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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.

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