Where Is Everybody

There is something beguiling about paradox. The impossible and paradoxical prints of Maurits Escher never fail to deceive the eye. Poems like Robert Graves' Warning to Children, which play with the paradox of infinite regress, make the head spin. Paradox lies at the heart of Joseph Heller's Catch-22, one of the 20th century's greatest novels. My favorite paradox, though, is that of Fermi.

I first came across the Fermi paradox in the summer of 1984. I had just graduated from Bristol University, and I should have spent the summer months studying Aitchison and Hey's Gauge Theories in Particle Physics — required reading before I started postgraduate studies at Manchester University. Instead, I spent my time enjoying the sunshine on the Bristol Downs, studying my favorite reading matter: Isaac Asimov's Science Fiction Magazine. (As with many people, SF sparked my interest in science. It was through reading the works of Isaac Asimov, Arthur Clarke and Robert Heinlein and watching films like Forbidden Planet that I became enamored with science.1) Two thought-provoking science-fact articles appeared in successive issues of IASFM that year. The first, by Stephen Gillett, was simply entitled The Fermi Paradox. The second, a forceful rebuttal by Robert Freitas, was entitled Fermi's Paradox: A Real Howler.2

Gillett argued in the following way. Suppose, as the optimists believed, that the Galaxy is home to many extraterrestrial civilizations. (To save typing, I shall often refer to an extraterrestrial civilization as an ETC.) Then, since the Galaxy is extremely old, the chances are good that ETCs will be millions or even billions of years in advance of us. The Russian astrophysicist Nikolai Kardashev proposed a useful way of thinking about such civilizations. He argued that ETCs would possess one of three levels of technology. A Kardashev type 1 civilization, or K1 civilization, would be comparable to our own: it could employ the energy resources of a planet. A K2 civilization would be beyond our own: it could employ the energy resources of a star. A K3 civilization could employ the energy resources of an entire galaxy. According to Gillett, then, most ETCs in the Galaxy would be of a K2 or K3 type. Now, everything we know about terrestrial life tells us that life has a natural tendency to expand into all available space. Why should extraterrestrial life be any different? Surely ETCs would want to expand from their home world and out into the Galaxy. The key point, however, is that a technologically advanced ETC could colonize the Galaxy in a few million years. They should already be here! The Galaxy should be swarming with life. And yet we see no evidence that ETCs exist. Gillett called this the Fermi paradox. (I learned why Fermi's name is attached to the paradox a few months later, when Eric Jones published a Los Alamos preprint describing the origins of the paradox; but more of this later.) For Gillett, the paradox pointed to a chilling conclusion: mankind is alone in the Universe.

Freitas thought this was all hogwash. He compared Gillett's logic to the following argument: Lemmings breed quickly — about 3 litters per year, with each litter containing up to 8 offspring. In just a few years the total mass of lemmings will be equal to the mass of the entire terrestrial biosphere. The Earth must be swarming with lemmings. And yet, most of us see no evidence that lemmings exist. Have you ever seen a lemming? The "Fermi paradox" line of reasoning would lead us to conclude that lemmings do not exist — yet, as Freitas pointed out, this would be absurd. More interestingly, he believed the lack of evidence for ETCs is not particularly strong: if small artificial probes were parked in the Asteroid Belt, say, or larger probes in the Oort Cloud, then we would have no chance of detecting them. Besides, he argued that the logic behind the so-called paradox is faulty. The first two steps in the argument are: (i) if aliens exist, then they should be here; (ii) if they are here, then we should observe them. The difficulty is those two "should"s. A "should" is not a "must," and therefore it is logically incorrect to reverse the arrow of implication. (In other words, the fact we have not observed them does not allow us to conclude they are not here, so we cannot conclude they do not exist.)

Until there is clear evidence to resolve a paradox, people are free to follow different lines of reasoning. This is what makes a paradox so interesting. In the case of the Fermi paradox, the stakes are so high (the existence or otherwise of alien intelligence) and the experimental input to the argument is so sparse (even now, we cannot be sure ETCs are not here) that arguments often become heated. In the Gillett-Freitas debate, I initially sided with Freitas. The main reason was sheer weight of numbers: there are perhaps as many as 400 billion stars in the Galaxy, and as many galaxies in the Universe as there are stars in the Galaxy. Ever since Copernicus, science has taught us there is nothing special about Earth. It followed, then, that Earth could not be the sole home to intelligent life. And yet...

I could not get Gillett's argument out of my mind. I had been reading about cosmic wonders since I was a child. The Galaxy-spanning civilization of the Foundation trilogy, the astroengineering wonders of Ringworld, the enigma of the vessel in Rendezvous with Rama — all these were part of my mental furniture. And yet where were these marvels? The imaginations of SF writers had shown me hundreds of possible universes, but my astronomy lecturers made it clear that so far, whenever we look out into the real Universe, we can explain everything we see in terms of the cold equations of physics. Put simply, the Universe looks dead. The Fermi question: where is everybody? The more I thought about it, the more the paradox seemed to be significant.

It seemed to me the paradox was a competition between two large numbers: the vast number of potential sites for life versus the vast age of the Universe.

The first number is simply the number of planets with suitable environments for the development of life. If we adopt the Principle of Mediocrity, and assume there is nothing at all about special about Earth, then it follows there are many millions of suitable environments for life in the Galaxy (and many billions of environments in the Universe). Given so many potential seeding grounds, life should be common.

The second number is simply the age of the Universe: the latest measurements suggest it is slightly more than 13 billion years old. To evoke a feeling for such a large time span, it is usual in these discussions to compress the entire history of the Universe into a standard length or interval. In this case, I will compress the current age of the Universe into a standard Earth year: in other words, the "Universal Year" compresses the entire history of the Universe into 365 days. On this timescale, a second of real time corresponds to 400 years; in other words, in the Universal Year, western science begins about 1 second before midnight on 31 December. The whole history of our species is much less than 1 hour of the Universal Year. The earliest ETCs, however, could have originated in the early summer months of the Universal Year. If the colonization of the Galaxy can take place in the equivalent of a few hours, then one would expect one or more of the advanced technological civilizations to have long since completed the job. At the very least, if they really were so far beyond us, one would expect to see or hear some evidence of their presence. But the Universe is silent. The Fermi paradox might not logically prove aliens do not exist, but surely it is

I was not the only one who found the Fermi paradox interesting. Over the years, many people have offered their resolutions to the paradox, and I developed the habit of collecting them. Although there is a fascinating range of answers to the question "where is everybody?," they all fall into one of three classes.

First, there are answers based around the idea that somehow the extraterrestrials are (or have been) here. This is probably the most popular resolution of the paradox. Certainly, belief in intelligent extraterrestrial life is widespread. In a CNN Internet poll on 1 July 2000, of the 6399 people who voted, 82% thought there is intelligent life elsewhere in the Universe. As of the 2001 summer solstice, 94% of the 94,319 respondents to a [email protected] poll believe life exists outside Earth. Several polls suggest the majority of American people believe flying saucers exist and are here; the proportion of believers seems to be less among Europeans, but is nevertheless high.

Second, there are answers suggesting ETCs exist, but for some reason we have not yet found evidence of their existence. This is probably the most popular category of answer among practicing scientists.

Third, there are answers purporting to explain why mankind is alone in the Universe, or at least in the Galaxy; we do not hear from extraterrestrial intelligence because there is no extraterrestrial intelligence.

The purpose of this book is to present and discuss 50 proposed solutions to Fermi's question. The list is not intended to be exhaustive; rather, I have chosen them because they are representative (and also because I think they are particularly interesting). The proposed solutions come from scientists working in several widely separated fields of science, but also from SF

a problem demanding solution.

table 1 In the "Universal Year," we compress 13 billion years into 365 days.

"Real" time

Time in a Universal Year

50 yrs 0.125 s

100 yrs 0.25 s

400 yrs 1 s

1000 yrs 2.5 s

2000 yrs 5 s

10 000 yrs 25 s

100 000 yrs 4mins10s

1 million yrs 41 mins 40 s

2 million yrs 1 hr 23 mins 20 s 10 million yrs 6 hr 56 min 40 s

100 million yrs 2 days 21 hr 26 min 40 s authors; in this topic, authors have been at least as industrious as scientists, and in many cases they have anticipated the work of scientists.

The outline of the book is as follows.

Chapter 2 gives a brief biography of Fermi, focusing on his scientific achievements; I then discuss the notion of paradox and present a brief discussion of the history of the Fermi paradox.

Chapters 3-5 present 49 of my favorite solutions to the paradox; not all of them are independent, and sometimes I revisit a solution in another guise, but all of them have been seriously proposed as answering Fermi's question. I arrange the answers according to the three classes mentioned above: Chapter 3 contains answers based around the idea that ETCs are here; Chapter 4 contains answers based around the idea that ETCs exist, but we have not yet found evidence of them; Chapter 5 contains answers based around the idea that we are alone. There is a logic to the arrangement of the solutions, but I hope the discussions are self-contained enough to allow readers to "dip into" the book and pick out solutions that particularly interest them. In the discussions I will try to be as even-handed as possible, even if I disagree with the solution (which I often do).

Chapter 6 contains the 50th solution: my own view of the resolution of the paradox. It is not an original suggestion, but it summarizes what I feel the Fermi paradox can tell us about the Universe in which we live.

Superscripted numbers, which appear throughout the book, refer to numbered items in Chapter 7; these items contain notes and suggestions for further reading. Since the material in this book covers a wide range of subjects from astronomy to zoology, and since the space for the discussions is necessarily limited (it works out at an average of about 5 pages per solution), I have also given a wide-ranging list of references. The references themselves, which are referred to in Chapter 7 by numbers in square brackets, appear in Chapter 8. They range from SF stories to primary research articles in scholarly journals. Many readers may find it difficult to access the more specialized references, but I hope they will at least find it possible to use these references to help find related information on the Web.

The book is specifically aimed at a popular audience. One of the beauties of the Fermi paradox is that it can be appreciated without the need for any mathematics beyond an understanding of exponential notation.3 It follows that anyone can present a resolution of the Fermi paradox; you do not need to have years of scientific and mathematical training to contribute to the debate. (Indeed, as I noted above, many of the best ideas have come from SF writers rather than scientists.) I hope that a reader of this book may devise a solution that no-one else has thought of. If you do — please write to me and share it!

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