Interstellar communication
Encyclopedia
Interstellar communication is the transmission of signals between planetary system
s. Sending Interstellar messages 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.
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 Alternative biochemistry
).
The regular radio pulses emitted by pulsar
s 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, see "Realized projects" at Active SETI
and Interstellar Radio Messages. 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. Further, a possible answer needs double the travel time
, i.e. tens of years (near stars) or 60,000 years (M13). Sending Interstellar Radio Messages between stars may prove to be optimal for many applications.
s 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 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 — known as Bracewell probe
s — 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
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 and 1982.
Planetary system
A planetary system consists of the various non-stellar objects orbiting a star such as planets, dwarf planets , asteroids, meteoroids, comets, and cosmic dust...
s. Sending Interstellar messages is potentially much easier than interstellar travel
Interstellar travel
Interstellar space travel is manned or unmanned travel between stars. The concept of interstellar travel in starships is a staple of science fiction. Interstellar travel is much more difficult than interplanetary travel. Intergalactic travel, or travel between different galaxies, is even more...
, 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.
Radio
The SETISETI
The search for extraterrestrial intelligence is the collective name for a number of activities people undertake to search for intelligent extraterrestrial life. Some of the most well known projects are run by the SETI Institute. SETI projects use scientific methods to search for intelligent life...
project has for the past several decades been conducting a search for signals being transmitted by extraterrestrial life
Extraterrestrial life
Extraterrestrial life is defined as life that does not originate from Earth...
located outside the solar system
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...
, primarily in the radio
Radio
Radio is the transmission of signals through free space by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...
frequencies of the electromagnetic spectrum
Electromagnetic spectrum
The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object....
. Special attention has been given to the Water Hole
Waterhole (radio)
The waterhole, or water hole, refers to an especially quiet band of the electromagnetic spectrum between 1,420 and 1,666 megahertz, corresponding to wavelengths of 21 and 18 centimeters respectively. The term was coined by Bernard Oliver. The strongest hydroxyl radical spectral line radiates at 18...
, the frequency of one of neutral hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
'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
Biochemistry
Biochemistry, sometimes called biological chemistry, is the study of chemical processes in living organisms, including, but not limited to, living matter. Biochemistry governs all living organisms and living processes...
(see Alternative biochemistry
Alternative biochemistry
Hypothetical types of biochemistry are forms of biochemistry speculated to be scientifically viable but not proven to exist at this time. While the kinds of living beings we know on earth commonly use carbon for basic structural and metabolic functions, water as a solvent and DNA or RNA to define...
).
The regular radio pulses emitted by pulsar
Pulsar
A pulsar is a highly magnetized, rotating neutron star that emits a beam of electromagnetic radiation. The radiation can only be observed when the beam of emission is pointing towards the Earth. This is called the lighthouse effect and gives rise to the pulsed nature that gives pulsars their name...
s 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, see "Realized projects" at Active SETI
Active SETI
Active SETI is the attempt to send messages to intelligent aliens. Active SETI messages are usually in the form of radio signals. Physical messages like that of the Pioneer plaque may also be considered an active SETI message...
and Interstellar Radio Messages. One of the earliest and most famous was the 1974 radio message sent from the largest radio telescope in the world, Arecibo
Arecibo
Arecibo may refer to:*Arecibo, Puerto Rico, a municipality located by the Atlantic Ocean*Arecibo Observatory, a very sensitive radio telescope located approximately south-southwest from the city of Arecibo...
, in Puerto Rico. An extremely simple message was aimed at a globular cluster of stars
Globular cluster
A globular cluster is a spherical collection of stars that orbits a galactic core as a satellite. Globular clusters are very tightly bound by gravity, which gives them their spherical shapes and relatively high stellar densities toward their centers. The name of this category of star cluster is...
known as M13
Messier 13
Messier 13 or M13 is a globular cluster of about 300,000 stars in the constellation of Hercules....
in the Milky Way Galaxy and at a distance of 30,000 light years from our solar system
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...
. These efforts have been more symbolic than anything else, however. Further, a possible answer needs double the travel time
Travel time
Travel time may refer to* Travel, movement of people between locations* Travel journal, record made by a voyager* Propagation speed, term in physics to measure things such as the speed of light or radio waves...
, i.e. tens of years (near stars) or 60,000 years (M13). Sending Interstellar Radio Messages between stars may prove to be optimal for many applications.
Other methods
It has also been proposed that higher frequency signals, such as laserLaser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...
s 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
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
from the perspective of its target.
Other more exotic methods of communication have been proposed, such as modulated neutrino
Neutrino
A neutrino is an electrically neutral, weakly interacting elementary subatomic particle with a half-integer spin, chirality and a disputed but small non-zero mass. It is able to pass through ordinary matter almost unaffected...
or gravitational wave
Gravitational wave
In physics, gravitational waves are theoretical ripples in the curvature of spacetime which propagates as a wave, traveling outward from the source. Predicted to exist by Albert Einstein in 1916 on the basis of his theory of general relativity, gravitational waves theoretically transport energy as...
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 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
Lag
Lag is a common word meaning to fail to keep up or to fall behind. In real-time applications, the term is used when the application fails to respond in a timely fashion to inputs...
), 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
Bandwidth (computing)
In computer networking and computer science, bandwidth, network bandwidth, data bandwidth, or digital bandwidth is a measure of available or consumed data communication resources expressed in bits/second or multiples of it .Note that in textbooks on wireless communications, modem data transmission,...
. The possibility of using interstellar messenger probes for interstellar communication — known as Bracewell probe
Bracewell probe
A Bracewell probe is a hypothetical concept for an autonomous interstellar space probe dispatched for the express purpose of communication with one or more alien civilizations. It was proposed by Ronald N...
s — was first suggested by Ronald N. Bracewell
Ronald N. Bracewell
Ronald Newbold Bracewell AO was the Lewis M. Terman Professor of Electrical Engineering, Emeritus of the at Stanford University.- Education :...
in 1960, and the technical feasibility of this approach was demonstrated by the British Interplanetary Society's starship study Project Daedalus
Project Daedalus
Project Daedalus was a study conducted between 1973 and 1978 by the British Interplanetary Society to design a plausible unmanned interstellar spacecraft. Intended mainly as a scientific probe, the design criteria specified that the spacecraft had to use current or near-future technology and had to...
in 1978. Starting in 1979, Robert Freitas
Robert Freitas
Robert A. Freitas Jr. is a Senior Research Fellow, one of four researchers at the nonprofit foundation Institute for Molecular Manufacturing in Palo Alto, California. He holds a 1974 Bachelor's degree majoring in both physics and psychology from Harvey Mudd College, and a 1978 Juris Doctor degree...
advanced arguments
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 and 1982.