The Fermi paradox is the apparent contradiction between high estimates of the probability of the existence of extraterrestrial civilizations and the lack of evidence for, or contact with, such civilizations.
The extreme age of the universe and its vast number of stars suggest that if the Earth is typical, extraterrestrial life should be common. Discussing this proposition with colleagues over lunch in 1950, the physicist Enrico Fermi asked: "Where are they?" (alternatively, "Where is everybody?") Fermi questioned why, if a multitude of advanced extraterrestrial civilizations exist in the Milky Way galaxy, evidence such as spacecraft or probes are not seen.
A more detailed examination of the implications of the topic began with a paper by Michael Hart in 1975, and it is sometimes referred to as the Fermi-Hart paradox. Another closely related question is the Great Silence - even if travel is hard, if life is common, why don't we detect their radio transmissions?
There have been attempts to resolve the Fermi Paradox by locating evidence of extraterrestrial civilizations, along with proposals that such life could exist without human knowledge. Counterarguments suggest that intelligent extraterrestrial life does not exist or occurs so rarely that humans will never make contact with it.
Explaining the paradox theoretically:
No other civilizations currently exist.
The simplest explanation is that the human species is alone in the galaxy.
No other civilizations have arisen.
Those who believe that extraterrestrial intelligent life does not exist argue that the conditions needed for life—or at least complex life—to evolve are rare or even unique to Earth. This is known as the Rare Earth hypothesis.
It is the nature of intelligent life to destroy itself.
Technological civilizations may usually or invariably destroy themselves before or shortly after developing radio or space flight technology.
It is the nature of intelligent life to destroy others.
Another possibility is that intelligent species beyond a certain point of technological capability will destroy other intelligence as it appears.
Human beings were created alone.
They do exist, but we see no evidence.
It may be that technological extraterrestrial civilizations exist, but that human beings cannot communicate with them because of various constraints: problems of scale or of technology; because their nature is simply too alien for meaningful communication; or because human society refuses to admit to evidence of their presence.
Intelligent civilizations are too far apart in space or time.
It may be that non-colonizing technologically capable alien civilizations exist, but that they are simply too far apart for meaningful two-way communication.
It is too expensive to spread physically throughout the galaxy.
Human beings have not been searching long enough.
Human beings are not listening properly.
Civilizations only broadcast detectable radio signals for a brief period of time.
It may be that alien civilizations are detectable through their radio emissions for only a short time, reducing the likelihood of spotting them. There are two possibilities in this regard: civilizations outgrow radio through technological advance or, conversely, resource depletion cuts short the time in which a species broadcasts.
They tend to experience a technological singularity.
Another possibility is that technological civilizations invariably experience a technological singularity and attain a posthuman (or postalien) character. Theoretical civilizations of this sort may have altered drastically enough to render communication impossible. The intelligences of a post-singularity civilization might require more information exchange than is possible through interstellar communication, for example. Or perhaps any information humanity might provide would appear elementary, and thus they do not try to communicate, any more than human beings attempt to talk to ants.
They choose not to interact with us.
Earth is purposely isolated (The zoo hypothesis).
It is possible that the belief that alien races would communicate with the human species is a fallacy, and that alien civilizations may not wish to communicate, even if they have the technical ability. A particular reason that alien civilizations may choose not to communicate is the so-called Zoo hypothesis: the idea that Earth is being monitored by advanced civilizations for study, or is being preserved in an isolated "zoo or wilderness area".
They are too alien.
Another possibility is that human theoreticians have underestimated how much alien life might differ from that on Earth. Alien psychologies may simply be too different to communicate with human beings, and they are unable or unwilling to make the attempt. Human mathematics, language, tool use, and other concepts and communicative capacity may be parochial to Earth and not shared by other life.
They are non-technological.
It is not clear that a civilization of intelligent beings must be technological. If an alien species does not develop technology, because it is difficult in its environment, because it chooses not to, or for any other reason, it will be very hard for human beings to detect.
They are here unobserved.
It may be that intelligent alien life forms not only exist, but are already present here on Earth. They are not detected because they do not wish it, human beings are technically unable, or because societies refuse to admit to the evidence.