Triplet Arp 274 [Courtesy NASA] Stone pseudo-magic square on exterior of La Sagrada Familia cathedral, Barcelona, Spain [Photo by DHB, (c) 2011]

Where are the extraterrestrial civilizations?

David H. Bailey
13 December 2017 (c) 2017


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 numerous vast, diverse ET civilizations (or even in just one ET civilization), each spanning multiple planets or stars, and each consisting of millions if not billions of individuals, 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 broadcast and is on its way to Earth, there is no way to call it back, within 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.
  2. They exist, but have lost interest in scientific research, exploration and expansion (the "beach bum" solution). Rejoinder: Darwinian evolution 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 having an in-depth scientific understanding of 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, 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, over a vast array of inhabited planets, each with billions of individuals, on communication targeted to emerging technological civilizations, that is absolutely 100% effective. As noted before, once a signal is on its way to Earth, it cannot be called back, within known laws of physics.

    Similar diversity arguments defeat a wide range of other proposed solutions:

  5. In a vast, diverse society, there will be exceptions to any rule. Thus claims that "all ET are like X" have no credibility, no matter what "X" is. It is ironic that while most scientists would reject stereotypes of religious, ethnic or national groups, some seem willing to hypothesize sweeping, ironclad stereotypes for ET societies.

  6. They exist, but are not aware of our existence yet --- our first TV signals have only passed 80 light years' distance (the "no evidence of humans" solution). As an example, physicist-cosmologist Lawrence Krauss declared [Reynolds2017]:
    Over the last four and a half billion years, the only evidence of intelligent life was in the last 50 to 60 years, by watching Star Trek or I Love Lucy, or whatever --- signals we sent out. So even if someone told you "look at that star, and look at the third rock from that star, that's where you will find life," even if [you] knew which object to look for, there is only a 50 year period over 5 billion years, almost, where you would be able to find intelligent life.

    Rejoinder: Ample evidence of an emerging technological civilization on Earth has been on display for much longer:

  7. 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:

    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?

  8. 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, galloping technology is destroying this solution to Fermi's paradox.
  9. Earth is a unique planet with characteristics fostering a long-lived biological regime leading to intelligent life (the "rare earth" solution). Rejoinder: Perhaps, although most recent discoveries point in the opposite direction:

  10. 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].

The grim mathematics of Fermi's paradox

Let p be the probability that an individual on a given planet in a given year launches an interstellar exploration, m be the number of individuals on a typical planet, n be the number of planets, and t be the number of years. Then the probability P that a civilization has explored the Milky Way can be estimated as P = 1 - (1 - p)m n t. Conservative estimates for the Milky Way are m > 109, n > 1011 and t > 5 x 109. For the universe as a whole, n > 1022 and t > 1010.

In other words, if the probability of the rise of a space-faring civilization anywhere is even microscopically nonzero (given the instance of human civilization), then after billions of years, on many billions of planets, with billions of individuals, ET should be everywhere. Where is everybody?

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]. Davies, in agreement with several other observers, concludes that the question of whether the universe is primed for life is central to the debate. Davies contrasts the view of French biochemist Jacques Monod, who declared "The universe is not pregnant with life nor the biosphere for man," with that of biologist Christian de Duve, who declared that there is a "cosmic imperative" for life [Davies2010, pg. 25].

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, although, as mentioned above in item 3, it is hardly credible that every individual and every ET society is not interested in us.

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 other scientists are beginning to question this orthodoxy, 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.

Was the universe made for us? Or is our understanding of the laws of physics fundamentally in error?

Either way, 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].