Because the Present Isn’t Complicated Enough
Time travel has long been a standby in writing a ripping tale. Dickens knew it; A Christmas Carol would not have been anywhere near as gripping a tale if it hadn’t involved Ebenezer Scrooge jumping to Christmas past, present and future with the help of some ghosts. H. G. Wells’ The Time Machine brought the word for a time-traveling device into the mainstream, and since then time travel has flourished in science fiction and other genres, despite physicists still not quite sure how it would be possible. But when has science stopped people from telling tales?
As a fan of Star Trek, I’ve noticed many of my favorite episodes and films involve time. Star Trek IV: The Voyage Home (1986) involves a 23rd century crew returning to 20th century San Francisco in order to round up some whales and save future Earth. “The City on the Edge of Forever” (1967), often considered the best episode of the original series, features Kirk and Spock going back in time to save their friend and set the timeline right. Once again, Star Trek: First Contact (1996) has the crew of the USS Enterprise travel back in time to stop some meddlesome Borg from conquering Earth in the 21st century.
As goes Star Trek, so does the rest of science fiction, but perhaps the flourishing of time travel as a plot device is worrisome–the more time travel stories, the less impact each individual one has. Regardless, each film and episode of any series that deals with time often follows a set of similar rules, appropriately bent for the story to take place.
Let’s take Star Trek: First Contact as one example. The 24th century crew of the Enterprise-E go back in time to the 21st century, a dark age in Earth history where humanity was at its weakest. An astronaut is about to attempt the first faster-than-light test run, which attracts the attentions of aliens known as Vulcans and leads to the glittering future Star Trek relates. The crew of the Enterprise realizes the Borg have altered history when Earth suddenly turns into a blackened Borg metropolis, but are saved from the effects of the alteration (which would have obliterated them) by being in the same path that the time-altering Borg took. Thus they can go back in time, fix the damage, and go forward again.
Years ago, a simple binding tenet of many time travel stories was the “grandfather paradox”. That is, a time traveller cannot go back in time and kill his grandfather; doing so would prevent the time traveller himself from being born, thus making it impossible for him to have gone back in time and killed his grandfather in the first place. Another example is Hitler. Say someone were to go back in time and kill or remove Hitler from power in the 1930’s. Nazi Germany might not have risen to such heights, or the Holocaust might not have happened, but either way, the resulting present would not have any reason for time travelers to go back in time to kill Hitler, because he hadn’t done anything; therefor, a time traveller wouldn’t have gone back in time in the first place. (Does your head hurt yet?)
One possible theory for avoiding the grandfather paradox is that time travelers could go back in time and alter history, but only in self-consistent ways; that is, the time traveller was part of history all along and wasn’t really changing anything. Another possible solution is that the traveler can change history, but not in ways that prevent him from doing so; therefor, a shot fired at the grandfather would somehow miss, because otherwise the time traveler could not have gone to the past to shot at his relative. A lighthearted example of this is the Futurama episode “Roswell That Ends Well”, where the character of Fry is so obsessed with protecting his grandfather he accidentally kills him in an atomic blast. Fry believes the man couldn’t have been his grandfather as per the paradox, and proceeds to sleep with his grandmother, becoming his own grandfather; the timeline’s continuity remains intact.
In some respects, First Contact is similar to the “self-consistency” example where people are part of history all along. In an episode of Star Trek: Enterprise, which takes place in the 22nd century, we learn that the warp drive’s inventor, Cochrane, told people about being attacked by aliens and assisted by future humans, but was not believed. This can be used to suggest that the Enterprise crew was always a part of history.
There’s another wrinkle writers have come up with, however, based on recent theories regarding quantum mechanics. To avoid all that paradox stuff, these new stories use alternative universes. In this version of time travel, when the traveler goes back in time and alters the past, he creates an alternate universe; the universe he came from continues on to his point of origin, but his actions have created a “breakaway” timeline with his alterations. Since according to the many-worlds theory of quantum physics, any possible action occurs in all possible ways, spread over countless parallel universes, then the time traveler would return to “his” present to find things had changed. In this way, the Enterprise crew returns to the present and finds they have altered history. This would mean that the Star Trek: Enterprise timeline is the altered timeline that the 24th century crew created.
Then again, while the Enterprise was sheltered from the effects of the timeline change by the Borg, if it fixed this future, wouldn’t they not remember it when they returned? This would be in keeping with the Star Trek: Voyager episode “Year of Hell”, in which the crew of the USS Voyager tries to stop an obsessed captain from obliterating populations from existence, altering timelines in a futile effort to bring back his dead wife. The crew of the timeship and eventually Voyager are immune to the time changes due to special shielding, but when the Voyager is rammed into the time ship, destroying both, the timeline is reset and the respective crews don’t realize the timeline has been altered or the events that transpired. I suppose the main reason that this sort of “we don’t remember” storyline is not too common (“The City on the Edge of Forever” left Kirk and company completely aware of what happened, despite the fact that history was changed so that there was no Enterprise) is simple convenience for storytelling’s sake. In one story it’s more dramatic to have everyone remember what happened; in others, it’s better to follow the grandfather or predestination paradoxes. Maybe one day when we actually do have time travel lots of these past science fiction books and shows will be outdated; until then, the wishful thinking of the author rules all.