Scott Westerfeld has an interesting post trying to identify the various types of time-travel elasticity in science fiction media and literature, notions of one absolute vs. multiple timelines, and whether or not you can effect change in your own timeline.
In contrast to Ray Bradbury's "butterfly effect," I believe it was Isaac Asimov, who in The End of Eternity, submitted the “wave” theory of elastic time, proposing that the effects of any change would be most noticable immediately after the point of interference/insertion, but would gradually diminish in intensity as you moved past the event. So, killing Hitler would radically alter the 20th century, marginally alter the 21st or 22nd century, and might leave the 23rd century or beyond virtually untouched and have no effect on the 30th century at all. Of course, the largest alterations would carry further forwards, while smaller ones would fade out more rapidly.
Oddly, this reminds me of a recent article I read on the resurgence of the “Great Man” theory of history. The idea that, at any given time, there are about 12 people who are creating the world had falled out of academic favor for some decades, replaced with the notion that economic, social, political trends were shaping events more than individuals, who were simply stepping in to fill roles dictated for them by larger forces. I.e., if you did assassinate Hitler, someone else would have stepped into the power vacuum in Germany and mobilized tensions there to similar effect. However, ironically, George W. has renewed interest in the “Great Man” theory. While the artile wasn’t suggesting W was in any way “great,” it pointed out that in almost-single handedly forcing a war that a) wasn’t necessary and b) wasn’t popular with congress, the people, or the world at large, he has demonstrated how much (catastrophic) effect one individual really can have on the course of history.
The other thing this discussion brings to mind is the way that science filters through into pop culture and effects our fictions even at the most visible layer. The original Star Trek very much adhered to the notion of “one timeline”, which, when broken, was always repaired - the break and it’s correction (as Spock points out in “City on the Edge of Forever”) always part of the design. This holds sway through the TNG episode, “Yesterday’s Enterprise,” in which Guinan senses the wrongness of a universe in with the Federation and Klingon Empire are at war, and impresses Picard on the necessity of repairing the damage. (Picard raises the question, “How do we know this timeline isn’t any more right than any other?” but Guinan steamrollers him into accepting her position.)
But this notion of a single timeline begins to break down with the latter episode “Parallels,” which sees Worf permeating through a variety of alternative timelines until everything culminates in a clusterfuck of thousands of Enterprises from a myriad different universes.
By Deep Space Nine, the notion of one time-line has radically broken down, as withness an episode whose title escapes me, in which Chief O’Brien is constantly teleporting back and forth to a future in which the station is destroyed. Despite the fact that each trip exposes him to radiation poisoning, he makes one final attempt to avert the encroaching disaster, meets himself of just minutes later on, dies, and sends his minutes-into-the-future self back in his stead. Upon his return, he wonders if he really has the right to call Kieko his wife, given that “her” O’Brien died in a timeline that was then prevented from occurring, and is reassured by his best friend that he’s still the Chief, even if his memories are out of whack by a few minutes. Since “most of him” is the same, that’s good enough for government work, what?
Finally, when we get to Voyager’s first few seasons (where my knowledge of Trek ends, as my viewing of Trek did too), time has become elastic, fractal, alterable, permeable, and generally good for twisting into any shape the writers need. The two-part episode “Future’s End” sees multiple versions of characters encountered with no attempt to match cause to effect. Here, a crash-landing in the past has resulted in a boom in 1990s computer technology (seemingly the boom we ourselves experienced - thus the alteration IS the correct time, was always meant to occur, etc..), but the 29th Century timecop that is sent to prevent it is re-encountered twice, once as a sane individual aware of and contributing to the outcome of the episode’s action, and once as a homeless man wandering deranged from the initial crash. Both versions co-exist in the same (final) timeline, and alterations and corrections made in the episode do not erase or negate the mad homeless version’s existence. I quite watching soon afterwards, but kept enough tabs on the show to know that they continued to play with multiple versions of their characters, and multiple co-existing and interacting timelines.
Finally, while I haven’t quite put my finger on it yet, I do think that the move from an absolute time to fluid/fractal timelines somehow coincides with blurring questions of individuality and identity unconsciously co-opted from the general zeitgeist. It used to give me fits when Jeri Taylor would tell me “absolutely the holographic doctor is a person” and Brannon Braga would turn around and tell me “absolutely he is not.” They didn’t know themselves, but unconsciously, a lot of that series was about ascribing “personhood” to inanimate, but sentient-seeming, objects, as witness the sympathy everyone gives the doctor when his holographic family breaks down, despite the fact that a simply tripping of a reset switch would have them all up and running again. This inability to distinguish between a long-running program who had achieved self awareness over time and the game-pieces conjured into existence on the holodeck typified the entire series, which, to me, was perfectly appropriate for - and indicative of - an age where a woman driver ran over a biker because her tamaguchi needed immediate food. As tragic as that was, one day I'm sure our robot masters will cite it as a watershed in human/machine empathy.