seem contradictory while in fact looking through the wormhole is looking through an other kind of space then looking through normal space. In relativity we have to talk about the Worldlines of objects, what the wormhole does is make a shortcut between the two worldlines of both mouths. No matter what happens in normal space with the mouths, they are always linked through wormhole space. No contradiction only two different spacetime contiua...



That's not correct. Even if D travels at 1.0c, by the time it gets to Vega, from Earth we shall observe it to be half-way there. If D travels slower than 1.0c, then from Earth it will appear to be even closer to Vega at the time it actually finishes the trip (e.g. if D travels at 0.33c, by the time it gets to Vega, we will on Earth receive the light it emitted when it was 1- 0.33/(1+0.33) = 0.75 percent of the way there.)

That is very probable but I was using alfa proxima as my example which is a bit more then 4 lightyears away from us (I thought). I didn't really check my math as I should :o but was using crude approximations, thank you for the correction however.



Ah, but therein lies the rub. Why should this particular phenomenon occur here? What prevents twin D from going through the wormhole, and finding twin S to still be 8 years older? In other words, why does the observation through the wormhole suddenly take precedence over observation through normal space, whereas they are supposed to be equipotent

No rub, see my previous answer. If this is possible to bring two mouths together who are timeshifted relative to each other, then they would still shortcut the two worldlines of the mouths, this shortcut would be more in time in then in space in this particular case. Thus making time a genuin dimension you can travel in OR would consist in the connection of two parallel worlds. Take your pick... :)

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"If I have been able to see further, it was only because I stood on the shoulders of giants."
Isaac Newton

Plato (VERY IMPORTANT),

OOOOOOPS, sorry I wrote that I had this text saved at work. I got it off the net and I'M NOT the author, so not claiming to be an expert. I surfed around and started to read ran out of time and saved the text. I only have a very basic understanding of these concepts. The text i have saved doesn't have the author's name otherwise I would have attached it. Again sorry. I just posted this stuff to show Boris that even Steven Hawkins changed his mind about time travel.

Boris,

you said:
"The stationary twin S can simultaneously observe D through normal space with one eye, and through the wormhole with another eye"

How can the twin view this simultaneously?, the image in normal space would take 4 years to reach him, so the normal space image would NOT have the other twin in it for another 4 years. Looking through the worm hole would then in affect be seing 4 years into the future would it not (if we assume the normal space image is the present rather then 4 year old light, but the former point is the one I'm trying to make this is just a side note).

[This message has been edited by Rambler (edited April 18, 2000).]

I have found a website with the correct formulas for a relativistic rocket that is under constant acceleration : http://phyhp.phy.ncku.edu.tw/mirrors/physicsfaq/rocket.html

So my previous problem, assuming there is 4 lightyears difference between Sol and Alfa Centauri and I have a constant acceleration of 1 g and half way I begin braking. It would take 3.56 years shiptime to get to full stop at Alfa Centauri, coming back to earth is again the same amount of years so actually the twins would be almost a year apart now and the wormhole would provide a means to a year in the future or past, depending what mouth you do in.
If I would stick to my previous assumption to brigde the distance of 4 lightyears in 1 year ship time, it would take a bit more then 5 g's. This is a bit to much for the human constitution to undergo for two consecutive years I'm afraid... ;)

However if we would go much further, we could still do this in one lifetime. Suppose we would go with one g acceleration to the Andromeda Galaxy, which is 2 million lightyears away. Assuming the same procedure with breaking halfway it would only take us a bit more then 28 years ! If you go back then you would have some timemachine taking you 4 million years into the future !

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"If I have been able to see further, it was only because I stood on the shoulders of giants."
Isaac Newton

Because everything is made up of straight lines and curves, time travelling is impossible. The physical representaion of time i.e. watches are made up of atoms, protons, neutrons (metals/or whatever). This shows that the fastest anything known can travel is the speed of light. To think differently about how machines are made is the answer to this problem. Think caveman and wheel, egyptians and pyramids.

Epitectus,

Ha?

Rambler, Plato,



How can the twin view this simultaneously?, the image in normal space would take 4 years to reach him, so the normal space image would NOT have the other twin in it for another 4 years. Looking through the worm hole would then in affect be seing 4 years into the future would it not (if we assume the normal space image is the present rather then 4 year old light, but the former point is the one I'm trying to make this is just a side note).




In relativity we have to talk about the Worldlines of objects, what the wormhole does is make a shortcut between the two worldlines of both mouths. No matter what happens in normal space with the mouths, they are always linked through wormhole space. No contradiction only two different spacetime contiua...


The point is not in the fact that the states represented by photons arriving through the wormhole and photons traversing normal space are not synchronized. The contradiction comes from the fact that judging from photons that come through the wormhole, the moving mouth experiences no time dilation, while the photons arriving through normal space indicate that the moving mouth is time-dilated. Thus, simultaneous observations indicate that the moving mouth is time-dilated and not time-dilated at the same time. (Btw, since the moving mouth starts moving from a vicinity of the stationary mouth, it will be visible during its entire journey, not 4 years later.)

And it's not two distinct spacetime continua; it's one single continuum with a topological twist in it. The contradiction is still there, and it is not at all clear what is really happening with the event rate at the moving mouth.

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I am; therefore I think.

[This message has been edited by Boris (edited April 20, 2000).]

Boris,

I think you got the problem of wormholes on the spot, the only way out of it is by saying that or wormholes simply don't exist and won't ever exist or that wormholes connect different universes...
From the moment the wormhole exist, the two mouths we see are not connected at all but each lead to a different universe so actually when you 'make' a wormhole you are making two of them. This last explanation is very awkward I know and I don't like it at all, but to tell you the truth I don't know enough about the details of wormholes to give a sensible answer... :(

However I see a small loophole, Since the photons going through the wormhole are with you much faster then the ones going through normal space, we are actually not talking about simultaneously created photons any more. So the image you have through the wormhole will always be younger then the one you have through normal space. This might be a reason why there is no real paradox. Maybe if you stretch the concept of 'simultaneity' enough there will be no such thing as a paradox...

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"If I have been able to see further, it was only because I stood on the shoulders of giants."
Isaac Newton

Plato,

If a wormhole links two distinct universes, then it's not truly a wormhole. Real wormholes are just supposed to be topological deformations of one and the same spacetime. And it's hard to say that wormholes don't exist since they arise as solutions to a variety of different scenarios under GR. There has been a time when people couldn't come to grips with neutron stars and black holes, and even Einstein spent a significant portion of his career trying to show why black holes can't exist ("abominations" he called them.) By analogy, a purported inexistence of wormholes would be resting on rather shaky ground.

Finally, the differential reception time scenario you describe doesn't solve the paradox. Imagine a setup where the moving mouth of the wormhole makes a full circle and zooms past the stationary mouth. Then there will be two configurations where the distance photons travel from the moving mouth through the wormhole to the stationary mouth, and from the moving mouth to the stationary mouth through normal space, is the same (i.e. photons emitted simultaneously at the transmitter located at the moving mouth, will arrive simultaneously at the receiver located at the stationary mouth.) So you see, there's no simultaneity trick here. We are concerned with photons emitted in a single reference frame, so we can indeed be sure that within that reference frame the photons were emitted simultaneously, and represented its instantaneous motion. So, the problem still remains: according to the observer at the stationary mouth, the moving mouth simultaneously is and is not time-dilated, which is a logical contradiction. I've searched and thought about this, and I still can't find a way out...other than to say that the "time" in GR (or SR for that matter) is an incorrect construct, and should rather be thought of as rate...and somehow, somewhere, an absolute reference frame must creep in. (did I hear a "DOHHH!"? :D)

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I am; therefore I think.

Boris,
I still don't see the paradox. I'm nowhere near Plato's or your level of understanding of GR so bear with me.

Could you eplain to me again (dumb it down as much as you can stomach) how you come to the conclusion that an image on one side of the wormhole would create a paradox to an observer on the otherside. Could you perhaps illustrate your above example with arbitary dimensions like say 100 meter long throught (inside wormhole) and .25 l/year normal space....or whatever you need to explain what make the paradox a little clearer. Also if you have the resources could you post some equations that deal with the dilation issues (Plato if you have them please post, as I have said before I'm at work and don't have the means).

Rambler,

First of all, you would be surprised if you would know what I don't know about general relativity. The things I do know are what I remember from my courses in university and the (popularised) articles I read about it. I never really studied it in dept...
Anyway I think you will find http://phyhp.phy.ncku.edu.tw/mirrors/physicsfaq/relativity.html very interesting indeed. It has a whole bunch of formulas and also explanations of weird relativistic effects.

Boris,
what can I say ? I'm as much at a loss as you are. The only thing I can say that to arrive at the paradox that you talk about assumes already a whole bunch of thing of which we don't have any data about !
To begin with is ther such a thing is negative mass ? I agree that wormhole are an essential part of GR but normal wormholes only exist at the quantumscales, we need negative mass to blow them up to our macro world. Then is it possible that timeshifted mouths can come close enough together to make an experiment that you are talking about ? One thing is for sure GR as we know it is certainly not a complete theory, String theory is in the making and I actually have my doubts if that will be the 'ultimate theory' that they want us to believe.
We don't have a wormhole so we can't test it and for me experiment stays the ultimate way to make sure we aren't bullshitting ourselves (pardon the expression) and making up stories about timetravel and wormholes.

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"If I have been able to see further, it was only because I stood on the shoulders of giants."
Isaac Newton

[This message has been edited by Plato (edited April 28, 2000).]

Rambler,

The paradox arises because of the way relativity treats reality. In relativity, you cannot assume something exists in some state in any way, unless you receive light (or some kind of signal bearing information) from whatever it is you are interested in describing. Then you could use the signal you received to describe the object it came from (and therefore the state of the object.)

Relativity uses light as a measuring stick both to measure distance and time (since lightspeed is constant and absolute). Because of that, it is fairly easy (well, at least in absense of gravitational mass, meaning a flat spacetime) to derive formulas that show time slowing down in a fast-moving object, as determined by a stationary external observer based on the light emitted by the moving object. This is what we call time dilation.

The paradox arises because, while sitting at the stationary mouth of the wormhole, we observe photons of light simultaneously emitted from something located at the moving mouth. Some of the photons go through the wormhole to reach us, while others traverse regular space. If we take a snapshot of this situation when the moving mouth of the wormhole is just as far away from the stationary mouth along normal space, as it is through the wormhole, we have photons simultaneously emitted from the moving mouth at that moment, traveling through both normal space and through the wormhole, and arriving at the stationary mouth simultaneously. With respect to the stationary mouth, the distance to the moving mouth <u>through the wormhole</u> does not change, while the distance to the moving mouth through regular space is changing since the moving mouth is indeed moving. Now, if the moving mouth is zooming at a relativistic speed, then as observed by the stationary mouth through normal space via photons received from the moving mouth, it is measurably time-dilated (meaning that its time rate has slowed down relative to that of the stationary moutn.) However, judging from photons received through the wormhole, the "moving" mouth is actually standing still, and is not time-dilated at all.

The task of the observer at the stationary mouth is to determine just what is happening with time at the moving mouth. And the point is that if this observer used just the photons that arrived through normal space, he/she would conclude that time at the moving mouth has slowed down. However, if the observer used just the photons that arrived through the wormhole, he/she would determine that time at the moving mouth is synchronized to his/her own time. These are two mutually contradictory conclusions, and yet according to relativity they must both be simultaneously true. So there is something very fishy going on here, and we do indeed have a paradox.

Now, it seems the "solution" to the paradox is to assert that due to the motion of the "moving" mouth, the wormhole establishes a time gradient across itself, so that even though the moving mouth is really time-dilated (as observed through "normal" spacetime), the signals it sends are being squeezed closer together in time as they pass through the wormhole, with the end result being that they appear not at all time-dilated at the stationary mouth (in essense, they travel into the future). What's particularly bizarre about this, is that the space inside the wormhole remains undisturbed through all of this "motion", so it's unclear where the time gradient comes from, as far as that region of spacetime is concerned. And in general, it's not clear why the spacetime inside the wormhole should be any less or more "privileged" than the space outside, in terms of induced time gradients. For example, imagine enlarging the wormhole until its internal volume is larger than "the rest" of the universe -- now what is the "inside" vs. the "outside"?

And at any rate, since I refuse to consider time a dimension, I tend to think that there is something fundamentally wrong with this entire construction...

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I am; therefore I think.

Boris,

Thanks for the excellent explanation, hope it wasn't too painfull going into so much detail :)

OK,
There is a stationary mouth and a moving mouth. The observer in (or on the edge) of the staionary mouth recieves a photon from the passing exit (through normal space) and concludes that time dilation has indeed occured. However you also say:

------------

"What's particularly bizarre about this, is that the space inside the wormhole remains undisturbed through all of this "motion", so it's unclear where the time gradient comes from, as far as that region of spacetime is concerned"

------------

Therefore the observer still sees the exit move at relativistic speed wheather the photon comes through normal space or through the wormhole. Both would appear dilated. If indeed the space inside the hole is the same as that outside and distances are the same. Therefore still no paradox (maybe????, I realise this is too simple to solve the dilema but I thought I'd post it anway)

Also,
The event you describe has one moment where the geometry is such that the photon travels an equal distance. I'm not going to attempt to comprehend the dynamics involved in a system which is being observed with relativitic speeds AND topological twists :rolleyes: but would I be right in assuming the paradox would only exist for the one moment where the path through the hole or normal space is equidistant. The reason I ask is because for that one moment we can assume the same reference frame wheather looking through the wormhole or through normal space?????????. However I would think that when the distance are no longer the same we would need a seperate reference frame and assume two seperate sytems of events.

On a side note one wormhole solution I have read about (in fact the first I time I had ever heard of them) suggested that if the EXIT is movig relative to the entrance (and this would always occur) then the solution (I suspect over large distance in normal space) as I understood it stated a photon (or whatever) would exit the wormhole in the past. i.e you would always go back in time. I wish I took more notice of the math it would probably come in handy here...



[This message has been edited by Rambler (edited April 29, 2000).]

okay, folks, i don't know who all took the time to read that article, but it doesn't say that they observed a wormhole, just that they "mathematically proved" that one could exist. it goes on to say not that it proves exotic matter exists, but that it assumes and requires its existence as a pretense. you can prove virtually anything through high-level math, and you can look in the religious forum under "theomatics" for an entertaining example of how something can be "100% proven" by math.

time travel becomes a possibility once you begin describing time as another physical dimension. although we do currently describe time as another dimension, problems occur when we begin to apply characteristics of our own three physical dimensions. imagine a two dimensional being in his two dimensions, "traveling through" time as a third dimension. you could say that for him, time travel would be possible if he were to bend some of his space three dimensionally back to another one of his two dimensional planes of existence in the past. unfortunately, this requires that we look at time as simply a series of still frames, causing all kinds of problems. furthermore, this requires that his third dimension of time act exactly as the other two, when we have every indication that it acts very differently. the issue is far more complicated than that, but we are bound by the limitations of our universe. we would have nothing to allow us to break those boundries, just as the two dimensional being would have no way of curving his universe in the third dimension as he could apply no force in that direction. no, i do not believe that time travel is possible, and we don't even have any solid theories on how this might be accomplished. however, as an open-minded individual i have to concied that our current understanding of the universe is far from adequate, so i cannot eliminate the possibility.

we have to think of time travel in one of two ways, and the "paradoxes" which are outlined here are only caused when you mix the two. the first way is that you look at it as one continuous timeline, and that anything you do while traveling back in time is included in reaching the time machine and sending yourself back. at this point, everything you do has already happened as determined history, i.e. you already didn't kill yourself when you travel back because you aren't dead now. the second way is to describe it as diverging timelines. at the exact point you travel back to in time, you begin to create a new timeline with potentially different results. at this point, you would completely dissapear from you original timeline never to be heard from again. you could only return to the alternate timeline because your "time-traveled" existence returning to the future requires your "time-traveling" existence in the past; you can't return with a bunch of memories without those memories having happened. in the original timeline, of course, none of those memories happened because you had not yet traveled back in time. this would explain why we have never been visited by time travelers, because we would first have to follow the path leading to that time traveling. however, this also allows you to now go back and kill your parents. you would simply return to a future with no record of your existence.

as for the wormhole discussion, i think this is a case of where we need to differentiate between reality and percieved reality. light does not dictate reality, and should not be treated as any more important than any other object traveling through the wormhole. when i look at a picture of a mountain on my computer, it isn't because i create a small mountain inside my monitor. it is because my video card knows how to create the correct pattern of light to fool my eyes. yes, you could stare through a wormhole and normal space and see two totally different images of the same object. so?

samus

Samus,



yes, you could stare through a wormhole and normal space and see two totally different images of the same object. so?


In general, I agree -- but go and try to tell that to quantum physicists or to relativists...

As for your assertion that one can prove anything with math, I'm afraid your are confusing the flabby drawl presented as math in the "theomatics" circles, and the rigid, axiomatic derivations employed in physics. Certaily, there are always assumptions embedded in any axyom; but in the case of phycics, those assumptions are often a result of measurement, or give rise to theories that feature amazing accuracy when their predictions are verified empirically. That means the model is not arbitrary, but indeed represents something very concrete and relevant about the world. Remember, black holes used to be just a crazy idea until recently -- and where did they come from? -- from the very same theory that is predicting wormholes.

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I am; therefore I think.

ok, after some reflection on GR (what some meditation sometimes can do ;) ) I have come to the conclusion that wormholes actually don't violate the equivalence principle of GR.
The question in GR is not am I time deleted or not, this is simply an effect due to transformation of coördinates. The question is : are the laws of physics the same for all observers, or in other words is there such a thing as a preferred reference system.
What we have at the mouth of a wormhole is 'simply' some deformed topology of spacetime. This deformation makes measurements of the surroundings path-dependent, this means measuring lightsignals is dependent of how the signals travel from their origan to your measuring device. This is also true in 'normal' space since for example light from stars gets bent when we observe the stars close to our sun (during a solar eclips for example). We could also ask ourselves where is the star really, this however is not a question addressed by GR.
So the question is there timedelation or not is mute, it all depends on through wich topology you are making your measurments : no paradox.

However something else troubles me a lot more and that are the conservation laws of energy and momentum. Suppose you have two mouths with a relative speed V, when I step trough a mouth my energy as opposed to my starting point has just increased with MV^2/2 with M my mass. Where does this energy come from ?

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"If I have been able to see further, it was only because I stood on the shoulders of giants."
Isaac Newton

Is it perhaps possible we (humans, life) can exist in lots of different histories at one time and that when people claim to see ghost that this in fact a glitch in the time continum and we are in fact looking at another history existing at the same time, if not instead of the present one?

Are wormholes similar to the coriolus effect

Hi Epitectus,

No, they are not similar; in fact, they are two completely different things.

The coriolis effect is caused by the rotation of the earth, and its consequence is that free moving objects (eg. clouds or a cannonball) get bent away from their straight path in a direction that depends on the initial way they were going and on what hemisphere you're on.

Wormholes are "bridges" in the spacetime continuum (Einstein predicted that the spacetime continuum gets bent when mass is present. With some imagination you can think two masses can bend spacetime that heavily, that the "dips" merge together - it's quite difficult to imagine that in 4 dimensions). They have not been observed in real life though: their existance is a theoretical consequence of the equations of general relativity from Einstein.

Bye!

Crisp

Plato,

You asked:

"Suppose you have two mouths with a relative speed V, when I step trough a mouth my energy as opposed to my starting point has just increased with MV^2/2 with M my mass. Where does this energy come from ?"

Well wouldn't the extra energy be from the "moving" mouth which you just stepped into. I see no difference in this to say standing still, then being taken by a chairlift on a snow field. Obviously in the real world you wouldn't be able to step into the mouth instantaneously, hence you would need to match the speed of the mouth first, which requires adding energy on your behalf.

Rambler,

there is a huge difference between the chairlift and the wormhole. In the chairlift you are staying in the same spacetime continuum, going through the wormhole is actually going inside a new spacetime continuum.
You see, the mouths are moving with respect to each other only through normal space but through their wormhole space they are standing perfectly still.
So if you are going through the wormhole no laws are violated through wormhole space but they are violated through normal space because in normal space you first had no kinetic energy and next you have suddenly an energy difference compared to where you left. There is no matching of speed involved because the mouths aren't moving through wormhole space.
Hence the paradox of course...

Honesty obliges me to say that my view on wormholes and time travel has changed since I first started this thread. If people have been following the discussions in the determinism vs. indeterminism thread they will know what I'm talking about.
I am now convinced that the equations of Einstein are in need of revision. The time symmetric aspect needs to be taken out. There are already people trying to do this very thing, some work is being done in the Texan Prigogine institute but this is all very cutting edge theory and to draw any conclusions now would be precausious, to say the least.
What I can say is that time travel won't work any more in the revised equations because there will be a distinct arrow of time emerging from the basic equations themselves. This means that everything is compelled to go in this one direction : from past to future, no turning back !

Miss Happy,

This would also do away with the notion of parallel dimensions I'm afraid. They were a concoction of people trying to explain the wave nature of quantum mechanics and strange things like the same particle going through two slits at once and thereby creating a diffraction pattern. Quantum theory also has the same lack as relativity : namely the time symmetrie in the equations. If you take the probabilities themselves as basic stuff and let the wave functions drop, this takes away all need for parallel dimensions.

This is actually a good thing because if you think about it, an infinite amound of parallel dimensions would mean an infinite amount of Plato's trying to explain something to an infinite amount of Miss Happy's, where is the real you ? This idea was crazy in the first place so I think it is best we can put it to rest now...

So in answer to my previous question that started the thread :
Is time travel possible theoretically ?
I would have to say : Yes, but only in one direction : forward !

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I err, therefore I exist !

Plato,

Ok you have said that there would be 2 different space-time continuum. Wouldn't that mean then for your comparison you would need to look at the system with 2 different refernece frames? And hence to do an accurate comparison of kinetic energies you would need to do some kind of "mapping" of the velocities as seen from each respective reference?

Here is what I see happening. Our guinea pig is standing in normal space watching a wormhole mouth zip past at velocity V(m)(for mouth) for the guinea pig to enter the mouth it needs to match the velocity of the mouth...i.e. raise its kinetic energy. Now conversly if the the guinea pig is inside the mouth the refernce frame its in tells it it has no kinetic energy, however the moment it exits the wormholes space-time, its transformed its refernce frame and sees that the extra energy it appears to have gained is a result of taking some of the energy the WHOLE wormhole system had... with it, and was oblivious to because of the reference it was in. Analogous to the velocity of a ball bouncing inside a train as seen by an observer inside the train, and by one outide the train.

Sorry to bug you on this Plato, you no doubt realise that I only read this stuff out of interest, my only formal training was from the perspective of an electronics engineer...so I had a limited study of quantum mechanics and even less relativity. As you can appreciate engineers only look at the stuff relevant to their work and how it can be exoploited..we never looked at it as a subject on its own.

Rambler,

actually I'm streching the word 'different' here cause 2 different space-time contiua would mean there is a discontinuity between them in order to make a distinction. This however is not the case because the mouth is not a discontinuity but a rather continuus change of the 'flat' geometry of normal space to the wierd folded geometry of the wormhole space. The difference lies in the geometry.

The guinea pig that watches a wormhole mouth zip by would be standing next to the other mouth that is at rest with respect to the guinea pig (that is my set up). Looking through the other mouth the zipping mouth stands perfectly still.
However your Whole wormhole system energy got me thinking : what happens to the internal geometry of the wormhole if the mouths move against each other ? It changes of course, this change must also take energy so perhaps going through a wormhole also involves an energy transfer. Where does all this energy come from ? From the mouths of course because that is the only connection the wormhole has with the universe. So I guess towing along a wormhole is not that easy as I first thought

Don't sweat it about bugging me about these things. Asking things always works both ways, the asker and the ask usually learn something out of it. ;)

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I err, therefore I exist !

[This message has been edited by Plato (edited May 26, 2000).]

According to Einstiens Theory of Relativity, time travel_is_possible.

(apparently) As you near the speed of light time slows down. So, say, travel to Wolf 339 at 98% the speed of light. Time will travel (for memory) 14% slower than on earth. Leave in 2050 and when you come back in 2065 you would only have aged around 13 years.

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"The credit belongs to the man who is actually in the arena, whose face is marred with sweat and dust and blood; who strives valiantly; who errs and comes short again and again; who knows the great enthusiasms, the great devotions, and spends himself in a worthy cause; who' if we wins' knows the triumph of high achievement; and who, if he fails, at least fails while daring greatly, so that his place shall never be with those cold and timid sould who know neither victory nor defeat."
-"The Man in the Arena"

Plato et al,

Consider the actual situation: you are standing in front of the stationary wormhole mouth, and looking through the wormhole (that is, through the moving mouth).

To you, it will seem that everything outside the moving mouth is moving, while you will actually think that the "moving" mouth is stationary. So at first glance it would seem that if you traversed the wormhole, by the time you were exiting from the moving mouth you should be moving with the same velocity. However, upon further consideration I do not see why that should be the case. Why should it not be that when you exit through the moving mouth, your kinetic energy is the same as when you started, and you emerge on the other side only to watch the wormhole mouth zoom away from you?

When a wormhole mouth moves, I should think it's more akin to a transverse wave traveling on a taut sheet of rubber. The individual rubber particles don't actually experience any "horizontal" motion. It's hard to describe, but imagine that the wormhole is visualized in 3D by taking two sheets of plastic, cutting a hole in each, and joining the edges of the holes so that the two sheets form a single surface, with a smooth "throat" connecting the two planes. Now, fix one end of the throat, and try to move the other one along its plane. Since plastic has to be conserved, you are going to have to shove plastic through the wormhole along its leading edge, and pull plastic out of the trailing edge -- to maintain the same geometry. This means that there will be spacetime "currents" evoked by the moving mouth at the stationary mouth (spacetime is going to be sucked into the stationary mouth from certain directions.) But regardless, the two motions of spacetime at the moving mouth (since they are opposite in direction) should cancel each other out near the mouth's middle, so that there is no overall spacetime movement at that point -- hence, no imparted velocity.

I don't know if any of this makes sense, but I really am having a hard time putting this in words without drawing pictures.

P.S.

In thinking about this typical wormhole analogy (with two flat sheets connected by a throat), I can't help but arrive at the conclusion that, if one of the wormhole mouths is moving while the other remains stationary, the actual distance (through the wormhole) of the two mouths cannot remain constant! That is, the trip through the wormhole will be either shortening, or lengthening -- though that should have nothing to do with time travel...

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I am; therefore I think.

[This message has been edited by Boris (edited May 27, 2000).]

“Time is but the stream I go fishing in. I drink at it, but while I drink I see the sandy bottom and detect how shallow it is. It's thin current slides away, but eternity remains.” - Henry David Thoreau

Indeed time travel is possible. This Clinton-era thread is proof.

In practice, time travel will never be possible, only perhaps in the weird quantum world of sub-atomic particles (and below).

Regardless of any theories already made, I think that time travel is not possible

You can never get to the past, because you didn't start your time machine before you started it.

Yep, it happened to me.
I landed earlier than I am departed.

they say it is easier to time travel to the future than it is to the past..

i say the proof of backwards time travel is in this thread..(7 posts ago it was 5-27-00)

:rolleyes::D

The O.P. asks if Time Travel is theoretically possible.

Define time:
Time, as described by Einstein in Relativity, is the measurement of change brought on by Motion. Within the Universe, motion and change are a constant (even if the rates of each are not); time is not a constant.
Since time is not a constant, it is not subject to alteration.
An analogy: Imagine you have a cup of coffee. That cup of the coffee is the Entire Universe.
You pick up the canister of powdered coffee creamer and sprinkle some onto the coffee. That powdered creamer represents 'matter' in the universe. It represents space and therefor: time.
The creamer sits on the top of the coffee, floating there motionlessly. You can see particulates, just sitting there. You pick up the coffee stirrer and start swirling it through the coffee. Motion has been added to the Universe. You can see the particulates dragging with currents and eddies, swirling around- Change is happening- time is measured as passing. Since time is not a constant, the amount of change measured is determinant only in pockets; you'll have different motion, and therefor rate of change, measured for different temperature and material consistencies in the coffee.
Differing motion against mass will result in different relatively perceived rates of change.

In order for Time travel to be possible, one would have to Undo Change. Literally. This "undoing of change" would have to be throughout the cosmos.

To travel backward in time, you must undo all motions and change within the entire universe- reverting it back to its exact state and position at the point of contact with the 'past.' Clearly, impossible for anything that exists within the Universe.
One must exit the Universe and build a machine that has enough energy and calculating power to alter all matter and states within the universe.

To accelerate change, you have only your own relative viewpoint to play with. You cannot accelerate change for all within the universe any more than travel back in time, but you CAN do it for yourself or anyone traveling with you by accelerating to near the speed of light. Since you cannot undo change, you cannot visit the future and return to the past. You're stuck there.

All of which renders the grandfather paradox moot.

As Prof. Farnsworth pointed out (With some help from the Harlem Globetrotters); The Doom Field has a self correcting paradox resolver.

Time is a concept of man that describes a rate, it is immaterial and therefore impossible to travel through. Of course, the only real way to "travel" time is to go to sleep and suddenly its morning when you wake up.

Of course, the only real way to "travel" time is to go to sleep and suddenly its morning when you wake up.

Wrong. It is not the only way.

A reasonable alternative is to drive by yourself down a country road, have the vehicle suddenly lose power, the dog barks and a bright light beams down from above.

You'll wake up later with a sore rectum and strange fuzzy images of wide eyed things and star maps at the back of your head -- In the Future -- having lost a few hours.

If you can travel faster than the speed of light, you can. Theoretically.

Sadly you would expand to the size of the universe, so technically you cant.

This is why light can have no mass at rest but because its in motion is has mass.

As we know of right now.

To me the question and physics behind it just unravels time as an illusion, so much of one i can barley explain it.

Time started when we recorded the patterns between seasons and years. Calendars implemented to record dates and overall a general history of civilization.

Gravity effects time as to gravity effects earth revolving the sun.

Hope I'm not beating a dead horse here but time travel to the future is most certainly possible. Traveling to past is highly unlikely however.

Hi! I'm writing to you from 18 minutes in the future. Hope you are here!
:wave:(wave of the future)

Time travel,what a Joke?
Do you know that there is a mad scientist who obtained Physcis degree only to create a time machine to meet his father who was murdered.Really crazy!

http://www.youtube.com/watch?v=oRWwI61so5Q

Well I think that chances of a time machine is allmost NO!

Talk of a time machine is even beyond talk of cold fussion and Gravitational energy powered engines.

Hi! I'm writing to you from 18 minutes in the future. Hope you are here!
:wave:(wave of the future)

LOL :bravo:

But seriously, I'm talking about time dilation which if you can get going fast enough would let you travel to the future. Of course you clothes will be horribly out of style when you get there. ;)

Well, I'm not moving at all, relative to the earth, and I'm still travelling into the future. My clothes were out of style when I left.

Well, I'm not moving at all, relative to the earth, and I'm still travelling into the future. My clothes were out of style when I left.

Go far enough in the future and they may come back in style :rolleyes::p

That's why I keep them.

that's why i keep them.

lol :d

Every one of us has probably pondered this question but do we really know what we are talking about if we say time travel ?
Suppose I invented a time machine and I went back to the time when I was making the device and destroyed all the plans and blueprints to make it. What would happen to me ?
When we ask such questions we are actually wondering whether or not the universe is deterministic and if there is only one universe or several. You see there are many ways out of this apparent paradox :
1. one might argue it would be somehow prevented that I destroyed the plans because otherwise I could never have traveled back to the past.
2. the whole thing is simply impossible because there is no way time travel is possible, this might involve some kind of presently unknown law like the one we know that prevents us to go faster then the speed of light
3. I do go back and destroy the plans but by doing this I destroy my future and therefor am stuck in the past
4. I don't really go back to 'my past' but to the past of a universe in a parallel dimension, anything I do there has no effect what so ever at my own universe therefore making time travelling more like dimension travelling.

One final remark before I let you all lose on this subject : if time travel is possible then where are all the travellers of the future ? Are they prevented to interact because of some mysterious mechanism like in explanation 1 ? Have they perhaps (like our famous aliens who controll the ufo's ;) ) some sort of non intervention pact ? Aren't they there because it is impossible to begin with ?

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"If I have been able to see further, it was only because I stood on the shoulders of giants."
Isaac Newton
you cant go back in time bc PAST doesnt exist except in our memories and pictures..
same with the Future..

you cant go back in time bc PAST doesnt exist except in our memories and pictures..
same with the Future.. Prove it! The past (as I conceive it) is all there is, because the present is not stable! You always look at the memory of what was present when you try to look at the present. And the future is not yet decided so...All that IS decided is the past.
You cant visit the past since you actually didnt visit it when it happened!
All you possibly can do (Hah!) is to visit a copy of the past.

Time travel is not possible.No scientist will agree with this.
No No,I am not conventional,but there is no logic behind time travel.How can present can be taken to past?How can I see future when Entropy keep on increasing.
The main obstacle to time travel is Incerase in Entropy of universe.

Talking about making of time machine will prove laws of thermodynamics wrong which can never happen.

Time travel is not possible.No scientist will agree with this.
No No,I am not conventional,but there is no logic behind time travel.How can present can be taken to past?How can I see future when Entropy keep on increasing.
The main obstacle to time travel is Incerase in Entropy of universe.

Talking about making of time machine will prove laws of thermodynamics wrong which can never happen.

Actually, evey scientist worthy of the title will tell you that time travel is 100% possible. You are currently traveling to the future at the rate of one second per second. You can speed that up considerably if you manage to go a large fraction of the speed light.

http://en.wikipedia.org/wiki/Time_dilation

I am not talking of present day normal time travel.I was talking about speeding up time heavily from same location or to go back in past ,which do not seem possible to me.

nobody has mentionend the GPS system yet who are most definitly time dilated.

All events exist simultaneously in potentia. It is only our relative position to those events that create the perception of time as a straight line. So to me time travel is possible in the sense that everything that has been or ever will be already exists, but then it is also not possible because the only moment that we ever truly exist in is the present.

No, you can't travel back in time, technically you can travel forward and slow down your perception of time, you would not go very far into the future. I had a thought the other day, if there was a big bang, what if it was caused by an attempt at time travel?

Yes. As it has already been highlighted, we are already moving through time. My current strategy is to MATCH the speed of time and travel into the future.

No, you can't travel back in time, technically you can travel forward and slow down your perception of time, you would not go very far into the future. I had a thought the other day, if there was a big bang, what if it was caused by an attempt at time travel?

LOL you mean you watched an eppisode of family guy yesterday.

if there was a big bang, what if it was caused by an attempt at time travel?

or from a Large Hadron Collider...

This subject is done to death in a number of threads, however there are still various points to make.

For instance one of the many misconceptions is "If you could travel faster than light, you'd end up being able to go back in time." That is a ridiculous notion, after all it isn't talking about two competitors racing to a finish line where if one runs faster will finish first. What you are actually assuming is that the fast at which you travel, the slower the universe will be as a counter observation to you. It doesn't mean the other competitor starts running backwards slowly, what it means is the fast you travel the closer you'd get to a universal state of "Rest" or absence of motion.

If you kept going until everything was at absolute rest, everything but the person attempting the observation would cease to exist.

It does generate an important question in regards to what occurs should "you attempt to return to normal speed", as if you reached that ultimate universal absolute rest, you will have broken "continuity", so you might never be able to return back to a normal universe. It generates a rather interesting discussion in both Philosophy and some area's of Theology considering such figurative's like "Maya", after all to exist in an occurance of absolute rest is a potential "space" to build a whole universe from scratch, "a beginning". While going to the point where entropy becomes so great that continuity can no longer follow suit is itself a form of universal destruction, "an End".

So it's figuratively a beginning and end, while also being a completely figurative illusion.

Forward time travel is. Traveling back is problematic in many ways- and on a logical level, not just on the level of application.

If in a "different universe" where causality wasn't a problem, perhaps then. Traveling back in time outside one's own sphere of causality is also potentially acceptable.