Messages
From the Moon, to Mars, to the Stars

The earliest speculations about communication with extraterrestrial intelligence (ETI) involved contact with the Moon and with other planets of our own solar system. In the 1800s, many astronomers thought that — at least theoretically — life might well exist throughout the solar system. But when people raised the question of whether we are really alone in the solar system, they began to imagine ways to find a very concrete answer.

Since it was still impossible to travel to other planets in the early 19th century, other methods not requiring face-to-face contact were suggested to answer the question "Is anyone out there?" For example, in the 1820s, one mathematician proposed that we could signal any inhabitants of the Moon by clearing massive stretches of forest in Siberia. In this plan, the forests would be cut down so that any beings viewing Siberia from the Moon would see a geometry lesson carved on the surface of the Earth. Surely, it was argued, a huge diagram of the Pythagorean theorem would tell any intelligent beings on the Moon that their next door neighbors have a basic grasp of mathematics.

Others suggested that we could use the same approach, even when the dark side of the Earth faced the Moon. In this case, huge canals might be dug out of the Sahara Desert, perhaps in the shape of a circle. When filled with kerosene and lit while it was nighttime in the Sahara, this signal would suggest to the Moon-folk that Earthlings have an appreciation for geometry and enough practice with pyrotechnics to get a good bonfire going.

Neither of these ideas was ever put into action. For such diagrams to be seen from the Moon, they would need to be many miles long on each side — a considerable feat of engineering and fundraising!

Even though we no longer look for ETI on the Moon, as we think about the types of messages that might be used for interstellar communication, we still hold out the hope that such messages would be based on concepts that may be universally understandable — concepts like those used in geometry and arithmetic.

Part 2

Some of the earliest proposals for communicating with extraterrestrial intelligence (ETI) imagined contact with inhabitants of our Moon. But by the 1870s, it was shown that the Moon is too small to sustain an atmosphere. Because the Moon is considerably smaller than the Earth, the Moon has a weaker gravitational field, and thus all gases escape from the Moon's surface. And with no atmosphere, it was argued, there could be no life on the Moon. The planet Mars, however, is considerably bigger than our Moon, and even in the late 1800s it seemed likely that Mars might have at least a thin atmosphere.

When people started thinking about how we might communicate with potential Martians, a new challenge arose. Since the Moon is relatively close to the Earth, earlier proposals to signal possible inhabitants of the Moon relied on huge diagrams "drawn" on the surface of the Earth. But a similar message, if visible at all from Mars, would be much harder to see clearly — simply because of the greater distance. Therefore, an alternative approach was suggested for interplanetary communication.

One example of this new method — proposed in 1920 — would rely on a variation of Morse code, using flashes of light to attempt communication with ETI. In this scheme, pictures would first be laid out on a grid, using only colored and white squares. By sending both short and long flashes, like the "dits" and "dahs" of Morse code, these pictures could be transmitted. As shown below, the orange squares are signaled as short flashes, and the white squares as long flashes.

examples of signal patterns

Most of these proposals to communicate with Martians remained theoretical and were never actually attempted. Today, we no longer believe that there is intelligent life on Mars, so astronomers turn their radio telescopes to the stars to listen for signs of ETI.

Part 3

In the early 1960s, when SETI was just getting started, not much was known about the history of earlier proposals to communicate with life beyond Earth. So when Frank Drake started to think about the sort of message we might some day receive from extraterrestrial intelligence (ETI), he had to start from scratch. The format he suggested in 1962 is similar to proposals from several decades earlier - proposals that had never gained wide circulation, and that had been forgotten in the meantime. Like his nineteenth-century predecessors, Drake also drew on mathematics, but instead of showing geometrical concepts, he focused on ways of communicating numbers to ETI. In addition, he drew on the likelihood that ETI would also know a lot about the physical universe. In fact, many SETI scientists today still believe that if extraterrestrials have an advanced radio technology, then they would also know something about chemistry and physics. Thus, science might provide a foundation for an interstellar "common language."

To help in decoding the message, Drake tried to make the format as obvious as possible. For starters, he made sure the number of characters in the message would provide a clue for displaying it as a two-dimensional picture. When he distributed this message to colleagues, to see if they could figure it out, he gave them a page of 551 0s and 1s.

Click on the image above to view larger version

The key to putting this string of numbers into the proper form is to recognize that 551 is the same as 19 multiplied by 29. And, in fact, those are the only two numbers by which 551 can be divided (except of course for 1 and 551, which don't provide much help). When this sequence of 0s and 1s is converted into black and white squares, and arranged in rows 19 squares long, stacked one on top of another for a total of 29 rows, you get the picture shown here.

Try your hand at interpreting this message from a fictional ETI.

hypothetical message

Message explanation as follows:

1. A hypothetical solar system is shown, with the star at the top, and nine planets lined up underneath.

2.Two chemical elements critical for life on Earth are carbon and oxygen. Apparently, the same holds true for this fictional ET biochemistry. The central nucleus of each atom is shown with four black squares. The carbon atom (on the left) has six electrons circling its nucleus, while the oxygen atom (on the right) has eight electrons.

3.The numbers from one to five, written in a binary notation. This section of the message provides the foundation for understanding the numbers elsewhere in the message.

4.The population of three of the planets, written in a binary number system. Each number is located on the same row as one of the planets. The second planet from their star has a population of 5 - perhaps the crew of a small expedition. The third planet has about 2,000 inhabitants - possibly indicating the establishment of a permanent colony on this planet. The fourth planet, with a population of about 4 billion, seems to be the home world of this species.

5.Although the legs of this hypothetical ETI are more splayed than we are used to seeing on humans, its general bipedal form is quite similar to our own. The number 31 is shown between brackets, indicating the height of the ETI. But in what units? The key is in the signal itself. The wavelength of the radio signal, about 10 centimeters, is used as the basic unit of measurement. Thus, the ETI is about ten feet tall. Underneath the ETI is its name: "Four Bits" for short. Whereas all the numbers have an odd number of bits, this name has an even number, setting it apart as distinct. A line connects the image at the bottom of the message with the numbers above.

Part 4

A picture is worth a thousand words, especially if you're trying to get your point across to someone who doesn't speak your language. At least that has been the assumption of many proposals for communicating with extraterrestrial intelligence (ETI). But would pictures necessarily be understood at interstellar distances?

When we consider the pictures drawn by diverse cultures right here on Earth, we can see that pictures don't merely reflect reality as it independently exists. Instead, any attempt to picture an object is based on certain assumptions about which aspects of an object to emphasize. What seems obvious in one culture might be incomprehensible in another. The Abelam artist from Papua, New Guinea, for instance, might represent the human form like this:

click image above to enlarge

To the initiated, some of the lines in this image represent arms and legs, while the circles represent eyes. But to the outsider unfamiliar with these artistic conventions, the picture may well be unintelligible. If people here on Earth have this much difficulty understanding works of art from other human cultures, imagine how much harder it would be to overcome the "communication gap" across interstellar space.

Might there be something even more fundamental than pictures? After all, even when pictures have been proposed for interstellar communication, often non-pictorial information has also been included. For example, Frank Drake relied heavily on numbers when he drafted both a hypothetical message from an ETI and a message actually transmitted from the Arecibo radio telescope. Indeed, some people have proposed entirely non-pictorial methods for interstellar communication.

For instance, at about the same time that Drake sketched his first message, the Dutch mathematician Hans Freudenthal advanced his own "cosmic language," aptly named Lingua Cosmica, or Lincos for short. Rather than relying on pictures, Freudenthal constructed a complex mathematical language that might be transmitted via radio signals. The principles of arithmetic are among the first lessons to be taught in his interstellar tutorial.

As an illustration, the process of addition can be conveyed through a series of radio pulses, with pulses of one type representing numbers, and different kinds of pulses acting as "+" and "=" signs. In this manner, we might communicate mathematical fundamentals such as "1 + 2 = 3" and "2 + 2 = 4" simply by transmitting a series of pulses. As shown below, numbers are indicated by long pulses - one pulse for a "1", two pulses for a "2", and so on. In contrast, mathematical concepts like addition and equality are shown by groups of short pulses - in this case, two short pulses stand for the "+" sign, and three short pulses indicate the "=" sign. In the same way, concepts like subtraction, multiplication, and division could be illustrated.

click image above to see animated example of 1 + 2 = 3

click image above to see animated example of 2 + 2 = 4

Using this sort of approach, the mathematician Carl L. DeVito has suggested that we can build up an interstellar language based on potentially universal concepts from mathematics and science. DeVito argues that any civilization with advanced radio technology would also know many of the same basic concepts of chemistry that we know. For instance, technologically advanced ETI should also know about the Periodic Table of Elements, which shows how the different chemical elements can be arranged according to their atomic structures. And because atomic structure can be described with numbers, basic chemistry might serve as a bridge between presumably universal mathematical principles and elaborate scientific explanations of the physical universe.

In the coming months, this web site will describe more recent proposals for interstellar messages that are being developed right now at the SETI Institute. For example, how might a technologically advanced civilization go beyond a description of shared math and science to describe its unique culture, such as its notions of art and ethics? If SETI succeeds, and we do some day detect a signal from a distant civilization, humankind will need to decide whether to send a reply, and if so, what to say. There has already been some discussion within the international SETI community on these topics, leading to the recommendation that no reply should be sent unless there is first international agreement to do so. In the meantime, it may be useful to think about these questions in very concrete terms, by considering the sorts of messages we might some day send in reply - if indeed we send any at all.

SETI

Messages From the Moon, to Mars, to the Stars

Part 2

Part 3

Part 4

Messages
From the Moon, to Mars, to the Stars