by Prof. Carl Sagan
Suppose we have arranged a meeting at an unspecified place in New York City with a stranger we have never met and about whom we know nothing – a rather foolish arrangement, but one that is useful for our purposes. We are looking for him, and he is looking for us. What is our search strategy? We probably would not stand for a week on the corner of Seventy-eighth Street and Madison Avenue.
Instead, we would recall that there are a number of widely known landmarks in New York City – as well known to the stranger as to us. He knows we know them, we know he knows we know them, and so on. We then shuttle among these landmarks: The Statue of Liberty, the Empire State Building, Grand Central Station, Radio City Music Hall, Lincoln Center, the United Nations, Times Square, and just conceivably, City Hall. We might even indulge ourselves in a few less likely possibilities, such as Yankee Stadium or the Manhattan entrance to the Staten Island Ferry. But there are not an infinite number of possibilities. There are not millions of possibilities; there are only a few dozen possibilities, and in time we can cover them all.
The situation is just the same in the frequency-search strategy for interstellar radio communication. In the absence of any prior contact, how do we know precisely where to search? How do we know which frequency or “station” to tune in on? There are at least millions of possible frequencies with reasonable radio bandpasses. But a civilization interested in communicating with us shares with us a common knowledge about radio astronomy and about our Galaxy. They know, for example, that the most abundant atom in the universe, hydrogen, characteristically emits at a frequency of 1,420 Megahertz. They know we know it. They know we know they know it. And so on. There are a few other abundant interstellar molecules, such as water or ammonia, which have their own characteristic frequencies of emission and absorption. Some of these lie in a region of the galactic radio spectrum where there is less background noise than others. This is also shared information. Students of this problem have come up with a short list of possibly a dozen frequencies that seem to be the obvious ones to examine. It is even conceivable that water-based life will communicate at water frequencies, ammonia-based life at ammonia frequencies, etc.
There appears to be a fair chance that advanced extraterrestrial civilizations are sending radio signals our way, and that we have the technology to receive such signals. How should a search for these signals be organized? Existing radio telescopes, even very small ones, would be adequate for a preliminary search. Indeed, the ongoing search at the Gorky Radiophysical Institute, in the Soviet Union, involves telescopes and instrumentation that are quite modest by contemporary standards.
The amiable and capable president of the Soviet Academy of Sciences, M. V. Keldysh, once told me, with a twinkle in his eye, that “when extraterrestrial intelligence is discovered, then it will become an important scientific problem.” A leading American physicist has argued forcefully with me that the best method to search for extraterrestrial intelligence is simply to do ordinary astronomy; if the discovery is to be made, it will be made serendipitously. But it seems to me that we can do something to enhance the likelihood of success in such a search, and that the ordinary pursuit of radio astronomy is not quite the same as an explicit search of certain stars, frequencies, bandpasses, and time constants for extraterrestrial intelligence.
But there are enormous numbers of stars to investigate, and many possible frequencies. A reasonable search program will almost certainly be a very long one. Such a search, using a large telescope full time, should take at least decades, by conservative estimates. The radio observers in such an enterprise, no matter how enthusiastic they may be about the search for extraterrestrial intelligence, would very likely become bored after many years of unsuccessful searching. A radio astronomer, like any other scientist, is interested in working on problems that have a high probability of more immediate results.
The ideal strategy would involve a large telescope that could devote something like half time to the search for extraterrestrial intelligent radio signals and about half time to the study of more conventional radioastronomical objectives, such as planets, radio stars, pulsars, interstellar molecules, and quasars. The difficulty in using several existing radio observatories, each for, say, 1 percent of their time, is that these activities would have to be pursued for many centuries to have a reasonable probability of success. Since the time on existing radio telescopes is mainly spoken for, larger allocations of time seem unlikely.
A wide variety of objects obviously should be examined: G-type stars, like our own; M-type stars, which are older; and exotic objects, which may be black holes or possible manifestations of astroengineering activities. The number of stars and other objects in our own Milky Way Galaxy is about two hundred billion, and the number that we must examine to have a fair chance of detecting such signals seems to be at least millions.
There is an alternative strategy to searching painfully each of millions of stars for the signals from a civilization not much more advanced than our own. We might examine an entire galaxy all at once for signals from civilizations much more advanced than ours . A small radio telescope can point at the nearest spiral galaxy to our own, the great galaxy M31 in the constellation Andromeda, and simultaneously observe some two hundred billion stars. Even if many of these stars were broadcasting with a technology only slightly in advance of our own, we would not pick them up. But if only a few are broadcasting with the power of a much more advanced civilization, we would detect them easily. In addition to examining nearby stars only slightly in advance of us, it therefore makes sense to examine, simultaneously, many stars in neighboring galaxies, only a few of which may have civilizations greatly in advance of our technology.
We have been describing a search for signals beamed in our general direction by civilizations interested in communicating with us. We ourselves are not beaming signals in the direction of some specific other star or stars. If all civilizations listened and none transmitted, we would each reach the erroneous conclusion that the Galaxy was unpopulated, except by ourselves. Accordingly, it has been proposed – as an alternative and much more expensive enterprise – that we also “eavesdrop”; that is, tune in on the signals that a civilization uses for its own purposes, such as domestic radio and television transmission, radar surveillance systems, and the like. A large radio telescope devoting half time to a rigorous search for intelligent extraterrestrial signals beamed our way would cost tens of millions of dollars (or rubles) to construct and operate. An array of large radio telescopes, designed to eavesdrop to a distance of some hundreds of lightyears, would cost many billions of dollars.
In addition, the chance of success in eavesdropping may be slight. One hundred years ago we had no domestic radio and television signals leaking out into space. One hundred years from now the development of tight beam transmission by satellites and cable television and new technologies may mean that again no radio and television signals would be leaking into space. It may be that such signals are detectable only for a few hundred years in the multibillion-year history of a planet. The eavesdropping enterprise, in addition to being expensive, may also have a very small probability of success.
The situation we find ourselves in is rather curious. There is at least a fair probability that there are many civilizations beaming signals our way. We have the technology to detect these signals out to immense distances – to the other side of the Galaxy. Except for a few back-burner efforts in the United States and the Soviet Union, we – that is, mankind – are not carrying out the search for extraterrestrial intelligence. Such an enterprise is sufficiently exciting and, at last, sufficiently respectable that there would be little difficulty in staffing a radio observatory designed for this purpose with devoted, capable, and innovative scientists. The only obstacle appears to be money.
While not small change, some tens of millions of dollars (or rubles) is, nevertheless, an amount of money well within the reach of wealthy individuals and foundations. In fact, there is in astronomy a long and proud history of observatories funded by private individuals and foundations: The Lick Observatory, on Mount Hamilton, California, by Mr. Lick (who wanted to build a pyramid, but settled for an observatory – in the base of which he is buried); the Yerkes Observatory in Williams Bay, Wisconsin, by Mr. Yerkes; the Lowell Observatory in Flagstaff, Arizona, by Mr. Lowell; and the Mount Wilson and Mount Palomar Observatories in Southern California, by a foundation established by Mr. Carnegie. Government money will probably be forthcoming for such an enterprise eventually. After all, it costs about the same as the replacement costs of U.S. aircraft shot down over Vietnam in Christmas week, 1972. But a radio telescope designed for communication with extraterrestrial intelligence and an attached institute of exobiology would make a very fitting personal memorial for someone.
—The cosmic connection; 163: 1973