Given a Search for Extraterrestrial Intelligence, SETI, it follows of necessity that a means of communicating with such intelligences, once discovered, ought to be available. This branch of SETI is known as Communication with Extraterrestrial Intelligence, CETI. If communication is defined as a means of conveying meaning from one person to another, then it is clear that communication with a non terrestrial intelligence is likely to present some problems that will not be solved by the means of communication that terrestrial intelligence has developed over the last many thousand years. Even between different Homo sapiens the communication has often proved troublesome, to put it mildly; between species separated not just by distance, but by billions of years of separate evolution, the problem is likely to be somewhat more complex.
The idea of sending messages to possible inhabitants of extraterrestrial worlds is not a new one and it predates the advent of modern day SETI. As far back as the 19th century, when many folk believed that there might be life on the Moon, Venus and Mars, ideas as to how earth could communicate with the inhabitants of those worlds were being thought of long before the advent of radio. Interesting suggestions included the creation of a giant triangle and three squares on the Siberian tundra, using wide strips of pine forests as the outlines and wheat or rye as the interiors, made by Carl Friedrich Gauss (1777-1855) the German mathematician. Another proposal was by Joseph Johann Littrow (1781-1840) the Austrian astronomer, who proposed that giant trenches in various geometrical shapes be dug in the Sahara and filled with water. Enough kerosene to burn for six hours would then be poured over the water and set ablaze at night. Using this method, different signals could be sent out every night. The inhabitants of other worlds viewing these clearly unnatural phenomena would deduce that they were the result of intelligence seeking to communicate. So great was the enthusiasm for extraterrestrial life, and communication with such life, that in 1900, the French Academy of Sciences announced a 100,000 franc prize, the Guzman prize, for the first person to establish interplanetary communication; Mars, which many believed at that time to harbour intelligent life, was specifically excluded as being not enough of a challenge!
Since then, things have moved apace; the search for intelligent extraterrestrial life moving away from our quiet solar neighbourhood into the farthest reaches of the galaxy and beyond. Various methods of communicating with putative extraterrestrial intelligences have been proposed.
The use of maths and logic symbols is one of the methods that have been suggested to facilitate communication on the grounds that the rules of these disciplines must always be the same wherever it is that a race inhabits in the universe. On the same premise, the basic facts of science e.g. the physical properties of elements etc. have been proposed as the basis of a common intelligible inter species language. One way in which CETI workers have sought to communicate with any extraterrestrial intelligence we may come in contact with is to represent mathematical and scientific facts as pictograms and transmit such into space.
The most famous of the interstellar messages broadcast from earth is the Arecibo broadcast which was transmitted from the Arecibo Observatory, Puerto Rico, in November 1974 and it was in form of a pictogram which included information such as the numbers one to ten, the atomic numbers of the most important elements from an earth point of view like hydrogen, nitrogen, oxygen, etc., the composition of deoxyribonucleic acid, DNA, etc. The Arecibo broadcast was transmitted in the direction of the globular cluster Messier 13, M13, in the Hercules Constellation.
Although the Arecibo broadcast is the most famous of the interstellar calls yet made from earth, there have been quite a few others. Cosmic Call, has sent out two messages in 1999 and 2003 respectively from the Evpatoria Planetary Radar, located in the Crimean peninsula, consisting of symbolic representations of numbers, mathematical operations, physical processes as well as objects such as basic elements. In addition, it contains a copy of the Arecibo message as well as text, video, audio and other image files submitted by ordinary folk from around the world. Intended destinations include stars in Cygnus, Cancer, Orion and Andromeda constellations, amongst others. The Teen Age message, in part composed by young people from all over Russia, was also sent from this facility in 2001 to six nearby stars in such constellations as Virgo, Draco etc.
But not all attempts at communication have been by means of radio transmission. Both the Pioneer 10 spacecraft, launched in 1972, and which is expected to reach the star Aldebaran in Taurus Constellation some two million years hence, and Pioneer 11, launched the following year, and which is expected to pass by one of the stars in the Aquila galaxy in some 4 or so million years, both had on board plaques which showed the position of earth and the solar system in the galaxy, the form of the human body (male and female), and the transition of the hydrogen atom. Similarly, both Voyager probes, 1 and 2, launched in 1977, contained golden records that carry pictures and sounds of earth, symbolic instructions for playing the record as well as data regarding our location in the galaxy. Although, given the time frames involved in physically travelling to other stars, the human race might well have become extinct before any communication is made, the effort would still be worthwhile if even one of the artefacts aboard these crafts ever fell into the "hands" of intelligent extraterrestrial beings.
We just called to say hello.