Since human beings are the only known form of fully sentient life, any encounter with nonhuman intelligence is necessarily speculative. Writers in the 17th and 18th centuries produced many tales of travel to and from other inhabited worlds, but works such as Voltaire’s Micromégas did not depict Saturnians as alien beings; they were men, though of Saturn-sized proportions.

A fuller knowledge of natural history enabled writers to imagine that life on other worlds might develop differently from life on Earth. In 1864 the astronomer and science popularizer Camille Flammarion published Les Mondes imaginaires et les mondes réels (“Imaginary Worlds and Real Worlds”), depicting otherworldly forms of life that could evolve within alien biological environments. This Gallic conceptual breakthrough was first exploited in fiction by J.H. Rosny Aîné, whose short story Les Xipéhuz (1887) describes an evolutionary war of extermination between prehistoric humans and a menacing crystal-based life-form.

Aliens were thus first conceived as Darwinian competitors with mankind, a scheme worked out in spooky Huxleyan detail by H.G. Wells, whose slimy, bloodsucking Martians possessed intellects “vast, and cool, and unsympathetic.” Wells’s The War of the Worlds (1898) was all the more successful for its implication that the highly advanced British Empire was finally experiencing from the other side the gunboat diplomacy that it had meted out to others. In 1938 Orson Welles’s radio adaptation of The War of the Worlds was mistaken by the gullible for actual news reportage of marauding Martians sacking and looting New Jersey. The episode provoked a famous attack of mass panic, making it perhaps the most famous radio drama of all time.

Wells’s The First Men in the Moon (1901) boosted antlike aliens into a sinister lunar analog for human society. The spate of alien invasion stories that followed were often strident in tone and genocidal in their predictions of coming doom. The “bug-eyed monster” became a staple of science fiction. Stanley G. Weinbaum won immediate and lasting acclaim with his more sophisticated approach in A Martian Odyssey (1934), which presented aliens whose behaviour, though whimsical, harmless, and colourful, was profoundly inexplicable to human mentality. In Raymond Z. Gallun’s Old Faithful (1934), the Martians tended to be quite decent sorts.

Authors of “serious” literature, such as Olaf Stapledon, also dealt with alien life forms. His Star Maker (1937) follows an Englishman whose disembodied mind travels across space and time, observing aliens as metaphysical actors in a fiery cosmic drama remote from all human concern, and encounters the creator of the universe (Star Maker). This critically acclaimed book is more a philosophical treatise on science, human nature, and God than a traditional novel. Stapledon’s descriptions and social-philosophical discourses on galactic empires, symbiotic alien life-forms, genetic engineering, ecology, and overpopulation inspired a number of SF writers, including Arthur C. Clarke, during the 1940s and ’50s.

As dramatic actors within a narrative, aliens pose unique difficulties. If too humanlike, they are of little use; if genuinely alien, they defy the fictional conventionalities of motive, conflict, and plot. In Stanisław Lem’s Solaris (1961; films 1972 and 2002), the sentience on an alien planet is so metaphysically distant from humanity that it causes its cosmonaut investigators to hallucinate and collapse. The Solaris alien is a permanent enigma, completely unframable by any human thought process. Hal Clement’s Mission of Gravity (1954) was a tour de force in that its hero is a tiny intelligent centipede-like creature who breathes poison gas in the crushing gravity of an alien world. This description alone makes it clear just how difficult imagining the alien can be. As a result, science fiction writers often centred their energies on a first contact with aliens, such as those found in Steven Spielberg’s film Close Encounters of the Third Kind (1977). In the “first contact” narrative, one can enjoy the novel thrill of alienness without having to confront the implications of everyday interactions with aliens.

Alien-invasion motifs persist in science fiction, as in the film Alien (1979) with its ruthless, parasitic monsters. Yet a distinct and growing trend within science fiction depicted aliens as coworkers, science officers, technical specialists, sidekicks, and even love interests. Two of the most prominent examples of this come from the various television shows, films, and novels based on the worlds of Star Trek and Alien Nation. It is also increasingly common for human characters to have undergone such extensive warping and mutation—as in Paul Di Fillipo’s Ribofunk (1996)—that they themselves are as exotic as aliens.

Aliens are supposed evolutionary products of life on different worlds, while intelligent robots are supposed mechanical, industrial creations. Robots and aliens therefore serve similar thematic purposes for science fiction. The first robots were introduced by Czech dramatist Karel Čapek as characters in his play R.U.R. (1921). In a rather standard alien-menace maneuver, Čapek’s robots outcompete humanity within the new milieu of industrial mass production and attempt to exterminate the human race.

Robots remain primarily theatrical inventions, but they are central figures in science fiction thought experiments intended to provoke debate about humanity’s place within a technological environment. Isaac Asimov, for example, devoted much effort to creating an ethical system for humans and robots. Asimov’s famous Three Laws of Robotics are as follows: “(1) a robot may not injure a human being or, through inaction, allow a human being to come to harm; (2) a robot must obey the orders given it by human beings except where such orders would conflict with the First Law; (3) a robot must protect its own existence as long as such protection does not conflict with the First or Second Law.”

Asimov was able to derive an entertaining set of novels and stories from these three premises—even though his imaginary laws have never been used for the control of any real-world robot. Quite to the contrary, 21st-century robotics are probably best represented by semiautonomous military devices such as the cruise missile, specifically designed to blow itself up as it reaches its target and to do considerable damage.

The robot as a reflection of humanity received a classic outing in Lester del Rey’s short story “Helen O’Loy” (1938). Helen was not the first female robot—her famous predecessor is the sinister celluloid robot Maria from the aforementioned film Metropolis (1927). Helen, by contrast, somehow establishes her womanhood by marrying her inventor and then sacrificing her own mechanical life upon her husband’s death. Male robots, in the hands of authors such as Tanith Lee (The Silver Metal Lover, 1981) and Marge Piercy (He, She, and It, 1991), became distorted images of human men.

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Humanoid robots, or androids, remain the photogenic darlings of SF cinema, appearing in a host of productions, including Westworld (1973), The Stepford Wives (1975 and 2004), Star Wars (1977), Bicentennial Man (1999), A.I. Artificial Intelligence (2001), and I, Robot (2004).

Space travel

Flight into outer space is the classic SF theme. Verne’s pioneering De la terre à la lune (1865; From the Earth to the Moon) was the first fiction to treat space travel as a coherent engineering problem—to recognize explicitly that gravity would cease, that there could be no air, and so forth. Because Verne found no plausible way to land his cannon-fired passengers on the lunar surface, they merely whiz by the Moon at close range, cataloging craters in a geographic ecstasy. At the conceptual dawn of space travel, it was enough just to be up there, escaping earthly bonds to revel in sheer extraterrestrial possibility. Given that Georges Méliès filmed a fictional trip to the Moon with his pioneering camera in 1902, science fiction cinema is as old as cinema itself.

A certain disenchantment with this theme necessarily set in after the actual Moon landing in 1969, for human life in outer space proved less than heavenly. Far from swashbucklers, astronauts and cosmonauts were highly trained technicians whose primary motive was to preserve their hardware. They grappled with strict limits in fuel, power, water, oxygen, and privacy, along with cramped personal quarters—a life more akin to submarine service than to a romantic flight aboard a luxury starship.

The SF works that treat space travel with nuts-and-bolts realism are a minority taste. Science fiction far more commonly omits the unromantic aspects of space travel, especially through one of the genre’s commonest stage devices, the “faster-than-light drive,” or “warp drive.” Although this imaginary technology is no more technically plausible than lifelike androids, it is a necessity for the alien-planet adventure story. Science fiction writers cheerfully sacrifice the realities of astrophysics in the service of imaginary worlds.

Much creative energy has been invested in “space opera,” science fiction at its most romantic. The space opera is an action adventure, commonly of galactic scale, of which the film cycle Star Wars (1977, 1980, 1983, 1999, 2002, 2005, 2015–17) is the best-known exemplar. It presents a unique type of “widescreen baroque,” with all the riches of pulp fiction in a single package. For example, Star Wars offers not only advanced scientific technology—presumably necessary to build the starships and orbiting battle stations—but also princesses, smugglers, robots, sword fights, mystical doctrines, levitating gurus, monsters, barroom brawls, heroes of dubious birth, elaborate chase scenes, and gothic death traps.

Like the black-clad figures who move the props in Japanese Noh theatre, the fantastic aspects of space opera are simply and gratefully accepted by its devotees. Writers of 20th-century space opera are among the most respected figures in science fiction. Their ranks include E.E. (“Doc”) Smith, Edmond Hamilton, John W. Campbell, Jack Williamson, A.E. Van Vogt, Jack Vance, Anne McCaffrey, Lois McMaster Bujold, and C.J. Cherryh. Nor is space opera by any means moribund, for a particularly extravagant form of space opera is the signature of the New British Science Fiction, the first SF literary movement of the 21st century. Introverted postimperial insularity had long characterized British science fiction, but in the 21st century a cluster of writers—including Iain M. Banks, Stephen Baxter, Justina Robson, Peter F. Hamilton, Charles Stross, and Ken MacLeod—reengineered the universe in gaudy bursts of star-smashing neo-cosmology.