While scientists debate whether fixed time or parallel dimensions better explain time travel, most stories use some variant of Replacement Theory. According to replacement theory, if you travel to the past you arrive in your own past, and what you do within that past may have consequences to your own future and the future of the universe. Thus you can change history and impact your own existence, but the consequence is that you can change history and impact your own existence.
Under replacement theory, it is assumed that no one can arrive in the past before he departs from the future, in a sequential sense; and that this sequential sequence is tied in the original history to temporal sequence. This means that there is always an original history, a version of events in which no one and nothing arrives from the future because the future has not yet been reached. The time traveler departing from the future then arrives in the past and alters that original history, in essence erasing it and replacing it with a new version. In so doing, he also undoes his own existence in the original history, and so at the moment that his departure time is reached he undoes that departure; this in turn undoes his arrival in the past--unless the version of himself in this altered history makes that same trip to the past, and so becomes the cause of his own arrival.
The theory is frequently highly rigorous, following strict rules related to maintaining causality. That is, it is certainly possible to travel to the past and kill Adolph Hitler, but you must be careful to ensure that you do not also prevent yourself from making that trip, because as easily as you can undo Hitler's legacy of horror you can also undo your own existence. Under rigorous versions of replacement theory, once you have undone your own departure to the past you have also undone whatever changes you made in the past, including the fact that you changed your own history. Time thus becomes caught in an infinity loop, something akin to a Mobius strip in time, alternating between two versions of history. Other possible anomalies under replacement theory include the sawtooth snap, cycling causality, and N-jump, each of which will be examined in detail in future articles, illumining the theory concepts further.
Some less rigorous versions of the theory tend to gloss over the problem of the impact a trip has on the traveler himself. That is, once the traveler has made changes he might undo his own birth, but he does not undo his own arrival in the past. These logical inconsistencies are answered by falling back on the notion that once the past has been formed it can only be changed by someone coming from the future, and so what the traveler did in the past cannot be undone except by someone traveling from the future. In short, the only event you cannot prevent is the arrival in the past of a time traveler, even if you successfully prevent his departure from the future. Such versions of the theory begin more to resemble divergent dimension theory, in that the traveler comes from another branch of time which still exists without him; he has changed history, but has not actually changed his history.
Other versions of the theory attempt to limit the impact a traveler has on himself, such that he has created his own history of the universe which applies only to him. Most of these are not significantly distinct from divergent dimension theory, but in others the traveler builds up for himself something (usually called "paradox" but distinct from the use of that word otherwise) which eventually can undo his own existence as he becomes progressively more separated from reality and connected to his own in some sense unreal universe. (It is unreal because it does not exist for anyone else, and thus the people in it are not the real versions of themselves.)
The long list of movies in which replacement theory of some form seems to be in view includes Back to the Future and Back to the Future Part III, the several Terminator movies, Millennium, Flight of the Navigator, Frequency, and probably Star Trek IV: The Voyage Home. Many stories which appear to work under fixed time theory and some which appear to work under parallel or divergent dimension theory work as well under replacement theory. The extensive award-winning web site Temporal Anomalies in Popular Time Travel Movies relies on this theory in its analyses of many familiar and lesser-known films, and is an excellent source for information concerning this theory.
Next time we will begin examining some of the terminology used in analyses of such stories, beginning with the phrase temporal anomaly.
Comments
Self-proclaimed expertise is usually a very bad sign. Therefore, I was very wary of even reading your post on your own superior time travel theory. I must state your schism on replacement theory is a fatalistic and thinly disguised parallel universe theory, poorly adapted and supported by mere verbosity and ad vercundium. "Spontaneous snapbacks" which recreate a timeline that had been "erased" and thrown into "oblivion" are illogical, arbitrary, and could only result from an act of God.
Replacement theory as you describe it,is non-sequential though erroneously labeled as "sequential." This problematic theory is inferior to anything I have read. Is there any evidence that someone has agreed with your journal besides some sort of internet editor who approved this without comprehension? If so, please list such sources or perhaps consider labeling this page as fantasy time travel.
The other pages are good but too similar to Wikipedia with poor synonyms such as "Sawtooth."
uhh dude... i think it's a sci-fi concept. wtf cares what kind of terminology is used? Plus, when observing a movie (in our flow of time), like The Terminator, one sees the events in one flow of time wherein there is instantly one history. This is because it is described in film. So while viewing one scene of the movie, one is fooled into getting a sense of a 'snap-back'. So, yes, it's really parallel universe theory, but the adoption of this theory is just a trick of the mind.
Thank you, Dr. A., for your comments. As to my "self-proclaimed" expertise, as far as I have been able to ascertain, although stories which seemed to follow replacement theory had been written and filmed, my own explanation of it was the first version published, and the site has been cited and recognized since it went live over a decade ago. It is an awkward situation, to some degree, to have to present the various theories one of which is "my own"; I cannot easily omit it, and my efforts to be impartial here apparently were not sufficient to your liking. I did not read the Wikipedia articles on this.
It should also be noted that the opening sentence of this article connects the theory to "stories", and the closing paragraph cites several which appear to follow it.
As to the logic of snap-backs, since those are discussed in future articles I refer you to the article on the Infinity Loop, where I address a similar concern.
Thanks again.
--M. J. Young
As a footnote, others have arrived independently to very similar understandings. Some of those are cited on the Temporal Anomalies web site linked above, and at least one other in this article (the Mobius Strip link).
--M. J. Young
Interestingly, in an independent thread on a forum I frequent (Gaming Outpost), someone cited Larry Niven on this subject:
"Niven’s Law of Time Travel...: 'If the universe of discourse permits the possibility of time travel and of changing the past, then no time machine will be invented in that universe.' The reason is that if it’s possible to change the past — and, thus the present — change will continue until it reaches the stable state in which no time machine is invented anywhere." The poster then cites Hans Moravec to the same effect.
That's clearly replacement theory; however, it overlooks the problem created by changing the changes. See the infinity loop article for more on that.
Thanks.
--M. J. Young
Your pogram against acceptable theory is a puerile effort to infect weak minds along with pompous claims that Hollywood has emulated your invented theory, which has been put forth by others I must say long before you with similar degrees of failure and lack of insight. Your baseless statements only further prove that there is no genuine support in the intellectual community for RT theory. One only needs to examine miyoungnet movie reviews to prove you fail to grasp the fundamental concepts of time travel. Sorry to burst your bubble so strongly but that is the inherent nature of such antiphonal blogs. At least you get the last statement to pretend that you can see clearly that you are correct in your own eyes and that everyone else must be of inferior intelligence unless they support RT theory. Dr. A
Wow. Thank you for your candor.
There being no experimental data in the field, all time travel models are theoretical. They are debated based on whether they are consistent with other knowledge. There are flaws with every model, and adherents to each find the others lacking. I have attempted to be even-handed in my treatments here; perfect impartiality is difficult for anyone.
You have me at a disadvantage, as you are cloaked in anonymity and so can snipe from hiding. My credentials, such as they are, are easily discoverable. Replacement theory is the only theory of time I know under which you can change your own history. There are disagreements concerning the scope of the consequences of doing so. I accept that I might be wrong; you do not accept that I might be right. That's fine. What's your theory?
--M. J. Young
Truce. I prefer theory that accepts that the existence of a timeline that once existed can never be completely erased. There must be some remaining detritus matter, some intellectual effulgence, or else it never truly existed. This is the only page I disagreed with, and so compliment you on the rest of your works. Public acceptance is a difficult bird to catch. Good hunting to you.
You use rather flowery language, but i suspect you're just a prude. Perhaps a literary man? It'd be prudent to make note of how many times you've used the word 'that' in each sentence. I agree with you, regardless.
Welcome to the conversation, Hajad Arid. It is not entirely clear to me, however, whose writing you are addressing here. Either way, a charge of euphuism is difficult to maintain. The subject itself being difficult, it is important to write about it clearly, and that sometimes requires such steps as the careful and clear use of pronouns and connectives. What Dr. A. says is stated clearly enough, and I hope that my own statements are similarly clear.
Thanks for your comment.
--M. J. Young
Truce accepted, and compliment also. I am intrigued by theories in which "detritus" from prior histories is retained, but I find them difficult. For example, with Back to the Future, Marty grew up in the home we see at the beginning of the film, then went to the past and changed it; he then grew up in the more affluent home, and also went to the past. Which Marty awakens at home the next morning? I say it has to be the affluent Marty, who is not at all surprised at the world in which he lives. But the original Marty is shown in the film, and he does not make sense to me.
Thanks for your comments.
--M. J. Young
I cant say I have any clever comment to your postings M.J Toung at the moment, but I found your theories and writings very interesting. I read almost all your articles of time travel movies from your site, and feel like your almost a guru :) I always been a huge fan og time travel movies, ever since i saw Back to the future for the first time. Keep up the good work!
Thank you, Lars. I'm always encouraged by those who appreciate what I've written. Let me know if there are movies you'd like analyzed, and I'll keep an eye open for them.
--M. J. Young
I am dismayed and alarmed that this can even be believed by human beings who owe their existence to a God that created them. Replacement theory flies in the face of a loving God who created us and sent His Son to this earth to live without sin, die a death that redeems us for eternity and who returned to Heaven to prepare a place for us. When He says enough sin in enough He will declare it at an end and come back to this earth to take the righteous to Heaven. After a 1000 years He will come back and reestablish this earth as it was in the beginning before sin and sorry started. This is from a book, The Bible, that can be proven reliable due to the prophecies that were made hundreds and even thousands of years before their fulfillment. Only those who accept Jesus Christ as their savior will inherit life. All others, sinners who have not repented, will be destroyed forever. The new earth will be the eternal home of the saved. I appeal to you to consider this for yourself.
Thank you for your comments, Hubert, and for your concern for me.
As I have written elsewhere, Christian "credentials" are never much use--they are never the things that matter to Christians. That said, permit me to give you some of mine. I have two undergraduate degrees in Biblical Studies, the second from Gordon College (the first from Luther College of the Bible and Liberal Arts in Teaneck, long gone), and I have taught undergraduate studies in the field. I serve as Chaplain of the international Christian Gamers Guild. My ministry "credits" include having written several books and more web pages, smuggling Bibles into Romania, performing with evangelistic and ministry bands from Maine to Maryland to Western Pennsylvania, and appearing regularly on several Christian radio stations. I cannot easily prove to you that I am a believer, but it is a fact about me that annoys a number of atheists and agnostics who can't understand how a person of my intelligence can hold such beliefs. (For more on this, http://www.mjyoung.net/bible/bible.htm will connect you to some of my more accessible writings.)
As to my belief in and defense of replacement theory, what I for one set of reasons expect and for another set hope on the subject is that time travel will never be possible. In a sense, that means I hope God will not permit it. However, there are many things people thought God would not permit which He apparently has--splitting the atom, flying to the moon, world war. It is not a particularly safe claim to make, that He will not permit this or that. Thus my approach has always been "if time travel is possible, what are the consequences?"
Perhaps, though, there is enough interest for me to write an article on the theological ramifications of time travel theory. That might be a bit off-topic for The Examiner, but since you've raised the question I'll give it some thought.
It is to be hoped that you are correct, that Jesus will return before we learn how to travel to the past. Again, while the hope of His coming is an important part of our faith, it is evident that we have been eagerly anticipating it "any day now", as Andre Crouch put it, since Paul brought the gospel to Rome. It would be a mistake to suppose that that return will save us from any specific possible future, particularly one that would be of our own making. We ought to be ready to go, but also to stay.
Thanks again.
--M. J. Young
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