A time loop or temporal loop is a common plot device in science fiction (especially in universes where time travel is commonplace) where a certain length of time (such as a few hours, or a few days) repeats over and over. When the time loop "resets", the memories of most characters are reset, and behave as though they're not aware of the loop. The plot is advanced by having one or more central characters retain their memory or become aware of the loop, sometimes through déjà vu.
One well-known example of this is in the 1993 film Groundhog Day, in which the main character is the only one affected by the time loop. Stories with time loops commonly center on correcting past mistakes or on getting a character to recognize some key truth; escape from the loop may then follow.
The closed loop in time, in which an event becomes its own cause, is the simplest narrative form of the time-paradox story, seized upon by several of the contestants invited by the editor of Amazing Stories to find a clever ending for Ralph Milne Farley's "The Time-Wise Guy" (1940). More notable examples include Ross Rocklynne's "Time Wants a Skeleton" (1941), Bester's "The Push of a Finger" (1942), P. Schuyler Miller's "As Never Was" (1952) and Mack Reynolds's "Compounded Interest" (1956). Greater ingenuity is exercised when these loops become more complicated, forming convoluted sealed knots. Two classic exercises in this vein were written by Robert A. Heinlein, "By His Bootstraps" (1941) as Anson MacDonald and "—All You Zombies—" (1959), the latter being a story whose central character moves back and forth in time and undergoes a sex-change in order to become his own mother and father.
In a physical time loop (rarely seen in the media), the spacetime loops around to form several closed timelike curves. Since the time in that region is looped, a person could escape it only by leaving the affected area. Also, there would be an infinite number of copies of any matter in the area, unless an object left the loop. In that case, there would only be as many copies of that object as many times it completed the loop. This type of time loop cannot be ended or destroyed.
Conscious Time Loop
In a conscious time loop, everyone's consciousness loops through time. In such a time loop, causality could easily be violated.
In popular culture
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The Day Room, a short play by Don DeLillo, features actors in a play who perform the roles of doctors, nurses, and patients in a "day room" of the Arno Klein Psychiatric Wing. However, the play is performed at unannounced locations and times.
In a story in The Decameron, a dead man (Guido degli Anastagi) is ordered to catch his dead recalcitrant beloved and tear her apart, every Friday.
The Details of Nikita Vorontsov's Life (1984) by Arkady Strugatsky, two friends investigate into the biography of a man who lives the life of the same personality many times and after his death always returns to a certain moment of this personality's childhood in late 1930s.
"Doubled and Redoubled", a short story by Malcolm Jameson that appeared in the February, 1941 issue of Unknown. Accidentally cursed by a witch, the protagonist repeats a "perfect" day, including a lucky bet, a promotion, a heroically foiled bank robbery, and a successful wedding proposal. This story was a precedent to Groundhog Day and 12:01 PM.
Lost in a Good Book (2002), the second of the Thursday Next novels by Jasper Fforde. The title character travels back in time to save her husband from being eradicated and experiences a time loop before returning to her present-day 1985.
Rendezvous (2013), an adventure novella by Nowick Gray, turns on a series of time loops accessed through a dreamed hall of doorways, from a cabin in a mountain pass. The seven doorways/loops represent multiple endings, only one leading to survival. Thus Rendezvous is also a variation of the Choose Your Own Adventure narrative.
Replay, a Ken Grimwood novel (1987) in which the main character suddenly shifts to much earlier in his life, then relives shorter and shorter periods.