Interstellar communication

Interstellar communication is the transmission of signals across planetary systems. Interstellar communication is potentially much easier than interstellar travel, being possible with technologies and equipment which are currently available. However, successful communication requires someone at the other end to communicate with, and in that regard it is currently not known whether interstellar communication is possible at this time.

Radio

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 1974 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.

Other methods

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.

Sending physical mail packets [1] (http://www.rfreitas.com/Astro/InterstellarProbesJBIS1980.htm) between stars may prove to be optimal for many applications. While mail packets would likely be limited to speeds far below that of electromagnetic or other light-speed signals (resulting in very high latency), the amount of information that could be encoded in only a few tons of physical matter could more than make up for it in terms of average bandwidth. The possibility of using interstellar messenger probes for interstellar communication was first suggested by Ronald N. Bracewell in 1960, and the technical feasibility of this approach was demonstrated by the British Interplanetary Society's starship study Project Daedalus in 1978. Starting in 1979, Robert Freitas advanced arguments [2] (http://www.rfreitas.com/Astro/InterstellarProbesJBIS1980.htm) [3] (http://www.rfreitas.com/Astro/ProbeMyths1983.htm) [4] (http://www.rfreitas.com/Astro/TheCaseForInterstellarProbes1983.htm) for the proposition that physical space-probes provide a superior mode of interstellar communication to radio signals, then undertook telescopic searches for such probes in 1979 [5] (http://www.rfreitas.com/Astro/SearchIcarus1980.htm) and 1982 [6] (http://www.rfreitas.com/Astro/SearchIcarus1983.htm).

FTL (Faster than Light) or STL (Slower than Light)

A question which has much impact on the way in that one would imagine future (interstellar) societies. How is society formed by the way it communicates? One could easily imagine alienation of different human tribes in remote star systems if it takes more than a thousand years to communicate.

Faster than light signals could also present the possibility of allowing communication with one's own past, according to current understanding of physics. This would present many of the same problems and opportunities provided for by physical time travel.