by Carl Sagan
We are like the inhabitants of an isolated valley in New Guinea who communicate with societies in neighboring valleys (quite different societies, I might add) by runner and by drum. When asked how a very advanced society will communicate, they might guess by an extremely rapid runner or by an improbably large drum. They might not guess a technology beyond their ken. And yet, all the while, a vast international cable and radio traffic passes over them, around them, and through them.
At this very moment the messages from another civilization may be wafting across space, driven by unimaginably advanced devices, there for us to detect them–if only we knew how. Perhaps the message will come via radio waves to be detected by large radio telescopes. Or perhaps by more arcane devices, the modulation of X-ray stars, gravity waves, neutrinos, tachyons, or transmission channels that no one on Earth will dream of for centuries. Or perhaps the messages are already here, present in some everyday experience that we have not made the right mental effort to recognize. The power of such an advanced civilization is very great. Their messages may lie in quite familiar circumstances.
Consider, for example, seashells. Everyone knows the “sound of the sea” to be heard when putting a seashell to one’s ear. It is really the greatly amplified sound of our own blood rushing, we are told. But is this really true? Has this been studied? Has anyone attempted to decode the message being sounded by the seashell? I do not intend this example as literally true, but rather as an allegory. Somewhere on Earth there may be the equivalent of the seashell communications channel. The message from the stars may be here already. But where?
We will listen for the interstellar drums, but we will miss the interstellar cables. We are likely to receive our first messages from the drummers of the neighboring galactic valleys–from civilizations only somewhat in our future. The civilizations vastly more advanced than we will be, for a long time, remote both in distance and accessibility. At a future time of vigorous interstellar radio traffic, the very advanced civilizations may be, for us, still insubstantial legends.
32. The Night Freight to the Stars
For three generations of human beings there was–as an ever-present, but almost unperceived, part of their lives–a sound that beckoned, a call that pierced the night, carrying the news that there was a way, not so very difficult, to leave Twin Forks, North Dakota, or Apalachicola, Florida, or Brooklyn, New York. It was the wail of the night freight, as haunting and evocative as the cry of the loon. It was a constant reminder that there were vehicles, devices, which, if boarded, could propel you at high velocity out of your little world into a vaster universe of forests and deserts, seacoasts and cities.
Especially in the United States, but perhaps over much of the world, few people today travel by train. There are whole generations growing up which have never heard that siren call. This is the moment of the homogenization of the world, when the diversities of societies are eroding, when a global civilization is emerging. There are no exotic places left on Earth to dream about.
And for this reason there remains an even greater and more poignant need today for a vehicle, a device, to get us somewhere else. Not all of us; only a few–to the deserts of the Moon, the ancient seacoasts of Mars, the forests of the sky. There is something comforting in the idea that one day a few representatives of our little terrestrial village might venture to the great galactic cities.
There are as yet no interstellar trains, no machines to get us to the stars. But one day they may be here. We will have constructed them or we will have attracted them.
And then there will once again be the whistle of the night freight. Not the antique sort of whistle, for sound does not carry in interplanetary space or in the emptiness between the stars. But there will be something, perhaps the flash from magnetobrehmstrahlung, as the starship approaches the velocity of light. There will be a sign.
Looking out on a clear night from the continent-sized cities and vast game preserves that may be our future on this planet, youngsters will dream that when they are grown, if they are very lucky, they will catch the night freight to the stars.
33. Astroengineering
In a by now much quoted and possibly even apocryphal story, the nuclear physicist Enrico Fermi asked, during a luncheon conversation at Los Alamos in the middle 1940s, “Where are they?” If there are vast numbers of beings more advanced then we, he was musing, why have we seen no sign of them–by a visitation to Earth, for example?
We have discussed this problem in Chapters 27 and 28. But there is another formulation of Fermi’s question. A civilization a hundred years in our technological future (assuming present rates of technological growth) would surely be able to communicate by radio, and possibly by other techniques, anywhere in the Galaxy and probably with other galaxies as well. A civilization thousands of years in our technological future will very likely be able to travel physically between the stars, although with the expenditure of considerable time and resources.
But what of civilizations tens of thousands or hundreds of thousands of years in our future, or even farther advanced? There are, after all, stars billions of years older than the Sun. The very oldest such stars lack heavy metals; probably their planets are similarly lacking. Such very old stars are inhospitable environments for the development of technological civilizations. But some stars one or two billion years older than the Sun have no such attendant difficulties. It is surely possible that there are at least a few civilizations hundreds of millions or billions of years in our technological future.
With prodigious energy resources, such civilizations should be able to rework the cosmos. We have discussed in Chapter 22 how life on Earth has already altered our planet significantly and how we can envision in the relatively near future making important changes in the environments of the nearby planets.
More major changes are possible in the somewhat more distant future. The mathematician Freeman Dyson, of the Institute for Advanced Study, offers a scheme in which the planet Jupiter is broken down piece by piece, transported to the distance of the Earth from the Sun, and reconstructed into a spherical shell–a swarm of individual fragments revolving about the Sun. The advantage of Dyson’s proposal is that all of the sunlight now wasted by not falling upon an inhabited planet could then be gainfully employed; and a population greatly in excess of that which now inhabits the Earth could be maintained. Whether such a vast population is desirable is an important and unsolved question. But what seems clear is that at the present rate of technological progress it will be possible to construct such a Dyson sphere in perhaps some thousands of years. In that case, other civilizations older than we may have already constructed such spherical swarms.
A Dyson sphere absorbs visible light from the Sun. But it does not continue indefinitely to absorb this light without re-radiating; otherwise, the temperature would become impossibly high. The exterior of the Dyson sphere radiates infrared radiation into space. Because of the large dimensions of the sphere, the infrared flux from a Dyson sphere should be detectable over quite sizable distances–with present infrared technology, over distances of hundreds to thousands of light-years. Remarkably enough, large infrared objects of roughly Solar System dimensions and of temperatures less than 1,000 degrees Fahrenheit have been detected in recent years. These, of course, are not necessarily Dyson civilizations. They may be vast dust clouds surrounding stars in the process of formation. But we are beginning to detect objects that are not dissimilar to the artifacts of advanced civilizations.
There are many phenomena in contemporary astronomy that are not understood. Quasars, for example, are one. The reported very high-intensity gravitational waves coming from the center of our galaxy are another. The list can be extended considerably. As long as we do not understand these phenomena, we cannot exclude the possibility that they are manifestations of extraterrestrial intelligence. This hardly demonstrates the likelihood of extraterrestrial intelligence, any more than our inability to understand seasonal changes on Mars (Chapter 19) provi
ded strong evidence for vegetation on that planet. As the Soviet astrophysicist I. S. Shklovskii says, “Following the principles of law, we should assume all astronomical phenomena natural until proven otherwise.”
Some scientists have asked, in the reformulation of Fermi’s question, why it is that advanced civilizations are not much more obvious. Why have stars not been rearranged into entirely artificial patterns in the sky–perhaps blinking advertising lights, detectable over intergalactic distances, for some cosmic soft drink? This particular example is, of course, not very tenable–one society’s soft drink may be another society’s poison. More seriously, the manifestations of very advanced civilizations may not be in the least apparent to a society as backward as we, any more than an ant performing his anty labors by the side of a suburban swimming pool has a profound sense of the presence of a superior technical civilization all around him.
34. Twenty Questions: A Classification of Cosmic Civilizations
To deal with the possibility of enormously advanced extraterrestrial civilizations, the Soviet astrophysicist N. S. Kardashev has proposed a distinction in terms of the energy available to a civilization for communications purposes.
A Type I civilization is able to muster for communications purposes the equivalent of the entire present power output of the planet Earth–which is now used for heating, electricity, transportation, and so on; a large variety of purposes other than communication with extraterrestrial civilizations. By this definition the Earth is not yet a Type I civilization.
The power usage of our civilization is growing at a rapid rate. The present power output of planet Earth is something like 1015 or 1016 watts; that is, a million billion to ten million billion watts. The standard exponential notation simply indicates the number of zeros following the 1. For example, 1015 means fifteen zeros after the 1. The concept of power in physics is that of an energy expenditure per unit time. One watt is ten million ergs of energy expended per second. All of the power used on the Earth is thus equivalent to lighting up, say, one hundred trillion hundred-watt bulbs. Especially if this energy were put out in the radio part of the spectrum, it might be detected over very sizable distances.
A Type II civilization is able to use for communications purposes a power output equivalent to that of a typical star, about 1026 watts. We already see particularly bright stars at optical frequencies in the nearest galaxies. A Type II civilization, putting out in our direction 1026 watts in some fairly narrow radio bandpass, could be detectable over vast inter-galactic distances. It would be easily detectable, if we used the right search procedures, were there only one such civilization in the nearest spiral galaxy to our own, M31, the great galaxy in the constellation Andromeda. M31 is by no means the largest galaxy. For example, an elliptical galaxy, M87–also known as Virgo A–contains perhaps 10 trillion stars.
Finally, Kardashev imagines a Type III civilization, which would use for communications purposes the energy output of an entire galaxy, roughly 1036 watts. A Type III civilization beaming at us could be detected if it were anywhere in the universe. There is no provision for a Type IV civilization, which by definition talks only to itself. There need not be many Type II or Type III civilizations for their presence to be felt once a search for extraterrestrial civilizations is organized in earnest. It may well be that a few Type II or Type III civilizations would be far more readily detectable than a large number of Type I civilizations–if they choose to signal us (see Chapter 31).
The energy gap between a Type I and a Type II civilization or between a Type II and a Type III civilization is enormous–a factor of about ten billion in each instance. It seems useful, if the matter is to be considered seriously, to have a finer degree of discrimination. I would suggest Type 1.0 as a civilization using 1016 watts for interstellar communication; Type 1.1, 1017 watts; Type 1.2, 1018 watts, and so on. Our present civilization would be classed as something like Type 0.7.
But there may be more significant ways to characterize civilizations than by the energy they use for communications purposes. An important criterion of a civilization is the total amount of information that it stores. This information can be described in terms of bits, the number of yes-no statements concerning itself and the universe that such a civilization knows.
An example of this concept is the popular game of “Twenty Questions,” as played on Earth. One player imagines an object or concept and makes an initial classification of it into animal, vegetable, mineral, or none of these three. To identify the object or concept, the other players then have a total of twenty questions, which can only be answered “Yes” or “No.” How much information can be discriminated in this manner?
The initial characterization can be thought of as three yes-no questions: Conceptual or objective? Biological or nonbiological? Plant or animal? If we agree that a particular game of “Twenty Questions” is in pursuit of something alive, we have, in effect, answered three questions already by the time the game begins. The first question divided the universe into two (unequal) pieces. The second question divided one of those pieces into two more, and the third divided one of those pieces into yet two more. At this stage we have divided the universe crudely into 2×2×2=23=8 pieces. When we have finished with our twenty questions, we have “divided the universe into 220 additional (probably unequal) pieces. Now, 210 is 1,024. We can perform such calculations fairly quickly if we approximate 210 by 1,000=103; therefore, 220 equals (210)2, which approximately equals (103)2=106. The total number of effective questions, twenty-three, has divided the universe into about 223, or approximately 107 pieces or bits. Thus, it is possible for skillful players to win at “Twenty Questions” only if they live in a civilization that has an information content of about 107 bits.
But, as I discuss below, our civilization is characterized by perhaps 1014 bits. Therefore, skillful players should win at “Twenty Questions” only about 107 out of 1014 times, or one in 107, or one in ten million times. That the game is won more often in practice is because there is an additional rule–usually unstated but well understood: Namely, that the object or concept being named should be one in the general cultural heritage of all the players. But this must mean that 107 bits can convey a great deal of information about a civilization, as indeed it can. Philip Morrison has estimated that the total written contribution to our present civilization from classical Greek civilization is only about 109 bits. Thus, a one-way message, containing what, by the standards of modern radio astronomy, is a very small number of bits, can contain a very significant amount of new information and can have a powerful influence on a society in the long run.
What is the total number of bits in an English word? In all the books in the world? There are in general English usage twenty-six letters and a sprinkling of punctuation marks. Let us estimate that there are thirty-two such effective “letters.” But 32=25; that is, there are something like five bits per letter. If a typical word has four to six letters (for an average of six letters a word, there would have to be a lot of fancy words), there will then be about twenty to thirty bits per word. A typical book–about three hundred words per page and about three hundred pages–would have about a hundred thousand words, or about three million bits. The largest libraries in the world, such as the British Museum, the Bodleian Library at Oxford, the New York Public Library, the Widener Library at Harvard, and the Lenin Library in Moscow, have no more than about ten million volumes. This is about 3×1013 bits.
A poor-quality low-resolution photograph may have a million bits in it. A quite complex caricature or cartoon might have only about a thousand bits. On the other hand, a large, high-quality color photograph or painting might have about a billion bits. Let us make allowances for the amount of fundamental information contained in graphics, photography, and art in our civilization, as well as the recorded oral tradition. Let us also try to estimate–this can be done only very crudely–the information we are born with about how to deal with the world. (Human beings are, relative to other animals,
born with very little such information–we deal with the world much more in terms of learned rather than inherited or instinctual information.) I estimate, then, that we and our civilization can be very well characterized by something like 1014 or 1015 bits.
Parenthetically, the ancient Chinese saying that a single picture is worth ten thousand words (three hundred thousand bits in English; but in Chinese?) is approximately correct–provided that the picture is not too complex.
We can imagine civilizations that have a much greater number of bits characterizing their society than characterizes ours. In general, we would expect a civilization high on the energy scale to be high on the information scale. But this need not necessarily be true. I certainly can imagine societies that are very complex and require many more bits to characterize them than our society requires–but that are not interested in interstellar communication. Characterization of interstellar civilizations requires us to characterize their information content as well.
If we have used numbers to describe energy, we should perhaps use letters to describe information. There are twenty-six letters in the English alphabet. If each corresponds to a factor of ten in the number of bits, there is the possibility of characterizing with the English alphabet a range of information contents over a factor of 1026–a very large range, which seems adequate for our purposes. I propose calling a Type A civilization one at the “Twenty Questions” level, characterized by 106 bits. In practice this is an extremely primitive society–more primitive than any human society that we know well–and a good beginning point. The amount of information we have acquired from Greek civilization would characterize that civilization as Type C, although the actual amount of information that characterized Periclean Athens is probably equivalent to Type E or so. By these standards, our contemporary civilization, if characterized by 1014 bits of information, corresponds to a Type H civilization.