A critique of the replacement theory of time travel
The replacement theory of time travel was developed by Mark Joseph Young to sort out various anomalies appearing in time travel stories. It is described in detail on his website Temporal Anomalies in Popular Time Travel Movies and illustrated by logical reconstructions of many complicated movie plots. The author recently had a lively email exchange with Mr. Young on the subject of time travel. On his suggestion this critique is posted here for comment and discussion. The article is self-contained, but the reader will benefit (and have fun) from reading Discussing Time Travel Theory section on the Temporal Anomalies website. The site also contains some previous discussion between Mr. Young and the author concerning time travel and the replacement theory (see A Critique of the Spreadsheet Theory).
Replacement theory
The gist of the theory is that if the past is changed by a time traveler from an original timeline, causes of changes must be replaced within the resulting new timeline. In particular, there has to be a person in this new timeline, who travels into the past and makes the changes that created it. Every time trip into the past, say from 2030 to 2000, terminates the original timeline at 2030 and rewinds the history back to 2000. The traveler then has until 2030 to re-justify his appearence in 2000 within the new timeline, e.g. convince his younger self to repeat the trip in 2030.
Otherwise, time snaps back to 2000 again to avoid a causality violation. Snap-backs are the main enforcement mechanism in the replacement theory and they are instrumental to its claim that a time traveler impacts his own life. Mr. Young sees this last point as a distinctive feature of true time travel as opposed to parallel dimension jumps. I will take up both ideas here and express my reservations about them. At the end, I will outline some approaches to modifying the theory.
By his own bootstraps
To get a feel for the replacement theory let us briefly illustrate how it deals with the classical time travel paradoxes. In cases with a single time trip to the past there are two basic types of them, often called bootstrap and grandfather paradoxes.
In a typical bootstrap paradox a traveler (Oldie) gives his younger self (Newbie) a book describing construction of time machines. Newbie reads the book, builds a time machine and goes back in time to hand over the book to himself. Where did the book come from? This is not a logical contradiction, but there is a mystery of that self-existing book with all its knowledge, which is forever trapped in a time loop.
The replacement theory is at its best in handling such bootstrap scenarios. In our example, it postulates existence of an original timeline, where Newbie invented time travel without Oldie’s help and wrote a book about it. Intending to save himself the trouble he travels to the past as Oldie and hands over the book. The original timeline is aborted as soon as he arrives at the past and time restarts from that point. In the new timeline, Oldie instructs Newbie to close the loop by returning the book to himself, and goes back to the future to live the rest of his life. This is the bootstrap timeline we started with. The genuine cause in the original is replaced by circular causality in the final timeline.
The entire process, called N-jump, can be pictured as a zig-zag of the original timeline terminating in 2030, snapping back to 2000, and continuing undisturbed into the future. Most of the reconstructions on the Temporal Anomalies website feature an iterated version of N-jump, where the first Newbie fails to close the loop, but after several intermidiate snap-backs one of his successors does.
Killing yourself the hard way
But suppose Newbie disregards good advice, throws out the book and turns his mind away from time machines. Or perhaps, Oldie discourages him from pursuing them. Then time is forced to snap back again from 2030 to 2000, the original timeline is replayed with invention of time machines, trip to the past, snap-back, new timeline with no time machines, snap-back, etc. This is replacement theory’s infinity loop that traps time forever between 2000 and 2030.
This is also a variation of the grandfather paradox. Classically, a time traveler goes back to kill his grandfather and prevent his birth, but we can do just as well with him killing himself. This is a true contradiction since he can kill himself if and only if he does not kill himself. To get what happens under the replacement theory simply replace ‘build a time machine’ with ‘kill yourself’ in the above example.
In fact, all grandfather paradoxes result in infinity loops and trapped time. Although the contradiction is removed, perpetual snap-backs do not give an appealing resolution. If time travel functioned according to the replacement theory it would likely trip up our universe soon after being discovered. If original inventors experiment with sending an ice cube one minute into the past and see it appear one minute prior to scheduled sending, they may well decide to see what happens if they do not send it after all. The result will be an entire universe forever trapped in a one minute infinity loop.
Time also gets trapped in more general situations, where instead of endless repetition there is endless production of new timelines, all aborted. These are called sawtooth snaps. In fact, iterated N-jumps ending in bootstrap timelines are the only scenarios having a satisfactory resolution. They are also the ones most commonly encountered in time travel movies and literature. But there are serious logical issues with the entire mechanism of snap-backs, which we discuss next.
Dead timeline walking
Time always moves from the past to point A and beyond to point B; at point B, when Traveler leaves time and heads to point A, time ends; but Traveler cannot arrive at point A, because he could not have been there in the original time line… The instant Traveler reaches point A, he destroys point A, replacing it with point C… Time will now continue to point D.
In this passage point A is year 2000 in the original timeline, point B is its year 2030, points C and D are counterparts of A and B in the new timeline. A time jump at B triggered a snap-back to C. The AB timeline has been aborted, obliterated, metaphysically erased. In my opinion, once AB sinks into oblivion its former existence should exert no influence over CD except through the traveler’s presence, which is available at C in the entirety of its relevance. One would think that time would not ‘know’ any different if the traveler simply popped out of nowhere as a result of some spontaneous fluctuation in space. After all, AB timeline is no more, vanished into the metaphysical past. In other words, one would expect time to be metaphysically memoryless. Not in the replacement theory.
Time has to make sure that traveler’s appearence at C is not a miracle, that it is supported by the new timeline. And there is a deadline for compliance — 2030. For an N-jump to occur the matters at D must stand as at B ‘in every pertinent way’, otherwise time snaps back again. In other words, time has to raise B from its metaphysical grave, compare D to B, decide if the differences are sufficiently pertinent and then snap-back or continue. There does not have to be any time travel attempt in 2030 for this to happen. When time is trapped in an infinity loop, no time machines are ever built in CD and nothing special happens in 2030. Somehow, AB timeline extends a dead hand from its grave and snaps time back. I understand the intent to preserve causality, but I do not see through what physical means such a snap-back might be accomplished.
Even if we accept that ‘nature abhors causality violations’ and the time traveler must reproduce circumstances of his appearence in 2000, why is he only afforded 30 years to do so? From CD’s point of view this due date is completely arbitrary. Oldie has left the scene, Newbie has better things to do, there are no time machines around. All that matters is that the traveler’s appearance in 2000 must eventually have a cause within the same timeline. This cause may be a time jump from 2030, or 2050, or 10327. At some point down the line the traveler or his clone must make a trip to 2000 to save causality, but other than that there are no restrictions. Perhaps, time patrol from a distant future will uncover the wrinkle, clone the time traveler, instruct him accordingly and at the appropriate age send him back to 2000. There is no immediate cause for the snap-back in 2030 within the CD timeline, which defeats the purpose of saving causality.
This is like a bill that comes due eventually but the payment can be deferred indefinitely into the future. Of course, in practice this means that the bill never comes due. The miraculous appearence in 2000 may remain a spontaneous act forever. There is no physical mechanism in the replacement theory to prevent that. Forcing a snap-back in 2030 is simply a deus ex machina.
Banana peels
This is not the only problem with spontaneous snap-backs. What if after dissuading his younger self from pursuing time machines our older traveler decides to visit 2040? Under the replacement theory there is no 2040 in this timeline, so no 2040 for him. But why? He would have had no trouble making the trip under the N-jump scenario, and his younger self still has time to change his mind and enact it. According to Mr. Young, travel to the future is benign and should raise no red flags on Time’s radar screen. But to stop the traveler, not only should CD know in advance that the history will end in a loop, it also has to keep diligent watch for any future travel attempts.
And what if after reaching whenever instead of 2040 our traveler decides to return and convinces his younger self to build the time machine after all? The timeline recoheres and 2040 that he allegedly did not reach exists. Then he did reach it despite what we thought before. If we maintain that the trip can happen then there is no snap-back in 2030. If it can not happen we need what is called a ‘banana peel mechanism’. There must be a strategically placed banana peel in the CD timeline, upon which Oldie slips, bumps his head and loses his memory before he can jump to 2040. We saw this mechanism at work in the fixed timeline theory. To paraphrase Mr. Young, Time is a very clever gentleman if it can do all that.
Replacement causes
It isn’t time travel unless the traveler arrives in his own past. That means he can impact his own life; and that means we need a theory that addresses what happens when someone impacts his own life.
In a bootstrap scenario a traveler does change his own life, but only to make it what it was. What one really wishes is to change the course of events into something other than what is remembered. The challenge is to make new events happen to the same person that made the changes, to transmute the time traveler into his younger self. What is vaguely imagined is exiting the flow of time, improving events in the past and then re-entering time as the same person under new circumstances. There are two visions of this re-entry. You can relive your own life from the time the changes were made, or you can skip part of it and slide back into the new you at a later date.
Both scenarios miss the target. Someone has to live through the new life, and if it is not you the only way to slide into it is to displace that new person. If on the other hand, you regress back into childhood and relive your life, then it is the old self that is displaced. In this version of the dream the memories of time travel vanish but its effects linger on. Perhaps, they retain some supernatural influence reminiscent of reincarnations in Buddhism with forgotten past lives and karma. But dreams have the luxury of ignoring logic. In the light of day, it what is asked can not be delivered, it is a logical impossibility. But it can be approximated.
Parallel worlds do the next best thing. They duplicate a person into a changer and a changee: the older traveler makes changes that affect his younger duplicate. This duplication destroys the possibility of changing own past even if you start with exactly the same past. The replacement theory tries to mitigate the problem using snap-backs. In my opinion, it does not succeed.
Duplication still happens as attested to by descriptions of infinity loops and sawtoth snaps, where the older and the younger self have separate origins. Spontaneous snap-backs are unleashed to save the appearences by preventing a meeting between them when they are of the same age. In N-jumps replacement causes are generated to make it look like the older and the younger self are one and the same. But all of this is smoke and mirrors. Having observed the original timeline, we know better. The true old self is gone, vanished into oblivion. The younger traveler growing up, jumping back and changing himself is a sleight of hand, an optical illusion like wheels of a car appearing to roll backwards in old movies.
In a way, the replacement theory fails better than its alternatives. If duplication does not produce an illusion of transmutation the entire timeline gets punished by being aborted at the original travel date. The replacement theory is a crime and punishment story with causality as the victim, time traveler as the perpetrator and Time itself as the police, the judge and the executioner. Many oddities of the theory can be traced to this forcing of Time into roles that it fiercly resists to perform.
Identity crisis
The time traveler in the timeline that marks the final, stable history of the world knows nothing about any previous history of the world… This time traveler has no first-hand knowledge of the original history; that version of him was erased…
It is rather mysterious how exactly the old traveler transforms into his replacement. In the new timeline everything is done by the new traveler who originates in it. The old one is shut down as soon as the old traveler steps out of it. So where exactly did he go, or rather wherewhen did he perform the changes that created the stable timeline with everything performed by his replacement? This is supposed to be explained by the spreadsheet illustration: “Although the cause of the value at A1 has changed, the value itself has remained the same, and all the values springing from it are likewise preserved”. If it means what I think it means, we have another optical illusion.
In the ‘first run’ along the new timeline it was the impostor (as in visitor from another timeline) that instructed the authentic youngster on the fine points of building time machines. After that he conveniently leaves the scene. As the youngster grows up and jumps back we get a ‘second run’. He ends up ‘having the same value although not the same cause’ as the impostor, and now instructs himself on the subject. Of course, we are not allowed to think about it this way, because the timeline is complete in its eternal glory and there are no ‘runs’.
As a result, there is no particular occasion at which the replacement happens or a process by which it happens. This reminds me a common experience of making two lampposts look like one by keeping them in the same line of sight. Identity of indiscernibles hardly applies to physical objects. Besides, I would argue that even metaphysically entities with different causes are discernible.
The curse of parallel worlds
Mr. Young sees duplication as a hallmark of parallel worlds and dismisses them as non-time-travel. Although I agree that traditional parallel worlds are probably not time travel, this broadening of the concept goes too far. According to the strictures of Mr. Young’s definition, his own theory is not time travel. Let me demonstrate.
Consider an N-jump: the original timeline terminates in 2030 and the jump takes the traveler back to 2000. This is the branch point. Replacement timeline then proceeds unhindered into the future. Since the original timeline is gone into oblivion the traveler can not return to it, but he can travel freely along the new timeline. Sounds familiar?
One may object that in parallel worlds the old timeline still exists, while here it is obliterated. First of all, in some versions of parallel worlds only one timeline can be active. But more importantly, if it is impossible to access the old timeline the question of its existence is a moot point.
In parallel worlds you could go back to the branch point and reactivate the abandoned timeline. You simply need to undo the changes made there. Well, under the replacement theory you can also jump back to 2000 and undo what you did. This will reproduce the old timeline in its fullness including the snap-back in 2030. Technically, this is a ‘new’ timeline while in the parallel worlds you reactivated an ‘old one’. But you will be none the wiser unless you are a philosopher preoccupied with identity of indiscernibles. I trust that the reader can perform similar analysis on infinity loops and sawtooth snaps.
Of course, 30 years into the new timeline our brave new traveler will go back to the branch point to satify the N-jump requirement. Then he can finally rest and admire his(?) handiwork. But do we really know that? Or perhaps, he deactivated the new branch and created a newer one just like it? Who knows, and more importantly who cares. In one respect, admitting the second and subsequent branches is an improvement since it would resolve the identity crisis discussed earlier.
As long as two theories make the same predictions their differences are irrelevant. In terms of predictions, the replacement theory is a variant of branching parallel worlds (broadly construed) with return to the branch one creates.
The only remaining distinction are the snap-backs that cap failed branches, but they are not particularly vital. The spirit is preserved if failed acausal branches are allowed to play out, they are just second class citizens.
Temporal kinetics
The idea of replacement causes seems to be the right one for time travel. If the past can be changed and existing causes removed the only way to preserve causality is to replace them. But the enforcemant mechanism of snap-backs is not only incompatible with relativity and quantum mechanics, but also almost inevitably leads to the pathology of trapped time. Snap-backs themselves should be replaced by more plausible physical processes.
A close analogy is the Le Chatelier principle in chemistry: if a system at equilibrium experiences a change, then the equilibrium shifts to counter-act the imposed change. Le Chatelier principle can predict outcomes of chemical reactions but it does not explain them. To explain the shift one needs to understand physical mechanisms that enforce it. Taken at face value, the principle seems to ascribe to a chemical system a mind of its own. In reality, it is a corollary of perfectly mindless equations of chemical kinetics. Now replace a system at equilibrium with a causal timeline, a change with a causality violation and counter-action with snap-backs that seek to establish a new equilibrium. To wit,
Le Chatelier principle of time travel: If causality is violated by a time travel event, then the timeline shifts to counter-act the violation.
This is the driving motive of the replacement theory, but its implementation there is only a first approximation. To continue the analogy, shift in the chemical equilibrium is not instantaneous. If we solve the kinetic equations we discover that in the process of shifting the system undergoes transient stages that are not equilibria. We can also trace chemical forces that drive the shift.
Temporal kinetics is what I am after. Non-equilibria are acausal timelines that appear as intermediate stages between causal ones. Mr. Young has such high intolerance of them that he is aborting them in the womb at the cost of spontaneous snap-backs with all their dubious physics and supernatural baggage. I am not so radical: as transient objects acausal timelines are not that threatening. We also have to account for a possibility that a new equilibrium can not be attained. In this case I would rather have a succession of acausal timelines than endless snaps and trapped time.
But most importantly, I would like to trace the mechanisms by which time polices itself. Anthropomorphic detection of causal violations with abrupt abortion of the violators at an arbitrary date is hardly satisfactory. It is plausible that in the extended theory we will see key features of the replacement theory reproduced, but the point is that they ought to be explained. Resistance of time to causal violations can not be postulated, it has to be derived. We need a mechanism by which time drives events within timelines towards restoration of causality.
Middle course
In the article Toward Two-Dimensional Time recently posted on Outpost Mr. Young suggests a possible alternative to the replacement theory. In the new theory once root changes are made in 2000 the entire timeline instantly changes to incorporate their consequences. If you kill yourself back in 2000 you will produce a history where your time machine was never built, and your murderous trip never took place. Time will follow through on all the consequnces and wipe out the old you from 2000 as well. But then who is going to kill the young you? Do we get an endless oscillation between two timelines again? Mr. Young himself finds this theory wanting.
But the problem with it is exactly the opposite of the previous one. In the replacement theory timelines are too rigid and snap back if time travelers fail to promptly replace uncaused causes. In the two-dimensional theory they are too flexible with no regard for existing events. There is no resistance at all to making changes, they can be made at will and spread across time instantly. In the replacement theory there is so much resistance that any residual violation snaps time’s back. Perhaps we should follow Daedalus’s advice to Icarus and fly the middle course.
The idea is that there ought to be temporal resistance to changes that do not conform with the future already in place. A good model of such resistance should explain how acausal timelines manage to recohere themselves. If a traveler or his successors fail to restore causality on their own there ought to be a mechanism that compels them or others to do so.
Let us apply this idea to the grandfather paradox. If you go back and kill yourself the timeline will start incorporating this change. Taken to the extreme, it should erase your presence completely. However, the previous history where you were in place, will resist alteration. Since time is not a reasoning entity it will likely take a path of least resistance, i.e. make minimal changes that restore causality. Your spot may be filled by others, they replace you as causes of events already in place. Since you conveniently removed yourself from the timeline in 2030 this does seem like an economical solution. Your double presence in 2000 remains a wrinkle to be smoothed out, but again someone else may take your place as your childhood killer.
This holds assuming that you did not go back after the self-killing but put a gun to your own head. Your return would change things. Now you are alive before 2000 and after 2030 but not in between. This will not do. It makes sense that the wave of changes is strongest near the point of origin, by 2030 it loses some of its strength. In contrast, your return to 2030 triggers a much stronger backlash wave. The two will have to balance out on events between 2000 and 2030. In the final stable timeline your childhood wound is not fatal and you recover to make your ill-conceived trip in 2030.
Working out a physical mechanism behind the time waves is beyond the scope of this article, but it does seem to be a worthy undertaking. It will allow a better reconstruction of time travel plots that do not easily conform to a bootstrap scenario. This includes stories where time seems to resist travelers’ efforts to change it as in the movies Deja Vu and Time Machine. It will also handle multiple interlocking time jumps more robustly and perhaps satisfy our curiosity as to what history ends up persisting. An example is the Terminator series, where the replacement theory predicts infinity loops over minor inconsistencies and does not tell us who ultimately has the timeline, the Skynet or John Connor.
June 15th, 2009 at 11:54 am
Mr. Koshkin was kind enough to provide a copy of this text last week, so I have already made some notes in reply; I encouraged him to post it publicly here so that we could have this discussion publicly, so others could participate and my our points won’t so soon be repeated by others in private.
At the moment I have a busy afternoon, and I would like to make my points as a series of responses to this post, but will need a bit of time to organize them thus.
–M. J. Young
June 15th, 2009 at 5:04 pm
When Sergiy Koshkin first wrote a critique of what he called “the Spreadsheet Theory”, and proposed his own notion of Supertime, with his permission I published his A Critique of the Spreadsheet Theory by Sergiy Koshkin on my Temporal Anomalies web site, and responded to the same in article form there, under the somewhat labored title A Response by M. Joseph Young to A Critique of the Spreadsheet Theory by Sergiy Koshkin. Mr. Koshkin also responded on this site to my article A Draft: Toward Two-Dimensional Time (reprinted on my own site as Toward Two-Dimensional Time), and our discussion appears there.
He now has followed up those discussions with another article; I encouraged him to post it here, so I could respond to it here, and I am now doing so in a series of reply posts, rather than as an article or page, for several reasons. One of those reasons is that I think much of this ground is already covered elsewhere. I further think that Mr. Koshkin and I have an irreconcilable difference, in that to my mind the preservation of causal chains is paramount to any theory of time travel, and to him they are very nearly inconsequential, and thus most of our disagreement arises from this one essential point.
I once read an interview with the famous scientist Freeman Dyson, in which the interviewer asked whether when he was growing up he ever wondered why he was so smart. He responded no, that the question that came to him was why everyone else was so stupid. I do not imagine myself to be in league with Mr. Dyson; yet from his comments and my own observations, I have come to realize that for everyone, it is natural to suppose that whatever comes easily to ourselves is easy, and whatever is difficult for us is generally difficult. Mr. Koshkin seems on the one hand to be very intelligent, and can undoubtedly propound on subjects on which I would be quite lost. The fact that certain points which seem simplicity itself to me escape him completely should not impugn his abilities in the least. I have great respect for anyone who would wade into these waters, whatever difficulties they have in understanding my theories or communicating their own.
In my sequenced responses, I am going to attempt to focus each reply on a rather narrow point from his article.
June 15th, 2009 at 5:05 pm
I should begin by addressing three minor points. The first is in response to his introductory remarks,
“The replacement theory of time travel was developed by Mark Joseph Young to sort out various anomalies appearing in time travel stories.”
It may be that Mr. Koshkin does not mean what that appears to say, but it was never my purpose to develop a methodology for unraveling time travel stories. Rather, it was always my purpose to understand how time might work if time travel were possible, and in so doing I gave consideration to the various stories others had created, delving into what made sense as a real theory and what might have happened had real people been able to do what the characters in the stories did.
Further, the first presentation of the theory was not in connection with the analysis of stories already created, but as part of the Multiverser Referee’s Rules, as a system for adjudicating outcomes of time travel experiments. That is, it was originally written with a forward view, so that in a role playing game situation if a player character were to travel back in time the referee could determine what the results would be, and play it accordingly. The application to movies and other stories followed this, as a way of illustrating the theory which had been presented for other purposes.
It is a minor point, and I am certain that Mr. Koshkin did not mean that I created a theory of time travel so that I could use it to unravel fictional accounts, but only that having created such a theory I did use it that way.
June 15th, 2009 at 5:09 pm
The second clarification arises from this: Koskin wrote,
“Most of the reconstructions on the Temporal Anomalies website feature an iterated version of N-jump, where the first Newbie fails to close the loop, but after several intermidiate snap-backs one of his successors does.”
It is true that in doing film analyses, every effort is made to find a way for the events depeicted to be possible. That is, it would seem that in the original Terminator movie if the machine travels to the past and kills Sarah Conner, John Conner is never born and cannot send Kyle Reese back even a few seconds later. All probabilities say that the Terminator will kill Sarah; yet there is the faintest chance that she might survive long enough to ahve a child, and since the story requiires that that child exists, the analysis says that against all odds she does manage to survive. It does not say that such an outcome is likely, but that it is necessary and possible. Filmmakers usually are not attempting to tell stories which are logically impossible (although sometimes they do), and so the analyses give the stories the benefits of every doubt.
It is also not the case, third, that
“When time is trapped in an infinity loop, no time machines are ever built in CD and nothing special happens in 2030.”
That is, if no time machines are built in the second timeline the result will be an infinity loop, but there are far more ways to create an infinity loop than that.
June 15th, 2009 at 5:10 pm
Now we approach the heart of Koshkin’s arguments.
“The AB timeline has been aborted, obliterated, metaphysically erased. In my opinion, once AB sinks into oblivion its former existence should exert no influence over CD except through the traveler’s presence, which is available at C in the entirety of its relevance.”
The traveler, though, must also cease ever to have existed.
Koshkin has a great deal of trouble with this concept, as he continues, writing
Koshkin is thinking too temporally and misses the sequential nature of causal chains. He accepts that we can change the past, but in so doing misses a critical point: either we can disrupt causal chains, or we cannot.
For example, he writes:
“All that matters is that the traveler’s appearance in 2000 must eventually have a cause within the same timeline. This cause may be a time jump from 2030, or 2050, or 10327. At some point down the line the traveler or his clone must make a trip to 2000 to save causality, but other than that there are no restrictions. Perhaps, time patrol from a distant future will uncover the wrinkle, clone the time traveler, instruct him accordingly and at the appropriate age send him back to 2000. There is no immediate cause for the snap-back in 2030 within the CD timeline, which defeats the purpose of saving causality.“
This causal chain requires that this person with these memories, personality, and character move from this moment to that moment. What Koshkin suggests is that a critical causal chain which broke can later be fixed. This is metaphorically similar to suggesting that if the fifth floor of a hundred story skyscraper is demolished, people on the fifthieth floor can go downstairs to fix it. If the fifth floor is gone, the fiftieth has fallen.
Let us imagine a house of cards. 2040 is built on 2039, which is in turn built on 2038. Our traveler in essence travels back to the lower level of 2000 and removes it. Everything above it falls.
Koshkin is with me this far; but he maintains that it takes time for the removal of the lower level to result in the collapse of the upper. This is where he misses the point: the time traveler is not really removing the lower level, but instead making it such that the lower level was never built. Thus the conceptualization is not that a change in the past must over time work through undoing the future, but rather that the past which was now never was, and the future built upon it now never was built. It can be described as the undoing of future history, but it is more accurately the failure of that history ever to have been created.
June 15th, 2009 at 5:12 pm
Still pursuing the same point, Koshkin turns his attention to another hypothetical:
“What if after dissuading his younger self from pursuing time machines our older traveler decides to visit 2040? Under the replacement theory there is no 2040 in this timeline, so no 2040 for him. But why? He would have had no trouble making the trip under the N-jump scenario, and his younger self still has time to change his mind and enact it. According to Mr. Young, travel to the future is benign and should raise no red flags on Time’s radar screen. But to stop the traveler, not only should CD know in advance that the history will end in a loop, it also has to keep diligent watch for any future travel attempts.”
What is the history of this 2040? The only history 2040 has that can exist is the one for which there is a stable past. The only time traveler who can reach it is the one for whom that stable past is his own history. That version of traveler can reach 2040 and return–but the problem is created by his return, which also alters history. In that original history the traveler who goes to 2040 never returns anyway, because 2040 must be supported by the history from which he was absent. His return creates the new anomaly.
Koshkin has trouble grasping this concept, that the older and younger travelers must be the same person at different points along the same life. He writes,
“Duplication still happens as attested to by descriptions of infinity loops and sawtoth snaps, where the older and the younger self have separate origins.”
In the final version of time, the older and younger selves will have the same origin. The only version of traveler which ever existed is the only version who exists, the final version.
“It is rather mysterious how exactly the old traveler transforms into his replacement. In the new timeline everything is done by the new traveler who originates in it. The old one is shut down as soon as the old traveler steps out of it. So where exactly did he go, or rather wherewhen did he perform the changes that created the stable timeline with everything performed by his replacement? This is supposed to be explained by the spreadsheet illustration: ‘Although the cause of the value at A1 has changed, the value itself has remained the same, and all the values springing from it are likewise preserved’. If it means what I think it means, we have another optical illusion.
I think it does not mean what he thinks it means. In any case, the Spreadsheet Illustration is not more than an illustration, a metaphor that is intended to aid some people in grasping the theory. As to the original traveler, he did not “go” anywhere. He never became himself, but became someone else.
June 15th, 2009 at 5:13 pm
“In the article Toward Two-Dimensional Time recently posted on Outpost Mr. Young suggests a possible alternative to the replacement theory. In the new theory once root changes are made in 2000 the entire timeline instantly changes to incorporate their consequences. If you kill yourself back in 2000 you will produce a history where your time machine was never built, and your murderous trip never took place. Time will follow through on all the consequnces and wipe out the old you from 2000 as well. But then who is going to kill the young you? Do we get an endless oscillation between two timelines again? Mr. Young himself finds this theory wanting.”
Perhaps my ability to express myself is wanting. Although indeed I do find this alternative theory wanting, there is no hint within it of oscillating timelines (as there is in the Replacement Theory). Rather, the point of Two-Dimensional Time is supposed to be that a change in the timeline can impact anything subsequent to it, but cannot impact anything prior to it, including the arrivals of other time travelers from points subsequent in a previous version of time. If there is any hope for such a theory, it will require much more consideration and development than I can currently devote to it, particularly given my lack of enthusiasm for it; but let us at least be clear as to the concepts of the theory as proposed. Two dimensional time might not work, but it does not create the kinds of temporal anomalies found in replacement theory. Rather, it provides something very like the kind of solution Koshkin wants, ignoring causal chains once their effects are established in the past.
I find two-dimensional time wanting for the same reasons I find Koshkin’s Supertime theory wanting: the failure of the causal chain. Those for whom causality is not important might find either theory appealing; I do not.
I thank Mr. Koshkin for contributing his interesting and challenging thoughts, and for permitting me to respond to them. I also thank our Gaming Outpost hosts for allowing us to have this discussion, at least a bit off the beaten track for a hobby games site, here on their space.
–M. J. Young
June 23rd, 2009 at 4:25 pm
I would like to thank Mark for carrying this fascinating discussion on time travel with me, and I would like to join Mark in thanking our Gaming Outpost hosts for letting us off-topic on their space.
“…To my mind the preservation of causal chains is paramount to any theory of time travel, and to him they are very nearly inconsequential, and thus most of our disagreement arises from this one essential point.”
This may be true, but our disagreement on causality is beside the point. Moreover, I am willing to accept any view of causality that works, I am a mathematician after all. This article is about the internal flaws of the replacement theory that condemn its current form regardless of philosophical positions. To recount, they are
1) Snap-backs presume absolute space and absolute time
2) There is no trigger for them (at least) in grandfather-type infinity loops
3) There is no plausible mechanism for preventing future time travel in aborted timelines
4) Snap-backs are hypersensitive to minutia like scientists changing their mind about sending ice cubes back in time
5) As a result, trapped time is nearly inevitable with or without random events
6) There is no resolution to grandfather paradoxes
“The traveler, though, must also cease ever to have existed. Koshkin has a great deal of trouble with this concept…
He accepts that we can change the past, but in so doing misses a critical point: either we can disrupt causal chains, or we cannot…”
To avoid confusion, let us adopt your view of Aristotelian causality: an effect exists only as long as it is ‘pushed’ by its cause. Remove the cause and the effect disappears as well. I have no trouble with this concept, but it eliminates any possibility of meaningful time travel. As soon as Marty McFly accelerates to 88 miles per hour in the DeLorean and his timeline is uncreated, he must cease to exist along with the DeLorean. This is a well-known Loose End problem, that plagues all time travel theories relying on Aristotelian causality. Once Marty successfully arrives in the past the Aristotelian causality goes out the window.
“What Koshkin suggests is that a critical causal chain which broke can later be fixed. This is metaphorically similar to suggesting that if the fifth floor of a hundred story skyscraper is demolished, people on the fifthieth floor can go downstairs to fix it.”
Actually, this is what you suggest, I am simply taking your suggestion to its logical conclusion. Marty does not cease to exist, recognizes that his existence is threatened and arranges for his parents to meet. Thus, Marty ‘repairs the damage he has done’. And if Marty can, why can’t time patrol? Even-numbered snap-backs in grandfather-type infinity loops are more loose ends, there is no Aristotelian cause for them.
You are free to postulate any principles, but when postulated they have to be applied consistently. Once you admit a loose end, you admit meta-causality, and once you admit meta-causality, you can not go back and claim that only Aristotelian causality is legitimate.
Indeed, your entire critique of the fixed timeline theory relies on meta-causality. “An event whose occurrence is dependent upon its own occurrence cannot occur”. Why not? From Aristotelian point of view this is a fallacy, his Unmoved Mover depends on his own occurrence. If we remove the Unmoved Mover, we get an infinite regress of causes, a causal chain as uncaused as a causal loop. Under the circular time doctrine adopted by many ancients, the entire universe is in a perpetual causal loop. Without meta-causality a question of why a causal loop exists is no more meaningful than the question of why anything exists. “…The history of the A-B timeline is necessary, and therefore exists”. Same here, under the Aristotelian doctrine there is no need for the A-B timeline at all, we can simply declare every loop to be an unmoved mover (or as physicists call it, self-existing object).
My conclusion is that in practice you do not take Aristotelian causality that seriously, declarations to the contrary notwithstanding. Nor can you, if time travel is to be had.
“In the final version of time, the older and younger selves will have the same origin. The only version of traveler which ever existed is the only version who exists, the final version.”
This is a strange statement. I hope it does not mean that aborted timelines exist in some metaphysically inferior sense than complete ones. Otherwise, aborted timelines being generic and almost never resolving in happy terminations, the entire time likely only semi-exists. In fact, I would not worry what happens ‘in the final version of time’ at all, it being so rare. The question is what happens in the aborted versions. And there the older and the younger travelers are not “the same person at different points along the same life”.
“The only history 2040 has that can exist is the one for which there is a stable past. The only time traveler who can reach it is the one for whom that stable past is his own history.”
This is all well and good, but the question was what specifically prevents future travel in aborted timelines, not why it should be prevented. Is it a metaphysical hand or banana peels? I can not help but to substitute ‘replacement’ for ‘Fixed Time’ in the next passage: “The Fixed Time theory does not resolve paradox. It ignores it. It makes the unfounded claim that paradox cannot happen. That is at best a hypothesis and at worst wishful thinking…”
“The first presentation of the theory was… as a system for adjudicating outcomes of time travel experiments… so that in a role playing game situation if a player character were to travel back in time the referee could determine what the results would be, and play it accordingly… The application to movies and other stories followed this…”
I can understand insistence on strict causality from this point of view. The referee must have an unambiguous set of rules to steer the game. Moreover, it should be possible to determine the outcomes in a straightforward manner. However, once the replacement theory is extended to other applications the requirements modify. One extension is to deal with various thought experiments from SF fiction and movies. Trapped time in all grandfather paradoxes is a lousy outcome (in fact, it is not so great even in role playing games).
“Filmmakers usually are not attempting to tell stories which are logically impossible (although sometimes they do), and so the analyses give the stories the benefits of every doubt.”
I do not believe this is the case. There are certainly many stories that are logically possible, but not under the current version of the replacement theory (Time Patrol). Even more often replacement reconstructions are artificial:
“I would rather conclude that John’s lieutenants were not killed, because then I have a working time travel story…”; or incomplete: in Minority Report precogs’ visions are dismissed as irrelevant, even though it is quite curious that they can see possible futures so accurately (if they just extrapolate how do they gather the necessary data?); in Deja Vu and Time Machine no special consideration is given to timeline’s seeming resistance to attempted changes. Time vision and time resistance may not be exactly time travel, but a theory of time can certainly be expected to address them.
The next step is to incorporate not just abstract logic, but the entire body of observations that are codified in our science. In particular, the theory must be made compatible with modern physics. This is not all bad, for example predictions no longer have to be as straightforward. Newton’s theory of gravity is well-established, but in realistic situations heavy calculations are needed to decide what it predicts. We do not even have a complete solution to the 3-body problem.
Aside from impossibility of snap-backs under relativity, the replacement theory fails a test required for any physical theory: it is ill-conditioned. In contrast, classical mechanics is perfectly deterministic, but well-conditioned. If you introduce small random disturbances its predictions usually do not change much. When they do, classical mechanics becomes inapplicable and is replaced by theories with averaging, like hydrodynamics, thermodynamics, etc.
But the replacement theory’s predictions are always drastically altered by minor events, e.g. turning N-jumps into infinity loops. This is well illustrated by the reconstruction of Terminator 3 that has a section with a telling title: Killed By Minutia. Even if our world is deterministic, we will never have enough data to take full advantage of it. From the practical human perspective there will always be random noise, if only apparent. Theories that are not robust to noise are of little practical use.
“I find two-dimensional time wanting for the same reasons I find Koshkin’s Supertime theory wanting: the failure of the causal chain. Those for whom causality is not important might find either theory appealing; I do not”.
I am not sure why ‘appealing’ is mentioned here. A theory must work, whether it is appealing or not is irrelevant. For instance, I find Lorentz’s Ether appealing, but it is a defective theory. Aristotelian causality comes into conflict not just with physics (it was discarded by science since Galileo), but even with the concept of time travel itself (by excluding loose ends). No matter how much it appeals to you it has to go. If you can fix problems 1)-6) while interpreting causality to your satisfaction I will be happy to discuss it. But if something got to give, surely experience takes precedence over metaphysical wishes.
August 7th, 2009 at 1:16 pm
It occurs to me to mention that a brief explanation of the Replacement Theory has been published at The Examiner.com this week under the title Temporal Theory 101: What is replacement theory?, part of an ongoing series on temporal theory and time travel there.
–M. J. Young
November 8th, 2009 at 12:07 pm
i agree with young
January 20th, 2010 at 2:26 pm
It seems to me that if Young can’t resolve the problems with relativity, the rest of his idea is irrelevant, because the whole thing is a non-starter. An interpretation of time travel that relies on the concept of a “universal now” is just as useless as an interpretation of quantum mechanics that relies on the concept of energy being continuous.
In Koshkin’s original critique, he says, “In relativity there is only here-and-now for each observer, but there is no universal here and certainly no universal now.”
Young replies, “This is a mistake that students of relativity make concerning their own theory.”
But he’s completely wrong here. This is not a mistake, but the cornerstone of relativity. Simultaneity is relative between reference frames (unless the entire universe is timelike-connected, but that means either a solipsistic universe or a completely acausal universe). It is perfectly plausible for event A to happen before event B from my reference from, but after event B. So, which one of us is right? Well, I’m right in my reference frame, and you’re right in yours, and, because there is no such thing as a privileged frame of reference, that’s all that can be said.
Using the example from the article: When a spaceship is traveling away from Earth at 0.71c, the space travelers experience 2:1 time dilation. (In fact, the article said 0.5c, but I’ll ignore that trivial error.) From your point of view on Earth, while you age 10 years, they age only 5. But at the same time, Earth is traveling away from the spaceship at 0.71c, which means you are experiencing time dilation. From the space travelers’ point of view, while they age 10 years, you age only 5.
How is this possible? You can’t both be aging at half speed, can you? One of you must be right, certainly! No, both of you are right, relative to your own reference frame. If one of you were uniquely right, that would mean you were at rest relative to some universal rest frame, and the fact that no such frame exists is the entire basis of special relativity. In fact, all of the rest of the theory can be trivially derived from this fact, plus the fact that you cannot communicate faster than light.
In special relativity, spacetime is (relative to any particular observer) simply a 4-dimensional grid, where each point is an event, and each object is a worldline through that grid. Objects at rest (that is, moving at the same speed as that observer) are a straight line from the past into the future, perfectly “vertical” (assuming you draw time as the vertical dimension). Objects moving at light speed are perfectly diagonal lines. Objects moving slower than light are slanted lines somewhere between those two extremes.
From the viewpoint of one of those space travelers, spacetime is a 4-dimensional grid where they are moving purely vertically. The axes tilt, meaning that from that viewpoint, you are moving diagonally.
The axis you see as “time” is to them an axis tilted 1/4th of the way into space (in the direction of their travel), and vice-versa. There is not even a universal axis of time, and therefore there can’t possibly be a universal now that is the same for all points in the universe for the exact same reason there is also no universal here (which is an obviously ludicrous concept).
But why, then, when the spacecraft returns, have the space travelers aged 10 years to your 20, even though you are now in the same frame? Doesn’t that mean you were right and they were wrong all along?
No, it means they accelerated and you did not.
If you suddenly accelerated the Earth to an even faster speed (~0.84c) and caught up to the time travelers, you would be the one who had aged 10 years while they aged 20.
Acceleration is where special relativity breaks down and general relativity becomes necessary, and this is a lot harder to explain.
When you observe an accelerating object, it moves not on a line in your coordinate system, but on a curve. Therefore, if motion is relative even in the context of acceleration, an accelerating reference frame must have curved axes. The amount by which time and space are intermixed is changing over time, and of course what “over time” means is changing as those axes curve. This is where the math gets difficult, but the concept should be understandable at least. If you’re accelerating, your future is curving.
Without getting into more details, general relativity explains why the travelers end up aging at half speed through their trip, equivalently (at least once you total everything up) from both your frame of reference and theirs. Which is a good thing, since at the end of the trip you’re both in (virtually) the same reference frame, so you’d better agree.
At any rate, general relativity has all of the same problems with a “universal now” as special relativity, and new ones besides, so you can’t hand-wave it away.
There is some good news, in that general relativity already includes time travel for free, and even resolves all of the causality problems (at the cost of requiring complete determinism). But, whether you use its mechanisms (wormholes, spinning universes, cosmic strings, etc.), quantum time travel, or something completely different, it’s still true that any theory that depends on a universal now is a non-starter, because there is no “same time” across reference frames.
January 20th, 2010 at 2:34 pm
I just realized that another piece of Young’s theory is just as problematic for relativity.
To quote his initial response to Koshkin: “Thus I think that objections based on the speed of light carrying information throughout the universe miss the point entirely. No information need move through space at all; it need only touch the non-spatial temporal dimension.”
There is no such thing as a universal and separate time dimension. There are simply four dimensions. As you draw the axes at any reference frame, there will be one labeled time (and with an opposite sign to the others), but it will not be parallel with the time axis in any other reference frame. So, there is no such thing as “only touch[ing] the non-spatial temporal dimension.”
Or, as Minkowski famously put it, “Henceforth space by itself, and time by itself, are doomed to fade away into mere shadows, and only a kind of union of the two will preserve an independent reality.”
January 20th, 2010 at 4:47 pm
I must thank A. Barnett for his comments on this thread. I am certain I do not grasp all of them, as my physics background is woefully insufficient for some of these questions. The lack of a universal now would be a serious challenge to the theory, although I am not completely certain that it would be fatal to the theory itself–only to my understanding of how time functions universally. For example, most of my efforts to understand and explain time travel stories relied on an image of time in motion, but I did eventually perceive that the theory works as well given a concept of time as static. It may be that my perception of the universal now is not an essential, but that not fully understanding the concept of a universe in which there is no such thing I do not yet have a solution to the problem.
–M. J. Young
January 21st, 2010 at 7:42 pm
I think you could understand all of this using only a Wikipedia-level understanding of the basics of special relativity, but I could be wrong.
But I think I’ve seen the deeper problem behind most of Koshkin’s objections. I think you need to be clearer about the ontological status of the idea of a timeline being replaced. His objections are all over the map and somewhat contradictory because the theory is all over the map and somewhat self-contradictory.
Forgive me in advance for what I’m sure will turn out to be a long, meandering, and poorly organized meander. But I think it’s worth following me–either I’m right, or you can figure out where I’m wrong and from there derive answers to a wide class of critiques.
I think that, however you resolve your theory, it’s going to have to be either Novikov (aka “Fixed Time”) in disguise, parallel universes in disguise, Novikov in disguise implemented on top of parallel universes in disguise, or something either inconsistent or incomplete. On top of that, you’re going to be stuck with a lot of unnecessary baggage that adds nothing to the theory.
Of course that’s not to say that it can’t be useful for pedagogical purposes (or RPG purposes), to describe how the universe looks from the reference point of a time traveler. (The fact that centripetal force is not an extra force doesn’t mean that it’s not sometimes a useful idea.)
I think that the central problem that’s keeping you from getting this straight is that you’re trying to imagine time travel without (general) relativity. GR inherently allows the possibility of closed timelike curves (CTCs). Someone traveling along such a curve can be moving locally into his own future all the time, and yet arrive at his own past. At that point, he could just go back around the curve again, but he doesn’t have to (at least not in 3 or more dimensions)–you can have a worldline that curves off to the side sharply enough that it comes back and intersects itself, and heads forward again normally from there.
For example, you could have a wormhole with one opening in your office in 2030, and the other end in your office in 2000. Just because you walk into it once doesn’t compel you to walk into it every time you get back to 2030. And, even if you do walk into it again, it’s a 30-year-older you following the original you.
Given this picture, everything becomes a lot simpler to understand. An N-Jump is just a worldline with a loop in the middle of it. An infinite loop is a worldline that terminates in a loop (or figure-8, or any other closed curve). A infinite sawtooth is a worldline that goes around and around in an endless helix. And a finite sawtooth just involves going around the loop a few times before going off somewhere else.
In no case is there any need to drag the rest of the world along through some sort of instantaneous change. You might have to drag part of the world into the curve with you, depending on the scenario (and I suppose you could even drag in the entire observable universe if you were really clever), but this can be easily done in a relativistic way. The normal causality of the universe flows out from the future light cone of that intersection (including flowing into the loop, if you’ve gone into your own past light cone).
Before Novikov (or at least before Kip Thorne), physicists believed that any universe with such a loop was impossible. Novikov’s Conjecture is that, as long as history viewed from outside the loop is self-consistent (which the N-jump, infinite loop, and sawtooth are, if envisioned as I described), and normal physics apply within the loop, then the universe can accommodate it with no problems. (Novikov didn’t prove this; he just showed that the known arguments against CTCs don’t apply if his consistency requirement holds, so it’s at least possible for them to exist, given our current understanding.)
This doesn’t necessarily require determinism. You could have an actual choice, upon returning to 2000, between continuing on away from the loop vs. going around a figure-8.
If spacetime is fixed, this choice is determined; if it’s not, it may not be. But either way, there’s no need for mystical forces or cosmic coincidences to prevent you from making the “wrong choice.” As long as every path you can possibly choose (and there could be any number of these, probably even an infinite number) is consistent, everything works. This is the same as our everyday experience. If the universe is determined, then there’s only one path for your worldline to follow; if not, there are multiple possible paths. Adding loops to the possibilities doesn’t change that.
It’s true that the paths are restricted to those that generate consistent histories. But I don’t believe RT can accommodate inconsistent choices any better than Novikov. There are only 3 possibilities in RT, and all 3 of these can be explained in a Novikov universe.
As a digression, I’d also like to point out that determinism and free will are not necessarily incompatible, and you could even argue that free will doesn’t make sense without determinism. But I can’t explain that, short of pointing to Daniel Dannett’s books “Elbow Room” and “Freedom Evolves” and the various philosophical debates that have followed on from them.
The biggest difference between TR and Novikov is that the original timeline never existed. There may be multiple paths that you can choose the second time around in 2000, but there is no path that involves you not being in 2000 if you find yourself in 2000. (Your local 2000, or even two different local 2000s, could easily be in the middle of the CTC, which would be necessary for certain endless loop scenarios. Otherwise, all of the stuff you interact with wouldn’t be part of the infinite loop. But even those aren’t parallel universes; there are just two areas of spacetime that are both 1 year later than 1999, both existing in the same global spacetime.)
That eliminates the ontological problems with RT. What does it mean for a timeline to cease to exist? Ceasing is something that happens in time; there’s no before and after outside of time. A segment of time that doesn’t exist just never happened. Moreover, if both timelines are in any sense ontologically real, then what you’re describing is a multiverse, not a single universe. For RT as described, you really have to pick one or the other, but neither one is acceptable.
It also eliminates a serious causality problem. If the time traveler remembers the original timeline (and aged, etc. along that timeline), but that timeline doesn’t exist, then there are effects without causes. If that timeline does exist, not only do you have multiple universes, you have cross-universe causation. But if time travel all happens along a single (locally timelike) timeline with a loop in it, through a single universe, as described by GR, normal causation works around the loop just fine.
In summary, the concept of universe replacement is either inconsistent with physics, a convoluted way of disguising perfectly normal causation in a universe with CTC and subject to Novikov, or a convoluted way of disguising multiple universes. You need to make it clear which one you mean–and, once you do so, you won’t need it anymore (except maybe for pedagogical or story-telling purposes).
On a side note, the supertime idea doesn’t seem to add anything useful. The issue you’re trying to solve is that you want local (total) causal ordering even when you don’t have chronological ordering. But that problem is irrelevant. You already have a total chronological ordering for any worldline without an infinite loop. And if you want to order an infinite loop for some reason, you can trivially transform it into an infinite helix where each iteration is indistinguishable from the last. And if supertime is actually a second time dimension–well, either you’ve got 3+2 relativity, or a 5th dimension that doesn’t participate with the other 3+1 (which is going to require some really odd topology), but either way this just makes it harder to create a total order, not easier. Plus, it’s hard to see how that’s any different from taking an array of parallel universes and slapping a metric on them.
January 21st, 2010 at 7:58 pm
By the way, on the static vs. dynamic time issue (does time flow, or do we move through time): These are really equivalent pictures, that can be derived from each other once you eliminate the usual semantic confusion.
As for which one is “right,” well, philosophers have been arguing that since the ancient Greeks (and, more usefully, since JME McTaggart’s “Unreality of Time” in the early 20th century clarified the issue).
In fact, many people studying quantum gravity argue that spacetime (and therefore time) can be derived (as Antiphon, Leibniz, etc. historically argued, but obviously couldn’t physically prove) from relations between events, and that both of these pictures are further-derived approximations.
And Julian Barbour in “The End of Time” (not the Doctor Who finale, BTW, although I did love that) argues that they are two different illusions that in a completely timeless mega-dimensional configuration space (as Parmenides and Buddha tried to articulate, but without the mathematical machinery).
So, don’t get too hung up on which one is “right”; just make sure you always know which view of time you’re using in a particular argument (not to mention for which observer).
January 22nd, 2010 at 8:19 pm
Again, thank you. I’m really pressed for time at the moment, so I can’t do this justice, but I wanted to address a few comments.
You wrote, “For example, you could have a wormhole with one opening in your office in 2030, and the other end in your office in 2000. Just because you walk into it once doesn’t compel you to walk into it every time you get back to 2030. And, even if you do walk into it again, it’s a 30-year-older you following the original you.”
My concern about the traveler entering it again is here: in 2000 I was 45, and in 2030 I will be 75. If I travel back to when I was 45, I am 75, but my 45-year-old self is there. Assuming I live to be 105, I will still be alive in 2030 when I departed for 2000, and my younger self will now be 75. What if he doesn’t enter the wormhole? What if he does? I expect that the outcomes must be different. Fixed time tells me that he cannot fail to do so, because I already arrived in the past; parallel dimension theory tells me that’s irrelevant because this is a different universe anyway. In replacement theory, it is possible for him not to enter the wormhole, and it has consequences to this universe.
You wrote, “As a digression, I’d also like to point out that determinism and free will are not necessarily incompatible, and you could even argue that free will doesn’t make sense without determinism.”
I am comfortable with that. I figure that whatever I choose is based on my free response to all the influences which impact me to that moment, and were I to live through that moment again with exactly the same history and influences, I would choose the same way. Not everyone is comfortable with that idea, but it is necessary for replacement theory to resolve.
You wrote, “If the time traveler remembers the original timeline (and aged, etc. along that timeline), but that timeline doesn’t exist, then there are effects without causes.”
It’s certainly complicated, and I’m not completely certain I am right on all this; but I do insist (see my Back to the Future analysis) that the only time traveler who exists after the point of departure is the one for whom through the final version of history is his only history, and thus unless there is some mechanism for transferring memories from one self to the next, there are no memories of the universe that no longer ever was.
I also agree that the concept of a history ceasing ever to have been is difficult to grasp in either temporal or atemporal terms, and is one of the most difficult aspects of replacement theory. I tend to analogize it to history being a recording on a video tape, in that as the new history is created the old is erased; but this does not explain the memories the time traveler might have if he came from the original history and created the second. That problem resolves once time stabilizes, though, since for time to stabilize the traveler who leaves from the future is identical to the one who arrives in the past, having lived through the same history, which is the final form.
I suspect that your objections are beyond what I can answer, but I hope this helps you understand a bit more clearly what I am saying.
–M. J. Young
February 2nd, 2010 at 3:11 pm
I realize that I am conversing with those whose grasp of relativity is perhaps an order of magnitude better than mine, and so any challenge I raise would be more that of the secondary school student to the college professor. However, one point that has been made against me is the notion any effects of changes made to time would be limited in the rate of propagation through space to the speed of light. It is understood that nothing can travel faster than that.
However, I have identified two situations in which this appears to be falsified under relativity.
The one of which I am more certain is the one that might prove less. It is established (theoretically) that an object passing through a wormhole emerges from the other end of that wormhole without passage of time from any perspective, “instantaneously”, without regard for the apparent distance. This suggests a sort of spacial folding, in most minds, that two points in space which appear to be at vast distances from each other are actually touching. The relative time of these two points is irrelevant to the question; using a wormhole it is possible for an object to impact a distant point in space sooner than would be possible at light speed.
The other is a quantum concept of which I admit having only a very weak grasp but which intrigues me greatly. According to quantum non-locality, it is possible for a particle to exist in two places at once. As it does so, whatever happens to it at either location happens at both locations. I am given to understand that distance is not a factor in this, and it follows logically that since it is the same object existing at different points, the impact on either would be instantaneously felt at the other.
These two concepts demonstrate the possibility that “instantaneous” transmission of information or effect is possible, escaping the limitation of the speed of light. I do not propose a relativistic or quantum mechanism by which time could be so universally affected; I only note that it is not a priori limited, as particularly Mr. Koshkin insists.
The problem of the lack of a universal “now” is a separate issue; but I am not addressing time travel as a physicist but as a metaphysician, trying to grapple with the issues of causality once temporal sequence is disrupted.
–M. J. Young
February 4th, 2010 at 7:48 pm
Wormholes:
There is a passage of time from the object’s perspective (or any other), namely the time needed to traverse the wormhole. Instantaneous wormholes can not exist, any wormhole has a finite length that can not be traveled faster than the speed of light. It may still be much faster than the light would take to travel through the “apparent” space, but this is not moving “faster than light”. It simply means that the observer missed the wormhole and the actual distance between its mouths is much shorter than she thought.
This concerns wormholes that already exist in the universe (none was ever seen). But transmission of information to prescribed locations would require creating wormholes at will. There are doubts that this is even theoretically possible, and at any rate it would take a long time to create one. Replacement theory on the other hand, needs creating wormholes to every other point in the universe instantaneously.
Quantum non-locality:
Quantum non-locality is a metaphor used to explain certain counter-intuitive features of quantum mechanics. Since we are used to classical objects it may be didactically helpful to say something like: if quantum particles were classical (which they are not) then they would behave non-locally. But no, it is not possible for a particle to exist in two places at once, or instantaneously feel a spacially distant impact. In fact, it can be proved that no transfer of energy or information is possible through ‘quantum non-locality’.
February 5th, 2010 at 3:40 pm
Re: wormholes: the point is that it is possible for a local action to impact a distant point in space at what appears to be a rate faster than light speed. It is not that replacement theory “creates wormholes”, but that if one means of connecting distant points in space such that effects can occur instantaneously (the object entering a wormhole theoretically passes through zero distance to the exit, and therefore requires no time to arrive at the other end) there might be another.
Re: quantum non-locality: and yet it was on an a Institute of Electrical and Electronics Engineers (I-triple-E) list of technologies likely to have practical application in the twenty-first century not so long ago, so it is more than a metaphor. What is possible has yet to be determined. (Teleporter technology has been suggested.)
–M. J. Young
March 9th, 2010 at 2:04 am
There is one version of time travel I have only seen once. It’s difficult for stories and extremely difficult to pull off in an RPG.
Time travel is possible, but you can’t change the past. Everything happens, because it has happened.
I saw this in a german pulp called ‘Perry Rhodan’.
A short version of what happened in a storyline:
They got into contact with a hostile empire from another galaxy. Travelling over there they studied their opponent and found out that there was a ‘realm of happyness’ 160.000 years ago. They also find bones of humans. Very old ones. Checking the genes of them with the archives leads nowhere. By accident they are sucked into a time travel by a group that wanted to change the past by stopping the empire from forming. Having their own experience with time travel, they know that they will loose people here. They already saw the skeletons (Which they did not check against living crewmembers. Who would?). So, they have a good view how the realm of happyness ends and the empire they have gotten to know emerges from it.
As the series is now going for nearly 50 years, the writers had time to build up their rules for time-travelling, integrate it into the story and make some very good plots with it.
You see, the main characters (being unaging thanks to a higher being) are not sure time-travel will always work like that. It’s only, that up to now this is how time-travel always worked out up to now. They of course also have the problem if someone come from the future and told them how their works will lead to a cruel regime someday or something equally dark. If time travel really works like this, they would not be able to change it. There is also the problem, that history may self-correct itself. Travel back and kill Hitler could result in another individual taking his place which leaves history unchanged.
All in all, this method stops all the technical problems with the usual way time travel works (timelines and such), but opens up a whole realm of interesting stories.
Travelling back to change history, only to discover something has negated the change for example and still having to deal with the original problem.
March 9th, 2010 at 3:14 pm
Marko–Thanks for your comments.
What you describe is called http://www.examiner.com/x-15701-Time-Travel-Movies-Examiner~y2009m7d27-Temporal-Theory-101nbsp-What-is-fixed-time-theory Fixed Time Theory, and there are many stories which are said to be Fixed Time, including http://www.mjyoung.net/time/monkeys.html 12 Monkeys, http://www.examiner.com/x-15701-Time-Travel-Movies-Examiner~topic655762-Terminator the original Terminator films, and http://www.mjyoung.net/time/somewher.html Somewhere in Time. (I disagree in each instance, holding that http://www.examiner.com/examiner/x-15701-Time-Travel-Movies-Examiner~y2009m8d6-Temporal-Theory-101nbsp-What-is-replacement-theory Replacement Theory better explains the events in each case.) It is one of the leading theories concerning time travel, but it’s difficult to tell interesting stories without incorporating something like http://www.examiner.com/examiner/x-15701-Time-Travel-Movies-Examiner~y2009m8d27-Temporal-Theory-101nbsp-What-is-a-predestination-paradox a Predestination Paradox, in which something in the past happens only because it causes the events in the future which cause it. (All three films mentioned contain such a paradox.)
I find the theory implausible because of causality and its rejection of free will. To accept that the past cannot be changed demands that the future also cannot be changed, that all events are predetermined and our lives are simply exercises in discovering what we did. I also find its failure of the causal chain irrational. That is, Tom is born in 1980, and in 2010 travels to 2000. The Tom who arrives in 2000 already experienced the events between 2000 and 2010, and therefore they already happened before he could arrive in 2000; yet from the perspective of 2000 they have not happened yet. Thus to my mind there must be a version of history between 2000 and 2010 which does not include the arrival of Tom who has, from his perspective, not yet departed from 2010; and that means that that version of history is altered the instant Tom arrives, from a history in which no one arrived from the future to a history in which he did.
Absent that, life is not a series of choices nor a chain of causes and effects, but simply a mosaic pieced together either by a brilliant divine artist or by a completely random scattering of pieces which only give the illusion of scientific order. I prefer to believe that we are actors in our lives, not puzzle pieces. The alternative is, of course, that you and I are not really “having a conversation” in the sense that we mean, but that you were destined to write your post and I to write mine, quite independently of each other. That’s what fixed time theory really means.
–M. J. Young