A time viewer is a hypothetical device, often featured in works of fiction, that can display events occurring in another time, usually in the past but also (less commonly) in the future.
In his short story "The Dead Past" (1956), Isaac Asimov called a similar device a chronoscope, which is also the name that the Victorian-era scientist Charles Wheatstone gave to his invention for measuring small intervals of time.
Father François Brune, a French Catholic priest and author, related in his book Le nouveau mystère du Vatican (2002) that an Italian priest supposedly invented a time viewer in the 20th century. He called the machine the chronovisor.[page needed]
In the 1947 novella E for Effort, T. L. Sherred describes a time viewer built by a poor genius who cannot get people to take him seriously. The genius uses his invention to create historical movies which he then shows in his decrepit theater. He is discovered by a Hollywood producer, who is able to exploit the viewer to create first movies, then historical reconstructions, and finally political documentaries. The last part is his undoing, as he exposes every crime committed in the name of patriotism and ideology by world leaders, resulting in the collapse of government, followed by nuclear war.
For the short story "Private Eye" (1949), Henry Kuttner and C.L. Moore (writing together as Lewis Padgett) envision a society in which time-viewing makes it virtually impossible to commit a murder undetected, but which allows pleas of temporary insanity and right of self-defense. The protagonist schemes to provoke an attack by his victim, and then kill the man in (ostensible) self-defense. The murder weapon is an antique scalpel used as a letter opener, whose presence between them is carefully orchestrated by the murderer. The story was dramatized for BBC1 as The Eye, an episode of the science fiction anthology series Out of the Unknown.
In Philip K. Dick's short story "Paycheck" (1953), Rethrick Construction recruits an electronic engineer to build a machine that can view the future. After the job is done, the man's memory is erased, and he finds that he is pursued by secret police. It was adapted as the film of the same title in 2003.
"The Dead Past" (1956) by Isaac Asimov concerns the clandestine invention of a time viewer after research into the subject is suppressed. The reason for this is revealed in the story's conclusion: Visual monitoring with a time viewer deprives others of privacy. A similar outcome features in Arthur C. Clarke and Stephen Baxter's The Light of Other Days (2000).
Damon Knight's 1976 short story "I See You" describes an invention that allows its operator to view anyone at any point in time.
For his 1983 novel Millennium, John Varley conceived a time viewer operated by time travellers. The viewer disallows its operators from viewing places where they have been or will be. When the viewer screens a temporal paradox, the image blurs as alternate futures overlap.
The novel Zig Zag (2006) by José Carlos Somoza describes a string theory-based technology that makes it possible to produce still images of past events.
Time viewers have a relatively minor part in the following novels and short stories: