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 SETI is a manifestation of man's drive to explore. This drive is one of the oldest and most fundamental aspects of our nature; the very origin of the hominidae as a distinct biological entity is owed, at least in part, to the boldness of our venturesome simian ancestors who abandoned their familiar forest environment to probe the savannah, there to seek fleet-footed prey. Our forebears pushed into almost every corner of the globe. They explored by climbing hills, by walking through forests, and even by crossing large bodies of water. Sometimes they may have had in mind some material purpose, but certainly they sometimes went where they did for no other purpose than to see what was there. Modern man still explores, but the arena for his exploration is now the planets and the stars beyond. But we are not limited to physical exploration. We can use the fruits of our intelligence to conduct exploration from a distance. Though some day we may wish to build space ships to travel there, to probe the stars now we need only telescopes. Yet the excitement and exhilaration that comes with this kind of exploration, as larger telescopes and more sensitive and sophisticated data acquisition techniques lead to discovery after discovery, is akin to that the Viking seamen must have felt.
Exploration has always required knowledge and understanding of the physical world. Viking boats could not have been built without knowledge of what wood to use; Viking navigation could not have been accomplished without an understanding of the winds and tides. We call this understanding of nature, which we gain from observation and experiment, scientific knowledge. To explore the stars in search of other intelligent life also requires scientific knowledge; indeed, because it can only be done using highly sophisticated technologies and methods, it requires more scientific knowledge than have man's classical explorations.
A SETI program should embrace not only a search for evidence of other civilizations, such as radio signals, but also a wide range of related scientific studies. We need knowledge of nature primarily for two purposes. One of these is to enable us to narrow the scope of the search by distinguishing promising volumes of search space; for example, we might be better able to identify promising target stars or frequency bands. The other purpose is to enable us to be able to interpret any evidence of other civilizations we obtain, and to decide what course we should follow once we are sure that other intelligent life has been discovered.
The scientific knowledge needed by a SETI program can perhaps best be illustrated in the context of the Drake equation, which relates the expected number N of intelligent, technologically advanced communicative species in the Galaxy to the product of several factors. It should be understood that the Drake equation is not a fundamental expression of the way nature behaves, as is, for example, the deceptively simple law, f = ma. Rather, the Drake equation is simply a device to enumerate the factors that influence N and hence must be considered in any attempt to estimate this number. One form of the Drake equation (there are several) is
 where R* is the average rate of star formation in the Galaxy, fg is the fraction of stars that are "good" stars in the sense of providing conditions thought to be necessary for life, fp is the fraction of good stars that have planets, ne is the number of suitable planets in a typical planetary system, fl is the fraction of suitable planets on which life starts, fi is the fraction of life starts that evolve intelligence, fc is the fraction of intelligent species that enter a communicative phase and L is the mean lifetime of the communicative phase. It is immediately obvious that the factors on the right-hand side of the equation have widely differing character. Some, such as R* and fp, involve only knowledge from one basic discipline, astrophysics. Others, such as fg, n, fl and to some extent fi involve questions spanning many disciplines, including astrophysics, prebiotic chemistry and biology. Finally, the terms fc and L involve considerations not generally found in the natural sciences, but which nevertheless lend themselves to scientific inquiry. The full panorama of the elements in the Drake equation is embodied in the concept of "cosmic evolution" (see Section II-1).
For convenience, we can take the Drake equation as a model and categorize the science of SETI under the headings of physical science, biological science, and social science. We begin with those aspects of SETI involving the physical sciences.
A very important question concerns the way stars are born. At present, our understanding of this stage in the evolution of matter is sketchy at best. We are able to calculate the mean rate of star formation over the age of the Galaxy (this number is generally used for R* in the Drake equation), but we are not yet able to specify in detail the conditions necessary for star formation nor are we able to describe or predict the course of events involved in the evolutionary path from a relatively low density interstellar cloud to an incandescent ball of gas supplying its own energy by nuclear fusion deep in its interior. Observational progress on this question has been forthcoming over recent years through infrared and radio studies of dark clouds and newly formed stars; however, many large gaps still exist in the fabric of our knowledge. Theoretical study of star formation is in an even more rudimentary state than is its observational counterpart, due principally to the intrinsically three-dimensional, non-analytic nature of the mathematical representation of the problem, and the inability to model or parameterize the role of turbulence and plasma dynamics in dense interstellar clouds. The advent of very rapid computers is making it possible now to undertake the formidable numerical problems involved in simulating the physics of star formation, and with adequate support we soon should be in a position to compare meaningfully theory and observation. Understanding this fundamental process of star formation is important not only because it provides the means for obtaining a more accurate value of R* in Drake's equation, but more significantly, it will provide the required construct within which we can understand the formation of planetary systems. An understanding of the formation of planetary systems could allow us to eliminate certain classes of stars as targets of the search.
Some stars, even though they have planetary systems, may be intrinsically hostile to life. There is some evidence, for example, that M-dwarf stars produce powerful flares with accompanying radiation levels inimical to living organisms. There is also the possibility that, as a consequence of the low luminosity of M-dwarfs, a planet in the stellar ecoshell would have to be so close to the star that its axial rotation period would become tidally locked to its orbital period about the star.
 However, since M-dwarfs are the most abundant stars, studies which will resolve these questions are important.
One of the primary scientific reasons for the growing feeling that a SETI program is a sound undertaking is the present consensus, based on the previously discussed fragmentary state of knowledge concerning star formation, that conditions conducive to the formation of planets are natural consequences of the birth of a star. We have learned much of the early history of the solar system through our space program. Consequently, we now have a clearer idea how the planets in the solar system formed and evolved. However, as with our understanding of star formation, much work needs to be done. Some of the important questions involve the formation and evolution of planetary atmospheres, and their interaction with their parent star. Do planets form and evolve atmospheres after their star has settled onto the main sequence, do they form and evolve before the star is fully developed, or do both form and evolve simultaneously?
An understanding of this sequence, as well as the details of planetary evolution as a function of mass, composition and its interaction with its environment is important, not only from the standpoint of subsequent biological evolution, but from the standpoints of detecting planets around other stars and identifying those that may have environments favorable to life. Jupiter and Earth emit intense, nonthermal radio bursts, while Venus emits nonthermal, perhaps masering, infrared molecular lines. In spite of their intensity, neither of those characteristic planetary emissions would be detectable over interstellar distances with present technology. Do these phenomena occur on other planets around other stars with sufficient intensity to be detected? If so, as noted in Section II-3, they could provide the best means for carrying out the detection program. Maser radiation from water or some organic compound, or nonthermal radiation arising in a planetary magnetic field might also provide a clue concerning the possible existence of life. We must seek to understand in more detail the nature of planetary formation, the chemistry of planetary atmospheres, and the interaction between star and planet.
Other scientific needs of a SETI program in general, and detection of other planetary systems in particular, involve questions concerning the frequency of binary stars and the stability of stellar properties. As noted in Section II-3, the fraction of main sequence stars that are involved in binary or multiple star-systems, particularly those of short period (periods 100 years), is important. Between two moderately close stars there would be no stable planetary orbits except those with either very short or very long orbital periods. For planets with short orbital periods it is inferred that the planetary surface would be strongly heated by radiation from its parent star. Exactly how hot the surface would be depends on the spectral type of the star and the details of the planet's orbit. For planets with long orbital periods, the opposite temperature extreme might be expected. For this reason, it is important to extend present binary frequency studies to cover a broad spectral range (the most complete study to date falls in the narrow spectral type range F5-G2). If it should turn out that most stars have short period binary companions, we would be forced to conclude that the solar system represents a rare phenomenon in the galaxy, and correspondingly lower the value of fg. However, such a finding would allow us to concentrate the search on a smaller number of targets.
 In connection with the stability of intrinsic stellar properties, it would be useful to be able to determine the long-term stability of stellar luminosities and radial velocities. It is somewhat surprising that we do not know very much about the stability of the apparent solar radial velocity for periods much in excess of a day, let alone months or years. If as discussed in Section II-3, radial velocity techniques are to be considered as potential candidates to be used in search for other planetary systems, we must have a basic understanding of the long-term (comparable to planetary orbital periods) stability of stellar radial velocity as a function of stellar type. This knowledge might also provide needed insight into the modes of internal motion of stars. The subject of stability of stellar luminosities is central to many of the biological aspects of the SETI program. Small scale variations in the luminosity of a star may not be detrimental (and could be beneficial) to the evolution of primitive life forms toward more advanced biological systems, but at present we possess no hard empirical data as to the magnitude or time scale of luminosity variations for any main sequence star, the Sun included. Advances in understanding of the luminosity stability of main sequence stars could also shed light on many fundamental questions related to stellar structure. Arguments have been advanced that the Sun's luminosity varies periodically in time, and that this variation can explain the current lack of neutrinos emitted by the Sun. We simply do not know whether the Sun's luminosity has been, is, or will be varying in time. A potential SETI search strategy could hinge on whether certain spectral types of main sequence stars have sufficiently stable or unstable luminosity to provide conditions conducive to biological evolution.
It will be necessary to develop efficient techniques to identify weak signals embedded in noise, to distinguish signals of artificial origin from natural phenomena, and to evaluate whatever information they might contain. These require studies of pattern recognition at low signal-to-noise ratios, and studies of decoding strategies.
In the biological and social science areas, there is a wide range of studies that are important to a SETI program. In contrast to the purely physical sciences, biological and social science studies have potential not only to contribute to the refinement of search strategy, but also to provide us with guidance with respect to actions that should be taken if another civilization is ultimately discovered. Moreover, the specific problems which SETI would need to have addressed would require new approaches that integrate data and methods from the physical sciences, and these could prove extremely stimulating.
Research is needed on the origin of life (see Section II-1). We now have a general understanding of the chemical evolution process. However, it is not clear in what ways this process depends upon the environment. Both the direction taken by chemical evolution and the time period required for it could be influenced by large numbers of environmental factors including the chemical composition of the planet's atmosphere, the existence and nature of a dense fluid medium, tides, tectonic activity, the degree of climatic diversity, the planetary rotation rate and orientation of the spin axis, the planetary magnetic field, and the energy spectrum of electromagnetic radiation and particles from the primary star. A large number of studies, including laboratory experiments, observations of chemical evolution on other planets in our own solar system, and theoretical studies, can shed light on the degree to which these and other environmental factors influence chemical evolution. Should chemical evolution prove to be very sensitive to a small  number of factors, then we will be able to develop programs to allow us to identify systems likely to contain one or more planets upon which those factors exist. Searches for evidence of intelligent life could then concentrate on these systems and ignore others.
The same considerations apply to studies of biological evolution. There probably are planet- or star-dependent environmental factors to which both its course and its rate will be sensitive. Identification of such factors will allow development of a more discriminating search strategy. Such identification can and should be attempted through a variety of chemical, biological and paleontological studies, both in the laboratory and in the field. However, it is important to develop a general theoretical model of biological evolution in the context of cosmic evolution (see Section II-1). The formalism of statistical thermodynamics or other areas of physics that deal with interacting many-body systems might provide useful insight here. Interactions between investigators in many of the physical and biological sciences will be required to develop such models, and such interactions should prove stimulating to all concerned.
But this is not the only contribution studies in the biological sciences can make to SETI. If the search should prove successful, we will face the problem of communicating or otherwise interacting with whatever civilization we discover. In this regard, studies in the area of the social sciences will also be important.
The history of the Earth advises us that interacting with another civilization will be no trivial problem. Members of one culture have often enough failed to appreciate the customs and beliefs of another, and this has led to unfortunate results. It is an observationally determined fact that men of equal intelligence but different cultural backgrounds may not understand one another. Given this, it is not clear that we will be able to decode all the information content of the first signals we receive. Certainly, there is no reason other than faith to believe that, just because both they and we are intelligent, communications that may ultimately take place between us will necessarily convey the intended meaning. To the degree cultural characteristics are determined by biological and evolutionary factors, so may we expect cultural differences between human and extraterrestrial societies to be greater than those observed among human societies. Great cultural differences imply greater potential for misunderstanding. The result of a misunderstanding could range from our failure to comprehend information intended to benefit us, to cessation of transmission.
Human relations, however, also advise us that if we take the trouble to he aware of cultural differences and treat them with respect, relationships can proceed in harmony. Thus our problem is to anticipate how intelligent extraterrestrial societies might be culturally different from us and to determine what communications problems these cultural differences might cause. This is not entirely a problem in metaphysical divination, but is susceptible to at least partial solution by scientific experiment and observation.
In the first place, we have before us the empirical evidence provided by the human cultures of Earth. It should be possible by appropriate studies to determine the factors responsible for their diversity; indeed, much of this work has undoubtedly been done, and needs no more than discussion and synthesis. Models of cultural development which focus on the interaction of a  society with its technology could be especially valuable since radio technology is the one characteristic which we believe we will share with any extraterrestrial civilization we are likely to discover. If statistical thermodynamics can supply a useful formalism for models of biological evolution, it may also prove adaptable to describing culture and its development. Moreover, we are not limited to studies of human cultures; evidence is also provided by animal societies. Little has been done to evaluate this evidence, but it may give us some very great insights into cultural evolution, social behavior, and the meaning of intelligence.
The Cultural Evolution Workshop (see Section II-2) emphasized that research since 1960 into animal behavior has completely altered our perception of these creatures. The Workshop members had no difficulty accepting the ideas that man is not the only intelligent animal on this planet; that chimpanzees, small cetaceans, and perhaps elephants should also be included in this category, and that these other species have complex social organizations and systems of communication. Indeed, our similarity to the end products of several other evolutionary lines was cited as one of the principal supports for the proposition that intelligence is likely to evolve on other planets, and is not just a fluke which occurred only here. Investigation of and attempts at communication with other intelligent terrestrial animals might establish the extent to which their "cultures" differ, both intraspecifically and interspecifically, and what factors biological, evolutionary, or social- are responsible for these differences. The general theoretical model of biological evolution needed for the identification of factors that produce intelligent life will also be important here. A thorough comparative understanding of behavior, encompassing at least men, chimpanzees, and dolphins might provide us at least a minimum of insight into what range of cultural behaviors we might expect among extraterrestrial societies. In particular, comparison of dolphins and primates might give us some feeling for the consequences of different primary means of sensory perception.
The investigation of the nature and origins of behavioral diversity among the intelligent and semi-intelligent higher animals, although not considered to directly affect the present ETI search strategy, nonetheless is considered to be an important parallel effort. Our goals should be (1) to catalog and classify behavioral patterns and cultural differences; (2) to determine how these are related to the environment, physiology, and evolutionary history of each species studied; (3) to determine what traits, if any, appear common to all intelligent animals; (4) to gain experience in actual communication with other terrestrial species; and (5) to develop theoretical models that will allow extrapolation to extraterrestrial cultures, and allow us to evaluate at least semi-quantitatively the uncertainties in such an extrapolation. To the extent that this approach might enable us to better understand human behavior, it could result in one of the most important benefits of the SETI program.
There is little doubt that the science germane to SETI would contribute in a significant way to our overall understanding of cosmic evolution. Positive feedback exists between these studies, such as a search for other planetary systems and analyses of cultural evolution, and a SETI effort. Each can be carried forward independently, but we should make every effort to apply the gains in knowledge from one study to the others.