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On more than one occasion in Back to the Future, Marty tries to warn 1955-Doc that he will be shot on the night he tests the DeLorean. The first time, he tells Marty to shut up and says something like "I don't want you to endanger my future the way you endangered your own future". But even from Marty's perspective in 1985, any potential consequences of telling the Doc about the shooting would only affect the "real" future, i.e., things that hadn't happened yet in any timeline.

This is very different from Marty's situation in 1955 - by changing the course of events, he was creating a very real possibility of negating his own existence (as well as the existence of his siblings). But 1955-Doc learning that Libyans would try to kill him in 30 years does not pose similar problems. No one who already exists will cease to exist, and the changes that would proceed from the Doc wearing body armor would have no effect on current events only things that had not yet come to pass (in any timeline). In fact, I can't see how it would change anything other than 1985-Doc dying.

Am I missing something here? How is Marty changing his own past anything like him changing the Doc's future? Is it actually dangerous for 1955-Doc to hear that he will be shot?

  • Thanks, Richard. I forgot we were talking about a time travel movie. :) – Wad Cheber Jun 15 '15 at 5:47
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    It's pretty clear that Marty's actions in the past can change the future. If he gives the Doc too much info, he might create a much larger paradox. The worst case scenario (from the Doc's perspective) is that he tells him something that gets him killed the next day – Valorum Jun 15 '15 at 5:48
  • @Richard That is something I hadn't considered, although I don't think "wear a bulletproof vest on this night in 1985" would be the kind of information that one would act upon immediately in 1955. On the other hand, this is the Doc we're talking about, and he would probably try to make his own vest rather than buy one with his vast Brown Estate fortunes. – Wad Cheber Jun 15 '15 at 5:51
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    @Richard - You just inspired me like I was Doc Brown falling off a toilet. If the Doc is so concerned about not changing the past, why the hell was he about to go back in time to visit himself? He's a frigging hypocrite! – Wad Cheber Jun 15 '15 at 5:58
  • See the question Richard inspired- scifi.stackexchange.com/questions/92602/… – Wad Cheber Jun 15 '15 at 6:21
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But 1955-Doc learning that Libyans would try to kill him in 30 years does not pose similar problems.

This isn't necessarily the case. From Doc's point of view, he didn't know the threat that was facing him. If he had known about the Libyan terrorists killing him, he may have decided not to go through with developing the time machine, as it may have got him to think about the problems a time machine would face (namely creating paradoxes) which (ironically) would have created the paradox where he wouldn't develop a time machine because of information which was given to him from someone operating his own time machine. Alternatively, he may have built the time machine and tried to source the plutonium from somewhere else, getting himself caught, meaning he couldn't show Marty the time machine (another paradox) or given up on the time machine realising it was too dangerous to try and use plutonium which was necessary for the 1.21 jigawatts. The potentials for creating a paradox is huge.

Hence, to Doc, information about the night of his death that Marty was trying to give him probably looked like it was too likely to cause a paradox.

Doc also didn't know what other information Marty was going to give him about the future - and information about the future can cause all sorts of trouble, which forms the basis of the plot with Biff in Back to the Future II

  • Of course, but a guy who builds a machine so he can go to the future has no business telling other people not to change the future. And the gist I'm getting from your answer is that it basically amounts to the Doc not trusting Marty enough to know whether the information he is trying to convey is potentially dangerous. – Wad Cheber Jun 15 '15 at 6:54
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    @WadCheber when the universe (or, if we're lucky the galaxy) is at stake someone's gotta step in ;) – Often Right Jun 15 '15 at 6:56
  • But the lack of trust is understandable, since he doesn't know Marty as anything more than a weird kid who showed up on his doorstep screaming about time travel and his mom trying to hook up with him. – Wad Cheber Jun 15 '15 at 6:57

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