In Back to The Future, Marty changes the future from the past. In reality, however, space and time is a continuum, which means that if he did something in the past, it is a fact and it's effects (His life changing) would apply before Marty goes back in time. Therefore, this movie has a large plot flaw that actually makes you rethink the entire movie!
Um, actually, it's not fact...it's theory. No one really knows what the effects would be of changing the past, since no one has (to anyone's knowledge) succeeded in time travel. One theory is that such a change would create an alternate universe...and Marty's traveling back to his time may lead him to that alternate universe since he would follow the time "path" that his changes set forth.
No! It does not become fact until he actually goes back then the new time line forms.If he doesn't go back, then nothing changes. Doc explains some of that in the next movie. That's the paradox.
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