The verdict
A puzzle about our own assumptions rather than about aliens, and the most productive question in the genre.
Quick facts
- Also called
- The Fermi paradox, the Great Silence
- The question
- If the galaxy is old and large, where is everybody?
- Origin
- Enrico Fermi, over lunch at Los Alamos, 1950
- The formal version
- The Drake equation, 1961, which organises the ignorance rather than solving it
- Leading explanations
- Life is rare, civilisations do not last, or they are there and we cannot tell
- Status
- Unresolved
The Fermi paradox is the gap between two things that both seem reasonable: the galaxy is old enough and large enough that civilisations should be everywhere, and we have found no sign of a single one. It is not a puzzle about aliens. It is a puzzle about our own assumptions, and nobody has solved it.
The question itself
The galaxy is around thirteen billion years old and contains hundreds of billions of stars, a large proportion of which are older than the Sun. If life arises readily, and if any civilisation survives long enough to travel between stars, then even at very modest speeds a single species could spread across the whole galaxy in a small fraction of its lifetime.
The paradox is that we see no evidence of any of it. No signals, no structures, no artefacts, nothing. Either something is preventing civilisations from arising, or something is preventing them from lasting, or something is preventing us from noticing.
Where the argument comes from
The name comes from a conversation. In 1950, over lunch at Los Alamos, Enrico Fermi and a group of colleagues fell to discussing flying saucers and interstellar travel, and Fermi asked the question that has carried his name ever since: where is everybody?
The formal version arrived in 1961, when Frank Drake set out an equation for estimating the number of communicating civilisations in the galaxy. Its value is not the number, which is unknowable, but the structure: it makes clear that the uncertainty sits in the terms about whether life starts and whether civilisations last, and not in the astronomy.
The leading explanations
They fall into three groups, and all three are live.
- Life is rare. The conditions for complex life may be far narrower than we assumed, in which case we are early or alone. This is the argument in the book Rare Earth, and it has got stronger as we have learned how much of Earth’s habitability depends on accidents.
- Civilisations do not last. The Great Filter is the name for whatever it is that stops a species from becoming interstellar, whether it sits before us or ahead of us. Robin Hanson put the term into circulation in 1998, and the uncomfortable version is that the filter may be the technology itself.
- They are there and we cannot tell. Detection windows may be short, signals may be narrow, or civilisations may deliberately stay quiet. Any of these makes a silent sky perfectly consistent with a crowded one.
There is also the possibility that we are looking at the wrong thing entirely. Every search so far has assumed that another civilisation would want to be found, or would leak something we can detect, and neither is guaranteed.
Why it matters
The paradox is a mirror. Almost every explanation says more about the person offering it than about the galaxy, and the two most popular readings lead to opposite moods: either we are precious and rare, or we are unremarkable and soon to be corrected.
It also has real scientific content. As exoplanet surveys have made the astronomical terms of Drake’s equation firmer, the uncertainty has concentrated on the biological and social terms, which has focused attention on questions about the origin of life that are genuinely being worked on.
The Fermi paradox in science fiction
It is the single most productive question in the genre, because it hands a writer a whole galaxy and then removes everybody from it. Revelation Space has the most complete answer on this site: something is out there, it is vast, it is indifferent, and it has been tidying up for a very long time.
The other classic is the dark forest, from The Three-Body Problem: the reason nobody answers is that answering is fatal. Pandora’s Star takes the opposite line, in which the first alien species humanity meets is aggressive in a way that makes the silence look like mercy.
Common misunderstandings
It is not evidence that we are alone, and it is not a proof of anything. A null result from a small search of a large galaxy is what you would expect whichever answer is true.
It is also not the same as the Drake equation. Drake gives you a way to organise the ignorance; Fermi is the observation that the answer appears to be zero.
Frequently asked questions
Who was Enrico Fermi?
An Italian physicist who built the first nuclear reactor and worked on the Manhattan Project. The question that carries his name was asked at lunch in 1950, in passing.
Has the Fermi paradox been solved?
No. Every proposed explanation is either untestable at present or depends on assumptions about life and civilisations that we cannot yet check.
What is the Great Filter?
A term from the economist Robin Hanson for whatever step prevents a species from becoming a galaxy-spanning civilisation. The uncomfortable question is whether that step is behind us or ahead of us.
What is the dark forest theory?
The idea that civilisations stay silent because any detectable species is a potential threat and will be destroyed. It is best known from Liu Cixin’s trilogy rather than from the scientific literature.
Are we alone?
Unknown, and nobody is close to a confident answer. The honest position is that the evidence is consistent with several very different galaxies.