Other mistake: After they take off from California, Kirk gives a heading to Alaska. He then tells Sulu "full impulse power", and Sulu says "aye, ETA 12 minutes." Full impulse is 1/4 the speed of light. No way they would use speeds like that to go a few thousand miles.
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Star Trek IV: The Voyage Home (1986)
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Directed by: Leonard Nimoy
Starring: William Shatner, George Takei, Leonard Nimoy, Walter Koenig, DeForest Kelley, James Doohan, Nichelle Nichols, Catherine Hicks
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Spock: Your use of language has altered since our arrival. It is currently laced with, shall we say, more colorful metaphors, "double dumb-ass on you" and so forth.
Kirk: Oh, you mean the profanity?
Spock: Yes.
Kirk: Well that's simply the way they talk here. Nobody pays any attention to you unless you swear every other word.
Question: What exactly was Scotty's reason as to why giving the Company boss the formula for the one inch glass wouldn't alter the future? He gave a brief response, but I honestly can't think of any reason why it wouldn't do any future damage.
Chosen answer: Scotty says "Why? How do you know he didn't invent the thing!" If the man was in fact the inventor, this would only cause a slight causality loop problem - he "invents" it because they gave it to him, but they only know it because he "invented" it. However, since Sulu said earlier in the movie that it was about 150 years too early for transparent aluminum, it would seem they do know this, so it wasn't a smart thing to do. Of course, the real flaw in the plot is that they need the tank to be transparent at all.
Answer: The crew is resigned to the fact that their mission forces them to alter history in some fashion or another. McCoy just wants to acknowledge the gravity of their actions before they go ahead and do it, and Scotty's response is a cheeky way of reassuring him, "Hey, maybe it won't be that bad."
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Answer: They only give him a schematic of the molecule. The man even says, "It would take years to decipher the matrix", or something like that.