The Fermi paradox

So where is everybody?

Fourteen answers to the Fermi paradox, from the serious to the speculative, with how each could be tested.

The question

In 1950, over lunch at Los Alamos, physicist Enrico Fermi asked: "Where is everybody?" The Milky Way has hundreds of billions of stars, most with planets, and it is billions of years older than Earth. Even slow colonists could cross it in a few million years. So why no visitors, no signals, no ruins?

Below are the main answers, grouped by how seriously scientists take them. More dots means more serious.

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Taken most seriously

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Mainstream ideas scientists actively test or model.

  1. 01

    We have barely looked

    Wright, Kanodia and Lubar, 2018

    All SETI searches so far add up to a tiny sample of the sky, radio frequencies and time. One estimate compared it to checking a hot tub of water for fish when the search space is all of Earth's oceans. Silence from such a small sample tells us very little.

    How it could be tested: Bigger, longer surveys: Breakthrough Listen, the Square Kilometre Array, optical and infrared laser searches.

    Read more: Wright et al. 2018 (arXiv)

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  2. 02

    Life, or intelligence, is rare

    Ward and Brownlee, "Rare Earth", 2000; Sandberg, Drexler and Ord, 2018

    Simple life may be common, but complex life and technology may need a long chain of lucky steps: a stable star, plate tectonics, a big moon, the jump to complex cells, and billions of quiet years. When the honest uncertainty in each Drake factor is included, a 2018 analysis found a large chance that we are alone in our galaxy.

    How it could be tested: Find life (even microbes) on a second world nearby. If life started twice in one solar system, it is probably common.

    Read more: Dissolving the Fermi paradox (arXiv)

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  3. 03

    The Great Filter

    Robin Hanson, 1996

    Somewhere between dead chemistry and a galaxy-spanning civilisation there is at least one step that almost nobody passes. If the filter is behind us (life starting, complex cells), we are lucky. If it is ahead of us (war, climate, runaway technology), that is a warning.

    How it could be tested: Finding simple life elsewhere would suggest the filter is not at the start, which moves the worry forward.

    Read more: The Great Filter (Hanson)

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  4. 04

    Space and time are too big

    Many authors; percolation model by Geoffrey Landis, 1998

    Civilisations may simply not overlap. The galaxy is 100,000 light years wide and 13 billion years old: two species could miss each other by a few million years or a few thousand light years. Our own radio leakage fades into the background within a few light years.

    How it could be tested: Look for long-lived signs instead of live signals: megastructures, waste heat, industrial gases in exoplanet air.

    Read more: Fermi paradox (Wikipedia)

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  5. 05

    We are early

    Loeb, Batista and Sloan, 2016; Hanson et al., "grabby aliens", 2021

    Small, long-lived red dwarf stars will shine for trillions of years, so most life in the universe may still be in the future. The grabby aliens model adds that expanding civilisations would fill the sky fast, so seeing none suggests we arrived early.

    How it could be tested: Learn whether red dwarf planets can host life (Webb is checking whether they keep atmospheres).

    Read more: Grabby aliens (arXiv)

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Plausible, untested

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Reasonable, but hard to check with today's tools.

  1. 06

    They don't use the signals we look for

    A common SETI caveat

    We mostly listen for narrow radio beams, a technology we have used for barely a century. Others might use lasers, neutrinos or tight beams we would never intercept, or not broadcast at all.

    How it could be tested: Broaden the search: technosignatures such as waste heat, light pollution or artificial gases.

    Read more: Technosignatures (NASA)

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  2. 07

    They destroy themselves

    Discussed since the 1960s (Sagan and Shklovskii)

    Technological species may not last long: nuclear war, engineered pandemics, climate collapse or misaligned AI. In the Drake equation this is a small L, the number of years a civilisation keeps signalling.

    How it could be tested: Hard to test from afar; our own history is the only data point.

    Read more: Drake equation (Wikipedia)

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  3. 08

    They are sleeping

    Sandberg, Armstrong and Ćirković, aestivation hypothesis, 2017

    Computation is cheaper when the universe is colder, so an advanced civilisation might wait billions of years before doing much. For now they would look like nothing at all.

    How it could be tested: They should leave resources untouched and guard them, which is hard to see.

    Read more: Aestivation hypothesis (arXiv)

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  4. 09

    Everyone is hiding: the dark forest

    Liu Cixin, "The Dark Forest", 2008 (fiction)

    If you can never be sure a neighbour is friendly, and attack is cheaper than defence, the safest strategy is silence. A popular idea from a novel, debated by game theorists; it also fuels the argument against sending loud messages (METI).

    How it could be tested: Untestable in practice, which is its weakness.

    Read more: Dark forest hypothesis (Wikipedia)

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  5. 10

    They go inward, not outward

    John Smart, transcension hypothesis, 2012

    Advanced civilisations may turn to dense computation and virtual worlds rather than expanding through space, leaving little to see.

    How it could be tested: Look for very compact, very hot or energy-hungry objects; so far nothing points this way.

    Read more: Transcension hypothesis (Wikipedia)

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Speculative

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Fun to think about; little or no way to test.

  1. 11

    The zoo hypothesis

    John Ball, 1973

    Others know we are here and leave us alone on purpose, like a nature reserve, perhaps until we reach some threshold. It needs every civilisation to follow the same rule forever, which many scientists find unlikely.

    How it could be tested: None, unless they decide to say hello.

    Read more: Zoo hypothesis (Wikipedia)

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  2. 12

    The "drywall" idea: kept apart on purpose

    An idea shared online and attributed to someone at OpenAI; we couldn't trace the original

    A modern twist on the zoo: we sit far from other civilisations by design, separated like rooms by a wall, so contact doesn't interfere with our own experience and development. Fun to think about, but with no evidence behind it and no way to test it, it belongs with the speculation.

    How it could be tested: None that we know of.

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  3. 13

    The planetarium, or a simulation

    Stephen Baxter, 2001; Nick Bostrom, 2003

    What we see of the universe could be an illusion built to look empty, or we may live in a simulation that only renders us. Philosophically interesting, scientifically almost impossible to test.

    How it could be tested: Proposals exist (glitches in physical constants), none has found anything.

    Read more: Simulation hypothesis (Wikipedia)

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  4. 14

    They are already here

    UFO and UAP believers

    Some say the paradox is solved because aliens already visit, and governments hide it. The official record has unexplained sightings but no craft, bodies or clean data.

    How it could be tested: Raw sensor data or physical material checked by independent labs.

    Read more: Our UFO and UAP verdicts

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Our read

The silence is real but weak evidence: we have searched very little, for a short time, in a narrow way. The most defensible answers today are the boring ones: space is big, we've barely looked, and life or intelligence may be rare. The tripwires are where any of this would change. Try your own numbers in the Drake equation calculator.