Time Travel

Time travel was popularized by H.G. Wells with The Time Machine (1888), though Edward Page Mitchell wrote "The Clock that Went Backwards" seven years before that. Characters travel to the past or future, or are visited by travelers from either end of the spectrum. Topics range from "Let's go see what the Pleistocene looked like," to issues of paradox (what if you traveled to the past and killed your own grandfather?) and "tampering" (could stepping on a butterfly in the Paleolithic profoundly alter the entire future?). A variant of this subgenre is the "alternate universes" theme, in which each change in the timestream spins off a new universe.