# Time travel into the past is logically possible

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 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?

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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 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 ) 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 traveling in the first place?

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

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.

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Staff Emeritus
Gold Member
Dearly Missed
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.

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
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 spatial dimension is free of time. The two-dimensional creature is as much "under the laws of time" as the ten-dimensional one.

Mentat
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... .

Ontoplankton

http://sheol.org/throopw/tachyon-pistols.html [Broken]

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.

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Mentat
Originally posted by Ontoplankton

http://sheol.org/throopw/tachyon-pistols.html [Broken]

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).

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Originally posted by Mentat
Not at all. No spatial 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.

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Originally posted by Mentat
Not at all. No spatial 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.

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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... .

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.

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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
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 spatial dimension that was free of time then it would not be orthogonal. That's why I disagreed that one could leave the 3 spatial dimensions that we are used to and thus travel back in time, since there is no spatial 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
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
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?

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

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 Hawking 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
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
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 Hawking 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).

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".

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
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 occurring 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
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 spatial 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 traveling 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.

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

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

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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 traveling 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?

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 occurring 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.

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 spatial 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).

Originally posted by Spacestar
You know, many persons think that traveling 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
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 into 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 spatial 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) spatial 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
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.

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.