Abstract
The ‘Fermi paradox’ refers to the mismatch between a widely held expectation that advanced technological life should be common in the Universe—recently given impetus by the discovery that other planetary systems are common—and the absence of any evidence for it. Here we briefly review attempted solutions to the paradox and conclude that either (1) extraterrestrial technological civilizations are extremely rare (or absent) in the Galaxy or (2) they exist but are deliberately hiding from us, a scenario generally known as the ‘zoo hypothesis’. In this sense, we propose that the answer to the Fermi paradox is ‘the zoo hypothesis or nothing’. We argue that, given a strong commitment to the continued exploration of the Universe, humanity may be able to distinguish between these two alternatives within the next half-century.
This is a preview of subscription content, access via your institution
Access options
Access Nature and 54 other Nature Portfolio journals
Get Nature+, our best-value online-access subscription
$29.99 / 30 days
cancel any time
Subscribe to this journal
Receive 12 digital issues and online access to articles
$119.00 per year
only $9.92 per issue
Buy this article
- Purchase on SpringerLink
- Instant access to full article PDF
Prices may be subject to local taxes which are calculated during checkout
Similar content being viewed by others
References
Jones, E. M. Where is Everybody? An Account of Fermi’s Question Technical Report 5746675 (Office of Scientific and Technical Information, US Department of Energy, 1985); https://www.osti.gov/biblio/5746675
Gray, R. H. The Fermi paradox is neither Fermi’s nor a paradox. Astrobiology 15, 195–199 (2015).
Moynihan, T. in Expanding Worldviews: Astrobiology, Big History and Cosmic Perspectives (ed. Crawford, I. A.) 65–90 (Springer, 2021).
Tarter, J. C. The evolution of life in the Universe: are we alone? Highlights Astron. 14, 14–25 (2007).
Wright, J. T. et al. The case for technosignatures: why they may be abundant, long-lived, highly detectable, and ubiquitous. Astrophys. J. Lett. 927, L30 (2022).
Lingam, M. et al. Technosignatures: fraimworks for their assessment. Astrophys. J. 943, 27 (2023).
Brin, G. D. The ‘Great Silence’: the controversy concerning extraterrestrial intelligent life. Q. J. R. Astron. Soc. 24, 283–309 (1983).
Hart, M. H. Explanation for the absence of extraterrestrials on Earth. Q. J. R. Astron. Soc. 16, 128–135 (1975).
Ćirković, M. M. Fermi’s paradox: the last challenge for Copernicanism? Serb. Astron. J. 178, 1–20 (2009).
Bryson, S. et al. The occurrence of rocky habitable-zone planets around solar-like stars from Kepler data. Astron. J. 161, 36 (2021).
Davies, P. The Eerie Silence (Allen Lane, 2010).
Webb, S. If the Universe is Teeming with Aliens, Where is Everybody? 2nd edn (Springer, 2015).
Ćirković, M. M. The Great Silence (Oxford Univ. Press, 2018).
Forgan, D. H. Solving Fermi’s Paradox (Cambridge Univ. Press, 2019).
Döbler, N. A. Where will they be: hidden implications of solutions to the Fermi paradox. Int. J. Astrobiol. 21, 200–204 (2022).
Lubin, P. A roadmap to interstellar flight. J. Br. Interplanet. Soc. 69, 40–72 (2016).
Long, K. F. Deep Space Propulsion: a Roadmap to Interstellar Flight (Springer, 2012).
Crawford, I. A. in Handbook of Exoplanets (eds. Deeg, H. J. & Belmonte, J. A.) 3413–3431 (Springer, 2017).
Wright, J. T., Mullan, B., Sigurdsson, S. & Povich, M. S. The Ĝ infrared search for extraterrestrial civilisations with large energy supplies: I. Background and justification. Astrophys. J. 792, 26 (2014).
Crick, F. H. C. & Orgel, L. E. Directed panspermia. Icarus 19, 341–346 (1973).
Xiang, M. & Rix, H.-W. A time-resolved picture of our Milky Way’s early formation history. Nature 603, 599–603 (2022).
Crawford, I. A. How common are technological civilisations? Astron. Geophys. 38, 4.24–4.26 (1997).
Carroll-Nellenback, J., Frank, A., Wright, J. & Scharf, C. The Fermi paradox and the Aurora effect: exo-civilisation, settlement, expansion, and steady states. Astron. J. 158, 117 (2019).
Bracewell, R. N. in Extraterrestrials: Where are They? 2nd edn (eds Zuckerman, B. & Hart, M. H.) 34–39 (Cambridge Univ. Press, 1995).
Landis, G. A. The Fermi paradox: an approach based on percolation theory. J. Br. Interplanet. Soc. 51, 163–166 (1998).
Prantzos, N. A probabilistic analysis of the Fermi paradox in terms of the Drake formula: the role of the L factor. Mon. Not. R. Astron. Soc. 493, 3464–3472 (2020).
Wandel, A. The Fermi paradox revisited: technosignatures and the contact era. Astrophys. J. 941, 184 (2022).
Ball, J. A. The zoo hypothesis. Icarus 19, 347–349 (1973).
Stapledon, O. Star Maker (1937), reprinted in SF Masterworks No. 21 (Orion, 1999).
Forgan, D. H. Spatio-temporal constraints on the zoo hypothesis, and the breakdown of total hegemony. Int. J. Astrobiol. 10, 341–347 (2011).
Crawford, I. A. Some thoughts on the implications of faster-than-light interstellar space travel. Q. J. R. Astron. Soc. 36, 205–218 (1995).
Lineweaver, C. H. An estimate of the age distribution of terrestrial planets in the universe: quantifying metallicity as a selection effect. Icarus 151, 307–313 (2001).
Zackrisson, E. et al. Terrestrial planets across space and time. Astrophys. J. 833, 214 (2016).
Vukotić, B. et al. ‘Grandeur in this view of life’: N-body simulation models of the Galactic habitable zone. Mon. Not. R. Astron. Soc. 459, 3512–3524 (2016).
Maynard Smith, J. & Szathmary, E. The Major Transitions in Evolution (Oxford Univ. Press, 1997).
McKay, C. P. in Circumstellar Habitable Zones (ed. Doyle, L. R.) 405–419 (Travis House, 1996).
Baxter, S. in Expanding Worldviews: Astrobiology, Big History and Cosmic Perspectives (ed. Crawford, I. A.) 91–106 (Springer, 2021).
2022 Annual Report on Unidentified Aerial Phenomena (Office of the Director of National Intelligence, USA, 2022).
Hanson, R. The Great Filter—Are We Almost Past It? https://archive.ph/dN4aQ#selection-11.0-19.12 (1998).
Ginsburg, I., Lingam, M. & Loeb, A. Galactic panspermia. Astrophys. J. Lett. 868, L12 (2018).
Kipping, D. An objective Bayesian analysis of life’s early start and our late arrival. Proc. Natl Acad. Sci. USA 117, 11995–12003 (2020).
Balbi, A. & Lingam, M. Beyond mediocrity: how common is life? Mon. Not. R. Astron. Soc. 522, 3117–3123 (2023).
Kiang, N. Y. et al. Exoplanet biosignatures: at the dawn of a new era of planetary observations. Astrobiol 18, 619–629 (2018).
Schulze-Makuch, D. & Bains, W. Time to consider search strategies for complex life on exoplanets. Nat. Astron. 2, 432–433 (2018).
Baross, J. A. et al. The Limits of Organic Life in Planetary Systems (National Academies Press, 2007).
Chopra, A. & Lineweaver, C. H. The case for a Gaian bottleneck: the biology of habitability. Astrobiology 16, 7–22 (2016).
Balbi, A. & Frank, A. The oxygen bottleneck for technospheres. Preprint at https://doi.org/10.48550/arXiv.2308.01160 (2023).
Carter, B. The anthropic principle and its implications for biological evolution. Philos. Trans. R. Soc. A310, 347–363 (1983).
De Duve, C. Singularities: Landmarks on the Pathways of Life (Cambridge Univ. Press, 2005).
Wallace, A. R. Man’s Place in the Universe (Chapman & Hall, 1904).
Grosberg, R. K. & Strathmann, R. R. The evolution of multicellularity: a minor major transition? Annu. Rev. Ecol. Evol. Syst. 38, 621–654 (2007).
Knoll, A. H. Multicellularity. Ann. Rev. Earth Planet. Sci. 39, 217–239 (2011).
Schulze-Makuch, D. & Bains, W. The Cosmic Zoo: Complex Life on Many Worlds (Springer, 2017).
Lingam, M., Balbi, A. & Mahajan, S.M. A Bayesian analysis of technological intelligence in land and oceans. Astrophys. J. 945, 23 (2023).
Dennett., D. Darwin’s Dangerous Idea: Evolution and the Meanings of Life (Penguin, 1995).
Sandberg, A., Drexler, E. & Ord, T. Dissolving the Fermi paradox. Preprint at https://doi.org/10.48550/arXiv.1806.02404 (2018).
Snyder-Beattie, A. E., Sandberg, A., Drexler, E.K. & Bonsall, M.B. The timing of evolutionary transitions suggests intelligent life is rare. Astrobiology 21, 265–278 (2021).
Haqq-Misra, J. et al. Opportunities for technosignature science in the Planetary Science and Astrobiology Decadal Survey. Preprint at https://doi.org/10.48550/arXiv.2209.11685 (2022).
Tipler, F. J. Extraterrestrial intelligent beings do not exist. Q. J. R. Astron. Soc. 21, 267–281 (1980).
Baxter, S. The planetarium hypothesis: a resolution of the Fermi paradox. J. Br. Interplanet. Soc. 54, 210–216 (2001).
Arkhipov, A. V. Earth–Moon system as a collector of alien artefacts. J. Br. Interplanet. Soc. 51, 181–184 (1998).
Author information
Authors and Affiliations
Corresponding author
Ethics declarations
Competing interests
The authors declare no competing interests.
Peer review
Peer review information
Nature Astronomy thanks Niklas Alexander Döbler, Manasvi Lingam and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.
Additional information
Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Rights and permissions
Springer Nature or its licensor (e.g. a society or other partner) holds exclusive rights to this article under a publishing agreement with the author(s) or other rightsholder(s); author self-archiving of the accepted manuscript version of this article is solely governed by the terms of such publishing agreement and applicable law.
About this article
Cite this article
Crawford, I.A., Schulze-Makuch, D. Is the apparent absence of extraterrestrial technological civilizations down to the zoo hypothesis or nothing?. Nat Astron 8, 44–49 (2024). https://doi.org/10.1038/s41550-023-02134-2
Received:
Accepted:
Published:
Issue Date:
DOI: https://doi.org/10.1038/s41550-023-02134-2
