en.m.wikipedia.org
The
Fermi paradox, named after
Italian-American physicist
Enrico Fermi, is the apparent contradiction between the lack of evidence for
extraterrestrial civilizations and various high estimates for their
probability (such as some optimistic estimates for the
Drake equation).
[1][2]

A graphical representation of the
Arecibo message, humanity's first attempt to use radio waves to actively communicate its existence to alien civilizations
The following are some of the facts that together serve to highlight the apparent contradiction:
- There are billions of stars in the Milky Way similar to the Sun.[3][4]
- With high probability, some of these stars have Earth-like planets.[5]
- Many of these stars, and hence their planets, are much older than the Sun.[6][7] If the Earth is typical, some may have developed intelligent life long ago.
- Some of these civilizations may have developed interstellar travel, a step humans are investigating now.
- Even at the slow pace of currently envisioned interstellar travel, the Milky Way galaxy could be completely traversed in a few million years.[8]
- And since many of the stars similar to the Sun are billions of years older, the Earth should have already been visited by extraterrestrial civilizations, or at least their probes.[9]
- However, there is no convincing evidence that this has happened.[8]
There have been many attempts to explain the Fermi paradox,
[10][11] primarily suggesting that
intelligent extraterrestrial beings are extremely rare, that the lifetime of such civilizations is short, or that they exist but (for various reasons) we see no evidence.
Although he was not the first to consider this question, Fermi's name is associated with the paradox because of a casual conversation in the summer of 1950 with fellow physicists
Edward Teller,
Herbert York and
Emil Konopinski. While walking to lunch, the men discussed recent
UFO reports and the possibility of
faster-than-light travel. The conversation moved on to other topics, until during lunch Fermi allegedly said suddenly, "But where is everybody?" (although
the exact quote is uncertain).
[12][13]