SECTION I: CONSENSUS

INTRODUCTION

Heaven and earth are large, yet in the whole of space they are but as a small grain of rice .... It is as if the whole of empty space were a tree, and heaven and earth were one of its fruits. Empty space is like a kingdom, and heaven and earth no more than a single individual person in that kingdom. Upon one tree there are many fruits, and in one kingdom many people. How unreasonable it would be to suppose that besides the heaven and earth which we can see there are no other heavens and no other earths?

Teng Mu, 13th Century philosopher
(translated by Joseph Needham)

In the enormous emptiness of space we can now recognize so many stars that we could count one hundred billion of them for each human being alive. Yet we know of only one inhabited planet, our Earth. The Earth has supported the development of life nurtured by one commonplace star, the nearby five-billion-year old Sun. We look out into the starry Universe quite unable to see within its compass any sign that we are not alone. The other planets near our Sun offer some hope to a search for other life, and indeed for many months Viking on the surface of Mars has been reporting the enigmatic chemical activity of the Martian soil. We remain uncertain, at the time of writing, whether the chemical changes are biological or inorganic in nature.

The web of life here on Earth is the consequence of a long complex sequence of natural selection by which life increased its scope and its variety, always exploiting the flood of energy bestowed directly or indirectly by the Sun. The Earth has seen fire and ice, yet it has provided steadily, for three billion years without a break, some environments to which life could adapt. Changes were never so drastic or so rapid that all survival became impossible, though particular species have arisen and died by the millions. Indeed, life has spread from its origins, probably near the seashore, to alpine peaks and ocean troughs, and has diversified almost beyond description. Our species and a few of our forebears have achieved considerable technological abilities and some degree of self-knowledge. Nor do we foresee any natural catastrophe ending this fabric of life until in due course the Sun itself runs out of nuclear fuel, some five billion years in the future.

We all know the starry sky at night, and on our deep photographs of the sky we see everywhere a dusting of small dots. Analysis of the light which caused those images, using its intensity and the details of its spectrum, has made it certain that such dots represent suns resembling our own, about which we know only that they are suns. Our own Sun with its cortege of planets would be just such another single dot, quite indistinguishable from a hundred million others at the distances we scan.

We have been able to understand in a general way how stars are born out of dense clouds of gas and dust in the interstellar spaces; we can see other stars in the transient stages of birth, as once was the Sun and its planets. Are planets always born in the spinning disk of gas out of which the

other suns form? Or is our own set of planets as rare as its central luminary is commonplace? We cannot now say, though we are sure that the processes that form stars and might have formed planets as well, were going on for billions of years before our solar system formed, and will outlast our Sun.

If around those other visible suns there spin other planets, hidden from us by the distances of space, it is at least possible that on some the work of natural selection has continued for times which could be five or ten billion years longer than the whole history of our Sun and Earth. We could conceive that life never arose on a given planet, or that it exhausted its resources of adaptability, to end in an algal monotony, or in total extinction. Or we can imagine the slow evolution of beings - not human - able to control their world and themselves and to know the Universe.

In evolutionary diversity there is still unity. Squid and human see with eyes that evolved quite differently, and yet resemble each other, for they perform similar tasks. The big tuna, the extinct icthyosaur, and the dolphin resemble each other closely in streamlined form, and even somewhat in behavior, They are distinct evolutionary solutions to the problem of earning a living by predation upon fast-swimming fish; the three, fish, reptile, and mammal, have been molded alike by natural selection to solve the single dynamical problem of fast pursuit in the sea. Similarly, the way of life of H. sapiens appears to spread and to succeed; it seems to us that if natural selection has once built so subtle and successful a scheme, it can do so again. Sapient beings on other planets would in no way be our biological kin for they would share with us no common ancestor. But they might have converged with us in behavior; they might have evolved to culture, and then, say, to radio telescopes! Culture is a workable way of life, like hunting schools of mackerel. Indeed, we have seen that human cultural evolution, also, often converges: no less a development than writing was independently achieved by the Aztecs, the Chinese, and the peoples of the Middle East. On this basis, it would be consistent with the historical development of the great ideas of science to postulate that for a time of unknown duration, near an unknown number of stars, deliberate radio beacons or unintended radio leakage are present. This is a hypothesis untested, but capable of verification by experiment.

It is not idle curiosity to inquire whether other intelligent life has arisen and survived near some distant sun, beings in no way our biological kin. For by some sign of that presence we might come to learn, in a way, our own possible future. Indeed, the one most solid result of the calculus of chance which governs our thoughts about such uncertainty is this: intelligent beings out there if they exist at all almost surely form societies which have endured for a time long compared to the history of our own civilization, a time which might even reach the span of geological time itself.

Astronomers have real hope of detecting planets near other stars, especially relatively neighboring ones, by new optical or infrared measurements from ground or orbit. But detection of plant or animal life implies a landing such as we made on Mars, and this is well beyond our capabilities over interstellar distances.

If we are to learn about distant life, it must make itself perceptible. As far as we can see, only life that has followed our own evolution to the extent of being able to send some mark of its presence across space can be found. This must mean that intelligence develops naturally out of evolving life, that it can make signals capable of traversing space, and that, for some period of time at least, it wants to make its presence known (or at least does not conceal it!). If these conditions exist anywhere, we might hope to detect creatures far older and more capable than ourselves. Exploration would then cross a new frontier; the frontier of an intelligence biologically wholly unrelated to our own.

How would such signals be made? Might super-Viking probes cross space? Might light flashes like stellar lighthouses show an intelligent presence? Much speculation has considered the situation (some of the variety of different ideas are presented in Complementary Document 1). The key facts seem to be that radio waves cross space well, and that the radio engineer has found means to detect extremely weak signals with large dishes and extremely sensitive receivers. Violent events on every scale, from explosions in galaxies to electrical instabilities on the planet Jupiter, have been recorded by radio astronomers. None of these signals appear to bear the marks of any but an astronomical origin, so far. Interesting as these have been, it remains true that radio energy compared to visible light is scarce in the Galaxy. Within the scope of present knowledge in our own Galaxy, a certain well-defined radio waveband (from about one meter to one centimeter) is, for natural reasons, the quietest region over the whole span of electromagnetic waves (see Section II-4).

This fact lies behind a remarkable event in human history. Almost imperceptibly, without really intending it, within the past two or three decades we have entered a new communicative epoch Until that time, we could have made no sound, no pattern or mark, no explosive flash of light on our small planet that could be detected far out among the stars by any means we understand. Space is too deep, and the stars are rivals too brilliant, for any mere faint human glow to become visible far away. Even the whole amount of sunlight reflected from a planet, a light source thousands of times more powerful than all the energy now at human disposal is still beyond our ability to pick out at the distance of a nearby star. But our radio technique, only a generation or so old, has now reached such maturity that a signal sent from an existing radio dish on Earth, with sending and receiving devices already at hand, could be detected with ease across the Galaxy by a similar dish, if only it is pointed in the right direction at the right time, tuned to the right frequency. Such a lucky observer or one who is patiently and systematically searching - would see us as unique, distinguished among all the stars, a strange source of coherent radio emission unprecedented in the Galaxy.

Or are we without precedent? Are we the first and only?

Or are there in fact somewhere among the hundred billion stars of the Galaxy other such beams, perhaps so many of them that our civilization, like our Sun, is to be counted as but one member of a numerous natural class? For such a radio beam cannot come, we think, from any glowing sphere of gas or drifting beam of particles. It can come only from something like our own complex artificial apparatus, far different from any star or planet, smaller, newer, much more

particular; something we would recognize as the product of other understanding and ingenious beings.

That is the topic of this technical report: the search for extraterrestrial intelligence, SETI.

We do not intend to send any signals out to add to those which have already gone out from our TV transmitters and our powerful radars. Rather, we want to listen, to search all the directions of space, the many channels of the radio (and other) domains, to seek possible signals. Perhaps it will be only an accidental signal, as we have made ourselves. That would be harder to find. Or perhaps there is a deliberate signal, a beacon for identification, or even a network of communication. There seems no way to know without trying the search. This is an exploration of a new kind, an exploration we think both as uncertain and as full of meaning as any that human beings have ever undertaken.

The search would be an expression of man's natural exploratory drive. The time is at hand when we can begin it in earnest. How far and hard we will need to look before we find a signal, or before we become at last convinced that our nature is rare in the Universe, we cannot now know.