The Search for Extraterrestrial Life

The Search answer personal questions about the significance of our role in the universe and what we can accomplish. Presum- ably, an answer might also show us how to obtain important scientific and technological information from other civilizations—knowledge we would not otherwise gain except by hundreds of years of hard, expensive effort. Thus the question of whether extraterrestrial intelligence exists is important for rea- sons that range from the most profound philosophical level to the very practical and technical levels. Nowadays we generally break the question into two parts. First we ask about our expectations for intelligent life in space based on what we know of biology, the arrangement of the universe, and the laws of physics. This part guides us about the difficulty of parameters, no gamma functions, no more than 10 per cent of the stars have the search, the possible closeness of the sines or cosines, or such—it tells us planetary systems. As yet, though, we nearest civilizations, the expected num- that all the things we need to know are haven’t confirmed that estimate to any ber of civilizations in the galaxy, and equally important. The chain of factors degree of certainty. Models of plane- the range of life that might be found. is only as strong as its weakest link. tary formation suggest there will be two These answers in turn serve as guidance The product of all the factors except planets in each system suitable for life, for the second, more practical ques- the last one gives the rate of production but there is no physical evidence other tion: What is the most promising way of potentially detectable civilizations in than our own system to support such an to search for that life? the galaxy. Using the best numbers we answer to this astrophysical question. have, which frequently are only crude Now we come to the questions about guesses, we arrive at a rate of about life. What fraction of potential life- How Many Other Worlds? one per year. If we now multiply by bearing planets give rise to life? The The number of detectable civilizations the mean longevity of highly technical experiments that have been done now in N that might exist in our galaxy can civilizations L, we find the total number a host of laboratories show a multitude be represented by the nice, neat equa- of detectable civilizations in the galaxy: of pathways giving rise to the chemicals tion shown above. In this equation the of life. Moreover, if that is not good “good suns” associated with fs are stars Now all these factors are, in fact, enough, these chemicals seem to be car- whose production of light has the length questions. The rate of star formation is ried in from outer space by comets and and constancy needed for the evolution an astronomical question, and we hap- asteroids. It is pretty clear that we know of intelligent life. Also, the fraction of pen to know the answer quite well— quite a bit about this question: the frac- intelligent species fc that achieve elec- about twenty per year since the birth of tion must be very close to one. tromagnetic communication are those the galaxy. Realistically, this number is The next three factors—the fraction that develop the technology needed to the only one we know well. We know of living systems that give rise to in- make them detectable in the galaxy. the fraction of good suns fairly well, but telligence, the fraction of those that The equation plays two roles. First, it the fraction of stars that have planets give rise to technology, and finally the tells us what we need to know: Looking is currently seen only through a glass longevity—are fascinating in them- at it, we see what the other unanswered darkly. Intimations of planet forma- selves, and I will dwell on these fac- questions about life are. Second, be- tion in the disks of dust around nearby tors a little more than on the previous cause the equation is a simple product— stars, in the detection of a brown dwarf ones. In fact, we usually quickly wave there are no exponential or logarithmic around a star, and so forth suggest that our hands over the last factor and move 52 Los Alamos Science Fellows Colloquium 1988 The Search on, but it is one of the most important BRAIN-BODY WEIGHT RATlO unanswered questions about life. Is Intelligence Inevitable? Until recently the fraction of systems that give rise to intelligence was quite controversial. Intelligence was believed to be the artifact of a limited series of freak events in the course of evolution and thus occurred only rarely in systems of living things. In other words, it was thought to be an anomaly in the universe—a view that some people still hold. However, over the years evidence has increased to support the idea that intelligence will arrive by one or another of many evolutionary paths. If one examines the fossil record, the only feature that one always sees increas- 0.01 1 100 10000 ing in power or size is the brain. We have had larger creatures, faster flying Body Weight (kilograms) creatures, faster running creatures, more vicious creatures, but we have never Fig. 1. A logarithmic plot of brain weight ver- thirds power relationship. The average statis- had creatures on the surface of the earth sus body weight for living mammals, living tical increase in brain mass for a given body that were smarter than the ones we have reptiles, archaic mammals, and archaic rep- weight over those 70 million years has been on today. In fact, many studies of the cra- tilea, where archaic means 70 million years the order of a factor of 10, but the 20-fold in- nial capacity of creatures have shown ago during the age of the dinosaurs. The plot crease for the mammals is significantly larger the increase in intelligence to be very shows that each group follows the typical two- than the 4-fold increase for the reptiles. monotonic over the course of time. Figure 1 shows the distribution of brain mass versus body weight both for weight and brain weight than the re- creature will appear on a life-supporting reptiles and mammals living now and gion occupied by the living reptiles. planet. In fact, if competition did not for archaic reptiles and mammals that But when we compare mammals to rep- take its toll, there would most likely lived about seventy million years ago tiles at a given body weight, we find the eventually be a large number of intelli- in the period before the Cretaceous- archaic mammals have approximately gent creatures on each planet. Tertiary boundary. Surprisingly, we four times the brain mass of the archaic An interesting bit of evidence for this find for each group not a scatter dia- reptiles, whereas the living mammals last conjecture is the recent discovery of gram but a well-defined relationship in have twenty times the brain mass of the dinosaurs that had relatively large brain which brain mass goes essentially as living reptiles. Although the change is masses. Their brains were not as large body weight to the two-thirds power, more dramatic for the mammals than as those of humans by any stretch of a rule that has applied through all evo- for the less evolutionarily advanced rep- the imagination, but the mass was larger lutionary eras for which we have good tiles, there has been a definite increase than that of the typical dinosaur brain. data. for both. One can make plots using dif- These creatures include one known as Each of the four groups shown in ferent or more finely divided time inter- an ostrich dinosaur that lived in Mongo- Fig. 1 occupies a clearly defined re- vals, and one always finds a monotonic lia and the northern United States, one gion. For example, the archaic reptiles increase in brain size with time. with the delightful name of Stenony- (the dinosaurs) were, of course, much The host of statistical evidence show- chosaurus inegualis that stood about larger than current reptiles, and they ing the increase supports the idea that, six feet tall and weighed a little over a occupy a region higher both in body in one way or another, an intelligent hundred pounds, and my favorite, a cute Los Alamos Science Fellows Colloquium 1988 53 The Search critter called Saurornithoides, which slowly rotate in space with its long means reptile with feet like a bird. axis pointed toward the sun (Fig. 3). The saurornithoides (Fig. 2) had about Huge mirrors, inclined to reflect sunlight the same height and weight as we do, through windows, would create daylight stood on its feet using its forearms as inside. Initially, one might think that a we use our arms, and had the equiv- space colony would be a horrible place alent of an opposable thumb. It used to live, but it could be delightful with its intelligence and its “hands” to catch landscapes, lakes, rivers, and a force its favorite food, rat-like animals—the resembling the earth’s gravity due to things that Jack Sepkoski calls vermin— A SAURORNITHOIDES the slow rotation (a few revolutions a that were the original mammals, our minute) of the colony. Moreover, you ancestors. Most interesting, the sauror- Fig. 2. This relatively intelligent dinosaur used could have a bug-free environment and nithoides had a rather large brain case: its “hands” and intelligence to catch vermin, made-to-order weather: open the mir- Its brain mass was not the few grams our mammalian ancestors.

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