"Time," a character explains in Ray Cummings's The Girl in the Golden Atom (stories 5 March 1919, 24 January-24 February 1920 All-Story Weekly; fixup 1921), "is what keeps everything from happening at once." This enormously broad sf theme is dealt with under several encyclopedia headwords, of which the most important is Time Travel. Physical travel through time is traditionally accomplished either by imaginary Technology in the form of a Time Machine, through the more or less "natural" accident of a Timeslip, or via a fixed-position Time Gate which may be either natural or artificial. Heedless use of time machines may – depending on the nature and stability of the fictional universe – lead to Time Paradoxes and less specific existential threats to the present, threatening to undermine its past and replace it with an Alternate History or Parallel World reality; in extreme cases, an Alternate Cosmos. To prevent such upheaval is a major function of science fiction's Time Police. Time Viewers allow the future to spy on the past or, more rarely, vice-versa; occasionally a science-fictional present or future attempts to communicate with the past using some form of Time Radio (see Tachyons).
The concept of Time in Reverse, either perceived or actual, has spawned a small subgenre of its own; more rarely, characters perceive Time Out of Sequence or are caught in repetitive Time Loops. Time Distortion is frequently deployed as a plot device, most often in the context of Relativity. No actual manipulation of time is implied by the term Time Abyss, used for the immense temporal gulfs separating us from the deep past and the Far Future, the Dying Earth era or the End of Time; the vastness of such spans is reliably evocative of Sense of Wonder. [DRL]
see also: Changewar; Jonbar Point; Malachronism; Perception; Stasis Field; Temporal Adventuress; Time Opera.
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