The Fermi Paradox

Sometimes I think we're alone. Sometimes I think we're not.

In either case, the thought is staggering.

Buckminster Fuller

Thanks to detective work by the Los Alamos physicist Eric Jones, whose report I draw heavily upon in this section, we know the genesis of the Fermi paradox.18

The spring and summer of 1950 saw the New York newspapers exercised over a minor mystery: the disappearance of public trash cans. This year was also the height of flying saucer reports, another subject that filled the column inches. On 20 May 1950, The New Yorker published a cartoon by Alan Dunn that made amusing reference to both stories.

Fermi was at Los Alamos in the summer of 1950. One day, he was chatting to Edward Teller and Herbert York as they walked over to Fuller Lodge for lunch. Their topic was the recent spate of flying saucer observations. Emil Konopinski joined them and told them of the Dunn cartoon. Fermi remarked wryly that Dunn's was a reasonable theory because it accounted for two distinct phenomena: the disappearance of trash cans and the reports of flying saucers. After Fermi's joke, there followed a serious discussion about whether flying saucers could exceed the speed of light. Fermi

figure 5 For reasons that make sense only to them, aliens are returning to their home planet with trash cans that are the property of New York's Department of Sanitation.

asked Teller what he thought the probability might be of obtaining evidence for superluminal travel by 1960. Fermi said that Teller's estimate of one-in-a-million was too low; Fermi thought it was more like one-in-ten.

The four of them sat down to lunch, and the discussion turned to more mundane topics. Then, in the middle of the conversation and out of the clear blue, Fermi asked: "Where is everybody?" His lunch partners Teller, York and Konopinski immediately understood that he was talking about extraterrestrial visitors. And since this was Fermi, perhaps they realized that it was a more troubling and profound question than it first appears. York recalls that Fermi made a series of rapid calculations and concluded that we should have been visited long ago and many times over.

Although neither Fermi nor the others ever published any of these calculations, we can make a reasonable guess at his thought processes. He must first have made an estimate of the number of ETCs in the Galaxy, and this is something we can estimate ourselves. After all, the question "How many advanced communicating extraterrestrial civilizations are there in the Galaxy?" is a typical Fermi question!

figure 6 Edward Teller (left) with Fermi in 1951, not long after Fermi first asked his question.

A Fermi Question: How Many Communicating Civilizations Exist?

Represent the number of communicating ETCs in the Galaxy by the symbol N. To estimate N we first need to know the yearly rate R at which stars form in the Galaxy. We also need to know the fraction fp of stars that possess planets and, for planet-bearing stars, the number ne of planets with environments suitable for life. We also need the fractionf of suitable planets on which life actually develops; the fractionf of these planets on which life develops intelligence; and the fraction fc of intelligent life-forms that develop a culture capable of interstellar communication. Finally, we need to know the time L, in years, that such a culture will devote to communication. Multiplying all these factors together will provide us with an estimate for N. We can write it as a simple equation:

The equation N = R xfp x ne xf xf xfc x L is no more a "proper" equation for the number of communicating ETCs than N = pc x nf x fp x nt x R is the equation for the number of piano tuners in Chicago. But if we assign reasonable values to the various factors in the equation — always with the understanding that such values can and will change as our knowledge

figure 7 Herbert York, one of Fermi's lunchtime companions.

increases — we will arrive at an estimate for the number of ETCs in the Galaxy. The difficulty we face is in our varying degrees of ignorance for the various terms in the equation. When asked to provide values for these terms, astronomers would provide responses ranging from "We're reasonably certain" (for the factor R) to "We're close to pinning it down" (for the factor fp) to "How the hell should we know?" (for the factor L). At least when we try to estimate the number of Chicago-based piano tuners, we can be reasonably confident that our various sub-estimates are not wildly in error; there can be no such confidence with our estimate for the number of communicating ETCs. Nevertheless, in the absence of any definite knowledge of ETCs, it is our only way to proceed. (The equation above has reached a certain iconic status in science; it is known as the Drake equation, after the radio astronomer Frank Drake who was the first to make explicit use of it.19 The Drake equation was the focal point of an extremely influential conference on the search for extraterrestrial intelligence, held at Green Bank in 1961 — 11 years after Fermi's remark.)

In 1950, Fermi would have known far less about the various factors in the above "equation," but he could have made some reasonable guesses

figure 9 The Drake equation is a means of estimating the number of communicative civilizations in the Galaxy. Drake developed the equation so that it could form the agenda for the first ever seti meeting (held at nrao Green Bank, WV, in 1961). This commemorative plaque is on the same wall that held the blackboard where the equation was first written.

figure 8 Emil Konopinski (far left), another one of Fermi's lunchtime companions.

figure 9 The Drake equation is a means of estimating the number of communicative civilizations in the Galaxy. Drake developed the equation so that it could form the agenda for the first ever seti meeting (held at nrao Green Bank, WV, in 1961). This commemorative plaque is on the same wall that held the blackboard where the equation was first written.

— guided, as he would have been, by the Principle of Mediocrity: there is nothing special about Earth or our Solar System. If he guessed at a rate of star formation of 1 star per year he would not have been too wrong. Values of fp = 0.5 (half the stars have planets) and ne = 2 (stars with planets on average each have 2 planets with environments conducive to life) seem to be "reasonable." The other factors are much more subjective; if he were an optimist, Fermi might have chosen fl = 1 (every planet that can develop figure 8 Emil Konopinski (far left), another one of Fermi's lunchtime companions.

life will develop life),f = 1 (once life develops, intelligent life will certainly follow), fc = 0.1 (1 in 10 intelligent life-forms will develop a civilization capable and willing to communicate) and L = 106 (civilizations remain in the communication phase for about 1 million years). Had he argued like that, he would have arrived at the estimate N = 106. In other words, there could right now be a million civilizations trying to communicate with us. So why do we not hear from some of them? In fact, why are they not already here? If some of the civilizations are extremely long-lived, then we might expect them to colonize the Galaxy — and have done so before multicellular life even developed on Earth. The Galaxy should be swarming with extraterrestrial civilizations. Yet we see no sign of them. We should already know of their existence, but we do not. Where is everybody? Where are they ? This is the Fermi paradox.

Note that the paradox is not that extraterrestrial intelligence does not exist. (I do not know whether Fermi believed in the existence of extraterrestrial intelligence, but I suspect that he did.) Rather, the paradox is that we see no signs of such intelligence when we might expect to. One explanation of the paradox is indeed that we are the only advanced civilization

— but it is only one of several explanations.

Asking why we see no evidence of extraterrestrial civilizations may seem like a trivial question but, as we might expect from a remark by Fermi, it is a profound puzzle. We can appreciate the strength of the paradox when we realize that it has been independently discovered four times: it might more properly be called the Tsiolkovsky-Fermi-Viewing-Hart paradox.

Konstantin Tsiolkovsky, a scientific visionary who worked out the theoretical basis of spaceflight as long ago as 1903, believed deeply in the monistic doctrine that ultimate reality is entirely of one substance. If all parts of the Universe were the same, it followed that there must be other planetary systems similar to our own, and that some of those planets would possess life.20 However, not unnaturally given his interest in the details of spaceflight, Tsiolkovsky also firmly believed that mankind would construct habitats in the Solar System and then move out into space. His feelings were revealed in his famous phrase: "Earth is the cradle of intelligence, but it is impossible to live forever in the cradle." The monist in him impelled him to argue that if we expand into space, then all those other species must do the same. The logic is inescapable, and Tsiolkovsky was aware that this led to a paradox when maintaining both that mankind will expand into space and that the Universe is brimful with intelligent life. In 1933, long before Fermi asked his question, Tsiolkovsky pointed out that people deny the existence of ETCs because (i) if such civilizations existed, then their representatives would have visited Earth, and (ii) if such civilizations existed, then they would have given us some sign of their existence. Not only is this is a clear statement of the paradox, Tsiolkovsky offered a solution: he believed that advanced intelligences — "perfect heavenly beings" — consider mankind to be not yet ready for a visitation.21

Tsiolkovsky's technical works on rocketry and spaceflight were widely discussed, but the rest of his output was generally ignored in the Soviet era. An appreciation of his discussion of the paradox therefore came only recently. (Fermi's own contribution did not fare much better. In their influential 1966 book Intelligent Life in the Universe, Sagan and Shklovsky introduce a chapter with the quote "Where are they?"; they attribute it to Fermi, but they incorrectly state that it was uttered in 1943. In a later paper, Sagan says that Fermi's quote was "possibly apocryphal.")

In 1975, English engineer David Viewing clearly stated the dilemma. A quote from his paper encapsulates it nicely: "This, then, is the paradox: all our logic, all our anti-isocentrism, assures us that we are not unique — that they must be there. And yet we do not see them." Viewing acknowledges that Fermi was first to ask the important question — "Where are they?" — and that this question leads to a paradox. To my knowledge, then, this paper is the first that refers directly to the Fermi paradox.22

However, it was a 1975 paper by Michael Hart in the Quarterly Journal of the Royal Astronomical Society that sparked an explosion of interest in the paradox.23 Hart demanded an explanation for one key fact: there are no intelligent beings from outer space on Earth at the present time. He argued that there are four categories of explanation for this fact. First, "physical explanations," which are based on some difficulty that makes space travel unfeasible. Second, "sociological explanations," which in essence suppose that extraterrestrials have chosen not to visit Earth. Third, "temporal explanations," which suggest that ETCs have not had time to reach us. Fourth, there are explanations arguing that perhaps they have been on Earth, but we do not see them now. These categories were meant to exhaust the possibilities. Hart then forcefully showed how none of these four categories provide a convincing account of the key fact, which led him to offer his own explanation: we are the first civilization in our Galaxy.

Hart's paper led to a vigorous debate, much of it appearing in the pages of the Quarterly Journal. It was a debate that anyone could enter — one of the earliest contributions came from the House of Lords at Westminster!24 Perhaps the most controversial offering came from Frank Tipler, in a paper with the uncompromising title "Extraterrestrial Intelligent Beings Do Not Exist." Tipler reasoned that advanced ETCs could use self-replicating probes to explore or colonize the Galaxy cheaply and in a relatively short time. The abstract to Tipler's paper sums it up: "It is argued that if extraterrestrial intelligent beings exist, then their spaceships must already be present in our Solar System."25 Tipler contended that the SETI program had no chance of success, and was therefore a waste of time and money. His argument poured oil on the fires of the debate and led to a further round of argument. The coolest and best summary of the arguments came from David Brin, who called the paradox the "great silence."26

In 1979, Ben Zuckerman and Michael Hart organized a conference to discuss the Fermi paradox. The proceedings were published in book form,27 and although the volume contains a variety of views it is difficult to read it without concluding that ETCs have the means, motive and opportunity to colonize the Galaxy. The means: interstellar travel seems to be possible, if not easy. The motive: Zuckerman showed how some ETCs would be forced into interstellar travel by the death of their star, and in any case it seems a wise idea for a species to expand into space to guard against the possibility of planetary disaster. The opportunity: the Galaxy is 13 billion years old, but colonization can take place over a period of only a few million years. Yet we do not see them. If this were a murder mystery, we would have a suspect but no body.

Not everyone was struck by the force of the argument. A recent book by the mathematician Amir Aczel makes the case for the probability of extraterrestrial life being 1.28 The physicist Lee Smolin wrote that "the argument for the non-existence of intelligent life is one of the most curious I have ever encountered; it seems a bit like a ten-year-old child deciding that sex is a myth because he has yet to encounter it."29 The late Stephen Jay Gould, referring to Tipler's contention that ETCs would deploy probe technology to colonize the Galaxy, wrote that "I must confess that I simply don't know how to react to such arguments. I have enough trouble predicting the plans and reactions of people closest to me. I am usually baffled by the thoughts and accomplishments of humans in different cultures. I'll be damned if I can state with certainty what some extraterrestrial source of intelligence might do."30

It is easy to sympathize with this outlook. When considering the type of reasoning employed with the Fermi paradox, I cannot help but think of the old joke about the engineer and the economist who are walking down a street. The engineer spots a banknote lying on the pavement, points to it, and says, "Look! There's a hundred-dollar bill on the pavement." The economist walks on, not bothering to look down. "You must be wrong," he says. "If there were money there, someone would already have picked it up."31 In science it is important to observe and experiment; we cannot know what is out there unless we look. All the theorizing in the world achieves nothing unless it passes the test of experiment.32

Nevertheless, surely Hart's key fact does require an explanation. We have been searching for ETCs for more than 40 years. And the continuing silence, despite intensive searches, is beginning to worry even some of the most enthusiastic proponents of SETI. We observe a natural universe when we could so easily observe an artificial universe. Why? Where is everybody? Fermi's question still demands an answer.

figure 10 Enrico Fermi, sailing off the island of Elba. The photograph was taken shortly before his death.

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