Fermi Paradox

Origins and the core argument

According to Jones's (1985) reconstruction from the recollections of those present, Fermi asked 'Where is everybody?' during a 1950 lunchtime conversation at Los Alamos about UFOs and faster-than-light travel. Hart (1975) turned the question into a rigorous argument: colonizing the galaxy would take only a few million years, far less than the galaxy's age, so if extraterrestrial civilizations existed they should already have reached Earth. Tipler (1980) strengthened the conclusion with self-replicating probes, while Sagan and Newman (1983) countered that the reasoning was too simplistic.

The Drake equation and leading hypotheses

Drake (1965) proposed an equation that estimates the number of communicating civilizations by multiplying the rate of star formation, the fraction of stars with planets, the probabilities that life and intelligence emerge, the lifetime of civilizations and other factors. Observations such as Petigura, Howard and Marcy's (2013) analysis of exoplanets, indicating that a substantial share of Sun-like stars host Earth-size planets in the habitable zone, have sharpened the paradox. Hanson's (1998) Great Filter hypothesis holds that some step on the path from life to a spacefaring civilization is extremely hard to pass, and whether that step lies behind us or ahead matters greatly for humanity's future. Ward and Brownlee's (2000) Rare Earth hypothesis argues that complex life is exceedingly rare, and Ball's (1973) zoo hypothesis suggests that civilizations deliberately avoid contact. Webb (2002) systematically catalogued the proposed solutions.

Recent reinterpretations

Sandberg, Drexler and Ord (2018) argued that once the scientific uncertainty in each term of the Drake equation is represented as a probability distribution, the probability that our galaxy contains no other civilization comes out substantial, so the paradox may be dissolved. The approach treats uncertainty through Bayesian reasoning. Frank and Sullivan (2016) estimated the likelihood that a technological species has arisen even once across cosmic history. Ćirković (2018) reviewed the paradox comprehensively at the intersection of science and philosophy, discussing observation selection effects and implications for humanity's long-term future.

FAQ

What is the Fermi paradox in simple terms?

The universe is huge and old, so it seems like alien civilizations should be common. The paradox asks why we haven't found any sign of them.

What is the Great Filter?

It's the idea that some step on the road from simple life to a civilization that spreads through space is extremely hard to get past. If that step is behind us, we're lucky; if it's ahead, it could be a warning about humanity's future.

Sources reviewed: 2026-10-26

References

1. Jones, E. M. (1985). 'Where is everybody?' An account of Fermi's question. Los Alamos National Laboratory Report LA-10311-MS.
2. Hart, M. H. (1975). An explanation for the absence of extraterrestrials on Earth. Quarterly Journal of the Royal Astronomical Society, 16, 128–135.
3. Drake, F. D. (1965). The radio search for intelligent extraterrestrial life. In G. Mamikunian & M. H. Briggs (Eds.), Current Aspects of Exobiology (pp. 323–345). Oxford: Pergamon.
4. Tipler, F. J. (1980). Extraterrestrial intelligent beings do not exist. Quarterly Journal of the Royal Astronomical Society, 21, 267–281.
5. Sagan, C., & Newman, W. I. (1983). The solipsist approach to extraterrestrial intelligence. Quarterly Journal of the Royal Astronomical Society, 24, 113–121.
6. Ball, J. A. (1973). The zoo hypothesis. Icarus, 19(3), 347–349.
7. Hanson, R. (1998). The Great Filter — Are we almost past it? Unpublished manuscript, George Mason University.
8. Ward, P. D., & Brownlee, D. (2000). Rare Earth: Why Complex Life Is Uncommon in the Universe. New York: Copernicus.
9. Webb, S. (2002). If the Universe Is Teeming with Aliens ... Where Is Everybody? New York: Copernicus.
10. Ćirković, M. M. (2018). The Great Silence: Science and Philosophy of Fermi's Paradox. Oxford: Oxford University Press.
11. Sandberg, A., Drexler, E., & Ord, T. (2018). Dissolving the Fermi paradox. arXiv:1806.02404.
12. Frank, A., & Sullivan, W. T. (2016). A new empirical constraint on the prevalence of technological species in the universe. Astrobiology, 16(5), 359–362.
13. Petigura, E. A., Howard, A. W., & Marcy, G. W. (2013). Prevalence of Earth-size planets orbiting Sun-like stars. Proceedings of the National Academy of Sciences, 110(48), 19273–19278.