A complement to travel through space is travel through time. A prototype of the time travel story is Charles Dickens’s A Christmas Carol (1843). The story features the Ghost of Christmas Yet to Come, who is magically able to immerse the hapless Scrooge in the dire consequences of his own ungenerous actions. But for all their familiarity, Scrooge’s time travels were mere ghostly dreammongering. The SF version of time travel arrived when H.G. Wells suggested in The Time Machine (1895) that the process might be done mechanically.

For a genre whose central issues involve processes of historical change, time travel is irresistibly attractive. For instance, time travel offers the edifying spectacle of “moderns” traveling into the past to remake the world closer to the heart’s desire. Mark Twain’s A Connecticut Yankee in King Arthur’s Court (1889) contrasts industrial ingenuity with feudal romance, to darkly hilarious effect. L. Sprague de Camp’s novel Lest Darkness Fall (1941) has an American archaeologist rescuing Imperial Rome in its decline, an act the hero carries out with such luminous attention to techno-historical detail that it resembles a World Bank bailout of an underdeveloped country. In Stephen King’s 11/22/63 (2011; television series 2016), attempts to stop the assassination of U.S. Pres. John F. Kennedy and thus restore a more-promising alternate history do not go according to plan.

Time tourism, a distinct subgenre, is a perennial SF theme. It is exemplified in Ray Bradbury’s A Sound of Thunder (1952; film 2005), in which a tiny misstep by dinosaur hunters grimly affects the consequent course of history. In Robert Silverberg’s Up the Line (1969), voyeuristic thrill seekers from the future infest the past.

Another variant on the time travel theme involves physical objects that become displaced in time. C.M. Kornbluth’s The Little Black Bag (1950) concerns a doctor’s bag from the future. Warring groups of time travelers battle one another up and down the time streams in Poul Anderson’s Guardians of Time (1960) and Fritz Leiber’s The Change War (1978). Barrington J. Bayley’s Fall of Chronopolis (1974) achieves the technicolour proportions of “time opera.” In John Kessel’s Corrupting Dr. Nice (1997), cynical exploiters from the future invade the past wholesale, kidnapping major historical figures and crassly employing them as underlings and talk-show hosts.

A one-way trip into the future is the staple of the suspended-animation story, the device behind the Buck Rogers stories and a host of consequent tales in which a hero of the present-day escapes the customary time-bound limits of human mortality. In Martin Amis’s Time’s Arrow (1991), the flow of time is entirely reversed, but life seems just as precarious as people solemnly march to a final end in their mother’s wombs.

The long-lived British television series Doctor Who (1963–89, 2005– ) involved an eccentric time traveler whose exotic mode of transport was disguised as a common blue police box. Periodically portrayed by different actors, the Doctor exhibited a popularity so perennial that he indeed seemed timeless. The popularity of the notion can be seen in any number of time-travel films, including The Time Machine (1960 and 2002), Slaughterhouse-Five (1972), Time Bandits (1981), Back to the Future (1985), Terminator (1984), Twelve Monkeys (1995), Primer (2004), and Looper (2012).

Alternate histories and parallel universes

Stories centred on time-travel paradoxes developed as a separate school of science fiction. If a human being broke free from the conventional chains of causality, intriguing metaphysical puzzles ensued. The classic SF version of these puzzles is the challenge posed by a man who travels back in time and kills his own grandfather, thus ensuring that he, the time traveler, can never be born in the first place. Time-travel paradoxes were usually resolved as ingeniously as locked-room murder mysteries.

Murray Leinster’s Sidewise in Time (1934) expanded the possibilities by suggesting a vast multiplicity of “histories,” all occurring at the same “time.” Under the scheme Leister proposed, one need not limit oneself to one past or one future but might travel between many alternate worlds existing in parallel. This new SF convention of a “multiverse” opened a vast potential canvas for fictional exploitation, with humanity’s universe just one undistinguished universe among many.

Narratives set in the future offered at least some potential connection to the real world. By contrast, the “parallel universe” was entirely conjectural and hypothetical. Initially, readers found parallel worlds an amusing but inconsequential conceit, just as they had once found works set within the future academic or absurd. They soon realized, however, that the notion of uchronia (or “no-times”) offered certain pleasures all its own, such as the ability to deploy actual historical figures as fictional characters. Well-known settings and events could be mutated and distorted at will.

The passage of time had a complex, uchronic effect on science fiction itself. Despite the passing of the year 1984 itself, a number of concepts presented in Nineteen Eighty-four—such as omnipresent video surveillance—were not so far-fetched at the turn of the 21st century, and Orwell’s political concerns remain painfully relevant. In addition to representing the uchronic effect of some works of science fiction, Nineteen Eighty-four is an excellent example of a uchronic novel; it is neither futuristic nor historical, existing in a peculiar uchronic netherworld. As time passes, growing numbers of SF classics fall into this conceptual category. It is a small step from this category to parallel worlds and alternate histories. Those concepts no longer seem abstract and improbable, but they have become part of the heritage of science fiction.

Even historical fiction has dealt with the “what if” posed by uchronias. In 1907 G.M. Trevelyan wrote an essay speculating on the consequences of a Napoleonic victory at Waterloo. Trevelyan’s work inspired J.C. Squires’s anthology If It Had Happened Otherwise (1931), in which such period worthies as Winston Churchill, André Maurois, and G.K. Chesterton speculated on counterfactual historical turning points. This was an intellectual parlour game of the type that science fiction liked to play.

Alternate histories existed well outside the customary bounds of science fiction, such as Len Deighton’s thriller SS-GB (1978), about the grim role of Nazi occupiers in Britain, and Vladimir Nabokov’s involved and elegant Ada (1969). Alternate histories tend to cluster around particularly dramatic and colourful junctures of history, with World War II and the American Civil War as particular favourites. Some ventured farther out, postulating a global Roman Empire or a world in which dinosaurs avoided extinction.

The film It’s a Wonderful Life (1946)—based on the story “The Greatest Gift” (1943) by Philip Van Doren Stern—is a perennial sentimental favourite. In the film, a man in despair learns that his life does matter when he sees that, without his presence, his hometown becomes an evil dystopia. It is an ultimate compliment to the individual when the universe rewrites itself around a fantasy of self-worth.

In some deep sense, all works of fiction must be alternate histories and parallel worlds, for their protagonists and described events do not in fact exist. As the tradition of fiction grew longer and deeper, presenting works ever more distant from the reader’s cultural framework, readers seemed more willing to accept work that was radically detached from local truisms of time and space.