I really don't think the time continuum makes assumptions. It's more like the time continuum plays out the events that would have happened after the time traveler left, and where the time traveler ends up, that would be their end result.
Originally Posted by Chyron8472
So when Marty, Doc and Jennifer went to 2015, it was the 2015 that had no other Doc. If Marty and Doc had gone together to 2015 first, the future would have been laid out without either of them just as it was laid out for one minute with no Einstein at the beginning of BTTF1.
While I understand what you're saying, that also is flawed. Think about it like this.
When Marty travels back to 1885 from 1955, he does so because of the Tombstone and seeing as how Doc was murdered. Now, when Marty went back in time, he altered the past and the Tombstone was then Blank, and then the tombstone disappeared. This, in itself, caused a paradox in which Marty should have no longer been in 1885 because the tombstone was no longer present, Doc wasn't murdered, and he had no real reason to return to 1885.
It seems as though, at certain points, BTTF time travelers live outside of the 4th dimension's logic and reasoning. And, if you think of time like a river, then there are some events that won't necessarily alter future events to the point where a paradox would occur.
I think, when Marty and Jennifer went to 2015, the time continuum did change, however with their pre-determined chain of events, the time continuum "knew" (for better lack of a term) that they would eventually return to 1985 and the future timeline wouldn't be interrupted. Hence why they could be in two places at the same time. Now, if 1985 Marty were to die in a horrible accident while visiting the future, a paradox would occur and the future would be erased.
So, it appears that one can travel to the future, a future where they still exist, so long as they do not do anything to endanger themselves to the point where they may alter the future, or risk not returning to the "present" at all.