The question of extraterrestrial intelligence and the existential question "Are we alone in the universe?" is one of the oldest and most popular in science and is a popular theme in science fiction. The basic assumption behind the existence of extraterrestrial intelligence inferred from the existence of human intelligence and the size of the known universe.
The Copernican principle is generalized to the relativistic concept that humans are not privileged observers of the Universe. Many prominent scientists, including Stephen Hawking have proposed that the sheer scale of the universe makes it improbable for intelligent life not to have emerged elsewhere. However Fermi's Paradox highlights the apparent contradiction between high estimates of the probability of the existence of extraterrestrial civilization and humanity's lack of contact with, or evidence for, such civilizations.
The Kardashev scale is a method of measuring a civilization's level of technological advancement, based on the amount of energy a civilization is able to utilize. The Drake equation is an estimation of how many planets in the Milky Way are inhabited by intelligent life forms.
The extraterrestrial hypothesis is the idea that some UFOs are vehicles containing or sent by extraterrestrial beings (usually called aliens in this context). As an explanation for UFOs, ETI is sometimes contrasted with EDI (extradimensional intelligence), for example by Allen Hynek.
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