The Arecibo Observatory in Puerto Rico is a radio astronomy facility that includes the largest single-dish radio telescope in the world, some 300 metres across and is operated by Cornell University on behalf of the United States National Science Foundation. Completed in 1963, it underwent certain upgrades which were completed in 1974. At a ceremony on 16th November 1974 to mark the completion of the upgrade what is now known as the Arecibo broadcast or message was sent out from the telescope. The message was a short one sent out into space for the benefit of putative intelligent aliens who might intercept the message and decipher it and learn one or two things about us.
The destination chosen was the globular cluster Messier 13 or M13, also designated NGC 6205 in the New General Catalog, which lies about 25,100 light years away from us in the constellation Hercules. The cluster is some 145 light years across and contains hundreds of thousands of stars. The message itself was made up of 1679 binary digits with a total size of some 230 bytes and was dispatched by means of frequency modulated radio waves. The choice of destination was fairly simple: the dish at the Arecibo Observatory is so large that it is not maneuverable as some other such telescopes and M13 was the largest relatively close collection of stars that was in the right position in the sky at the time and place of the ceremony.
The Arecibo message was not designed as part of a potential conversation between Earth and any putative aliens. In the first place, it will take the broadcast 25 or so thousand years to arrive at its destination and the same time for any reply to arrive back here. Secondly, and perhaps more important, by the time that the message arrives, M13 will no longer be in that position, along with any life, intelligent or otherwise, that it might harbor. In fact, as confirmed by Cornell University in a press release of November 1999, the real purpose of the broadcast was to demonstrate the capabilities of the newly upgraded equipment.
But, even if M13 has moved in the time it takes the message to reach its destination, it is not beyond the bounds of possibility that someone else will be there to receive and decipher the message. What, then, were the Cornell scientists telling whoever finds the metaphorical bottle containing this message on behalf of Earth?
First, the mode chosen to convey the message, 1679 binary numbers arranged in rectangular form of 73 rows and 23 columns, ought to convey a message to the recipients. The number 1679 is what is called a semi-prime or bi-prime number i.e. it is a product of two prime numbers (a prime number is a number that is not divisible by any whole number other than itself and one) and the message encoded in these numbers, make sense, at least from a human viewpoint, only in the form in which it is framed; framed as 23 rows by 73 columns, it comes out as just nonsense.
The message proper seeks to convey seven different points to the recipients.
First, the message sets out the numbers one to ten in binary form which informs the recipients that although the message itself is couched in binary form, we are actually a species that lives and work in base 10.
Second, it seeks to give the atomic numbers of the elements that comprise the human DNA i.e. hydrogen, carbon, nitrogen, oxygen and phosphorous.
Once the recipients have the basic elements figured out, the next set of digits set out the composition of the sugars and bases that make DNA i.e. deoxyribose, phosphate, adenine, thymine, cytosine and guanine.
The chemical composition is followed by a graphical representation of DNA which shows its double helix shape. Embedded in the DNA is a number showing approximately how many nucleotides (the sugars and bases) are contained in the human DNA: approximately 6 billion.
The message then provides a graphical representation depicting the human shape and average size which is calculated by reference to the speed of light. Anyone who has had no trouble deciphering so far will not find it difficult to decipher the size indicated by matching the transmission frequency to the speed of light and getting the size indicated. In addition, the approximate population of humans on Earth in 1974 is embedded in this section.
Yet another graphic shows the solar system, the sun and all her daughters, with earth raised and directly below the representation of a human which ought to indicate that the sending species is from the third planet, insignificant though that planet may seem when compared to the giants of the system.
Finally, the message ends with a graphical representation of the Arecibo radio telescope from which the message was sent indicating its size.
Will we ever get a reply? We'll know in just another 25,000 years or so.