Further BttF problem I thought of, let's accept that they CAN travel through time and end up right where they travelled from, it's still not possible to suddenly introduce more mass into the world like that, there is air, dust, viruses, micro-organisms, they may be small but they are occupying that space at that time, so by appearing there suddenly like that you would introduce matter into matter, thereby destroying everything involved at the least, and existance at most. Though I think they tried to cover that with the 3 booms, I figure the time machine would have to create a vaccum to enter into the new "time-space".
As far as the "Dissapearing Marty" impossible, any effects of time travel would not occur to the one actually going through time as they orriginated in a time line where thier life fully occured. So basically, you could go back in time, and (if you were sick enough in the head) replace your parent in your other parents life. sure you'd have strange and deformed babies that would technically be your sister-daughter, but you would only create a new timeline where you were never born. Anything you brought with you, and you yourself would be fine, no change due to the fact that you DO exist. (if this is at all unclear just message me, I'd be glad to try and clarify) Basically it comes down to Marty replacing his dad in his mom's heart would not make him dissapear. And if he did, it would have been instantaneous.
Of course, it's just a movie. Maybe we shouldn't be picking it apart like that. Maybe we should just sit back and enjoy it. Right?
