Max quoting me wrote
In short, my AB self did not travel to the past, but to a different universe that only looks like the past.
What if it is the same universe that looks like the past?
I was describing multiple dimension theory in that quote (either parallel or divergent dimensions); if the time traveler lands in the past of the same universe, then it is either fixed time theory and he cannot change anything and will only discover that whatever he does here is the cause of the events he wanted to undo, or it is replacement theory and we have to deal with the anomalies according to the rules of that theory of time. The latter is my default assumption in all time travel, and what the book recommends if the world description does not specify otherwise.
Max further wrote:
The AB self have been born in the time before the rewriting takes place.
This is the disconnect. The AB self was born in the original AB history, the "time before the rewriting takes place"; but the "rewriting" involves erasing everything that happened in the AB history. That means that anyone born in that time was never born; and anyone who was never born does not exist. The AB time traveling self has been erased and replaced by the CD time traveling self. This happens over the course of the CD timeline, but by point D, the AB version not only no longer exists, he now never has existed. Anything he ever did was done by the CD version, or it was never done.
This will beg the question who is the real self right?
That depends on the time machine.
It does not depend on the time machine at all. It depends on the nature of time itself.
But it occurs to me that when you said
What if it is the same universe that looks like the past?
You might have meant something more like
What if no time travel has occurred at all, but instead by some means the present state of the world has been altered to match what the universe that looked like at a specified moment in the past?
That would mean that in 2010 I decided I wanted to go back to 2000, but instead traveling back to 2000 I caused everyone to forget everything that happened over the course of that decade, undid the births of everyone born in that time and the deaths of everyone who died in that time, undid every cause and effect which occurred, realigned all the stars and planets to their positions in 2000, and youthened everyone except myself to our ages at that time.
It is an absolutely insane notion, but it might be possible. It raises some issues concerning the natures of time and space and their relationship to each other, and whether you can undo entropy that way (perhaps if you are using magic, you could draw an immense about of supernatural energy to do so; a technological solution is definitionally impossible here). If you missed something--radioactive decay somewhere in the universe which could be and/or had been detected and recorded, for example--you would leave traces to prove that the universe lost ten years.
I would argue that this is not time travel, though, and so if that's what you mean we'll have to address it as "something that spoofs time travel".
If somehow AB me gets past point D, he ceases ever to have been; only CD me (or the final version, whatever version that is) has the ability to reach the future beyond the end point of the anomaly.
That depends on the time machine.
No, again, it depends on the nature of time, not the nature of the time machine.
If time works by erasing what happened and replacing it with the new version (replacement theory), then the version of the time traveler who came from the AB history has erased his own existence, not as a function of the time machine but as a function of time itself: he was never born, and so he cannot now be.
If time works by preventing any change to the past, then the question is moot: the AB timeline has always included the presence of the traveler from the future, and there is no CD timeline nor a CD version of the character, and nothing that the time traveler can do will change anything that ever happened in the world from which he came.
If time works by shunting supposed time travelers to a different universe, then the traveler vanishes from the original universe never to return and duplicates a version of himself in the target universe. If that duplicate version also makes the same trip, everyone in this universe is deceived into believing that the original universe version is the second universe version arriving from the future, and in all probability the same thing will happen in the third universe; but the first universe will realize that time travel is a failed experiment, that the traveler simply disintegrated never to be seen again.
It is also more likely that the first universe traveler will be different enough from his second universe version that the second universe version will never make that trip, which means they will both exist as independently existing people, akin to identical twins but for being different ages and having the same fingerprints.
That depends on how you define time. Time might be only an illusion of moving space. If a space is reversed then do you define that as time travel?
I think I can demonstrate that time is a dimension very similar to the three spatial dimensions. For our purposes here, though, let's just say that "reversing space" is another word for "reversing the direction of the movement of the universe through time", which is a form of time travel. Yes, I can reverse causality such that the glass which just spilled and broke rises and reforms, but doing so is not different from making time go backwards in the area of the glass. How that impacts the rest of the universe might be a significant issue (as is whether having reversed time for the glass it can then avoid the accident which lies in its future), but it's not unresolvable.
If there is no past/future and there is only the present, "time travel" would have to be redefine to "space/time causality manipulation". This is because if the "time traveler" perceive that they come back to the past, they still have experienced what they have experience before and what will happen next will not change their previous experience.
This would for game purposes be time travel; it would involve isolating the traveler from the causal stream, which constitutes isolating him from time, and then adjusting the flow of the causal stream as desired, which constitutes temporal mechanics. Simply redefining the concepts does not get you out of the problems of time travel.
If for example the time traveler causality depends on the action the time traveler will take in the future, this will make the time travel itself improbable because there is no future yet for CD time line. How can the time traveler said that he travels from the future if the future have not happened yet?
Max, I've got an entire web site addressing issues like this, and I feel foolish using my time to address them again here when I could just tell you to go read the hundred or so pages of time travel work I've already done.
In one sense, the AB time traveler is "instantly" unmade at the moment he enters the CD timeline; in another sense, that event happens at the moment he leaves the AB timeline. But I've put a lot of work into explaining all that elsewhere, and should be running game threads here, not answering questions I've already answered.
For example, the time traveler uses the time machine and succeeds in going back to the past but then dies without changing anything.
This is a fundamental error a lot of people make. If the time traveler succeeded in arriving in the past, he changed the past. In the original version he was not there; in this version, he was. His death does not prevent him from having left an impact in the form of his decaying corpse. If somehow his corpse was snatched away, his mass has caused ripples in the atmosphere and through the earth's crust. If he traveled non-corporeally, his image may have been seen by some creature who moved to avoid him. Anything which arrives in the past changes the past simply by its arrival.
The error is confusing "significant" changes with any changes. Because of the butterfly effect, we can't really know what changes will be "significant". We hope that whatever the time traveler did does not change anything that "matters", and in temporal terms what "matters" is anything that is necessary to support what the time traveler did. That is, if the AB time traveler went from 2010 to 2000 and then died, then the CD traveler must also go from 2010 to 2000 and die, because if he doesn't then we have two different histories, one in which the time traveler arrived and died, and the other in which he never arrived, and even if that is the only difference between the two histories, as long as each supports the other we cannot have a single stable version of history and thus we cannot have a future built on a single stable version of history. Time ceases.
I hope this helps.
--M. J. Young