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I remember a story or movie from my childhood where the protagonist goes back in time to retrieve some object, and while doing so he inadvertently steps on something small (like a fly or butterfly). When he returns to the present, he finds the future has changed quite a bit.

Does anybody know the story I'm talking about?

Also, does anybody know about the scientific law/paradox which governs this phenomenon?

marked as duplicate by Otis, Politank-Z, Ward, KharoBangdo, Paulie_D Dec 28 '16 at 9:07

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    Please edit this to focus on one story, and ask the other one separately. – Adamant Dec 26 '16 at 3:42
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    @shirish Choosing a winner should be based on who answered the question in proper form, as opposed to a comment. – Marakai Dec 26 '16 at 5:57
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    The butterfly effect is the name of the paradox you are talking about. The short story is called: The Sound of Thunder. It is one of several common time travel paradoxes including, the Grandfather Paradox, and the Bootstrap Paradox. There are also the inconsistent temporal loop, and the multiple universes theory which negates the paradox with a trip down a nearby and nearly similar timeline. It is a paradox because you can never correct the initial cause, just create a new timeline. – Thaddeus Howze Dec 27 '16 at 21:08
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    "The butterfly effect" is another name for "sensitive dependence on initial conditions." It is often invoked to explain that weather is unpredictable. It could be conceivably invoked with regard to how small changes to the past could result in enormous changes to the present, but that's not it's main purpose. – Jim Conant Dec 27 '16 at 21:59
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    Relevant meta discussion: Why did a moderator delete this question about a paradox?. – Valorum Dec 28 '16 at 12:29
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Not sure about the paradox name (probably needs to be a separate question anyway), but the story is A Sound of Thunder, short story by Ray Bradbury.

  • @Mischa Rpsmasj . There was a similar one of Captain Janeway of Star Trek fame, where whole civilizations are at stake, it had paralle universes, a tear in space-time which was sending whole civilization to death. – shirish Dec 26 '16 at 4:15
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Time travel stories rely on several "laws" or paradoxes, sometimes in conflict with one another:

  • the butterfly effect (as seen in Bradbury's A Sound of Thunder, the one about the dinosaur hunter stepping on a, literal, butterfly and coming back to a totalitarian future). This requires time travelers to be exceedingly careful and usually require lots of observation and fine-tuning (e.g. Asimov's Eternity).

  • the inertia effect (I remember Star Trek novel Ishmael) where small changes are actually absorbed and reversed by the time stream. For example, you go back to Sumer and kill a king. Back in the XXI century, all that it changes is that some symbols in a shard of pottery in a forgotten museum rearrange themselves. To have effect on the future, a change must be enormous enough to overcome Time's "inertia". This may mean that you can do almost anything in the past, provided it's sufficiently far back, with all the ethically sinister implications of that "anything".

  • the Grandfather Paradox. You go back in time and kill your own grandfather before he reproduces, or marry your mother and maybe even get her pregnant. Possibly then you change sex, go back again in time and are your own mother. How is this even possible? Solutions range from "it isn't", to the point that trying to build a time machine wrecks your civilisation or makes the Sun go supernova, to "it is and it isn't" (not completely resolved in David Weber's Apocalypse Troll) since every time you go back in time, you actually split the timestream in two, one where you did not appear (and is the one that gave birth to you) and one where you did (and had effect on the future). You can come back from a future where you can't exist because they are two completely separated futures. Both theories conflate in one story by... Greg Bear, possibly?, in which the timestream you left finds itself irredeemably short some 80 kg of mass, and the one you arrive in finds itself with an identical excess. Both continua are then doomed - in one the Sun goes out, in the other it balloons and ultimately goes nova.

The timeline split plus butterfly effect is the plot device at the basis of Eric Flint's 1632 (aka Ring of Fire) series, where a sizeable area of Earth's surface is swapped with a different area some four centuries earlier. The up-timers quickly conclude that they might as well do their best with what they have, and actually endeavour to change their would-be future as much as possible, rather than the reverse, since either they are in a parallel time-stream or they've already trashed that future just by being.

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