But along with the burgeoning dedication to a serious search, a slightly negative note has emerged which is nevertheless very interesting. A few scientists have lately asked a curious question: If extraterrestrial intelligence is abundant, why have we not already seen its manifestations? Think of the advances by our own technical civilization in the past ten thousand years and imagine such advances continued over millions or billions of years more. If only a tiny fraction of advanced civilizations are millions or billions of years more advanced than ours, why have they not produced artifacts, devices or even industrial pollution of such magnitude that we would have detected it? Why have they not restructured the entire Galaxy for their convenience?
Skeptics also ask why there is no clear evidence of extraterrestrial visits to Earth. We have already launched slow and modest interstellar spacecraft. A society more advanced than ours should be able to ply the spaces between the stars conveniently if not effortlessly. Over millions of years such societies should have established colonies, which might themselves launch interstellar expeditions. Why are they not here? The temptation is to deduce that there are at most a few advanced extraterrestrial civilizations-either because statistically we are one of the first technical civilizations to have emerged or because it is the fate of all such civilizations to destroy themselves before they are much further along than we.
It seems to me that such despair is quite premature. All such arguments depend on our correctly surmising the intentions of beings far more advanced than ourselves, and when examined more closely I think these arguments reveal a range of interesting human conceits. Why do we expect that it will be easy to recognize the manifestations of very advanced civilizations? Is our situation not closer to that of members of an isolated society in the Amazon basin, say, who lack the tools to detect the powerful international radio and television traffic that is all around them? Also, there is a wide range of incompletely understood phenomena in astronomy. Might the modulation of pulsars or the energy source of quasars, for example, have a technological origin? Or perhaps there is a galactic ethic of noninterference with backward or emerging civilizations. Perhaps there is a waiting time before contact is considered appropriate, so as to give us a fair opportunity to destroy ourselves first, if we are so inclined. Perhaps all societies significantly more advanced than our own have achieved an effective personal immortality and lose the motivation for interstellar gallivanting, which may, for all we know, be a typical urge only of adolescent civilizations. Perhaps mature civilizations do not wish to pollute the cosmos. There is a very long list of such “perhapses,” few of which we are in a position to evaluate with any degree of assurance.
The question of extraterrestrial civilizations seems to me entirely open. Personally, I think it far more difficult to understand a universe in which we are the only technological civilization, or one of a very few, than to conceive of a cosmos brimming over with intelligent life. Many aspects of the problem are, fortunately, amenable to experimental verification. We can search for planets of other stars, seek simple forms of life on such nearby planets as Mars, and perform more extensive laboratory studies on the chemistry of the origin of life. We can investigate more deeply the evolution of organisms and societies. The problem cries out for a long-term, open-minded, systematic search, with nature as the only arbiter of what is or is not likely.
If there are a million technical civilizations in the Milky Way Galaxy, the average separation between civilizations is about 300 light-years. Since a light-year is the distance that light travels in one year (a little under 6 trillion miles), this implies that the one-way transit time for an interstellar communication from the nearest civilization is some 300 years. The time for a query and a response would be 600 years. This is the reason that interstellar dialogues are much less likely-particularly around the time of first contact-than interstellar monologues. At first sight, it seems remarkably selfless that a civilization might broadcast radio messages with no hope of knowing, at least in the immediate future, whether they have been received and what the response to them might be. But human beings often perform very similar actions as, for example, burying time capsules to be recovered by future generations, or even writing books, composing music and creating art intended for posterity. A civilization that had been aided by the receipt of such a message in its past might wish similarly to benefit other emerging technical societies.
For a radio search program to succeed, the Earth must be among the intended beneficiaries. If the transmitting civilization were only slightly more advanced than we are, it would possess ample radio power for interstellar communication-so much, perhaps, that the broadcasting could be delegated to relatively small groups of radio hobbyists and partisans of primitive civilizations. If an entire planetary government or an alliance of worlds carried out the project, the broadcasters could transmit to a very large number of stars, so large that a message is likely to be beamed our way, even though there may be no reason to pay special attention to our region of the sky.
It is easy to see that communication is possible, even without any previous agreement or contact between transmitting and receiving civilizations. There is no difficulty in envisioning an interstellar radio message that unambiguously arises from intelligent life. A modulated signal (beep, beep-beep, beep-beep-beep…) comprising the numbers 1, 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31-the first dozen prime numbers-could have only a biological origin. No prior agreement between civilizations and no precautions against Earth chauvinism are required to make this clear.
Such a message would be an announcement, or beacon signal, indicating the presence of an advanced civilization but communicating very little about its nature. The beacon signal might also note a particular frequency where the main message is to be found, or might indicate that the principal message can be found at higher time resolution at the frequency of the beacon signal. The communication of quite complex information is not very difficult, even for civilizations with extremely different biologies and social conventions. Arithmetical statements can be transmitted, some true and some false, each followed by an appropriate coded word (in dahs and dits, for example), which would transmit the ideas of true and false, concepts that many people might guess would be extremely difficult to communicate in such a context.
But by far the most promising method is to send pictures. A repeated message that is the product of two prime numbers is clearly to be decoded as a two-dimensional array, or raster-that is, a picture. The product of three prime numbers might be a three-dimensional still picture or one frame of a two-dimensional motion picture. As an example of such a message, consider an array of zeros and ones which could be long and short beeps or tones on two adjacent frequencies, or tones of different amplitudes, or even signals with different radio polarizations. In 1974 such a message was transmitted to space from the 305-meter antenna at the Arecibo Observatory in Puerto Rico, which Cornell University runs for the National Science Foundation. The occasion was a ceremony marking the resurfacing of the Arecibo dish, the largest radio/radar telescope on the planet Earth. The signal was sent to a collection of stars called M13, a globular cluster comprising about a million separate suns which happened to be overhead at the time of the ceremony. Since M13 is 24,000 light-years away, the message will take 24,000 years to arrive there. If any responsive creature is listening, it will be 48,000 years before we receive a reply. The Arecibo message was clearly intended not as a serious attempt at interstellar communication, but rather as an indication of the remarkable advances in terrestrial radio technology.