The fact that Time Travel into the past disrupts the pattern of causality, changing or cancelling matters of known fact, has not caused stories of this kind to be banished from the sf field; instead it has led to the growth of a subgenre of stories celebrating the peculiar aesthetics of such Paradoxes. The essential paradoxicality of time travel is often dramatized by asking: "What would happen if I went back in time and killed my own grandfather?" – a question to which sf writers have provided many different answers. A time-paradox story usually leads either to a singularly appropriate reductio ad absurdum or to a cunning literary move which appears to resolve the paradox by removing or avoiding the seemingly inevitable contradiction. F Anstey's pioneering fantasy The Time Bargain (1891; vt Tourmalin's Time Cheques) provided a prototype for the first kind of story by dismissing its ultimate, unresolvably complex tangle as no more than a dream. Fritz Leiber's "Try and Change the Past" (March 1958 Astounding) is a good example of the second kind, with its postulate that history is resistant to alteration owing to the "Law of Conservation of Reality": a man laboriously saved from being shot in the head is nevertheless killed by a micrometeorite which inflicts a closely similar wound. Sf writers frequently invoke sweeping metaphysical hypotheses in the cause of accommodating potential paradoxes; Alfred Bester's "The Men who Murdered Mohammed" (October 1958 F&SF) does so by providing every individual with his or her own personal continuum. There are several notable stories and series about Time Police who try to protect the world – or, quite often, a whole series of Alternate History worlds – from temporal upset.
The closed loop in time (see Time Loop), 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" (May 1940 Amazing). More notable examples include Ross Rocklynne's "Time Wants a Skeleton" (June 1941 Astounding), Alfred Bester's "The Push of a Finger" (May 1942 Astounding), P Schuyler Miller's "As Never Was" (January 1944 Astounding), Murray Leinster's "The Gadget had a Ghost" (June 1952 Thrilling Wonder) and Mack Reynolds's "Compounded Interest" (August 1956 F&SF). 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" (October 1941 Astounding) as by Anson MacDonald and "All You Zombies –" (March 1959 F&SF), 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.
The second fundamental variant of the time-paradox story is that in which the present from which the time-travellers start is replaced by an alternative because of the effect (often trivial and unintended) which they have had upon the past. Nat Schachner's "Ancestral Voices" (December 1933 Astounding) is an early story which uses such a device to expose the absurdities of ancestor-worship and racism, but the best known example is Ray Bradbury's moral fable "A Sound of Thunder" (28 June 1952 Collier's), in which a time-tourist who treads on a prehistoric butterfly alters the Politics of the present for the worse. Eando Binder's "The Time Cheaters" (March 1940 Thrilling Wonder) suggests that time might have stubbornly ingenious ways of taking care of such threatened contradictions, and William Tenn's "Brooklyn Project" (Fall 1948 Planet Stories) points out that observers who change with the world would not notice such alterations, however drastic they became. In many stories the good intentions of would-be history-changers go sadly and ironically awry. L Sprague de Camp's "Aristotle and the Gun" (February 1958 Astounding) is a fine example; others are Poul Anderson's "The Man Who Came Early" (June 1956 F&SF) and Kirk Mitchell's Never the Twain (1987). Works in which such ideas are further extrapolated and intensively recomplicated tend to feature Changewars fought through time by the representatives of alternate worlds ambitious to demolish their competitors. Jack Williamson's The Legion of Time (May-July 1938 Astounding; rev 1952) opened up such imaginative territory for further exploration in Fritz Leiber's Change War series and Barrington J Bayley's spectacular The Fall of Chronopolis (1974); the long Timewars series by Simon Hawke of exuberantly extravagant stories in this vein, begun with The Ivanhoe Gambit (1984), is still continuing.
The potential which time-travellers have to exist twice in the same time is considered so uniquely unreasonable (see Doppelgangers) as to be specifically proscribed in stories like Wilson Tucker's The Lincoln Hunters (1957), where the restriction opens up potential for ingenious plotting, as it does also in John Varley's elaborate paradox-avoidance story Millennium (1983). However, other writers – including such non-genre writers as Osbert Sitwell in The Man Who Lost Himself (1929) and Eliot Crawshay-Williams in "The Man Who Met Himself" (in The Man Who Met Himself and Other Stories, coll 1947) – have been particularly intrigued by the possible psychological effects of a person's meeting with a later version of his or her own self. Ralph Milne Farley's "The Man Who Met Himself" (August 1935 Top-Notch) is an early example from the sf Pulp magazines. Later sf writers have casually extended this notion to its absurd limits, displayed by Barry N Malzberg in "We're Coming Through the Window" (August 1967 Galaxy) as by K M O'Donnell, and David Gerrold in The Man Who Folded Himself (1973), the latter being a notable if silly story which conscientiously attempts to compile a narrative portmanteau of all possible time paradoxes.
Sf writers who have made particularly prolific and ingenious use of time-paradox plots include Charles L Harness, whose many works in this vein extend from the early "Time Trap" (August 1948 Astounding) and "Stalemate in Space" (Summer 1949 Planet Stories; vt "Stalemate in Time" August 1966 New Worlds) to Krono (1988) and Lurid Dreams (1990), and Robert Silverberg, whose even more numerous contributions range from the early "Hopper" (October 1956 Infinity; exp as The Time-Hoppers 1967) and Stepsons of Terra (1958) through the convoluted Up the Line (1969) to the neat "Many Mansions" (in Universe 3, anth 1973, ed Terry Carr) and the smooth "The Far Side of the Bell-Shaped Curve" (March 1982 Omni).
The time-paradox story may have posed an attractive challenge to sf writers but it has also been something of a wasting asset. All the elementary changes have been rung, and it now requires considerable cunning to find a new twist or even to redeploy an old one in more pointed or poignant fashion. Nevertheless, there still remains a good deal of life in the subgenre: Bob Shaw's Who Goes Here? (1977) slickly exploits the comic potential of the theme; Hilbert Schenck's A Rose for Armageddon (1982) is a brilliantly recomplicated Timeslip romance; Walter Jon Williams's Days of Atonement (1991) interrelates time paradox and quantum physics; and John Crowley's Great Work of Time (in Novelty, coll 1989; 1991) cleverly recombines several well worn themes to striking quasi-surreal effect. [MJE/BS]
see also: Maniac Mansion: Day of the Tentacle.
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