Triplet Arp 274 [Courtesy NASA]

Where are the extraterrestrial civilizations? (Fermi's paradox)

David H. Bailey
Updated 3 July 2021 (c) 2021


While having lunch with colleagues Edward Teller and Herbert York, who were chatting about a recent cartoon in the New Yorker depicting aliens abducting trash cans in flying saucers, physicist Enrico Fermi suddenly blurted out, "Where is everybody?" [Webb2002, pg. 17-18]. His question is now known as Fermi's paradox.

Behind Fermi's question was this line of reasoning: (a) There are likely numerous other technological civilizations in the Milky Way galaxy; (b) if a society is less advanced than us by even a few decades, they would not be technological, so any other technological civilization is, almost certainly, many thousands or millions of years more advanced; (d) Within a few million years after becoming technological (an eye-blink in cosmic time), a society could have explored and/or colonized most if not all of the Milky Way; (e) so why don't we see evidence of the existence of even a single extraterrestrial civilization?

Clearly the question of whether other civilizations exist is one of the most important questions of modern science. And any discovery of a distant civilization, say by analysis of microwave data, would certainly rank as among the most significant and far-reaching of all scientific discoveries. For one thing, it would lend credence to the suggestion by some that the universe is primed for the emergence of life. As Freeman Dyson memorably declared in 1979, "As we look out into the universe and identify the many accidents of physics and astronomy that have worked together to our benefit, it almost seems as if the universe must in some sense have known we were coming." [Dyson1979, pg. 250].

The question of the existence of intelligent life also has religious implications. As Paul Davies observes, "The search for alien beings can thus be seen as part of a long-standing religious quest as well as a scientific project." [Davies1995, pg. 138].

The SETI Project

In the wake of these analyses, scientists proposed the Search for Extraterrestrial Intelligence (SETI) project, which would search the skies for radio transmissions from distant civilizations in a region of the electromagnetic spectrum thought to be best suited (because of low background noise) for interstellar communication. Initially, the SETI project used existing radiotelescopes, but more recently it uses a large interconnected array devoted to the search for alien civilizations constructed in northern California, financed in part from funds donated by former Microsoft executive Paul Allen. The SETI project has by now searched the radio spectrum for several decades, at higher and higher frequency resolution, with ever-more-sophisticated equipment and computer processing facilities.

But the bottom line of all this effort is that after 50 years of searching, nothing has been found. If there are indeed numerous technological civilizations in the Milky Way, as suggested by Drake's equation, why haven't we yet been able to detect any signals or other evidence of their existence? At the very least, if some distant civilization exists, they certainly have not made it very easy for us to find them.

Proposed solutions to Fermi's paradox

Numerous scientists have examined Fermi's paradox and have proposed solutions. Below is a brief listing of some of the proposed solutions, and common rejoinders [Webb2002, pg. 27-231]. In the following, we will assume only that: (a) the laws of physics, as currently understood, apply over several billion light-years of space and several billion years of time, and (b) any technological extraterrestrial (ET) civilization consists of millions if not billions of individuals, has arisen via Darwinian evolution, and thus is subject to principles of diversity and natural selection. We do NOT assume that ET individuals are carbon- or water-based (although Lewis, Barnes and others have shown that it is quite likely that they are [Lewis2016]), or that their biology is based on DNA, or that ET societies have invented exotic communication or transportation technologies (e.g., space-warp travel) beyond what we can envision from well-known physics. (But if they have, Fermi's paradox is even further compounded.)
  1. They exist, but are under strict orders not to communicate with a civilization such as Earth (the "zookeeper" solution). Rejoinder: In a vast, diverse ET civilization (and much more so if there are are numerous such ET civilizations), each very likely consisting of many millions of individuals and spanning multiple planets or stars, it is hardly credible that a galactic society could impose a global ban on communication to Earth that is absolutely 100% effective. Note that once a signal has been sent on its way to Earth, it cannot be called back, according to known laws of physics. And for a civilization that is thousands or millions of years more advanced than us, such communication would be vanishingly cheap, even for a single individual or group of individuals.

  2. They exist, but have lost interest in scientific research, exploration and expansion (the "beach bum" solution). Rejoinder: As mentioned before, diversity is a fundamental principle of Darwinian evolution, and evolution also strongly favors organisms that think, explore and expand. Thus it is hardly credible that every individual in every ET civilization has lost interest in scientific research, exploration and expansion, or that a global ban on such activities is absolutely 100% effective. What's more, any ET society's long-term existence crucially hinges on scientific research to uncover all potential perils in its cosmic environment, including asteroids, meteorites, solar flares, supernovas, gamma ray bursts, neutron star mergers, potentially dangerous biological systems and potentially hostile neighbors.

  3. They exist, but have no interest in a primitive, backward society such as ours; to them, we are as ants (the "humans are ants" solution). Rejoinder: Perhaps 99.99% of an ET society is not interested in primitive societies such as ours. But, as before, from a fundamental diversity consideration it is hardly credible that every individual in every ET civilization has no interest. In our society, perhaps 99.99% of the public has little or no interest in ants. But many thousands do. There is even a full-fledged scientific field (myrmecology) to study ants, and researchers have meticulously catalogued and studied every known species.

  4. They exist, but have progressed to more sophisticated communication technologies (the "advanced communication" solution). Rejoinder: This does not apply to signals that are specifically targeted to societies such as ours, in a form (optical, microwaves) that could be easily recognized by a newly technological society. Again, it is hardly credible that a galactic society could enforce a global ban on communication targeted to Earth that is absolutely 100% effective. As noted before, once a signal is on its way to Earth, it cannot be called back, according to known laws of physics. Similar diversity arguments defeat a broad range of other proposed solutions (see below).

  5. They exist, but are not aware of our existence yet, since our first radio/TV and radio telescope signals have only passed roughly 50 light years' distance (the "no evidence of humans" solution) [Reynolds2017]. Rejoinder: Ample evidence of an emerging technological civilization on Earth has been on display for a much longer time period. In particular, networks of lights have been visible on Earth for hundreds of years, other evidence of civilization has been visible for thousands of years, large animal species (including early hominins) have been visible for millions of years, and atmospheric signatures of life have been evident for billions of years.

  6. They exist, but travel and communication are too difficult (the "technological" solution). Rejoinder: Recent dramatic and largely unanticipated developments in technology in the past few years have all but destroyed this solution: new energy sources [Bailey2015]; new propulsion systems [Ion2016, Foster2004, Slough2013]; new space exploration vehicles [Drake2017]; fleets of nanocraft to visit nearby stars [Billings2016]; supercomputers (currently run at 1017 flop/s), quantum computing and artificial intelligence [AlphaGo2017,Parloff2016]; robotics, 3-D printing and nanotechnology; exoplanet detection and analysis technology [Exoplanet2019]; gravitational lenses (see below); and von Neumann probes (see below). If we are on the verge of deploying such technologies today, what is stopping societies and even individuals that are thousands or millions of years more advanced than us? See also "Exploration of the Milky" below.

  7. Civilizations like us invariably self-destruct before becoming a space-faring society (the "self-destruct" solution). In 200 years of technological adolescence, we have not yet destroyed ourselves through a nuclear, environmental or biological catastrophe. Further, we have developed sophisticated supercomputer simulations to foresee and control future perils. Thus it is hardly credible that societies such as ours invariably self-destruct before they become space-faring society, without any exceptions whatsoever. In any event, within a few years human civilization will spread to the Moon, Mars and elsewhere, and then its long-term survival will be largely impervious to calamities on the home planet. As before, accelerating technology is seriously eroding this solution to Fermi's paradox, although it remains one of the more credible.

  8. Earth is a unique planet with characteristics fostering a long-lived biological regime leading to intelligent life (the "rare earth" solution) [Ward2000,Gribbin2018]. Rejoinder: This is arguably one of the most credible solutions (see below), although many recent discoveries point in the opposite direction: the universe contains over 100 billion galaxies; the Milky Way contains over 100 billion stars; thousands of exoplanets have been found (more than 40 in the habitable zone) (see below); recent work in biogenesis indicates that the origin of life was not a particularly unlikely event (also indicated by recent fossil finds, which show life arose almost immediately after the formation of Earth, over 3.8 billion years ago).

  9. WE ARE ALONE, within the Milky Way galaxy if not beyond (the "solitary" solution). Rejoinder: It hardly seems credible that we are unique even in the Milky Way (with over 100 billion stars and planets), much less the entire universe (with over 100 billion galaxies). This solution may be consistent with Occam's razor, but it is an extreme violation of the "Copernican principle," namely the hypothesis that there is nothing special about Earth or humanity. Has the Copernican principle been completely overturned? Many recoil at this solution (including the author), but what is the alternative?
Numerous other proposed solutions and rejoinders are given at [Webb2002]. A more recent review of these issues is given in [Gribbin2011, Gribbin2018 and Forgan2019].

Diversity and Fermi's paradox

As mentioned above, diversity arguments defeat a wide range of proposed solutions. Consider: In a vast, diverse society, there will be exceptions to any rule. Thus claims that "all ET are like X" (e.g., "all ETs have lost interest in exploration and communication") have no credibility, no matter what "X" is. It is ironic that while most scientists and others would reject stereotypes of religious, ethnic, racial or national groups, some seem willing to hypothesize sweeping, ironclad stereotypes for ET societies.

Exploration of the Milky Way

As mentioned above (item 6, the "technological" solution) argues that exploration and communication is simply too difficult. However, in addition to the developments listed above, a distant society could deploy "von Neumann probes," self-replicating robotic spacecraft that travel to a star system, send video and scientific data back to the home planet, and then manufacture several copies of themselves, which are launched to even more distant systems.

Von Neumann probe scenarios have been studied at length. In the latest such analysis, researchers at the University of Edinburgh employed a computer simulation to explore the scenario where each probe travels at fairly modest speed under powered flight (roughly 10 km/sec), but employs a "slingshot" technique (i.e., passes by one star to give itself a gravitational boost to another star) to enhance its speed and reduce its need for fuel (as several spacecraft, including Voyager I and II, have already done). These researchers found that with this scenario, 99% of all star systems in the Milky Way could be explored in only about five million years, which, as mentioned above, is an eyeblink in the multi-billion-year age of the Milky Way [Nicholson2013].

"Exploring" the Milky Way telescopically, at least for reasonably close stars, is even easier, by taking advantage of the fact that the Sun can act as a "gravitational lens," according to the equations of general relativity. Magnifications of 1015 may be achieved. All that is required is to transport the equivalent of the Hubble Space Telescope (although a more modest version would do), together with a facility to relay images and messages back to Earth, to a point beyond the solar system that is the focal point of the Sun's "lens" for a given distant star. With such a facility, which is nearly feasible at the present time, we could obtain rather high-resolution images of distant planets, and even listen in to their microwave transmissions, such as from the equivalent of cell phones, and respond in kind. We could also send messages to them with the same equipment [Landis2016].

As mentioned above, if we are on the verge of deploying such technologies today, what is stopping societies and even individuals that are thousands or millions of years more advanced than us? Are other civilizations using gravitational lenses to see close-up images of Earth? Or even to send messages to Earth? Why not?

Non-biological intelligences

In 2010, physicist Paul Davies published an extensive analysis of Fermi's paradox, together a detailed discussion of the potential philosophical and religious implications of a discovery of extraterrestrial intelligence [Davies2010]. One intriguing possibility mentioned by Paul Davies is the notion that extraterrestrial intelligences exist, but have advanced to a "post-biological" or even "post-material" state, and now exist only as an extremely advanced computer program somewhere, possibly spending their time exploring and proving ever-more sophisticated mathematical theorems [Davies2010, pg. 160-168]. SETI astronomer Seth Shostak recently expressed a similar idea: "Once any society invents the technology that could put them in touch with the cosmos, they are at most only a few hundred years away from changing their own paradigm of sentience to artificial intelligence." [McCormack2010]. If so, perhaps the solution to Fermi's paradox is simply that we have nothing useful to say to such advanced entities. On the other hand, this solution is also vulnerable to a diversity argument -- as mentioned above in item 3, it hardly seems credible that every individual and every ET society is not interested in us.

How rare is the Earth and human life?

As mentioned above, one common rejoinder to the "rare Earth" solution (#8 above) is the growing catalogue of extrasolar planets, which now total more than 4000 [NASA-Exo2019]. Most of these are either too large or too close to their sun to possess liquid water, much less complex carbon-based compounds, so researchers have been on the lookout for exoplanets in the circumstellar habitable zone about a star, defined as a region where a planet might have a temperature regime capable of supporting liquid water. A recent study estimated that there are between 5 billion and 10 billion exoplanets in the Milky Way that reside in the habitable zone about their respective stars by this criterion [McFall-Johnsen2019]. Does this mean that the discovery of Earth 2.0 is inevitable? Are the really billions of life-cradling exoplanets?

Unfortunately, there are many reasons to hold the champagne. As a recent New Scientist article points out, most likely none of the current list of 4000 exoplanets is capable of hosting life [McFall-Johnsen2019]. This is because life needs much more than a water-friendly temperature regime. For example, one major problem is that most of the "habitable" planets identified so far are planets orbiting red dwarf stars, which as an August 2019 Scientific American article points out, are notorious for frequent flares with x-rays and high-energy UV radiation that almost certainly would sterilize any planet in the "habitable" zone [Mann2019]. Other difficulties with currently catalogued exoplanets are listed in Exoplanets.

In addition to Earth being special, the Sun and Solar System are also unusual in many ways. In most of the recently discovered exoplanet systems, planets tend to be of the same size. This is in stark contrast to our Solar System, which features tiny planets such as Mercury and huge planets such as Jupiter, with roughly 20 times the radius (and 8000 times the volume) of Earth. The existence of a large planet such as Jupiter is now thought to be crucial to clearing out debris from the inner planets in the Solar System's early life. Additionally, our system's position in the Milky Way is also quite favorable: at roughly 27,000 light-years from the galactic center, our Solar System strikes a good balance between being close enough to the center to have a critical concentration of heavier elements for complex chemistry, and yet not so close as to be bathed in sterilizing radiation -- only about 7% of the galaxy is in a "galactic habitable zone" by these criteria. See this 2018 Scientific American article by John Gribbin [Gribbin2018] and Exoplanets for additional facts and discussion.

In short, while both the public and the astronomy community are currently excited by the recent discovery of thousands of exoplanets, including some which appear to be in the "habitable zone" (at least for the existence of liquid water), a more sober reality is that virtually none of these exoplanets are likely to be cradles for life. Thus the "rare Earth" solution of Fermi's paradox is growing in credibility.

The great filter

Some writers have suggested that there is a "great filter" that explains the silence -- some major barrier to a society becoming so advanced that it can thoroughly explore the Milky Way. Possibilities here range from the hypothesis that it might be extraordinarily unlikely for life to begin at all, or that the jump from prokaryote to eukaryote cells is similarly unlikely, or that our combination of planetary dynamics and plate tectonics is exceedingly unlikely, or, as suggested above, that civilizations like ours invariably self-destruct, or that some future calamity, such as a huge gamma-ray burst from a nearby star, invariably ends societies like ours before they can explore the cosmos.

One disquieting aspect of this line of thinking is that it then follows that either (a) we are first such technological society (the great filter is behind us), or else (b) we are in deep trouble (the great filter, possibly a great catastrophe, is still ahead of us). Along this line, Nick Bostrom, among others, hopes that the search for extraterrestrial life (e.g., on Mars) comes up empty-handed, because if found, this would reduce the number of possible candidates of the great filter being behind us, and it would increase the likelihood that the great filter is still ahead of us [Bostrom2008].

On the other hand, there are problems with the "great filter" solution as well. For example, given that no gamma-ray burst or neutron star merger has destroyed Earth to date (over 4.5 billion years), it seems exceedingly unlikely that this will happen within the next 20-50 years, during which time we will have ventured to the cosmos.


With every new research finding of extrasolar planets, potential life-friendly environments within the solar system, and, especially, with every new advance of human technology, the mystery of Fermi's paradox deepens. Indeed, "Where is everybody?" has emerged as one of the most significant scientific and philosophical questions of our time. Numerous scientists have traditionally opined that in such an enormous galaxy (and universe), there must be countless instances of extraterrestrial life, and almost as many full-fledged technological civilizations.

But in light of more sober reading of recent scientific findings, such as presented above in "How rare is the Earth and the human species?," a number of leading scientists are beginning to question this prevailing mindset, saying out loud that we may be alone, at least in the Milky Way galaxy if not beyond. Max Tegmark, a prominent Swedish-American cosmologist, argues [Tegmark2017, pg. 241] that "this assumption that we're not alone in our Universe is not only dangerous but also probably false." He adds, "This is a minority view, and I may well be wrong, but it's at the very least a possibility that we can't currently dismiss, which gives us a moral imperative to play it safe and not drive our civilization extinct."

Paul Davies concludes his latest book on the topic by stating his own assessment [Davies2010, pg. 207-208]:

[M]y answer is that we are probably the only intelligent beings in the observable universe and I would not be very surprised if the solar system contains the only life in the observable universe. [Nonetheless,] I can think of no more thrilling a discovery than coming across clear evidence for extraterrestrial intelligence.

John Gribbin, a prominent British scientist, largely agrees with Davies' stark assessment. He concludes his recent book on the topic in these uncompromising terms [Gribbin2011, pg. 205]:

They are not here, because they do not exist. The reasons why we are here form a chain so improbable that the chance of any other technological civilization existing in the Milky Way Galaxy at the present time is vanishingly small. We are alone, and we had better get used to it.
If we are truly alone in the Milky Way or beyond, this greatly magnifies the paradox of fine tuning -- not only do we reside in an incredibly fortunate universe, but we occupy an incredibly unique time and place within that universe (see Fine tuned). Even if we are "only" extremely rare in the universe, this is a most important finding, with truly cosmic implications.

Either way, it is becoming clear that human existence is far more significant than anyone could have imagined even a few years ago.

See also Drake's equation and Fine tuned.


[See Bibliography].