plot explanation - In Back to the Future Part II, how could old Biff have made it back to 2015? - Movies & TV

In Back to the Future Part II, Doc picks up Marty in 1985 and takes him to 2015. Then old Biff realizes the Delorean is a time machine. Old Biff steals the Delorean and travels from 2015 to 1955 to give the 1955 version of himself the 2015 sports almanac.

When old Biff then returned to 2015 from 1955, shouldn't he have arrived in a way-different 2015? Instead, he arrives in the same 2015 that he departed, where Doc and Marty are still running around as if nothing changed.

Here's pic of Doc Brown in BTTF2 explaining how Old Biff created new timeline:

I think this is a problem created by perspective. When Old Biff takes the Delorean back in time, we viewers do not go along for the ride. We're left hanging in a 2015 that becomes obsolete when Old Biff changes 1955. Seems our 2015 has two options: continue to exist as an alternate to the 2015 Old Biff created, or cease to exist. Either way, still seems our 2015 would be unreachable by Old Biff and the Delorean on their return from 1955.

Answer

Speculation: Old Biff returns to the changed 2015 (designated 2015A). Doc and Marty "transitioned" to 2015A without noticing. Why didn't they notice? Maybe because they were out on the street when it happened and Hilldale looks the same in 2015A.

BTW, time travelers "transitioning" between changed timelines is implied by Doc Brow when he tells Marty not to worry about leaving Jennifer on the porch in "alternate 1985". After they restored the timeline Jennifer was still sleeping on the porch.*

Also, from Futurpedia:

In scenes deleted from the film, Biff Tannen of 2015 fades out of existence (much like Marty was in the original movie when interfering with his parents' meeting) once arriving in the future, having been erased from existence in that time period.

This can be explained for two different reasons:

• First, these events created a time paradox. Having created an alternate timeline by giving his younger self the almanac, the old Biff from 2015 no longer existed, once 2015A became part of the 1985A timeline.




• Another possibility is that Lorraine, in 1996 of the alternate timeline, shot and killed Biff.

These events are never depicted in the trilogy, although Robert Zemeckis and Bob Gale suggest this theory as a way of explaining Biff's "disappearance."

I took some screenshots from a YouTube clip: