The Fermi paradox is the apparent contradiction between the lack of evidence for extraterrestrial civilizations and various high estimates for their probability. Named after Italian-American physicist Enrico Fermi.
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?”
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.
- With high probability, some of these stars have Earth-like planets.
- 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, the Earth should have already been visited by extraterrestrial civilizations, or at least their probes.
- However, there is no convincing evidence that this has happened.
The first aspect of the Fermi paradox is a function of the scale or the large numbers involved: there are an estimated 200–400 billion stars in the Milky Way (2–4 × 1011) and 70 sextillion (7×1022) in the observable universe.
Even if intelligent life occurs on only a minuscule percentage of planets around these stars, there might still be a great number of extant civilizations, and if the percentage were high enough it would produce a significant number of extant civilizations in the Milky Way. This assumes the mediocrity principle, by which the Earth is a typical planet.
The second aspect of the Fermi paradox is the argument of probability: given intelligent life’s ability to overcome scarcity, and its tendency to colonize new habitats, it seems possible that at least some civilizations would be technologically advanced, seek out new resources in space, and colonize their own star system and, subsequently, surrounding star systems. Since there is no significant evidence on Earth, or elsewhere in the known universe, of other intelligent life after 13.8 billion years of the universe’s history, there is a conflict requiring a resolution.
Some examples of possible resolutions are that intelligent life is rarer than we think, that our assumptions about the general development or behavior of intelligent species are flawed, or, more radically, that our current scientific understanding of the nature of the universe itself is quite incomplete.
There are two parts of the Fermi paradox that rely on empirical evidence—that there are many potential habitable planets, and that we see no evidence of life.
The first point, that many suitable planets exist, was an assumption in Fermi’s time but is now supported by the discovery that exoplanets are common. Current models predict billions of habitable worlds in our galaxy.
The second part of the paradox, that we see no evidence of extraterrestrial life, is also an active field of scientific research. This includes both efforts to find any indication of life, and efforts specifically directed to finding intelligent life. These searches have been made since 1960, and several are ongoing.
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) we see no evidence.
Some hypothetical explanations for the paradox:
- Extraterrestrial life is rare or non-existent.
- Extraterrestrial intelligence is rare or non-existent.
- Periodic extinction by natural events.
- Intelligent alien species haven’t developed advanced technologies.
- It is the nature of intelligent life to destroy itself.
- It is the nature of intelligent life to destroy others.
- Colonization is not the cosmic norm.
- Alien species may have only settled part of the galaxy.
- Alien species may not live on planets.
- Intelligent life may be too far away.