Originally Posted by HDMe2
Something that occurred to me after seeing the previews for next week... and also a plot-hole (only one??) in Back to the Future.
Digressing for a moment...
In Back to the Future. Marty goes back in time... makes some slight changes... then comes back to the present to find immediate differences (his family is better off).
In Back to the Future II, while in the future, Old Biff goes back in time and makes a major change then comes back to his future and nothing has changed! The change only kicks in when Marty & the Professo
r go "back" to the present time.
So why did the past immediately affect the future one instance but had a delayed one in the other?
As a Back to The Future expert (sad... I know...) I'll answer this part.
In I, Marty made changes in 1955, and traveled along the new time line to 1985 (but didn't go right away....).
In II, the intention of the writers was that the 'time ripple' took time to reach 2015 - there is a cut scene where you see Old Biff fade away after bringing the DeLorean back (likely because he was no longer alive in the 2015 he created) - that explains why he looked like he was in pain or something when he was leaving the car.
So Marty and Doc basically went back in time at the most perfect possible time as they crossed over time lines.
Their existence in the alternate 1985 was a time paradox, as their alternates were there (Alt-Marty in Europe and Alt-Doc institutionalized) - had they stuck around long enough - the Futurama Time Paradox theory might have taken effect (or they could've caused a chain reaction that would unravel the very fabric of the space time continuum, and destroy the entire universe! Granted, that's a worse case scenario. The destruction might in fact be very localized, limited to merely our own galaxy).
As for Jennifer and Einstein - Marty was right, they should have been more concerned - I'd like to understand why Doc would just assume everything would change around them.... (This goes into your question on what happens to the other 4 in Eureka...)
EDIT - side note - "The Professor"??? really?