Bananas

[Ontoplankton]

Time travel into the past is logically possible, provided that your presence in the past only causes things to happen that have, in fact, already happened. In other words: if time travel is restricted to only one universe (as in general relativity), then there is only one version of the universe that actually happened -- it's never the case that one thing happens in the first "draft" and another in the second "draft".

So, it's not possible to go back in time and kill your grandfather, thereby causing yourself not to have been born. If you try this, you will be stopped by freak accidents, such as tripping on a banana peel.

In other words: if you look at all initial conditions, and time-evolve them forward, then the vast majority will lead to inconsistencies (blatant ones as in the grandfather case, or subtler ones such as a blade of grass being in the wrong place). The only ones left with time travel involve amazing coincidences.

My question to all of you is: does this make time travel extremely improbable? The paper Bananas Enough for Time Travel (http://www.vuw.ac.nz/phil/njjs/bett.pdf) argues it doesn't, but I'm not sure I'm convinced. I think to solve this question, you would have to know how to assign probabilities to various boundary conditions in General Relativity, which I don't.

So, are there bananas enough for time travel?

[Mentat]

Originally posted by Ontoplankton
Time travel into the past is logically possible, provided that your presence in the past only causes things to happen that have, in fact, already happened. In other words: if time travel is restricted to only one universe (as in general relativity), then there is only one version of the universe that actually happened -- it's never the case that one thing happens in the first "draft" and another in the second "draft".

So, it's not possible to go back in time and kill your grandfather, thereby causing yourself not to have been born. If you try this, you will be stopped by freak accidents, such as tripping on a banana peel.

In other words: if you look at all initial conditions, and time-evolve them forward, then the vast majority will lead to inconsistencies (blatant ones as in the grandfather case, or subtler ones such as a blade of grass being in the wrong place). The only ones left with time travel involve amazing coincidences.

My question to all of you is: does this make time travel extremely improbable? The paper Bananas Enough for Time Travel (http://www.vuw.ac.nz/phil/njjs/bett.pdf) argues it doesn't, but I'm not sure I'm convinced. I think to solve this question, you would have to know how to assign probabilities to various boundary conditions in General Relativity, which I don't.

So, are there bananas enough for time travel?

Wasn't this "fulfilling of the past" idea originally concieved by Kurt Godel? I have two problems with such a concept:

1) It requires predestination.

2) It requires backward time travel (obviously), and there is logical flaw with traveling to any time before the time you start traveling.

[Ontoplankton]

Originally posted by Mentat
1) It requires predestination.

What comes out of a time machine depends on events that will happen in the future. In that sense, the future already has to be "fixed" (though not by anything outside the universe). I don't see this as a problem, since this is true in general relativity (or for that matter, Newtonian mechanics), or at least the obvious way to look at the theory (block universe, spacetime manifold, and so on).

2) It requires backward time travel (obviously), and there is logical flaw with traveling to any time before the time you start traveling.

Namely?

[Mentat]

Originally posted by Ontoplankton
What comes out of a time machine depends on events that will happen in the future. In that sense, the future already has to be "fixed" (though not by anything outside the universe). I don't see this as a problem, since this is true in general relativity (or for that matter, Newtonian mechanics), or at least the obvious way to look at the theory (block universe, spacetime manifold, and so on).


Yes, my problem with the predestination thing is mainly just personal preference.


Namely?

In my thread, "Why we can't 'go back'", I used the following circumstance to explain what I believe is a logical flaw in the concept of backward time travel (please note: in my analogy, the minute is the smallest increment of time. This is for the purpose of convenience, and says nothing about the analogy's practicality in actual time) :

Let's say that the time traveller get's into his time machine at 5:00 P.M. He then pushes the button (or pulls the lever, or however he starts the thing [6)]) at 5:01 P.M. and the time machine starts to go backward in time. Well, now, the time before 5:01 is 5:00, so this should be the first point in time that he reaches, but here is where the problem begins. At 5:00, he was not traveling - he was still "strapping himself into" the time machine. So, how is it that he is now traveling through time, at a point in time before he ever began travelling in the first place?

IOW, he would have to be traveling before he ever starts traveling. Not really logical is it?

[radagast]

Mentat,
You are assuming that the time traveler is staying in our 3D plane. If our 'experienced' universe exists as a 3D hyperplane within four spatial dimensions, then moving our time traveler along that 4'th dimension axis, before traveler start backwards on the time axis, then back (into our 3D hyperplane) at the termination of the journey will circumvent your logical problems.

This does assume a 4th spatial dimension. When making a logical argument, it's good for you to make your assumptions explicit.

[selfAdjoint]

Likewise if a traveler procedes FTL from his base to a remote station moving away from the base at high sublight velocity, comes to rest at that station, and then returns FTL to his base, he comes back to a date in his original time line earlier than his departure, and depending on the distances and speeds, maybe quite a bit earlier. But he does not need to traverse his original timeline backwards to do so.

[radagast]

SelfAdjoint,
I'm not that familiar with the implications of Einsteins relativity equations. That said, the last time I played with them it seemed you got a negative it (along an imaginary time axis), not just a negative t. If this is the case, doesn't it imply that FTL travel forces a negative delta on a time axis that's orthoganol to our normal time axis?

[Mentat]

Originally posted by radagast
Mentat,
You are assuming that the time traveler is staying in our 3D plane. If our 'experienced' universe exists as a 3D hyperplane within four spatial dimensions, then moving our time traveler along that 4'th dimension axis, before traveler start backwards on the time axis, then back (into our 3D hyperplane) at the termination of the journey will circumvent your logical problems.


Not at all. No spacial dimension is free of time. The two-dimensional creature is as much "under the laws of time" as the ten-dimensional one.

[Mentat]

Originally posted by selfAdjoint
Likewise if a traveler procedes FTL from his base to a remote station moving away from the base at high sublight velocity, comes to rest at that station, and then returns FTL to his base, he comes back to a date in his original time line earlier than his departure, and depending on the distances and speeds, maybe quite a bit earlier. But he does not need to traverse his original timeline backwards to do so.

But this is not logical. If a person arrives at a time that is before the time that he left, then he is not arriving at all, since he's never left...[g)].

Help, please.

[Ontoplankton]

selfAdjoint is right:

Tachyon Pistols (http://sheol.org/throopw/tachyon-pistols.html)

This does lead to inconsistencies, so again this will either not be possible or there will be banana peels preventing people from traveling FTL in different reference frames.

[Mentat]

Originally posted by Ontoplankton
selfAdjoint is right:

Tachyon Pistols (http://sheol.org/throopw/tachyon-pistols.html)

This does lead to inconsistencies, so again this will either not be possible or there will be banana peels preventing people from traveling FTL in different reference frames.

The author of that illustration (about the tachyon pistols) missed one very valid point: B wouldn't just be outraged that A fired before 8 seconds were up, but also that he (B) was hit in the back before A ever fired (at least, that is how it happened in B's reference frame). At least, that's how it seems to me (please tell me if I'm wrong).

[radagast]

Originally posted by Mentat
Not at all. No spacial dimension is free of time.

If 'No spatial dimension is free of time" then that implies time isn't orthoganol to the spatial dimensional axes, therefore time isn't a dimensional axis. Since it is defined that way, I don't agree. If you imply that all spatial dimensions are not outside of time, then you are correct, but neither is time outside of any of the other spatial dimensions - that is the definition of a dimensional space. Theoretically (and this is only considering the math, not real physics) translation can occur within any of the dimensions and not any or all of the others - each dimensional axis is orthoganol to all others.

It is possible to have two time axes (theoretically), such that change along one time axis is unchanged yet continues along another. It seems that the relativity formulas imply such a dimensional axis at velocity's higher than c.

[radagast]

Originally posted by Mentat
Not at all. No spacial dimension is free of time.

Irrelevant to my argument. It was stated that "going back in time was impossible, because the time traveler would run into 'things' in the real environment" (paraphrased). I solved this by moving the traveler outside the three spatial dimensional plane containing these 'things' which would be run into. I didn't take the traveler outside of the time axis - I couldn't since he is required to travel 'backwards' along such an axis if he is to appear prior to when he left.

[radagast]

Originally posted by Mentat
But this is not logical. If a person arrives at a time that is before the time that he left, then he is not arriving at all, since he's never left...[g)].

Help, please.

Though it certainly may have causal implications, I fail to see your reason(s) for saying it's illogical. The person departed at one point in time, and arrived at a different point in time. This happens all the time, with the caveat that it is usually the case that the arrival point is after the departure point.

[selfAdjoint]

Originally posted by Mentat
The author of that illustration (about the tachyon pistols) missed one very valid point: B wouldn't just be outraged that A fired before 8 seconds were up, but also that he (B) was hit in the back before A ever fired (at least, that is how it happened in B's reference frame). At least, that's how it seems to me (please tell me if I'm wrong).

At 4 seconds in B's proper time, he is hit, turrns instantly and looks at A. What he sees is A at A's 2 seconds, according to the Lorentz transform. A is there pointing the gun at him because in A's frame, actually 8 seconds have passed. A's action is time shifted in B's frame. And this is real, not an illusion.

But the idea you have is a good one. By a little rearrangemnt and the kind offices of a friend, B can feel himself shot by A before the duel ever starts.

[Mentat]

Originally posted by radagast
If 'No spatial dimension is free of time" then that implies time isn't orthoganol to the spatial dimensional axes, therefore time isn't a dimensional axis.


Not at all, in fact it implies quite the opposite. If there were a spacial dimension that was free of time then it would not be orthogonal. That's why I disagreed that one could leave the 3 spacial dimensions that we are used to and thus travel back in time, since there is no spacial dimension that is free of time.


Since it is defined that way, I don't agree. If you imply that all spatial dimensions are not outside of time, then you are correct, but neither is time outside of any of the other spatial dimensions - that is the definition of a dimensional space.


But that's what I said.

[Mentat]

Originally posted by radagast
Irrelevant to my argument. It was stated that "going back in time was impossible, because the time traveler would run into 'things' in the real environment" (paraphrased). I solved this by moving the traveler outside the three spatial dimensional plane containing these 'things' which would be run into. I didn't take the traveler outside of the time axis - I couldn't since he is required to travel 'backwards' along such an axis if he is to appear prior to when he left.

Ah, I see. I guess I hadn't sen the objection that "he'd run into things", though I disagree that this would happen, even in our own dimensions (since nothing that used to exist in the past still exists now, it's the past, everything has "moved on").

[Mentat]

Originally posted by radagast
Though it certainly may have causal implications, I fail to see your reason(s) for saying it's illogical. The person departed at one point in time, and arrived at a different point in time. This happens all the time, with the caveat that it is usually the case that the arrival point is after the departure point.

Then apparently you haven't tried to conceptualize it. Let me help you out (again, in my illustrations, the minute is the smallest incrimant of time).

If I tell someone, at 4:30, that I plan to leave for Detroit, Michigan at 5:00, and then call them at 4:31 to tell them that I have arrived (even as I stand right beside them still, since it is not yet 5:00, and so I will not yet depart), a great infraction of logic appears to have occured, doesn't it?

[radagast]

Originally posted by Mentat
Not at all, in fact it implies quite the opposite. If there were a spacial dimension that was free of time then it would not be orthogonal. That's why I disagreed that one could leave the 3 spacial dimensions that we are used to and thus travel back in time, since there is no spacial dimension that is free of time.

You are mis-stating my argument. I didn't say anything about leaving 3 dimensions, only leaving (our) three dimensional hyperplane. In a five dimensional space (four spatial and one time) our three dimensional universe (that we percieve) would be a three dimensional hyperplane, which could be left, just as a two dimensional plane can be left via motion along the (perpendicular) third dimension.

But that's what I said

That is not how I interpreted your statement.

As I reiterate, I was responding, with a plausible scenerio, regarding colliding with objects in our three dimensional hyperplane when attempting travel back along the time axis.

[radagast]

Originally posted by Mentat
Then apparently you haven't tried to conceptualize it. Let me help you out (again, in my illustrations, the minute is the smallest incrimant of time).

I have been considering the implications of time travel for forty years. I will override my initial impression and assume you are not attempting to be condescending. I would kindly ask you to avoid the appearance of such condescension in the future.

A lot of very intelligent writers and scientists have considered time travel and have not come to the same conclusions you have. Asimov, Cramer, and Hawkings are few that come to mind. It's funny that I haven't seen any of them mentioning the illogical aspects that you have. Causality and paradox problems, yes, logical contradictions in the travel itself, no. Considering they have not come to the conclusions you have, perhaps some respect for this side of the argument is warranted, at least in any apparent emotional content.


If I tell someone, at 4:30, that I plan to leave for Detroit, Michigan at 5:00, and then call them at 4:31 to tell them that I have arrived (even as I stand right beside them still, since it is not yet 5:00, and so I will not yet depart), a great infraction of logic appears to have occured, doesn't it?


Your example may seem a violation of common sense, but isn't a violation of logic.

You seem to imply that having two of you in existence, at the same point in time is illogical. It isn't when you consider that the two of you in existense would not be at the same point in three space, nor the same point in experiential time - the time-traveler would be older.

This may be outside our normal experience, true, but not illogical. You have failed to show even one violation of logic in time travel, except thru unspoken assumptions.

Obviously, the time traveler's experiential time doesn't follow the standard time axis, otherwise time travel becomes meaningless and the universe can be seen as simply a four dimensional object (3D spatial+ 1D time), with our experience being the only thing giving the arrow of time it's direction. True time travel seems to imply a second time axis.

If you wish to show that time travel is illogical, then giving common sense examples will most likely fail you, given time travel is outside common experience, and not within typical 'common sense'.

[Mentat]

Originally posted by radagast
You are mis-stating my argument. I didn't say anything about leaving 3 dimensions, only leaving (our) three dimensional hyperplane. In a five dimensional space (four spatial and one time) our three dimensional universe (that we percieve) would be a three dimensional hyperplane, which could be left, just as a two dimensional plane can be left via motion along the (perpendicular) third dimension.


Oh, so do you mean that we could leave the hyperplane of spacetime itself? If so, I disagree since, as we've already discussed, all of the dimensions of space are directly connected to time.


As I reiterate, I was responding, with a plausible scenerio, regarding colliding with objects in our three dimensional hyperplane when attempting travel back along the time axis.

Yes, and I'm sorry that I hadn't realized that that was what you were responding to earlier. However, I don't think we can "run into" anything when traveling backward in time anyway, since there is nothing there. As Deslaar (I'm pretty sure it was Deslaar) used to point out repeatedly, there is no past. That which used to be the present is the past, and that which will be the present is the future, but the future doesn't exist yet and the past doesn't exist anymore.

[Mentat]

Originally posted by radagast
I have been considering the implications of time travel for forty years. I will override my initial impression and assume you are not attempting to be condescending. I would kindly ask you to avoid the appearance of such condescension in the future.


My apologies. I certainly did not mean to sound condescending, and am sorry that I did. I will try to avoid that more carefully in the future.


A lot of very intelligent writers and scientists have considered time travel and have not come to the same conclusions you have. Asimov, Cramer, and Hawkings are few that come to mind. It's funny that I haven't seen any of them mentioning the illogical aspects that you have. Causality and paradox problems, yes, logical contradictions in the travel itself, no. Considering they have not come to the conclusions you have, perhaps some respect for this side of the argument is warranted, at least in any apparent emotional content.


Of course, though I happen to know that, on many occasions, the experts seem to ignore the more important problems.

You see, it still hasn't been explained to me how someone can travel to a time that no longer exists. Nor has it been explained to me how a person can be traveling if they have yet to "take off".


Your example may seem a violation of common sense, but isn't a violation of logic.

You seem to imply that having two of you in existence, at the same point in time is illogical.


Not at all, merely that the one of me that chose to travel has yet to do so. So how is it that one can arrive if they have yet to travel? Deductive logic seems to (seems to, mind you) rule that this cannot be the case. The very term "arrive" implies having traveled.


It isn't when you consider that the two of you in existense would not be at the same point in three space, nor the same point in experiential time - the time-traveler would be older.

This may be outside our normal experience, true, but not illogical. You have failed to show even one violation of logic in time travel, except thru unspoken assumptions.

Obviously, the time traveler's experiential time doesn't follow the standard time axis, otherwise time travel becomes meaningless and the universe can be seen as simply a four dimensional object (3D spatial+ 1D time), with our experience being the only thing giving the arrow of time it's direction. True time travel seems to imply a second time axis.


But our experience is not what gives "the arrow of time" it's direction. We are traveling forward in time because of the Laws of physics, and a being that is capable of "experiencing" these things didn't evolve (or, alternately: wasn't created) until very recently (in geological terms).

[radagast]

Originally posted by Mentat
Oh, so do you mean that we could leave the hyperplane of spacetime itself? If so, I disagree since, as we've already discussed, all of the dimensions of space are directly connected to time.

No, No, No. There is no "THE" hyperplane. If there is a 4 spatial dimensions, then a 3D hyperplane would be one of many. You leave our 3D hyperplane along a dimensional axis perpendicular to height, width, length, time, into a parrallel and unoccupied hyperplane. Move along the time axis, not running into anything physical (because it's empty), then retrace your translation along the fourth spatial dimensional axis back into our 3D hyperplane.



Yes, and I'm sorry that I hadn't realized that that was what you were responding to earlier. However, I don't think we can "run into" anything when traveling backward in time anyway, since there is nothing there. As Deslaar (I'm pretty sure it was Deslaar) used to point out repeatedly, there is no past. That which used to be the present is the past, and that which will be the present is the future, but the future doesn't exist yet and the past doesn't exist anymore.

If there is no past, then there can be no time travel - again you have used an axiomatic assumption to help argue toward the same point your arguing to.

If there is no past, then time is not a dimensional axis. If time is a dimensional axis, then it has points along that axis prior to "now".

[radagast]

My apologies. I intended to post this yesterday, but the site wasn't very cooperative.

Originally posted by Mentat
You see, it still hasn't been explained to me how someone can travel to a time that no longer exists.

You are assuming that time and matter in that 4D (3spatial, 1Time) space do not exists, except at our point in time. This is an assumption that cannot, currently, be proven or disproven. It also seems to dismiss time as a dimension, since if I were at coordinates x, y, z, t where t is at some point in the past, then traveling back to that point in space time, I would have to be at that point.


Nor has it been explained to me how a person can be traveling if they have yet to "take off".

Well, assuming that the past does exists in a 5D (4spatial1Time) coordinate system, taking off would start with moving (thru some unknown mechanism), perpendicular to our current 4 dimensions, into a different hyperplane, then moving (thru some unknown mechnism), back in time.

Not at all, merely that the one of me that chose to travel has yet to do so. So how is it that one can arrive if they have yet to travel? Deductive logic seems to (seems to, mind you) rule that this cannot be the case. The very term "arrive" implies having traveled.

You are assuming that the traveler is 'regressing' in time. This couldn't happen, if time travel is assumed to be possible. The time traveler exists at four dimensional coordinates (x,y,z,t) on take off, and is transported to (x,y,z,t') such that t' is prior to t, and assuming the traveler doesn't traverse the x,y,z points between t and t'. The traveler at t-1 (just prior to take off) is not the traveler at t - they both have to exist in our x,y,z,t coordinate system if time travel is to be possible.


But our experience is not what gives "the arrow of time" it's direction. We are traveling forward in time because of the Laws of physics, and a being that is capable of "experiencing" these things didn't evolve (or, alternately: wasn't created) until very recently (in geological terms).

There is no way to know this. The universe may exist as a four dimensional solid (3S, 1T), where space, past, and future all exist, and only our experience is traveling along the "arrow of time", much as a tape players read head traverses the changes in magnetic field as the tape is passed over it.

There is no way to know if this is true, or if we live in a three dimensional spatial space, constantly moving along the time axis where the future is unwritten. The laws of physics would appear the same either

[Mentat]

Originally posted by radagast
No, No, No. There is no "THE" hyperplane. If there is a 4 spatial dimensions, then a 3D hyperplane would be one of many. You leave our 3D hyperplane along a dimensional axis perpendicular to height, width, length, time, into a parrallel and unoccupied hyperplane. Move along the time axis, not running into anything physical (because it's empty), then retrace your translation along the fourth spatial dimensional axis back into our 3D hyperplane.


Ok. One more question though, if the t dimension is (as you say) "empty", then what is there to travel to (except empty space)?


If there is no past, then there can be no time travel - again you have used an axiomatic assumption to help argue toward the same point your arguing to.


Well, I didn't mean for it to be axiomatic, it is an observation based on the very semantics of the issue. The very word, "past", means that which used to exist. If it is that which used to exist, then it doesn't exist anymore.


If there is no past, then time is not a dimensional axis. If time is a dimensional axis, then it has points along that axis prior to "now".

Of course it has points prior to the "now", but that doesn't mean that anything is occuring on those points. A useful analogy (that isn't completely consisten with science and was given to me by a layman, but that I still like) is that the t dimension is like a copper wire, and all "current" occurances, are like a spark that is traveling along this wire. So, only that which exists where the "spark" is "now" exists, there is nothing else, except empty points in space.

[Mentat]

Originally posted by radagast
You are assuming that time and matter in that 4D (3spatial, 1Time) space do not exists, except at our point in time. This is an assumption that cannot, currently, be proven or disproven.


Why not? It may not be scientifically verifiable, but it seems that it must logically be so. After all, if something is in the "past" then it has already happened. All of that which "is happening now" is happening in the present. Thus, nothing is happening in the past.

Where is the flaw?


It also seems to dismiss time as a dimension, since if I were at coordinates x, y, z, t where t is at some point in the past, then traveling back to that point in space time, I would have to be at that point.


Yes, you would certainly be at that point, but nothing else would be since reality did not travel backward with you (did it?).


You are assuming that the traveler is 'regressing' in time. This couldn't happen, if time travel is assumed to be possible. The time traveler exists at four dimensional coordinates (x,y,z,t) on take off, and is transported to (x,y,z,t') such that t' is prior to t, and assuming the traveler doesn't traverse the x,y,z points between t and t'. The traveler at t-1 (just prior to take off) is not the traveler at t - they both have to exist in our x,y,z,t coordinate system if time travel is to be possible.


I can understand that. However, if I leave our hyperplane altogether (btw, does this involve going into a different dimension of time or just a different set of spacial dimensions?) and then re-enter at a different point in time, then there shouldn't be anything else there (except that which I took with me in my travel, should there?

[Spacestar]

You know, many persons think that travelling trough time involve paradoxes. . Their arguments normally are of this kind: "If you did have a time machine right now, and you could step into it and travel back to some earlier time. Your actions in that time might then prevent yourself from ever growing up and becoming a time traveller, and thus not step into the time machine. So, the claim that there could be a time machine is self-contradictory". People, this argument is fallacious. Does the very concept of time travel entail contradictions? Does the possibility of murdering yourself as a child show that time travel is an impossibility?
The answer is: there is NO POSSIBILITY, if you travel into the past, of murdering yourself as a child. The very fact that you are here now logically guarantees that no one - neither you nor anyone else - murdered you as a child, for there is no possibility of changing the past.
This notion that you can’t change the past needs careful attention. There is nothing special about the past in this particular regard. For you can no more change the past than you can change the present or change the future. And yet this is not fatalism or determinism. I am not saying that our actions are pointless.
I cannot change the future - by anything I have done, am doing, or will do - from what it is going to be. But I can change the future from what it might have been. I may carefully consider the appearance of my garden, and after a bit of thought, mulling over a few alternatives, I decide to cut down the apple tree. By so doing, I change the future from what it might have been. But I do not change it from what it will be. Indeed, by my doing what I do, I - in small measure - contribute to making the future the very way it will be. Similarly, I cannot change the present from the way it is. But I can change the present from the way it might have been, from the way it would have been were I not doing what I am doing right now. And finally, I cannot change the past from the way it was. In the past, I changed it from what it might have been, from what it would have been had I not done what I did. We can change the world from what it might have been; but in doing that we contribute to making the world the way it was, is, and will be.
If one travels into the past, then one doesn’t change the past; one does in the past only what in fact happened. If you are alive today, having grown up in the preceding years, then you were not murdered. If, then, you or anyone else travels into the past, then that time traveller simply does not murder you. What does that time traveller do in the past? From our perspective, looking backward in time that traveller does whatever in fact happened, and that - since you are alive today - does not include murdering you.
Time travel involves no intrinsic contradiction. The appearance of contradiction arises only if one illicitly hypothesizes that the time traveller can change the past from what it was. But that sort of contradiction has nothing whatever to do with time travel per se. One would encounter the same sort of contradiction if one were to hypothesize that someone now were to change the present from the way it is or someone in the future were to change the future from the way it will be.
Nothing true is made false; there is no logical contradiction.

And for a better comprehension on relativity and hyperframes read this:

http://www.ximensions.uklinux.net/articles/rpt1.htm

[Mentat]

Welcome to the PFs, Spacestar! [:)]

Great post, and I agree with you, but I have one question: Since we can't change anything in the past, then we must also have been predestined to travel into the past, since we arrived in the past in the past, right?

IOW, if a person travels into the past then they were predestined to do so, since they already appeared in the past, and thus not only the past is set but also some (perhaps all) of the future.

Also, I see how your reasoning disqualifies the "Grandfather paradox" and all such so-called paradoxes, but it doesn't really effect the paradox that I've been talking about (which is that, if you travel to a moment before the time you start travelling, then you haven't started travelling yet, so how did you get to where you are?). Can you see some way how it could be applied to my paradox? Or are they unrelated?

[radagast]

Originally posted by Mentat
Ok. One more question though, if the t dimension is (as you say) "empty", then what is there to travel to (except empty space)?

The t dimension was the time axis. The forth spatial dimension isn't a place, it's a dimension. If we exists in 4 spatial dimensions, then we are in all. What I said was the you travel along the forth spatial dimension to a parrallel 3D hyperplane (where nothing exists, or at least is highly sparse). Just because there is nothing there, doesn't mean you can't go there. There's nothing in Phenix City, Alabama either, but I've been there several times. What you travel to is the space, not a thing.


Well, I didn't mean for it to be axiomatic, it is an observation based on the very semantics of the issue. The very word, "past", means that which used to exist. If it is that which used to exist, then it doesn't exist anymore. And if that's a given, then time travel would be impossible.


Of course it has points prior to the "now", but that doesn't mean that anything is occuring on those points. A useful analogy (that isn't completely consisten with science and was given to me by a layman, but that I still like) is that the t dimension is like a copper wire, and all "current" occurances, are like a spark that is traveling along this wire. So, only that which exists where the "spark" is "now" exists, there is nothing else, except empty points in space.

I certainly don't want to appear insulting, but you don't seem to grasp the concept of time as a dimension. If an event occurred at point x,y,z, t then traveling to that point in time and space (well close to that in space), would bring you to all aspects that happened. To traverse backwards in time, the 'now' has to change backwards, just as movement in space changes the 'here'. Without this, then time cannot be a dimensional axis.

[radagast]

Originally posted by Mentat
Why not? It may not be scientifically verifiable, but it seems that it must logically be so. After all, if something is in the "past" then it has already happened. All of that which "is happening now" is happening in the present. Thus, nothing is happening in the past.

Where is the flaw?

You are not treating time as a dimensional axis. If there is only the 'now' then there can be no other points on the time axis, hence, no axis at all.

Yes, you would certainly be at that point, but nothing else would be since reality did not travel backward with you (did it?).

For time to be a dimensional axis, then everything at that point along the time axis would be just exactly as it had been. Again, you don't seem to grasp the concept of time as an axis. If nothing were there, then nothing would have ever been there at that point in time and space.

If I draw half a parabola, one axis representing distance along a spatial axis (say height), and one representing time. When I draw the line, I represent where something is at that point in time. To go to that point in time means that 'that something' was there. That's what time time axis and graph implies.


I can understand that. However, if I leave our hyperplane altogether (btw, does this involve going into a different dimension of time or just a different set of spacial dimensions?) and then re-enter at a different point in time, then there shouldn't be anything else there (except that which I took with me in my travel, should there?

One extra spatial dimension. I don't think it requires a second time axis, but that doesn't imply
our personal time is decoupled from the time axis - an implied requirement of time travel.

The way to visualize it is to assume we are two dimensional creatures living in an infinitely thin sheet of paper, within a stack of papers. To move into a parrallel sheet of paper requires us to move perpendicular to all dimensions we are aware of, just enough to get into the next sheet of paper. It would appear as a completely new/different universe. Then traversal backwards in time means that the natural motions of things (as time flows backwards) ensures nothing will run into us (being our new plane is empty, as we specified earlier).

[radagast]

Originally posted by Spacestar
You know, many persons think that travelling trough time involve paradoxes.

Spacestar,
In the simplest view of time travel, paradoxes are a given possibility. If (time travel) paradoxes could exist in reality is something we couldn't know until we have time travel to play with. Since reality does tend to abhore paradoxes, I suspect you are right, however just assuming that a paradox cannot occur is an assumption. You cannot use this assumption to prove itself.


Their arguments normally are of this kind: "If you did have a time machine right now, and you could step into it and travel back to some earlier time. Your actions in that time might then prevent yourself from ever growing up and becoming a time traveller, and thus not step into the time machine. So, the claim that there could be a time machine is self-contradictory". People, this argument is fallacious. Does the very concept of time travel entail contradictions? Does the possibility of murdering yourself as a child show that time travel is an impossibility?
The answer is: there is NO POSSIBILITY, if you travel into the past, of murdering yourself as a child.

You seem to have discovered paradoxes and are trying to prove that it cannot occur because it is a paradox. The whole reason things are paradoxes is that they produce a logical impossibility. By this reasoning, all paradoxes have NO POSSIBILITY of occurring. This is an argument flaw. You cannot acknowledge that paradoxes are defined as logical impossibilities, then use that definition to prove them impossible. The key aspect of paradoxes involves the exacting conditions that require them. This generally involves mathematically precise declaration of conditions - under such conditions a paradox is required under the conditions specified. That doesn't mean that the paradox will translate into reality.

In truth, all paradoxes, that appear to be testable, have been shown that there is some underlying assumption that is fallacious. Zeno's paradox involved an assumption that you couldn't perform an infinite number of actions (the actions are not discrete). The apparent assumption, causing trouble in the twin paradox, revolves around the effects of acceleration.


..snip...
I cannot change the future - by anything I have done, am doing, or will do - from what it is going to be. ..snip..

The implication of the above statement is time travel isn't possible, since the time traveler just being there (in the past), is a change in what occurred. A presumption that this is not the case is required to consider time travel.

[Mentat]

Originally posted by radagast
And if that's a given, then time travel would be impossible.


Then time travel's impossible (which is what I've been saying anyway), unless you can see a way out of such a semantic problem


I certainly don't want to appear insulting, but you don't seem to grasp the concept of time as a dimension. If an event occurred at point x,y,z, t then traveling to that point in time and space (well close to that in space), would bring you to all aspects that happened.


With all due respect, I think you have failed to grasp the concept; you see, you are referring to traveling along all four dimensions to a point on all four dimensions. Now, try to put this into practice. You see, if I were to travel from the seat that I am sitting in to the desk across the room, I will have gone to a different point on spacetime, but will have been going "forward" in time the entire time (inspite of my movement in space). So, I can travel to any point on spacetime freely, but there are three dimensions (possibly more) of space, and only one of time. What I'm saying is that my movement is constantly "forward" in time (along with everything else in the known Universe), but my movement is free in the spacial dimensions. The only known way to travel in any other direction on the one time axis is to travel faster than c in space.

Now, the reason I mention this is because you speak of things that "exist at certain points of the four dimensions". However, according to the very semantics of the issue, nothing exists at previous points on the t dimension. All of space and matter and energy has been moving "forward" in time along with me, and I would thus have nothing to "go back to".


To traverse backwards in time, the 'now' has to change backwards, just as movement in space changes the 'here'. Without this, then time cannot be a dimensional axis.

Sure, this is possible in principle, but (again) in practice it is quite illogical, since I have three (or more) spacial dimensions which allow me to "turn around" and "go back". Alas, there is only one dimension of time, and thus no way to "turn around", so the only way to travel backward is to leave the rest of reality and regress in time (which regression you yourself have said was impossible).

[Mentat]

Originally posted by radagast
You are not treating time as a dimensional axis. If there is only the 'now' then there can be no other points on the time axis, hence, no axis at all.


But there are other points on the axis, there's just nothing at these points.


For time to be a dimensional axis, then everything at that point along the time axis would be just exactly as it had been. Again, you don't seem to grasp the concept of time as an axis. If nothing were there, then nothing would have ever been there at that point in time and space.


Not true. Remember the analogy of the spark on the wire. If the spark continues to travel along the wire, there are points that it "has traversed but is no longer traversing" and points that it "has yet to traverse". If all of space and matter and energy are the "spark" in the analogy, then there is nothing at any of the other points on the time axis.


If I draw half a parabola, one axis representing distance along a spatial axis (say height), and one representing time. When I draw the line, I represent where something is at that point in time. To go to that point in time means that 'that something' was there. That's what time time axis and graph implies.


Yes, something WAS there, but it is not anymore. Think of the analogy you are using further: If you draw a parabola then all points on the line are manifest at the same time. However, this is not so with time, because of the very semantics of the issue. How can all points of time exist "at the same time"?


One extra spatial dimension. I don't think it requires a second time axis, but that doesn't imply
our personal time is decoupled from the time axis - an implied requirement of time travel.

The way to visualize it is to assume we are two dimensional creatures living in an infinitely thin sheet of paper, within a stack of papers. To move into a parrallel sheet of paper requires us to move perpendicular to all dimensions we are aware of, just enough to get into the next sheet of paper. It would appear as a completely new/different universe. Then traversal backwards in time means that the natural motions of things (as time flows backwards) ensures nothing will run into us (being our new plane is empty, as we specified earlier).

This is a very good analogy, and it only serves to prove my point further: in the analogy, all of reality (which is two-dimensional, in the analogy) moves into different sheets of paper on a constant basis. Thus, if some crazy 2D being decided he wanted to travel backward in time (and somehow accomplished it), he would go to a vacant piece of paper.

[radagast]

Originally posted by Mentat
Yes, something WAS there, but it is not anymore. Think of the analogy you are using further: If you draw a parabola then all points on the line are manifest at the same time. However, this is not so with time, because of the very semantics of the issue. How can all points of time exist "at the same time"?


Of course something WAS there AT THAT POINT IN TIME, the same point in time your traveler wants to visit. If he goes back to that point in time and nothing is at x,y,z,t then it was never there!!!! That's what time travel is - going back to where and when things were.

AGAIN, you are assuming time travel doesn't or cannot exist, implicitly in your discourse.

Again, think about the parabola describing an object's one dimensional motion - if the parabola didn't cross the point x,t then the object was never at x at time t. Let me put it differently. Just because the object isn't there now, if we went back and time and it wasn't there at time 't', then there would be a gap in the graph - the graph shows where the object was (in x) at that point in time. To go back in time means the object had to be there or there could be no continuity of the graph at that point in time.

Mentat, I am in no way trying to insult you, but what approx level of education have you had, with respect to math and physics? Perhaps I'm just taking a mental block on your part for a lack of grounding in the areas I'm talking about.

If you have the basic function f(t)=x, then when you change the time t, backwards, you get where it was at x - if you go back to time t, it has to be at x, otherwise - by definition, you haven't traveled back to time t.

The same could be said of function f(t)=x,y,z.

The reason you cannot visualize objects existing thru a duration of time is it put's the world squarely into 4 dimensions, and we have a hard time thinking in four dimensions.

[radagast]

Originally posted by Mentat
Then time travel's impossible (which is what I've been saying anyway), unless you can see a way out of such a semantic problem


If you believe time travel is impossible, fine. But, you cannot take time travel as impossible as a given, then try and use things you derive from that as proof that time travel is impossible. It's a circular logic flaw.

[radagast]

quote:
--------------------------------------------------------------------------------
Originally posted by radagast
You are not treating time as a dimensional axis. If there is only the 'now' then there can be no other points on the time axis, hence, no axis at all.

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


But there are other points on the axis, there's just nothing at these points.


Then all graphs that have time as an axis would have no points except at the present.

If the only active point on a time axis (ever) is the present, that can be used to locate an object, then it is not an axis.

Dimensional axes enable us to locate things in space (or space-time). If you are saying that going back to a point, in space-time, where something existed, and it isn't there, then you haven't traveled in time to that point in space. That would be what time travel is.

[radagast]

Originally posted by Mentat
--------------------------------------------------------------------------------

For time to be a dimensional axis, then everything at that point along the time axis would be just exactly as it had been. Again, you don't seem to grasp the concept of time as an axis. If nothing were there, then nothing would have ever been there at that point in time and space.

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


Not true. Remember the analogy of the spark on the wire. If the spark continues to travel along the wire, there are points that it "has traversed but is no longer traversing" and points that it "has yet to traverse". If all of space and matter and energy are the "spark" in the analogy, then there is nothing at any of the other points on the time axis.

Extended analogy flaw. One) sparks don't travel down wires, electrons and eletromotive force do, but I digress. Two) You are simply defining motion. Change in position with respect to time - yet you still fail to see that reversing the time, reverses the motion (AS defined by the X,Y,Z,T coordinate system we refer to as space-time). Disregarding IF time travel is possible, positing it as possible, forces things to return to all those same positions as the position in time reverses. Otherwise you're saying the past is an illusion that never really existed. To travel back in time means you're in the past only from the perspective of the present. To the traverler, he would experience it all as his current now.


This is a very good analogy, and it only serves to prove my point further: in the analogy, all of reality (which is two-dimensional, in the analogy) moves into different sheets of paper on a constant basis. Thus, if some crazy 2D being decided he wanted to travel backward in time (and somehow accomplished it), he would go to a vacant piece of paper.

He only time-travels within the parallel (Y) plane. The only purpose for the two dimensional creature to shift into the parallel plane, was to avoid colliding with himself as he attempts to travel backwards in time. Since he doesn't exist in the parallel plane at the same place he starts, he cannot spatially collide with himself (occupy the same sets of coordinates in X,Y,T), if he shifts to a different position along Y.

[radagast]

quote: Mentat
The very word, "past", means that which used to exist. If it is that which used to exist, then it doesn't exist anymore.


But using the phrase 'used to exist' is using a subtle time reference that's of dubious meaning when refering to existence in time. A more dimensional definition would be the word past refers to things at a prior point in time. They don't exist at this point in time like they did at a prior point in time, but that doesn't mean they don't exist in the four dimensional manifold of space time, at that point in time.

[radagast]

Originally posted by Mentat
With all due respect, I think you have failed to grasp the concept; you see, you are referring to traveling along all four dimensions to a point on all four dimensions. Now, try to put this into practice. You see, if I were to travel from the seat that I am sitting in to the desk across the room, I will have gone to a different point on spacetime, but will have been going "forward" in time the entire time (inspite of my movement in space). So, I can travel to any point on spacetime freely, but there are three dimensions (possibly more) of space, and only one of time. What I'm saying is that my movement is constantly "forward" in time (along with everything else in the known Universe), but my movement is free in the spacial dimensions. The only known way to travel in any other direction on the one time axis is to travel faster than c in space.

You are only bringing up that we have no current mechanism for traveling back in time. No argument there - but that wasn't the initial discussion. The initial discussion had to do with the logic of traveling back in time. The above just concerns never having experienced it and that we don't know how to.


Now, the reason I mention this is because you speak of things that "exist at certain points of the four dimensions". However, according to the very semantics of the issue, nothing exists at previous points on the t dimension. Prove it.

My point being is this is an unsupported statement that violates all current concepts of dimensional usage.

If we say something existed at x,y,z two days ago, then by traveling back to two days ago, existed changes to exists for the traveler. This is basic coordinate geometry - just because there have been special behaviours observed for a coordinate axis doesn't mean we cease to treat all the points, but one, as non-existent.

All of space and matter and energy has been moving "forward" in time along with me, and I would thus have nothing to "go back to". Again, prove it. This is another unsupported statement. Traveling back in time implies to everything at that point in time. If I am at x,y,z at 10 am then move ten feet along the x axis at 11 am, time travel dictates that I existed at 10am at x,y,z - so to travel back in time, so that I could observe point x,y,z requires that my previous self is at x,y,z at 10 AM. That's what the coordinate system using time as a coordinate requires.


Sure, this is possible in principle, but (again) in practice it is quite illogical, since I have three (or more) spacial dimensions which allow me to "turn around" and "go back".

Logic is not the issue - you are bring up an issue of how, i.e. what mechanism. That is outside the discussion since we have said this was a discussion of being illogical. You are failing to show that going back in time is illogical. Please stick to standard informal logical arguments and demostrate this is so.


[quote][b] Alas, there is only one dimension of time, and thus no way to "turn around", so the only way to travel backward is to leave the rest of reality and regress in time (which regression you yourself have said was impossible).

Again, a how, not an illogical impossibility. Saying there is no way to turn around either has to be shown as impossible, or is simply that we currently have no such mechanism. Theoretically we have the mechanism of jumping to faster than c, but that's still a 'how'. Perhaps it's a practical impossibility if no how is determined, but not an error in the logic of what is currently presented.

[Spacestar]

Thank you, Mentat, I’m glad to be here.

I think “predestined” is not precisely the term to be used here, since it implies that events are set on stone, or that we cannot alter time events because they are already determined. But as I said before, it is exactly our deliberations and actions that determine the way things are, were or will be. I’m not arguing that whatever we do is futile.

For example: suppose a visitor were to arrive here and now from the year 2045. He shakes my hand, and then sits and chats with me about what is in store during the next years. I take notes and record them in my diary. A year from now, I even publish some of these notes. The visitor from the future (year 2045) has not changed the past (i.e. the past relative to the year 2045): he has contributed to making the past just the way it was. By traveling back to the year 2003, he caused certain events to occur in 2003 and in 2004. Nothing was changed from the way it was; but the past was changed from the way it would have been if he had not traveled back from 2045 to 2003.

…it doesn't really effect the paradox that I've been talking about (which is that, if you travel to a moment before the time you start travelling, then you haven't started travelling yet, so how did you get to where you are?). Can you see some way how it could be applied to my paradox? Or are they unrelated?
I see that what you’re talking about is not the same kind of time travel that most of us here are referring to. You’re referring to time travel from your own three dimensional perspective, that is, to time travel in a manner that you would revive past experiences in a first person basis, but that sort of time travel is meaningless. Since your mind and rest of the body, would also be pushed backward or forward in time, therefore you wouldn’t remember of whether you time traveled or not. Actually, from that perspective you haven’t time traveled at all (no pun intended).

In the simplest view of time travel, paradoxes are a given possibility. If (time travel) paradoxes could exist in reality is something we couldn't know until we have time travel to play with. Since reality does tend to abhore paradoxes, I suspect you are right, however just assuming that a paradox cannot occur is an assumption. You cannot use this assumption to prove itself.
Well, at least it is theoretically right until confirmed by hard evidence.

You seem to have discovered paradoxes and are trying to prove that it cannot occur because it is a paradox. The whole reason things are paradoxes is that they produce a logical impossibility. By this reasoning, all paradoxes have NO POSSIBILITY of occurring. This is an argument flaw. You cannot acknowledge that paradoxes are defined as logical impossibilities, then use that definition to prove them impossible. The key aspect of paradoxes involves the exacting conditions that require them. This generally involves mathematically precise declaration of conditions - under such conditions a paradox is required under the conditions specified. That doesn't mean that the paradox will translate into reality.
Does it seem that way? Believe me, I didn’t take my conclusions out of such a vague reasoning. In fact, the reason why I consider the grandfather paradox as illogical is because the argument explicitly commits a modal fallacy. Let me explain:
Since it is possible that someone should have prevented your grandparents from having met one another, and since it is impossible for you to travel into the past and to have prevented your grandparents from having met one another, you conclude that it is thus impossible to travel into the past. Let "P" stand for "preventing your grandparents from meeting" and "T" stand for "travel into the past" (patched up as needed to be proper statements). Then the argument is:

#P
~#(T & P)
---------------------
therefore ~#T

The argument is invalid. From the conjoint impossibility of T and P, and the possibility of P, the impossibility of T does not follow. (Just to drive the point home, now let "T" stand for "the coffee table is four-sided" and let "P" stand for "the coffee table is six-sided".)

Arguments like the “grandfather paradox” have been persisting for ages, they are especially tricky because they involve what are called modal concepts, in particular the notions of possibility and impossibility.

Of course a time machine which allowed one to change the past is logically impossible, but making that the (unrealizable) goal is to trivialize the problem, to pose it in a question-begging manner, to stack the deck. Even if one does stack the deck in this way, there still remains the question: Is it logically possible to build a time machine which allows travel into the past where one does not change the past? And the answer to this latter, nontrivial, version of the question is: yes.

What is logically impossible is that BOTH one travels into the past AND changes the past (from what it was). But so long as one does not change the past, there is no logical contradiction in positing travel into the past.

The implication of the above statement is time travel isn't possible, since the time traveler just being there (in the past), is a change in what occurred. A presumption that this is not the case is required to consider time travel.
Allow me to say that your quoting of my statement is incomplete, because I also said that although we cannot change the past, we can change the past from the way it might have been. When I argue for the logical possibility of travel into the past, I do not mean as a disembodied watcher of past events. I mean as a 'real' participant in the activity of the past.

[radagast]

Originally posted by Spacestar
Does it seem that way? Believe me, I didn’t take my conclusions out of such a vague reasoning. In fact, the reason why I consider the grandfather paradox as illogical is because the argument explicitly commits a modal fallacy. Let me explain:
Since it is possible that someone should have prevented your grandparents from having met one another, and since it is impossible for you to travel into the past and to have prevented your grandparents from having met one another, you conclude that it is thus impossible to travel into the past.

Excuse me? I have never argued that time travel was impossible because of paradoxes. In fact I've heard extremely few ever argue that. I suspect, before you start arguing what I am claiming and not claiming, you read my posts.


Let "P" stand for "preventing your grandparents from meeting" and "T" stand for "travel into the past" (patched up as needed to be proper statements). Then the argument is:

#P
~#(T & P)
---------------------
therefore ~#T

The argument is invalid. From the conjoint impossibility of T and P, and the possibility of P, the impossibility of T does not follow. (Just to drive the point home, now let "T" stand for "the coffee table is four-sided" and let "P" stand for "the coffee table is six-sided".)


Straw man argument flaw. It's a straw man, because you are putting arguments in my mouth that I've not made.

Of course a time machine which allowed one to change the past is logically impossible,

This is a statement, not an argument. You have not shown the above to be true, unless of course, you are stating that paradoxes, being logically impossible, force time travel to be logically impossible.



What is logically impossible is that BOTH one travels into the past AND changes the past (from what it was). But so long as one does not change the past, there is no logical contradiction in positing travel into the past. While Mentat may have made that statement, I have not.


Allow me to say that your quoting of my statement is incomplete, because I also said that although we cannot change the past, we can change the past from the way it might have been. When I argue for the logical possibility of travel into the past, I do not mean as a disembodied watcher of past events. I mean as a 'real' participant in the activity of the past.

Pardon me, but to say that you can travel into the past and not change the past, yet be a real participant in the past AND can change the past from the way it might have been, sounds like semantic double talk. I will assume there is just a breakdown in communication and wait for elaboration.

[Spacestar]

Originally posted by Spacestar
Does it seem that way? Believe me, I didn’t take my conclusions out of such a vague reasoning. In fact, the reason why I consider the grandfather paradox as illogical is because the argument explicitly commits a modal fallacy. Let me explain:
Since it is possible that someone should have prevented your grandparents from having met one another, and since it is impossible for you to travel into the past and to have prevented your grandparents from having met one another, you conclude that it is thus impossible to travel into the past.
Excuse me? I have never argued that time travel was impossible because of paradoxes. In fact I've heard extremely few ever argue that. I suspect, before you start arguing what I am claiming and not claiming, you read my posts.
Hold on, I think we have a misunderstanding here.

I was explaining what I think to be the general feeling on the so-called grandfather paradox. I wasn’t exactly addressing your statements at that point. I’m not necessarily saying that is your point of view. I’m sorry about the confusion.


Let "P" stand for "preventing your grandparents from meeting" and "T" stand for "travel into the past" (patched up as needed to be proper statements). Then the argument is:

#P
~#(T & P)
---------------------
therefore ~#T

The argument is invalid. From the conjoint impossibility of T and P, and the possibility of P, the impossibility of T does not follow. (Just to drive the point home, now let "T" stand for "the coffee table is four-sided" and let "P" stand for "the coffee table is six-sided".)
Straw man argument flaw. It's a straw man, because you are putting arguments in my mouth that I've not made.
Let me offer a counter:

Pretend ad hominem flaw. Instead of staying on the point that I’m trying to make, you begin to act hurt, as if I’m viciously giving a false imprint of your statements, changing the focus.

This is a statement, not an argument. You have not shown the above to be true, unless of course, you are stating that paradoxes, being logically impossible, force time travel to be logically impossible.
Yes, it is a statement, but I have explained before why I believe the statement to be true.

Pardon me, but to say that you can travel into the past and not change the past, yet be a real participant in the past AND can change the past from the way it might have been, sounds like semantic double talk. I will assume there is just a breakdown in communication and wait for elaboration.
I suggest that you take a look at the example in the beginning of my last post, which illustrates clearly my point.

[radagast]

Originally posted by Spacestar
Hold on, I think we have a misunderstanding here.

I was explaining what I think to be the general feeling on the so-called grandfather paradox. I wasn’t exactly addressing your statements at that point. I’m not necessarily saying that is your point of view. I’m sorry about the confusion.

OK, that wasn't clear. Misunderstanding acknowledged.

Just as an aside, I do feel compelled to say I've never heard someone use the grandfather paradox used to justify that time travel was logically impossible.


...Straw man argument flaw....
Let me offer a counter:

Pretend ad hominem flaw. Instead of staying on the point that I’m trying to make, you begin to act hurt, as if I’m viciously giving a false imprint of your statements, changing the focus.

Acting hurt?!? I am pretty think skinned when it comes to online activities, so I really don't think you hit the mark, there.

Let me clarify, I don't defend statements I'm not making or stand behind. I would think that was common sense. If you want to fight that fight, find someone that believes it. If you are in a debate, and attack issues I've not raised, nor back up, then there is no onus for me to defend/debate them.

Just so I'm crystal clear, I see no reason to disagree with the statement that the grandfather paradox doesn't logically invalidate time travel (assuming that was what you were saying).



Yes, it is a statement, but I have explained before why I believe the statement to be true.


I suggest that you take a look at the example in the beginning of my last post, which illustrates clearly my point.

Perhaps I'm just missing your points, but I've not seen where you made the statements that you say you have. If they were made in another thread, then I will freely admit I didn't see them. If in this thread, then I haven't recognised them for what you say they are.

[Spacestar]

Just as an aside, I do feel compelled to say I've never heard someone use the grandfather paradox used to justify that time travel was logically impossible.
But, the paradoxes (including the grandfather paradox) play an important part in the analysis of the logical/physical possibility of time travel.

It is to my knowledge that most people think of that kind of time travel that meets the demand “ Ho, I wish I could go back and make Adolf Hitler slip on a banana peel and die at the age of seven." So, that is a notion of time travel that properly translates into the modal argument, since it means a logical impossibility, and probably also a physical impossibility for the reason that it violates causation.

Let me clarify, I don't defend statements I'm not making or stand behind. I would think that was common sense. If you want to fight that fight, find someone that believes it. If you are in a debate, and attack issues I've not raised, nor back up, then there is no onus for me to defend/debate them.
Basically, I’ve joined this message board to share my ideas and premises, but I’m also here to discuss other points of view. Mainly because such conduct refreshes current theories, allowing new insights and mental connections not previously made. Thus I personally see a great importance in debating new theories; even if they are contrary to your convictions. Essentially, that is what makes science go on.

Just so I'm crystal clear, I see no reason to disagree with the statement that the grandfather paradox doesn't logically invalidate time travel (assuming that was what you were saying).
All right, so please tell me, what makes you to not disagree with the statement?

[radagast]

Originally posted by Spacestar
But, the paradoxes (including the grandfather paradox) play an important part in the analysis of the logical/physical possibility of time travel.

It is to my knowledge that most people think of that kind of time travel that meets the demand “ Ho, I wish I could go back and make Adolf Hitler slip on a banana peel and die at the age of seven." So, that is a notion of time travel that properly translates into the modal argument, since it means a logical impossibility, and probably also a physical impossibility for the reason that it violates causation.


You are correct that most view time travel as just what you say. This has been helped by the fact that virtually every SF story dealing with time travel does would, in fact, trigger a paradox.

This view of time travel assumes the most basic idea of time travel, with instant propagation of changes from an event, a single time axis, a single universe, and most importantly that other laws don't permit the exact type of paradox to occur. The twin paradox is a paradox within the constraints of the special theory of relativity. Just because it is actually a paradox doesn't invalidate all of the special theory - this is what I am understanding you to say.

Let me clarify, I don't defend statements I'm not making or stand behind. I would think that was common sense. If you want to fight that fight, find someone that believes it. If you are in a debate, and attack issues I've not raised, nor back up, then there is no onus for me to defend/debate them. Basically, I’ve joined this message board to share my ideas and premises, but I’m also here to discuss other points of view. Mainly because such conduct refreshes current theories, allowing new insights and mental connections not previously made. Thus I personally see a great importance in debating new theories; even if they are contrary to your convictions. Essentially, that is what makes science go on.

Debate is fine, however, as I stated earlier, if you are debating points I don't disagree with, it's not much of a debate.



All right, so please tell me, what makes you to not disagree with the statement?

Since, at this point, which statement is not clear, I'll review things as I've seen them stated, leaving out any side issues.

OK, to begin with you state that many people think time travel involves paradoxes.

To which I state that under the most basic ideas of time travel, they do. You have to understand that under the most basic ideas of time travel, time travel (into the past) is a given, and the ability to interact in that past is also a given.

Now, to say that a paradox invalidates this because they are logically impossible, only reiterates the definition of a paradox. Many things appear to be paradoxes that are no, in real life. Zeno's paradox. The idea that an electron could, apparently, be a particle and a wave also appeared to be a paradox.

[elas]

Surely you should differentiate between "Time Travel" and "History Travel". While it may be possible to travel back in time that does not necessarily mean you are also traveling back in history.
A body that lives for 60yrs go back 40yrs and live another 50yrs will be 50yrs old with a 110yr history. This treats time in the manner used by Dr Brownoski author of "The Ascent of Man".

[radagast]

Originally posted by elas
Surely you should differentiate between "Time Travel" and "History Travel". While it may be possible to travel back in time that does not necessarily mean you are also traveling back in history.
A body that lives for 60yrs go back 40yrs and live another 50yrs will be 50yrs old with a 110yr history. This treats time in the manner used by Dr Brownoski author of "The Ascent of Man".

Obviously, going back in time, when it is the time travelers personal time, is of little value, since they would simply regress, losing all they had gained in knowledge, experience, et. al.

Most, when referring to time travel, are speaking of traveling back to a prior point along the time axis (i.e. in history, with reference to the travelers starting point), without a corrosponding change in the travelers personal time (i.e. the traveler doesn't regress in age, memories, or personally experienced history).

I have not read Dr. Brownoski's book, but that sounds more like age regression than time travel.

[Spacestar]

The twin paradox is a paradox within the constraints of the special theory of relativity. Just because it is actually a paradox doesn't invalidate all of the special theory - this is what I am understanding you to say.
Yes, basically that’s what I’m saying. Paradoxes arise when we don’t take into account (out of unawareness or disregard) all the aspects concerning the issue. Not that they indeed occur in reality.
For instance, the twin paradox comes to mind when we consider time to be absolute, but when we comprehend the relative nature of time, the paradox doesn’t occur. Moreover, I’d like to add that such a paradox can be resolved quite simply by a careful and correct application of the principles of relativity. As a matter of fact, it is relativity that turns time travel into a physical possibility.

Surely you should differentiate between "Time Travel" and "History Travel". While it may be possible to travel back in time that does not necessarily mean you are also traveling back in history.
A body that lives for 60yrs go back 40yrs and live another 50yrs will be 50yrs old with a 110yr history. This treats time in the manner used by Dr Brownoski author of "The Ascent of Man".
I agree with Radagast, that does not fit the definition of time travel.

How you’d quantify that? You would have to be in another reference frame, that is, in a position that would suit the actual definition of time travel (as you say “history travel”).

I have to say that your argument is rhetorical.

[radagast]

Originally posted by Spacestar
Yes, basically that’s what I’m saying. Paradoxes arise when we don’t take into account (out of unawareness or disregard) all the aspects concerning the issue. Not that they indeed occur in reality.
For instance, the twin paradox comes to mind when we consider time to be absolute, but when we comprehend the relative nature of time, the paradox doesn’t occur. Moreover, I’d like to add that such a paradox can be resolved quite simply by a careful and correct application of the principles of relativity. As a matter of fact, it is relativity that turns time travel into a physical possibility.


Perhaps we were much closer to agreeing than was apparent.

That said, with specific reference to the twin paradox: I did believe the twin paradox was more based on the effects of accelleration (GR) invalidating certain aspects of special relativity. As I understand it, after one of the twins left earth (or whereever they started), each twin could say that the other was the one travelling close to the speed of light [since in their frame of reference, it could appear that the other was the traveler], so the other would see the other as younger. Then once the twins returned to the same frame of reference, each would expect the other to actually be younger. Obviously, once their frames of reference merge, this cannot be, hence the paradox. General relativity, removed the fact that one could always see the other as the one traveling at a high fraction of c, when it came down to measuring apparent age at the end of the round trip.

Perhaps I've just misinterpreted what you've said, or it's always possible, with my limited understanding of relativity, that the twin paradox is much more trivial than I'd thought.

[elas]

Obviously, going back in time, when it is the time travelers personal time, is of little value, since they would simply regress, losing all they had gained in knowledge, experience, et. al.

It's obvious that you do not have Windows XP or you would know that you can regress without loss of memory!.
Bronowski was a great mathematician who worked for universities in several countries usually by invitation. He was also a great educator whose TV lectures remain unsurpassed. His one hour lecture on time contained several definitions, each of which were analysed; his own preference was indeed decay reversal.
One aspect not mentioned by Bronowski that struck me with my chapel upbringing, is that decay reversal matches the two statements on time given in the New Testament.
This is not my subject and I certainly do not want to start a religous debate, so I will retreat to the audience, thanks for letting me get a word in.

[pelastration]

My problems with the possibility of 'Time Travel'.

First I refer to a post on a old-old mkaku forum :
"Subject: time travel
Guys,
I am here and now in Belgium. I drink now a glass of wine. White wine from China ... made some years ago ... smooth, tasty and cool too.
That bottle contains about 600 billion atoms. These atoms move in an exciting way, ... it seems the bottle smiles even to me ... but most of those atoms will disappear soon in my body and a complex enzyme process will starts. Maybe I will have tomorrow an headache ;-) causing traffic jams in the microtubules in my brain proteins. How many billion brain cells with have flashing connections ... 16 billion?
That's only the wine and my body ... but there are billions of people, cars, trillions of drops in the Pacific and decay events like the fires in California. Time travel will reverse them all ... what an energy! It's now time ... to take another glass of wine and phone Einstein. Cheers. Dirk"

Time travel would implicate in a dynamic model of the Universe: a reverse (!) of all those particles movements and a reversal of there combinations in atoms and molecules, and reverse of all local and universal processes. And each is related to moves of fundamental particles each with a numbers of specific characteristics such as spin, electron orbitals, emission of photons, etc.
In time travel there are also questions such as: Would time travel be a local event? So does the total Universe continues to progress during the traveling back of a local reference frame (the traveller).

In Time travel the term: "Past" is oversimplified.
"Past" includes (1) all historical events which are still active now in the "Present", but (2) Past includes also historical events which decayed. All must reversed. :/

To finalize: We may not mix "Time" and "Grow". The velocity in Growth and Decay can be expressed in a Time sequence. Time is a local expression of that growth/decay process view from an observer/participant in his reference frame. But Time is not the process itself.
The reverse of a local growth process is called a 'decay' but it doesn't means the reverse (i.e. a sudden anti-spin) of the molecules, atoms and particles. Decay just means that the some couplings don't hold anymore. Time continues while the decay process progresses.

So TT? Impossible.

Dirk

[radagast]

Originally posted by elas
Obviously, going back in time, when it is the time travelers personal time, is of little value, since they would simply regress, losing all they had gained in knowledge, experience, et. al.

It's obvious that you do not have Windows XP or you would know that you can regress without loss of memory!.

XP doesn't 'go back in time', it merely recovers the state prior to a given point. It does, however, 'lose memory', since it's the memory of a virus, registry modification problem, or some other such problem that you are trying to get XP to forget.

Bronowski was a great mathematician who worked for universities in several countries usually by invitation. He was also a great educator whose TV lectures remain unsurpassed. His one hour lecture on time contained several definitions, each of which were analysed; his own preference was indeed decay reversal.
While intriguing, these don't match regressing in personal time, since everything, including memory, would regress without clarifying the important differences between reversing a persons personal history (time) and the 'decay reversal' Bronowski speaks of.

[Spacestar]

That said, with specific reference to the twin paradox: I did believe the twin paradox was more based on the effects of accelleration (GR) invalidating certain aspects of special relativity. As I understand it, after one of the twins left earth (or whereever they started), each twin could say that the other was the one travelling close to the speed of light [since in their frame of reference, it could appear that the other was the traveler], so the other would see the other as younger. Then once the twins returned to the same frame of reference, each would expect the other to actually be younger. Obviously, once their frames of reference merge, this cannot be, hence the paradox. General relativity, removed the fact that one could always see the other as the one traveling at a high fraction of c, when it came down to measuring apparent age at the end of the round trip.

Perhaps I've just misinterpreted what you've said, or it's always possible, with my limited understanding of relativity, that the twin paradox is much more trivial than I'd thought.
In addition to the GR approach, the better way to clarify why the paradox does not occur, is to realize that the two situations are not symmetrical. The apparent paradox occurs when the situation is faultily analyzed from the moving observer point of view. All the frames of reference are not equivalent, only the so-called inertial frames (in which Newton’s laws hold). The moving observer frame is not inertial while it is accelerating. And this is detectable by carrying out relativistic calculations using a sequence of inertial frames that at each instant during acceleration are momentarily at rest to the moving observer. Because these frames have different velocities, the systematic difference in the way that clocks are synchronized from one frame to the next must be taken into account. Reference frames are only equivalent when at rest with respect to each other (inertial frames, relatively nonaccelerating ).

I think the twin paradox inadvertently emphasizes the consequences of relativity.

It's obvious that you do not have Windows XP or you would know that you can regress without loss of memory!
Are you saying that windows XP can regress in time?!?

Wow, windows XP is always surprising me [:)]

…his own preference was indeed decay reversal.
Decay reversal is not time reversal; it is reforming.

Decay and growth are two continuous processes that take time like any other process, not that they are the ‘arrows’ of time itself.

Time travel would implicate in a dynamic model of the Universe: a reverse (!) of all those particles movements and a reversal of there combinations in atoms and molecules, and reverse of all local and universal processes. And each is related to moves of fundamental particles each with a numbers of specific characteristics such as spin, electron orbitals, emission of photons, etc.
In time travel there are also questions such as: Would time travel be a local event? So does the total Universe continues to progress during the traveling back of a local reference frame (the traveller).

In Time travel the term: "Past" is oversimplified.
"Past" includes (1) all historical events which are still active now in the "Present", but (2) Past includes also historical events which decayed. All must reversed. :/
Time travel is not a physical impossibility if you consider time as a dimension. But it sounds impossible if you consider time as a moving entity that you would need to rewind or invert its spin in order to time travel.

[Mentat]

My apologies for the delay in response.

Originally Posted by Radagast:
Of course something WAS there AT THAT POINT IN TIME, the same point in time your traveler wants to visit. If he goes back to that point in time and nothing is at x,y,z,t then it was never there!!!! That's what time travel is - going back to where and when things were.


Not good enough, IMO, since to say that something "was" there, but isn't there in the present time, precludes it's being there now (obviously). IOW, you cannot expect to find something "still there", if it's the past, since all things that are "still there" (by the very semantics of the issue) are in the present.

Think of this, if the prospective time traveller has been travelling forward in time (note: not replicating infinite copies of himself along the t dimension, but continuing to move along it as you would along a spacial dimension) for his whole life, and has not travelled into a void of nothingness, then (obviously) the rest of the Universe has moved along with him (note again: not replicating infinite copies of itself along the t dimension, just moving along it like anything would move along the spacial dimensions).

And, if the rest of the Universe has moved on, then what is there to "go back" to?


Again, think about the parabola describing an object's one dimensional motion - if the parabola didn't cross the point x,t then the object was never at x at time t. Let me put it differently. Just because the object isn't there now, if we went back and time and it wasn't there at time 't', then there would be a gap in the graph - the graph shows where the object was (in x) at that point in time. To go back in time means the object had to be there or there could be no continuity of the graph at that point in time.


I think you may be missing the point, radagast (no offense). A graph of things shows what exists now (this is obvious, because the graph exists now). Thus, a graph may be able to represent many points (and objects on these points) on the spacial dimensions, but the temporal dimension cannot be shown on the graph, since it only shows what exists, and things that "used to exist" but don't "now" (in the "present") do not exist (again, by the very semantics of the issue).

Here, let me use another analogy, and try to make my point more clear. We will assume that time is the fourth dimension for us, and so for the "Flatlander" it is the third dimension. Now, let's imagine that the entire Universe is "Flatland". So, in normal occurance, all things traveling below c are traveling forward in time, but no Flatlander can concieve of time, because that is the third dimension, and is beyond their conceptual abilities. Anyway, we can imagine that all of Flatland (which is all of the Universe) is a piece of paper - and the movement along the "time" axis is someone's picking up the paper (since "up" is "forward" movement in the t dimension, in our analogy). Now, the entire Universe has to move along this axis, more or less together. So, if everything is moving up this axis, then the poor doomed flatlander who happens to find a way to travel backward along the up/down axis (which is "time" in the analogy), will find himself seperated from the Universe, wont he?


Mentat, I am in no way trying to insult you, but what approx level of education have you had, with respect to math and physics? Perhaps I'm just taking a mental block on your part for a lack of grounding in the areas I'm talking about.


No offense, taken, I have a reasonable grasp of mathematics and physics, but there is perhaps a mental block to understanding what you are trying to tell me. I'm sorry if this frustrates you, but I thank you for your patience.


If you have the basic function f(t)=x, then when you change the time t, backwards, you get where it was at x - if you go back to time t, it has to be at x, otherwise - by definition, you haven't traveled back to time t.

The same could be said of function f(t)=x,y,z.

The reason you cannot visualize objects existing thru a duration of time is it put's the world squarely into 4 dimensions, and we have a hard time thinking in four dimensions.

Well, that's why I used the "Flatland" analogy, so that we could conceive of it in three dimensions, like we're used to.

Anyway, you speak of just changing the function of t, but ignore the logical barrier to treating it as a spacial dimension (which is basically what you are doing, btw, or so it seems to me), and that logical barrier is brought up in the previous analogy.

[elas]

It seems to me that you are implying that time cannot be changed (forwards or backwards) without changing the other dimensions. Does this not make time unique in that it is not only different from other dimensions but also has a different set of rules, if so, why?

[radagast]

Originally posted by Mentat
My apologies for the delay in response.



Not good enough, IMO, since to say that something "was" there, but isn't there in the present time, precludes it's being there now (obviously).



Of course it is not there now, it is there 'then'. That is the whole point of time travel. Now isn't then. If you go back in time, the 'then' becomes the time traveler's now. At that point in time, things are there, otherwise we don't have a past. You are mixing up the present time and the past. The past isn't a place in space, but in time. If you travel back to that place in time, then all the 'things' that were at that place at that time are at that place at that time - it is the basic definition of 'back in time'.

Been busy myself, don't know how much of the responses I'll be able to get to.

[radagast]

Originally posted by Mentat


I think you may be missing the point, radagast (no offense). A graph of things shows what exists now (this is obvious, because the graph exists now). Thus, a graph may be able to represent many points (and objects on these points) on the spacial dimensions, but the temporal dimension cannot be shown on the graph, since it only shows what exists, and things that "used to exist" but don't "now" (in the "present") do not exist (again, by the very semantics of the issue).


Though the temporal dimension cannot be shown, it can be represented in a graph. I've done it countless times in physics and math.

Plotting y=40s - 32s2 does exactly that. It is representing where in along the y axis our hypothetical point/object/falling ball was at the point when s was equal to the point along the time axis. True, the object isn't at that point now, but then again, now is not in the past - where the time traveler is going.

The time traveler has a destination of where something "used to exist". That is a destination in space-time. Once he gets to that point in space time, the things that were there will be there. If they weren't, then he didn't get to that point in space time. There is no concept of 'nothing is there, now' because now doesn't refer to the past, and certainly not to a point, in space-time, in the past.

It sounds like what your trying to say, once it's boiled down is that 'you cannot travel into the past, because it is the past'.

[Mentat]

Originally posted by radagast
Though the temporal dimension cannot be shown, it can be represented in a graph. I've done it countless times in physics and math.

Plotting y=40s - 32s2 does exactly that. It is representing where in along the y axis our hypothetical point/object/falling ball was at the point when s was equal to the point along the time axis. True, the object isn't at that point now, but then again, now is not in the past - where the time traveler is going.

The time traveler has a destination of where something "used to exist". That is a destination in space-time. Once he gets to that point in space time, the things that were there will be there. If they weren't, then he didn't get to that point in space time. There is no concept of 'nothing is there, now' because now doesn't refer to the past, and certainly not to a point, in space-time, in the past.

It sounds like what your trying to say, once it's boiled down is that 'you cannot travel into the past, because it is the past'.

That is pretty much what I'm saying, but I still think it's a valid point, since you cannot change the past from being the past. I'm saying this because:

1) If you could move the "present" back to another point in time, then everyone else would be in the future.

2) If, instead, you move yourself out of the present, and into the past, then you have the same argument that I was using before (you cannot be in the past, because to "be" is to be now).

BTW, the problem with moving the "present" into the past, with everyone else in the future, is that nothing that they "are doing" (please take note of the "are doing" part, since it's written in the present tense) has happened yet. You effectively destroy everything but yourself, since they have yet to come into existence (they are in the future).

Also, as to representing where something was on a graph, that is no big effort when compared to trying to plot a point where the object, A, exists and then plot a point where all other objects exist. We do not all exist in the past (unless you subscribe to the idea that we are constantly replicating new versions of ourselves, and that that is what traveling through time really is - in which case, I have some logical and scientific problems with that for you), and so the time traveller is removing himself from the rest of us to...where?

[radagast]

Originally posted by Mentat
That is pretty much what I'm saying, but I still think it's a valid point, since you cannot change the past from being the past. I'm saying this because:

1) If you could move the "present" back to another point in time, then everyone else would be in the future.
Assuming that the people from the present existed at the time in the past you plan to visit, then their former selves (and even your own former self) would exist where you went. The history of their existence states that they were at a given set of coordinates in space time, assuming you go back to a given temporal offset and close to the same physical coordinates as they existed in 'our' history, then you, as the time traveler would have to see them. With you, the time traveler, in the past and given the general rules of cause and effect, the future would then be somewhat fluid, since any changes you effected by being in the past, ripple through time altering events that followed those points in space-time.



2) If, instead, you move yourself out of the present, and into the past, then you have the same argument that I was using before (you cannot be in the past, because to "be" is to be now).

This is a statement, not an argument. Your initial argument was that time travel was illogical, inherently. This is only a statement that you cannot go into the past because it's in the past.

If you remember, one of the premises of this discussion was that we, hypothetically, have a method to go back into the past. Given that, you need to show the logical inconsistency that this sets up. If it always created a paradox, then that would be something forming a good argument, but just travel into the past doesn't create a true paradox.


BTW, the problem with moving the "present" into the past, with everyone else in the future, is that nothing that they "are doing" (please take note of the "are doing" part, since it's written in the present tense) has happened yet. You effectively destroy everything but yourself, since they have yet to come into existence (they are in the future).

Also, as to representing where something was on a graph, that is no big effort when compared to trying to plot a point where the object, A, exists and then plot a point where all other objects exist.

If time is an a dimension, then we are required to have all those versions of ourselves in the past. Spacetime describes a set of four coordinates that can locate things in space and time. If 'Sam' existed at x,y,z,t - where t is in the past, then if backwards travel in time is possible, and we were able to travel to t at coordinates very close to x,y,z, then by the definitions of a coordinate system 'Sam' would have to be there. The act of visiting the past could, as mentioned prior, alter events leading to the present, as well as the state of the present.

We do not all exist in the past (unless you subscribe to the idea that we are constantly replicating new versions of ourselves, and that that is what traveling through time really is - in which case, I have some logical and scientific problems with that for you), and so the time traveller is removing himself from the rest of us to...where?

The idea of replicating is not accurate. If I move from place x to x+5. At some point in time I existed along all points between x and x+5 (assuming straight line movement). I didn't replicate myself. If I had been a point, then my motion would have produced a line in space and time. I hadn't replicated myself, I just existed at all those points in space-time. In fact, had my 'point' self had not moved, but existed thru a span of time, it would still form a line. A line orthoganol to all three spatial dimensions, but parrallel to the time access (but I digress). Going back in time is like producing a line (not a geometric line, just a contiguous set of points) that is discontinous in time at the point my 'point' self jumped back in time. It proceeds from a position in the present, jumping (or moving outside of our normal three-dimensional hyperplane) to a position in time previous. The prior self existed back there just as the my 'point' existences motion thru time produced a line

Let's add a dimension.

Let's use an example of two dimensional space with time as a third. We will use a cartoon, where all the cartoons are written like a child would draw them - with no overlap - this will be our bounded two dimensional space, with each frame being an instant in time. If we pick a particular point in the cartoon as 'now', then reverse the film a number of frames, that reverses time. You'll notice that the position in the film corrosponds to time and that there are things back in time. If we take a character from the present of our film, draw him into the past frames (as he looked in the present) then he has traveled to the past (given his past is defined by the positions, on the frames prior, in the film).

Now for a little depth.

Now let's generalize the film, making each frame of our movie, three dimensional. True, the film will take a lot more space[:)], but still the analogy will hold. Time travel would be going back to a previous frame in the picture. The only difference between that time travel and the one we speak of is 'cause and effect' - by going back to a point in time previous to now, we start causing/changing things that were not events in our original history. If there is something that was changed that would effect the time traveler or his decision/ability to travel back in time, then we have a paradox.

There is some conjecture that the entire universe is basically the described 4D movie, with each frame being the hypothetical smallest unit of time (the time it takes light to traverse the distance of Planck's length) by the physical dimensions of the universe, and that all the past and future already exists, and only our consciousness is stepping thru the frames. In this scenerio, time travel would be logically impossible.

[radagast]

content deleted: remedying a carelessly clicked quote button.

[radagast]

Originally posted by elas
It seems to me that you are implying that time cannot be changed (forwards or backwards) without changing the other dimensions. Does this not make time unique in that it is not only different from other dimensions but also has a different set of rules, if so, why?

You may need to add more context. I don't know if you are addressing some of my arguments or mentat's.

Also, you may want to clarify your terms. I assume you mean change a position in time or position along some of the other dimensional axis. Changing a dimension, versus a position along that dimensional axis, is something that would have to be defined.

[radagast]

deleted

[Spacestar]

It seems to me that you are implying that time cannot be changed (forwards or backwards) without changing the other dimensions. Does this not make time unique in that it is not only different from other dimensions but also has a different set of rules, if so, why?
When you change time, you also change the three spatial dimensions (y, x, z), for the reason that all the dimensions of space exist in time. That’s why it’s called space/time. In space/time geometry the ‘distance’ between events is given by the by sum of the squares of the space offsets minus the square of the time difference.

Time travel would be going back to a previous frame in the picture. The only difference between that time travel and the one we speak of is 'cause and effect' - by going back to a point in time previous to now, we start causing/changing things that were not events in our original history.

To my understanding, what you’re stating is not traveling to the past, but to an alternate past. To travel to the past implies that you follow your space/time line or line of causation. But, if you are considering that traveling to the past obligatorily means that you have to change it from what it was, then you are violating ‘cause and effect’.

Alternatively, let’s say that you do travel to a past where things happen differently, my question is: How you got there?

[radagast]

Originally posted by Spacestar
When you change time, you also change the three spatial dimensions (y, x, z), for the reason that all the dimensions of space exist in time. That’s why it’s called space/time. In space/time geometry the ‘distance’ between events is given by the by sum of the squares of the space offsets minus the square of the time difference.

Not to jump into another's discussion, but don't you mean a change in position (or offsets)within the dimensional space. Otherwise I'd like someone to explain what 'changing a dimension' means.



To my understanding, what you’re stating is not traveling to the past, but to an alternate past. To travel to the past implies that you follow your space/time line or line of causation. But, if you are considering that traveling to the past obligatorily means that you have to change it from what it was, then you are violating ‘cause and effect’.

I don't know if you are bringing up a semantic discussion. In other words, when you travel to a point in your past and change it, then semantically, it ceases to be your past, given defining the past as the history we remember/or would remember with perfect knowledge and memory.

Alternate past could mean an, initially, identical past in a parallel universe/parallel dimensional plane. Or it could mean that you have altered your own past (making it an alternate past) by traveling to it. If there is a third meaning, then you'll have to elaborate.

If not, as I see it, then the only violation of 'cause and effect' occurs if a paradox occurs - you go back in time change something such that your present self no longer needs to go back to the past, since he doesn't, the past isn't changed and the present self does decide to go back to the past...


Alternatively, let’s say that you do travel to a past where things happen differently, my question is: How you got there?

Two wormholes will get, potentially, you into the past, according to current theory. Your able to do some interesting things when you are can both create two wormholes and move one of the wormhole's mouth position in space. Tangentially mentioned in http://www.npl.washington.edu/AV/altvw69.html.

[Mentat]

Originally posted by radagast
Assuming that the people from the present existed at the time in the past you plan to visit, then their former selves (and even your own former self) would exist where you went. The history of their existence states that they were at a given set of coordinates in space time, assuming you go back to a given temporal offset and close to the same physical coordinates as they existed in 'our' history, then you, as the time traveler would have to see them. With you, the time traveler, in the past and given the general rules of cause and effect, the future would then be somewhat fluid, since any changes you effected by being in the past, ripple through time altering events that followed those points in space-time.


This is interesting though. You say that our "fomer selves" should all be at these different points in time. That implies that we must be producing a "new self" at every one of the smallest intervals of time (otherwise we could run into certain points in time where we all do exist, and certain points where we don't), but this cannot be so if our cells are decaying at different rates, and at different times, can it?


This is a statement, not an argument. Your initial argument was that time travel was illogical, inherently. This is only a statement that you cannot go into the past because it's in the past.

If you remember, one of the premises of this discussion was that we, hypothetically, have a method to go back into the past. Given that, you need to show the logical inconsistency that this sets up. If it always created a paradox, then that would be something forming a good argument, but just travel into the past doesn't create a true paradox.


To travel into the past is to imply the existence of the past. You have to have something to go to and, as I've already said before, the past doesn't exist now.


If time is an a dimension, then we are required to have all those versions of ourselves in the past. Spacetime describes a set of four coordinates that can locate things in space and time. If 'Sam' existed at x,y,z,t - where t is in the past, then if backwards travel in time is possible, and we were able to travel to t at coordinates very close to x,y,z, then by the definitions of a coordinate system 'Sam' would have to be there. The act of visiting the past could, as mentioned prior, alter events leading to the present, as well as the state of the present.


Why is it that you think time works so differently from space? There are not infinite copies of me in space, so why should there be in time? Besides, the fact that I was there at that time, doesn't mean that I'm still there. In fact, it means that I am definitely not still there. All of space and matter/energy has moved on from that point, hasn't it?

Also, with regard to changing the present, that's not completely accurate. We are actually changing the future, since the present is where you are.


The idea of replicating is not accurate. If I move from place x to x+5. At some point in time I existed along all points between x and x+5 (assuming straight line movement). I didn't replicate myself. If I had been a point, then my motion would have produced a line in space and time. I hadn't replicated myself, I just existed at all those points in space-time. In fact, had my 'point' self had not moved, but existed thru a span of time, it would still form a line. A line orthoganol to all three spatial dimensions, but parrallel to the time access (but I digress). Going back in time is like producing a line (not a geometric line, just a contiguous set of points) that is discontinous in time at the point my 'point' self jumped back in time. It proceeds from a position in the present, jumping (or moving outside of our normal three-dimensional hyperplane) to a position in time previous. The prior self existed back there just as the my 'point' existences motion thru time produced a line


I disagree. A point's movement through space might produce a "line" but not its movement through time. I say this because a the only reason there is a discernable "line" in space, is because all parts of the line, along with the actual point itself, exist in together in the present. If there were a "line" or "path" through time, then the time axis would all have to exist in the present, which is self-contradictory.


Let's add a dimension.

Let's use an example of two dimensional space with time as a third. We will use a cartoon, where all the cartoons are written like a child would draw them - with no overlap - this will be our bounded two dimensional space, with each frame being an instant in time. If we pick a particular point in the cartoon as 'now', then reverse the film a number of frames, that reverses time. You'll notice that the position in the film corrosponds to time and that there are things back in time. If we take a character from the present of our film, draw him into the past frames (as he looked in the present) then he has traveled to the past (given his past is defined by the positions, on the frames prior, in the film).


That's the problem. All of the people who believe that time travel is possible try to think of time in terms of frames, but this just doesn't follow. If all of time were frames, then the frames would have to all exist - and the term "exist" is in the present tense.


Now for a little depth.

Now let's generalize the film, making each frame of our movie, three dimensional. True, the film will take a lot more space[:)], but still the analogy will hold. Time travel would be going back to a previous frame in the picture. The only difference between that time travel and the one we speak of is 'cause and effect' - by going back to a point in time previous to now, we start causing/changing things that were not events in our original history. If there is something that was changed that would effect the time traveler or his decision/ability to travel back in time, then we have a paradox.


But that's the point! Don't you realize that set frames of time couldn't possibly be changed?! The fact that going into the past means you change everything after the point you travelled to proves that normal travel along the t axis is not the constant production of new "frames of reality".

[radagast]

Originally posted by Mentat
This is interesting though. You say that our "fomer selves" should all be at these different points in time. That implies that we must be producing a "new self" at every one of the smallest intervals of time (otherwise we could run into certain points in time where we all do exist, and certain points where we don't), but this cannot be so if our cells are decaying at different rates, and at different times, can it?

A line is a point, existing through a span along a single dimension. The point doesn't have new selves. The problem your running into is that you assume that we are not truly 4D entities (3spatial, 1Time). As a 4D entity/object we have existance bounded by three spatial dimensions and one time dimension. Our existence in the time dimension starts at our creation and ends with our dissolution. It's impossible to quite visualize, but the idea of a long worm, stretching throughout the span of ones existence is about as close to visualizing it as I can get. This is one of the requirements of a time dimension. It certainly does raise question about the immutability of future events and free will, but it doesn't logically invalidate time travel.

The problem with your above example is that it violates an initial 'given'. The initial premise is that time travel is possble, with your argument being finding the logical inconsistencies that this produces. Your example 'assumes' that time travel is impossible (just the statement that we don't exist in the past implies exactly such). If you wish to assume such, fine, but it does circumvent the debate, as I see it.

Why is it that you think time works so differently from space? There are not infinite copies of me in space,

This is something which may seem to be common sense, but cannot possibly be known. If you exist as an object with size in all four dimensions, and are only aware of three of them, with the time dimension only experienced as the moment, then it's not copies of you, but the continuation of you, just as the continuation of a point thru a span along single dimension is a line segment.

so why should there be in time? Besides, the fact that I was there at that time, doesn't mean that I'm still there. In fact, it means that I am definitely not still there. All of space and matter/energy has moved on from that point, hasn't it? Not when time is considered a dimension.

Also, with regard to changing the present, that's not completely accurate. We are actually changing the future, since the present is where you are. Depends on your temporal frame of reference. A person can only take action in the present of his personal temporal frame of reference - this has an effect/alteration on future events/reality from that point in time forward.


I disagree. A point's movement through space might produce a "line" but not its movement through time. Again, you are treating time as something other than a dimension. It does exactly produce a line segment, just a line perpendicular to the three spatial dimensions. You cannot treat time, in the abstract, as totally different from the other dimensions.


That's the problem. All of the people who believe that time travel is possible try to think of time in terms of frames, but this just doesn't follow. If all of time were frames, then the frames would have to all exist - and the term "exist" is in the present tense.

You cannot use semantics to alter reality. If the past exists, as stated, then it exists and the common usage of the word needs to be expanded.

But that's the point! Don't you realize that set frames of time couldn't possibly be changed?! The fact that going into the past means you change everything after the point you travelled to proves that normal travel along the t axis is not the constant production of new "frames of reality".

Just because it seems impossible or absurd doesn't make it so, any more than the apparent absurdity that time moves at different speeds, depending on what frame of reference you observe, or the apparent absurdity that you cannot precisely state a neutrino's position and momentum.

I say this because a the only reason there is a discernable "line" in space, is because all parts of the line, along with the actual point itself, exist in together in the present. If there were a "line" or "path" through time, then the time axis would all have to exist in the present, which is self-contradictory.

Such people as Stephen Hawkings, John Cramer, and Michio Kaku haven't raised the concerns that you are raising, so perhaps the arguments on the other side of the coin are not quite as illogical as you seem to elude to.

You continually contradict the assumption that time is a dimension of reality, and you seem to ignore the given at the start of this debate. With the two of us deviating on such a basic points, I see further debate to be unfruitful.

[Spacestar]

Not to jump into another's discussion, but don't you mean a change in position (or offsets)within the dimensional space. Otherwise I'd like someone to explain what 'changing a dimension' means.
I suppose elas is referring about how and why changing one’s position in time would affect the spatial dimensions.

I don't know if you are bringing up a semantic discussion. In other words, when you travel to a point in your past and change it, then semantically, it ceases to be your past, given defining the past as the history we remember/or would remember with perfect knowledge and memory.
The past it’s obviously what happenens in a prior point to our current position in the space/time line (clearly what we would remember). How can the past be a set of events and be another set of events at the same point in time?

Alternate past could mean an, initially, identical past in a parallel universe/parallel dimensional plane. Or it could mean that you have altered your own past (making it an alternate past) by traveling to it. If there is a third meaning, then you'll have to elaborate.

If not, as I see it, then the only violation of 'cause and effect' occurs if a paradox occurs - you go back in time change something such that your present self no longer needs to go back to the past, since he doesn't, the past isn't changed and the present self does decide to go back to the past...
What I mean is that you appear to be saying that the very act of traveling to the past automatically changes the past from what it was, creating thus an alternate/parallel space/time line. That’s what essentially violates the line of causation; since to get to a previous point in the space/time continuum that you are in, obviously one has to track back the same line of action. And the journey would alter only your personal time, not the space/time continuum itself. Accordingly, your presence in the past is a part of the cause that makes your space/time continuum the way it “is”.

Two wormholes will get you into the past, according to current theory. Your able to do some interesting things when you are can both create two wormholes and move one of the wormhole's mouth position in space.
That’s not what I’m asking.
Please let me reformulate the question: How you’d get in a different past, if you you’re following your space/time line?

And, I’d like to add that the current theory also states that you would need exorbitant amounts of energy (as the mass of Jupiter) to control and generate wormholes. Which, of course is very unpractical. Besides, the shifting the spatial position of the wormholes does not modify your position in the space/time dimensions. Actually, it puts you out of phase with your original dimensions. That’s more like interdimensional travel than time travel.

[radagast]

Originally posted by Spacestar
The past it’s obviously what happenens in a prior point to our current position in the space/time line (clearly what we would remember). How can the past be a set of events and be another set of events at the same point in time?

Any non-trivial time travel into the past would require the travellers personal memory/history/timeline to remain intact - in effect removing him from a point in time where he still had his experiences and memory and plopping him into a time previous to the time of departure. Assuming no quantum annihilation of the wormholes, wormhole physics predicts this would be possible.


What I mean is that you appear to be saying that the very act of traveling to the past automatically changes the past from what it was, creating thus an alternate/parallel space/time line. That’s what essentially violates the line of causation; since to get to a previous point in the space/time continuum that you are in, obviously one has to track back the same line of action. And the journey would alter only your personal time, not the space/time continuum itself. Accordingly, your presence in the past is a part of the cause that makes your space/time continuum the way it “is”.
Some conjectures state that that is exact what would happen, branching a parrallel timeline. This certainly handles any problems of paradox. However, violating causation assumes that all history that the traveller lived through is inviolate (assuming I'm understanding you correctly). Causation is based on very basic things we have observed in working in the natural universe. Time travel, assuming one timeline, would tip causation on it's ear. You cannot say that because everything we've seen (in regards to causation) wouldn't remain the same with time travel, therefore time travel is irrational is taking the cart before the horse - causation is considered extremely hard and fast, because we seen not contraditions - however, assuming we come up with a method of time travel - which we haven't seen either - means our 'laws of the universe ' have to expand to account for them. If you are speaking of the specifics of a paradox, then all bets are off. A paradox is, by it's definition, a logical inconsistency.


That’s not what I’m asking.
Please let me reformulate the question: How you’d get in a different past, if you you’re following your space/time line?

As mentioned earlier, the traveller cannot follow his own 'personal' timeline back - assuming we are speaking of none-trivial time travel. I was speaking of just the same kind of time travel as theorized by using timewise wormholes.




And, I’d like to add that the current theory also states that you would need exorbitant amounts of energy (as the mass of Jupiter) to control and generate wormholes. Which, of course is very unpractical. No argument here. This has always been, from my point of view, a hypothetically and theoretically oriented debate.

Besides, the shifting the spatial position of the wormholes does not modify your position in the space/time dimensions. Actually, it puts you out of phase with your original dimensions. That’s more like interdimensional travel than time travel.

I am going completely by memory on how this was to be done - it seems like it was a from something written by Robert L. Forward. I've also seen references to the same by John Cramer and Stephen Hawkings. They referred to them as time travel, so I bow to their greater knowledge in the area. Since Hawkings was concerned with paradoxes and the potential causability problems of a timewise wormhole, I assume that he regarded these as true time travel, not interdimensional travel.

[By interdimensional travel, I presume you mean between 3D hyperplanes??]

[Mentat]

radagast, with regard to your previous response to my post,

I agree with your point about semantics, the terms would need to be expanded for newly discovered realities, not the other way around. I understand, also, that the great minds of science do not see the same problem that I do with time travel (probably because the problem doesn't really exist, and I just keep running into it due to some predisposition toward deductive logic instead of purely scientific inquiry).

However, I do have one point to mention. If I go back a few days into the past, there would (logically) be another "me" at that point, since I existed at that point, right? If this is so, then at that point in time (which is before this point in time) there were two of me (how long this circumstance existed is irrelevant for now). But then, in the near future, when I decide to go into the past, I will go back to the time where there are already two of me (as it seems there must be, according to the previous sentence) and thus there will be three of me at that point in time. But then, since there are three of me at that point in time, then there have always been three of me at that point in time, and so when I, in the near future decide to go back into the past, I will find three of me already there, and there will thus be four of me. This should go on infinitely, I think.

Also, I don't recall how it is you explain the problem of logic that I presented much earlier in this thread (that if you go back in time to any point before the time you leave, you have not yet left, and so are not yet travelling and thus have no means by which to arrive at this "past" point in time). Could you please address this one again (please forgive me if you've already covered it sufficiently before)?

[radagast]

Mentat,
Just to clarify, I am withdrawing from the debate with you, on this subject. No insult or emnity intended, but our positions haven't changed for some time, and just debating to debate isn't something I see much point in. I'm sure we will lock horns on other (perhaps less intractible) issues in the future.

[Mentat]

Originally posted by radagast
Mentat,
Just to clarify, I am withdrawing from the debate with you, on this subject. No insult or emnity intended, but our positions haven't changed for some time, and just debating to debate isn't something I see much point in. I'm sure we will lock horns on other (perhaps less intractible) issues in the future.

Alright, if that's what you want to do. I would much appreciate it if you addressed the points in my previous post, but you (of course) don't have to if you don't want to.

[radagast]

I will try to answer, since it seems to be a point of clarification, not a point I'm trying to argue.

Think of a person as a string, where the width and breadth of the string represent the width and breadth of a person - as well as it's position with respect to the two dimensions (this example will have to dispense height since I'll be borrowing that dimension for time and humans are poor at visualizing 4D objects). A person would not keep 'replicating themselves, but it would be the continuation of their 4D self. Now, without time travel, the string would be required to always be both continuous and differentiable with respect to the time axis - no catastrophes allowed. Once time travel is introduced, either that string becomes discontinous at the point where the traveler travels back in time, or shifts into a parallel 3D plane to travel backwards in time. At this point the string would double back on itself (thru the parallel hyperplane), until it reenters normal time flow (thus the leading edge again points back in the direction of the flow of time's arrow) and our hyperplane. Yes, the leading edge of the string could encounter points where it's prior existence is located, assuming their spatial coordinates are close and the time coordinate matches (time with respect to some arbitrary measure along the time axis, not the travelers personal/experiential time). Both the traveler and his 'previous' self would and could interact since their experiential time front's (experiential moments) would coincide within time. This would be fraught with the danger of a paradox, assuming the earlier aspect of the traveler could decide not to travel back in time.

This was meant as way of presenting the concept I've been trying to get across, not as a point of argumentation.

[selfAdjoint]

Why doe the string have to double back? It can gradually (smoothly) turn and head backwards without intersecting itself, then when it has gone back far enough, turn again and rejoin itself. Things like this can happen in solutions of the general relativity equations. They are called closed timelike curves, or CTCs.

[radagast]

Double back only referred to the change of direction with respect to the time axis. How sharply that change or orientation takes place was outside the scope of the discussion. I never meant to imply the string/person ever intersected itself in all four coordinates.

[selfAdjoint]

Okay, but in that case I don't see your point about a singularity. Could you clarify that?

[radagast]

Originally posted by selfAdjoint
Okay, but in that case I don't see your point about a singularity. Could you clarify that?

Singularity? I've not commented on a singularity, could you elaborate on what you believe I've said.

[selfAdjoint]

This is what I was referring to.
Now, without time travel, the string would be required to always be both continuous and differentiable with respect to the time axis - no catastrophes allowed. Once time travel is introduced, either that string becomes discontinous at the point where the traveler travels back in time, or shifts into a parallel 3D plane to travel backwards in time. At this point the string would double back on itself (thru the parallel hyperplane), until it reenters normal time flow (thus the leading edge again points back in the direction of the flow of time's arrow) and our hyperplane

The singularity I meant was the failure of continuity or differentiability in the worldline, or at least the proper time (your string, or mapping of it into the time axis).

[radagast]

To be quite candid, I'm still not exactly certain what exactly your question is asking, but I'll address something that has bothered me and with luck it will be related.

When I've considered the consequences of time travel, it always seems that their has to be a second time dimension - such that there is usually no change (motion, but no change) in t' as the universe progresses along t, until backward time travel (in t). This also handles the problems of paradoxes, since the timeline has changed as it progresses along t' with every backwards t incursion.

From my limited playing with the relativity equations it seems that the idea of a second time axis pops out when speeds exceed c, as negative value on an imaginary time axis. An imaginary time would be orthoganol to our primary time axis, and as far as I can see, identical to the previously mentioned t'.

[Spacestar]

I’m sorry for my delay. Hope you guys are still around.
Any non-trivial time travel into the past would require the travellers personal memory/history/timeline to remain intact - in effect removing him from a point in time where he still had his experiences and memory and plopping him into a time previous to the time of departure. Assuming no quantum annihilation of the wormholes, wormhole physics predicts this would be possible.
If we are considering time travel in its accurate meaning, of course that the traveler’s memory/history/time line must remain intact. Or should I say, the aspiration to surf one’s time line. But that doesn’t mean that moving back in time is the same as changing the past from what we memorize. Time traveling could mean contributing to the framework of time as well. The possibility of wormholes may corroborate the idea of time travel, not its consequences.

Some conjectures state that that is exact what would happen, branching a parrallel timeline. This certainly handles any problems of paradox. How that would discard a paradox? To my understanding, that involves a paradox, since that would imply giving two futures to a point in space/time (the future is and it isn’t) – from the perspective of someone in the past, the traveler would come from the future, yet not from the future?! Logically that produces a paradox.

However, violating causation assumes that all history that the traveller lived through is inviolate (assuming I'm understanding you correctly). Causation is based on very basic things we have observed in working in the natural universe. Time travel, assuming one timeline, would tip causation on it's ear. You cannot say that because everything we've seen (in regards to causation) wouldn't remain the same with time travel, therefore time travel is irrational is taking the cart before the horse - causation is considered extremely hard and fast, because we seen not contraditions - however, assuming we come up with a method of time travel - which we haven't seen either - means our 'laws of the universe ' have to expand to account for them. If you are speaking of the specifics of a paradox, then all bets are off. A paradox is, by it's definition, a logical inconsistency.
The possibility of time travel is also based on our understanding of nature and its laws. So at this point I’m not certain how “natural” time travel is, but the theory of relativity indicates that time travel is pretty “natural”, since according to Einstein’s theory, time is relative by nature. A good example is to imagine the speed and acceleration of a particle orbiting near a black hole. The gravitational force of the black hole would induce the particle to speeds near the speed of light (or even beyond, seeing as even light can escape when near a black hole), causing the particle to literally time travel naturally. Which, according to relativity does not violate causation, as we know. Also, saying that time travel might eventually alter causation does not support your point of view, because you are appealing to an argument to the future, that we don’t know if will be valid.
I am going completely by memory on how this was to be done - it seems like it was a from something written by Robert L. Forward. I've also seen references to the same by John Cramer and Stephen Hawkings. They referred to them as time travel, so I bow to their greater knowledge in the area. Since Hawkings was concerned with paradoxes and the potential causability problems of a timewise wormhole, I assume that he regarded these as true time travel, not interdimensional travel.

[By interdimensional travel, I presume you mean between 3D hyperplanes??]
Yes, which are identically the same as orthogonal universes, or "hyperframes". The physics behind orthogonal dimensions or levels (multiple valued basic dimensional functions) are either not very known or severely disregarded on the current interpretation of relativity (of course that doesn’t take its consistency). Nonetheless, it was approached by Hugh Everett for his doctoral thesis in 1956, and the thesis was published in 1957 (See Hugh Everett, III, The Many-Worlds Interpretation of Quantum Mechanics: A Fundamental Exposition, with papers by J. A. Wheeer, B. S. DeWitt, L. N. Cooper and D. Van Vechten, and N. Graham; eds. Bryce S. Dewitt and Neill Graam, Princeton Series in Physics, Princeton University Press, 1973.).

[hey.like]

If the time system isn't sole, it is formed in mass system and a beautiful universy. but the later is absolute and semi-dimension. to travel in time is in this semi-dimension . if to travel in mass systems time , it is puzzle very still.

[hey.like]

Only a light particle, it is everywhere. but it is different from this world. only it is not exist , we can get it information. as the time or space dimension action, naturely , it is not enable to get some information in it , only it is not exist but its action is clear. in this way the different unit space-time is ease to do the time travel , but it is not in this world, it is errors to get in some another world and get the world space-time exist or unexist hold way. Naturely it is safety for this world like so many light particle.