Solution The Fermi Paradox Resolved

When facts are few, speculations are most likely to represent individual psychology.

Carl Gustav Jung

The paradox resolved? Well, not really. The topic remains so intangible that honest people can reach quite opposite conclusions. The reader is free to choose one of the solutions presented earlier, or to originate his or her own. Here, though, I present the solution that makes most sense to me.

There is just one gleaming, hard fact in the whole debate: we have not been visited by ETCs, nor have we heard from them. So far, the universe remains silent to us. Those who would deny this fact of course have a ready solution to the Fermi paradox (and presumably stopped reading this book after the first few pages). The job for the rest of us is to interpret this lone fact.

As the above quotation suggests, with just one piece of evidence to play with, our biases will come to the fore. My own biases, such as I can identify them, include optimism about our future. I like to think our scientific knowledge will continue to expand and our technology to improve; I like to think mankind will one day reach the stars — first by sending messages and then later, perhaps, by sending ships. I like to think something akin to the Galaxy-spanning civilization described by Asimov in his classic Foundation stories might one day come to pass. But these biases collide with the Fermi paradox: if we are going to colonize the Galaxy, why have they not already done so? They have had the means, the motive and the opportunity to establish colonies, yet they appear not to have done so. Why?

Of the suggestions discussed in Chapter 4, only Solutions 16,17 and 20 strike me as plausible resolutions of the paradox; I suspect that most SETI scientists would agree that some combination of these ideas is likely to be correct. (Strictly, these are solutions to the "great silence" question: why do we not hear from ETCs? To explain why ETCs have not visited us, or why we see no evidence of their existence, we must take further suggestions into account — that interstellar travel is impossible, for example.) But the only position that is consistent with the observed absence of extraterrestrials and that at the same time supports my prejudices — the only resolution of the

Fermi paradox that makes sense to me — is that we are alone.

If you look up at the sky on a clear moonless night and gaze with the naked eye at the myriads of stars and the vastness of space, it is difficult to believe we might be alone. We are too small and the universe is too big for this to make sense. But appearances can be deceptive: even under ideal observing conditions you are unlikely to see more than about 3000 stars, and few of those would provide conditions hospitable to our form of life. The gut reaction we perhaps all feel when we look at the night sky — that there must be intelligent life somewhere out there — is not a good guide. We have to be guided by reason, not gut reaction, when discussing this matter. Well... reason tells us there are a few hundred billion stars in our Galaxy alone, and perhaps a hundred billion galaxies in the Universe. Just one sentient species when there is such an immense number of places life might get started? Come on ... surely I cannot be serious?

When discussing some of the different types of paradox, I noted Rapoport's observation that the shock of a paradox may compel us to discard an old (perhaps comfortable) conceptual framework. I believe the Fermi paradox provides a shock that forces us to examine the widespread notion that the vast number of planets in existence is sufficient to guarantee the existence of extraterrestrial intelligent life. In fact, we need not be too surprised. The Drake equation is a product of several terms. If one of those terms is zero, then the product of the Drake equation will be zero; if several of the terms are small, then the product of the Drake equation will be very small. We will be alone.

If one factor in the Drake equation is close to zero, then we can reasonably identify that factor as being the solution to the Fermi paradox. For example, as we saw in Solution 30, some scientists argue that the emergence of life was an almost miraculous fluke, a 1-in-10100 event (a number that dwarfs the number of planets in the Universe, and when expressed as a probability becomes, for practical purposes, indistinguishable from zero). Other scientists argue, perhaps more convincingly, that the improbability of the prokaryote-eukaryote transition (Solution 44) explains the paradox. Rather than there being a single solution to the paradox, however, I suspect there is a combination of factors — a product of various solutions we have discussed in this book — resulting in the uniqueness of mankind.

It is usual at this point to pick some numbers favorable to one's position, plug them into the Drake equation, and then put forward the required result. I would prefer to present here a more pictorial approach.

When I was a schoolboy, I was fascinated by the Sieve of Eratosthenes.236 Eratosthenes was a Greek astronomer and mathematician, famed for being head of the Library at Alexandria and for being the first to provide an accurate measurement of Earth's circumference. He also developed a technique — his "sieve" — for finding all prime numbers less than some given number N. Primes — numbers evenly divisible only by themselves and 1 — are extremely important in mathematics; they are like atoms, from which we can compose all other numbers through multiplication. If you are given a number at random, it can be difficult to know whether it is composite or prime. The Sieve of Eratosthenes is a technique for sifting out the composite numbers and leaving only the prime numbers standing.

Suppose you are a Greek mathematician who wants to find all primes less than or equal to 100. First, you take a sheet of papyrus and write down the numbers from 1 to 100. The number 1 is special, so ignore it. The number 2 is prime so leave it; but go through the list and cross out all its multiples: 4,6,8, ... 100. Repeat the process, using the next smallest remaining number, 3; leave it because it is prime, but cross out its multiples all the way up to 99. Continue until you reach the end of the list. Remarkably quickly, you find all the numbers up to 100 have been deleted — except for the 25 prime numbers, which are still standing. Even for a computer, the Sieve of Eratosthenes is the quickest way of finding all primes less than about 108.

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figure 72 This figure shows what happens when you apply the Sieve of Eratosthenes to a grid of numbers up to 100. The bold numbers are primes, which are left untouched by the procedure. The gray boxes are composite numbers — those that can be created by multiplying two or more prime numbers. The subscript on a composite number indicates the smallest divisor ofthe number — the first prime that sifted out the number. The number 1 is special, and is not considered prime.

As a schoolboy, I was intrigued by the way the Sieve caught more and more of the large numbers. The technique was inexorable: on large grids I found myself chopping down number after number. Since the distribution of primes thins out quickly the higher you count, there are long stretches where all the numbers have been crossed out — numbers that have failed to make it through the Sieve.

I picture something similar happening with the Fermi paradox. Imagine writing down a grid of numbers, from 1 to 1000,000,000,000, with each number representing an individual planet in the Galaxy. (I arrive at this number by multiplying the number of stars in the Galaxy, which is about 1011, with an assumed average of 10 planets per star. In fact, the number of stars is probably greater than this, with some estimates suggesting that our Galaxy contains as many as 400 billion stars. On the other hand, the average number of planets per star is likely to be less than 10. So although a figure of 1012 planets is a rough guess, it may not be too wrong — and anyway, this hardly matters when all the other numbers in the problem are so vague.) We assign Earth the number 1, since the Earth is special: it is the only planet on which we know intelligent life exists. Now start applying a sieve — let us call it the Sieve of Fermi. (The process I describe here is not meant to be the only way of working the numbers. You may prefer different numerical values for the quantities I describe, but the process shows why we should not be surprised if we discover that we are alone.)

Step 1 In Solution 36 we briefly discussed the notion of a galactic habitable zone (GHZ) in which a star must reside before it can give rise to a viable planetary system. A recent suggestion is that the GHZ contains only 20% of the stars in the Galaxy. So cross out those numbers corresponding to planets not orbiting a star in the GHZ: with 10 planets per star, 2 x 1010 planets remain. Now make a second application of the Sieve.

Step 2 The bright O and B stars die too quickly for life to evolve around them; the dull K and M stars are too miserly with their energy for life to prosper. For life as we know it, we need consider only stars like the Sun. (As I stressed in earlier sections, this assumption may be an expression of chauvinism — or a failure of scientific imagination. But I think it is the best assumption we can make at this time.) Only about 5% of stars in our Galaxy are like the Sun; cross out numbers corresponding to planets not orbiting a Sun-like star, and 108 planets remain.

Step 3 Life as we know it requires a terrestrial planet to remain in the continuously habitable zone (chz) for billions of years. We discussed the narrow width of the CHZ in Solution 36. We also discussed some factors that may cause Earth-like planets to be more rare than we might suppose, such as the migration of Jupiters to the inner parts of a planetary system (Solution 37) and the possible scarcity of rocky planets (Solution 35). My own guess is that only 1% of planets will be both suitable for life and remain in a CHZ for billions of years. You may think a different figure is in order here (and one could argue for higher or lower figures), but 1% seems reasonable to me. So cross out numbers corresponding to planets that do not remain in a CHZ: 106 planets remain.

Step 4 Of the million planets that orbit in the CHZ of a Sun-like star that is itself in the GHZ, how many are home to life? If you believe the genesis of life is exceptionally rare (Solution 30), then the answer is: none. If you believe a special set of circumstances is required, such as life originating on a planet like Mars and then being transported via impact ejecta to an Earth-like planet (Solution 43), then the answer is: not many. I prefer to believe that life is a probable occurrence: that if conditions are suitable, then there is a good chance of cells evolving. Let us say that the chance is 0.5. Cross out numbers corresponding to planets on which life does not arise, and 5 x 105 planets remain. Half a million planets with life!

Step 5 The Universe is a dangerous place. We saw how destruction can come from the depths of space (Solution 39) and from closer to home (Solution 40). We also discussed how the rate of planetary disaster may be significant (Solution 38). On many planets, life may be snuffed out — or at least prevented from evolving into complex life-forms — by some disaster. My guess is that as many as 20% of planets may suffer such a fate. (This is just a guess, and it may be an overestimate.) So cross out numbers corresponding to planets on which disaster strikes: 105 planets remain.

Step 6 We saw how Earth's system of plate tectonics was important in the development of life (Solution 41) and also how the Moon plays a role (Solution 42). If both these factors are necessary for the evolution of complex life, then the number of planets with the sentient species we are searching for may be small. However, although I believe these phenomena are important in some ways, I have no feel for the numbers involved. So I will ignore these factors, and at this stage of the sifting process all the planets make it through: 105 planets still remain.

Step 7 Cross out numbers corresponding to planets where life never evolves beyond the prokaryotic grade (Solution 44). The development of the modern eukaryotic cell took aeons on Earth, which perhaps indicates that this step is far from inevitable. No one knows what fraction of planets with prokaryotes will go on to host complex multicellular life-forms; my own estimate of one in ten may be very generous. We are left with 104 numbers — ten thousand planets possessing complex multicellular life. Does that mean the Galaxy contains ten thousand ETCs? Unfortunately not, because we must make several further applications of the Sieve before we arrive at the number of species with whom we can communicate. Let us combine all these into one last pass through the sifting process.

Step 8 Cross out numbers corresponding to planets on which advanced life-forms do not develop tool use and the ability to continuously improve their technology (Solutions 45 and 46). Cross out numbers corresponding to planets on which advanced life-forms do not develop the type of abstract high-level intelligence we are familiar with (Solution 47). Finally, and to my mind crucially, cross out numbers corresponding to planets on which advanced life-forms do not develop complex, grammatical language (Solution 48). How many planets remain? Of course, no one knows; it is impossible to assign accurate probabilities to these matters. My feeling is that many of these developments were far from inevitable. The feeling arises because, of the 50 billion speciation events in the history of our planet, only one led to language — and language is the key that enabled all our other achievements to take place. My own guess, then, is that none of the planets make it through this final sifting process.

After applying the Sieve of Fermi I believe that all grid numbers will be crossed out, except the number 1. Only Earth remains. We are alone.

I believe that the Fermi paradox tells us mankind is the only sapient, sentient species in the Galaxy. (We are probably also unique in our Local Group of galaxies, since many Local Group galaxies are unlikely to possess a GHZ. Perhaps we are even unique in the whole Universe — although the finite speed of light means ETCs could now exist in very distant galaxies without us yet being aware of them.) Yet the Galaxy need not be sterile. The picture I have is of a Galaxy in which simple life is not uncommon; complex, mul-ticellular life is much rarer, but not vanishingly rare. There may be tens of thousands of exceptionally interesting biospheres out there in the Galaxy. But only one planet — Earth — has intelligent life-forms.

Such a picture is often criticized as violating the Principle of Mediocrity. The picture seems to suggest that Earth, and mankind, is special. Is this not the height of arrogance?

Paradoxically, at least to my mind, the expectation that other sentient species must be out there itself smacks of arrogance. Or rather, it achieves the tricky feat of being both self-important and self-effacing at the same time. At the core of this expectation is the belief that human adaptations, attributes such as creativity, and general intelligence, that we think important, are qualities to which other Earth organisms aspire and alien creatures may possess in even more abundance. Allow us a few more million years, so the logic seems to go, and we might evolve into the cognitively, technologically and spiritually superior beings that already exist out there. But the converse of this position is surely false. Give chimps another few million years, so the reasoning goes, and they too will be as intelligent and creative as us. But why should they be? Chimpanzees are good at being chimpanzees; dolphins are good at being dolphins; elephants are good at being elephants . . . Rather than patronizing these species for not exhibiting human characteristics, we should respect them on their own terms for earning a living in a harsh world that cares not whether they live or die.

On the other hand it is undeniable that mankind is profoundly different from every other species on Earth. We alone have language, a high level of self-consciousness, and a moral sense. We are special. But surely our uniqueness could not have arisen by mere chance, by the blind and random groping of evolution, could it? Well, why not?

As Stephen Jay Gould pointed out in a delightful analogy, we can account for any growth in the complexity of living organisms through a drunkard's walk effect.237 Imagine a drunk leaning against a wall. A few meters to his right is a gutter. If the drunk takes random equal-sized steps to his left or to his right, then he inevitably ends up in the gutter. No force propels him to his right; he moves randomly, and at any time he is as likely to move to his left as to his right. But the wall eventually stops his leftward motion; over time, there is only one direction in which to move. Eventually, completely by chance, the drunk stumbles into the gutter. The same effect can explain any advance we might observe in the complexity of organisms. At one end we have a wall of minimum complexity that organisms can possess and still be alive. This wall is where life began, and where most life on Earth remains. Over time, evolution tinkers with more advanced organisms; when life itself was young, that was the only available possibility — evolution could not try out simpler designs, because its path was blocked by the wall of minimum complexity. Some of the new designs worked, in the sense that the organisms were adapted well enough in their immediate environments to survive long enough to reproduce. And so evolution staggered on, like a blind drunk, tentatively producing organisms of greater complexity. After almost 4 billion years of random tinkering, we end up with the living world we see today. But there was nothing inevitable about the process; the purpose of evolution was not to produce us. Play the tape of history again, and there is no reason to suppose Homo sapiens — or any equivalent sentient species — would play any role at all.

Many eminent scientists argue that Mind is in some way predestined in this Universe. That far from being the outcome of chance, Mind is an inevitable outcome of deep laws of self-complexity. They argue that, over aeons, organisms will inevitably self-complexify and form a "ladder of progress": prokaryote to eukaryote to plants to animals to intelligent species like us. It is a comforting idea, but I know of no definite evidence in its favor, and I believe the silence of the Universe argues against it.

The famous French biologist Jacques Monod wrote that "evolution is chance caught on the wing." Even more evocatively, he wrote that "Man at last knows he is alone in the unfeeling immensity of the Universe, out of which he has emerged only by chance."238 It is a melancholy thought. I can think of only one thing sadder: if the only animals with self-consciousness, the only species that can light up the Universe with acts of love and humor and compassion, were to extinguish themselves through acts of stupidity. If we survive, we have a Galaxy to explore and make our own. If we destroy ourselves, if we ruin Earth before we are ready to leave our home planet . . . well, it could be a long, long time before a creature from another species looks up at its planet's night sky and asks: "Where is everybody?"

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