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Marty has always been inexplicably protected from the changes in the timeline. Thus he operates with a memory of the original timeline in which the "time machine" was never invented (or never used).

In Back to the Future when Marty returns from the past; Marty is unaware of the following changes to the timeline that his meddling has caused:

  • that his father wrote a book
  • the socio-economic status of his family members
  • the truck

These things don't make a lot of sense if Marty is traveling through time in the same universe, because the instant he changed something he should immediately be converted into the updated Marty which already knows all these things. But he isn't. Marty is just lucky he arrived in a universe where his doppelganger (the Marty native to that universe) also jumps in a "time machine" and disappears with a flash.

In Back to the Future 2 Marty travels to the future and his actions result in an old Biffy traveling back to 1955 and changing history. Subsequently When Doc Brown, Jennifer, Einstein, and Marty attempt to return to 1985 they think they are in an alter timeline. Things that suggest Marty is in parallel universes:

  • Marty encounters himself in the future, but our Marty never returns from this future, so this future Marty shouldn't exist (he should have disappeared in 1985 never to be seen again, just like the original universe from which original Marty left).
  • When Marty returns to 1985 there is another Marty in this world already. The one who is attending boarding school somewhere.
  • In this universe Doc Brown never created the "time machine" and Marty never went back in time.

I'm thinking when old Biffy went back in time, he did so to another universe, one which he changed but for which events didn't depend on the creation of a "time machine."

I'm putting "time machine" in quotes because it now seems to me that the machine really travels between parallel universes AND time.

In Back to the Future 3 Marty travels to 1885. This same Marty (and Doc Brown) have knowledge of Clayton Ravine and its history, despite their meddling which causes this future to no longer exist.

Has Marty been hopping through parallel universes all this time, never returning to his original one?

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5 Answers 5

No.

Marty is not traveling to parallel universes. If he were doing so, preventing his own birth in parallel universes would cause no paradox. In the movies, when he has failed to correct the disturbances he causes, he starts to become translucent and "fade away". This would not be the case in a parallel universe, where causing an alteration to the past would merely prevent a parallel doppleganger from being born.

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good point, but then his memories should keep chaning all the time. –  user1129682 Apr 8 '13 at 19:02
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Strictly speaking this would be true, but we've already seen that he has hours or days to correct alterations. It could be that memories lag even further. Additionally, since it was written for mass appeal, the finer points of logic might be glossed over so as not to confuse less sophisticated viewers, certainly it could have impacted the popularity of the film to include such details. –  John O Apr 8 '13 at 19:06
    
It is true that the parallel universe does not explain the changing photographs. –  Jack B Nimble Apr 8 '13 at 19:07
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The memories must lag quite a bit behind indeed then. From the end of BttF1 to the end of BttF3 at least a week had passed (roughly the same amount of time Marty spent in 1955 in BttF1), yet Marty never give any indication that he suddenly "remembers" his changed 18 years, growing up with his altered parents from BttF1. –  Xantec Apr 8 '13 at 19:14
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Also, not all changes are slow moving. When Marty's father took back his mother on the dance floor, Marty immediately sprung up whole and the picture was completely restored. Logically the restoration should have taken the same amount of time as the dissolution, and during that time Marty then could have then been altered along with everything else. –  Xantec Apr 8 '13 at 19:16

Yes, he is/has.

Futurepedia, the Back to the Future Wiki, makes a well documented case for 8 different parallel universes. In part II of the series, Doc himself explains that when one travels back in time and modifies history, a new alternate universe is created at the point-of-entry. To prevent paradoxes, the previous timeline is erased from that point on, even though their events can be remembered by the time travelers.

There seems to be some "lag" in the destruction of the voided timeline, however, as can be seen when Marty's brother ans sister, and eventually himself, only disappear from the picture after a good amount of time, and not immediately. This is because the picture is from a now defunct alternate universe and the changes need to somehow travel back to it before they take effect.

This video contains some relevant bits from the movies regarding this issue, especially the beginning where Doc traces the timeline on the board while attempting to explain the changes brought by Biff's travel through time.

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Doc does not ever say the timelines are "parallel", the multiple timelines can be explained in terms of the idea of the timeline "changing" in some kind of meta-time dimension, as in the "type 3" time travel at xibalba.demon.co.uk/jbr/chrono.html ...in this case other timelines did exist "earlier" in meta-time, but they cease to exist when changes are made. Whereas truly parallel timelines would continue to exist even after you had changed the past and ended up in an alternate timeline. –  Hypnosifl Mar 13 at 21:10
    
Also, if BTTF was explained in terms of parallel universes, then when old Biff went back and gave himself the almanac in 1955, when he returned to the future he should have returned to a different "2015-B" than the "2015-A" he had left, while Marty/Doc/Jennifer stayed in 2015-A since they were there when he left and didn't go back with Biff. But the official BTTF FAQ at bttf.wikidot.com/official-bttf-faq (written by Bob Gale and Robert Zemeckis) suggest in answer 1.9 that 2015 had actually altered around M/D/J via the ripple effect, so "original" 2015-A may no longer exist. –  Hypnosifl Mar 13 at 21:12

In many stories pertaining to time travel, it is usually assumed that the traveler is somewhat protected from alterations and changes that they inadvertently make, due to the fact they are simply a visitor. So technically memories shouldn't change (look at Time Cop), but then again, shouldn't Marty also be protected from disappearing from the first one if his parents didn't hook up?

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I think you're on the right track with the first portion of your answer, but you should remove the part where you start asking your own question. Open a new one if you feel it is distinctly different enough to warrant it. –  phantom42 May 7 '13 at 15:20

I suppose the flux capacitor keeps him from the very likely overwelmimg disturbances his actions in the past are causing. But apparently his memories never catch up, so yes I think he keeps reappearing in different parallel universes, just as Doc explains the obvious 1985A parallel universe.

The Doc even explains the time machine can only travel back and forward along the timeline (TM). So if his actions cause other turns to be taken by individuals, the time machine has to follow these new paths.

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Doc Brown wasn't a particularly good inventor. And even when he hit upon one that worked (the time machine) he had to spend his entire fortune to build it. Considering the first test of the machine was with Einstein in BttF1, I wouldn't really call the Doc an expert on what it could or couldn't do. –  Xantec Apr 8 '13 at 19:19

No, he traveled into alternate timelines, not universes. As explained here.

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We generally try to discourage answers which are essentially just links to other sites. Please at least try to sum up the relevant information. –  phantom42 Jul 2 '13 at 14:31

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