The Drake equation is a probabilistic argument used to estimate the number of active, communicative extraterrestrial civilizations in the Milky Way Galaxy.
The Fermi paradox is the discrepancy between the lack of conclusive evidence of advanced extraterrestrial life and the apparently high likelihood of its existence. As a 2015 article put it, "If life is so easy, someone from somewhere must have come calling by now."
The Hainish Cycle consists of a number of science fiction novels and stories by Ursula K. Le Guin. It is set in a future history in which civilizations of human beings on planets orbiting a number of nearby stars, including Terra ("Earth"), are contacting each other for the first time and establishing diplomatic relations, and setting up a confederacy under the guidance of the oldest of the human worlds, peaceful Hain. In this history, human beings did not evolve on Earth but were the result of interstellar colonies planted by Hain long ago, which was followed by a long period when interstellar travel ceased. Some of the races have new genetic traits, a result of ancient Hainish experiments in genetic engineering, including people who can dream while awake, and a world of hermaphroditic people who only come into active sexuality once a month, not knowing which sex will manifest in them. In keeping with Le Guin's style, she uses varied social and environmental settings to explore the anthropological and sociological outcomes of human evolution in those diverse environments.
Apocalyptic and post-apocalyptic fiction is a subgenre of science fiction in which the Earth's civilization is collapsing or has collapsed. The apocalypse event may be climatic, such as runaway climate change; astronomical, an impact event; destructive, nuclear holocaust or resource depletion; medical, a pandemic, whether natural or human-caused; end time, such as the Last Judgment, Second Coming or Ragnarök; or any other scenario in which the outcome is apocalyptic, such as a zombie apocalypse, cybernetic revolt, technological singularity, dysgenics or alien invasion.
Strata is a 1981 science fiction comedy novel by Terry Pratchett. It is one of Pratchett's first novels and one of the few purely science fiction novels he wrote, along with The Dark Side of the Sun.
The Majipoor series is a series of novels and stories by American writer Robert Silverberg, set on the planet Majipoor. The setting is a mixture of science fiction and fantasy elements.
The concept of self-replicating spacecraft, as envisioned by mathematician John von Neumann, has been described by futurists and has been discussed across a wide breadth of hard science fiction novels and stories. Self-replicating probes are sometimes referred to as von Neumann probes. Self-replicating spacecraft would in some ways either mimic or echo the features of living organisms or viruses.
The Noon Universe is a fictional future setting for a number of hard science fiction novels written by Arkady and Boris Strugatsky. The universe is named after Noon: 22nd Century, chronologically the first novel from the series and referring to humanity reaching its noon in the 22nd century.
Root races are concepts in the esoteric cosmology of Theosophy. As described in Helena Petrovna Blavatsky's book The Secret Doctrine (1888), these races correspond to stages of human evolution, and existed mainly on now-lost continents. Blavatsky's model was developed by later theosophists, most notably William Scott-Elliot in The Story of Atlantis (1896) and The Lost Lemuria (1904). Annie Besant further developed the model in Man: Whence, How and Whither (1913). Both Besant and Scott-Elliot relied on information from Charles Webster Leadbeater obtained by "astral clairvoyance". Further elaboration was provided by Rudolf Steiner in Atlantis and Lemuria (1904). Rudolf Steiner, and subsequent theosophist authors, have called the time periods associated with these races Epochs.
The Saga of Seven Suns is a series of seven space opera novels by American writer Kevin J. Anderson, published between 2002 and 2008. The books are set in a not-too-distant future where humans have colonized a number of other planets across the galaxy, thanks in part to technological assistance from an ancient alien race, the Ildirans. The series chronicles the universe-spanning war that erupts when humans inadvertently ignite the fury of a hidden empire of elemental aliens known as the hydrogues. Internal conflict is sparked within both the human and Ildiran empires as other ancient elemental races reappear to renew their own ancient war with the hydrogues.
Subterranean fiction is a subgenre of speculative fiction, science fiction, or fantasy which focuses on fictional underground settings, sometimes at the center of the Earth or otherwise deep below the surface. The genre is based on, and has in turn influenced, the Hollow Earth theory. The earliest works in the genre were Enlightenment-era philosophical or allegorical works, in which the underground setting was often largely incidental. In the late 19th century, however, more pseudoscientific or proto-science-fictional motifs gained prevalence. Common themes have included a depiction of the underground world as more primitive than the surface, either culturally, technologically or biologically, or in some combination thereof. The former cases usually see the setting used as a venue for sword-and-sorcery fiction, while the latter often features cryptids or creatures extinct on the surface, such as dinosaurs or archaic humans. A less frequent theme has the underground world much more technologically advanced than the surface one, typically either as the refugium of a lost civilization, or as a secret base for space aliens.
The legendary island of Atlantis has often been depicted in literature, television shows, films and works of popular culture.
The Helliconia trilogy is a series of science fiction books by British writer Brian W. Aldiss, set on the Earth-like planet Helliconia. It is an epic chronicling the rise and fall of a civilisation over more than a thousand years as the planet progresses through its incredibly long seasons, which last for centuries.
The Alliance–Union universe is a fictional universe created by American writer C. J. Cherryh. It is the setting for a future history series extending from the 21st century into the far future.
City of the Chasch is a science fiction novel by American writer Jack Vance, the first in the adventure tetralogy Planet of Adventure. It follows the attempts of a man stranded on the distant planet Tschai to return to Earth.
Shadow World is a high-fantasy campaign setting situated on the fictional planet of Kulthea. Originally produced for the Rolemaster role-playing game system, Shadow World is fairly compatible with other d100 games such as High Adventure Role Playing (HARP), Middle-earth Role Playing (MERP) or Against the Darkmaster. The setting blends traditional fantasy elements, such as elves, dwarves, and magic, with science fiction, including such elements as space and time travel, and, to a degree, futuristic technology.
The Viagens Interplanetarias series is a sequence of science fiction stories by L. Sprague de Camp, begun in the late 1940s and written under the influence of contemporary space opera and sword and planet stories, particularly Edgar Rice Burroughs's Martian novels. Set in the future in the 21st and 22nd centuries, the series is named for the quasi-public Terran agency portrayed as monopolizing interstellar travel, the Brazilian-dominated Viagens Interplanetarias. It is also known as the Krishna series, as the majority of the stories belong to a sequence set on a fictional planet of that name. While de Camp started out as a science fiction writer and his early reputation was based on his short stories in the genre, the Viagens tales represent his only extended science fiction series.
Ancient astronauts have been addressed frequently in science fiction and horror fiction. Occurrences in the genres include:
Dying Earth is a subgenre of science fantasy or science fiction which takes place in the far future at either the end of life on Earth or the end of time, when the laws of the universe themselves fail. Dominant themes include world-weariness, innocence, idealism, entropy, heat death of the universe, exhaustion or depletion of many or all resources, and the hope of renewal. A related subgenre set in the distant future of entropic decay is called entropic romance.
Golden Witchbreed is a novel by Mary Gentle published in 1983.
