Science fiction (abbreviated SF or sci-fi with varying punctuation and capitalization) is a broad genre of fiction that often involves speculations based on current or future science or technology. Science fiction is found in books, art, television, films, games, theatre, and other media. In organizational or marketing contexts, science fiction can be synonymous with the broader definition of speculative fiction, encompassing creative works incorporating imaginative elements not found in contemporary reality; this includes fantasy, horror, and related genres.
Science fiction differs from fantasy in that, within the context of the story, its imaginary elements are largely possible within scientifically established or scientifically postulated laws of nature (though some elements in a story might still be pure imaginative speculation). Exploring the consequences of such differences is the traditional purpose of science fiction, making it a "literature of ideas".Science fiction is largely based on writing entertainingly and rationally about alternate possibilities in settings that are contrary to known reality.
These may include:
- A setting in the future, in alternative time lines, or in a historical past that contradicts known facts of history or the archeological record
- A setting in outer space, on other worlds, or involving aliens
- Stories that involve technology or scientific principles that contradict known laws of nature
- Stories that involve discovery or application of new scientific principles, such as time travel or psionics, or new technology, such as nanotechnology, faster-than-light travel or robots, or of new and different political or social systems (e.g. a dystopia) 
Time travel stories have antecedents in the 18th and 19th centuries, and this subgenre was popularized by H. G. Wells's novel The Time Machine. Stories of this type are complicated by logical problems such as the grandfather paradox. Time travel is a popular subject in novels, and in television series, either as individual episodes within more general science fiction series, for example, "The City on the Edge of Forever" in Star Trek, or as one-off productions such as The Flipside of Dominick Hide.Terminator,time cop,back to the future ,and many more
Alternate history stories are based on the premise that historical events might have turned out differently. These stories may use time travel to change the past, or may simply set a story in a universe with a different history from our own. Classics in the genre include Bring the Jubilee by Ward Moore, in which the South wins the American Civil War and The Man in the High Castle, by Philip K. Dick, in which Germany and Japan win World War II. ,back to the future ,The Sidewise Award acknowledges the best works in this subgenre; the name is taken from Murray Leinster's early story Sidewise in Time. Harry Turtledove is one of the most prominent authors in the subgenre and is often called the "master of alternate history."
Military science fiction is set in the context of conflict between national, interplanetary, or interstellar armed forces; the primary viewpoint characters are usually soldiers. Stories include detail about military technology, procedure, ritual, and history; military stories may use parallels with historical conflicts. Heinlein's Starship Troopers is an early example,robocop along with the Dorsai novels of Gordon Dickson. Joe Haldeman's The Forever War is a critique of the genre, a Vietnam-era response to the World War II-style stories of earlier authors.Prominent military SF authors include David Drake, David Weber, and S. M. Stirling. Baen Books is known for cultivating military science fiction authors.
Superhuman stories deal with the emergence of humans who have abilities beyond the norm. This can stem either from natural causes such as in Olaf Stapledon's novel Odd John, or be the result of intentional augmentation such as in A.E. Van Vogt's novel Slan. These stories usually focus on the alienation that these beings feel as well as society's reaction to them. These stories have played a role in the real life discussion of human enhancement.,x-men,Superman,Batman,Spiderman,the list is endless
Apocalyptic fiction is concerned with the end of civilization through nuclear war, plague, or some other general disaster or with a world or civilization after such a disaster. Typical of the genre are George R. Stewart's novel Earth Abides and Pat Frank's novel Alas, Babylon. Apocalyptic fiction generally concerns the disaster itself and the direct aftermath, while post-apocalyptic can deal with anything from the near aftermath to hundreds or thousands of years in the future, such as in Russell Hoban's novel Riddley Walker and in George Orwell's classic book, Nineteen Eighty-Four,The day after tomorrow,Deep impact,Armageddon to name but a few
Space opera emphasizes romantic, often melodramatic adventure, set mainly or entirely in space, generally involving conflict between opponents possessing powerful (and sometimes quite fanciful) technologies and abilities. Perhaps the most significant trait of space opera is that settings, characters, battles, powers, and themes tend to be very large-scale. These stories typically follow the Homeric tradition, in which a small band of adventurers are cast against larger-than-life backdrops of powerful warring factions. Alastair Reynolds' Revelation Space series and the immensely popular Star Wars trilogies are examples of this genre.Star Trek etc fit into this
Space Westerns could be considered a sub-genre of Space Opera that transposes themes of the American Western books and film to a backdrop of futuristic space frontiers. These stories typically involve "frontier" colony worlds (colonies that have only recently been terraformed and/or settled) serving as stand-ins for the backdrop of lawlessness and economic expansion that were predominate in the American west such as Firefly (TV series) by Joss Whedon and the accompanying movie Serenity (film).
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