Murmurs of Earth

by Carl Sagan

  About six months after the Green Bank meeting, it occurred to me that a powerful way to send unambiguous interstellar messages was to transmit pictures resembling ordinary television pictures, a procedure very similar to the procedure by which human infants learn to speak. Infants are shown objects and told what the names of these objects are. Would it not make sense in the interstellar context to send images of things and attach to them some linguistic representation, which could then be used to construct sophisticated texts? With a little experimenting, I found that quite a good picture could be made by simply drawing the picture with black and white spots. It is of course useful to have intermediate shades of gray, but in fact the prime characteristics of an image can be expressed quite adequately in nothing more than black and white. The picture could be drawn by taking a rectangular grid of points and making the points black or white, thus constructing the desired pictorial representation. The sequence of black and white spots that make up the picture could then be sent as a sequence of two characters (one for black and one for white), two tones, or perhaps dots and dashes. It should be quite trivial for any intelligent civilization to decode.

  As an example of how this might work, I constructed a picture made up of 551 characters. Why 551? Well, it is the product of 19 and 29—prime numbers—and I thought this would help extraterrestrials understand how to lay out the message. It was made in a rectangular grid 29 units high and 19 units across. When extraterrestrials recognize that the number 551 can be divided by 19 and 29 and by nothing else except 1 and 551, it should suggest that the sequence of characters received in the message should be laid out in a format of either 19 rows of 29 characters each, or 29 rows of 19 characters. Which is right must be discovered by trial and error, but it takes only a few minutes to do that.

  The message constructed to demonstrate this procedure is shown in Figure 1. At the time, I did it almost as a joke, just to show that if one doesn’t try to be too highfalutin, one can come up with a pretty good interstellar language. As a game, I sent out the message, no hints given, to all the members of the Order of the Dolphin and challenged them to deciper its contents. It has surprised me ever since that this exercise in interstellar linguistics, which started out primarily as a source of entertainment, has over the years taken on the aura of a great breakthrough in human intellectual achievements.

  In fact, the 551-character message contained a great deal of information—indeed, more than would be allowed by the conventional wisdom of information theory. According to that theory, 551 characters should contain the same information content as about 25 English words. Yet there is much more than 25 words’ worth in the message. This is because the message as constructed uses shared concepts of physics and astronomy to impart information, as it were, in a shorthand way.

  The decryption is shown in Figure 2, in which the message has been laid out in 29 groups of 19 and the 1’s have been made black and the 0’s white. What is the message content? I tried to depict what was then thought to be of most interest to another civilization. We see an image of the intelligent creature who composed the message; we see along the left-hand margin of the message a diagram of the creature’s solar system, starting with the star at the top, four minor planets, one intermediate planet, two major planets, an intermediate planet, and a final minor planet. In the upper right-hand corner, we see drawings of the atoms of carbon and oxygen, an indication that the creature’s life system is like ours, based on carbon, and that the creature’s chemistry depends on oxygen in an important way just as ours does. This would indicate to us, of course, that the creature is chemically similar to us.

  Figure 1

  The 551-character “message” constructed to show how a television picture might be sent simply, and to test the skills of the members of the Order of the Dolphin. The 0 and 1 characters are used merely to indicate that only two characters are used. In practice, the message might consist of dots and dashes or pulses and spaces or two different tones, as in the Arecibo message of 1974.

  Figure 2

  The decryption of the 551-character message, in which the characters of Figure 1 have been laid out in 29 rows of 19 characters, the 1’s have been made into black squares and the 0’s have been made white.

  The characters just below and to the right of the sun are the most difficult to decipher. In fact they are the numerals 1, 2, 3, 4, and 5 written in a binary code. Indeed, this is the simplest numbering system: numbers are based on two rather than the bizarre number ten derived from our fingers. An extra character is added as necessary such that the number of black characters is always odd. In this way, a numbering system is established, and the means by which one can identify what is a number from what is not are clearly identified. If there is an odd number of characters or “bits,” then that is a numeral. This method allows us to interpret the three large groups below the atomic symbols. These are groups having an odd number of black characters; therefore we know they are numerals. They are, from the top, 5, 2,000, and 4 billion, approximately. The lowest group is connected by a diagonal line to the figure of the intelligent creature. We can’t be sure what these numbers mean, but we notice that they are written alongside planets 2, 3, and 4, suggesting perhaps that there are 4 billion of the creatures on planet 4, evidently its home planet; 2,000 on planet 3, indicating the existence of a large colony on that planet; and finally a group of 5 on planet 2, implying that an exploration on the planet is proceeding. Below the creature is a character sequence; it has an even number of bits, and therefore it can’t be a numeral. What else can it be? A word? We can’t be sure, but we can guess that the creature is giving us a name, four bits, so that in future messages when the creature wishes to refer to itself it doesn’t have to draw a picture but can simply give us the name Four Bits.

  Lastly, on the lower right-hand margin, there is a diagram that evidently tells us the size of the creature. The creature is thirty-one somethings high. What can these somethings be? The only length we have in common is the wavelength in the radio spectrum on which this message was transmitted. It is believed that the optimum wavelengths for interstellar communication are about ten centimeters, which would make the creature about ten feet high, and this would, of course, make us feel ten feet high for understanding the contents of this message.

  And now, a depressing shock: almost none of the elite members of the Order of the Dolphin were able to interpret this message. Never having seen this kind of message before, it just didn’t occur to them to try the picture format. Nowadays thousands of people are aware of this possible format and readily decode it. In any case, in those ancient times, my letters to the members of the Order of the Dolphin enclosing this message elicited only one reply. Back from Barney Oliver came a new message consisting of a sequence of 0’s and 1’s. But at least I knew what to look for, and indeed it was another very simple and inspiring message, containing just one image: that of a martini glass with an olive in it!

  The 551-character message was perhaps quite difficult to interpret even after one did discover the picture format. This was because the message was very tightly structured with all of the symbols and sketches very close together. To one not familiar with such messages it was very difficult to tell what part of the picture was connected to another part of the picture, or what was in fact unrelated to something in the picture. Taking this into account, Barney soon constructed a new message utilizing 1,271 bits, as shown in Figure 3. Its decryption is shown in Figure 4. As is obvious, it gives a much clearer picture. Of course, the information content is not so high, since a lot of the message is devoted to depicting nothing but blank space. But then the amount of ambiguity is much less. As can be seen by looking at this picture, even a small increase in the number of characters has allowed the transmission of much more information and in fact more sophisticated information. In the images of the intelligent creatures here, we see two sexes and a child. It becomes clear that the creatures are not born full-size but develop from smaller versions of the
mature ones.

  The construction of these two simple binary-code-picture messages demonstrated that there is a good means for reliable interstellar communication without prior contact. We need not fear a linguistic problem when we encounter other civilizations even if the means of contact are limited to very simple radio transmissions.

  Figure 3

  Barney Oliver’s 1,271-character message, written in the same way as the message of Figure 1.

  Figure 4

  The decryption of the 1,271-character message.

  In December 1969 our Arecibo Observatory sponsored a meeting of the American Astronomical Society in a big resort hotel in San Juan, Puerto Rico. During a coffee break, I was chatting with Carl Sagan when he related a fact realized by almost no one: the Pioneer 10 spacecraft soon to be launched on a mission to Jupiter would in fact swing by Jupiter and in the process gain enough velocity to escape the solar system. It would travel indefinitely among the stars to the farthest reaches of the galaxy. This point had first been recognized by a writer for the Christian Science Monitor, Eric Burgess, and a planetarium expert, Richard Hoagland, who had suggested that since this would be the first object to leave the solar system, it might well carry on it some message to any intelligent civilization that might someday happen to intercept the spacecraft. Carl had enthusiastically volunteered to construct such a message for NASA. The plan was to engrave something on a metal plate, the idea being that such a metal engraving in a space environment would remain recognizable for a period of perhaps billions of years.

  Now in this case, as many as 100,000 characters might possibly be inscribed on the plate if they were crowded in, and so it appeared that a much more sophisticated message than those of the early binary-code pictures could be sent. This time we desired to send some information about the nature of life on Earth and in particular to give the time and place of the Pioneer 10 launching.

  Carl asked me how we might express these facts well. In those few minutes before the meeting resumed, we talked about several methods, including the possibility of showing a map of the galaxy with a large number of binary stars indicated, thus perhaps establishing the location of Earth in the galaxy. Carl suggested the possibility of presenting a map showing the Big Dipper and a few other constellations. This would establish the time of launching to within about 10,000 years and the place of launch to within twenty or thirty light-years. At that time, I was very active in research on pulsars, the rapidly pulsating radio sources each of which has a distinct pulsing frequency, and it occurred to me that it would be a much more powerful approach to give the location of the earth with respect to a number of prominent pulsars. The characteristic pulsing frequency of each pulsar would very clearly identify which pulsar was which on the map. More than that, although the pulsing frequencies are well defined, they are gradually changing by a very small amount, which can be accurately measured, small as it is. Some change their pulsing periods by much less than a billionth of a second a day. The amount of change in the pulsar frequency between that shown in the engraving and that determined by those capturing the Pioneer spacecraft would give the time that had elapsed since the picture was drawn. Thus using a pulsar map would allow us to locate the place of origin of the spacecraft in the Milky Way galaxy and would also indicate with very high accuracy how long the spacecraft had been in flight before interception. We adopted this means of achieving the main goals of the Pioneer 10 plaque. This same map is shown on the cover of the Voyager record, and in the picture sequence, to mark the birthplace and cosmic birthday of the Voyager.

  Carl asked me if I could construct the pulsar map quickly because time was very short. I said yes and proceeded to do so. As I was doing this, Carl added a diagram of the solar system with relative distances of the planets and the Pioneer trajectory, and his wife Linda drew perhaps the most important information on the plaque, the figures of a man and a woman standing in front of a drawing of the spacecraft itself to give scale. The figures contained characteristics of all human races. The plaque is shown in Figure 5. One of these plaques was launched on each of the Pioneer 10 and 11 spacecraft. Both of these spacecraft have now flown past Jupiter and are on their way into the interstellar space as planned.

  The Pioneer 10 and 11 plaque, simple as it was, elicited a public reaction both amusing and amazing. The news and television media were confronted with the problem of showing this plaque in all its details despite the fact that naked humans were depicted on it. For some of the media this was a challenging first. In the Chicago Sun-Times, the editors made a frantic effort to airbrush out the sexual parts of the nude figures. For one edition to the next, all on the same day, one titillating bit of anatomy after another disappeared. In the Los Angeles Times, there were angry letters to the editor denouncing NASA for using taxpayers’ money to send “smut” into space. There were letters from outraged feminists protesting that the woman on the plaque appeared to be subservient to the man. Was she not in fact standing behind him, and why for heaven’s sake was the man’s hand raised and not hers? This all came as a shock to the artist, Linda Sagan, who felt herself to be a liberated woman.

  Figure 5

  The Pioneer 10 and 11 plaques. The image seen here was engraved on a six-by nine-inch gold anodized aluminum plate.

  Surprise and alarm was expressed by some who felt that the human figures resembled too closely their own race, whatever it might be. Surprisingly, this objection was voiced by members of all races; there must be some deep psychological truth here. Perhaps most important were complaints that this message was constructed by a very limited group of humans—in fact, three humans—and thus was neither representative of the human race as a whole nor perhaps as informative as it could be. There were editorials published in the British press demanding that any future similar enterprise be engineered by a large international ecumenical group of scientists and lay people.

  After this barrage of criticism, we decided that most of these criticisms had no merit, and we felt that no great mistake had been made. Still, they caused us to look toward the art of constructing interstellar messages with much more humility. We had discovered that there was widespread serious human concern about the content of interstellar messages, even though the chance of reception was very small.

  These thoughts were on my mind when the next opportunity to transmit an interstellar message appeared. It was in 1974, and we, at the National Astronomy and Ionosphere Center, had just finished installing a new reflector surface on the Arecibo 1,000-foot radio telescope, and a new transmitter with a power of about half a million watts. The power from this transmitter, when focused by the giant reflector, was to become the strongest signal leaving Earth. In fact, the signal was to be so strong that it was perhaps a million times brighter than the sun at a comparable wavelength, a signal easily detectable by radio telescopes no more sensitive than ours across interstellar distances of many thousands of light-years. A dedication ceremony was to be held at the Arecibo Observatory in November, and it seemed that a striking way to dedicate the “new” telescope would be to transmit an interstellar message from it. This time I sought advice from many more people—most of them scientists, to be sure, but still a much broader cross section than before.

  Figure 6

  The Arecibo message of November 1974. In this case, the two characters were sent by switching the radio transmitter between two radio frequencies about 75 cycles per second apart. The basic radio frequency was 2380 Megahertz. The characters were sent at a rate of 10 per second.

  Figure 7

  The decryption of the Arecibo message. In this case, the message is written as 73 groups of 23 characters.

  In this message, the same format was used as in the early messages—a black-and-white television picture. We wished to send about ten characters a second, both because this would cause the signal to be detectable to very large distances and because such a signal would sound pleasing to the dedication audience. We thought that about three minutes of such a transmiss
ion would be enough—more might be boring—and so a message consisting of 1,679 characters was chosen. The number 1,679 is the product of the prime numbers 73 and 23. The characters would be sent as two tones. The movie Close Encounters of the Third Kind uses a communication method that mimics this message, but on a trivial level.

  I took the ideas, which were suggested by a number of people, and combined them to produce the final message, which is shown in Figures 6 and 7. The first characters appear in the upper right-hand corner of the decryption, and the message reads from right to left and down. This message is considerably more informative than the previous messages, but still is probably among the simplest to decode. We were learning.

  The message starts with the numbers 1 through 10, again written in binary code, but intentionally squeezed against the top margin so that we were forced to adopt some convention to indicate how large numbers are to be written where there is not enough space to do so. In this way, the convention would be clearly established. This has to be done carefully and clearly because numbers appear in the message, written not only on several lines but in three orientations. Then follow the numbers 1, 6, 7, 8, and 15, placed in prominent isolation. This is probably the most enigmatic part of the message. The only self-consistent interpretation of these numbers is that they are the atomic numbers of the elements hydrogen, carbon, nitrogen, oxygen, and phosphorus. This makes sense particularly when one explores the next characters in the message, which are sets of numbers in the same locations as the atomic numbers of the various elements. These make up chemical formulas—in fact the formulas of the components of the molecule that determines the nature of terrestrial life, the DNA molecule. Its composition is shown here and even its form, that of a double helix. When one looks at the usual sketches of the complicated DNA molecule, it may not seem possible to depict it clearly and succinctly with the limited imaging capabilities of this type of message, yet when we set out to do it, it turned out to be rather simple after all.