Apologies to Harlan and Roddenberry both, but that is some lousy science. Or is it?
Later, when time is altered and then set right, our good Vulcan rationalizes their belief in one true timeline by suggesting that both the alteration and it's correction were predestined.
This was pretty much the Star Trek position for the original series, the original cast films, and into the start of The Next Generation, right up through the episode "Yesterday's Enterprise" (and Denise Crosby's only decent performance) . But then something happened. The Graham-Everett-Wheeler many worlds interpretation of quantum mechanics finally broke through into the popular consciousness.
Then we got episodes like season seven's "Parallels," in which a quantum fissure in space-time propels Worf through a series of alternate universes. The episode's climax sees a myriad different versions of the Enterprise converging, all from their own realities, all equally "real" to their inhabitants, some radically different from our own. The singular time line is gone.
This plays out even more in Deep Space Nine, especially in the episode "Trials and Tribble-ations," in which we learn that - far from having it's own self-correcting mechanisms as Mr. Spock proposed a century ago - time actually requires Federation protection in the form of the "Department of Temporal Investigations."
By the time of Voyager's two-part episode "Future's End," the notion of one sacrosanct timeline has been shot all to hell, and the writers don't even bother with resolving or explaining the multiple temporal paradoxes they introduce. This will become a staple of Voyager episodes, with multiple stories showing the plurality of existence, and the subsequent loss of individuality that results. Of course, some of this is just lazy writing, but the impact of the Graham-Everett-Wheeler depiction of the multiverse was certainly still reverberating through the science fiction writers' zeitgeist.
And now this....
Physicists Daniel Greenberger of the City University of New York and Karl Svozil of the Vienna University of Technology in Austria claim to have demonstrated that the most basic features of quantum theory may inherently rule out paradox. Essentially, their argument resolves around a quantum object's ability to behave as a wave. Ordinarily, such an object split into component waves and traveling through space-time is most likely to end up in places where its component waves recombine "constructively." But, when the component waves travel into the past they behave surprisingly different, their various components "interfering destructively", canceling each other out and actively preventing anything from happening in contradiction to that which has already taken place. If I'm wrapping my head around this correctly, this means that you can travel into the past, but only that past which results in your future.
Basically, you can't alter the timeline, because the past you travel to is the one in which you don't.
It seems that all those heavy-handed, self-correcting mechanisms science fiction writers used to put in their stories showing the universe itself acting to prevent paradox might actually have a basis in reality. And Harlan Ellison's "river of time," far from looking like a hackneyed plot device to get Kirk and Spock where they needed to be, is seeming a bit damn prescient about now.