Let's say you're a human with a big radio transmitter, who wants to send a message to the putative aliens out there light-years away, listening by their radio receivers. What would your message say? How would you format it, since you don't have any concept of the recipient's language or cognitive abilities? What would be most interesting and salient to a completely unknown civilization? Given the limits of the exercise, there are only a few known factors about the recipient: you can assume that the recipient is technologically advanced enough to have built a radio, received the message, and recognized that it's actually a message rather than noise. But aside from that, a world of questions surround the issue.

Carl Sagan dramatized this problem (in reverse) in his 1985 book Contact. His novel was likely based on his own experience more than ten years earlier, he was faced with the challenge in real life. In 1974, astrophysicist Frank Drake proposed sending just such a message -- and Sagan was recruited to help write and format it.

Drake, Sagan, and others developed a message to be broadcast by the Arecibo radio telescope, using a mathematical scheme they hoped could be decoded by an alien civilization. The message itself consisted of just 1,679 binary digits (1's and 0's). The digits were broadcast one per second, on November 16, 1974. The telescope was pointed at the M13 cluster, some 25,000 light years away. The broadcast was never repeated -- hopefully someone will be listening when the message arrives in deep space (for what it's worth, by the time 25,000 years pass, M13 will no longer be where it was when we sent the message -- so our transmission will miss whoever lives there). But let's get back to brass tacks -- what did the message say? Well, using binary encoding, the message carried the information below. (A colorized version of the message, rendered as blocks, is also presented at left.)

1. the numbers one (1) through ten (10)
2. the atomic numbers of the elements hydrogen, carbon, nitrogen, oxygen, and phosphorus, which make up deoxyribonucleic acid (DNA)
3. the formulas for the sugars and bases in the nucleotides of DNA
4. the number of nucleotides in DNA, and a graphic of the double helix structure of DNA
5. a graphic figure of a man, the dimension (physical height) of an average man, and the human population of Earth
6. a graphic of Earth's solar system
7. a graphic of the Arecibo radio telescope and the dimension (the physical diameter) of the transmitting antenna dish

It's clear that the transmission was more a symbolic event than an actual attempt at communication -- if we were attempting to communicate, we'd probably send the message more than once, or to more than one spot in the sky. (A 1999 press release said as much, with Cornell Professor Donald Campbell explaining, "It was strictly a symbolic event, to show that we could do it.") But the possibility remains that some intelligence could intercept the message and perhaps decode it -- and maybe, just maybe, reply. In August of 2001, a crop circle appeared in farmland near the Chibolton radio telescope in Hampshire, UK. Known to crop circle aficionados as the Arecibo reply, the pattern looked like a modified version of the original transmission, showing a big-headed alien and adding silicon to the list of elements, among other changes. While it's clearly a hoax, it's a clever one, and took a lot of work to put together.

Further reading: the Arecibo message at Wikipedia, a mathematical explanation of the message, and more on Frank Drake.

So let's hear it: if you were sending a message into the unknown depths of space, what would you say?