Time travel into the past is logically possible

[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 traveled 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 existence 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 traveled 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 Hawking 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 traveling 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 Hawking 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 systems is not sole

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

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.