A review of the theory of interstellar communication

Abstract Sixteen years have elapsed since Cocconi and Morrison suggested that attempts might be made to detect communications from extraterrestrial civilizations. In this time there have been three major types of activity. First, the general theory of the estimation of the number of technological civilizations has been developed and expanded. Second, studies have been made of the designs of systems of sufficient sensitivity to give reasonable chances of intercepting signals. Third, attempts have been made, on a small scale, to listen specifically for intelligent signals. Two additional events should be mentioned: the Pioneer 10 spacecraft, the first man-made object to escape the solar system, is carrying a plaque depicting some basic data about Earth and human life; and a signal with similar information was recently sent from the Arecibo telescope to the Messier 13 galaxy. In view of this activity, it is reasonable to ask whether there are now grounds for suggesting that a serious attempt be made to design a system to search for extraterrestrial intelligent life. Such grounds may indeed now exist. The number of coexisting technological civilizations is related, in the formulation proposed by Drake, to the rate of star formation in the galaxy, the fraction of stars presently having planetary systems, the suitability of the stellar system, the number of planets in the life zone of each star, the probability of the emergence of life on suitable planets, the fraction of biological systems becoming intelligent, the number of intelligent species attempting communication, and the average lifetime of a communicative civilization. The last factor appears to have the greatest uncertainty. Since the original formula was proposed, there have been a number of advances. It has been proposed by one of us (Oliver) that situations will have arisen in galactic history where a civilization is sufficiently close to other planets on which life has developed that discovery of that life is highly probable. These situations are of four types: first, civilizations could arise independently on the planets of single stars that are separated by only a few light years. Even if the lifetimes of the civilizations are measured only in thousands of years, there will still have been a significant number of instances of coexisting communicative civilizations which are likely to have discovered each other. Second, civilizations could have developed simultaneously on planets around binary stars, with an even greater likelihood of contact. Third, a civilization on a planet of one star of a binary system might have traveled to planets around the second star and discovered non-intelligent life. Fourth, a civilization in a single solar system might have discovered non-intelligent life on a planet in the same system. This would be equivalent to our own discovery of primitive or extinct life on Mars. The probability that one or more of these discoveries has taken place is high. Such a discovery would provide a strong stimulus to the civilization in question to attempt to contact more distant civilizations, with the evolution of a network of communicating cultures across the galaxy. It is possible that the rapid increases in knowledge facilitated by such communication would accelerate the evolution of the participants to the point where they are able to achieve stability and longevity. In contrast, those civilizations which do not attempt communication may retrogress. The achievement of interstellar communication, with subsequent membership in the galactic community, could represent a decisive step in the continuation of biological evolution. Attempts have been made with existing radiotelescopes, over the last 15 years, to detect coherent signals characteristic of intelligent life. Project Ozma, in 1960, was the first attempt. Subsequent experiments are described. None has been successful to date, which is perhaps not surprising in view of the low sensitivity of the receiving system and the limited frequency range employed. Most of the activity has been in the microwave region of the spectrum. Other approaches, such as laser communication, interstellar probes, or the detection of infrared emissions from stellar system engineering projects, have been suggested and will be reviewed. The first attempt to study in detail the design of a system for contacting extraterrestrial intelligent life was made in the summer of 1971 by a team of scientists at the NASA Ames Research Center, under the direction of two of us (Oliver and Billingham). The concept, known as Project Cyclops, is a large array of phased radiotelescopes on the Earth, listening in the microwave region of the spectrum for coherent signals from star systems within 1000 light years. The microwave region was chosen because galatic and quantum noise are at a minimum, and because the spectral lines of hydrogen and hydroxyl (1420 and 1680 MHz) fall in the center of the region. The data-processing system examines simultaneously all 0.1-Hz channels within this band and can detect signals that are 90 db below the integrated noise level in the band. The phased array would be pointed at each likely star for a period of 20 minutes, and then be switched to the next star. A few million candidate stars are available within 1000 light years, and all could be examined within a few decades by the Cyclops system. In conclusion, it is suggested that the discovery of extraterrestrial intelligent life could be of such profound importance to mankind that further studies should be carried out. The technology is available, and the costs seem reasonable in comparison with other major endeavors in science and exploration.