A time loop or temporal loop is a scientific theory, used as 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.
Types of Time Loops
Physical Time Loop
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 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
"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.
- Time loop logic
- Butterfly effect
- Eternal return
- Grandfather paradox
- List of television series that include time travel
- List of films featuring time loops
- Ontological paradox
- Predestination paradox
- Time slip
- Time travel
- Time travel television series