Proposing the Microwave Search
My wartime service as neutron combat engineer over, by the mid -fifties I had made my way to high-energy astronomy. After the success of radio astronomy, the notion of opening new channels was appealing. In 1958 I came to see that gamma rays promised another new channel, and worked out early predictions. One point was their easy crossing of the entire dusty plane of the Galaxy, unlike starlight yet at light speed. My ingenious friend and Cornell colleague, Giuseppe Cocconi, came to me with a question. "We already make gamma- ray beams. (The electron synchrotron at Cornell was then brand-new). Why not send them out across space to see if anyone out there can detect them?" It was a surprising question, but most stimulating. My reply was that we should look at the whole spectrum, radio to gamma rays, and choose the best band for such signals.
By mid-1959 we had learned enough to propose microwave radio as the band best suited for listening—not yet sending-- to any Others as finite as ourselves. Cornell’s own big radio dish near Arecibo in Puerto Rico would soon to be able to detect another dish like itself at interstellar distances if well-aimed and powered, and a counterpart of the transmitter readied for Arecibo would be able to reach well across the Galaxy. Gamma rays were far less mastered, and optical light had to compete with the stars. Arecibo microwaves, with directed beam and sharp-tuned frequency, would far outshine the sun’s diffuse microwaves, and the Galaxy’s as well, at 1420 megahertz (21 centimeters), the hydrogen line, best-studied of natural radio emissions.
The Galaxy has changed little in 40 years, but our technology and our insight have changed a lot. Strange but true, among Earth’s many radio dishes Arecibo remains the biggest, and arguably the best to search with. Microwave receivers have improved, but not by orders of magnitude. Interfering artificial signals in microwave are far more numerous now , but experience has shown us ways to mitigate their impact (perhaps one day to listen only from a crater bottom on the unseen side of the moon, shadowed from most interfering signals!)
To search you must decide where to point, when to point, and how long to dwell at each chosen direction, what frequencies to cover, how weak a signal to delve for, and of what form? Only a handful of systematic searches have been made mostly with marginal resources, for a task we are still defining. We are constrained both by the knowable properties of the natural world, and by the intangible choices any putative sender might make. The first of all SETI efforts was carried out in 1960 (independent of our own proposal, but fully compatible with it) by Frank Drake at Green Bank, a real radio astronomer. He pointed a good- sized dish at a few of the nearest sun-like stars, one by one —and found transient interference from unidentified human sources.
Consider the rough scale of the task. To search a wide patch of sky is possible without a big dish, but it demands incredible power from the transmitter that must light up many directions at once. The Arecibo microwave receiving beam can discriminate several thousand directions in a patch of sky one degree on edge. 40,000 such patches cover the sky, (don’t forget the southern sky hemisphere, where we have lately looked for a while.) Would you look at all directions, or choose some special points? So far it has seemed best to try both: narrow beams one after another to multiple identifiable targets like near-by stars, and a full sky coverage again spot by spot for less sensitive searches. It is not easy to foresee the power our ambitious unknown counterparts might assign for transmissions. Most of Galactic space is far away, of course, and might have rare but powerful senders among so many stars, while close-by stars are few by comparison, and do not need rare capabilities.
The physics of the dilute gases adrift between the stars in the Milky Way implies that even the sharpest signals will smear out in frequency rather soon as they travel, perhaps to acquire an enforced width of 0.1 hertz; if that holds, it is not much use to look for still narrower ones. Even if we accept our old naïve recommendation of the 1420 megahertz band, a hundred million dial settings are none too many for a plausible search of that one band. Multiply frequency choices by directions, and a really full search requires a trillion brief periods of listening. Of course we will consider laser-made signals in the infra-red and optical bands--a modest try is being made now—and follow other leads along the entire spectrum.
Only one visit to each possible choice? A sender sweeping the stars to economize on power has to face the fatal chance that his blind choice of time at the right direction and the right frequency has ended any chance of success. The answer is to repeat, repeat, repeat, or burn power steadily. These alternatives have been studied, and plausible choices made—doubtless to be remade as we learn more.
SETI@home participants themselves demonstrate the major change in technology since 1960, not new dishes, new receivers, or even new knowledge of stars and their medium. It is the multiplicity of choice implied by the amazing rise in computer power. The early proposals expected a thousand channels at once to be recorded during the search , a good start at shortening the search time. Today we operate rather inexpensive systems that can receive data in one hundred million channels all at once. Nor is the limit clearly at hand. The million and more volunteer CPUs put to use now to help analyze a backlog of recent search data from Arecibo is only a sign of what lies ahead in the next century of signal processing.
Searching in Time
SETI seeks one day to search the space of this Galaxy, a home to a few hundred million suitable suns. An extragalactic reach opens so many possibilities that an experimenter is daunted, even though his pencil beam covers a large area of stars all at once in a distant galaxy.. All of them are millions of light years away. Any sender way out there has to wait out the round trip as a minimum time for an answer. So much does this transcend our idea of history—our own species is maybe 100,000 years old-- that we find it hard to plan. One brief search was made at Arecibo years ago of the nearest big galaxy, the Andromeda spiral ; it brought no signal. How little we understand of what to expect and how to act over such depths of time!
But there is a much smaller time delay --still more than we ordinarily face in experiment design--when we restrict our search within our own home spiral, the Milky Way. First of all the nearest 100 stars, most of them faint red dwarf stars much dimmer than the sun, occupy a sphere about 50 light years across. Before 1960 or so, we had no way to know whether or not every nearby star was actively sending our way. We are less worried now about a crowded dial. Those common fainter stars do not promise much; they seem unlikely to warm a planet steadily and safely, and there are sun-like stars by the billion farther out .
The marvelous 1999 discovery of a planet in orbit around a distant sun-like stable star has shown that planets resembling our own neighbor Jupiter accompany a few percent of all the sun-like stars we have examined near us, out as far as 150 light years or so. We know about two dozen such sun-planet systems, though not yet one earth-like planet, for our present methods are too crude to detect such small rocky planets as Earth even if they are present. We can presently find only the gas giants. That should change within the next decade or so, once we have launched new space-borne instruments able to detect earths-- if they are there.
Looking for Peers
Begin with symmetry, which promises the possibility of life and astronomers resembling ourselves in goal, if not in appearance. To find 100 candidates we need to examine many planetary systems, even if we make the most optimistic guess that earths –so far not seen near any star but our Sun —will be found, and not merely the hot Jupiters in tight orbits about their star that now fill our initial lists. An optimist would propose looking among the billion stars up to 1000 light years away, our near galactic neighbors. It follows at once that we should listen for hundreds of years before we need try to send, on the good grounds that we are not likely to be the first astronomers among hundreds of star-warmed earths. Even though we cannot exclude our priority , we cannot claim any evidence for it, save our own single example. It makes sense to listen for a century or two before we enter after consensus on a serious phase of transmission on our own —systematic sending is far more costly than listening . So we say "keep on listening", and improve our efforts, possibly with other signal types, until some day, maybe in the year 2100, the issue of sending might be raised.
But we know this: we did not begin radio astronomy (or lasers or gammas or neutrinos or what you will) on a new planet. Rather, life grew here on Earth for about half the age of the galaxy before we humans even knew that the sun was a star among the stars. Our single species itself was five hundred centuries old before we knew our place in the sky. Reflection has led me to argue that we had to number on the order of a billion thinking human beings before our earth could become home to such devices as sensitive microwave receivers, longer still for the alternatives. Only a population at such scale can have given rise to the innumerable special discoveries, skills, insights and resources that comprise modern technology: from copper to mathematics, with theory and practice worldwide that underlie all of astronomy and its imaginative dreams. But can that specialization appear if many more people still have not long been growing crops, digging in the mines, voyaging, writing, drawing-- yes, and dreaming. A hunting band, however wise its individuals, is not persuasive as a realistic basis for interstellar signaling, our SETI. A billion humans lived on earth around 1800 (of course, I mean only that magnitude, not a precise figure). Technology is a social phenomenon spread among billions whose diverse lifework led to what we now can do. IQ does not alone create means for detecting signals from the stars. Intelligence is necessary but it is insufficient. The first firemakers of the caves, the Cro-Magnon flint knappers, the Europeans around Galileo and Newton—admirable discoverers all, but none could make an interstellar search with any chance of success.
A rough estimate of one social, adept, thoughtful species of a billion members implies a few billion years of evolution as far as we now know—true, though from only one example. That tentatively defines the time scale. Without a synchronizing feature we do not notice at all in the Galaxy, we cannot expect a good match to our quite new status, for we have known of radio astronomy for less than one century out of all earth history. One sees at once that detectable counterparts are likely to be ahead of our level of technology, while the rest of them, still silent and undetectable, are far behind on the human scale of time.
What we see as possible is an ambitious project, old by any human standards, defined by a timescale we do not know, undertaken perhaps intermittently by some curious, effective, but by no means all-powerful, species of billions of beings of our technological kind dwelling somewhere out there among the many, many stars. They too must pay the energy bills and await an answer, perhaps not for the first time. Their presence is a conjecture only. Most likely they reside among the tens of millions of planetary systems which we now expect through a very small, close-by sample of two dozen gaseous planets. A planet near a stable sun is the lowest rung of a ladder of still only conjectural nature that may follow the one case we now know by upward rung after rung to another earth, to life, to evolution, and eventually—perhaps-- to sentience, curiosity, and capability. SETI is an audacious but direct search for the top rung of the ancient ladder.
Digging for more evidence, finally the physical signals we hope for, SETI is the task of our time. It will last many years, until we succeed or at least until we accept the other amazingly unlikely case, that we are the very first to attempt distant exchange among the 400 billion stars of the Milky Way. After all, if we are to hear any authentic signal among the stars that functionally resembles what we do, such conjectures must have been true and enacted in reality somewhere out there in earlier times.
Close with a salutation very old among our clever forebears: Good hunting!