The universe contains hundreds of billions of galaxies and, in our own Milky Way alone, an estimated 100 to 400 billion stars. Many of those stars have planets. Given so many chances for life to appear, why do we see no evidence of anyone else? This puzzle is called the Fermi paradox, and it is one of the best-known open questions in science. It is also a useful, honest counterweight to the viral claims about an alien arrival that we address in Will Aliens Arrive in 2029?
Where does the name come from?
The question is attributed to the physicist Enrico Fermi. According to accounts from colleagues, in the summer of 1950 Fermi was having lunch at Los Alamos National Laboratory with fellow physicists when the conversation turned to reports of flying saucers and the possibility of interstellar travel. Partway through, Fermi is said to have asked, "Where is everybody?" The remark became shorthand for the contradiction between the high estimated odds of other civilizations and the absence of evidence for them. Michael Hart published an influential 1975 paper arguing that the absence of visitors suggests intelligent life is rare.
The argument in three steps
Many places for life. The Milky Way has hundreds of billions of stars, and surveys show that planets are common. More than 5,000 exoplanets are confirmed, and statistics suggest that a large fraction of stars have planets, including some in the habitable zone.
Plenty of time. The galaxy is roughly 13 billion years old, and our solar system formed about 4.6 billion years ago. Many stars are billions of years older than the Sun, so other civilizations could have had a long head start.
Expansion looks possible. Even at modest fractions of the speed of light, calculations suggest a civilization could spread across the galaxy in a few million years, which is short compared with the age of the galaxy.
Put together, this suggests the galaxy should already be visibly occupied. It does not appear to be.
Possible explanations
There is no consensus answer. Researchers group the proposals roughly like this:
1. They are rare or absent
Rare Earth. Complex life may need an unusual combination of conditions: a stable star, a large moon, plate tectonics, liquid water for billions of years. If that combination is very rare, we may be nearly alone.
The Great Filter. Economist Robin Hanson proposed in the 1990s that at least one step on the path from dead matter to a space-faring civilization is extremely unlikely. The filter could be behind us (for example, the origin of life or of complex cells), which would be good news, or ahead of us, which would not.
2. They exist but we cannot see them
Distance and time. Space is vast, light takes time to travel, and civilizations may appear and vanish at different eras, never overlapping.
Different technology. Their signals might not look like anything we know how to detect, or they might not use radio at all.
We have barely looked. This is a strong and sober point. Searches so far have covered only a tiny fraction of the possible sky, frequencies and times. Absence of evidence is weak evidence when the search has been this limited.
3. They are here, or hiding
Ideas such as the "zoo hypothesis" suggest that others deliberately avoid contact. These are not testable with current evidence. Official investigations of unidentified anomalous phenomena have not found verifiable evidence of extraterrestrial technology, as we explain in UFO Sightings Explained.
Is it really a paradox?
Some scientists argue that it is not a paradox but a set of estimates with huge uncertainties. We do not know how likely life is to start, how often it becomes intelligent, or how long technological civilizations last. A 2018 analysis by researchers at the University of Oxford's Future of Humanity Institute argued that, once those uncertainties are properly propagated, it is quite plausible that we are alone in the observable universe. Others disagree. The honest position is that we have one example of life, our own, and cannot yet draw strong statistical conclusions from a sample of one.
How science is trying to answer it
Exoplanet surveys are measuring how common Earth-size planets in habitable zones are, a step explored in the Drake equation.
Atmosphere studies with telescopes such as the James Webb Space Telescope look for chemical combinations that could indicate biology.
SETI programs search for radio and optical signals.
Solar system exploration checks Mars, Europa, Enceladus and other worlds for signs of past or present life. Missions like OSIRIS-REx, which returned material from Bennu, also analyze the organic building blocks found on asteroids. See OSIRIS-REx and Bennu.
Key takeaways
The Fermi paradox contrasts the high estimated odds of other civilizations with our lack of evidence for them.
Explanations range from "life is rare" to "we have not searched enough," and none is proven.
It gives no support to claims of a specific alien arrival date, and no scientific evidence supports such claims.
Sources and further reading
Facts in this article are drawn from public scientific sources. For primary information, see: