The most common science fiction themes are those of alternate realities, possible future worlds, and of characters that have abilities beyond those of ordinary humans. Contact with extraterrestrial beings is also common in this genre of literature, and many science fiction books explore ideas of how life would dramatically change if humans meet and interacted with aliens. Other science fiction themes have underlying ideas of how a future world could possibly be different if humanity continues on its current path in the present, and this kind of narrative usually has a setting in either a dystopia or a utopia depending on the author's point of view. Technology often has an important role in science fiction stories as a tool for either good or evil.
The plot of a science fiction novel can often cause readers to question what could be possible beyond the confines of day-to-day existence. Stories of alternate realities sometimes describe phenomena such as rips in the fabric of the linear space-time continuum. These kinds of science fiction themes frequently explore the nature of time itself. Alternate realities can also raise questions of how much characters may or may not have control over what happens in their futures. Science fiction prose written with these themes often has some limited basis in factual scientific studies of phenomena such as black holes, worm holes, and dark matter.
Science fiction themes surrounding alien contact are frequently among the most popular stories. Many of these narratives describe worlds where humans live alongside alien races that have equal or often superior intelligence. Some of the most optimistic themes explore how humans could overcome their common shortcomings through the enlightened teachings of such an extraterrestrial race. Other stories detail the human race abandoning existing prejudices and infighting to unite in battle against an alien threat.
Additional science fiction themes concern the limits of the human mind. Characters in these stories frequently have abilities such as telepathy or psychokinesis, and whether they use these talents for good or evil explores more elements of human nature. When these abilities result from scientific experiments with advanced technology, the underlying theme of the story is a cautionary one that humans should not interfere with their own inborn capabilities.
A science fiction story set in a future world often raises ideas of where certain collective decisions and actions will eventually take the human race. A narrative about a grim dystopia often serves as a warning about the present. A science fiction story concerning a utopia often presents a much more optimistic view of the human race.