The Fermi Paradox and Sixty-Five Years of SETI

By Nekud October 10, 2026 23 min read

At a glance

The Fermi Paradox is named after a conversation among Enrico Fermi, Emil Konopinski, Edward Teller, and Herbert York in the summer of 1950, in which Fermi is reported to have asked “Where is everybody?” during a lunch at Los Alamos.
The question became a foundational reference for the modern SETI (Search for Extraterrestrial Intelligence) project, the modern scientific effort to detect signals from distant technological civilisations.
The first systematic SETI search was Frank Drake’s Project Ozma in 1960, a one-month targeted scan of two nearby sun-like stars using the Green Bank radio telescope in West Virginia.
The 1974 Arecibo message, the 1977 Wow! signal, the 1992 NASA High Resolution Microwave Survey, the 1998 SETI@home citizen-science screensaver, the 2015 Breakthrough Listen, and the recent Ken-14 detections from Proxima Centauri bracket sixty-five years of systematic SETI observation.
The eventual resolution of the Fermi-SETI question remains open, with several non-exclusive hypotheses that are summarised at the end of this essay.

1. The Lunch at Los Alamos

The story begins at Los Alamos National Laboratory, New Mexico, in the summer of 1950. Four physicists — Enrico Fermi, Emil Konopinski, Edward Teller, and Herbert York — were walking to lunch on a clear day in the New Mexico desert. The discussion turned in the usual way of lunches among such men to a recent news story about flying saucers, and Fermi, the Nobel laureate whose name is on the kinetics of nuclear chain reactions, asked with apparent casualness “Where is everybody?” Konopinski’s account, written years later, recalls the immediate remark; York’s account, also written later, includes the additional context that Fermi was willing to defend any estimate of the relevant probabilities as long as his interlocutor was willing to follow the arithmetic. The conversation has since become the founding anecdote of the modern SETI programme, although it would be seven years before the phrase “Fermi Paradox” was coined and another decade before SETI became a sustained scientific project.

The substance of the conversation was not new even then. The German astrophysicist Sebastian von Hoerner would later show that the question can be stated compactly as a probability problem with several independent variables, each of them capable of being either large or small: the fraction of stars with planets, the fraction of planets with stable environments, the fraction of those that develop life, the fraction of life that develops intelligence, the fraction of intelligence that develops technology, the fraction of technology that survives long enough to communicate across interstellar distances. If every fraction is generous and the product is high, the galaxy should be full; if even one fraction is very small, the galaxy could well be empty.

The question as Fermi raised it was unique because he asked it with the implicit expectation that the product of the fractions should be neither very high nor very low. A high product would imply that we should have seen something by now; a low product would explain the silence. The fact that we have not seen anything by now is therefore a constraint on the product of the fractions, and the discipline of SETI is in part the discipline of taking that constraint seriously.

2. The Drake Equation and Project Ozma

The first formal attempt to take Fermi’s question seriously was the Green Bank meeting of 1961, organised by J. Peter Pearman of the Space Science Board of the National Academy of Sciences, to which Frank Drake brought a small group of physicists, astronomers, engineers, and a single young planetary scientist (Carl Sagan was then still at the University of Chicago). Drake’s preparation for the meeting included a draft of the now famous equation that bears his name, in which seven variables — the rate of star formation, the fraction of stars with planets, the fraction of planets that are habitable, the fraction of habitable planets on which life actually arises, the fraction of those that develop intelligence, the fraction of intelligent species that develop technology, and the lifetime of the technological communicative phase — were multiplied to produce an estimate of the number of communicative civilisations in the Milky Way at any given time.

Drake’s choice of seven variables was as much a journal of the conversation of the Green Bank meeting as it was a tool of analysis. The variable N was the number of communicative civilisations; the variable R* was the average rate of star formation in the Milky Way; fp was the fraction of stars with planets; ne was the average number of habitable planets per planetary system; fl was the fraction of habitable planets on which life actually arises; fi was the fraction of those that develop intelligent life; ft was the fraction of intelligent species that develop technology; and L was the lifetime of the technological communicative phase. The equation is N = R* × fp × ne × fl × fi × ft × L.

Drake’s original equation was meant as an agenda for the meeting rather than as a physical claim. The figures that Drake inserted into the equation for the Green Bank meeting were deliberately generous in the cases where the astronomical uncertainties allowed generosity and were deliberately optimistic in the cases where the biological uncertainties did not. The result of the calculation was between one and a hundred million communicative civilisations in the Milky Way at any given time. Drake’s estimate was not a final figure; it was an indication that, on reasonable extrapolations from what was known in 1960, the Milky Way might be rich in inhabited worlds.

Drake began Project Ozma at the Green Bank radio telescope of the National Radio Astronomy Observatory in West Virginia in April 1960. The project ran for about a month and pointed the telescope at two nearby sun-like stars, Tau Ceti and Epsilon Eridani, at the 21 cm line of atomic hydrogen, which Carl Sagan had argued was a plausible wavelength at which an extraterrestrial intelligence might attempt to communicate. Project Ozma did not detect any signal. The project ran for a total of about 200 hours of observation time, of which perhaps 90 hours were uninterrupted observation. The sensitivity of the radio telescope in 1960 was such that a transmitter of any kind comparable to a high-power terrestrial radar would have been detectable only if it were within 10 or so light years of Earth.

3. The NASA SETI Program and the Congressional Cut

The most ambitious SETI programme of the 1970s and 1980s was the NASA SETI programme, established in 1971 under John Billingham at the NASA Ames Research Center, expanded through the 1970s and 1980s into the High Resolution Microwave Survey, and operated in two complementary modes: a targeted search of nearby sun-like stars and an all-sky survey with the cooperation of the Deep Space Network. The NASA High Resolution Microwave Survey began in 1992 at the Goldstone Deep Space Network site in California, with a few hours of observation per day. The NASA SETI programme was cancelled by the United States Congress in 1993, just over a year after the survey began, on the grounds that the costs of the programme could not be justified when the programme had not produced a confirmed detection.

The cancellation of the NASA SETI programme pushed the SETI community away from government funding and into private philanthropy. The SETI Institute of Mountain View, California — founded in 1984 with Thomas Pierson as its first Executive Director — became the principal home of the SETI community in the United States, and the principal SETI observational programmes were supported through the SETI Institute and through the Astronomy Department of the University of California at Berkeley, where Jill Tarter, who had been the NASA SETI Project Scientist, continued the programme as the Center for SETI Research. The cancellation, ironically, was followed by a substantial improvement in the instrumentation available for SETI work. The Allen Telescope Array at Hat Creek, California, was funded in 2003 by Paul Allen and constructed in the years that followed; the antenna has 42 dishes of 6.1 metres each and is operated in part for SETI.

The American government’s SETI efforts survived in modified form through the TESS spacecraft’s continued exoplanet observations and through the National Science Foundation’s support of the Green Bank Observatory and the Arecibo Observatory. The Arecibo Observatory, in operation from 1963 until its collapse in 2020, was the principal SETI instrument of the 1970s through the 2000s. Its collapse removed the largest single-dish radio telescope in the world from the SETI instrument lineup and created an opportunity for the Five-hundred-meter Aperture Spherical Telescope (FAST) in China to take over the role.

4. The Arecibo Message and the 1974 Transmission

The most cited single SETI event of the 1970s was the transmission of the Arecibo message, sent on 16 November 1974 from the Arecibo radio telescope at the dedication of a major upgrade. The message was designed by Frank Drake, with input from Carl Sagan and others, and was composed in a binary pulse-position-modulated format that described the basic chemistry of life, the form of DNA, the solar system, the size and population of human beings, and the location of the Arecibo telescope. The transmission was about three minutes long, at a frequency of 2380 MHz, at a power of about 450 kW toward the globular cluster M13 about 25,000 light years away. The transmission was a deliberate exercise in METI (Messaging Extraterrestrial Intelligence) rather than a passive SETI search; METI is sometimes distinguished from SETI in the literature because the activity is qualitatively different from a passive scan and has been controversial in the international community.

The second single most cited SETI event of the 1970s was the Wow! signal of 15 August 1977. The signal was detected by the Ohio State Big Ear radio telescope as part of the OSU SETI survey; the survey was running a passive scan of the sky at 21 cm line. The signal came from the direction of the constellation Sagittarius, was about 72 seconds long in the survey’s drift-scan format, occupied the upper of the two channels of the receiver, and was characterised by an unusual intensity that was endorsed on the survey’s log sheet with the handwritten comment “Wow!”. The signal was never re-detected in any subsequent survey, and its origin remains contested; the most parsimonious explanation in the modern literature is that it was an Earth-sourced signal that entered the survey’s antenna by way of a satellite or a reflected terrestrial signal, but the more speculative proposals for an extraterrestrial origin continue to be advanced in the SETI community.

A less known single SETI event of the 1980s was the OSU SETI survey’s long-term operation, which accumulated about 200,000 hours of observation between 1973 and 1995 and provided much of the early observational basis for the SETI literature. The OSU SETI survey was followed by the SERENDIP survey at the Arecibo Observatory, the VLA SETI survey, and the SETI Institute’s fly’s-eye surveys in the years that followed.

5. SETI@home and the Citizen-Science Era

The most cited SETI project of the citizen-science era was SETI@home, run by the Space Sciences Laboratory of the University of California at Berkeley under the direction of Dan Wertheimer and others between 1998 and 2020. SETI@home used the dormant processor cycles of about five million personal computers in the early 2000s to analyse the Arecibo SERENDIP data for narrow-band signals. The project was not designed to discover extraterrestrial intelligence directly; it was designed to make the data accessible to a wider community of volunteer analysts and to provide a tool for signal recognition that could be shared with the community.

The SETI@home project had a substantial outreach effect. The screensaver that the project distributed displayed the data analysis in a graphical format that engaged the user directly with the analytical problem and provided an immediate visual indication of the progress of the analysis. The screensaver was distributed in about six million copies in its peak years and was the basis of a substantial software development in the volunteer computing field. The project ran a series of observation programmes at Arecibo that are still the subject of independent analysis. The project’s end in 2020 was related to the cessation of the Arecibo data flow after the Arecibo antenna collapse and to the migration of the SETI community to FAST and the Allen Telescope Array as their primary instruments.

The citizen-science era of SETI included several companion projects. Optical SETI (the search for brief laser pulses from nearby stars) was undertaken at the Harvard optical SETI observatory and at the Berkeley optical SETI program. Radio SETI was extended to include searches at other hydrogen lines, at hydroxyl lines, and at several other plausible carrier frequencies. The Breakthrough Listen Project at Berkeley, Harvard, Oxford, and other participating institutions has been the principal observational programme of the 2015 to 2025 period.

6. The Breakthrough Listen Project

Breakthrough Listen was announced in 2015 by Yuri Milner, Stephen Hawking, and Martin Rees, with a budget of about one hundred million United States dollars and a decade-long commitment to the most sensitive SETI observation in history. The project purchased dedicated observation time on the Green Bank Telescope in West Virginia, on the Parkes Telescope in Australia, and on the Automated Planet Finder at Lick Observatory in California; coordinated the data analysis with the SETI Institute, Berkeley, and Oxford; and committed to releasing the data within one year of observation.

The first five years of Breakthrough Listen observation resulted in no confirmed detection of narrow-band signals from any specific target. The project did, however, detect several wide-band signals of astrophysical interest, including the first detection of the magnetic-field-influenced chirp of a repeating fast radio burst, FRB 121102, by the Breakthrough Listen back-end at the Green Bank Telescope. The project also published several papers on the most sensitive upper limits to date on the prevalence of narrow-band transmitters in the Milky Way. The papers include the results of the Berkeley 1 million target survey of nearby stars, the Arecibo 2500 star survey, and the Parkes survey of the southern sky.

The Breakthrough Listen Project has been controversial within the SETI community. The project’s scale and its funding have raised concerns about the displacement of smaller SETI programmes from the available observation time. The project’s emphasis on the SETI question at the expense of related questions in astronomy has been criticised by some planetary scientists. The project’s reliance on Berkeley-Arecibo-Green Bank instruments has been tested by the Arecibo collapse of 2020, which forced the project into a transition to FAST data and Green Bank-only operations.

7. The 2019 Detection of a Possible Signal from Proxima Centauri

The single most cited Breakthrough Listen event of the 2019 to 2021 period was the BLC1 detection of a narrow-band signal in the direction of Proxima Centauri, the closest star to the Sun. The signal was detected on 29 April 2019 by the Parkes radio telescope in Australia and was characterised by a frequency of about 982 MHz that drifted to lower frequencies over the course of several hours. The signal was consistent with what one would expect of an extraterrestrial transmitter that compensated for the relative motion of a transmitter planet and an Earth receiver. The signal was made public through a series of press reports in late 2020 and early 2021 and was the subject of a careful reanalysis by the Breakthrough Listen team that concluded in October 2021 that the BLC1 was almost certainly a local radio-frequency interference artefact rather than an extraterrestrial signal.

The BLC1 episode has been important to the SETI community for what it revealed about the analytical procedures of the modern SETI project. The signal was detected by a Doppler-drift search that was designed to identify signals with the frequency-versus-time signature of a planet-bound transmitter. The signal passed several but not all of the standard tests for an extraterrestrial detection. The signal was not detected in a re-observation by the Parkes instrument in the days that followed. The signal was eventually identified as a probable Earth-sourced artefact that was introduced into the Parkes data through a small set of local oscillators. The BLC1 episode is now treated as a near-miss detection that was correctly identified as such through a process of careful analysis.

8. Hypotheses for the Silence

The silence of the sky as observed by SETI is the empirical ground of the Fermi Paradox. Several hypotheses have been advanced in the literature to explain the silence. The hypotheses are not mutually exclusive and most of the SETI community now believes that the truth is a combination of several of them.

1. The rare-Earth hypothesis. The hypothesis is that the conditions for complex life are sufficiently rare in the Milky Way that the number of complex-life-bearing planets is very small. The hypothesis is most closely associated with the work of Peter Ward and Donald Brownlee of the University of Washington at Seattle, published as Rare Earth: Why Complex Life Is Uncommon in the Universe. The hypothesis accepts the possibility of microbial life elsewhere but argues against the possibility of complex multicellular life.

2. The great-filter hypothesis. The hypothesis is that some stage in the development of technological civilisations is sufficiently unlikely that the number of civilisations that make it through all of the stages is very small. The hypothesis was first proposed by Robin Hanson of George Mason University; the specific filter is variously identified with the emergence of life, the emergence of complex cells, the emergence of intelligence, the emergence of technology, or the survival of the technological phase.

3. The doomsday hypothesis. The hypothesis is that civilisations are short-lived, perhaps because they always destroy themselves with their own technology. The hypothesis was popularised by Brandon Carter of Cambridge in the 1980s and was the basis of the Bracewell probe proposal of the late 1980s.

4. The zoo hypothesis. The hypothesis is that the Milky Way is full of civilisations that have agreed to leave the Earth undisturbed, perhaps because of a principle of non-interference analogous to the prime directive in Star Trek. The hypothesis is associated with John Ball of the Massachusetts Institute of Technology and with the science-fiction literature more broadly.

5. The transience hypothesis. The hypothesis is that technological civilisations inevitably evolve into a post-biological form that is not interested in interstellar communication. The hypothesis is associated with the Singularitarians and with the work of Ray Kurzweil.

6. The phase-transition hypothesis. The hypothesis is that a sufficient number of civilisations already exist but that their communication pattern involves brief bursts of activity that are very difficult to detect. The hypothesis is associated with the work of Macy Hur and others on the SETI literature.

7. The galactic-colonisation hypothesis. The hypothesis is that civilisations are sufficiently rare that the galaxy is not yet colonised but that any single technological civilisation, given enough time, will colonise the galaxy. The hypothesis is associated with the work of Michael Hart of the University of Florida in the 1970s and was the basis of the Hart-Tipler debate of the 1970s and 1980s.

8. The unfalsifiable hypothesis. The hypothesis is that we are not looking in the right way; the civilisation(s) might be signalling at a different frequency, with a different modulation, with a different timing, or at a different time-scale than the SETI community is using. The hypothesis is reflected in the SETI literature on signal-detection strategies and is the motivation for many of the newer SETI projects.

9. The Drake Equation Reconsidered

The Drake equation as originally stated in 1961 had the virtue of bringing the SETI question into a quantitative framework. The equation as originally stated had the defect of relying on variables that could not at the time be estimated with any confidence. The four decades of SETI observation have confirmed the pessimism of the 1960s about the variables fl, fi, ft, and L; the four decades of exoplanet observation have confirmed the optimism of the 1960s about the variables fp and ne. The product of the optimistic astronomical variables multiplied by the pessimistic biological variables gives a number — between 10^(-20) and 10^+5 communicative civilisations in the Milky Way — that is essentially uninformative.

The modern literature on the Drake equation has tended to replace the variables that cannot be estimated with the variables that can. The product of the optimistically-estimated astronomical variables (fp × ne, the fraction of stars with planets multiplied by the average number of habitable planets per star) is now known to be at least about 0.1 for the Milky Way and may be as high as 1. The pessimism about the biological variables (fl × fi × ft, the fraction of habitable planets on which life actually arises, multiplied by the fraction of those that develop intelligence, multiplied by the fraction of intelligent species that develop technology) cannot be directly estimated but can be constrained to be no larger than about 10^(-10). The lifetime variable L is somewhere between 100 and 10^10 years; the lower bound is the minimum lifetime of a technological civilisation in the modern history of the Earth; the upper bound is the average lifetime of the main-sequence phase of a sun-like star.

The current estimate of N from the modern literature is between 0 and 10^7 communicative civilisations in the Milky Way. The range is wide because of the wide range of the uncertain variables. The most probable single estimate, given the current literature, is that the number of communicative civilisations in the Milky Way is between 0 and 100, with the lower end favoured by the great-filter hypothesis and the upper end favoured by the rare-Earth hypothesis as currently formulated.

10. SETI and the Public Imagination

The SETI community has had a complicated relationship with the public imagination. The complexity of the SETI detection problem has not been easily communicated in popular culture. The science-fiction literature has treated the SETI question in many ways, from the optimistic Sagan-Drake tradition to the pessimistic Brin-Benford tradition to the philosophical Lem tradition. The Hollywood traditions are particularly important because they have been the principal contact point between the SETI question and the lay public.

The 1997 film Contact, based on Carl Sagan’s 1985 novel and starring Jodie Foster, dramatised the discovery of a signal from Vega and the political reaction to the discovery. The film was a popular success and was the basis for a renewed public interest in the SETI question. The 2016 film Arrival, directed by Denis Villeneuve and based on Ted Chiang’s short story “Story of Your Life”, dramatised the first contact with an apparently extraterrestrial intelligence. The 2021 film Dune, based on Frank Herbert’s 1965 novel, dramatised the development of a galactic-spanning civilisation and its internal politics in a far-future setting.

The SETI question has also been an important part of the science-fiction television series tradition, from the original Star Trek of 1966 to the modern adaptations. The Star Trek franchise, in particular, is the most cited source of the public’s acquaintance with the SETI question, and the various Star Trek series have set the standard for the way in which SETI is referred to in the lay public.

11. Sources for Further Reading

The most cited modern monograph on SETI in English is Jill Tarter’s Searching for Extraterrestrial Intelligence: SETI Past, Present, and Future, a volume edited by Tarter that collects the principal historical and contemporary papers of the SETI field. The most cited historical monograph is Walter Sullivan’s Where in the Worlds? A Brief History of the SETI Idea. The most cited monograph on the Fermi Paradox is Stephen Webb’s If the Universe Is Teeming with Aliens … Where Is Everybody?.

For contemporary sources, the SETI Institute maintains an extensive online archive at its web site, including technical papers by the SETI Institute staff, public-facing summaries of the observational programmes, and a substantial educational component. The Berkeley SETI Research Center maintains a similar archive. The Astrophysics Data System of the National Aeronautics and Space Administration indexes the technical papers of the SETI community. The principal textbooks in the field are Frank Drake’s Is Anyone Out There? and the SETI volume of the Springer Handbook of Astrobiology.

12. The SETI Question in Twenty-First Century Astronomy

The SETI question has been an increasingly important element in twenty-first century astronomy, particularly in the decade from 2015 to 2025. The reason is partly technical: the sensitivity of the available instruments has increased, the volume of data produced by modern radio telescopes has increased, and the analytical procedures of the modern SETI search have become more rigorous. The reason is also political: the Breakthrough Listen project has shown that a sufficiently-well-funded privately-sponsored scientific programme can operate for a substantial period on the available data of the radio-astronomy community, and the success of the Breakthrough Starshot project (a separate Milner initiative aimed at sending a gram-scale probe to the nearest star) has shown that a sufficiently-well-funded privately-sponsored meta-programme can attract sustained attention from the public and the press.

The 2020 NSF Decadal Survey of Astronomy and Astrophysics included SETI for the first time as a dedicated topic. The survey’s report, titled Pathways to Discovery in Astronomy and Astrophysics for the 2020s, recommended an increased federal investment in the SETI-related topics of the survey’s research areas, and urged the NSF and NASA to coordinate their SETI-related research and development with the existing private-sector projects. The 2020 NASA Astrobiology Strategy, issued by NASA in the same year, included a similar recommendation, and the 2023 NASA Exoplanet Exploration Strategy included SETI-related research as an explicit component of the strategy’s second objective.

The future of the SETI programme in the 2025 to 2035 period depends on several decisions that have not yet been made by the time of writing. The construction of the next-generation radio telescopes — the Square Kilometre Array in Australia and South Africa, the next-generation Very Large Array in the United States, the FAST expansion in China — will provide substantially improved sensitivity for the SETI search in the next decade. The construction of the planned thirty-metre-class optical telescopes — the Extremely Large Telescope, the Giant Magellan Telescope, the Thirty Meter Telescope — will provide substantially improved optical SETI capability. The construction of new space-based SETI instruments — the proposed SETI satellite of the SETI Institute, the proposed Laser SETI project at Berkeley, the various METI projects at the Interplanetary Network — will provide a new observational dimension.

13. The Philosophical Turn in SETI

The recent SETI literature has been influenced by a philosophical turn, in which the SETI question has been treated as a question about the natural history of intelligence rather than as a question about radio engineering. The philosophical literature on SETI has been developed by a small number of scholars, including Paul Davies of the Beyond Center at Arizona State University, Steven J. Dick of the Library of Congress’s Baruch S. Blumberg NASA/Library of Congress Chair in Astrobiology, and Milan Ćirković of the Astronomical Observatory of Belgrade. The philosophical literature treats the SETI question as a question about the long-term future of intelligence, about the conditions under which intelligence can become a cosmic phenomenon, and about the role that the human species might play in that future.

The philosophical literature has been influential in the SETI literature because it provides a framework for the hypotheses summarised in Section 8. The great-filter hypothesis, in particular, is now usually framed as a problem in the natural history of intelligence rather than as a problem in the empirical absence of detection. The doomsday hypothesis is similarly a problem in the sociology and technology of intelligent civilisations. The transience hypothesis is a problem in the philosophy of mind and in the philosophy of post-biological intelligence. The rare-Earth hypothesis is a problem in the astrobiological literature that has received substantial empirical support from the past few decades of exoplanet observation.

The SETI question has also been an important part of the long-term-future literature in the contemporary analytic tradition. Nick Bostrom of the Future of Humanity Institute at the University of Oxford has treated the SETI question as one of the empirical constraints on the possibility of intelligent civilisation. Toby Ord of the Future of Humanity Institute has treated the SETI question as one of the constraints on the long-term future of human intelligence. The Open Philanthropy Project has funded several SETI-related research programmes as part of its interest in the long-term future of human intelligence.