The Fermi paradox, named after Italian-American physicist Enrico Fermi,
is the apparent contradiction between the lack of evidence for extraterrestrial life
and various high estimates for their probability
(such as some optimistic estimates for the Drake equation).
The following are some of the facts and hypotheses
that together serve to highlight the apparent contradiction:
There are billions of stars in the Milky Way similar to the Sun.
With high probability,
some of these stars have Earth-like planets in a circumstellar habitable zone.
Many of these stars, and hence their planets, are much older than the Sun.
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.
And since many of the stars similar to the Sun are billions of years older,
Earth should have already been visited by extraterrestrial civilizations,
or at least their probes.
However, there is no convincing evidence that this has happened.
There have been many attempts to explain the Fermi paradox,
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) humans 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).