View source for Astronomy:Search for extraterrestrial intelligence

{{short description|Effort to find civilizations not from Earth}}

The '''search for extraterrestrial intelligence''' ('''SETI''') is a collective term for scientific searches for intelligent [[Astronomy:Extraterrestrial life|extraterrestrial life]], for example, monitoring [[Physics:Electromagnetic radiation|electromagnetic radiation]] for signs of [[Astronomy:Interstellar communication|transmissions]] from [[Social:Civilization|civilization]]s on other planets.<ref name="Schenkel_2006">{{cite magazine | title=SETI Requires a Skeptical Reappraisal | last=Schenkel | first=Peter | magazine=Skeptical Inquirer |date=May–June 2006 | access-date=June 28, 2009  |volume=30 |number=3 | url=http://www.csicop.org/si/show/seti_requires_a_skeptical_reappraisal/ |archiveurl=https://web.archive.org/web/20091105105946/http://www.csicop.org/si/show/seti_requires_a_skeptical_reappraisal/ |archivedate=2009-11-05|url-status=live}}</ref><ref>{{cite magazine | title=Why SETI is science and UFOlogy is not | last=Moldwin | first=Mark | magazine=Skeptical Inquirer | date=November 2004 | url=https://cdn.centerforinquiry.org/wp-content/uploads/sites/29/2004/11/22164643/p40.pdf | access-date=2022-06-11 | archive-date=2022-06-21 | archive-url=https://web.archive.org/web/20220621035247/https://cdn.centerforinquiry.org/wp-content/uploads/sites/29/2004/11/22164643/p40.pdf | url-status=live }}</ref><ref name="NYT-20170628">{{cite news |last=Johnson |first=Steven |title=Greetings, E.T. (Please Don't Murder Us.) |url=https://www.nytimes.com/2017/06/28/magazine/greetings-et-please-dont-murder-us.html |date=28 June 2017 |work={{wipe|New York Times}} |access-date=28 June 2017 |archive-date=19 July 2018 |archive-url=https://web.archive.org/web/20180719013213/https://www.nytimes.com/2017/06/28/magazine/greetings-et-please-dont-murder-us.html |url-status=live }}</ref>

Scientific investigation began shortly after the advent of [[Social:Radio|radio]] in the early 1900s, and focused international efforts have been ongoing since the 1980s.<ref name="SETI at 50"/> In 2015, [[Biography:Stephen Hawking|Stephen Hawking]] and Israeli billionaire [[Biography:Yuri Milner|Yuri Milner]] announced the [[Astronomy:Breakthrough Listen|Breakthrough Listen]] Project, a $100 million 10-year attempt to detect signals from nearby stars.<ref name="AP-20150720">{{cite news |last=Katz |first=Gregory |title=Searching for ET: Hawking to look for extraterrestrial intelligence |url=http://apnews.excite.com/article/20150720/eu--britain-extraterrestrials-e52c157915.html |date=July 20, 2015 |work=AP News |access-date=July 20, 2015 |archive-date=July 22, 2015 |archive-url=https://web.archive.org/web/20150722045619/http://apnews.excite.com/article/20150720/eu--britain-extraterrestrials-e52c157915.html |url-status=live }}</ref>

== History ==

===Early work===
There have been many earlier searches for extraterrestrial intelligence within the [[Astronomy:Solar System|Solar System]]. In 1896, [[Biography:Nikola Tesla|Nikola Tesla]] suggested that an extreme version of his wireless electrical transmission system could be used to contact beings on [[Astronomy:Mars|Mars]].<ref>{{cite book| last =Seifer| first =Marc J.| title =Wizard : the life and times of Nikola Tesla: biography of a genius| chapter-url =https://archive.org/details/wizardlifetimeso0000seif| chapter-url-access =registration| publisher =Carol Pub.| date =1996| location =Secaucus, New Jersey| page =[https://archive.org/details/wizardlifetimeso0000seif/page/157 157]|chapter=Martian Fever (1895–1896)| isbn = 978-1-55972-329-9 | oclc = 33865102}}</ref> In 1899, while conducting experiments at his Colorado Springs experimental station, he thought he had detected a signal from Mars since an odd repetitive static signal seemed to cut off when Mars set in the night sky. Analysis of Tesla's research has led to a range of explanations including: 
*Tesla simply misunderstood the new technology he was working with,<ref>{{cite book| last =Spencer| first =John| title =The UFO Encyclopedia| publisher =Avon Books| date =1991| location =New York| isbn = 978-0-380-76887-5| oclc = 26211869 }}</ref>
*that he may have been observing signals from Marconi's European [[Social:Radio|radio]] experiments,
*and even speculation that he could have picked up naturally occurring radio noise caused by a moon of [[Astronomy:Jupiter|Jupiter]] ([[Astronomy:Io (moon)|Io]]) moving through the [[Astronomy:Magnetosphere of Jupiter|magnetosphere of Jupiter]].<ref>W. Bernard Carlson, ''Tesla: Inventor of the Electrical Age'', Princeton University Press – 2013, pp. 276–278.</ref>

In the early 1900s, [[Biography:Guglielmo Marconi|Guglielmo Marconi]], [[Biography:William Thomson, 1st Baron Kelvin|Lord Kelvin]] and David Peck Todd also stated their belief that radio could be used to contact [[Astronomy:Martian|Martian]]s, with Marconi stating that his stations had also picked up potential Martian signals.<ref>{{cite book| last=Corum| first=Kenneth L.| author2=James F. Corum| title=Nikola Tesla and the electrical signals of planetary origin| date=1996| pages=1, 6, 14| url=http://www.teslasociety.com/mars.pdf| oclc=68193760|  access-date=2010-08-23| archive-date=2010-11-29| archive-url=https://web.archive.org/web/20101129065430/http://teslasociety.com/mars.pdf| url-status=live}}</ref><ref>{{Cite book |last=Brown |first=Michael |url=http://worldcat.org/oclc/56198770 |title=Transmitting the past: historical and cultural perspectives on broadcasting |publisher=University of Alabama Press |year=2005 |isbn=0-8173-1453-9 |editor-last=Winn |editor-first=J. Emmett |chapter=Radio Mars: The Transformation of Marconi's Popular Image, 1919–1922 |oclc=56198770 |editor-last2=Brinson |editor-first2=Susan}}</ref>

On August 21–23, 1924, Mars entered an [[Astronomy:Opposition (planets)|opposition]] closer to Earth than at any time in the century before or the next 80 years.<ref>{{cite web |author=Lasker |first=Jacques |title=A Primer on Mars Oppositions |url=http://www.imcce.fr/Equipes/ASD/mars/oppo_en.html |url-status=live |archive-url=https://web.archive.org/web/20111113100554/http://www.imcce.fr/Equipes/ASD/mars/oppo_en.html |archive-date=2011-11-13 |access-date=2014-01-01}}</ref> In the United States, a "National Radio Silence Day" was promoted during a 36-hour period from August 21–23, with all radios quiet for five minutes on the hour, every hour. At the United States Naval Observatory, a radio receiver was lifted {{convert|3|km|abbr=off}} above the ground in a [[Physics:Airship|dirigible]] tuned to a [[Wavelength|wavelength]] between 8 and 9&nbsp;km, using a "radio-camera" developed by [[Organization:Amherst College|Amherst College]] and [[Biography:Charles Francis Jenkins|Charles Francis Jenkins]]. The program was led by David Peck Todd with the military assistance of Admiral Edward W. Eberle (Chief of Naval Operations), with William F. Friedman (chief [[Cryptography|cryptographer]] of the United States Army), assigned to translate any potential Martian messages.<ref name="dick-bio-univ">{{cite book | first = Steven| last = Dick |title= The Biological Universe: The Twentieth Century Extraterrestrial Life Debate | date = 1999 | publisher = Cambridge University Press | isbn = 978-0-521-34326-8}}</ref><ref>[http://www.lettersofnote.com/2009/11/prepare-for-contact.html Prepare for Contact] {{Webarchive|url=https://web.archive.org/web/20181021165947/http://www.lettersofnote.com/2009/11/prepare-for-contact.html |date=2018-10-21 }}. Letters of Note (2009-11-06). Retrieved on 2011-10-14.</ref>

A 1959 paper by [[Biography:Philip Morrison|Philip Morrison]] and [[Biography:Giuseppe Cocconi|Giuseppe Cocconi]] first pointed out the possibility of searching the [[Physics:Microwave|microwave]] spectrum. It proposed [[Physics:Frequency|frequencies]] and a set of initial targets.<ref>{{cite journal |author=Cocconi, Giuseppe |author2=Philip Morrison |name-list-style=amp |title=Searching for interstellar communications |journal=Nature |volume=184 |number=4690 |date=1959 |pages=844–846 |url=http://www.coseti.org/morris_0.htm |doi=10.1038/184844a0 |bibcode=1959Natur.184..844C |s2cid=4220318 |access-date=2012-07-03 |archive-date=2008-05-09 |archive-url=https://web.archive.org/web/20080509125403/http://www.coseti.org/morris_0.htm |url-status=live }}</ref><ref>{{cite web|url=http://www.bigear.org/vol1no1/interste.htm|title=Cosmic Search Vol. 1, No. 1|access-date=1 October 2014|archive-date=23 October 2008|archive-url=https://web.archive.org/web/20081023190411/http://www.bigear.org/vol1no1/interste.htm|url-status=live}}</ref>
[[File:Wow signal.jpg|thumb|left|[[Astronomy:Wow! signal|The Wow! Signal]]<br />Credit: The Ohio State University Radio Observatory and the North American AstroPhysical Observatory (NAAPO).]]
In 1960, [[Organization:Cornell University|Cornell University]] astronomer Frank Drake performed the first modern SETI experiment, named "[[Astronomy:Project Ozma|Project Ozma]]" after the Queen of Oz in L. Frank Baum's fantasy books.<ref>{{Cite web |date=2011-02-20 |title=Science: Project Ozma – TIME |url=http://www.time.com/time/magazine/article/0,9171,874057,00.html |access-date=2023-10-06 |archive-url=https://web.archive.org/web/20110220022144/http://www.time.com/time/magazine/article/0,9171,874057,00.html |archive-date=2011-02-20 }}</ref> Drake used a radio telescope {{convert|26|m}} in diameter at Green Bank, West Virginia, to examine the stars [[Astronomy:Tau Ceti|Tau Ceti]] and [[Astronomy:Epsilon Eridani|Epsilon Eridani]] near the 1.420 [[Hertz#SI multiples|gigahertz]] marker frequency, a region of the radio spectrum dubbed the "[[Engineering:Water hole (radio)|water hole]]" due to its proximity to the [[Chemistry:Hydrogen|hydrogen]] and [[Chemistry:Hydroxyl radical|hydroxyl radical]] spectral lines. A 400 kilohertz band around the marker frequency was scanned using a single-channel receiver with a bandwidth of 100 hertz. He found nothing of interest.

[[Place:Soviet Union|Soviet]] scientists took a strong interest in SETI during the 1960s and performed a number of searches with [[Engineering:Omnidirectional antenna|omnidirectional antenna]]s in the hope of picking up powerful radio signals. Soviet astronomer [[Biography:Iosif Shklovsky|Iosif Shklovsky]] wrote the pioneering book in the field, ''Universe, Life, Intelligence'' (1962), which was expanded upon by American astronomer [[Biography:Carl Sagan|Carl Sagan]] as the best-selling book ''Intelligent Life in the Universe'' (1966).<ref name="sagan_shklovskii">{{cite book |last1=Sagan |first1=Carl |title=Intelligent Life in the Universe |last2=Shklovskii |first2=Iosif |date=1966 |publisher=Pan Books |isbn=978-0-330-25125-9}}</ref>

In the March 1955 issue of ''Scientific American'', John D. Kraus described an idea to scan the [[Astronomy:Cosmos|cosmos]] for natural radio signals using a flat-plane radio telescope equipped with a parabolic [[Astronomy:Reflector (antenna)|reflector]]. Within two years, his concept was approved for construction by [[Organization:Ohio State University|Ohio State University]]. With a total of {{wipe|United States dollar}}71,000 ({{Inflation|US|71000|1957|fmt=eq}})
in grants from the [[Organization:National Science Foundation|National Science Foundation]], construction began on an {{convert|20|acre|ha|adj=on|order=flip|0}} plot in Delaware, Ohio. This Ohio State University Radio Observatory telescope was called "Big Ear". Later, it began the world's first continuous SETI program, called the Ohio State University SETI program.

In 1971, [[Organization:NASA|NASA]] funded a SETI study that involved Drake, [[Biography:Bernard M. Oliver|Barney Oliver]] of Hewlett-Packard Laboratories, and others. The resulting report proposed the construction of an Earth-based radio telescope array with 1,500 dishes known as "[[Astronomy:Project Cyclops|Project Cyclops]]". The price tag for the Cyclops array was US$10 billion. Cyclops was not built, but the report<ref>{{cite web | title=Project Cyclops: A Design Study of a System for Detecting Extraterrestrial Intelligent Life | publisher=NASA | date=1971 | access-date=October 12, 2014 | url=https://seti.berkeley.edu/sites/default/files/19730010095_1973010095.pdf | url-status=dead | archive-url=https://web.archive.org/web/20150920204912/https://seti.berkeley.edu/sites/default/files/19730010095_1973010095.pdf | archive-date=September 20, 2015 }}</ref> formed the basis of much SETI work that followed.

The Ohio State SETI program gained fame on August 15, 1977, when Jerry Ehman, a project volunteer, witnessed a startlingly strong signal received by the telescope. He quickly circled the indication on a printout and scribbled the exclamation "Wow!" in the margin. Dubbed the ''[[Astronomy:Wow! signal|Wow! signal]]'', it is considered by some to be the best candidate for a radio signal from an artificial, [[Astronomy:Extraterrestrial life|extraterrestrial]] source ever discovered, but it has not been detected again in several additional searches.<ref name="gray_2012">{{cite book |author=Gray |first=Robert H. |title=The Elusive WOW: Searching for Extraterrestrial Intelligence |publisher=Palmer Square Press |year=2012 |isbn=978-0-9839584-4-4 |location=Chicago, Illinois |language=en-us}}</ref>

On 24 May 2023, a test extraterrestrial signal, in the form of a "coded radio signal from Mars", was transmitted to radio telescopes on Earth, according to a report in ''{{wipe|The New York Times}}''.<ref name="NYT-20230524">{{cite news |last=Overbye |first=Dennis |date=24 May 2023 |title=This Is Not an Extraterrestrial Signal. This Is Just a Test. – You too could be Jodie Foster as astronomers organize a practice run in communicating with aliens. |work={{wipe|The New York Times}} |url=https://www.nytimes.com/2023/05/24/science/extraterrestrial-signal-test.html |url-status=live |accessdate=24 May 2023 |archive-url=https://archive.today/20230524142954/https://www.nytimes.com/2023/05/24/science/extraterrestrial-signal-test.html |archive-date=24 May 2023 |}}</ref>

===Sentinel, META, and BETA===
In 1980, [[Biography:Carl Sagan|Carl Sagan]], [[Biography:Bruce C. Murray|Bruce Murray]], and [[Biography:Louis Friedman|Louis Friedman]] founded the U.S. [[Organization:The Planetary Society|Planetary Society]], partly as a vehicle for SETI studies.<ref name="NYT-20170628" />

In the early 1980s, [[Organization:Harvard University|Harvard University]] physicist [[Biography:Paul Horowitz|Paul Horowitz]] took the next step and proposed the design of a [[Physics:Spectrum analyzer|spectrum analyzer]] specifically intended to search for SETI transmissions. Traditional desktop spectrum analyzers were of little use for this job, as they sampled frequencies using banks of analog filters and so were restricted in the number of channels they could acquire. However, modern integrated-circuit [[Digital signal processing|digital signal processing]] (DSP) technology could be used to build [[Autocorrelation|autocorrelation]] receivers to check far more channels. This work led in 1981 to a portable spectrum analyzer named "Suitcase SETI" that had a capacity of 131,000 narrow band channels. After field tests that lasted into 1982, Suitcase SETI was put into use in 1983 with the {{convert|26|m|ft|adj=on|sp=us}} Harvard/Smithsonian radio telescope at Oak Ridge Observatory in Harvard, Massachusetts. This project was named "Sentinel" and continued into 1985.

Even 131,000 channels were not enough to search the sky in detail at a fast rate, so Suitcase SETI was followed in 1985 by Project "META", for "Megachannel Extra-Terrestrial Assay". The META spectrum analyzer had a capacity of 8.4 million channels and a channel resolution of 0.05 hertz. An important feature of META was its use of frequency [[Physics:Doppler effect|Doppler shift]] to distinguish between signals of terrestrial and extraterrestrial origin. The project was led by Horowitz with the help of the Planetary Society, and was partly funded by movie maker Steven Spielberg. A second such effort, META II, was begun in {{wipe|Argentina}} in 1990, to search the southern sky, receiving an equipment upgrade in 1996–1997.<ref>{{Cite web |last=Lemarchand |first=Guillermo |title=Progress in the Search for Ultra-Narrow Band Extraterrestrial Arti cial Signals from Argentina |url=http://tux.iar.unlp.edu.ar/SETI/bio99-1.pdf |website=Argentina en el Proyecto SETI}}</ref><ref>{{Cite web |last=Lemarchand |first=Guillermo |title=15 Years Developing SETI from a Developing Country |url=http://tux.iar.unlp.edu.ar/SETI/seti-boston.pdf |website=Argentina en el Proyecto SETI}}</ref>

The follow-on to META was named "BETA", for "Billion-channel Extraterrestrial Assay", and it commenced observation on October 30, 1995. The heart of BETA's processing capability consisted of 63 dedicated [[Fast Fourier transform|fast Fourier transform]] (FFT) engines, each capable of performing a 2<sup>22</sup>-point [[Complex number#Signal analysis|complex]] FFTs in two seconds, and 21 general-purpose personal computers equipped with custom [[Digital signal processing|digital signal processing]] boards. This allowed BETA to receive 250 million simultaneous channels with a resolution of 0.5 hertz per channel. It scanned through the microwave [[Spectrum|spectrum]] from 1.400 to 1.720 gigahertz in eight hops, with two seconds of observation per hop. An important capability of the BETA search was rapid and automatic re-observation of candidate signals, achieved by observing the sky with two adjacent beams, one slightly to the east and the other slightly to the west. A successful candidate signal would first transit the east beam, and then the west beam and do so with a speed consistent with [[Astronomy:Earth|Earth]]'s [[Astronomy:Sidereal time|sidereal]] rotation rate. A third receiver observed the horizon to veto signals of obvious terrestrial origin. On March 23, 1999, the 26-meter radio telescope on which Sentinel, META and BETA were based was blown over by strong winds and seriously damaged.<ref>{{cite web |author=MacRobert |first=Alan M. |date=29 March 2009 |title=SETI Searches Today |url=https://skyandtelescope.org/astronomy-news/seti-searches-today/ |url-status=live |archive-url=https://web.archive.org/web/20210416201521/https://skyandtelescope.org/astronomy-news/seti-searches-today/ |archive-date=16 April 2021 |access-date=16 April 2021 |publisher=Sky and Telescope}}</ref> This forced the BETA project to cease operation.

===MOP and Project Phoenix===
[[File:NASA-SETI-Sensitivity.jpg|thumb|upright=1.3|Sensitivity vs range for SETI radio searches. The diagonal lines show transmitters of different effective powers. The x-axis is the sensitivity of the search. The y-axis on the right is the range in light-years, and on the left is the number of Sun-like stars within this range. The vertical line labeled SS is the typical sensitivity achieved by a full sky search, such as BETA above. The vertical line labeled TS is the typical sensitivity achieved by a targeted search such as Phoenix.<ref name="CP2156a">{{cite web |last=Wolfe |first=J. H. |display-authors=0 |date=1979 |title=CP-2156, Chapter 5.5. SETI – The Search for Extraterrestrial Intelligence: Plans and Rationale |url=https://history.nasa.gov/CP-2156/ch5.5.htm |url-status=live |archive-url=https://web.archive.org/web/20090916162650/http://history.nasa.gov/CP-2156/ch5.5.htm |archive-date=September 16, 2009 |access-date=July 1, 2009 |publisher=NASA}}</ref>]]

In 1978, the NASA SETI program had been heavily criticized by Senator William Proxmire, and funding for SETI research was removed from the NASA budget by Congress in 1981;<ref name="garber">{{cite journal |last1=Garber |first1=S. J. |year=1999 |title=Searching for Good Science – the Cancellation of NASA's SETI Program |journal=Journal of the British Interplanetary Society |volume=52 |issue=1 |pages=3 |bibcode=1999JBIS...52....3G}}</ref> however, funding was restored in 1982, after [[Biography:Carl Sagan|Carl Sagan]] talked with Proxmire and convinced him of the program's value.<ref name="garber" /> In 1992, the U.S. government funded an operational SETI program, in the form of the NASA Microwave Observing Program (MOP). MOP was planned as a long-term effort to conduct a general survey of the sky and also carry out targeted searches of 800 specific nearby stars. MOP was to be performed by radio antennas associated with the [[Astronomy:NASA Deep Space Network|NASA Deep Space Network]], as well as the {{convert|140|ft|m|adj=on}} radio telescope of the [[Astronomy:National Radio Astronomy Observatory|National Radio Astronomy Observatory]] at Green Bank, West Virginia and the {{convert|1000|ft|m|adj=on}} [[Astronomy:Arecibo Telescope|radio telescope]] at the [[Astronomy:Arecibo Observatory|Arecibo Observatory]] in Puerto Rico. The signals were to be analyzed by spectrum analyzers, each with a capacity of 15 million channels. These spectrum analyzers could be grouped together to obtain greater capacity. Those used in the targeted search had a bandwidth of 1 hertz per channel, while those used in the sky survey had a bandwidth of 30 hertz per channel.

[[File:Arecibo Observatory Aerial View.jpg|thumb|upright=1.2|left|Arecibo Telescope in Puerto Rico with its {{convert|abbr=on|300|m|ft}} dish was one of the world's largest filled-aperture (i.e. full dish) radio telescope and conducted some SETI searches.]]

MOP drew the attention of the United States Congress, where the program was ridiculed<ref>{{cite news | url=https://www.nytimes.com/1993/10/07/us/ear-to-the-universe-is-plugged-by-budget-cutters.html?pagewanted=1%2F | work=The New York Times | title=Ear to the Universe Is Plugged by Budget Cutters | date=October 7, 1993 | access-date=May 23, 2010 | archive-date=May 12, 2013 | archive-url=https://web.archive.org/web/20130512201727/http://www.nytimes.com/1993/10/07/us/ear-to-the-universe-is-plugged-by-budget-cutters.html?pagewanted=1%2F | url-status=live }}</ref> and canceled one year after its start.<ref name="garber" /> SETI advocates continued without government funding, and in 1995 the nonprofit [[Organization:SETI Institute|SETI Institute]] of Mountain View, California resurrected the MOP program under the name of Project "Phoenix", backed by private sources of funding. In 2012 it cost around $2 million per year to maintain SETI research at the SETI Institute and around 10 times that to support different SETI activities globally.<ref>{{cite news |date=May 22, 2012 |title=Searching for Intelligent Aliens: Q&A with SETI Astronomer Jill Tarter |work=Space.com |url=http://www.space.com/15803-jill-tarter-seti-search-retirement-qanda.html |url-status=live |access-date=August 5, 2012 |archive-url=https://web.archive.org/web/20120814200337/http://www.space.com/15803-jill-tarter-seti-search-retirement-qanda.html |archive-date=August 14, 2012}}</ref> [[Astronomy:Project Phoenix (SETI)|Project Phoenix]], under the direction of [[Biography:Jill Tarter|Jill Tarter]], was a continuation of the targeted search program from MOP and studied roughly 1,000 nearby [[Astronomy:Sun|Sun]]-like stars until approximately 2015.<ref>{{Cite web |last=Shostak |first=Seth |date=2021 |title=Project Phoenix |url=https://www.seti.org/seti-institute/project/details/project-phoenix |website=SETI Institute}}</ref> From 1995 through March 2004, Phoenix conducted observations at the {{convert|64|m|ft|adj=on|sp=us}} [[Astronomy:Parkes Observatory|Parkes radio telescope]] in {{wipe|Australia}}, the {{convert|140|ft|m|adj=on}} radio telescope of the [[Astronomy:National Radio Astronomy Observatory|National Radio Astronomy Observatory]] in Green Bank, West Virginia, and the {{convert|1000|ft|m|adj=on}} radio telescope at the Arecibo Observatory in Puerto Rico. The project observed the equivalent of 800 stars over the available channels in the frequency range from 1200 to 3000&nbsp;MHz. The search was sensitive enough to pick up transmitters with 1&nbsp;GW EIRP to a distance of about 200 [[Astronomy:Light-year|light-year]]s.

== Ongoing radio searches ==
[[File:TerrestrialMicrowaveWindow.jpg|thumb|upright=1.6|Microwave window as seen by a ground based system. From NASA report SP-419: SETI – the Search for Extraterrestrial Intelligence]]
Many radio frequencies penetrate Earth's atmosphere quite well, and this led to [[Astronomy:Radio telescope|radio telescope]]s that investigate the cosmos using large radio antennas. Furthermore, human endeavors emit considerable electromagnetic radiation as a byproduct of communications such as television and radio. These signals would be easy to recognize as artificial due to their repetitive nature and narrow [[Bandwidth (signal processing)|bandwidths]]. Earth has been sending radio waves from broadcasts into space for over 100 years.<ref>
{{cite web |url= https://www.sciencealert.com/humanity-hasn-t-reached-as-far-into-space-as-you-think |title= How Far Radio Signals have travelled |date= Jan 20, 2021 |website= Science Alert |access-date= Jan 20, 2021 |archive-date= January 26, 2021 |archive-url= https://web.archive.org/web/20210126152134/https://www.sciencealert.com/humanity-hasn-t-reached-as-far-into-space-as-you-think |url-status= live }}
</ref> These signals have reached over 1,000 stars, most notably [[Astronomy:Vega|Vega]], [[Astronomy:Aldebaran|Aldebaran]], [[Astronomy:Barnard's Star|Barnard's Star]], [[Astronomy:Sirius|Sirius]], and [[Astronomy:Proxima Centauri|Proxima Centauri]]. If intelligent alien life exists on any planet orbiting these nearby stars, these signals could be heard and deciphered, even though some of the signal is garbled by the Earth's [[Astronomy:Ionosphere|ionosphere]].

Many international radio telescopes are currently being used for radio SETI searches, including the Low Frequency Array (LOFAR) in Europe, the [[Astronomy:Murchison Widefield Array|Murchison Widefield Array]] (MWA) in Australia, and the [[Astronomy:Lovell Telescope|Lovell Telescope]] in the United Kingdom.<ref name="SR-20150929">{{cite web |last=Siemion |first=Andrew |date=September 29, 2015 |title=Prepared Statement by Andrew Siemion – Hearing on Astrobiology Status Report – House Committee on Science, Space, and Technology |url=http://www.spaceref.com/news/viewsr.html?pid=47887 |access-date=October 19, 2015 |work=SpaceRef.com}}{{Dead link|date=June 2023|bot=InternetArchiveBot|fix-attempted=yes}}</ref>

===Allen Telescope Array===
{{Main|Astronomy:Allen Telescope Array}}
The SETI Institute collaborated with the [[Organization:Radio Astronomy Laboratory|Radio Astronomy Laboratory]] at the [[Organization:Berkeley SETI Research Center|Berkeley SETI Research Center]] to develop a specialized radio telescope array for SETI studies, similar to a mini-cyclops array. Formerly known as the One Hectare Telescope (1HT), the concept was renamed the "Allen Telescope Array" (ATA) after the project's benefactor, [[Biography:Paul Allen|Paul Allen]]. Its sensitivity is designed to be equivalent to a single large dish more than 100 meters in diameter, if fully completed. Presently, the array has 42 operational dishes at the Hat Creek Radio Observatory in rural northern California.<ref name="Allen">{{cite web | url = http://www.seti.org/site/pp.asp?c=ktJ2J9MMIsE&b=179146 | title = Allen Telescope Array General Overview | publisher = [[Organization:SETI Institute|SETI Institute]] | access-date = 2006-06-12 |archive-url = https://web.archive.org/web/20060428102322/http://www.seti.org/site/pp.asp?c=ktJ2J9MMIsE&b=179146 |archive-date = 2006-04-28}}</ref><ref>{{cite journal|last1=Welch|first1=Jack |display-authors=0.|title=The Allen Telescope Array: The First Widefield, Panchromatic, Snapshot Radio Camera for Radio Astronomy and SETI|journal=Proceedings of the IEEE|date=August 2009|volume=97|issue=8|pages=1438–1447|doi=10.1109/JPROC.2009.2017103|bibcode=2009IEEEP..97.1438W |arxiv = 0904.0762 |s2cid=7486677 }}</ref>

The full array (ATA-350) is planned to consist of 350 or more offset-[[Astronomy:Gregorian telescope|Gregorian]] radio dishes, each {{convert|6.1|m|ft|abbr=off|sp=us}} in diameter. These dishes are the largest producible with commercially available satellite television dish technology. The ATA was planned for a 2007 completion date, at a cost of US$25 million. The SETI Institute provided money for building the ATA while University of California, Berkeley designed the telescope and provided operational funding. The first portion of the array (ATA-42) became operational in October 2007 with 42 antennas. The DSP system planned for ATA-350 is extremely ambitious. Completion of the full 350 element array will depend on funding and the technical results from ATA-42.

ATA-42 (ATA) is designed to allow multiple observers simultaneous access to the [[Astronomy:Astronomical interferometer|interferometer]] output at the same time. Typically, the ATA snapshot imager (used for astronomical surveys and SETI) is run in parallel with a [[Beamforming|beamforming]] system (used primarily for SETI).<ref>{{cite book |last1=Gutierrez-Kraybill |first1=Colby |title=Proceedings of the SPIE |year=2010 |editor1-last=Radziwill |editor1-first=Nicole M. |series=Software and Cyberinfrastructure for Astronomy |volume=7740 |pages=77400Z |language=en-us |chapter=Commensal observing with the Allen Telescope array: Software command and control |bibcode=2010SPIE.7740E..0ZG |doi=10.1117/12.857860 |display-authors=0. |editor2-last=Bridger |editor2-first=Alan |arxiv=1010.1567 |s2cid=119183681}}</ref> ATA also supports observations in multiple synthesized pencil beams at once, through a technique known as "multibeaming". Multibeaming provides an effective filter for identifying false positives in SETI, since a very distant transmitter must appear at only one point on the sky.<ref>Harp, G. R. "Customized beam forming at the Allen Telescope Array." ATA Memo Series 51 (2002), available at http://www.seti.org/sites/default/files/ATA-memo-series/memo51.pdf {{Webarchive|url=https://web.archive.org/web/20150924101735/http://www.seti.org/sites/default/files/ATA-memo-series/memo51.pdf |date=2015-09-24 }}.</ref><ref>{{cite journal|doi=10.1029/2010RS004442|title=Real-time beamforming using high-speed FPGAs at the Allen Telescope Array|journal=Radio Science|volume=46|issue=1|year=2011|last1=Barott|first1=William C.|pages=n/a|display-authors=0.|bibcode=2011RaSc...46.1016B|url=http://commons.erau.edu/cgi/viewcontent.cgi?article=1001&context=db-electrical-computer-engineering|doi-access=free|access-date=2019-09-20|archive-date=2019-12-21|archive-url=https://web.archive.org/web/20191221084320/http://commons.erau.edu/cgi/viewcontent.cgi?article=1001&context=db-electrical-computer-engineering|url-status=live}}</ref><ref>{{Cite journal|arxiv=1309.3826|doi=10.1029/2004RS003133|last1= Harp|first1= G. R.|title= Using Multiple Beams to Distinguish Radio Frequency Interference from SETI Signals|journal= Radio Science|year=2013 |volume=40|issue=5|pages=n/a|bibcode=2005RaSc...40.5S18H|s2cid=117428022 }}</ref>

SETI Institute's Center for SETI Research (CSR) uses ATA in the search for extraterrestrial intelligence, observing 12 hours a day, 7 days a week. From 2007 to 2015, ATA identified hundreds of millions of technological signals. So far, all these signals have been assigned the status of noise or radio frequency interference because a) they appear to be generated by satellites or Earth-based transmitters, or b) they disappeared before the threshold time limit of ~1 hour.<ref name="PP">{{Cite journal |doi=10.1016/j.actaastro.2009.08.014|title=The first SETI observations with the Allen telescope array|journal=Acta Astronautica|volume=68|issue=3–4|pages=340–346|year=2011|last1=Tarter|first1=Jill|display-authors=0.|bibcode=2011AcAau..68..340T}}</ref><ref>{{cite journal |bibcode=2010AAS...21540302B|title=The ATA Galactic Center Survey: SETI Observations in 2009|journal=American Astronomical Society|volume=215|pages=403.02|last1=Backus|author2=Allen Telescope Array Team|first1=Peter R.|year=2010}}</ref> Researchers in CSR are working on ways to reduce the threshold time limit, and to expand ATA's capabilities for detection of signals that may have embedded messages.<ref>Harp, Gerald R., et al. ''A new class of SETI beacons that contain information.'' Communication with Extraterrestrial Intelligence. State University of New York Press, 2011.</ref>

Berkeley astronomers used the ATA to pursue several science topics, some of which might have transient SETI signals,<ref>{{cite journal |arxiv=1006.2003 |bibcode= 2010ApJ...719...45C|doi=10.1088/0004-637X/719/1/45|title= The Allen Telescope Array Twenty-Centimeter Survey—A 690 Deg2, 12 Epoch Radio Data Set. I. Catalog and Long-Duration Transient Statistics|journal= The Astrophysical Journal|volume= 719|issue= 1|pages= 45–58|year= 2010|last1= Croft|first1= Steve|s2cid= 118641366|display-authors=0.}}</ref><ref>{{cite journal |last1=Siemion |first1=Andrew P. V. |display-authors=0. |year=2012 |title=The Allen Telescope Array Fly's Eye Survey for Fast Radio Transients |journal=The Astrophysical Journal |volume=744 |issue=2 |pages=109 |arxiv=1109.2659 |bibcode=2012ApJ...744..109S |doi=10.1088/0004-637X/744/2/109 |s2cid=118713622}}</ref><ref>{{cite journal |bibcode=2011AAS...21724006S|title=Results from the Fly's Eye Fast Radio Transient Search at the Allen Telescope Array|journal=American Astronomical Society|volume=217|pages=240.06|last1=Siemion|first1=Andrew|display-authors=0.|year=2011}}</ref> until 2011, when the collaboration between the University of California, Berkeley and the SETI Institute was terminated.

CNET published an article and pictures about the Allen Telescope Array (ATA) on December 12, 2008.<ref>Terdiman, Daniel. (2008-12-12) [http://news.cnet.com/8301-13772_3-10121889-52.html SETI's large-scale telescope scans the skies | Geek Gestalt – CNET News] {{Webarchive|url=https://web.archive.org/web/20140201143419/http://news.cnet.com/8301-13772_3-10121889-52.html |date=2014-02-01 }}. News.cnet.com. Retrieved on 2011-10-14.</ref><ref>[http://news.cnet.com/2300-11397_3-6248324-1.html?tag=mncol Rendering of 350 image – Photos: Searching the heavens for life – CNET News]{{Dead link|date=October 2022 |bot=InternetArchiveBot |fix-attempted=yes }}. News.cnet.com (2008-12-12). Retrieved on 2011-10-14.</ref>

In April 2011, the ATA was entered an 8-month "hibernation" due to funding shortfalls. Regular operation of the ATA resumed on December 5, 2011.<ref name="NatureSETI">{{cite web |url=http://blogs.nature.com/news/thegreatbeyond/2011/04/seti_scope_suspends_search.html |title=SETI scope suspends search |access-date=26 April 2011 |editor=The Great Beyond. Nature Blogs |date=25 April 2011 |archive-date=2 May 2011 |archive-url=https://web.archive.org/web/20110502152007/http://blogs.nature.com/news/thegreatbeyond/2011/04/seti_scope_suspends_search.html |url-status=dead }}</ref><ref>{{cite web | url = http://www.seti.org/node/905 | title = SETI Search Resumes at Allen Telescope Array | publisher = [[Organization:SETI Institute|SETI Institute]] | access-date = 2019-07-24 | archive-url = https://web.archive.org/web/20111208040312/http://www.seti.org/node/905 |archive-date = 2011-12-08}}</ref>

In 2012, the ATA was revitalized with a $3.6 million donation by Franklin Antonio, co-founder and Chief Scientist of QUALCOMM Incorporated.<ref>{{cite web |author=Arthur |first=Damon |title=New Hat Creek receivers will let SETI delve deeper into space |url=http://www.redding.com/news/2012/dec/06/new-hat-creek-receivers-will-let-seti-delve-into/ |url-status=dead |archive-url=https://web.archive.org/web/20140330064010/http://www.redding.com/news/2012/dec/06/new-hat-creek-receivers-will-let-seti-delve-into/ |archive-date=2014-03-30}}</ref> This gift supported upgrades of all the receivers on the ATA dishes to have (2× to 10× over the range 1–8&nbsp;GHz) greater sensitivity than before and supporting observations over a wider frequency range from 1–18&nbsp;GHz, though initially the radio frequency electronics only go to 12&nbsp;GHz. As of July 2013, the first of these receivers was installed and proven, with full installation on all 42 antennas being expected for June 2017. ATA is well suited to the search for extraterrestrial intelligence (SETI) and to discovery of [[Astronomy:Astronomical radio source|astronomical radio source]]s, such as heretofore unexplained non-repeating, possibly extragalactic, pulses known as [[Astronomy:Fast radio burst|fast radio burst]]s or FRBs.<ref>{{Cite journal |last1=Siemion |first1=Andrew P. V. |last2=Bower |first2=Geoffrey C. |last3=Foster |first3=Griffin |last4=McMahon |first4=Peter L. |last5=Wagner |first5=Mark I. |last6=Werthimer |first6=Dan |last7=Backer |first7=Don |last8=Cordes |first8=Jim |last9=van Leeuwen |first9=Joeri |date=2011-12-20 |title=The Allen Telescope Array Fly's Eye Survey for Fast Radio Transients |url=http://dx.doi.org/10.1088/0004-637x/744/2/109 |journal=The Astrophysical Journal |volume=744 |issue=2 |pages=109 |arxiv=1109.2659 |doi=10.1088/0004-637x/744/2/109 |issn=0004-637X |s2cid=118713622}}</ref><ref>{{Cite journal |last1=Harp |first1=G. R. |last2=Richards |first2=Jon |last3=Tarter |first3=Jill C. |last4=Dreher |first4=John |last5=Jordan |first5=Jane |last6=Shostak |first6=Seth |last7=Smolek |first7=Ken |last8=Kilsdonk |first8=Tom |last9=Wilcox |first9=Bethany R. |last10=Wimberly |first10=M. K. R. |last11=Ross |first11=John |last12=Barott |first12=W. C. |last13=Ackermann |first13=R. F. |last14=Blair |first14=Samantha |title=Seti Observations of Exoplanets with the Allen Telescope Array |date=2016-11-22 |journal=The Astronomical Journal |volume=152 |issue=6 |pages=181 |doi=10.3847/0004-6256/152/6/181 |arxiv=1607.04207 |bibcode=2016AJ....152..181H |s2cid=118561727 |issn=1538-3881 |doi-access=free }}</ref>

=== SERENDIP ===
{{Main|Astronomy:SERENDIP}}
SERENDIP (Search for Extraterrestrial Radio Emissions from Nearby Developed Intelligent Populations) is a SETI program launched in 1979 by the [[Organization:Berkeley SETI Research Center|Berkeley SETI Research Center]].<ref name="SERENDIP">{{cite web | url = https://seti.berkeley.edu/SERENDIP.html | title = SERENDIP | publisher = UC Berkeley | access-date = 2006-08-20 | archive-date = 2017-07-09 | archive-url = https://web.archive.org/web/20170709232306/http://seti.berkeley.edu/SERENDIP.html | url-status = live }}</ref><ref>{{Cite journal |last1=Werthimer |first1=Dan |last2=Anderson |first2=David |last3=Bowyer |first3=C. Stuart |last4=Cobb |first4=Jeff |last5=Heien |first5=Eric |last6=Korpela |first6=Eric J. |last7=Lampton |first7=Michael L. |last8=Lebofsky |first8=Matt |last9=Marcy |first9=Geoff W. |last10=McGarry |first10=Meghan |last11=Treffers |first11=Dick |date=2001-08-03 |title=Berkeley radio and optical SETI programs: SETI@home, SERENDIP, and SEVENDIP |url=http://dx.doi.org/10.1117/12.435384 |journal=The Search for Extraterrestrial Intelligence (SETI) in the Optical Spectrum III |publisher=SPIE |volume=4273 |pages=104–109 |bibcode=2001SPIE.4273..104W |doi=10.1117/12.435384 |s2cid=122003925 |editor-first1=Stuart A. |editor-first2=Ragbir |editor-last1=Kingsley |editor-last2=Bhathal}}</ref> SERENDIP takes advantage of ongoing "mainstream" [[Astronomy:Radio telescope|radio telescope]] observations as a "piggy-back" or "commensal" program, using large radio telescopes including the NRAO 90m telescope at Green Bank and, formerly, the [[Astronomy:Arecibo Telescope|Arecibo 305m telescope]]. Rather than having its own observation program, SERENDIP analyzes deep space radio telescope data that it obtains while other [[Astronomy:Astronomer|astronomer]]s are using the telescopes. The most recently deployed SERENDIP spectrometer, SERENDIP VI, was installed at both the [[Astronomy:Arecibo Telescope|Arecibo Telescope]] and the [[Astronomy:Green Bank Telescope|Green Bank Telescope]] in 2014–2015.<ref>{{Cite web |title=Mid June Update (Jun 23 2015) |url=https://setiathome.berkeley.edu/forum_thread.php?id=77575 |access-date=2023-02-01 |website=setiathome.berkeley.edu}}</ref>

=== Breakthrough Listen ===

Announced in July 2015, the project is observing for thousands of hours every year on two major radio telescopes, the [[Astronomy:Green Bank Observatory|Green Bank Observatory]] in West Virginia, and the [[Astronomy:Parkes Observatory|Parkes Observatory]] in {{wipe|Australia}}.<ref name="Sample0720">{{Cite news|url = https://www.theguardian.com/science/2015/jul/20/breakthrough-listen-massive-radio-wave-project-scan-far-regions-for-alien-life|title = Anybody out there? $100m radio wave project to scan far regions for alien life|journal = The Guardian|last = Sample|first = Ian|date = 20 July 2015|access-date = 20 July 2015|archive-date = 20 July 2015|archive-url = https://web.archive.org/web/20150720155541/http://www.theguardian.com/science/2015/jul/20/breakthrough-listen-massive-radio-wave-project-scan-far-regions-for-alien-life|url-status = live}}</ref> Previously, only about 24 to 36 hours of telescope time per year were used in the search for alien life.<ref name="Merali0720" /> Furthermore, the [[Astronomy:Automated Planet Finder|Automated Planet Finder]] at [[Organization:Lick Observatory|Lick Observatory]] is searching for optical signals coming from laser transmissions. The massive data rates from the radio telescopes (24 GB/s at Green Bank) necessitated the construction of dedicated hardware at the telescopes to perform the bulk of the analysis.<ref>{{Cite journal|last1=MacMahon|first1=David H. E.|last2=Price|first2=Danny C.|last3=Lebofsky|first3=Matthew|last4=Siemion|first4=Andrew P. V.|last5=Croft|first5=Steve|last6=DeBoer|first6=David|last7=Enriquez|first7=J. Emilio|last8=Gajjar|first8=Vishal|last9=Hellbourg|first9=Gregory|date=2017-07-19|title=The Breakthrough Listen Search for Intelligent Life: A Wideband Data Recorder System for the Robert C. Byrd Green Bank Telescope|journal=Publications of the Astronomical Society of the Pacific|volume=130|issue=986|pages=044502|arxiv=1707.06024|doi=10.1088/1538-3873/aa80d2|bibcode=2018PASP..130d4502M |s2cid=59378232}}</ref> Some of the data are also analyzed by volunteers in the [[Astronomy:SETI@home|SETI@home]] volunteer computing network.<ref name="Sample0720" /> Founder of modern SETI Frank Drake was one of the scientists on the project's advisory committee.<ref>{{Cite web|url=https://breakthroughinitiatives.org/Leaders/1|title=Breakthrough Initiatives|website=breakthroughinitiatives.org|language=en|access-date=2017-09-22|archive-date=2017-09-04|archive-url=https://web.archive.org/web/20170904071147/http://breakthroughinitiatives.org/Leaders/1|url-status=live}}</ref><ref name="washingtonp1" /><ref name="Merali0720" />

In October 2019, Breakthrough Listen started a collaboration with scientists from the TESS team ([[Astronomy:Transiting Exoplanet Survey Satellite|Transiting Exoplanet Survey Satellite]]) to look for signs of advanced extraterrestrial life. Thousands of new planets found by TESS will be scanned for [[Astronomy:Technosignature|technosignature]]s by Breakthrough Listen partner facilities across the globe. Data from TESS monitoring of stars will also be searched for anomalies.<ref>{{Cite web|url=https://breakthroughinitiatives.org/news/27|title=Breakthrough Initiatives|website=breakthroughinitiatives.org|access-date=2019-11-12|archive-date=2019-11-09|archive-url=https://web.archive.org/web/20191109041910/https://breakthroughinitiatives.org/news/27|url-status=live}}</ref>

=== FAST ===
{{Main|Astronomy:Five-hundred-meter Aperture Spherical Telescope}}
China's 500 meter Aperture Spherical Telescope (FAST) lists ''detecting interstellar communication signals'' as part of its science mission. It is funded by the [[Finance:National Development and Reform Commission|National Development and Reform Commission]] (NDRC) and managed by the National Astronomical observatories (NAOC) of the Chinese Academy of Sciences (CAS). FAST is the first radio observatory built with SETI as a core scientific goal.<ref>{{cite web |date=8 November 2017 |title=What Happens If China Makes First Contact? |url=https://www.theatlantic.com/magazine/archive/2017/12/what-happens-if-china-makes-first-contact/544131/ |url-status=live |archive-url=https://web.archive.org/web/20180605190918/https://www.theatlantic.com/magazine/archive/2017/12/what-happens-if-china-makes-first-contact/544131/ |archive-date=5 June 2018 |access-date=5 June 2018 |website=The Atlantic}}</ref> FAST consists of a fixed {{convert|500|m|abbr=on}} diameter spherical dish constructed in a natural depression sinkhole caused by [[Earth:Karst|karst processes]] in the region. It is the world's largest filled-aperture radio telescope.<ref name="Brinks">{{cite web
  | last = Brinks
  | first = Elias
  | title = China Opens the Aperture to the Cosmos
  | work = The Conversation
  | publisher = U.S. News & World Report
  | date = 11 July 2016
  | url = https://www.usnews.com/news/best-countries/articles/2016-07-11/china-builds-worlds-largest-radio-telescope
  | access-date = 12 August 2016
  | archive-date = 26 August 2016
  | archive-url = https://web.archive.org/web/20160826032054/http://www.usnews.com/news/best-countries/articles/2016-07-11/china-builds-worlds-largest-radio-telescope
  | url-status = live
  }}</ref>
According to its website, FAST can search to 28 light-years, and is able to reach 1,400 stars. If the transmitter's radiated power were to be increased to 1,000,000 MW, FAST would be able to reach one million stars. This is compared to the former Arecibo 305 meter telescope detection distance of 18 light-years.<ref>{{cite web| url = http://fast.bao.ac.cn/en/science_SETI.html| title = Science SETI, FAST website| access-date = 2017-02-19| archive-date = 2017-10-17| archive-url = https://web.archive.org/web/20171017203930/http://fast.bao.ac.cn/en/science_SETI.html| url-status = live}}</ref>

On 14 June 2022, astronomers, working with China's [[Astronomy:Five-hundred-meter Aperture Spherical Telescope|FAST telescope]], reported the possibility of having detected artificial (presumably alien) signals, but cautioned that further studies were required to determine if a natural radio interference may be the source.<ref name="ES-20220614">{{cite news |last=Byrd |first=Deborah |title=Has China's FAST telescope detected alien intelligence? |url=https://earthsky.org/space/did-chinas-fast-telescope-detect-alien-intelligence/ |date=4 June 2022 |work=Earth & Sky |accessdate=15 June 2022 |archive-date=15 June 2022 |archive-url=https://web.archive.org/web/20220615154041/https://earthsky.org/space/did-chinas-fast-telescope-detect-alien-intelligence/ |url-status=live }}</ref> More recently, on 18 June 2022, Dan Werthimer, chief scientist for several SETI-related projects, reportedly noted, "These signals are from radio interference; they are due to radio pollution from earthlings, not from E.T.".<ref name="NYT-20220618">{{cite news |last=Overbye |first=Dennis |title=A Chinese Telescope Did Not Find an Alien Signal. The Search Continues. China's astronomers have been initiated into the search for extraterrestrial intelligence with the kind of false alarm that others in the field have experienced for decades. |url=https://www.nytimes.com/2022/06/18/science/china-alien-signal.html |date=18 June 2022 |work={{wipe|The New York Times}} |accessdate=19 June 2022 |archive-date=16 January 2023 |archive-url=https://web.archive.org/web/20230116230912/https://www.nytimes.com/2022/06/18/science/china-alien-signal.html |url-status=live }}</ref>

=== UCLA ===
Since 2016, University of California Los Angeles (UCLA) undergraduate and graduate students have been participating in radio searches for technosignatures with the Green Bank Telescope. Targets include the [[Astronomy:Kepler Space Telescope|Kepler]] field, [[Astronomy:TRAPPIST-1|TRAPPIST-1]], and solar-type stars.<ref>{{cite web|title=Researchers Just Scanned 14 Worlds From the Kepler Mission for "Technosignatures", Evidence of Advanced Civilizations|url=https://www.universetoday.com/138516/researchers-just-scanned-14-worlds-kepler-mission-technosignatures-evidence-advanced-civilizations/|website=Universe Today|access-date=2020-05-02|date=9 February 2018|archive-date=2018-02-10|archive-url=https://web.archive.org/web/20180210180315/https://www.universetoday.com/138516/researchers-just-scanned-14-worlds-kepler-mission-technosignatures-evidence-advanced-civilizations/|url-status=live}}</ref> The search is sensitive to Arecibo-class transmitters located within 420 light years of Earth and to transmitters that are 1,000 times more powerful than Arecibo located within 13,000 light years of Earth.<ref>{{cite journal |last1=Margot |first1=Jean-Luc |last2=Greenberg |first2=Adam H. |last3=Pinchuk |first3=Pavlo |last4=Shinde |first4=Akshay |last5=Alladi |first5=Yashaswi |last6=MN |first6=Srinivas Prasad |last7=Bowman |first7=M. Oliver |last8=Fisher |first8=Callum |last9=Gyalay |first9=Szilard |last10=McKibbin |first10=Willow |last11=Miles |first11=Brittany |last12=Nguyen |first12=Donald |last13=Power |first13=Conor |last14=Ramani |first14=Namrata |last15=Raviprasad |first15=Rashmi |last16=Santana |first16=Jesse |last17=Lynch |first17=Ryan S. |title=A Search for Technosignatures from 14 Planetary Systems in the Field with the Green Bank Telescope at 1.15–1.73 GHz |journal=The Astronomical Journal |date=25 April 2018 |volume=155 |issue=5 |pages=209 |doi=10.3847/1538-3881/aabb03 |bibcode=2018AJ....155..209M|arxiv=1802.01081 |s2cid=13710050 |doi-access=free }}</ref>

== Community SETI projects ==
[[File:SETI Beta.gif|thumb|upright=1.6|Screen shot of the screensaver for SETI@home, a former [[Volunteer computing|volunteer computing]] project in which volunteers donated idle computer power to analyze radio signals for signs of extraterrestrial intelligence.]]
=== SETI@home ===
{{Main|Astronomy:SETI@home}}

The SETI@home project used [[Volunteer computing|volunteer computing]] to analyze signals acquired by the [[Astronomy:SERENDIP|SERENDIP]] project.

SETI@home was conceived by David Gedye along with Craig Kasnoff and is a popular [[Volunteer computing|volunteer computing]] project that was launched by the [[Organization:Berkeley SETI Research Center|Berkeley SETI Research Center]] at the [[Organization:University of California, Berkeley|University of California, Berkeley]], in May 1999. It was originally funded by [[Organization:The Planetary Society|The Planetary Society]] and Paramount Pictures, and later by the state of {{wipe|California}}. The project is run by director [[Biography:David P. Anderson|David P. Anderson]] and chief scientist Dan Werthimer. Any individual could become involved with SETI research by downloading the [[Software:Berkeley Open Infrastructure for Network Computing|Berkeley Open Infrastructure for Network Computing]] (BOINC) software program, attaching to the SETI@home project, and allowing the program to run as a background process that uses idle computer power. The SETI@home program itself ran signal analysis on a "work unit" of data recorded from the central 2.5&nbsp;MHz wide band of the SERENDIP IV instrument. After computation on the work unit was complete, the results were then automatically reported back to SETI@home [[Server (computing)|server]]s at University of California, Berkeley. By June 28, 2009, the SETI@home project had over 180,000 active participants volunteering a total of over 290,000 computers. These computers gave SETI@home an average computational power of 617 [[FLOPS|teraFLOPS]].<ref>{{cite web | title=SETI@home&nbsp;– Credit Overview | last=de Zutter | first=Willy | publisher=BOINCstats | access-date=June 28, 2009 | url=http://www.boincstats.com/stats/project_graph.php?pr=sah | url-status=dead | archive-url=https://web.archive.org/web/20091215135415/http://boincstats.com/stats/project_graph.php?pr=sah | archive-date=December 15, 2009 }}</ref> In 2004 radio source SHGb02+14a set off speculation in the media that a signal had been detected but researchers noted the frequency drifted rapidly and the detection on three SETI@home computers fell within random chance.<ref name="BBC">{{cite news |work = BBC News |url = http://news.bbc.co.uk/1/hi/sci/tech/3621608.stm |title = Astronomers deny ET signal report |date = 2004-09-02 |first = David |last = Whitehouse |access-date = 24 April 2013 |archive-date = 2006-06-15 |archive-url = https://web.archive.org/web/20060615112727/http://news.bbc.co.uk/1/hi/sci/tech/3621608.stm |url-status = live }}</ref><ref>{{cite news |publisher = [[Organization:The Planetary Society|The Planetary Society]] |title = SETI@home Leaders Deny Reports of Likely Extraterrestrial Signal |url = http://planetary.org/news/2004/0902_SETIhome_Leaders_Deny_Reports_of.html |date = 2004-09-02 |last = Alexander |first = Amir |access-date = 2006-06-12 |archive-url = https://web.archive.org/web/20110726074250/http://planetary.org/news/2004/0902_SETIhome_Leaders_Deny_Reports_of.html |archive-date = 2011-07-26 }}</ref>

By 2010, after 10 years of data collection, SETI@home had listened to that one frequency at every point of over 67 percent of the sky observable from Arecibo with at least three scans (out of the goal of nine scans), which covers about 20 percent of the full celestial sphere.<ref>Alan M. MacRobert. [http://www.skyandtelescope.com/resources/seti/3304561.html?page=5&c=y "SETI Searches Today"] {{Webarchive|url=https://web.archive.org/web/20140103004454/http://www.skyandtelescope.com/resources/seti/3304561.html?page=5&c=y |date=2014-01-03 }}. ''Sky and Telescope'' (2010?).</ref> On March 31, 2020, with 91,454 active users, the project stopped sending out new work to SETI@home users, bringing this particular SETI effort to an indefinite hiatus.<ref>{{Cite news|last=Overbye|first=Dennis|url=https://www.nytimes.com/2020/03/23/science/seti-at-home-aliens.html|title=The Search for E.T. Goes on Hold, for Now|date=2020-03-23|work=The New York Times|access-date=2020-03-23|language=en-US|issn=0362-4331|archive-date=2020-04-07|archive-url=https://web.archive.org/web/20200407093808/https://www.nytimes.com/2020/03/23/science/seti-at-home-aliens.html|url-status=live}}</ref>

===SETI Net===
SETI Network was the only fully operational private search system.<ref>{{Cite web |date=2020-11-27 |title=Searching For Extraterrestrial Intelligence (SETI) with a HackRF |url=https://www.rtl-sdr.com/searching-for-extraterrestrial-intelligence-seti-with-a-hackrf/ |access-date=2022-05-30 |website=rtl-sdr.com |language=en-US |archive-date=2022-05-29 |archive-url=https://web.archive.org/web/20220529011446/https://www.rtl-sdr.com/searching-for-extraterrestrial-intelligence-seti-with-a-hackrf/ |url-status=live }}</ref> The SETI Net station consisted of off-the-shelf, consumer-grade electronics to minimize cost and to allow this design to be replicated as simply as possible. It had a 3-meter parabolic antenna that could be directed in azimuth and elevation, an LNA that covered 100&nbsp;MHz of the 1420&nbsp;MHz spectrum, a receiver to reproduce the wideband audio, and a standard [[Engineering:Personal computer|personal computer]] as the control device and for deploying the detection algorithms. The antenna could be pointed and locked to one sky location in Ra and DEC which enabling the system to integrate on it for long periods. The [[Astronomy:Wow! signal|Wow! signal]] area was monitored for many long periods. All search data was collected and is available on the Internet archive.

SETI Net started operation in the early 1980s as a way to learn about the science of the search, and developed several software packages for the amateur SETI community. It provided an astronomical clock, a file manager to keep track of SETI data files, a spectrum analyzer optimized for amateur SETI, remote control of the station from the Internet, and other packages.

SETI Net went dark and was decommissioned on 2021-12-04. The collected data is available on their website.

=== The SETI League and Project Argus ===
Founded in 1994 in response to the United States Congress cancellation of the NASA SETI program, The SETI League, Incorporated is a membership-supported nonprofit organization with 1,500 members in 62 countries. This grass-roots alliance of amateur and professional radio astronomers is headed by executive director emeritus [[Biography:H. Paul Shuch|H. Paul Shuch]], the engineer credited with developing the world's first commercial home satellite TV receiver. Many SETI League members are licensed radio amateurs and microwave experimenters. Others are digital signal processing experts and computer enthusiasts.

The SETI League pioneered the conversion of backyard satellite TV dishes {{convert|abbr=on|3|to(-)|5|m|ft|0}} in diameter into research-grade radio telescopes of modest sensitivity.<ref name="Chown 28">{{cite journal |last=Chown |first=Marcus |journal=New Scientist |title=The Alien Spotters |date=April 1997 |page=28 |url=https://www.newscientist.com/article/mg15420784.500-the-alien-spotters.html |access-date=2008-04-13 |archive-date=2023-01-16 |archive-url=https://web.archive.org/web/20230116230912/https://www.newscientist.com/article/mg15420784-500-the-alien-spotters/ |url-status=live }}</ref> The organization concentrates on coordinating a global network of small, amateur-built radio telescopes under Project Argus, an all-sky survey seeking to achieve real-time coverage of the entire sky.<ref>{{cite web |author=Shuch |first=H. Paul |title=The SETI League, Inc.: Project Argus |url=http://www.setileague.org/argus/ |url-status=live |archive-url=https://web.archive.org/web/20141229104106/http://www.setileague.org/argus/ |archive-date=2014-12-29 |access-date=2014-12-30}}</ref> Project Argus was conceived as a continuation of the all-sky survey component of the late NASA SETI program (the targeted search having been continued by the SETI Institute's Project Phoenix). There are currently 143 Project Argus radio telescopes operating in 27 countries. Project Argus instruments typically exhibit sensitivity on the order of 10<sup>−23</sup> Watts/square metre, or roughly equivalent to that achieved by the Ohio State University Big Ear radio telescope in 1977, when it detected the landmark "Wow!" candidate signal.<ref>{{Cite web|url=http://www.setileague.org/articles/capri.htm|title=Project Argus and the Challenge of Real-Time All-Sky SETI|website=www.setileague.org|access-date=2019-12-13|archive-date=2019-11-28|archive-url=https://web.archive.org/web/20191128064120/http://www.setileague.org/articles/capri.htm|url-status=live}}</ref>

The name "Argus" derives from the mythical Greek guard-beast who had 100 eyes, and could see in all directions at once. In the SETI context, the name has been used for radio telescopes in fiction (Arthur C. Clarke, ''"Imperial Earth"''; Carl Sagan, ''"Contact"''), was the name initially used for the NASA study ultimately known as "Cyclops," and is the name given to an omnidirectional radio telescope design being developed at the Ohio State University.<ref>{{Cite web|url=https://www.skyandtelescope.com/astronomy-news/the-future-of-seti/|title=The Future of SETI|last=Shostak|first=Seth|date=2006-07-19|website=Sky & Telescope|language=en-US|access-date=2019-12-13|archive-date=2019-05-24|archive-url=https://web.archive.org/web/20190524193142/https://www.skyandtelescope.com/astronomy-news/the-future-of-seti/|url-status=live}}</ref>

==Optical experiments==
<!-- This section is linked from [[Astronomy:Fermi paradox|Fermi paradox]] -->
While most SETI sky searches have studied the radio spectrum, some SETI researchers have considered the possibility that alien civilizations might be using powerful [[Physics:Laser|laser]]s for interstellar communications at optical wavelengths.<ref>{{Cite journal |last=Kingsley |first=Stuart A. |editor-first1=Stuart A |editor-last1=Kingsley |date=1993-08-06 |title=&lt;title&gt;The search for extraterrestrial intelligence (SETI) in the optical spectrum: a review&lt;/title&gt; |url=http://dx.doi.org/10.1117/12.150129 |journal=SPIE Proceedings |volume=1867 |pages=75–113 |publisher=SPIE |doi=10.1117/12.150129|s2cid=119615013 }}</ref><ref>{{Cite journal |last1=Schuetz |first1=Marlin |last2=Vakoch |first2=Douglas A. |last3=Shostak |first3=Seth |last4=Richards |first4=Jon |title=Optical Seti Observations of the Anomalous Star Kic 8462852 |date=2016-06-24 |journal=The Astrophysical Journal |volume=825 |issue=1 |pages=L5 |doi=10.3847/2041-8205/825/1/l5 |arxiv=1512.02388 |bibcode=2016ApJ...825L...5S |s2cid=119194869 |issn=2041-8213 |doi-access=free }}</ref><ref name="SPIE">{{Cite book |last1=Wright |first1=Shelley A. |last2=Horowitz |first2=Paul |last3=Maire |first3=Jérôme |last4=Chaim-Weismann |first4=Samuel A. |last5=Cosens |first5=Maren |last6=Drake |first6=Frank D. |last7=Howard |first7=Andrew W. |last8=Marcy |first8=Geoffrey W. |last9=Siemion |first9=Andrew P. V. |last10=Stone |first10=Remington P. S. |last11=Treffers |first11=Richard R. |last12=Uttamchandani |first12=Avinash |last13=Werthimer |first13=Dan |title=Ground-based and Airborne Instrumentation for Astronomy VII |chapter=Panoramic optical and near-infrared SETI instrument: Overall specifications and science program |editor-first1=Luc |editor-first2=Christopher J |editor-first3=Hideki |editor-last1=Simard |editor-last2=Evans |editor-last3=Takami |date=2018-07-23 |chapter-url=http://dx.doi.org/10.1117/12.2314268 |volume=10702 |page=201 |publisher=SPIE |doi=10.1117/12.2314268|arxiv=1808.05772 |bibcode=2018SPIE10702E..5IW |isbn=9781510619579 |s2cid=55939829 }}</ref> The idea was first suggested by R. N. Schwartz and Charles Hard Townes in a 1961 paper published in the journal ''Nature'' titled "Interstellar and Interplanetary Communication by Optical Masers". However, the 1971 Cyclops study discounted the possibility of optical SETI, reasoning that construction of a laser system that could outshine the bright central star of a remote star system would be too difficult. In 1983, Townes published a detailed study of the idea in the United States journal ''Proceedings of the National Academy of Sciences'',<ref name="Townes1983">{{cite journal|last1=Townes|first1=C. H.|title=At what wavelengths should we search for signals from extraterrestrial intelligence?|journal=Proceedings of the National Academy of Sciences|volume=80|issue=4|year=1983|pages=1147–1151|doi=10.1073/pnas.80.4.1147|pmid=16593279|bibcode = 1983PNAS...80.1147T |pmc=393547|doi-access=free}}</ref> which was met with interest by the SETI community.

There are two problems with optical SETI. The first problem is that lasers are highly "monochromatic", that is, they emit light only on one frequency, making it troublesome to figure out what frequency to look for.<ref>{{Cite journal |last1=Marcy |first1=Geoffrey W. |last2=Tellis |first2=Nathaniel K. |last3=Wishnow |first3=Edward H. |date=2022-07-13 |title=A search for monochromatic light towards the Galactic Centre |url=http://dx.doi.org/10.1093/mnras/stac1933 |journal=Monthly Notices of the Royal Astronomical Society |volume=515 |issue=3 |pages=3898–3910 |arxiv=2208.13561 |doi=10.1093/mnras/stac1933 |issn=0035-8711}}</ref> However, emitting light in narrow pulses results in a broad spectrum of emission; the spread in frequency becomes higher as the pulse width becomes narrower, making it easier to detect an emission.

The other problem is that while radio transmissions can be broadcast in all directions, lasers are highly directional. Interstellar gas and dust is almost transparent to near infrared, so these signals can be seen from greater distances, but the extraterrestrial laser signals would need to be transmitted in the direction of Earth in order to be detected.<ref name="PhysOrg 2016">{{Cite web |last=University of California – San Diego |title=Search for extraterrestrial intelligence extends to new realms |url=https://phys.org/news/2015-03-extraterrestrial-intelligence-realms.html |access-date=2023-02-01 |website=phys.org |language=en-us}}</ref><ref>{{Cite journal |last1=Clark |first1=James R. |last2=Cahoy |first2=Kerri |date=2018-11-05 |title=Optical Detection of Lasers with Near-term Technology at Interstellar Distances |journal=The Astrophysical Journal |volume=867 |issue=2 |pages=97 |doi=10.3847/1538-4357/aae380 |bibcode=2018ApJ...867...97C |s2cid=125463857 |issn=1538-4357|hdl=1721.1/135859 |hdl-access=free |doi-access=free }}</ref>

Optical SETI supporters have conducted paper studies<ref>{{cite book |title=SETI 2020: A Roadmap for the Search for Extraterrestrial Intelligence |date=2003 |publisher=SETI Press |isbn=978-0-9666335-3-5 |editor=Exers |editor-first=Ronald |editor2=Cullers |editor-first2=D. |editor3=Billingham |editor-first3=J. |editor4=Scheffer |editor-first4=L.}}</ref> of the effectiveness of using contemporary high-energy lasers and a ten-meter diameter mirror as an interstellar beacon. The analysis shows that an infrared pulse from a laser, focused into a narrow beam by such a mirror, would appear thousands of times brighter than the Sun to a distant civilization in the beam's line of fire. The Cyclops study proved incorrect in suggesting a laser beam would be inherently hard to see.

Such a system could be made to automatically steer itself through a target list, sending a pulse to each target at a constant rate. This would allow targeting of all Sun-like stars within a distance of 100 light-years. The studies have also described an automatic laser pulse detector system with a low-cost, two-meter mirror made of carbon composite materials, focusing on an array of light detectors. This automatic detector system could perform sky surveys to detect laser flashes from civilizations attempting contact.

Several optical SETI experiments are now in progress. A Harvard-Smithsonian group that includes Paul Horowitz designed a laser detector and mounted it on Harvard's {{convert|155|cm|in|abbr=off|adj=on|sp=us}} optical telescope. This telescope is currently being used for a more conventional star survey, and the optical SETI survey is "piggybacking" on that effort. Between October 1998 and November 1999, the survey inspected about 2,500 stars. Nothing that resembled an intentional laser signal was detected, but efforts continue. The Harvard-Smithsonian group is now working with [[Organization:Princeton University|Princeton University]] to mount a similar detector system on Princeton's 91-centimeter (36-inch) telescope. The Harvard and Princeton telescopes will be "ganged" to track the same targets at the same time, with the intent being to detect the same signal in both locations as a means of reducing errors from detector noise.

The Harvard-Smithsonian SETI group led by Professor [[Biography:Paul Horowitz|Paul Horowitz]] built a dedicated all-sky optical survey system along the lines of that described above, featuring a 1.8-meter (72-inch) telescope. The new optical SETI survey telescope is being set up at the Oak Ridge Observatory in Harvard, Massachusetts.

The University of California, Berkeley, home of [[Astronomy:SERENDIP|SERENDIP]] and [[Astronomy:SETI@home|SETI@home]], is also conducting optical SETI searches and collaborates with the [[Astronomy:NIROSETI|NIROSETI]] program. The optical SETI program at Breakthrough Listen was initially directed by [[Biography:Geoffrey Marcy|Geoffrey Marcy]], an extrasolar planet hunter, and it involves examination of records of spectra taken during extrasolar planet hunts for a continuous, rather than pulsed, laser signal. This survey uses the [[Astronomy:Automated Planet Finder|Automated Planet Finder]] 2.4-m telescope at the [[Organization:Lick Observatory|Lick Observatory]], situated on the summit of Mount Hamilton, east of San Jose, California.<ref name="arxiv.org">{{Cite journal |last1=Vogt |first1=Steven S. |last2=Radovan |first2=Matthew |last3=Kibrick |first3=Robert |last4=Butler |first4=R. Paul |last5=Alcott |first5=Barry |last6=Allen |first6=Steve |last7=Arriagada |first7=Pamela |last8=Bolte |first8=Mike |last9=Burt |first9=Jennifer |last10=Cabak |first10=Jerry |last11=Chloros |first11=Kostas |last12=Cowley |first12=David |last13=Deich |first13=William |last14=Dupraw |first14=Brian |last15=Earthman |first15=Wayne |date=April 2014 |title=APF – The Lick Observatory Automated Planet Finder |journal=Publications of the Astronomical Society of the Pacific |volume=126 |issue=938 |pages=359–379 |doi=10.1086/676120|arxiv=1402.6684 |bibcode=2014PASP..126..359V |s2cid=12067979 }}</ref> The other Berkeley optical SETI effort is being pursued by the Harvard-Smithsonian group and is being directed by Dan Werthimer of Berkeley, who built the laser detector for the Harvard-Smithsonian group. This survey uses a 76-centimeter (30-inch) automated telescope at Leuschner Observatory and an older laser detector built by Werthimer.

The SETI Institute also runs a program called '[[Astronomy:Laser SETI|Laser SETI]]' with an instrument composed of several cameras that continuously survey the entire night sky searching for millisecond singleton laser pulses of extraterrestrial origin.<ref>{{Cite news |last=SETI |first=Institute |title=Why We Need a New Type of SETI Instrument |url=https://www.seti.org/laserseti |access-date=2022-05-30 |archive-date=2021-12-26 |archive-url=https://web.archive.org/web/20211226111322/https://www.seti.org/laserseti |url-status=live }}</ref><ref>{{Cite web |author1=David |first=Leonard |date=2017-08-07 |title=New 'Laser SETI' Approach Seeks Crowdfunding to Seek Out Alien Life |url=https://www.space.com/37690-search-for-life-laser-seti-project.html |url-status=live |archive-url=https://web.archive.org/web/20230116230912/https://www.space.com/37690-search-for-life-laser-seti-project.html |archive-date=2023-01-16 |access-date=2022-05-30 |website=Space.com |language=en-us}}</ref>

In January 2020, two Pulsed All-sky Near-infrared Optical SETI (PANOSETI) project telescopes were installed in the Lick Observatory Astrograph Dome. The project aims to commence a wide-field optical SETI search and continue prototyping designs for a full observatory. The installation can offer an "all-observable-sky" optical and wide-field near-infrared pulsed technosignature and astrophysical transient search for the northern hemisphere.<ref>{{Cite web |title=PANOSETI |url=https://oirlab.ucsd.edu/PANOSETI.html |access-date=2023-02-01 |website=oirlab.ucsd.edu}}</ref><ref name="SPIE"/>

In May 2017, astronomers reported studies related to laser light emissions from stars, as a way of detecting technology-related signals from an [[Philosophy:Extraterrestrial intelligence|alien civilization]]. The reported studies included [[Astronomy:Tabby's Star|Tabby's Star]] (designated KIC 8462852 in the [[Astronomy:Kepler Input Catalog|Kepler Input Catalog]]), an oddly dimming star in which its unusual starlight fluctuations may be the result of interference by an artificial megastructure, such as a [[Astronomy:Dyson sphere#Dyson swarm|Dyson swarm]], made by such a civilization. No evidence was found for technology-related signals from KIC 8462852 in the studies.<ref name="ATL-20170417">{{cite web |last=Koren |first=Marina |date=17 April 2017 |title=Searching the Skies for Alien Laser Beams – A new study scanned 5,600 stars for tiny emissions of light, which may be the best way for an extraterrestrial civilization to signal its existence. |url=https://www.theatlantic.com/science/archive/2017/04/laser-seti-extraterrestrial/523104/ |url-status=live |archive-url=https://web.archive.org/web/20170615113559/https://www.theatlantic.com/science/archive/2017/04/laser-seti-extraterrestrial/523104/ |archive-date=15 June 2017 |access-date=3 June 2017 |work=The Atlantic}}</ref><ref name="AXV-201704">{{cite journal |last1=Tellis |first1=Nathaniel K. |last2=Marcy |first2=Geoffrey W. |date=April 2017 |title=A Search for Laser Emission with Megawatt Thresholds from 5600 FGKM Stars |journal=The Astronomical Journal |volume=153 |issue=6 |page=251 |arxiv=1704.02535 |bibcode=2017AJ....153..251T |doi=10.3847/1538-3881/aa6d12 |s2cid=119088358 |doi-access=free }}</ref><ref name="TAJ-201770512">{{cite journal |last1=Tellis |first1=Nathaniel K. |last2=Marcy |first2=Geoffrey W. |date=12 May 2017 |title=A Search for Laser Emission with Megawatt Thresholds from 5600 FGKM Stars |journal=[[Astronomy:The Astronomical Journal|The Astronomical Journal]] |volume=153 |page=251 |arxiv=1704.02535 |bibcode=2017AJ....153..251T |doi=10.3847/1538-3881/aa6d12 |number=6 |s2cid=119088358 |doi-access=free }}</ref>

==Quantum communications==
In a 2021 [[Preprint|preprint]], an astronomer described for the first time how one could search for [[Physics:Quantum network|quantum communication]] transmissions sent by [[Philosophy:Extraterrestrial intelligence|ETI]] using existing telescope and receiver technology. He also provides arguments for why future searches of ETI should also target interstellar quantum communication networks.<ref>{{cite news |title=We could detect alien civilizations through their interstellar quantum communication |url=https://phys.org/news/2021-04-alien-civilizations-interstellar-quantum.html |access-date=9 May 2021 |work=phys.org |language=en |archive-date=9 May 2021 |archive-url=https://web.archive.org/web/20210509144009/https://phys.org/news/2021-04-alien-civilizations-interstellar-quantum.html |url-status=live }}</ref><ref>{{cite journal |last1=Hippke |first1=Michael |title=Searching for Interstellar Quantum Communications |journal=The Astronomical Journal |date=13 April 2021 |volume=162 |issue=1 |page=1 |doi=10.3847/1538-3881/abf7b7 |arxiv=2104.06446 |bibcode=2021AJ....162....1H |s2cid=233231350 |doi-access=free }}</ref>

A 2022 paper noted that interstellar quantum communication by other civilizations could be possible and may be advantageous, identifying some potential challenges and factors for detecting [[Astronomy:Technosignature|technosignature]]s. They may, for example, use X-ray photons for remotely established quantum communication and [[Quantum teleportation|quantum teleportation]] as the communication mode.<ref>{{cite news |last1=Conover |first1=Emily |title=Aliens could send quantum messages to Earth, calculations suggest |url=https://www.sciencenews.org/article/alien-quantum-communication-extraterrestrial-communication-signal |access-date=13 July 2022 |work=Science News |date=5 July 2022 |archive-date=13 July 2022 |archive-url=https://web.archive.org/web/20220713125941/https://www.sciencenews.org/article/alien-quantum-communication-extraterrestrial-communication-signal |url-status=live }}</ref><ref>{{cite journal |last1=Berera |first1=Arjun |last2=Calderón-Figueroa |first2=Jaime |title=Viability of quantum communication across interstellar distances |journal=Physical Review D |date=28 June 2022 |volume=105 |issue=12 |pages=123033 |doi=10.1103/PhysRevD.105.123033|arxiv=2205.11816|bibcode=2022PhRvD.105l3033B |s2cid=249017926 }}</ref>

==Search for extraterrestrial artifacts==
The possibility of using interstellar messenger probes in the search for extraterrestrial intelligence was first suggested by Ronald N. Bracewell in 1960 (see [[Astronomy:Bracewell probe|Bracewell probe]]), 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>{{cite web | title=Interstellar probes&nbsp;– A new approach to SETI | last=Freitas | first=Robert A. Jr. | date=1980 | access-date=June 28, 2009 | url=http://www.rfreitas.com/Astro/InterstellarProbesJBIS1980.htm | archive-date=January 1, 2019 | archive-url=https://web.archive.org/web/20190101151515/http://www.rfreitas.com/Astro/InterstellarProbesJBIS1980.htm | url-status=live }}</ref><ref>{{cite web |last=Freitas |first=Robert A. Jr. |date=1983 |title=Debunking the Myths of Interstellar Probes |url=http://www.rfreitas.com/Astro/ProbeMyths1983.htm |url-status=live |archive-url=https://web.archive.org/web/20160814012941/http://www.rfreitas.com/Astro/ProbeMyths1983.htm |archive-date=August 14, 2016 |access-date=June 28, 2009}}</ref><ref>{{cite web | title=The Case for Interstellar Probes | last=Freitas | first=Robert A. Jr. | date=1983 | access-date=June 28, 2009 | url=http://www.rfreitas.com/Astro/TheCaseForInterstellarProbes1983.htm | archive-date=March 13, 2020 | archive-url=https://web.archive.org/web/20200313145008/http://www.rfreitas.com/Astro/TheCaseForInterstellarProbes1983.htm | url-status=live }}</ref> for the proposition that physical space-probes are a superior mode of interstellar communication to radio signals (see [[Astronomy:Voyager Golden Record|Voyager Golden Record]]).

In recognition that any sufficiently advanced interstellar probe in the vicinity of Earth could easily monitor the terrestrial [[Internet]], 'Invitation to ETI' was established by Allen Tough in 1996, as a Web-based SETI experiment inviting such spacefaring probes to establish contact with humanity. The project's 100 signatories includes prominent physical, biological, and social scientists, as well as artists, educators, entertainers, philosophers and futurists. [[Biography:H. Paul Shuch|H. Paul Shuch]], executive director emeritus of The SETI League, serves as the project's Principal Investigator.

Inscribing a message in matter and transporting it to an interstellar destination can be enormously more energy efficient than communication using electromagnetic waves if delays larger than light transit time can be tolerated.<ref>{{cite journal |author=Rose |first1=C. |last2=Wright |first2=G. |name-list-style=amp |date=2 September 2004 |title=Inscribed matter as an energy efficient means of communication with an extraterrestrial civilization |url=http://www.winlab.rutgers.edu/~crose/papers/nature.pdf |url-status=live |journal=Nature |volume=431 |issue=7004 |pages=47–9 |bibcode=2004Natur.431...47R |doi=10.1038/nature02884 |pmid=15343327 |s2cid=4382105 |archive-url=https://web.archive.org/web/20050512081549/http://www.winlab.rutgers.edu/~crose/papers/nature.pdf |archive-date=12 May 2005 |access-date=29 July 2005}}</ref> That said, for simple messages such as "hello," radio SETI could be far more efficient.<ref>{{cite journal |author=Sullivan |first=Woodruff T. |date=2 September 2004 |title=Message in a bottle |journal=Nature Magazine |volume=431 |issue=7004 |pages=27–28 |bibcode=2004Natur.431...27S |doi=10.1038/431027a |pmid=15343314 |s2cid=5464290}}</ref> If energy requirement is used as a proxy for technical difficulty, then a solarcentric Search for Extraterrestrial Artifacts (SETA)<ref>{{cite web |last=Freitas |first=Robert A. Jr. |date=November 1983 |title=The Search for Extraterrestrial Artifacts (SETA) |url=http://www.rfreitas.com/Astro/SETAJBISNov1983.htm |url-status=live |archive-url=https://web.archive.org/web/20090917060101/http://www.rfreitas.com/Astro/SETAJBISNov1983.htm |archive-date=September 17, 2009 |access-date=June 28, 2009}}</ref> may be a useful supplement to traditional radio or optical searches.<ref>{{cite news |date=8 September 2004 |title=New York Times Editorial |work={{wipe|The New York Times}} |url=https://www.nytimes.com/2004/09/08/opinion/08wed3.html?_r=1&ex=1252382400&en=5d6483f134ede447&ei=5090&partner=rssuserland |url-status=live |access-date=18 February 2017 |archive-url=https://web.archive.org/web/20230116230912/https://www.nytimes.com/2004/09/08/opinion/the-search-for-livable-worlds.html |archive-date=16 January 2023}}</ref><ref>{{cite web | title=Cosmic Communications | last=Rose | first=Christopher | date=September 2004 | access-date=August 1, 2010 | url=http://www.winlab.rutgers.edu/~crose/cgi-bin/cosmicB.html | archive-date=July 1, 2010 | archive-url=https://web.archive.org/web/20100701215805/http://www.winlab.rutgers.edu/%7Ecrose/cgi-bin/cosmicB.html | url-status=live }}</ref>

Much like the "preferred frequency" concept in SETI radio beacon theory, the Earth-Moon or Sun-Earth [[Astronomy:Libration|libration]] orbits<ref name="Earth-Moon"/> might therefore constitute the most universally convenient parking places for automated extraterrestrial spacecraft exploring arbitrary stellar systems. A viable long-term SETI program may be founded upon a search for these objects.

In 1979, Freitas and Valdes conducted a photographic search of the vicinity of the Earth-Moon triangular libration points {{L4}} and {{L5}}, and of the solar-synchronized positions in the associated halo orbits, seeking possible orbiting extraterrestrial interstellar probes, but found nothing to a detection limit of about 14th magnitude.<ref name="Earth-Moon">{{cite web |last1=Freitas |first1=Robert A. Jr. |last2=Valdes |first2=Francisco |date=1980 |title=A Search for Natural or Artificial Objects Located at the Earth-Moon Libration Points |url=http://www.rfreitas.com/Astro/SearchIcarus1980.htm |url-status=live |archive-url=https://web.archive.org/web/20190826152815/http://www.rfreitas.com/Astro/SearchIcarus1980.htm |archive-date=August 26, 2019 |access-date=June 28, 2009}}</ref> The authors conducted a second, more comprehensive photographic search for probes in 1982<ref>{{cite web |last1=Valdes |first1=Francisco |last2=Freitas |first2=Robert A. Jr. |date=1983 |title=A Search for Objects near the Earth-Moon Lagrangian Points |url=http://www.rfreitas.com/Astro/SearchIcarus1983.htm |url-status=live |archive-url=https://web.archive.org/web/20190831162847/http://www.rfreitas.com/Astro/SearchIcarus1983.htm |archive-date=2019-08-31 |access-date=2004-11-12}}</ref> that examined the five Earth-Moon [[Astronomy:Lagrangian point|Lagrangian positions]] and included the solar-synchronized positions in the stable L4/L5 libration orbits, the potentially stable nonplanar orbits near L1/L2, Earth-Moon {{L3}}, and also {{L2}} in the Sun-Earth system. Again no extraterrestrial probes were found to limiting magnitudes of 17–19th magnitude near L3/L4/L5, 10–18th magnitude for {{L1}}/{{L2}}, and 14–16th magnitude for Sun-Earth {{L2}}.

In June 1983, Valdes and Freitas used the 26 m radiotelescope at Hat Creek Radio Observatory to search for the tritium hyperfine line at 1516&nbsp;MHz from 108 assorted astronomical objects, with emphasis on 53 nearby stars including all visible stars within a 20 light-year radius. The tritium frequency was deemed highly attractive for SETI work because (1) the isotope is cosmically rare, (2) the tritium hyperfine line is centered in the SETI [[Engineering:Water hole (radio)|water hole]] region of the terrestrial microwave window, and (3) in addition to beacon signals, tritium hyperfine emission may occur as a byproduct of extensive [[Physics:Nuclear fusion|nuclear fusion]] energy production by extraterrestrial civilizations. The wideband- and narrowband-channel observations achieved sensitivities of 5–14{{e|&minus;21}}&nbsp;W/m<sup>2</sup>/channel and 0.7&ndash;2{{e|&minus;24}}&nbsp;W/m<sup>2</sup>/channel, respectively, but no detections were made.<ref>{{cite web |last1=Valdes |first1=Francisco |last2=Freitas |first2=Robert A. Jr. |date=1986 |title=A Search for the Tritium Hyperfine Line from Nearby Stars |url=http://www.rfreitas.com/Astro/TritiumSearch1986.htm |url-status=live |archive-url=https://web.archive.org/web/20050206001607/http://rfreitas.com/Astro/TritiumSearch1986.htm |archive-date=2005-02-06 |access-date=2004-11-12}}</ref>

Others have speculated, that we might find traces of past civilizations in our very own Solar System, on planets like [[Astronomy:Venus|Venus]] or [[Astronomy:Mars|Mars]], although the traces would be found most likely underground.<ref name="Koren 2017">{{cite web | last=Koren | first=Marina | title=Was the Solar System Previously Home to Another Intelligence? | website=The Atlantic | date=2017-04-26 | url=https://www.theatlantic.com/science/archive/2017/04/was-there-anybody-out-there/524235/ | access-date=2023-04-21|quote="For all we know, maybe Venus had cities all over it a billion years ago and now they’re gone," he [Jason Wright] said.}}</ref><ref name="Wright 2017 pp. 96–100">{{cite journal | last=Wright | first=Jason T. | title=Prior indigenous technological species | journal=International Journal of Astrobiology | publisher=Cambridge University Press (CUP) | volume=17 | issue=1 | date=2017-06-01 | issn=1473-5504 | doi=10.1017/s1473550417000143 | pages=96–100| arxiv=1704.07263 | s2cid=119028817 }}</ref>

==Technosignatures==
Technosignatures, including all signs of technology, are a recent avenue in the search for extraterrestrial intelligence.<ref name="WP-20201231">{{cite news |last=Frank |first=Adam |title=A new frontier is opening in the search for extraterrestrial life - The reason we haven't found life elsewhere in the universe is simple: We haven't really looked until now. |url=https://www.washingtonpost.com/outlook/2020/12/31/breakthrough-listen-seti-technosignatures/ |date=31 December 2020 |newspaper={{wipe|The Washington Post}} |access-date=1 January 2021 |archive-date=27 December 2021 |archive-url=https://web.archive.org/web/20211227105342/https://www.washingtonpost.com/outlook/2020/12/31/breakthrough-listen-seti-technosignatures/ |url-status=live }}</ref><ref name="NYT-20170628" /> Technosignatures may originate from various sources, from megastructures such as [[Astronomy:Dyson sphere|Dyson sphere]]s<ref>{{Cite journal |last1=Zackrisson |first1=Erik |last2=Calissendorff |first2=Per |last3=Asadi |first3=Saghar |last4=Nyholm |first4=Anders |date=2015-08-27 |title=Extragalactic SETI: the Tully–fisher Relation as a Probe of Dysonian Astroengineering in Disk Galaxies |url=http://dx.doi.org/10.1088/0004-637x/810/1/23 |journal=The Astrophysical Journal |volume=810 |issue=1 |pages=23 |doi=10.1088/0004-637x/810/1/23 |arxiv=1508.02406 |bibcode=2015ApJ...810...23Z |s2cid=118642500 |issn=1538-4357}}</ref><ref>{{Cite journal |last1=Hsiao |first1=Tiger Yu-Yang |last2=Goto |first2=Tomotsugu |last3=Hashimoto |first3=Tetsuya |last4=Santos |first4=Daryl Joe D |last5=On |first5=Alvina Y L |last6=Kilerci-Eser |first6=Ece |last7=Wong |first7=Yi Hang Valerie |last8=Kim |first8=Seong Jin |last9=Wu |first9=Cossas K-W |last10=Ho |first10=Simon C-C |last11=Lu |first11=Ting-Yi |date=2021-07-01 |title=A Dyson sphere around a black hole |url=https://doi.org/10.1093/mnras/stab1832 |journal=Monthly Notices of the Royal Astronomical Society |volume=506 |issue=2 |pages=1723–1732 |doi=10.1093/mnras/stab1832 |issn=0035-8711|arxiv=2106.15181 }}</ref> and space mirrors or [[Earth:Space sunshade|space shaders]]<ref>{{cite journal |last=Korpela |first=Eric |date=2015 |title=Modeling Indications of Technology in Planetary Transit Light Curves – Dark-side illumination |journal=The Astrophysical Journal |volume=809 |issue=2 |page=139 |arxiv=1505.07399 |bibcode=2015ApJ...809..139K |doi=10.1088/0004-637X/809/2/139 |s2cid=119113487}}</ref> to the atmospheric contamination created by an industrial civilization,<ref name=almar-2011>{{cite journal|doi=10.1016/j.actaastro.2011.05.036|author=Almár, Iván|volume=69|date=2011|pages=899–904|title=SETI and astrobiology: The Rio Scale and the London Scale|journal=Acta Astronautica|issue=9–10|bibcode = 2011AcAau..69..899A }}</ref> or city lights on extrasolar planets, and may be detectable in the future with large hypertelescopes.<ref name=colossus>{{cite news |url=http://www.space.com/21480-extraterrestrial-civilization-heat-seeking-telescope.html |title=Heat-Seeking, Alien-Hunting Telescope Could Be Ready In 5 Years |work=Space.com |date=2013-06-07 |access-date=2013-07-10 |archive-date=2013-07-01 |archive-url=https://web.archive.org/web/20130701071334/http://www.space.com/21480-extraterrestrial-civilization-heat-seeking-telescope.html |url-status=live }}</ref>

Technosignatures can be divided into three broad categories: astroengineering projects, signals of planetary origin, and spacecraft within and outside the [[Astronomy:Solar System|Solar System]].

An astroengineering installation such as a [[Astronomy:Dyson sphere|Dyson sphere]], designed to convert all of the incident radiation of its host star into energy, could be detected through the observation of an infrared excess from a solar analog star,<ref name="search">{{cite journal |author=Dyson |first=Freeman J. |date=1960 |title=Search for Artificial Stellar Sources of Infra-Red Radiation |url=http://www.islandone.org/LEOBiblio/SETI1.HTM |url-status=dead |journal=Science |volume=131 |issue=3414 |pages=1667–1668 |bibcode=1960Sci...131.1667D |doi=10.1126/science.131.3414.1667 |pmid=17780673 |s2cid=3195432 |archive-url=https://web.archive.org/web/20190714215002/http://www.islandone.org/LEOBiblio/SETI1.HTM |archive-date=2019-07-14 |access-date=2012-09-25}}</ref> or by the star's apparent disappearance in the visible spectrum over several years.<ref>{{Cite web |last=Hall |first=Shannon |title=Impossible vanishing stars could be signs of advanced alien life |url=https://www.newscientist.com/article/2095785-impossible-vanishing-stars-could-be-signs-of-advanced-alien-life/ |access-date=2023-02-02 |website=New Scientist |language=en-US}}</ref> After examining some 100,000 nearby large galaxies, a team of researchers has concluded that none of them display any obvious signs of highly advanced technological civilizations.<ref>{{cite web |author=Billings |first=Lee |title=Alien Supercivilizations Absent from 100,000 Nearby Galaxies |url=http://www.scientificamerican.com/article/alien-supercivilizations-absent-from-100-000-nearby-galaxies |url-status=live |archive-url=https://web.archive.org/web/20150418163044/http://www.scientificamerican.com/article/alien-supercivilizations-absent-from-100-000-nearby-galaxies/ |archive-date=2015-04-18 |access-date=2015-04-18 |work=Scientific American}}</ref><ref>{{cite journal|last1=Griffith|first1=Roger L.|last2=Wright|first2=Jason T.|last3=Maldonado|first3=Jessica|last4=Povich|first4=Matthew S.|last5=Sigurđsson|first5=Steinn|last6=Mullan|first6=Brendan|title=The Ĝ Infrared Search for Extraterrestrial Civilizations with Large Energy Supplies. III. The Reddest Extended Sources in WISE|journal=The Astrophysical Journal Supplement Series|date=15 April 2015|volume=217|issue=2|pages=25|arxiv=1504.03418|doi=10.1088/0067-0049/217/2/25|bibcode=2015ApJS..217...25G|s2cid=118463557}}</ref>

Another hypothetical form of astroengineering, the Shkadov thruster, moves its host star by reflecting some of the star's light back on itself, and would be detected by observing if its [[Astronomy:Transit|transit]]s across the star abruptly end with the thruster in front.<ref name=Villard2013>{{Cite web | title = Alien 'Star Engine' Detectable in Exoplanet Data? | url = http://news.discovery.com/space/alien-life-exoplanets/planet-searches-might-stumble-across-an-alien-stellar-engine-130625.htm | date = 2013 | author = Villard, Ray | access-date = 2013-07-08 | archive-date = 2013-07-05 | archive-url = https://web.archive.org/web/20130705174638/http://news.discovery.com/space/alien-life-exoplanets/planet-searches-might-stumble-across-an-alien-stellar-engine-130625.htm | url-status = dead }}</ref> [[Astronomy:Asteroid mining|Asteroid mining]] within the Solar System is also a detectable technosignature of the first kind.<ref>{{Cite journal |last1=Forgan |first1=Duncan |last2=Elvis |first2=Martin |date=2011 |title=Extrasolar Asteroid Mining as Forensic Evidence for Extraterrestrial Intelligence |journal=International Journal of Astrobiology |volume=10 |issue=4 |pages=307 |arxiv=1103.5369 |bibcode=2011IJAsB..10..307F |doi=10.1017/S1473550411000127 |s2cid=119111392}}</ref>

Individual extrasolar planets can be analyzed for signs of technology. [[Biography:Avi Loeb|Avi Loeb]] of the Center for Astrophysics {{!}} Harvard & Smithsonian has proposed that persistent light signals on the night side of an exoplanet can be an indication of the presence of cities and an advanced civilization.<ref>{{cite web |url=http://www.upi.com/Science_News/2011/11/03/SETI-search-urged-to-look-for-city-lights/UPI-89301320361336/#ixzz2HRkOcctY |title=SETI search urged to look for city lights |publisher=UPI.com |date=2011-11-03 |access-date=2013-07-10 |archive-date=2013-11-09 |archive-url=https://web.archive.org/web/20131109200554/http://www.upi.com/Science_News/2011/11/03/SETI-search-urged-to-look-for-city-lights/UPI-89301320361336/#ixzz2HRkOcctY |url-status=live }}</ref><ref>Extrasolar Planets: Formation, Detection and Dynamics, Rudolf Dvorak, p. 14, John Wiley & Sons, 2007</ref> In addition, the excess infrared radiation<ref name=colossus /><ref>{{cite news |last=Povich |first=Matthew |url=http://astrobiology.com/2014/08/infrared-search-for-extraterrestrial-civilizations-with-large-energy-supplies.html |title=Infrared Search for Extraterrestrial Civilizations with Large Energy Supplies |work=astro-ph.GA |publisher=Astrobiology Web |date=11 August 2014 |access-date=2014-08-19 |archive-date=2014-08-19 |archive-url=https://archive.today/20140819232641/http://astrobiology.com/2014/08/infrared-search-for-extraterrestrial-civilizations-with-large-energy-supplies.html |url-status=live }}</ref> and chemicals<ref>{{cite web |url=http://www.esa.int/Our_Activities/Observing_the_Earth/Satellite_sniffs_out_chemical_traces_of_atmospheric_pollution |title=Satellite sniffs out chemical traces of atmospheric pollution / Observing the Earth / Our Activities / ESA |publisher=Esa.int |date=2000-12-18 |access-date=2013-07-10 |archive-date=2013-11-09 |archive-url=https://web.archive.org/web/20131109203331/http://www.esa.int/Our_Activities/Observing_the_Earth/Satellite_sniffs_out_chemical_traces_of_atmospheric_pollution |url-status=live }}</ref><ref>{{cite news |url=http://www.space.com/21470-saturn-moon-titan-haze-cassini.html |title=Haze on Saturn's Moon Titan Is Similar to Earth's Pollution |work=Space.com |date=2013-06-06 |access-date=2013-07-10 |archive-date=2013-07-13 |archive-url=https://web.archive.org/web/20130713115302/http://www.space.com/21470-saturn-moon-titan-haze-cassini.html |url-status=live }}</ref> produced by various industrial processes or [[Earth:Terraforming|terraforming]] efforts<ref name=choi-2012>{{cite news |url=http://www.space.com/18624-alien-life-search-hairspray.html |title=Alien Hairspray May Help Us Find E.T. |work=Space.com |date=2012-11-26 |access-date=2013-07-10 |archive-date=2021-03-09 |archive-url=https://web.archive.org/web/20210309170829/https://www.space.com/18624-alien-life-search-hairspray.html |url-status=live }}</ref> may point to intelligence.

Light and heat detected from planets need to be distinguished from natural sources to conclusively prove the existence of civilization on a planet. However, as argued by the Colossus team,<ref>{{cite web |url=http://www.the-colossus.com/press/2013_06_Astronomy_ETC |title=How to Find ET with Infrared Light |publisher=Astronomy.com |date=June 2013 |url-status=dead |archive-url=https://web.archive.org/web/20131109190959/http://www.the-colossus.com/press/2013_06_Astronomy_ETC |archive-date=2013-11-09 }}</ref> a civilization heat signature should be within a "comfortable" temperature range, like terrestrial urban heat islands, i.e., only a few degrees warmer than the planet itself. In contrast, such natural sources as wild fires, volcanoes, etc. are significantly hotter, so they will be well distinguished by their maximum flux at a different wavelength.

Other than astroengineering, technosignatures such as artificial satellites around [[Astronomy:Exoplanet|exoplanet]]s, particularly such in [[Astronomy:Geostationary orbit|geostationary orbit]], might be detectable even with today's technology and data, and would allow, similar to [[Biology:Fossil|fossil]]s on Earth, to find traces of extrasolar life from long ago.<ref name="Shostak Astronomer 2018">{{cite web |last1=Shostak |first1=Seth |date=2018-04-16 |title=Alien satellites might offer a new way to find E.T. |url=https://www.seti.org/alien-satellites-might-offer-new-way-find-et |url-status=live |archive-url=https://web.archive.org/web/20221127014614/https://www.seti.org/alien-satellites-might-offer-new-way-find-et |archive-date=2022-11-27 |access-date=2022-09-22 |website=SETI Institute}}</ref>

Extraterrestrial craft are another target in the search for technosignatures. [[Physics:Magnetic sail|Magnetic sail]] interstellar spacecraft should be detectable over thousands of light-years of distance through the [[Physics:Synchrotron radiation|synchrotron radiation]] they would produce through interaction with the [[Astronomy:Interstellar medium|interstellar medium]]; other interstellar spacecraft designs may be detectable at more modest distances.<ref>{{cite conference | title=Detection of Extraterrestrial Civilizations via the Spectral Signature of Advanced Interstellar Spacecraft | publisher=Astronomical Society of the Pacific | author=Zubrin, Robert | editor=Shostak, Seth | book-title=Astronomical Society of the Pacific Conference Series | date=1995 | conference=Progress in the Search for Extraterrestrial Life | pages=487–496 | bibcode=1995ASPC...74..487Z}}</ref> In addition, robotic probes within the Solar System are also being sought with optical and radio searches.<ref name=freitas-1983>{{cite journal|journal=Journal of the British Interplanetary Society|title=The Case for Interstellar Probes|volume=36|date=November 1983|pages=490–495|url=http://www.rfreitas.com/Astro/TheCaseForInterstellarProbes1983.htm|author=Freitas, Robert|bibcode=1983JBIS...36..490F|access-date=2004-11-12|archive-date=2020-03-13|archive-url=https://web.archive.org/web/20200313145008/http://www.rfreitas.com/Astro/TheCaseForInterstellarProbes1983.htm|url-status=live}}</ref><ref name=tough-1998-2>{{cite journal|journal=Journal of the British Interplanetary Society|title=Small Smart Interstellar Probes|author=Tough, Allen|volume=51|pages=167–174|date=1998|url=http://www.ieti.org/tough/articles/jbis51.pdf|access-date=2012-06-02|archive-date=2021-02-26|archive-url=https://web.archive.org/web/20210226125550/http://www.ieti.org/tough/articles/jbis51.pdf|url-status=live}}</ref>

For a sufficiently advanced civilization, hyper energetic neutrinos from Planck scale accelerators should be detectable at a distance of many Mpc.<ref>{{cite arXiv |eprint=1503.01509 |class=(astro-ph.HE |first=Brian C. |last=Lacki |title=SETI at Planck Energy: When Particle Physicists Become Cosmic Engineers |year=2015}}</ref>

==Fermi paradox==
{{Main|Astronomy:Fermi paradox}}
Italian physicist [[Biography:Enrico Fermi|Enrico Fermi]] suggested in the 1950s that if technologically advanced civilizations are common in the universe, then they should be detectable in one way or another. According to those who were there, Fermi either asked "Where are they?" or "Where is everybody?"<ref>{{cite web |last=Jones |first=Eric |date=March 1985 |title="Where is everybody?", An account of Fermi's question |url=https://fas.org/sgp/othergov/doe/lanl/la-10311-ms.pdf |url-status=live |archive-url=https://web.archive.org/web/20151105211510/http://www.fas.org/sgp/othergov/doe/lanl/la-10311-ms.pdf |archive-date=November 5, 2015 |access-date=June 28, 2009 |publisher=Los Alamos National Laboratory}}</ref>

The Fermi paradox is commonly understood as asking why extraterrestrials have not visited Earth,<ref name=Zuckerman>Ben Zuckerman and Michael H. Hart (editors), ''Extraterrestrials: Where Are They?'' Elsevier Science & Technology Books (1982), {{ISBN|9780080263427}}</ref> but the same reasoning applies to the question of why signals from extraterrestrials have not been heard. The SETI version of the question is sometimes referred to as "the Great Silence".

The Fermi paradox can be stated more completely as follows:
{{blockquote |The size and age of the universe incline us to believe that many technologically advanced civilizations must exist. However, this belief seems logically inconsistent with our lack of observational evidence to support it. Either (1) the initial assumption is incorrect and technologically advanced intelligent life is much rarer than we believe, or (2) our current observations are incomplete, and we simply have not detected them yet, or (3) our search methodologies are flawed and we are not searching for the correct indicators, or (4) it is the nature of intelligent life to destroy itself.|sign=|source=}}

There are multiple explanations proposed for the Fermi paradox,<ref>Stephen Webb, ''Where is Everybody? Fifty Solutions to Fermi's Paradox,'' Copernicus, 2002 edition, 978-0387955018</ref> ranging from analyses suggesting that intelligent life is rare (the "[[Astronomy:Rare Earth hypothesis|Rare Earth hypothesis]]"), to analyses suggesting that although extraterrestrial civilizations may be common, they would not communicate with us, would communicate in a way we have not discovered yet, could not travel across interstellar distances, or destroy themselves before they master the technology of either interstellar travel or communication.

The German astrophysicist and radio astronomer [[Biography:Sebastian von Hoerner|Sebastian von Hoerner]] suggested<ref>{{cite journal |last=von Hoerner |first=Sebastian |title=The Search for Signals from Other Civilizations |journal=Science |volume=134 |issue=3493 |date=December 8, 1961 |pages=1839–43 |doi=10.1126/science.134.3493.1839 |pmid=17831111 |bibcode=1961Sci...134.1839V |issn=0036-8075}}</ref> that the average duration of civilization was 6,500 years. After this time, according to him, it disappears for external reasons (the destruction of life on the planet, the destruction of only rational beings) or internal causes (mental or physical degeneration). According to his calculations, on a habitable planet (one in three million stars) there is a sequence of technological species over a time distance of hundreds of millions of years, and each of them "produces" an average of four technological species. With these assumptions, the average distance between civilizations in the [[Astronomy:Milky Way|Milky Way]] is 1,000 light years.<ref>{{cite encyclopedia |url=https://www.daviddarling.info/encyclopedia/H/Hoerner.html |title=Hoerner, Sebastian von (1919–2003) |website=David Darling's Encyclopedia of Science |access-date=2020-07-26 |archive-date=2020-07-20 |archive-url=https://web.archive.org/web/20200720125240/https://www.daviddarling.info/encyclopedia/H/Hoerner.html |url-status=live }}</ref><ref>{{cite book |last=Shklovsky |first=Iosif |title=Вселенная. Жизнь. Разум |url=http://nuclphys.sinp.msu.ru/books/pop/%D0%A8%D0%BA%D0%BB%D0%BE%D0%B2%D1%81%D0%BA%D0%B8%D0%B9.htm |publisher=Наука |place=Moscow |year=1987 |pages=250–252 |language=ru |access-date=2020-07-26 |archive-date=2020-11-25 |archive-url=https://web.archive.org/web/20201125172153/http://nuclphys.sinp.msu.ru/books/pop/%D0%A8%D0%BA%D0%BB%D0%BE%D0%B2%D1%81%D0%BA%D0%B8%D0%B9.htm |url-status=live }}</ref><ref>{{cite book |last=Lem |first=Stanisław |author-link=Stanisław Lem |chapter=Space Civilizations |title=Summa Technologiae |publisher=University of Minnesota Press |location=Minneapolis |year=2013 |isbn=978-0816675777}}</ref>

Science writer Timothy Ferris has posited that since galactic societies are most likely only transitory, an obvious solution is an interstellar communications network, or a type of library consisting mostly of automated systems. They would store the cumulative knowledge of vanished civilizations and communicate that knowledge through the galaxy. Ferris calls this the "Interstellar Internet", with the various automated systems acting as network "servers". If such an Interstellar Internet exists, the hypothesis states, communications between servers are mostly through narrow-band, highly directional radio or laser links. Intercepting such signals is, as discussed earlier, very difficult. However, the network could maintain some broadcast nodes in hopes of making contact with new civilizations.

Although somewhat dated in terms of "information culture" arguments, not to mention the obvious technological problems of a system that could work effectively for billions of years and requires multiple lifeforms agreeing on certain basics of communications technologies, this hypothesis is actually testable (see below).

===Difficulty of detection===
A significant problem is the vastness of space. Despite piggybacking on the world's most sensitive radio telescope, astronomer and initiator of SERENDIP Charles Stuart Bowyer noted the then world's largest instrument could not detect random radio noise emanating from a civilization like ours,{{Citation needed|date=February 2023}} which has been leaking radio and TV signals for less than 100 years.<ref>{{cite web |date=1979 |title=Eavesdropping on the Earth |url=https://visns.neocities.org/EotE.pdf |url-status=live |archive-url=https://web.archive.org/web/20180814233011/https://visns.neocities.org/EotE.pdf |archive-date=2018-08-14 |access-date=2018-08-14 |publisher=astrosociety.org}}</ref> For [[Astronomy:SERENDIP|SERENDIP]] and most other SETI projects to detect a signal from an extraterrestrial civilization, the civilization would have to be beaming a powerful signal directly at us. It also means that Earth civilization will only be detectable within a distance of 100 light-years.<ref>{{cite web | title=SETI Insentitive To Earth-like Signals | publisher=spacedaily.com | date=1998 | access-date=February 8, 2013 | url=http://www.spacedaily.com/news/seti-98a.html | archive-date=April 13, 2013 | archive-url=https://web.archive.org/web/20130413083021/http://www.spacedaily.com/news/seti-98a.html | url-status=live }}</ref>

==Post-detection disclosure protocol==
The [[Organization:International Academy of Astronautics|International Academy of Astronautics]] (IAA) has a long-standing SETI Permanent Study Group (SPSG, formerly called the IAA SETI Committee), which addresses matters of SETI [[Science|science]], [[Engineering:Technology|technology]], and international policy. The SPSG meets in conjunction with the [[Organization:International Astronautical Congress|International Astronautical Congress (IAC)]], held annually at different locations around the world, and sponsors two SETI Symposia at each IAC. In 2005, the IAA established the SETI: Post-Detection Science and Technology Taskgroup (chairman, Professor [[Biography:Paul Davies|Paul Davies]]) "to act as a Standing Committee to be available to be called on at any time to advise and consult on questions stemming from the discovery of a putative signal of extraterrestrial intelligent (ETI) origin."<ref>{{Cite web |last=Norris |first=Ray |date=2022-12-14 |title=Terms of reference of the SETI Post-Detection Committee |url=https://www.atnf.csiro.au/people/Ray.Norris/SETI/terms.html |access-date=2023-02-02 |website=www.atnf.csiro.au |language=en}}</ref>

However, the protocols mentioned apply only to radio SETI rather than for METI ([[Astronomy:Active SETI|Active SETI]]).<ref>{{Cite web |title=What to do if we find extraterrestrial life |url=https://www.nbcnews.com/id/wbna39675346 |access-date=2023-02-02 |website=NBC News |date=15 October 2010 |language=en}}</ref> The intention for METI is covered under the SETI charter "Declaration of Principles Concerning Sending Communications with Extraterrestrial Intelligence".

In October 2000 astronomers Iván Almár and [[Biography:Jill Tarter|Jill Tarter]] presented a paper to The SETI Permanent Study Group in Rio de Janeiro, Brazil which proposed a scale (modelled after the [[Astronomy:Torino scale|Torino scale]]) which is an ordinal scale between zero and ten that quantifies the impact of any public announcement regarding evidence of extraterrestrial intelligence;<ref name="Rio scale">{{cite web | url=http://avsport.org/IAA/rioscale.htm | title=The Rio Scale | publisher=International Academy of Astronautics | access-date=2016-08-29 | archive-date=2016-09-02 | archive-url=https://web.archive.org/web/20160902040412/http://avsport.org/IAA/riocalc.htm | url-status=dead }}</ref> the [[Astronomy:Rio scale|Rio scale]] has since inspired the 2005 [[Astronomy:San Marino Scale|San Marino Scale]] (in regard to the risks of transmissions from Earth) and the 2010 London Scale (in regard to the detection of extraterrestrial life).<ref name="The London Scale">{{cite journal |author=Almár |first=Iván |date=November–December 2011 |title=SETI and Astrobiology: The Rio Scale and the London Scale |journal=Acta Astronautica |volume=69 |issue=9–10 |pages=899–904 |bibcode=2011AcAau..69..899A |doi=10.1016/j.actaastro.2011.05.036}}</ref> The Rio scale itself was revised in 2018.<ref>{{cite web| url = https://phys.org/news/2018-07-scientists-rio-scale-alien-encounters.html| title = Scientists revise the Rio Scale for reported alien encounters. July 2018| access-date = 2018-07-24| archive-date = 2018-07-24| archive-url = https://web.archive.org/web/20180724143818/https://phys.org/news/2018-07-scientists-rio-scale-alien-encounters.html| url-status = live}}</ref>

The SETI Institute does not officially recognize the [[Astronomy:Wow! signal|Wow! signal]] as of extraterrestrial origin as it was unable to be verified, although in a 2020 tweet the organization stated that <nowiki>''an astronomer might have pinpointed the host star''</nowiki>.<ref>{{Cite web |last=SETI Institute |date=2020 |title=Where did the famous mystery Wow! signal, detected in 1977, come from? |url=https://twitter.com/setiinstitute/status/1334972506487316483 |access-date=2022-05-30 |website=Twitter |language=en |archive-date=2022-05-30 |archive-url=https://web.archive.org/web/20220530051817/https://twitter.com/setiinstitute/status/1334972506487316483 |url-status=live }}</ref> The SETI Institute has also publicly denied that the candidate signal Radio source SHGb02+14a is of extraterrestrial origin.<ref>{{cite news | publisher = [[Organization:The Planetary Society|The Planetary Society]] | title = SETI@home Leaders Deny Reports of Likely Extraterrestrial Signal | url = http://planetary.org/news/2004/0902_SETIhome_Leaders_Deny_Reports_of.html | date = 2004-09-02 | last = Alexander | first = Amir | access-date = 2006-06-12 | url-status = dead | archive-url = https://web.archive.org/web/20110726074250/http://planetary.org/news/2004/0902_SETIhome_Leaders_Deny_Reports_of.html | archive-date = 2011-07-26 }}</ref><ref>{{cite news | work = BBC News | url = http://news.bbc.co.uk/1/hi/sci/tech/3621608.stm | title = Astronomers deny ET signal report | date = 2004-09-02 | first = David | last = Whitehouse | access-date = 2006-06-12 | archive-date = 2006-06-15 | archive-url = https://web.archive.org/web/20060615112727/http://news.bbc.co.uk/1/hi/sci/tech/3621608.stm | url-status = live }}</ref> Although other volunteering projects such as Zooniverse credit users for discoveries, there is currently no crediting or early notification by SETI@Home following the discovery of a signal.

Some people, including [[Biography:Steven M. Greer|Steven M. Greer]],<ref>{{Cite web |last=Vance |first=Ashlee |title=SETI urged to fess up over alien signals |url=https://www.theregister.com/2006/07/31/signals_seti/ |access-date=2023-02-02 |website=www.theregister.com |language=en}}</ref> have expressed cynicism that the general public might not be informed in the event of a genuine discovery of extraterrestrial intelligence due to significant vested interests. Some, such as [[Biography:Bruce Jakosky|Bruce Jakosky]]<ref>{{Cite web |date=2004-11-01 |title=NAI News Article: The Meaning of Life |url=http://nai.nasa.gov/news_stories/news_print.cfm?ID=138 |access-date=2023-02-02 |archive-url=https://web.archive.org/web/20041101002359/http://nai.nasa.gov/news_stories/news_print.cfm?ID=138 |archive-date=2004-11-01 }}</ref> have also argued that the official disclosure of extraterrestrial life may have far reaching and as yet undetermined implications for society, particularly for the world's [[Religion:Religion|religion]]s.

== Active SETI ==
{{main|Astronomy:Active SETI}}
[[Astronomy:Active SETI|Active SETI]], also known as messaging to extraterrestrial intelligence (METI), consists of sending signals into space in the hope that they will be detected by an alien intelligence.

=== Realized interstellar radio message projects ===
In November 1974, a largely symbolic attempt was made at the Arecibo Observatory to send a message to other worlds. Known as the Arecibo Message, it was sent towards the [[Astronomy:Globular cluster|globular cluster]] [[Astronomy:Messier 13|M13]], which is 25,000 light-years from Earth. Further IRMs [[Astronomy:Cosmic Call|Cosmic Call]], [[Astronomy:Teen Age Message|Teen Age Message]], Cosmic Call 2, and A Message From Earth were transmitted in 1999, 2001, 2003 and 2008 from the Evpatoria Planetary Radar.

=== Debate ===
[[Image:Pictorial_message_to_proxima_centauri_active_SETI.jpg|thumb|right|upright=2.5|Example of a high-resolution pictorial message to potential ETI. These messages usually contain information about the location of the solar system in the Milky Way.]]
Whether or not to attempt to contact extraterrestrials has attracted significant academic debate in the fields of [[Physics:Space ethics|space ethics]] and [[Social:Space policy|space policy]].<ref>{{Cite journal |last1=Billingham |first1=J. |last2=Benford |first2=J. |date=2016 |title=Costs and difficulties of interstellar 'Messaging' and the need for international debate on potential risks |url=https://bis-space.com/shop/product/reviewing-meti-a-critical-analysis-of-the-arguments/ |journal=Journal of the British Interplanetary Society |volume=19 |pages=17–23}}</ref><ref>{{Cite journal |last=Gertz |first=J. |date=2016 |title=Reviewing METI: A critical analysis of the arguments |url=https://bis-space.com/shop/product/reviewing-meti-a-critical-analysis-of-the-arguments/ |journal=Journal of the British Interplanetary Society |volume=69 |pages=31–36 |arxiv=1605.05663 |bibcode=2016JBIS...69...31G}}</ref><ref>{{Cite journal |last1=Haramia |first1=Chelsea |last2=DeMarines |first2=Julia |date=2019-01-02 |title=The Imperative to Develop an Ethically-Informed METI Analysis |url=http://dx.doi.org/10.1080/14746700.2019.1557800 |journal=Theology and Science |volume=17 |issue=1 |pages=38–48 |doi=10.1080/14746700.2019.1557800 |s2cid=171404637 |issn=1474-6700}}</ref> Physicist [[Biography:Stephen Hawking|Stephen Hawking]], in his book ''[[Astronomy:A Brief History of Time|A Brief History of Time]]'', suggests that "alerting" extraterrestrial intelligences to our existence is foolhardy, citing humankind's history of treating its own kind harshly in meetings of civilizations with a significant technology gap, e.g., the extermination of Tasmanian aborigines. He suggests, in view of this history, that we "lay low".<ref>{{Cite journal |last=Vakoch |first=Douglas A. |date=2017-04-03 |title=Hawking's fear of an alien invasion may explain the Fermi Paradox |url=http://dx.doi.org/10.1080/14746700.2017.1299380 |journal=Theology and Science |volume=15 |issue=2 |pages=134–138 |doi=10.1080/14746700.2017.1299380 |issn=1474-6700 |s2cid=219627161}}</ref> In one response to Hawking, in September 2016, astronomer [[Biography:Seth Shostak|Seth Shostak]] sought to allay such concerns.<ref name="TG-20160927">{{cite news |last=Shostak |first=Seth |title=Why Stephen Hawking is light years from the truth about 'dangerous aliens' |url=https://www.theguardian.com/commentisfree/2016/sep/27/stephen-hawking-light-years-dangerous-aliens |date=27 September 2016 |work={{wipe|The Guardian}} |access-date=28 September 2016 |archive-date=27 September 2016 |archive-url=https://web.archive.org/web/20160927125846/https://www.theguardian.com/commentisfree/2016/sep/27/stephen-hawking-light-years-dangerous-aliens |url-status=live }}</ref> Astronomer [[Biography:Jill Tarter|Jill Tarter]] also disagrees with Hawking, arguing that aliens developed and long-lived enough to communicate and travel across interstellar distances would have evolved a cooperative and less violent intelligence. She however thinks it is too soon for humans to attempt active SETI and that humans should be more advanced technologically first but keep listening in the meantime.<ref>{{cite web|last1=Orwig|first1=Jessica|title=A world leading scientist on the search for extraterrestrials pointed out a flaw in Stephen Hawking's fear of finding intelligent aliens|url=http://www.businessinsider.com/jill-tarter-says-stephen-hawking-is-wrong-about-aliens-2016-1|website=Business Insider|access-date=8 February 2018|archive-date=21 February 2018|archive-url=https://web.archive.org/web/20180221020808/http://www.businessinsider.com/jill-tarter-says-stephen-hawking-is-wrong-about-aliens-2016-1|url-status=live}}</ref>

==Criticism==

As various SETI projects have progressed, some have criticized early claims by researchers as being too "euphoric". For example, Peter Schenkel, while remaining a supporter of SETI projects, wrote in 2006 that:
{{blockquote|[i]n light of new findings and insights, it seems appropriate to put excessive euphoria to rest and to take a more down-to-earth view{{nbsp}}[...] We should quietly admit that the early estimates—that there may be a million, a hundred thousand, or ten thousand advanced extraterrestrial civilizations in our galaxy—may no longer be tenable.<ref name="Schenkel_2006"/>}}

Critics claim that the existence of extraterrestrial intelligence has no good [[Biography:Karl Popper|Popperian]] criteria for [[Philosophy:Falsifiability|falsifiability]], as explained in a 2009 editorial in ''Nature'', which said:
{{blockquote|Seti... has always sat at the edge of mainstream astronomy. This is partly because, no matter how scientifically rigorous its practitioners try to be, SETI can't escape an association with UFO believers and other such crackpots. But it is also because SETI is arguably not a falsifiable experiment. Regardless of how exhaustively the Galaxy is searched, the null result of radio silence doesn't rule out the existence of alien civilizations. It means only that those civilizations might not be using radio to communicate.<ref name="SETI at 50"/>}}

''Nature'' added that SETI was "marked by a hope, bordering on faith" that aliens were aiming signals at us, that a hypothetical alien SETI project looking at Earth with "similar faith" would be "sorely disappointed", despite our many untargeted radar and TV signals, and our few targeted Active SETI radio signals denounced by those fearing aliens, and that it had difficulties attracting even sympathetic working scientists and government funding because it was "an effort so likely to turn up nothing".<ref name="SETI at 50"/>

However, ''Nature'' also added, "Nonetheless, a small SETI effort is well worth supporting, especially given the enormous implications if it did succeed" and that "happily, a handful of wealthy technologists and other private donors have proved willing to provide that support".<ref name="SETI at 50">{{cite journal |title=SETI at 50 |doi=10.1038/461316a |pmid=19759575 |journal=Nature |date=2009 |volume=416|bibcode = 2009Natur.461..316.|issue=7262|pages=316|quote=Indeed, SETI is marked by a hope, bordering on faith, that not only are there civilizations broadcasting out there, but that they are somehow intent on beaming their signals at Earth. An alien SETI project relying on a similar faith in Earth would be sorely disappointed. It's true that a random mix of radar and television signals has been expanding outwards from Earth at the speed of light for the past 70 years. But there have been only a few short-lived attempts to target radio messages at other stars – with each attempt arousing concerns over alien reprisals. Understandably, many scientists who support SETI in spirit have instead pursued astronomical targets more likely to offer positive data – and tenure. Governments have also been averse to funding an effort so likely to turn up nothing.|doi-access=free}}</ref>

{{anchor|Rare Earth Hypothesis}}Supporters of the Rare Earth Hypothesis argue that advanced lifeforms are likely to be very rare, and that, if that is so, then SETI efforts will be futile.<ref name=RareEarth2007a>{{cite book |last1=Ward |first1=Peter D. |last2=Brownlee |first2=Donald |author-link2=Donald Brownlee |url=https://books.google.com/books?id=cHwpBAAAQBAJ&q=%22Rare+Earth+Hypothesis%22+SETI&pg=PA250 |title=Rare Earth: Why Complex Life is Uncommon in the Universe |publisher=Springer |year=2007 |page=250 |isbn=9780387218489 |quote=Unfortunately, it is very difficult to know if SETI is an effective use of resources. If the Rare Earth Hypothesis is correct, then it clearly is a futile effort. |access-date=2020-10-10 |archive-date=2023-01-16 |archive-url=https://web.archive.org/web/20230116230917/https://books.google.com/books?id=cHwpBAAAQBAJ&q=%22Rare+Earth+Hypothesis%22+SETI&pg=PA250 |url-status=live }} Revised edition (first published in 2000)</ref><ref name=Denton+Restivo2008a>{{cite book |last1=Denton |first1=Peter H. |last2=Restivo |first2=Sal |author-link2=Sal Restivo |url=https://books.google.com/books?id=lbVzCgAAQBAJ&q=%22Rare+Earth+Hypothesis%22+SETI&pg=PA403 |title=Battleground: Science and Technology [2 volumes] |publisher=ABC-CLIO |year=2008 |page=403 |isbn=9781567207439 |quote=SETI enthusiasts believe that the human race is characterized by mediocrity rather than excellence. According to Frank Drake and his followers, this means that intelligent life is common in the universe. Peter Ward and Donald Brownlee challenge the principle of mediocrity with the rare Earth hypothesis in their book Rare Earth: Why Complex Life Is Uncommon in the Universe (2000). |access-date=2020-10-10 |archive-date=2023-01-16 |archive-url=https://web.archive.org/web/20230116230919/https://books.google.com/books?id=lbVzCgAAQBAJ&q=%22Rare+Earth+Hypothesis%22+SETI&pg=PA403 |url-status=live }}</ref><ref name=Losch2017a>{{cite book |last1=Losch |first1=Andreas |url=https://books.google.com/books?id=P9AoDwAAQBAJ&q=%22Rare+Earth+Hypothesis%22+SETI&pg=PA167 |title=What is Life? On Earth and Beyond |publisher=Cambridge University Press |year=2017 |page=167 |isbn=9781107175891 |quote=In many ways, the rare-Earth hypothesis has since become somewhat of a default position in many astrobiological circles, and – since it predicts the absence of rationale for SETI – a mainstay of SETI scepticism.{{nbsp}}[...] There are many criticisms rightly raised against the rare-Earth Hypothesis, but here I shall mention just one. |access-date=2020-10-10 |archive-date=2023-01-16 |archive-url=https://web.archive.org/web/20230116230919/https://books.google.com/books?id=P9AoDwAAQBAJ&q=%22Rare+Earth+Hypothesis%22+SETI&pg=PA167 |url-status=live }}</ref> However, the Rare Earth Hypothesis itself faces many criticisms.<ref name=Losch2017a/>

{{anchor|Roy Mash}}In 1993, Roy Mash stated that "Arguments favoring the existence of extraterrestrial intelligence nearly always contain an overt appeal to big numbers, often combined with a covert reliance on generalization from a single instance" and concluded that "the dispute between believers and skeptics is seen to boil down to a conflict of intuitions which can barely be engaged, let alone resolved, given our present state of knowledge".<ref name="Mash1993-01a">{{cite journal |last1=Mash |first1=Roy |year=1993 |title=Big numbers and induction in the case for extraterrestrial intelligence |journal=Philosophy of Science |volume=60 |issue=2 |pages=204–222 |doi=10.1086/289729 |jstor=188351 |s2cid=120672390}}</ref> In response, in 2012, [[Biography:Milan M. Ćirković|Milan M. Ćirković]], then research professor at the [[Astronomy:Belgrade Observatory|Astronomical Observatory of Belgrade]] and a research associate of the [[Organization:Future of Humanity Institute|Future of Humanity Institute]] at the [[Organization:University of Oxford|University of Oxford]],<ref name="Cirkovic2012-about">{{Cite book |last1=Ćirković |first1=Milan M. |url=https://books.google.com/books?id=vrgf3tWgUrwC&q=seti+as+religion+Basalla |title=About the author (2012) |date=2012-06-21 |publisher=Cambridge University Press |isbn=9780521197755 |quote=Milan M. Ćirković is a research professor at the Astronomical Observatory of Belgrade, (Serbia) and a research associate of the Future of Humanity Institute at the University of Oxford. |access-date=2017-12-13 |archive-url=https://web.archive.org/web/20230116230920/https://books.google.com/books?id=vrgf3tWgUrwC&q=seti+as+religion+Basalla |archive-date=2023-01-16 |url-status=live}}</ref> said that Mash was unrealistically over-reliant on excessive abstraction that ignored the empirical information available to modern SETI researchers.<ref name="Cirkovic2012-01c">{{Cite book |last1=Ćirković |first1=Milan M. |url=https://books.google.com/books?id=vrgf3tWgUrwC&q=mash&pg=PA175 |title=The Astrobiological Landscape: Philosophical Foundations of the Study of Cosmic Life |date=2012-06-21 |publisher=Cambridge University Press |isbn=978-0-521-19775-5 |language=en}}</ref>

{{anchor|George Basalla}}[[Biography:George Basalla|George Basalla]], [[Social:Emeritus|Emeritus]] Professor of History at the [[Organization:University of Delaware|University of Delaware]],<ref name=Basalla2006-about>{{Cite book |url=https://books.google.com/books?id=oMQIQMmAgoIC |title=About the author (2006) |access-date=2017-12-13 |isbn=9780198038351 |date=2006-01-19 |last1=Basalla |first1=George |publisher=Oxford University Press |archive-date=2023-01-16 |archive-url=https://web.archive.org/web/20230116230922/https://books.google.com/books?id=oMQIQMmAgoIC |url-status=live }}</ref> is a critic of SETI who argued in 2006 that "extraterrestrials discussed by scientists are as imaginary as the spirits and gods of religion or myth",<ref name="Basalla2006-01a">{{cite book |author=Basalla |first=George |url=https://archive.org/details/civilizedlifeinu0000basa |title=Civilized Life in the Universe: Scientists on Intelligent Extraterrestrials |publisher=Oxford University Press |year=2006 |isbn=9780198038351 |page=[https://archive.org/details/civilizedlifeinu0000basa/page/14 14] |quote=Despite all their scientific trappings, the extraterrestrials discussed by scientists are as imaginary as the spirits and gods of religion or myth. |access-date=2017-12-13 |url-access=registration}}</ref><ref name=Cirkovic2012-01a/> and was in turn criticized by [[Biography:Milan M. Ćirković|Milan M. Ćirković]]<ref name=Cirkovic2012-about/> for, among other things, being unable to distinguish between "SETI believers" and "scientists engaged in SETI", who are often sceptical (especially about quick detection), such as [[Biography:Freeman Dyson|Freeman Dyson]] and, at least in their later years, Iosif Shklovsky and Sebastian von Hoerner, and for ignoring the difference between the knowledge underlying the arguments of modern scientists and those of ancient Greek thinkers.<ref name="Cirkovic2012-01a">Ćirković (2012), [https://books.google.com/books?id=vrgf3tWgUrwC&dq=seti+as+religion+Basalla&pg=PA172 p. 172] {{Webarchive|url=https://web.archive.org/web/20230116230915/https://books.google.ie/books?id=vrgf3tWgUrwC&pg=PA172&dq=seti+as+religion+Basalla&hl=en&sa=X&ved=0ahUKEwjTyt3KqIbYAhWBD8AKHZcrCwwQ6AEIJzAA#v=onepage&q=seti%20as%20religion%20Basalla&f=false|date=2023-01-16}}, "It is Basalla, the critic of SETI and not its practitioners, who violates the spirit of Hull's dictum, for instance, when he writes that 'extraterrestrials discussed by scientists are as imaginary as the spirits and gods of religion or myth'.[54] Second, the approach to this sociology of science criticism is obviously marred by Basalla's insistence on personal quirks and idiosyncrasies as the main motivation of scientific activity, an attitude that is not only demeaning to the many scientists mentioned, ..."</ref>

{{anchor|Massimo Pigliucci}}[[Biography:Massimo Pigliucci|Massimo Pigliucci]], Professor of [[Philosophy:Philosophy|Philosophy]] at CUNY – City College,<ref name="Pigliucci2010-about">{{cite web |date=2015-07-05 |title=Cuny – City College – Philosophy Department |url=http://www.ccny.cuny.edu/philosophy/ |url-status=live |archive-url=https://web.archive.org/web/20200820050136/https://www.ccny.cuny.edu/philosophy/ |archive-date=2020-08-20 |access-date=2017-12-13}}</ref> asked in 2010 whether SETI is "uncomfortably close to the status of [[Unsolved:Pseudoscience|pseudoscience]]" due to the lack of any clear point at which negative results cause the [[Hypothesis|hypothesis]] of Extraterrestrial Intelligence to be abandoned,<ref name="Pigliucci2010-01a">{{cite book |author=Pigliucci |first=Massimo |url=https://books.google.com/books?id=aC8Baky2qTcC&q=seti+as+pseudoscience&pg=PA34 |title=Nonsense on Stilts: How to Tell Science from Bunk |publisher=University of Chicago Press |year=2010 |isbn=9780226667874 |page=34 |quote=But in the case of SETI, negative results are what is expected most of the time, perhaps even forever, regardless of the truth of the central hypothesis. This raises the question: when will SETI researchers think that enough negatives have been accumulated to reject the hypothesis of existence of other technological civilizations? If the answer is that such hypothesis can never be rejected, regardless of the empirical results, that pushes SETI uncomfortably close to the status of pseudoscience. There is another way to look at the problem, based on an additional element (besides empirical evidence and testability) ... |access-date=2017-12-13 |archive-url=https://web.archive.org/web/20230116231002/https://books.google.com/books?id=aC8Baky2qTcC&q=seti+as+pseudoscience&pg=PA34 |archive-date=2023-01-16 |url-status=live}}</ref> before eventually concluding that SETI is "almost-science", which is described by Milan M. Ćirković<ref name=Cirkovic2012-about/> as Pigliucci putting SETI in "the illustrious company of [[Physics:String theory|string theory]], [[Physics:Interpretations of quantum mechanics|interpretations of quantum mechanics]], [[Philosophy:Evolutionary psychology|evolutionary psychology]] and history (of the 'synthetic' kind done recently by [[Biography:Jared Diamond|Jared Diamond]])", while adding that his justification for doing so with SETI "is weak, outdated, and reflecting particular philosophical prejudices similar to the ones described above in Mash<ref name=Mash1993-01a/> and Basalla<ref name=Basalla2006-01a/>".<ref name=Cirkovic2012-01b>Ćirković (2012), [https://books.google.com/books?id=vrgf3tWgUrwC&dq=seti+as+pseudoscience&pg=PA175 p175] {{Webarchive|url=https://web.archive.org/web/20230116230915/https://books.google.ie/books?id=vrgf3tWgUrwC&pg=PA175&dq=seti+as+pseudoscience&hl=en&sa=X&ved=0ahUKEwjaj8Tk6IbYAhUCLMAKHXW3Ch8Q6AEIRDAF#v=onepage&q=seti%20as%20pseudoscience&f=false |date=2023-01-16 }}, "However, in the second chapter, tellingly entitled 'Almost Science', the author (a distinguished philosopher, mainly involved in the philosophy of biology) devotes several subsections to the fields which are, in his opinion, neither pseudosciences, nor fully legitimate members of the scientific family. Here he puts SETI studies in the illustrious company of string theory, interpretations of quantum mechanics, evolutionary psychology and history (of the 'synthetic' kind done recently by Jared Diamond). While the club is fun to be in – and only a staunch conservative does not expect great breakthroughs to come out of one or more of these domains in the next few decades – the justification offered by Pigliucci in the case of SETI is weak, outdated, and reflecting particular philosophical prejudices similar to the ones described above in Mash and Basalla.[60]"</ref>

Richard Carrigan, a particle physicist at the Fermi National Accelerator Laboratory near Chicago, Illinois, suggested that passive SETI could also be dangerous and that a signal released onto the Internet could act as a [[Computer virus|computer virus]].<ref>{{cite web |last=Carrigan |first=Robert A. Jr. |date=June 2006 |title=Do potential SETI signals need to be decontaminated? |url=http://home.fnal.gov/~carrigan/SETI/SETI_Hacker_AC-03-IAA-8-3-06.pdf |url-status=live |archive-url=https://web.archive.org/web/20080828215027/http://home.fnal.gov/~carrigan/SETI/SETI_Hacker_AC-03-IAA-8-3-06.pdf |archive-date=2008-08-28 |access-date=2009-06-29 |publisher=Fermi National Accelerator Laboratory}}</ref> Computer security expert Bruce Schneier dismissed this possibility as a "bizarre movie-plot threat".<ref>{{cite web |title=A Science-Fiction Movie-Plot Threat |url=http://www.schneier.com/blog/archives/2005/11/a_sciencefictio.html |url-status=live |archive-url=https://web.archive.org/web/20110604063600/http://www.schneier.com/blog/archives/2005/11/a_sciencefictio.html |archive-date=June 4, 2011 |access-date=March 13, 2011}}</ref>

===Ufology===
{{anchor|Stanton Friedman}} [[Unsolved:Ufology|Ufologist]] Stanton Friedman has often criticized SETI researchers for, among other reasons, what he sees as their unscientific criticisms of Ufology,<ref name=StantonFriedman2002a>{{cite web|author=Friedman, Stanton T.|title=UFOs: Challenge to SETI Specialists|date=2002-05-13|url=http://www.stantonfriedman.com/index.php?ptp=articles&fdt=2002.05.13|access-date=2017-12-17|archive-date=2017-12-17|archive-url=https://web.archive.org/web/20171217132321/http://www.stantonfriedman.com/index.php?ptp=articles&fdt=2002.05.13|url-status=live}}</ref><ref name=StantonFriedman2009a>{{cite web|author=Friedman, Stanton T.|author-link=Stanton Friedman|title=Pseudo-Science of Anti-Ufology|publisher=The UFO Chronicles|date=2009-05-30|url=http://www.theufochronicles.com/2009/05/pseudo-science-of-anti-ufology.html|access-date=2017-12-17|archive-date=2017-09-18|archive-url=https://web.archive.org/web/20170918063759/http://www.theufochronicles.com/2009/05/pseudo-science-of-anti-ufology.html|url-status=live}}</ref> but, unlike SETI, Ufology has generally not been embraced by academia as a scientific field of study,<ref name=denzler2003-p69>{{cite book |author=Denzler, Brenda |title=The lure of the edge: scientific passions, religious beliefs, and the pursuit of UFOs |publisher=University of California Press |page=69|date=2003 |isbn=978-0-520-23905-0}}</ref><ref name="ufos">{{Cite web |url=http://www.redorbit.com/news/science/102455/a_space_science_perspective_on_boundaries/ |archiveurl=https://web.archive.org/web/20110523035856/https://www.redorbit.com/news/science/102455/a_space_science_perspective_on_boundaries/ |archivedate=2011-05-23 |url-status=dead |title=Why SETI Is Science and UFOlogy Is Not – A Space Science Perspective on Boundaries |first=Mark |last=Moldwin |date=12 November 2004}}</ref> and it is usually characterized as a partial<ref>{{cite book |author= Tuomela, Raimo |title=Science, action, and reality |publisher=Springer |date=1985 |page=234|isbn=978-90-277-2098-6 }}</ref> or total<ref>{{cite book|author= Feist, Gregory J. |title=The psychology of science and the origins of the scientific mind |url= https://archive.org/details/psychologyscienc00feis |url-access= limited |publisher=Yale University Press |date=2006 |page=[https://archive.org/details/psychologyscienc00feis/page/n239 219]|isbn=978-0-300-11074-6 }}</ref><ref>{{cite book |author=Restivo, Sal P. |title=Science, technology, and society: an encyclopedia |publisher=Oxford University Press US |date=2005 |page=[https://archive.org/details/sciencetechnolog0000unse_m5y0/page/176 176] |isbn=978-0-19-514193-1 |url=https://archive.org/details/sciencetechnolog0000unse_m5y0/page/176 }}</ref> [[Unsolved:Pseudoscience|pseudoscience]]. In a 2016 interview, [[Biography:Jill Tarter|Jill Tarter]] pointed out that it is still a misconception that SETI and UFOs are related.<ref name=TramielTarter01a>{{cite web|last1=Tramiel|first1=Leonard|title=Life as We Know It: An Interview with Jill Tarter|url=https://www.csicop.org/specialarticles/show/life_as_we_know_it_an_interview_with_jill_tarter|website=[[Organization:Committee for Skeptical Inquiry|CSI]]|date=23 August 2016|publisher=[[Organization:Center for Inquiry|Center for Inquiry]]|access-date=1 January 2018|archive-date=2 January 2018|archive-url=https://web.archive.org/web/20180102013222/https://www.csicop.org/specialarticles/show/life_as_we_know_it_an_interview_with_jill_tarter|url-status=live}}</ref> She states, "SETI uses the tools of the astronomer to attempt to find evidence of somebody else's technology coming from a great distance. If we ever claim detection of a signal, we will provide evidence and data that can be independently confirmed. UFOs—none of the above."<ref name=TramielTarter01a/> [[Astronomy:The Galileo Project|The Galileo Project]] headed by Harvard astronomer [[Biography:Avi Loeb|Avi Loeb]] is one of the few scientific efforts to study UFOs or UAPs.<ref>{{cite web |title=Activities |url=https://projects.iq.harvard.edu/galileo/activities |website=projects.iq.harvard.edu |access-date=25 July 2021 |language=en |archive-date=25 July 2021 |archive-url=https://web.archive.org/web/20210725062213/https://projects.iq.harvard.edu/galileo/activities |url-status=live }}</ref> Loeb criticized that the study of UAP is often dismissed and not sufficiently studied by scientists and should shift from "occupying the talking points of national security administrators and politicians" to the realm of science.<ref>{{cite news |last1=Loeb |first1=Avi |title=A Possible Link between 'Oumuamua and Unidentified Aerial Phenomena |url=https://www.scientificamerican.com/article/a-possible-link-between-oumuamua-and-unidentified-aerial-phenomena/ |access-date=25 July 2021 |work=Scientific American |language=en |archive-date=29 July 2021 |archive-url=https://web.archive.org/web/20210729152307/https://www.scientificamerican.com/article/a-possible-link-between-oumuamua-and-unidentified-aerial-phenomena/ |url-status=live }}</ref> The Galileo Project's position after the publication of the 2021 UFO Report by the U.S. Intelligence community is that the [[Philosophy:Scientific community|scientific community]] needs to "systematically, scientifically and transparently look for potential evidence of extraterrestrial technological equipment".<ref>{{cite web |title=The Galileo Project for the Systematic Scientific Search for Evidence of Extraterrestrial Technological Artifacts |url=https://projects.iq.harvard.edu/galileo |website=projects.iq.harvard.edu |access-date=25 July 2021 |language=en |archive-date=24 July 2021 |archive-url=https://web.archive.org/web/20210724200203/https://projects.iq.harvard.edu/galileo |url-status=live }}</ref>

==See also==
{{div col|colwidth=30em}}
* {{annotated link|Astronomy:Arecibo message}}
* {{annotated link|Astronomy:Astrobiology}}
* {{annotated link|Engineering:Astrobiology Science and Technology for Exploring Planets}}
* {{annotated link|Astronomy:Breakthrough Listen}}
** {{annotated link|Astronomy:BLC1}}
* {{annotated link|Astronomy:Communication with extraterrestrial intelligence}}
* {{annotated link|Astronomy:Detecting Earth from distant star-based systems}} – e.g. detectability to SETI programs by extraterrestrials
* {{annotated link|Astronomy:Kardashev scale}}
* {{annotated link|Astronomy:Metalaw}}
* {{annotated link|Organization:Nexus for Exoplanet System Science}}
* {{annotated link|Astronomy:OpenSonATA}} Open SETI on the Allen Telescope Array
* {{annotated link|Astronomy:Potential cultural impact of extraterrestrial contact}}
* {{annotated link|Astronomy:Quiet and loud aliens}}
* {{annotated link|Astronomy:SETIcon}}
* {{annotated link|Astronomy:SetiQuest}}
* {{annotated link|Unsolved:Xenoarchaeology}}
{{div col end}}

==References==
{{Reflist|20em}}

==Further reading==
{{Library resources box}}
* {{cite book |author=Campbell |first=John B. |chapter=Archaeology and direct imaging of exoplanets |date=2006 |pages=247 ''ff'' |isbn=978-0-521-85607-2 |editor=C. Aime & F. Vakili |chapter-url=http://eprints.jcu.edu.au/1027/1/S1743921306009392a.pdf |title=Proceedings of the International Astronomical Union |publisher=Cambridge University Press |url-status=dead |archive-url=https://web.archive.org/web/20090326065433/http://eprints.jcu.edu.au/1027/1/S1743921306009392a.pdf |archive-date=2009-03-26}}
* {{cite journal |url=http://spsr.utsi.edu/articles/markjbis2007.pdf |title=Detecting Patterns of a Technological Intelligence in Remotely Sensed Imagery |author=Carlotto |first=Mark J. |journal=Journal of the British Interplanetary Society |volume=60 |pages=28–39 |date=2007 |bibcode=2007JBIS...60...28C |access-date=2009-03-03 |archive-date=2016-09-09 |archive-url=https://web.archive.org/web/20160909220124/http://spsr.utsi.edu/articles/markjbis2007.pdf |url-status=dead}}
* {{cite book | last = Catran | first = Jack | title = Is There Intelligent Life on Earth? | date = 1980 | publisher = Lidiraven Books | isbn = 978-0-9361-6229-4 | url = https://archive.org/stream/pressontechnocra00unse_3#page/n0/mode/2up}}
* {{cite book |first=Milan M. |url=https://books.google.com/books?id=vrgf3tWgUrwC |title=The Astrobiological Landscape: Philosophical Foundations of the Study of Cosmic Life |publisher=Cambridge University Press |author=Ćirković |year=2012 |isbn=9780521197755 |series=Cambridge Astrobiology |issn=1759-3247}}
* {{Cite book |last=Cooper |first=Keith |title=The Contact Paradox: Challenging Our Assumptions in the Search for Extraterrestrial Intelligence |publisher=Bloomsbury Publishing |year=2019 |isbn=9781472960443}}
* {{Cite book |last=DeVito |first=Carl L. |title=Science, Seti, and Mathematics |publisher=Berghahn Books |year=2013 |isbn=9781782380702}}
* {{Cite book |last=Maccone |first=Claudio |title=Evo-SETI: Life Evolution Statistics on Earth and Exoplanets |publisher=Springer International Publishing |year=2022 |isbn=9783030519339}}
* {{cite book | last = McConnell | first = Brian |author2=Chuck Toporek | title = Beyond Contact: A Guide to SETI and Communicating with Alien Civilizations | date = 2001 | publisher = O'Reilly | isbn = 978-0-596-00037-0}}
* Phillip Morrison, John Billingham, & John Wolfe: ''[https://history.nasa.gov/SP-419/sp419.htm The search for extraterrestrial intelligence—SETI]''. NASA SP, Washington 1977
* {{Cite book |last=Oberhaus |first=Daniel |title=Extraterrestrial Languages |publisher=The MIT Press |year=2019 |isbn=9780262043069}}
* {{Cite book |last=Roush |first=Wade |title=Extraterrestrials |publisher=MIT Press |year=2020 |isbn=9780262538435}}
* David W. Swift: ''Seti Pioneers: Scientists Talk about Their Search for Extraterrestrial Intelligence.'' University of Arizona Press, Tucson, Arizona, 1993, {{ISBN|0-8165-1119-5}}
* Frank White: ''The Seti Factor: How the Search for Extraterrestrial Intelligence Is Changing Our View of the Universe and Ourselves''. Walker & Company, New York 1990, {{ISBN|978-0-8027-1105-2}}
* {{cite book |title=All These Worlds Are Yours: The Scientific Search for Alien Life |author=Jon Willis |publisher=Yale University Press |year=2016 |isbn=978-0300208696}}

==External links==
* SETI official website
* [http://seti.harvard.edu Harvard University SETI Program] {{Webarchive|url=https://web.archive.org/web/20110810063801/http://seti.harvard.edu/ |date=2011-08-10 }}
* [http://seti.berkeley.edu University of California, Berkeley SETI Program]
* [http://www.nhao.jp/~narusawa/oseti/project-dorothy.html Project Dorothy], a Worldwide Joint SETI Observation to Commemorate the 50th Anniversary of Project OZMA
* {{Cite web |url=https://libraries.ideasroadshow.com/issues/jill-tarter-2013-04-19 |title=SETI: Astronomy as a Contact Sport - A conversation with Jill Tarter |work=Ideas Roadshow |date=April 19, 2013}}
* [http://archive.seti.org/epo/news/features/rio-scale.php The Rio Scale] {{Webarchive|url=https://web.archive.org/web/20150914063643/http://archive.seti.org/epo/news/features/rio-scale.php |date=2015-09-14 }}, a scale for rating SETI announcements
* [https://www.youtube.com/watch?v=HPQz-kdaxNo&t=73s 2012 Interview of SETI Pioneer] Frank Drake by astronomer [[Biography:Andrew Fraknoi|Andrew Fraknoi]]
* Now dark SETI Net station archives ([https://www.seti.net/OriginalIdex.php www.seti.net)]

{{Astrobiology}}
{{Radio-astronomy|state=collapsed}}
{{Extraterrestrial life}}
{{Exoplanet search projects}}
{{Interstellar messages}}

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[[Category:Search for extraterrestrial intelligence| ]]
[[Category:Astrobiology]]
[[Category:Distributed computing projects]]
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