Suppose that we are not the sole intelligent life-form in the Universe. Discovery of another life-form and subsequent communication with it would be one of the most important events in the history of the human race. But how should we go about searching for such a life-form, let alone communicate with it? The question of search can be answered to a limited extent, but only within the framework of our present technology. The problem of communication is far more severe in view of our total ignorance regarding the possible nature of such life-forms. Nor is it a simple matter of science: there are economic problems too. It is certain that an efficient search for extraterrestrial intelligences would cost a great deal of money: a sum comparable with the Apollo Moon programme could be involved. Moreover such a search may take decades before extraterrestrial intelligence is discovered or before it can be fairly well established that civilizations like our own are very rare. It is clearly difficult to justify such a vast expenditure of money when the success of such a venture can hi no way be guaranteed. So at present the search for extraterrestrial life must be a by-product of more general astronomical exploration.
There have been a few searches using present telescopes for evidence of extraterrestrial civilizations which may be trying to communicate their existence to others. The first such search was project Ozma conducted at the National Radio Astronomy Observatory in Virginia in 1960. The stars observed were ? Ceti and ? Eridani. Since then there have been more observations, generally of nearby G. K or M stars. The searches have not yielded anything that could be construed as a signal, but they do lead us to discuss a number of very interesting problems. We might ask ourselves how many stars we might expect to observe using a given technique before obtaining a result. What is the optimal wavelength to search at ? What constitutes a ‘signal’ and how shall we recognize it as such? Then there are questions like: suppose we discover evidence of an extraterrestrial civilization, how will we understand its communication? What would be the result of such contact between civilizations? Our terrestrial experience of contact between different civilizations is a history of the decimation of the less advanced societies. Extraterrestrial communication does not involve physical contact, so the effect may be more akin to the influence of ancient Greek philosophers and Biblical prophets on present European society, and that is certainly not insignificant!
Such problems have been considered and debated at considerable length. International conferences have been organized to bring together the thoughts of scientists in many disciplines from many countries. In the face of our total ignorance about extraterrestrials, and of our own rapidly growing but evidently primitive technology, such discussions must be regarded as remarkably ingenuous. How¬ever, one is sobered somewhat by (and one may derive amusement from) the thought of how similar debates might have proceeded only a hundred years ago! Bearing this in mind, let us make a few comments on these questions.
Consider first the question of the likelihood of discovering an extraterrestrial civilization. That depends on many things. It depends on the probability of the existence of life on a planet orbiting a given star, on the extent to which the civilization has developed, on the nature of the instrument used in the search and on whether or not the civilization is trying to signal its existence to others. We have already discussed the evolution of rudimentary life on Earth, and the development of technological civilization, so the reader can judge the likelihood of the same process occurring elsewhere. An important factor here is how long such civilizations can survive, once established. If, for example, the lifetime is a mere hundred years following the development of nuclear technology, we would have to observe that civilization during that short period between the birth of its technology and the annihilation of its society. Unless technologically-advanced societies are stable over periods of perhaps millions of years, we may never discover any. Of course, if the civilization continues to advance it will become more readily detectable simply by virtue of the energy it radiates in the maintenance of its civilization.
The problem of what kind of instruments should be built to optimize the search for extraterrestrial civilizations is a difficult one . Even if we assume that some civilization has established a beacon to let others know of its existence, we have no way of guessing in advance what frequency to look at. The 1420 MHz line of neutral hydrogen has been suggested on the grounds that it is an obvious choice in the absence of any information, but really there is no reason to exclude the possibility of signalling with some other radio line (which may be less noisy), or at optical or even X-ray frequencies. In fact, since searches for extraterrestrials will in the foreseeable future be based on existing equipment, the search frequency may be dictated more by terrestrial conditions than anything else. The most ambitious project proposed to date is PROJECT CYCLOPS, an array of more than a thousand 100-metre dishes. The waveband to be searched by this instrument would be from 1420 MHz to the OH-transition at 1662 MHz, a band known picturesquely as the ‘waterhole’. The main advantage of such an array would be that it can be added to as money for the project becomes available, and will be capable of doing important radio astronomy at the same time.
If extraterrestrial civilizations do not set up beacons, they could be detected by virtue of the energy they emit in the maintenance of their society. The problem is then even more difficult, for not only are we ignorant of the best frequency to use, but we would have to eavesdrop and the power emitted may be rather low in comparison with what a specially constructed beacon might achieve. Moreover, in the absence of any special signal, it will be more difficult to establish that an extraterrestrial civilization has in fact been observed. If, for example, a civilization radiates most energy away at infrared wavelengths, we have the problem of distinguishing this infrared source from hundreds of others that are due to circumstellar dust shells.
Another way to look for evidence of extraterrestrial civilizations is to look here on Earth! It is conceivable that a highly-developed super-civilization may have had time to visit most stars in the Galaxy, and in particular may have visited the Earth. If the Earth provided a suitable environment for the visitors, they may have colonized the Earth. The absence of any obvious extraterrestrials on Earth indicates either that the Earth was a hostile environment and not colonized, or that there are no civilizations in the Galaxy with the capability of such extensive exploration. Visiting extra¬terrestrials, finding the environment hostile, may nevertheless have left evidence of their visit, either accidentally or intentionally. It is not clear what form such evidence might take. To be found now it would have to survive many thousands or even millions of years, and not get lost as a result of geological activity. One possibility that comes to mind is an artificial satellite orbiting the moon while emitting occasional strong bursts of radiation. What does seem clear, however, is that such a technologically advanced civilization would not need to construct landing sites or astronavigation aids of a kind resembling the archaeological relics found all over the world today. Whatever the correct interpretation of such sites or of petroglyphs, it makes no sense whatever to ascribe such things to an extraterrestrial civilization with the technological capability of interstellar space travel. Of course, there may be evidence around that we are not yet capable of recognizing. The previous example of a moon-orbiting satellite could not have been recognized until this century, and it is certainly conceivable that a highly-advanced civilization has left a sign that only comparable intelligence could appreciate. There is yet another possibility that we were visited and our visitors were, in their wisdom, careful not to leave any sign for fear of disturbing the development of a primitive life-form. Such visitors might even observe us rather as we might observe animals in a zoo, and this ingenious idea has been called the zoo hypothesis. The hypothesis is, of course, irrefutable and so is not therefore a properly scientific one. The possibilities are enormous, and speculation is part of the life-form. Such visitors might even observe us rather as we might observe animals in a zoo. and this ingenious idea has been called the zoo hypothesis. The hypothesis is. of course, irrefutable and so is not therefore a properly scientific one.
The possibilities are enormous, and speculation is part of the intrigue of science. The search for extraterrestrials must begin somewhere if we are to ascertain our true position in the Universe. We now have the technology to begin a search, and to signal our awakening to other civilizations. Accordingly, we should award some of our scientific effort to this problem now, even if it is only as a part of scientific research. It has taken several centuries for us to benefit from the dawn of the age of science and technology. It may take a long time to reap the benefit of interstellar communication, but if we are to succeed, it may herald the birth of a new era in human civilization.