The SETI project has for the past several decades been conducting a search for signals being transmitted by extraterrestrial life located outside the solar system, primarily in the radio frequencies of the electromagnetic spectrum. Special attention has been given to the "water hole", the frequency of one of neutral hydrogen's absorption lines, due to the low background noise at this frequency and its symbolic association with the basis for what is likely to be the most common system of biochemistry (see carbon chauvinism for a discussion of potential alternatives).
The regular radio pulses emitted by pulsars were briefly thought to be potential intelligent signals; the first pulsar to be discovered was originally designated "LGM-1", for "Little Green Men." They were quickly determined to be of natural origin, however.
Several attempts have been made to transmit signals to other stars as well. One of the earliest and most famous was the l974 radio message sent from the largest radio telescope in the world, Arecibo, in Puerto Rico. An extremely simple message was aimed at a globular cluster of stars known as M13 in the Milky Way Galaxy and at a distance of 30,000 light years from our solar system. These efforts have been more symbolic than anything else, however.
It has also been proposed that higher frequency signals, such as lasers operating at visible light frequencies, may prove to be a fruitful method of interstellar communication; at a given frequency it takes surprisingly small energy output for a laser emitter to outshine its local star from the perspective of its target.
Other more exotic methods of communication have been proposed, such as modulated neutrino or gravitational wave emissions. These would have the advantage of being essentially immune to interference by intervening matter, but are very difficult to generate or detect with current technology.Radio
Other methods