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.
[1] Discussing this proposition over lunch in 1950, the
physicist Enrico 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 H. Hart in 1975, and it is sometimes referred to as the
Fermi-Hart paradox.
[2] Another closely related question is the
Great Silence[3]—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.
Starting with Hart, a great deal of effort has gone into developing scientific theories about, and possible models of, extraterrestrial life, and the Fermi paradox has become a theoretical reference point in much of this work. The problem has spawned numerous scholarly works addressing it directly, while various questions that relate to it have been addressed in fields as diverse as
astronomy,
biology,
ecology, and
philosophy. The emerging field of
astrobiology has brought an interdisciplinary approach to the Fermi paradox and the question of extraterrestrial life.