The one questionable moment of potentially altering the future comes when Scotty and McCoy make a deal with Plexicorp’s Dr. Nichols (Alex Henteloff) to produce the transparent aluminum they need to carry the whales safely into the future. With that material not existing in the ‘80s, Scotty basically gives Dr. Nichols the formula for free, with McCoy questioning if this is going to ruin the future timeline.
Interestingly enough, Scotty waves it off in Star Trek IV: The Voyage Home’s cinematic incarnation, by asking his shipmate, “How do we know he didn’t invent the thing?” However, [in the film’s novelization](https://memory-alpha.fandom.com/wiki/StarTrekIV:TheVoyageHome(novel), Mr. Scott knows damned well that Dr. Nichols did invent Transparent Aluminum, which means history was never changed in this instance as it was always the way things were meant to pan out. Time travel fans will recognize this as a “Predestination Paradox,” where a trip in time was always built into the fabric of history to begin with.
