View source for Astronomy:Interstellar communication

{{Short description|Communication between planetary systems}}
'''Interstellar communication''' is the transmission of signals between [[Astronomy:Planetary system|planetary system]]s. Sending interstellar messages is potentially much easier than [[Engineering:Interstellar travel|interstellar travel]], being possible with technologies and equipment which are currently available. However, the distances from [[Astronomy:Earth|Earth]] to other potentially inhabited systems introduce prohibitive delays, assuming the limitations of the [[Physics:Speed of light|speed of light]]. Even an immediate reply to radio communications sent to stars tens of thousands of light-years away would take many human generations to arrive.

==Radio==
The SETI project has for the past several decades been conducting a search for signals being transmitted by [[Astronomy:Extraterrestrial life|extraterrestrial life]] located outside the [[Astronomy:Solar System|Solar System]], primarily in the [[Social:Radio|radio]] frequencies of the [[Physics:Electromagnetic spectrum|electromagnetic spectrum]]. Special attention has been given to the Water Hole, the frequency of one of neutral [[Chemistry:Hydrogen|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 [[Biology:Biochemistry|biochemistry]] (but see alternative biochemistry).

The regular radio pulses emitted by [[Astronomy:Pulsar|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 [[Astronomy:Active SETI|Active SETI]].) One of the earliest and most famous was the [[Astronomy:Arecibo message|1974 radio message]] sent from the largest radio telescope in the world, the [[Astronomy:Arecibo Observatory|Arecibo Observatory]] in Puerto Rico. An extremely simple message was aimed at a [[Astronomy:Globular cluster|globular cluster of stars]] known as [[Astronomy:Messier 13|M13]] in the Milky Way Galaxy and at a distance of 30,000 [[Light year|light year]]s from the Solar System. These efforts have been more symbolic than anything else, however.{{Citation needed|date=May 2022}} Further, a possible answer needs double the [[Physics:Time of flight|travel time]], i.e. tens of years (near stars) or 60,000 years (M13).

==Other methods==
[[File:Innovative Interstellar Explorer interstellar space probe .jpg|thumb|right|NASA's Vision Mission for the Innovative Interstellar Explorer considered using optical-laser communication, as did the 1980s era [[Engineering:TAU (spacecraft)|TAU]] probe]]
It has also been proposed that higher frequency signals, such as [[Physics:Laser|laser]]s operating at [[Physics:Visible light|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 [[Astronomy:Star|star]] from the perspective of its target.{{Citation needed|date=February 2008}}

Other more exotic methods of communication have been proposed, such as modulated [[Physics:Neutrino|neutrino]] or [[Astronomy:Gravitational wave|gravitational wave]] emissions. These would have the advantage of being essentially immune to interference by intervening matter.

Sending physical mail packets between stars may prove to be optimal for many applications.<ref name="probes">{{cite journal
|url=http://www.rfreitas.com/Astro/InterstellarProbesJBIS1980.htm
|title=Interstellar Probes: A New Approach To SETI
|first=Robert A. Jr.
|last=Freitas
|journal=Journal of the British Interplanetary Society
|volume=33
|pages=95–100
|date=1980|bibcode = 1980JBIS...33...95F }}</ref> While mail packets would likely be limited to speeds far below that of electromagnetic or other light-speed signals (resulting in very high [[Engineering:Latency (engineering)|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 (computing)|bandwidth]]. The possibility of using interstellar messenger probes for interstellar communication &mdash; known as [[Astronomy:Bracewell probe|Bracewell probe]]s &mdash; 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 [[Engineering:Project Daedalus|Project Daedalus]] in 1978.  Starting in 1979, Robert Freitas advanced arguments<ref name="probes"/>
<ref>{{cite journal
|url=http://www.rfreitas.com/Astro/ProbeMyths1983.htm
|title=Debunking the Myths of Interstellar Probes
|first=Robert A. Jr.
|last=Freitas
|date=July–August 1983
|journal=AstroSearch
|volume=1
|pages=8–9}}</ref>
<ref>{{cite journal
|url=http://www.rfreitas.com/Astro/TheCaseForInterstellarProbes1983.htm
|title=The Case for Interstellar Probes
|first=Robert A. Jr.
|last=Freitas
|journal=Journal of the British Interplanetary Society
|volume=36
|pages=490–495
|date=November 1983|bibcode = 1983JBIS...36..490F }}</ref> 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<ref>{{cite journal
|url=http://www.rfreitas.com/Astro/SearchIcarus1980.htm
|title=A Search for Natural or Artificial Objects Located at the Earth-Moon Libration Points
|first=Robert A. Jr.
|last=Freitas
|author2=Francisco Valdes
|journal=Icarus
|volume=42
|pages=442–447
|date=1980
|doi=10.1016/0019-1035(80)90106-2
|issue=3
|bibcode=1980Icar...42..442F}}</ref> and 1982.<ref>{{cite journal
|url=http://www.rfreitas.com/Astro/SearchIcarus1983.htm
|title=A Search for Objects near the Earth-Moon Lagrangian Points
|first=Francisco
|last=Valdes
|author2=Robert A. Freitas Jr.
|journal=Icarus
|volume=53
|pages=453–457
|date=1983
|doi=10.1016/0019-1035(83)90209-9
|issue=3
|bibcode=1983Icar...53..453V}}</ref>

==See also==
* [[Interplanetary Internet]]
* [[Astronomy:List of interstellar radio messages|List of interstellar radio messages]]
* [[Universal translator]]
* [[Astronomy:Gravitational lens|Gravitational lens]]

==References==
<references/>

==External links==

{{Extraterrestrial life}}
{{Interstellar messages}}

[[Category:Search for extraterrestrial intelligence]]
[[Category:Interstellar messages|*]]

{{Sourceattribution|Interstellar communication}}