Science fiction [and extraterrestrials]
Inhabitants of the Moon were described in stories by Lucian around 150 AD and Johannes Kepler in 1634--and in both cases were closely modelled on terrestrial organisms. Interest in fiction about extraterrestrials increased greatly at the end of the 1800s--perhaps because by then few parts of the Earth remained unexplored. And as science fiction developed, accounts of the future sometimes treated extraterrestrials as commonplace--and sometimes did not mention them at all. Most often extraterrestrials have been easy to recognize, being little more than simple combinations of terrestrial animals (and occasionally plants)--though fairly often with extra features like telepathy. Some stories have nevertheless explored extraterrestrial intelligence based for example on solids, gases or energy fields. An example is Fred Hoyle's 1957 The Black Cloud in which a large cloud of hydrogen gas achieves intelligence by exchanging electromagnetic signals between rocks whose surface molecular configurations store memories.
The most common fictional scenario for first contact with extraterrestrials is the arrival of spacecraft--often induced by us having passed a technology threshold such as radio, nuclear explosions or faster-than-light travel. Other scenarios sometimes considered include archeological discovery of extraterrestrial artifacts and receipt of radio signals.
In the movie 2001 a black cuboid with side ratios 1:4:9 detected on the Moon through its anomalous magnetic properties sends a radio pulse in response to sunlight. Later there are also a few frames of flashing octahedra, presumably intended to be extraterrestrial artifacts, or perhaps extraterrestrials themselves.
In The Black Cloud intelligence is suggested by responsiveness to radio stimuli. Communication is established--as often in science fiction--by the intelligence interpreting material that we supply, and then replying in the same format.
The movie Contact centers on a radio signal with several traditional SETI ideas: it is transmitted at 1420 π MHz, and involves a sequence of primes to draw attention, an amplified TV signal from Earth and a description of a machine to build.
The various Star Trek television series depict many encounters with "new life and new civilizations". Sometimes intelligence is seen not associated with something that is considered a lifeform.
Particularly in short stories various scenarios have been explored where it is difficult ever to recognize intelligence. These include one-of-a-kind beings that have nothing to communicate with, as well as beings with inner intellectual activity but no effect on the outside world. When there are extraterrestrials substantially more advanced than humans few efforts have been made to describe their motives and purposes directly--and usually what is emphasized is just their effects on humans.
(See also page 1184.)