Time travel in Back To The Future

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Back To The Future is one of two films, along with Bill & Ted's Excellent Adventure, which forms the bedrock of what most people of my generation understand about time travel. Back To The Future is so iconic that its model of time travel forms a kind of universal canon. Even people who've not seen it seem to have picked the rules up through osmosis. And why shouldn't they? This is the gold standard of time travel stories, ranking up there with the very best of popular science fiction.
This model of time travel is so widely known, in fact, that it can be safely used as the basis for many further time travel stories, with a relatively minimal amount of explanation. And it frequently is. Back To The Future's model almost seems to be the default model of time travel in most time travel fiction, and certainly there's a wide collection of stories whose models can be understood well by comparing them qualitatively with this one. How could there not be? Ho...