Where are the extraterrestrial civilizations?


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 million years or so (an eyeblink in cosmic time) after becoming technological, a society could have explored or even colonized many distant planets in the Milky Way; (e) so why don’t we see evidence of the existence of even a single extraterrestrial civilization?

The Drake equation

One of the first conferences to study the possibility of extraterrestrial intelligent civilizations was held in Green Bank, West Virginia in 1960. At this meeting, Frank Drake and others formulated what now is commonly known as the Drake equation, which estimates the number of civilizations in the Milky Way galaxy with which we could potentially communicate:

N = R* fp ne fl fi fc L


N = number of civilizations in our galaxy that can communicate
R* = average rate of star formation per year in galaxy
fp = fraction of those stars that have planets
ne = average number of planets that can support life, per star that has planets
fl = fraction of the above that eventually develop life
fi = fraction of the above that eventually develop intelligent life
fc = fraction of civilizations that develop technology that signals existence into space
L = length of time such civilizations release detectable signals into space.

The values used by Drake in 1960 were R = 10, fp = 0.5, ne = 2, fl = 1, fi = 0.01, fc = 0.01, L = 10,000, so that N = 10 x 0.5 x 2 x 1 x 0.01 x 0.01 x 10,000 = 10.

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. But after 50 years of searching, nothing has been found.

Proposed solutions to Fermi’s paradox

Numerous scientists have examined Fermi’s paradox and have proposed solutions. Here is a brief listing of some of the proposed solutions, and common rejoinders [Webb2002, pg. 27-231]:

  1. They are here, or at least are observing us, but are under strict orders not to disclose their existence.

    Common rejoinder: This type of explanation falls prey to the inescapable fact that it just takes one member of an extraterrestrial society to break the pact of silence. Given our experience with human society, it seems utterly impossible to impose such uniformity on a vast civilization that almost certainly consists of many billions of individuals, dispersed over numerous homelands and cultures, and, very likely, living on multiple planets and/or solar systems. These difficulties are further compounded if there is (as Drake’s equation suggests) more than just one extraterrestrial civilization in the Milky Way.

  2. They have been here and planted seeds of life, or perhaps left messages in DNA.

    Common rejoinder: The notion that life began on earth from bacterial spores or the like that originated elsewhere is known as the “panspermia” theory, and has been advanced by some well-recognized scientists, including, for instance, Francis Crick, the co-discoverer of DNA. One problem with this theory is that it really does not solve the problem of the origin of life — it just pushes it away to some other star system. And after years of searching through many gigabytes of data, there is no evidence in DNA sequences of anything artificial.

  3. They exist, but are too far away.

    Common rejoinder: Once a civilization is sufficiently advanced, it could send what have been termed “von Neumann probes” to distant stars, which could scout out suitable planets, land, and then construct additional copies of themselves, using the latest software beamed from earth. Simulations of this scheme indicate that a single society could “visit” (via its probes) the entire Milky Way galaxy within at most a few million years, which is a tiny fraction of the galaxy’s lifetime (see below). So it doesn’t seem likely that explanations such as this are sufficient to explain the eerie silence.

  4. They exist, but have lost interest in interstellar communication and/or exploration.

    Common rejoinder: This is conceivable, but all it takes is one exception — one small group that yearns to reach out to the cosmos — and this “solution” falls.

  5. They are calling, but we do not recognize the signal.

    Common rejoinder: This seems reasonable, but it doesn’t apply to signals that are sent with the express purpose of communicating to nascent technological societies. Indeed, the current SETI project program assumes that the remote civilization is making some effort to signal its existence using technology that is detectable by young civilizations like ours. And as with item #1, it is hard to see how a galactic society could forever enforce, without any exceptions, a global ban on such targeted communications.

  6. Civilizations like us invariably self-destruct.

    Common rejoinder: This contingency is already figured into the Drake equation in the L term (the average length of a civilization). In any event, from our experience we have survived at least 100 years of technological adolescence, and have managed to not yet destroy ourselves in a nuclear or biological apocalypse. And soon the human civilization will have spread to the Moon and to Mars, and then its long-term existence will be largely impervious to calamities on earth.

  7. The earth is a unique planet in fostering a long-lived biological regime that ultimately results in the emergence of intelligent life. Our existence results from a conjunction of numerous favorable factors, ranging from the stability of planetary orbits in the solar system, to the presence of tectonic plates regulating carbon levels, all of which happen not to be found in combination anywhere else [Ward2000].

    Common rejoinder: Such arguments may have some merit, but the latest studies, in particular the detections of extrasolar planets (see below), point in the opposite direction, namely that environments like ours appear to be quite common.

  8. We are alone, at least within the realm of the Milky Way galaxy. Some scientists in this camp further conclude that we are alone in the entire observable universe.

    Common rejoinder: This conclusion flies in the face of the “principle of mediocrity,” namely the presumption, dominant since the time of Copernicus, that there is nothing special about the human society or environment.

Numerous other proposed solutions and rejoinders are given at [Webb2002, pg. 27-231].

Extrasolar planets

Two key terms in the Drake equation are fp (the fraction of stars that have planets) and ne (the average number of planets that can support life, per star that has planets). Scientists once thought that stable planetary systems in general, and earth-like planets in particular, were a rarity. But beginning in the 1990s, scientists detected unmistakable evidence of planets orbiting around other stars. More recently there has been a veritable explosion in the number of detected extrasolar planets. For example, NASA’s Kepler spacecraft searches for planets circulating other stars by measuring small fluctuations in their light reaching earth. In the most recent announcement from the Kepler project (7 Jan 2013), researchers announced that they have identified 2740 planet “candidates” orbiting 2036 different stars, including 351 “earth-sized” planets [SD2013a; SD2013b]. Based on these findings, researchers estimate that there are 100 billion such “alien planets” in the Milky Way galaxy alone, 17 billion of which are roughly the size of the earth.

Until quite recently, though, there was scant evidence of earth-like planets in habitable zones that potentially could support life. But now researchers have identified numerous planets in the habitable zone. Some of these planets are in our own “back yard” — five orbit the star Tau Ceti, which is just 11.9 light-years from earth, and one of these five appears to be in the habitable zone [Wall2012].

In short, among the factors in the Drake equation, two that have proven amenable to experimental study so far have been found to have entirely reasonable values, roughly in keeping with what Drake and his colleagues first estimated in the 1960s.

Exploration of the Milky Way

As mentioned above, one frequently proposed solution to Fermi’s paradox is that such extra-terrestrial civilizations may in fact be quite numerous in the Milky Way, but they are too far away for any of them to have visited us or to have left any trace of their existence. However, researchers point out that a society could deploy “von Neumann probes,” namely self-replicating spacecraft that travel to a nearby star system and send video and scientific data back to the home planet. These probes would then find a suitable planet or asteroid, extract necessary materials, manufacture several copies of themselves, and launch these craft to even more distant systems. Such 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].

Such technologies may seem futuristic, but keep in mind, as emphasized above, that other technological civilizations are almost certainly many thousands or millions of years more advanced than our own. And futuristic technologies such as three-dimensional printing are already being deployed in industry.

Along this line, some have suggested that von Neumann probe technologies and the like might be feasible, but that a global galactic society has enacted a ban on their deployment. But as with claims that each and every extraterrestrial civilization has lost interest in any interstellar travel and communication, or that a galactic society has declared earth to be absolutely off-limits for any interstellar travel or communication, all it takes is one small group of one single civilization to launch these space probes, and this “explanation” falls. If we cannot uniformly and permanently enforce laws within our own earthly society, why should we think that there are “laws” that are uniformly and permanently enforced in a galactic society of many distinct civilizations, each with a different biology and culture, spanning thousands of light-years in physical extent?

Religious implications

If someone eventually does discover unmistakable evidence of extraterrestrial intelligence, many believe this would shake up the world’s religious faiths. It is true that few religious movements have seriously discussed the implications of other civilizations. Paul Davies discusses this topic at length, and then concludes “Christian theology is in a frightful muddle when it comes to extraterrestrial beings,” and “a positive result from SETI would immediately open up a horrible can of worms. … In any event, it is clear than any theology with an insistence on human uniqueness would be doomed.” [Davies2010, pg. 192-193].

On the other hand, these concerns may be overblown. A 2010 study conducted by Ted Peters, Director of the Center for Theology and the Natural Sciences in Berkeley, California, found that while such a discovery would be very interesting, only 8% of Americans surveyed felt that such a discovery would result in a personal crisis of faith. An evangelical Christian commented in the survey “Why should we repudiate the idea that God may have created other civilizations to bring him glory in the same way?” An Islamic participant commented “only arrogance and pride would make one think that Allah made this vast universe only for us to observe.” A Buddhist speculated that “ETs would be, essentially, no different from other sentient beings.” Several LDS participants responded that their religion already accommodates extraterrestrials [Griggs2010; Peters2010].


With every new research finding in the area of extrasolar planets and possible extraterrestrial living organisms, the mystery of Fermi’s paradox deepens. Indeed, “Where is everybody?” has emerged as one of the most significant scientific questions of our time.

Paul Davies concludes his latest book on the topic by stating his own assessment: “my 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, Davies reflects, “I can think of no more thrilling a discovery than coming across clear evidence for extraterrestrial intelligence.” [Davies2010, pg. 207-208].

John Gribbin, a prominent British scientist, agrees with Davies’ stark assessment. He concludes his recent book on the topic as follows [Gribbin2011, pg. 205]:

On a planet like the Earth, life may only get one shot at technology — we have exhausted the easily accessible supplies of raw materials, so if we destroy ourselves the next intelligent species, if there is one, won’t have the necessary raw materials to get started. There are no second chances. And that is the last piece of evidence that completes the resolution of the Fermi paradox. 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.

For additional details and discussion, see Fermi’s paradox.

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