IV is a pre-destination paradox. What happened is supposed to happen. Though we might be able to argue the glasses exist in two places at once from 1987 onward since they obviously must have a creator.
But, what would happen if the glasses that McCoy bought were the San Francisco pair, and not the London pair? After all, the SF pair could continue to exist into the 23rd century just as easily as the London pair.
I wonder if man had any idea there would be so many discussions and theories when he invented time to track how long a day is and how much sunlight he had to do his outdoor stuff.
But, what would happen if the glasses that McCoy bought were the San Francisco pair, and not the London pair? After all, the SF pair could continue to exist into the 23rd century just as easily as the London pair.
Interesting point. Consider this as a possibility. The universe had to get to the 23rd century before McCoy bought the glasses. Therefore there must have been a period of time in which there was only the one pair of glasses in existence, the ones we are calling the London pair. So they must have been the ones McCoy bought. He couldn't have bought the San Francisco pair, as they didn't exist until after Kirk sold them to the antiques dealer!
Wibbly-wobbly timey-wimey!
But the introduction of a second pair of spectacles isn't in itself a significant event. With a few notable exceptions, Star Trek doesn't have alternative timelines. The past itself can be changed.
When the Borg travelled back to assimilate Earth in First Contact, Picard didn't shrug his shoulders and say "Never mind, that's just a different reality, let's find a way back to ours." When McCoy prevented Edith's death in COTEOF, he changed history so that the Nazi's won. Not created an alternate timeline where it happened, but changed the past itself.
So in TVH, Kirk and his crew made a few changes to Earth's history. But those didn't create a whole new time line, they were just alterations to the past.
Save for a few isolated instances at rare points in my life, "The Voyage Home" never really appealed to me. I never liked Trek when they went back in time to the 20th century, that's not why I tuned in to watch in the first place.
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