Doc knows he will invent a time machine 30 years ahead and it doesn't change the future

Question

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

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

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

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

I know this is not one specific question, rather a few confused questions, but I hope you understand what to me is vague about this movie.

Answer 1

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.

Answer 2

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.

Answer 3

First of all, the Doc we see at the start of the movie did not receive a visit from Marty in 1955, so it wouldn't affect his reaction to it working. But in any case, if you had spent 30 years working on something, wouldn't you still be delighted (and maybe even a little surprised) that the day had finally come when it worked, even if you had evidence that there was never any doubt? The Doc might not have even kept the exact date in mind.

Secondly, as to the pace of the invention, Marty didn't actually give the Doc too much information on how the device works. He had already come up with the idea for the flux capacitor before Marty came to his door, so the only information he got really is that

1. It's going to be use a DeLorean
2. It'll require 1.21 GW (and hence some plutonium)
3. It'll have to drive at 88 mph

... the first two of which will not be available to him until relatively close to the completion date.

Finally, the Doc is well aware of the dangers of altering the timeline, and would probably endeavour not to "cheat" by using information he got from Marty to take shortcuts, and try to complete it within the same time as he had originally done. And moving the demo to a different location might not have helped him avoid the Lybians - presumably, they didn't just happen across him by accident at the Twin Pines mall, they had been searching for him.