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In Back to the Future, Doc invented time machine in 1985 and he was really excited and little surprised that it worked. But later in movie, Marty goes back and tells young Doc about it.

Why was Doc so surprised about it, when he knew he is inventing it in 1985 since 1955?

But more serious question, when he knew it since 1955, wouldn't this knowledge change the date of first time travel (Make it sooner, because he had lots of information from the future, or even make it later, because he "knew" he will invent so he could stop trying)?

Or why he didn't change the place of meeting with Marty, to prevent attack from Libyans (any events in past could change that moment, when Marty escaped only because the gun has jammed). Is it because he do not want to ruin time-space-continuum?

I know this is not one specific question, more few confused questions, but I hope you understand what is blurry about this movie for me.

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    Good question, but there are no set of rules for time travel . I'd say they set a few rules in the movie for that universe, but they violated them for the sake of the movie. – DustinDavis Apr 9 '13 at 18:38
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    Wibbly-Wobbly-Timey-Wimey.....stuff – Tablemaker Apr 9 '13 at 19:21
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    In the first part the doc specifically states he doesn't want to mess up the space-time continuum. That's why he lets the Libians think they killed him to avoid changing things (like them pursuing him further). In the later parts however the doc himself changes a lot of things! – Farhan Ahmed Apr 9 '13 at 21:02
  • I considered editing this down to just one question, but after actually writing an answer, I think that it's all related and so there's no reason not to leave the original wording. – Donald.McLean Apr 10 '13 at 14:16
  • There are some who say they aren't just traveling through time but also to alternate dimensions... scifi.stackexchange.com/questions/34038/… – Jack B Nimble Apr 11 '13 at 22:17
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Technically, in 1955, Doc Brown didn't invent time travel, he got the idea for the flux capacitor "which makes time travel possible." Doc Brown would first need to actually build a flux capacitor, and then he would need to work out how to use it to build a time machine. Just knowing that something is possible or that it will work, does not tell you how to actually make it work.

As to the rest of your question, there are many different theories of time travel and how it can affect the time line. One of the areas of contention (for which we do not have an answer) is the overall stability of the time line. Some theories say that even a minute change will cause a magnified ripple effect of larger and larger side effects - this is commonly known as the butterfly effect. At the opposite end of the spectrum, some science fiction folks speculate that a time stream is inherently stable and events and circumstances will tend to stay, as closely as possible, to what they were originally. Some theories even hold that the time stream will constantly work to repair itself and, eventually, only the largest and most significant changes will have any lasting effects.

It seems that, for Back to the Future, the theory of time travel in operation is more towards the stable time-line end of the continuum, though definitely not self-repairing. For the most part, things will tend to be very much like they were before the changes introduced by Marty's time traveling. As a result, the completion of the time machine, and the first trip will end up on the same day and in the same place, and probably not as a result of any conscious decision by Doc Brown.

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Back to the Future does not seem to operate on a "stable time loop" theory, which is the idea that changes made to the past will always have happened, so the events in the future must lead up to it. The simplest way to see this is the name of the shopping mall. At the start of the movie it's called 'Twin Pines', but during the course of BTTF1 one of the pines is knocked down. When they go back to the future, the mall is now called 'Lone Pine'.

As such, Doc Brown being told of the flux capacitor in 1955 didn't effect 1985 until the precise moment Marty told him about it, which changed history.

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