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Introduction

Behind Fermi's question was this line of reasoning: (a) There are likely numerous other technological civilizations in the Milky Way galaxy; (b) it is extremely unlikely that some other technological civilization is exactly as advanced as we are; (c) if a society is less advanced than us by even a few decades (which is an eyeblink in galactic time), they would not be technological, so any other technological civilization is, almost certainly, many thousands or millions of years more advanced; (d) in a few thousand years (certainly less than one million years) after becoming technological, a society could have explored or even colonized many distant planets; (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 Drake equation

As I planned the meeting, I realized a few day[s] ahead of time we needed an agenda. And so I wrote down all the things you needed to know to predict how hard it's going to be to detect extraterrestrial life. And looking at them it became pretty evident that if you multiplied all these together, you got a number, N, which is the number of detectable civilizations in our galaxy.

The result of these musings, further refined at the conference, 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:

The values used by Drake in 1960 were R = 10, f_p = 0.5, n_e = 2, f_l = 1, f_i = 0.01, f_c = 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. Initially, the SETI project used existing radiotelescopes, such as the Very Large Array in New Mexico, but funding for these searches was terminated by the U.S. Congress in 1994 (as depicted in the movie "Contact"). Subsequently a large interconnected array devoted to the search for alien civilizations was constructed in northern California, financed in part from funds donated by former Microsoft executive Paul Allen. The bottom line of the SETI project is that after 50 years of searching, nothing has been found. So if there are indeed ten or more technological, communicating 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?

Proposed solutions to Fermi's paradox

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

   Common rejoinder: The suggestion that aliens are among us but that we cannot detect them is an unscientific notion, since it is not falsifiable by any empirical test (and smacks of sci-fi fantasy). A weaker form of hypothesis, namely that extraterrestrial civilizations are observing us but are under strict orders not to contact us or disclose their existence (often termed the "zookeeper's theory") is preferred by some scientists including, for instance, the late astronomer Carl Sagan. However, 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 given that the universe is only 13.7 billion years old, and that second-generation star systems are required to provide carbon for life, this theory does not provide much more time for the origin of life. With regards to the notion that extraterrestrials have left messages in DNA, the proliferation of cheap genome sequencing technology has permitted DNA sequences of many organisms to be studied in detail, from primitive bacteria to humans. There is no evidence in these 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. 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 it is hard to see how any civilization could forever remain out of the range of the numerous earth- and space-based telescope systems that are being used to search for signs of extraterrestrial civilizations. More importantly, as with item #1, this explanation requires that each and every member of each and every alien civilization forever lacks interest in communication and exploration. And 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: At first this seems like a reasonable response. Human communication even now is moving very rapidly to compressed digital transmission, which, when broadcasted via radio, is by definition nearly indistinguishable from radio noise. Thus, as one scientist recently suggested, we may be sitting in the midst of a "Galaxy-Wide Web" of alien chatter, yet not be able to recognize it [Hallman2011]. However, this analysis 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. Global warming presents a major challenge at the present time, but we now understand the situation fairly well and there is no indication that it will prove to be an absolutely insoluble problem, provided that world governments soon take action. Besides, within a few decades 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.

Extrasolar planets

A breakthrough came in September 2010, when Steven S. Vogt of the University of California, Santa Cruz, and R. Paul Butler, of the Carnegie Institution in Washington, discovered evidence of a planet only three or four times the mass of earth orbiting in the "habitable zone" of a star (i.e., at a distance from a star where water could exist) about 20 light-years away from earth. As Butler noted, "This is really the first Goldilocks planet." [Overbye2010]. More recently, NASA deployed the Kepler spacecraft, which searches for planets circulating other stars by measuring small fluctuations in their light reaching earth. In December 2011, Kepler project scientists announced the discovery of two planets that are definitely earth-sized orbiting a star 950 light-years away, along with 2326 other potential planets near other stars [Overbye2011d].

In short, among the factors in the Drake equation, two that have proven amenable to experimental study have been found to have reasonable values, although not quite as optimistic as Drake and his colleagues first estimated.

The search for life on other planets in the solar system

One very intriguing possibility along this line is that life may exist (or may have existed) on Mars. In 1976 NASA's Viking spacecraft performed experiments that produced tantalizing results, but which were subsequently interpreted as not indicative of life. Some researchers, however, were never satisfied with these findings. Interest in these results was revived in September 2010, when scientists analyzing results of NASA's Phoenix mission in 2008 found that a highly reactive perchlorate compound was present in the Martian soil. Since the Viking spacecraft had heated its samples to high temperature, it is likely that organic material, if present, would have been destroyed in the process. In other words, the original conclusion "no organics present" is drawn into question by the existence of this perchlorate compound in the Martian soil [Kaufman2010a].

In 1996, NASA scientists David McKay and Everett Gibson announced evidence that the ALH84001 meteorite, which had previously been established as having originated on Mars, may have once fostered living organisms. For example, the following photo shows structures reminiscent of primitive bacteria (courtesy NASA):

But other scientists were not convinced. They argued, for instance, that these structures could have arisen from carbonate materials decomposing under high temperatures, such as during the impact of a large meteorite on Mars that may have ejected the ALH84001 sample. But just as with the Viking findings, some scientists were not so sure about this negative conclusion. Their view was bolstered in 2009, when a new study by McKay's team found that there was no plausible geological scenario that would be consistent with the claim that these structures arose from carbonate materials decomposing under high temperatures and pressures [Fisher2009]. Final resolution of this dispute will most likely need to await a Mars sample return mission, now scheduled for launch in 2018 (with return of samples in 2020-2022).

Other planets and moons are also being investigated as possible havens for life. In 2010 a team led by Darrell Strobel of Johns Hopkins University found that acetylene is anomalously absent from the surface of Titan (Saturn's largest moon), and hydrogen is anomalously scarce there, lending strength to a 2005 conjecture by Chris McKay of NASA's Ames Research Center that microbial life might exist on Titan [Shiga2010].

A "second genesis" on earth?

One interesting possibility that Davies explores is whether there was a "second genesis" here on earth. If so, this would confirm de Duve's more optimistic view [Davies2010, pg. 42]. Along this line, in December 2010 a team of researchers led by Felisa Wolfe-Simon, a young NASA astrobiologist, startled the scientific world with an announcement that they had coaxed a species of bacteria originally found on the shores of Mono Lake in California to utilize arsenic as a substitute for phosphorus, one of the six primary elements of all known life on earth [Overbye2010a. However, the Wolfe-Simon paper has already generated significant controversy, with some scientists questioning whether the NASA team's conclusions are justified based on their experimental results [Hayden2012]. For additional discussion, see Origin.

Post-biological intelligences

Religious implications

On the other hand, these concerns may be overblown. A recent 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].

Conclusion

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.