[Note: A condensed and revised version of this article was published here in The Conversation, an online forum of academic research headquartered in Melbourne, Australia.]
During a lunch in the summer of 1950, physicists Enrico Fermi, Edward Teller and Herbert York were chatting about a recent New Yorker cartoon depicting aliens abducting trash cans in flying saucers. Suddenly, Fermi suddenly blurted out, “Where is everybody?”
Behind Fermi’s question was this line of reasoning: Since there are likely many other technological civilizations in the Milky Way galaxy, and since in a few tens of thousand of years at most they could have explored or even colonized many distant planets, why don’t we see any 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, would certainly rank as among the most significant and far-reaching of all scientific discoveries.
The Drake equation
At one of the first conferences to study the possibility of extraterrestrial intelligent civilizations, Frank Drake (1930 — ) sketched out 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. That is, he estimated that ten such civilizations were out somewhere in the Milky Way.
In the wake of these analyses, scientists proposed the Search for Extraterrestrial Intelligence (SETI) project, to search the skies for radio transmissions from distant civilizations in a region of the electromagnetic spectrum thought to be best suited for interstellar communication. But after 50 years of searching, using increasingly powerful equipment, nothing has been found. So where is everybody?
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:
- They are here, or at least are observing us, but are under strict orders not to disclose their existence. Common rejoinder: This explanation (often termed the “zookeeper’s theory”) is preferred by some scientists including, for instance, the late astronomer Carl Sagan. But it falls prey to the inescapable fact that it just takes one member of an extraterrestrial society to break the pact of silence.
- 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, known as the “panspermia” theory, only pushes the problem of the origin of life to some other star system. More controversially, Francis Crick has suggested “directed panspermia,” but few scientists take his theory seriously. With regards to DNA, scientists see no evidence in DNA sequences of anything artificial.
- They exist, but are too far away. Common rejoinder: Once a civilization is sufficiently advanced, it could send 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. In this way the entire Milky Way galaxy could be explored within at most a few million years.
- They exist, but have lost interest in interstellar communication and/or transportation. Common rejoinder: As with item #1, this explanation requires that each and every member of these civilizations forever lacks interest in communication and transportation. And all it takes is one exception, and this “solution” falls.
- They are calling, but we do not recognize the signal. Common rejoinder: This may be, but this explanation doesn’t apply to signals that are sent with the direct purpose of communicating to nascent technological societies. And as with item #1, it is hard to see how a galactic society could enforce a global ban on such targeted communications.
- 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 not yet to destroy ourselves in a nuclear or biological apocalypse. Besides, soon we will colonize the Moon and Mars, and our long-term survival will no longer rely solely on planet Earth.
- The earth is a unique planet in fostering a long-lived biological regime that ultimately results in the emergence of intelligent life. 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.
- 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, popular since the time of Copernicus, that there is nothing special about the human society or environment.
Numerous other proposed solutions and rejoinders are given in [Webb2002].
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.
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.”
More recently, NASA deployed the Kepler spacecraft, which searches for planets circulating other stars by measuring small fluctuations in their light reaching earth. In some of the initial findings, announced in February 2011, 1325 planets have been found orbiting around the 150,000 stars surveyed. Of these planets, 68 are earth-sized, and most appear to be in the habitable region around their respective stars. Extrapolation from this data suggests that as many as 10% of all stars in the Milky Way may have earth-sized planets orbiting them [Hooper2011]. A separate team of scientists, using a telescope in Chile and a different detection technique (radial velocity), recently announced the discovery of 50 new planets orbiting distant stars, several of which are approximately the earth’s mass and near the habitable zone around their respective stars [Vastag2011a].
We should add, however, that many Kepler sightings in particular remain to be ‘confirmed.’ Thus one might legitimately wonder how mathematically robust are the underlying determinations of velocity, imaging, transiting, timing, micro-lensing, etc.?
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.
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.
Astronomer 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].
Similarly, British scientist John Gribbin concludes his recent book by saying “We are alone, and we had better get used to the idea.” [Gribbin2011, pg. 205].
- [Davies2010] Paul Davies, The Eerie Silence: Renewing Our Search for Alien Intelligence, Houghton Mifflin Harcourt, New York, 2010.
- [Gribbin2011] John Gribbin, Alone in the Universe: Why Our Planet Is Unique, John Wiley, New York, 2011.
- [Hooper2011] Rowan Hooper, “Exoplanet explosion sparks philosophical debate,” New Scientist, 21 Feb 2011, available at Online article.
- [Vastag2011a] Brian Vastag, “New ‘super-Earth’ that is 36 light-years away might hold water, astronomers say,” Washington Post, 12 Sep 2011, available at Online article.
- [Webb2002] Stephen Webb, If the Universe Is Teeming with Aliens… Where Is Everybody? Fifty Solutions to Fermi’s Paradox and the Problem of Extraterrestrial Life, Copernicus Books, New York, 2002.
[Updated 10 Feb 2013:] Since this was originally posted, the Fermi puzzle has further deepened with the discovery of thousands of planets, many of them in the “habitable zone” around their respective suns. Nonetheless, a study published in February 2013 concludes, based on targeted studies of candidate planets, that fewer than one in a million stars in the Milky Way have planetary civilizations advanced enough to transmit beacons we could detect. There still may be thousands of such planets, though. For details, see UC Berkeley news release.
[A version of this article appeared in The Conversation.]