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Friday, September 23, 2011

CHAPTER 8D ARE ADVANCED CIVILIZATIONS LIKELY? What events or processes would make it possible for our technological civilization to be the onl



What events or processes would make it possible for our technological civilization to be the only one in the galaxy? First of all, it may turn out that there are very few favorable planets. Our rather speculative theories on the formation of planetary systems could be wrong, for example, although today the evidence tends to be favorable. The fact that Mars and Venus are so different from the Earth indicates that there may be only a small band in a solar system where a terrestrial planet could give rise to life. Thus, even if planets of terrestrial size form, they may be just outside of that "green band," as Mars and Venus might be. Still that may leave many terrestrial planets at the appropriate distance from their stars. Some new computer models, however, predict that rocky planets like Earth are likely to be thrown out of their solar systems, or that few of them would have moons large enough to stabilize their rotations, and thus provide a climate favorable for life. But surveys of actual exo-terrestrial planets found suggest that there should be many rocky planets in the galaxy. But let me make allowances here and concentrate instead on some crucial assumptions at the heart of the issue.


Travel teaches us not only about other places and people but also about ourselves. Likewise trying to understand what other intelligent life might be like teaches us about our own intelligence. And trying to understand how alien intelligence may view nature teaches us about what our own views of nature amount to. In SETI we find almost bare many common assumptions about the origin, development, and nature of science. Thus from an analysis of SETI we may be able to draw some interesting philosophical lessons.[2]

In this section I will be concerned mainly with three notions that are frequently advanced by SETI proponents. The first notion is that once life appears on a planet, intelligent life is also very likely. The second is that once intelligence appears on a planet, science itself is likely. The third is that all scientific civilizations have something in common (i.e., an overlap in their scientific views of the world) and thus the basis for the beginning of communication between them[3]. The first two notions are advanced to support the contention that there is probably someone to look for. The third gives us hope that contact, if we make any, will be productive.

In spite of their initial plausibility, I will argue that these notions are plagued with less than obvious assumptions at many levels, and that they lead to a questionable account of our views of nature.

SETI proponents believe that life can begin elsewhere, that once it begins it is likely to become more complex, and that complexity produces intelligence. Presumably, as time goes on, intelligence will improve its attempts to understand what the world is like -- thus begins the almost inevitable road to a technological civilization. Whether life can begin elsewhere is a matter of great controversy, as we have seen. But I will grant for the sake of argument that it could. In the same spirit I will grant that, at least for some time, the complexity of life may increase; and I will also grant that intelligence is the result of certain complex biological organizations. But granting all these crucial assumptions of SETI is not the same as granting that alien technological civilizations are very likely.

Let me take stock of what I have granted. After some primitive form of life appears on a planet, it will not remain uniform for long. Small variations in the environment and other factors will bring about diversity. Of course, diversity is not the same as complexity, but it gets us on the road to it. For diversity means that there will be different kinds of biological structures and different ways of interacting with the environment. And the possibility then arises that eventually some of these structures and functions will combine. A and B may come to work together and a new structure C will arise to coordinate their work. And now A, B, and C together will form a new whole that is more complex than either A or B as separate individuals. During the first couple of billion years of life on Earth, prokaryote cells were the most prevalent, perhaps the only form of life. These cells in which the chromosomes are not protected inside a membrane (the nucleus) eventually led to cells with nuclei (eukaryotes), which are more complex. According to Lynn Margulis, this important step came about by the symbiosis of different kinds of prokaryote cells.[4] In any event, once cells with nuclei appeared it was possible to form organisms that combine many of these cells, sometimes billions of them. These organisms are very complex wholes of eukaryote cells that perform many different but coordinated functions. Although after billions of years the increase in the complexity of life can be considerable, complexity is not always bound to increase with time. Changes in the environment of a planet, some of them caused by life itself, may make it very difficult for all but simple organisms to survive on that planet.

Let me concentrate now on a particularly interesting kind of complexity. Eventually some Earth animals developed intricate patterns of muscles and bones so they could move about, external senses to give them information about the world, and internal senses to monitor a variety of organs. It does not take much to see the advantage of coordinating these functions. A successful predator not only sees the prey but also can move so as to catch it. On Earth, a popular answer to this problem of coordination is the central nervous system. And this is an interesting answer because it is in connection with a highly complex central nervous system that intelligence becomes conspicuous.

A highly complex central nervous system is not limited to just one way of handling the information that it receives from the world: It can rout and combine information in a variety of ways; it can compare sense modalities; it can store information and consider alternative actions; that is, it can make use of memory and imagination.

Visual perception is a good illustration of this kind of complexity. At a very primitive level we may suppose that the detection of light is enough for a certain organism, in order to move towards or avoid the light. The next step comes when the organism gains an advantage by being able to discriminate visually between objects, which may be achieved by making internal representations of those objects. These representations grow in sophistication, and the corresponding nervous structures in complexity, when the "input" from the eyes is coordinated with that from other senses. For example, when we are looking at a painting of a group of people our eyes are not stationary. First of all, the eye muscles make the eyes scan continuously. Second, our heads move sideways as well as up and down. Our whole bodies may also move, carrying our heads, and thus our eyes along. But the images of those people remain stationary. This is very different from, say, a video camera, whose images do move up and down or sideways, the more so the more unskilled we are at shooting with it. The reason our perceived images remain stationary is that the brain takes into account the automatic movements of the eyes as well as our body position in order to arrive at a perception that we can handle. The brain takes into account our body position by receiving information from the inner ear, which keeps track of the inclination of the body with respect to the Earth's gravitational lines of force, and from hundreds of skeletal muscles.

Visual perception is also easily affected by the other senses. Take hearing. As we walk down a dark street at night we may perceive some bundles a few steps ahead. But at least one of those bundles suddenly becomes a sharp image when we hear the distinct growling of a guard dog. Perception also takes into account memory and imagination. An artist well trained in the history of art may see many more details in the painting and many more relationships between different elements of the painting than most of us can, just as a well trained naturalist can detect a rare bird in a bush where most of us can see only foliage.

The more complex the central nervous system, the more complex the relationship between the organism and the environment, for the organism gains more degrees of freedom. Thus intelligence arises out of perception and other biological structures as the complexity of those structures increases. This account agrees with Piaget's description of intelligence as an instrument of adaptation not necessarily tied to the immediate and momentary demands of the environment (human beings, for example can figure out solutions to problems that will confront them far away and years hence).

Let me sketch now the main hurdles that life has to overcome on its way to an advanced technological civilization. On this account, to say that intelligence is adaptive is to say that a highly complex central nervous system (or its equivalent) is adaptive. But then intelligence is adaptive only for certain kinds of organisms and not for others. It would be adaptive for primates, for example, but not for cockroaches. Let me illustrate the point by means of an analogy. It is well known that the opposable thumb is a highly adaptive feature of human beings. But it would not be so for horses. And it does not even make any sense to ask whether it would be for cockroaches, since roaches do not have the kinds of physical structures to which opposable thumbs can be attached.

We might think that roaches would be better off if they were smarter. But to put the point properly we have to consider whether roaches would be better off with more complex brains. And now we may begin to see the difficulty: There is a price to pay all along the way to intelligence. The price is that a complex brain demands a high metabolism. In a minor way the same point may be made about sight, which also seems to be quite an advantage. Imagine that a population of small mammals has come to live in dark caves. The brain structures of sight use a lot of energy, and so these mammals have to spend much time and work getting that energy. Since sight is of marginal advantage in the dark caves, the mammals that preserve sight are not as competitive as others that use only a fraction of the energy to enjoy improved hearing, touch and smell. It would be nice to have sight, but a mammal of that size can't afford the price to keep it. And for a population of sightless mammals it would make no sense to develop it.

Likewise, an increase in the complexity of the brain requires that the organisms of the species in question gain some advantages that compensate for the price in metabolism that they have to pay. In the case of many species on Earth, including ours, those advantages have been there. But we should not expect that they would be there on any other planet where life may evolve.

Consider our kind of intelligence: mammalian intelligence. If the dinosaurs had not become extinct, mammals would have remained small vermin. Large mammals could not evolve because an increase in size would make it easier for dinosaurs to prey on them. But the price that mammals would have to pay for a bigger and more complex brain would probably be a bigger body. The point is that a species, or some other taxonomical category, can be successful enough on a planet to preclude the adaptation by other species that could some day evolve into creatures of high intelligence. In our own day, we ourselves are a cap on the development of intelligence by others. Suppose once again that raccoons become increasingly intelligent. As it was pointed out earlier, they would become such pests that humans would probably hunt them to extinction. Our very way of life tends to wipe out animals that enter into close competition with us.

Let us imagine a planet very similar to our own. Let us suppose that in that planet also the conquest of the land by fishes would have provided the necessary opportunities for an increase in the complexity of the brain. But let us also suppose that on that planet insects had already appeared on the land and were even more successful than on Earth. Because of their physical constitution, insects are not likely to grow large enough to develop the sort of large brain associated with intelligence. But insects have many adaptive features that serve them quite well. Thus they can be successful without being smart. In that planet they rule the land: Any fish that crawls out of the water will be eaten by insects, and if perchance eggs from that species are not only laid but hatched, the young fishes will be devoured. Intelligence as we know it is not likely to arise. The smartest being on that planet would be some kind of octopus. (It does no good to point to whales and dolphins--those are mammals and would have never evolved if vertebrates had not developed on land to begin with).

Thus on other planets the cap may come from many different kinds of beings, even if their own intelligence is rather modest by our standards. All it takes is that in some other respects they can adapt first to the land, or whatever key environment we consider. But what enters into that timing? Most often just accidents of natural history. For example, it is possible that the disappearance of the dinosaurs may be traced in large measure to the collision of a gigantic asteroid with the Earth. But there is no guarantee, let alone a law of nature, that accidents of natural history are going to favor the development of high intelligence.

Let me imagine, nonetheless, that on some planets central nervous systems as complex as ours, or more complex, do evolve. Will technological civilization then come about? Not automatically. It has to be the right kind of intelligence: technological intelligence. The evolution of human intelligence is tied to the use of tools for hunting and many other purposes. But the evolution of this mode of interaction with the world makes sense only if you have the right kind of body. Dolphins, for example, which are creatures with complex brains and perhaps high intelligence (even if not in our class), have no hands, to say nothing of opposable thumbs. There is a clear sense in which we express our intelligence by having the appropriate bodily interaction with the environment. A technological intelligence would not be adaptive unless the right kind of body developed along with it. Spears may have been a sensible option for our ancestors, but harpoons would not have been so sensible an option for the ancestors of dolphins.

Nevertheless let me suppose that technological intelligence does arise and takes over a planet. Technological civilization still does not follow automatically. A technological civilization is in part the result of complex social processes, thus the required type of intelligence must be not only technological but also social. But even if we have the evolution of this kind of intelligence, a highly advanced technological civilization may not arise. One reason is that high technology may well require the development of science. On our own planet a turning point came when the new science, culminating in Newton, was able to bring together astronomy and physics. But in a planet very similar to ours but perennially covered by clouds (or in a solar system traveling through a dust cloud) a comparable development of astronomy would be most unlikely.

Imagine, though, that we have a favorable physical environment where intelligent beings (both socially and technologically) can receive the inspiration and rewards that would take them on scientific paths blessed with the right kinds of intellectual breaks. We still cannot expect an advanced technological civilization. For having the right physical environment is not enough. Social factors may still prevent the development of science as we know it (let alone a more advanced science). It is plausible to suppose that the progress of science requires that ideas may be criticized and that alternative conceptions of the world be developed and defended even if a majority in a group do not agree with them. But in a species biologically inclined to a degree of social cohesion greater than ours, the criticism of the metaphysics of the society (e.g. of their account of the origin and nature of the world) may be seen, or felt, as a threat to the cohesion of the society and put down at once. It seems that in our world science barely made it; on that other planet science would have no chance.

I do not wish to argue that a technological civilization could not arise on a different planet. My intent is merely to point out that the process is by no means automatic, that it requires many good breaks from natural history.[5] A critic may argue that natural selection could have gotten around most if not all of the obstacles I have mentioned. All it takes is a bit of imagination, and we know how imaginative natural selection can be. For example, one of the reasons why advanced technology seems to need a social milieu to exist is that no one human being can fully develop a theory as comprehensive as, say, Newtonian mechanics (it took centuries), to say nothing of all the other branches of physics, chemistry, and so on. But even within one school it is difficult enough to come up with a few good ideas. To be able to see their flaws, possible means of improvement, or their connections to other areas of science often requires that we look at those ideas from many different points of view. One human being could not do all this. Science and advanced technology require a division of labor.

Imagine, however, a planet on which a single organism--not a single species, a single organism--comes to dominate even more than human beings do on Earth. This would be a strange organism that covers the environment like a comforter and grows larger by creating more branches of itself until it has finally covered much of the planet (if the food supply decreases this intelligent organism will either "farm" differently or drop off a few branches). Instead of a central brain, this organism has something that rather resembles a network of ganglia (large, complex ganglia to be sure). Although the action of the ganglia tends to be coordinated, in a network that large there must also exist a fair degree of decentralization. In that case ideas may be brought up by one particular ganglion and criticized by other ganglia, and so on. The concept of self of this organism may be quite different from ours, but the point is that in a single organism we may find the equivalent of a whole species. So this organism could develop an advanced technology even though, strictly speaking, it is not really social.

It is clear, then, that if certain avenues of development are closed to life, natural selection may find others. But the price of alternative natural histories would be alternative forms of intelligence and eventually alternative ways of formulating views of the world. The reason is that the brains (or their equivalent) that would result from such radically different natural histories would arise from entirely different biological structures, and thus, in coordinating these structures, the developing brain would face different evolutionary problems and would have different solutions and opportunities at hand. In the neurological ward of a hospital we find people whose brain structures have been altered and who thus have peculiar ways of perceiving and conceiving of the world. Of course, their modes of thought are maladaptive, just as skeletal structures that deviate from the norm may be maladaptive in a human. But for different creatures, different brain structures and their corresponding modes of thought may be as adaptive as their different skeletal structures are. The consequence of this point is that the science of a species, or kind of organism, may be relative to its natural and social history. Thus species with very different natural histories may have little overlap in their scientific views of the world. If this is so, there would be much less in common to serve as the basis for interstellar communication with other technological civilizations than the proponents of SETI make it out to be.

Defenders of the SETI program often assume that advanced sciences and technologies must exhibit a high degree of convergence. The grounds for this assumption are presumably that, as science grows in scope, the brains that produce that science must reckon with all-pervasive features of the universe. Just as dolphins and fishes have very different evolutionary histories but similar shapes because they both live in water, so sciences that deal successfully with the basic forces of the universe must come to similar views. Nature presumably already offers many cases of convergence: placental and marsupial wolves, and camera eyes in squids and mammals, to mention only two of the most striking.[6] Furthermore, when it comes to communication with advanced technological civilizations, we are talking about species that at a bare minimum have built means of electromagnetic transmission and may also have embarked in a program of space exploration. Their views may be superior to ours (having been around longer) but surely they must overlap with ours to some extent, for at least to some extent they and we are successfully applying the laws of electromagnetism.

Nevertheless the matter is not this straightforward. We must realize that even all-pervasive features of the universe would be interpreted differently by different scientific intelligences. As we have seen, a highly complex brain can deal with the environment in a very flexible and indirect manner. Moreover, it is not one brain but an ensemble of brains in very complex social relations that deal with the universe through science and technology. Whereas in the case of the ocean we had direct pressure (selection) on aquatic animals, in the case of the deep forces of nature we have many different ways of handling the pressure (indeed, a double tier of evolutionary slack). Even in the case of the ocean, animals with very different evolutionary histories have different shapes, as we can tell just by looking at crabs and salmon (fishes and dolphins are much more closely related). The very same "feature" of an environment impinges very differently on different organisms. A hot spring may kill some fish while making bacteria thrive. It is a mistake, therefore, to describe the situation as if different brains were dealing with the same problems. We rather have different brains dealing with different problems. Indeed those different brains will have (1) different starting points for inspiration, (2) different motivations, and (3) different social means of dealing with conceptual matters.

As for the overlap in electromagnetic theory, we should guard against confusing an overlap in performance with an overlap in content. For in a limited domain two radically different views may allow us to do pretty much the same. As a guide to navigation, the astronomy of the ancients was not surpassed by the astronomy of Copernicus and Newton until long after Newton's death; and it remained competitive until the advent of recent technology. But according to the ancient view, the immobile Earth sat at the center of the universe while the stars were fixed on a gigantic sphere that rotated around the Earth. By keeping the stars in that sphere it was possible to calculate very precisely their position in the sky at any time of the year. And by reference to that position a sailor or an explorer could chart his course. In many respects it is still easier to apply the ancient view. In any event, to some extent the ancient and the modern views give us very similar practical guidance; they allow us, in a limited context, similar performances. But the views are not only different, they actually contradict each other: One forbids the motion of the Earth around the sun; the other requires it. If perchance we receive electromagnetic transmissions from another species, we should not conclude that those beings must have the equivalent of Maxwell's laws of electromagnetism. We may need Maxwell's laws in order to describe, to ourselves, what those beings do. But their actual "laws," if they even think in such terms, may not be any more equivalent to Maxwell's than the Greeks' lack of motion of the Earth is equivalent to Copernicus' motion of the Earth around the sun.

We see then that there is no inevitable, nor highly probable, connection between the appearance of life and that of intelligence; nor between the appearance of intelligence and that of an advanced technological civilization.

None of preceding rules out the possibility that extraterrestrial science exists. And even a small chance that it does may perhaps warrant a program to search for it, as we will see below. My aim up to this point has been to use the assumptions behind the optimism prevalent in SETI to investigate the conditions that make human science possible. We have seen that those conditions are many, and that to a large extent they depend on the vagaries of natural and social history.

[1] This section was excerpted and published as Chapter 2 of my Evolution and the Naked Truth, Ashgate, 1998.

[2] An earlier version of this section appeared in Explorations in Knowledge, Vol. VI, No. 2, 1989. It was later reprinted as Chapter 2 of my Evolution and the Naked Truth, Ashgate, 1998, pp. 23-32. The conceptual underpinnings were first worked out in my Radical Knowledge, Hackett, 1981 (Avebury in the U.K.). Two other philosophers have developed views in a similar spirit: Lewis Beck, “Extraterrestrial Intelligent Life,” Presidential Address, American Philosophical Association, December 1971 (Reprinted in the APA Proceedings, 1971, pp. 5-21); and Nicholas Rescher, “Extraterrestrial Science,” Chapter 11 of his The Limits of Science, University of California Press, 1984, pp. 174-205.

[3] See, for example, C. Sagan, Communication with Extraterrestrial Intelligence, op. cit.

[4] L. Margulis, Symbiosis in Cell Evolution, W.H. Freeman Co, 1981. Symbiosis is a plausible way for complexity to arise, but it need not be the only way.

[5] See also S.J. Gould, “SETI and the Wisdom of Casey Stangel, in his book The Flamingo’s Smile, Norton, 1985.

[6] New findings suggest that proto-eyes are very ancient and thus that instead of convergence we have here a case of common ancestry.

Saturday, September 3, 2011




Even apart from the wisdom of making contact, we have seen why assuming the principle of mediocrity serves the opponents of SETI well. SETI depends on the possible transmission of signals by extraterrestrials. That is the extent of the search: To listen to the universe with radio telescopes in the hope that an artificial combination of pulses may be identified. And then, of course, we would try to decipher such a signal and perhaps to respond, thereby initiating the most extraordinary communication in the history of the human species. That is the program of SETI. Now, to show the urgency of the matter, the principle of mediocrity is invoked: There may well be a whole club of civilizations out there, and with just a little effort we might be able to join them. But if we assume the principle of mediocrity very literally, and consider the age of the galaxy, we must wonder why the extraterrestrials are not here.

Quite apart from such concerns, this principle deserves examination. Let us begin with the motivation for invoking it. What could we have learned from the Copernican revolution in the first place? Surely not that we are average. At best that we had no reason to assert that we are special. This is not the same as to say that we are not special, for it may well turn out that we are, even if we have no reason at this time to think so. I may have no reason to assert that the respectable looking man walking by my window is a criminal, even though perchance he might indeed be one. At most, then, we should simply gain a healthy skepticism about claims of human privilege.

Moreover, although we have now reasons to believe that the sun is an average star and that the Earth is not the center of the universe, we cannot say that we have similar reasons about our own standing in the realm of life. In the one relevant aspect--intelligence--we are clearly not average in the domain that we have been able to observe.

The principle of mediocrity is prompted, I suspect, by the notion that our belief that the Earth was the center of the universe and that we were the pinnacle of creation sprung from some primitive anthropocentric view of the world later reinforced by religion. Remove the notion of man at the center of things, and it becomes imperative to face up to our average nature. But however convenient for their religion, our ancestors did have good reasons for thinking that the Earth was the center of the universe. It took a lot of ingenuity and good timing to overcome devastating objections to the idea of the motion of the Earth (cf. the treatment of the Tower Argument in Chapter 3). Nor did they think that the Earth was at the center of the universe because it was special in any commendable way. On the contrary, the heavens were eternal, and unchanging, our example of perfection. Change and corruption could take place only in the lowly Earth. Copernicus himself resurrected the Pythagorean claim that the sun should be at the center of the universe since it was obviously so much nobler a body than the Earth.

The existence of extraterrestrial intelligence should thus be discussed without the burden of the principle of mediocrity. On the other hand, the principle of mediocrity cannot be used by the opponents of SETI either. The arguments against ETIs can no longer assume that if there are any, they should be so strikingly similar to us that we can make reliable, quasi-probabilistic guesses about them based on intuitions about ourselves. To be acceptable, the arguments must include a wide range of considerations from biology and space science. With this in mind, we need to explore two questions at both ends of the issue. First: how is it possible that ETIs exist but we have no evidence for them? As we have seen, the answer to this first question is that ETIs may exist without our knowing about them. And second: what events or processes could make it possible for our technological civilization to be the only one in the galaxy?

But before we embark on the task of answering this second question, it is useful to cast a critical eye on some practices that reflect on the field of SETI. One of them is the use of what some proponents of SETI call "subjective probability," which they think it permits them to arrive at their rosy conclusions about the chances for the existence of ETI. According to T. Fine, the subjective interpretation of probability "maintains that probability statements are derived through a largely unassisted process of introspection and are then applied to the selection of optimal decisions or acts."[1] Furthermore this subjective view "encourages the holder to fully use his informal judgment, beliefs, experience in arriving at probability estimates."[2] Although personal, such estimates are presumably not arbitrary because "there are reasonable axioms of internal consistency between assessments and constraints that force the user to learn from experience in a reasonably explicit way."[3]

This view of probability, together with the principle of mediocrity, has indeed encouraged some SETI enthusiasts to make highly optimistic pronouncements about the likelihood of planets with life, intelligence and technological civilizations, based on the fact that the Earth has life, intelligence and a technological civilization. But can these scientists justify what amounts to giving a statistical distribution from only one case?

If I think it is likely that I will survive intact a jump from the Golden Gate Bridge because I cannot believe that harm can come to me at this stage of my life, my estimate will be as wrong as it is arbitrary. Nonetheless the constraints of the experience (serious injury or death) will most definitely be inconsistent with my assessment. And if I do survive, such inconsistency will force me to learn a valuable lesson. Even so the arbitrariness of my initial assessment is not thereby removed.

The intuition behind subjective probability is that a scientist who has already learned from experience, and who is in a situation to which his expertise is relevant, may come up with reliable hunches as to what is the right action to take. Indeed we may measure such probability by determining how much he is willing to bet on a course of action over its alternatives. I think that this notion of probability has serious problems even under the best of circumstances. But in any event it does not apply in the case of SETI. On this subject we have learned nothing from experience because we have had no experience to learn from, nor can we use our expertise about the Earth because our theories are not yet developed enough to make decent guesses about how representative the Earth is. In a few years we are likely to, if we continue to increase the sophistication of telescopes in orbit. We may begin by detecting terrestrial planets at the right distance from their suns to have liquid water; and then we might be lucky enough to find one or more such planets with the right spectrum in their atmosphere (e.g., appropriate percentages of oxygen, methane, etc.) to make us believe that we have detected the “signature” of life. But so far we do not quite have instrumentation that refined. And we do not know if, once we have it, we will ever find such planets.

A related misuse of probability comes in the practice of splitting the difference. The optimist will use his subjective probability to estimate that in every mature planetary system there will be at least one planet with life (the probability of life is one), the pessimist will say that the probability is zero because life could have arisen only on Earth. And then there are those congenial types who declare that the truth must fall somewhere in between, and so decide that a probability of one half (or one fourth or one sixteenth) is a "conservative" or "reasonable" estimate.

Imagine, however, that I am given a photograph of a building that could be either Fort Knox or an empty warehouse, and that I am asked to estimate how much wealth that building contains. Suppose that I know that there are 200 billion dollars in gold in Fort Knox. And now, since I have no idea which building it is, I split the difference and estimate that there are 100 billion dollars in it. Whichever building it turns out to be, my estimate will be off by 100 billion, not a small mistake. In the case of ETI our estimates of probability should be based on our knowledge of the universe, not on reaching a compromise between the uneducated guesses of interested parties. As space science advances, we will have more insightful things to say about the chances for extraterrestrial life. For extraterrestrial intelligence we will have to take a few additional steps.

To see what those steps are, in the next posting I will discuss briefly the second question listed above: What events or processes would make it possible for our technological civilization to be the only one in the galaxy?

[1] T. Fine, “Nature of Probability Statements in Discussions of the Prevalence of Extraterrestrial Intelligence,” in C. Sagan, Communication with Extraterrestrial Intelligence, The MIT Press, 1973, p. 360

[2] Ibid.

[3] Ibid.