While scientists may be able to distinguish natural from artificial signals arriving from a particular location, what will be much more difficult will be the decoding of the signals which are identified as intelligent - a task not too dissimilar from that undertaken by cryptologists studying ancient writing.

At the same time, since we have allowed our radio-waves to leak out into space, why should we hesitate to send a deliberate message to other civilizations? Whatever the pros and cons of this suggestion, some astronomers have already taken the plunge and sent a few messages to our unknown cosmic neighbours.

Fig.28 The Arecibo message.

On November 16, 1974, the above message was transmitted by the 300 metre dish of the Arecibo Observatory and directed toward the globular cluster, M13 in Hercules, about 25,000 light years away (so a reply cannot be expected for at least another 50,000 years). The message contained 1679 bits of information (or 'on' or 'off' pulses of two particular frequencies) and was sent out at 2380 MHz, with a bandwidth of 10 Hz, continuous correction for the Doppler effect, and an output power of 3 x 10¹² watts.

The number 1679 was chosen because it is the product of the two prime numbers 23 and 73 and has no other factors. This would suggest, to anyone with a modest knowledge of mathematics, that the bits should be arranged in 73 columns of 23 bits each with the zeroes replaced by white squares and the ones replaced by black squares. When this is done, it can be seen to produce an interesting pictorial array (the other way round - 23 x 73 - is possible, but produces no discernible pattern). A little careful study clearly reveals that the message contains nine items (see Fig.29).

Fig.29 The decoded Arecibo message.

• The top part of the message is a lesson in the number system - the binary system - to be used in the rest of the message and lists the numbers 1 to 10, with a 'number marker' that indicates when a symbol represents a number.

• Under that are the numbers 1, 6, 7, 8, and 15 (in binary code), the atomic numbers of hydrogen, carbon, nitrogen, oxygen and phosphorus, the essential elements of life.

• Below this we find 12 groups of five numbers, each representing the chemical formula of various molecules significant to life and including the sugars and bases in nucleotides of DNA - thymine, guanine, adenine, cytosine, phosphate and the sugar deoxyribose.

• Further down is the chemical structure of DNA, with a double helix wound around the number of pair bases that exist in a single human chromosome (about 4 billion).

• The double helix leads to the head of a crude picture of a human being, indicating the connection between DNA and intelligent life. To the right of the figure is a line extending from head to foot, together with the number 14, indicating that the human is 14 units tall. The only unit we share with the recipient of the message will be the wavelength of the of the message-carrying photons (12.6 cm.), thus making the human 176.4 cm. tall (about 5 ft. 10 inches).

• To the left of the human is a number representing the human population at the time the message was sent - about 4 billion (not exact, because the binary system doesn't round off in the same way as the decimal system does).

• Below the human is a sketch of our solar system, with the sun to the right and the nine planets to the left. The third planet from the sun, the Earth, is displaced towards the human to indicate that this is our home.

• Finally, below the solar system, is a sketch of a radiotelescope (the function of the 'telescope' being indicated by focussing rays to a point) beaming away from the Earth. Below the sketch is the number 2430, representing the diameter (2430 x 12.6 cm.) of the telescope.

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