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The ripple effect caused by Marty in 1955, takes a week to reach 1968 in which he was born and begin to make him disappear.

Why the ripple effect caused by Doc in 1885 only takes a few seconds to reach 1955? Making the Western Union messenger appears almost instantly.

  • Because it made for an interesting story? I really don't think you can make any kind of time-travel story with people slowly disappearing logically consistent. – svick Sep 11 '15 at 22:58
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    I thought it had to do with certainty. It wasn't certain that 1955 events would break up his parents yet. Doc in 1885 was certain event. But it might be a plot hole why his picture from the future didn't show them on a better vacation. – swstephe Sep 12 '15 at 4:26
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    Maybe because one event had bigger repercussions (prepercussions? Damn time travel verbage) than the other. A message being sent would minimally alter time while two people getting married and having a kid would have immense effect on time? – cde Sep 12 '15 at 19:50
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The ripple effect of 1955 takes weeks in the year 1955. This is because in those weeks there is still a finite probability that the mess can get corrected. (and it does). The photo and Martin that are disappearing are in 1955. Had there been a way to fast forward to 1968, there would be no slow fading ripple effect in 1968

In the same way, the doc sending out a letter is in the year 1885 and any ripple effect would be felt then in 1885. Doc planning to send that courier is not shown in the movie hence we don't know how long the slow ripple effect may have been. The courier is received in 1955 is basically equal to fast forwarding to the point of impact. There would be no slow ripple effect at that moment in 1955, it is instant.

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    Makes perfect sense. Ripple - the further the rings from the source, the closer together they become. – user1103 Nov 9 '15 at 8:37

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