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Where are the extraterrestrial civilizations? (Fermi's paradox)
David H. Bailey
Updated 27 November 2022 (c) 2022
In 1950, while having lunch with colleagues Edward Teller and Herbert York, who were chatting about a recent cartoon in the New Yorker depicting aliens, physicist Enrico Fermi suddenly blurted out, "Where is everybody?," a question now known as Fermi's paradox. This article presents background on Fermi's paradox, explains why many of the proposed solutions are not viable, and mentions a few promising new results and directions.
Behind Fermi's question was this line of reasoning: (a) Given the vast number of stars in the Milky Way (not to mention the larger universe), there are likely numerous other technological civilizations; (b) if a society is less advanced than us by even a few decades, they would not be technological, so any other technological civilization almost certainly is many thousands or millions of years more advanced; (d) within a few million years after becoming technological (an eyeblink in cosmic time), a society could have explored and/or colonized most if not all of the Milky Way; (e) so why do we not see evidence of even a single extraterrestrial civilization?
Clearly the question of whether other civilizations exist is one of the most important questions of modern science. Any discovery of a distant civilization, say by analysis of microwave data or telescopic spectra, would certainly rank 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 seems primed for the emergence of intelligent 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].
There is a rich literature on Fermi's paradox; some good readable references include [Davies2010; Fermi-paradox2022; Forgan2019; Gribbin2011; Gribbin2018; Webb2002].
The Drake equation and the SETI Project
In 1961, at a meeting of researchers investigating extraterrestrial life, Frank Drake proposed his now-famous Drake equation, which attempted to quantify our present ignorance as to the number of extraterrestrial civilizations -- see [Drake2022]. At about the same time, scientists proposed the Search for Extraterrestrial Intelligence (SETI) project to search the skies for radio transmissions from distant civilizations in regions of the electromagnetic spectrum thought to be best suited for interstellar communication. Initially, the SETI project used existing radiotelescopes, but more recently it uses a large interconnected array constructed in northern California. 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 over 50+ years is that nothing has been found. While searching will continue (certainly encouraged by the present author), it raises the question that if there are indeed numerous technological civilizations in the Milky Way, why has it been so difficult to detect signals or other evidence of their existence?
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. 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 has arisen via Darwinian evolution, consists of a large number (at least millions, if not billions or more) of individuals, and thus is subject to principles of diversity and natural selection. We do not assume that ETs are carbon-based (although it is very likely that they are [Lewis2016]), or that their biology is based on DNA, or that ET societies have invented exotic communication or transportation technologies beyond what we can envision from well-known physics today (but if they have, Fermi's paradox is even further compounded).
Numerous other proposed solutions and rejoinders, mostly variations of the above, are given at [Davies2010; Fermi-paradox2022; Forgan2019; Gribbin2011; Gribbin2018; Webb2002].
- They exist, but are under strict orders not to communicate with a new 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), spanning multiple planets or stars, it is hardly credible that a society could impose a global ban on communication to a planet such as 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. See also "Diversity and Fermi's paradox" below.
- They exist, but have lost interest in scientific research, exploration and expansion (the "beach bum" solution). Rejoinder: 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 requires continuing scientific research to uncover potential perils in its cosmic environment, ranging from asteroids, meteorites, solar flares, supernovas, gamma ray bursts and neutron star mergers, to dangerous biological systems and hostile neighbors. See also "Diversity and Fermi's paradox" below.
- 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 ETs are not interested in primitive societies such as ours. But from a diversity point of view it is hardly credible that every individual in every ET civilization has no interest. In human 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 thousands of species. See also "Diversity and Fermi's paradox" and "Post-biological intelligences" below.
- 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 (e.g., optical or microwave) that could be easily recognized by a newly technological society. Again, it is hardly credible that a diverse galactic society could enforce a global ban on communication targeted to Earth that is absolutely 100% effective, and, as noted before, once a signal is on its way to Earth, it cannot be called back, according to known laws of physics. See also "Diversity and Fermi's paradox" below.
- They exist, but are not aware of us yet, since our first radio/TV/microwave signals have only passed about 80 light years' distance (the "no evidence of humans" solution) [Reynolds2017]. Rejoinder: Although radio/TV/microwave broadcasts may be recent, 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; pyramids, roads and other structures have 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.
- They exist, but travel and communication are too difficult (the "technological difficulty" solution). Rejoinder: Recent advances in modern technology in the past decade or two have seriously undermined this solution. These include new energy sources [Waldman2022]; new propulsion systems [Ion2016, Foster2004, Slough2013]; new space exploration vehicles [Drake2017]; fleets of nanocraft to visit nearby stars [Billings2016]; supercomputers (currently run at 1018 flop/s) [Clark2022]; quantum computing and artificial intelligence [AlphaGo2017, Parloff2016, Wood2022]; 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 Way" below.
- Civilizations like us invariably self-destruct before becoming a space-faring society (the "self-destruct" solution). Rejoinder: In over 200 years of technological adolescence, we have not yet destroyed ourselves through a nuclear, environmental, biological or military catastrophe. Further, we have developed sophisticated environmental monitoring technology and 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, 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, advancing technology is significantly eroding this solution to Fermi's paradox, although it remains one of the more credible. See also "The great filter" below.
- Earth is an exceedingly rare planet with characteristics fostering a long-lived biological regime, which enabled the unlikely rise of intelligent life (the "rare Earth" solution) [Ward2000, Gribbin2018]. Rejoinder: Although this is one of the most credible solutions, several recent developments point in the opposite direction. In particular, thousands of exoplanets have been found, including more than 40 in the habitable zone, and it is estimated that at least five billion habitable zone planets exist in the Milky Way alone [Hab-Exo2019]. For a more detailed discussion, see "How rare is the Earth?" and "How rare is intelligent life?" below.
- 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 in the entire observable 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 overturned? Many recoil at this solution (including the present author), even at the Milky Way level, but what is the alternative?
Diversity and Fermi's paradox
It is clear from the above list of solutions and rejoinders that diversity arguments defeat a wide range of proposed solutions. Consider:
In any vast, diverse society (an essential prerequisite for advanced technology), there will be exceptions to any rule. Thus claims that "all ET are X" (e.g., "all ETs lose interest in exploration and communication" or "all ETs isolate themselves") have little credibility, no matter what "X" is. It is ironic that while most scientific researchers would strenuously reject stereotypes of religious, ethnic, racial or national groups in human society, many seem willing to hypothesize sweeping, ironclad stereotypes for ET societies.
- Darwinian evolution is the only known or hypothesized mechanism whereby high-information organisms and species, carbon-based or not, can form.
- Diversity is a fundamental law of Darwinian evolution, without which natural selection could not occur.
- Diversity is also a law of economics, political science, organizational behavior, and even physics (quantum superposition, sum over histories, chaos, anisotropy in the cosmic microwave background, etc.).
- Highly conformist species, societies, governments, businesses and organizations inevitably fail.
- All great figures of history were nonconformists: Albert Einstein, Martin Luther King, Susan B. Anthony, Nelson Mandela, Steve Jobs (Jobs' motto was "think different").
The great filter
As noted in item 7 above, several 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 include that the origin of life or the rise of intelligent life are extraordinarily unlikely (see "Five obstacles to the rise of intelligent life" below), that gamma-ray bursts or neutron star mergers invariably destroy societies such as ours before they can explore the cosmos, or that civilizations like ours invariably self-destruct in some nuclear or biological apocalypse (see [Jiang2022] for an up-to-date discussion). For example, suppose most civilizations eventually discover some form of planet-destroying technology; such civilizations might not last very long after that.
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 in over four billion years, it seems exceedingly unlikely that this will happen within the next 200 years or so, during which time we will have ventured to the cosmos. Similarly, if human society on Earth can avoid destroying itself, say through the end of this century, then its long-term survival will be much more likely, since by that time we very likely will be a multi-planet species.
One intriguing possibility mentioned by physicist 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, which is a variation of item 3 ("humans are as ants") above, is also vulnerable to a diversity argument, since it is hardly credible that every individual in every ET society is uninterested in communication with a newly technological biological species such as us.
Exploration of the Milky Way
Item 6, the "technological difficulty" solution, is that interstellar exploration and communication are simply too difficult. One rejoinder to this argument, as mentioned above, is simply to observe the many remarkable advances in human technology over the past 20 years or so. In addition to the advances listed above in item 6, an ET society could deploy "von Neumann probes," namely self-replicating robotic spacecraft that travel to a star system, send photos and scientific data back to the home planet (relayed by other probes as necessary), and then manufacture several copies of themselves, which are launched to even more distant systems. Such probes could initiate communication with any promising societies they encounter.
Von Neumann probe scenarios have been studied at length. In one 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].
Another interesting approach is to take advantage of the fact that the Sun can act as a "gravitational lens," according to the equations of general relativity. Magnifications of up to 1015 may be achieved by transporting a moderately powerful space telescope 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, we could obtain rather high-resolution images of distant planets, and even observe microwave transmissions and respond in kind [Landis2016; Gasparini2022].
As mentioned above, if we are nearly capable of deploying such technologies today, what is stopping societies and even individuals that are thousands or millions of years more advanced than us? Why have they or their probes not initiated communication with us?
How rare is the Earth?
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 5,000 [Exoplanets2022], suggesting that the Earth is not particularly rare. Most of these exoplanets are either too large or too close to their sun to possess liquid water, so researchers have searched for exoplanets in the circumstellar habitable zone about a star, where a planet might support liquid water. A recent study estimated that there are between 5 billion and 10 billion habitable exoplanets in the Milky Way by this criterion [McFall-Johnsen2019]. Does this mean that the discovery of Earth 2.0 is inevitable? Are there really billions of life-cradling exoplanets in the Milky Way?
Unfortunately, there are many reasons to hold the champagne. Most likely few or none of the current list of 5,000 exoplanets is actually capable of hosting life [McFall-Johnsen2019], 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 orbit red dwarf stars, which 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]. In addition, the evolution of life on Earth has required a favorable environment not just at the present epoch, but continuously for the last four billion years, facilitated in part by plate tectonics, the geomagnetic field, and (for the last 600 million years) by the ozone shield [Webb2002]. Such considerations severely limit the number of candidates for inhabited 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 about a given star tend to be of roughly the same size. This is in 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 8,000 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 [Gribbin2018] for additional discussion.
How rare is intelligent life?
It is also not sufficient for a planet and solar system merely to be conducive for life -- life must actually appear. And the sobering reality is that despite decades of research, biochemists still do not understand how the original self-reproducing biomolecules on Earth arose -- far from it. For example, while researchers have recently discovered a plausible chemical pathway to form the four bases of RNA [Castelvecchi2019], it appears exceedingly unlikely that more than a handful of these bases could have spontaneously assembled into an RNA chain [Totani2020]. The fact that life was present at least 3.8 billion years ago, not long after the surface of the Earth solidified, may suggest that the origin of life might have been moderately likely. But the continuing failure to find a credible pathway in the laboratory, combined with the fact that this event only happened once in Earth's history (according to the latest phylogenetic studies), indicates that the origin of life might well have been an exceedingly improbable fluke, perhaps not repeated anywhere else in the Milky Way if not beyond [Totani2020]. See [Abiogenesis2022] for additional details.
Once life began on Earth, there were several other major obstacles to overcome, including the origin of photosynthesis (1.5 billion years after Earth's formation); the origin of eukaryotic cells with a distinct nucleus and other organelles (2.7 billion years after formation); the origin of complex animals at the Cambrian explosion (4 billion years after formation); and, finally, the rise of human intelligence (4.5 billion years after formation). As University of Bath biologist Nicholas Longrich observes, "That these innovations are so useful but took so long to evolve implies that they're exceedingly improbable." [Longrich2019].
Along this line, many have presumed that once complex life appears (here or elsewhere), it will steadily grow smarter until it reaches human-level intelligence. Others, after noting various instances of "convergent evolution" (instances where similar features have developed in more than one line), have argued for a similar effect with intelligence. However, as Charles Lineweaver of the Australian National University has observed, there have been numerous independent "experiments" testing this hypothesis on Earth. For instance, Australia, Madagascar, New Zealand and South America have each been isolated from each other and other continents for nearly 100 million years, roughly 40 times longer than the last 2-3 million years during which human intelligence has evolved, yet there is no indication in any of these places of species evolving towards human-level intelligence [Lineweaver2008; Lineweaver2022]. As Lineweaver explains:
If human-like intelligence were so useful, we should see many independent examples of it in biology, and we could cite many creatures who had [e]volved on independent continents to inhabit the "intelligence niche". But we can't. Human-like intelligence seems to be what its name implies -- species specific. ... [T]he fossil record strongly suggests that human-like intelligence is not a convergent feature of evolution.
Lineweaver adds [Lineweaver2022]:
Our existence on Earth can tell us little about the probability of life in the Universe because even if this probability were infinitesimally small and there were only one life-harboring planet in the Universe, we would, of necessity, find ourselves on that planet.
Five obstacles to the rise of intelligent life
In short, while both the scientific community and the general public are currently excited by the recent discovery of thousands of exoplanets, and while many have presumed that the origin of life is straightforward on such a planet and that the evolution of human-level intelligence is inevitable once life is initiated, a more sober reality is:
Note that any one of these five items may constitute a virtually insuperable obstacle -- a "great filter" as described above. Compounded together they suggest that the origin and rise of human-level technological life on Earth may well have been an exceedingly singular event in the cosmos, unlikely to be repeated anywhere else in the Milky Way if not beyond. Thus the "rare Earth" solution of Fermi's paradox is growing in credibility, and, in the present author's opinion, ranks as the most likely solution.
- Very few (or possibly none) of the currently known exoplanets are likely to be truly habitable (temperate, low radiation, water, dry land, position in galaxy, etc.), so Earth's role as a cradle for life may well have been exceedingly rare.
- Even given a habitable environment, the origin of life on Earth is not well understood, in spite of decades of study; it only occurred once, so it might well have been an exceedingly improbable event not repeated anywhere else in the Milky Way.
- Earth's continual habitability over four billion years, facilitated in part by plate tectonics, geomagnetism and the ozone shield, might well be exceedingly rare.
- Even after life started, numerous key steps (photosynthesis, complex cells, complex structures) were required before advanced life could appear on Earth; each of these required many millions or even billions of years, indicating that they were highly improbable.
- Even after the rise of complex creatures, the evolution of human-level intelligence may well be a vanishingly rare event; on Earth this happened only once among numerous continental "experiments" over the past 100 million years.
With every new development in extrasolar planets, biochemistry and 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. As mentioned above, the "rare Earth" solution, with its disquieting conclusion "we are alone," may be the most credible answer at the present time, although there is no way to know for sure, and the situation could change at any time with new discoveries.
Numerous scientists have opined that in such an enormous galaxy and universe, there must be countless instances of full-fledged technological civilizations. But in light of a more sober reading of recent scientific findings, such as those mentioned above, a number of leading scientists are beginning to question this prevailing Copernican paradigm, saying out loud that we may be alone, at least in the Milky Way galaxy if not beyond. Max Tegmark, a 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 book on the topic in these terms [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 agrees [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 in a larger realm of the observable universe, this clearly presents a challenge to the prevailing Copernican principle that has guided scientific thinking for many years. It also magnifies the paradox of fine tuning [Lewis2016] -- not only do we reside in an incredibly fortunate universe, but we occupy an incredibly unique time and place within that universe. Even if we are "only" quite rare in the universe, as the preponderance of evidence currently suggests, this is a most important finding, with truly cosmic implications.
While we eagerly await additional scientific evidence that could sway the conclusion either way, at the very least it is becoming clear that human existence is far more significant than anyone could have imagined even a few years ago. This emerging picture also adds considerable urgency to the quest to better govern human society, stop violence and war, and take much better care of the Earth's environment, since both the Earth itself and human society on Earth are precious beyond measure.