ParadoxesVIII
The Fermi paradox
The galaxy is vast and old, and life arose quickly on Earth. Even slow colonisers could have filled the Milky Way millions of years ago. So where is everybody?
- Reading time
- 4 min
- Difficulty
- Introductory
- Branch
- Reality
- First posed
- Enrico Fermi, Los Alamos, 1950; developed by Michael Hart, 1975
The puzzle
In 1950, over lunch at Los Alamos, the physicist Enrico Fermi was chatting with colleagues about flying saucers and the possibility of faster-than-light travel. Suddenly he asked: 'But where is everybody?'
The reasoning behind his question goes like this. The Milky Way contains a few hundred billion stars, many with planets, and many of those planets lie in zones where liquid water could exist. The galaxy is about 13 billion years old; Earth is only about 4.5 billion. Life appeared on Earth relatively soon after conditions allowed. If even a small fraction of planets developed technological civilisations, some should be billions of years ahead of us.
And spreading across the galaxy does not seem to require exotic technology. A civilisation sending out ships at a small fraction of the speed of light, settling new star systems which send out ships of their own, could cross the galaxy in a few million to tens of millions of years, which is a blink in cosmic time.
Yet we see no signs. No signals, no probes, no megastructures, no visitors. The Fermi paradox is the conflict between the apparent likelihood of extraterrestrial civilisations and the complete absence of evidence for them.
Why it matters
The answer bears on our own future. If civilisations commonly destroy themselves, that is a warning. If we are alone, our survival may matter on a cosmic scale.
It is also a lesson in reasoning under deep uncertainty. Many of the numbers involved, such as how often life arises or how long civilisations last, are unknown to within many orders of magnitude, which makes the paradox both compelling and slippery.
Key terms
- Drake equationA formula proposed by Frank Drake in 1961 that multiplies estimates such as the rate of star formation and the fraction of planets that develop life to estimate how many communicating civilisations exist in our galaxy.
- Great FilterRobin Hanson's idea that some step on the road from dead matter to galaxy-spanning civilisation is extremely improbable, which would explain the silence of the sky.
- Anthropic reasoningReasoning that takes into account the fact that we exist to make an observation. For example, we should not be surprised to find the universe compatible with life, since we could not observe one that was not.
- Fermi paradoxThe conflict between estimates suggesting intelligent life should be common in our galaxy and the absence of any evidence of it.
Proposed solutions
We are rare or alone
Life, or complex life, or intelligence, is far rarer than optimists assume. We may be the only technological civilisation in the galaxy, or the observable universe.
The Great Filter
Something stops almost all life from becoming a galaxy-spanning civilisation. The question is whether that filter is behind us or ahead of us.
They are out there, but quiet or far
Civilisations exist but we cannot detect them: they are too distant, too brief, choose not to expand, or deliberately avoid contact.
We are early
The universe is young relative to how long it will support life. We may simply be among the first.
Each solution, tested
We are rare or alone
- How it resolves the paradox
- Peter Ward and Donald Brownlee argued in Rare Earth (2000) that complex life requires an unusual combination of conditions: a stable star, a large moon, plate tectonics, a protective giant planet and more. The origin of life itself might be extraordinarily improbable. A 2018 analysis by Anders Sandberg, Eric Drexler and Toby Ord found that, once uncertainty in the Drake equation is handled properly, there is a substantial probability that we are alone in the observable universe.
- Why it doesn’t fully work
- We have a sample size of one. Life on Earth arose quickly, which some see as evidence that it is easy, not rare.
The Great Filter
- How it resolves the paradox
- Robin Hanson proposed that there is a Great Filter, some step in the path from dead matter to interstellar civilisation that is extremely unlikely. If it is behind us (the origin of life, complex cells, intelligence), we are lucky. If it is ahead of us (nuclear war, engineered pandemics, environmental collapse, misaligned technology), the silence is a grim warning. For this reason, some have said finding simple life on Mars would be bad news: it would make an early filter less likely.
- Why it doesn’t fully work
- The Great Filter is less a solution than a framework for asking where the improbability lies. We do not know which step it is.
They are out there, but quiet or far
- How it resolves the paradox
- We have searched only a tiny fraction of the sky, frequencies and time. Civilisations might not use radio for long, might prefer to stay home, or might deliberately keep quiet for fear of hostile neighbours, as in the 'dark forest' idea popularised by the novelist Liu Cixin. The 'zoo hypothesis' suggests they may be observing us without interfering.
- Why it doesn’t fully work
- These explanations need to apply to nearly every civilisation, for billions of years. It only takes one expansionist species to fill the galaxy. Explanations based on motives seem unlikely to be universal.
We are early
- How it resolves the paradox
- Stars like the Sun will keep forming for trillions of years, and long-lived red dwarf stars far longer. Robin Hanson and colleagues argued in 2021 that 'grabby' expanding civilisations, if they exist, would quickly fill the universe, so observers like us are likely to arise early, before the grabby ones reach us.
- Why it doesn’t fully work
- This depends on contested assumptions about how life and civilisations arise and spread, and about which observers we should consider ourselves typical of.
Who argued what
- We are rare or alone Peter Ward, Donald Brownlee, Anders Sandberg
- The Great Filter Robin Hanson, Nick Bostrom
- They are out there, but quiet or far Carl Sagan, John Ball
- We are early Robin Hanson, Avi Loeb
Variations
The Drake equation
In 1961 Frank Drake wrote an equation estimating the number of communicating civilisations in our galaxy, by multiplying the rate of star formation, the fraction of stars with planets, the number of habitable planets per system, and the fractions that develop life, intelligence and detectable technology, by how long such civilisations last. Each factor beyond the first few is deeply uncertain, which is why estimates range from less than one to millions.
Would we recognise them?
Suppose an advanced civilisation were made of computation running on structures we would mistake for natural phenomena. Would we know? Some argue that the paradox assumes aliens would be like us: expansionist, visible, and interested in contact.
Go deeper
What happened at lunch
Accounts of Fermi's lunch come from letters written decades later by those present, including Emil Konopinski, Edward Teller and Herbert York. They agree on the gist, a casual conversation about UFO reports, a joke cartoon, and Fermi's sudden question, but differ on details. Fermi apparently followed his question with quick calculations about how long expansion across the galaxy should take.
Related ideas
Further reading and sources
- EncyclopediaStanford Encyclopedia of Philosophy: search “Fermi paradox”
- WikipediaFermi paradox
- WikipediaDrake equation
- BookRare Earth, Peter Ward and Donald Brownlee
- BookIf the Universe Is Teeming with Aliens... Where Is Everybody?, Stephen Webb
- BookThe Precipice, Toby Ord
Open questions
- Would finding alien microbes be good news or bad news for us?
- Is it more unsettling to be alone, or not to be?
- What would it take for our civilisation to last a million years?