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Search for extraterrestrial intelligence

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"SETI" redirects here. For other uses, seeSETI (disambiguation).

Thesearch for extraterrestrial intelligence(SETI) is a collective term for scientific searches for intelligentextraterrestrial life,for example, monitoringelectromagnetic radiationfor signs oftransmissionsfromcivilizationson other planets.[1][2][3]

Scientific investigation began shortly after the advent ofradioin the early 1900s, and focused international efforts have been ongoing since the 1980s.[4]In 2015,Stephen Hawkingand Israeli billionaireYuri Milnerannounced theBreakthrough ListenProject, a $100 million 10-year attempt to detect signals from nearby stars.[5]

History

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Early work

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There have been many earlier searches for extraterrestrial intelligence within theSolar System.In 1896,Nikola Teslasuggested that an extreme version of his wireless electrical transmission system could be used to contact beings onMars.[6]In 1899, while conducting experiments at hisColorado 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,[7]
  • that he may have been observing signals from Marconi's Europeanradioexperiments,
  • and even speculation that he could have picked up naturally occurring radio noise caused by a moon ofJupiter(Io) moving through themagnetosphere of Jupiter.[8]

In the early 1900s,Guglielmo Marconi,Lord KelvinandDavid Peck Toddalso stated their belief that radio could be used to contactMartians,with Marconi stating that his stations had also picked up potential Martian signals.[9][10]

On August 21–23, 1924, Mars entered anoppositioncloser to Earth than at any time in the century before or the next 80 years.[11]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 theUnited States Naval Observatory,a radio receiver was lifted 3 kilometres (1.9 miles) above the ground in adirigibletuned to awavelengthbetween 8 and 9 km, using a "radio-camera" developed byAmherst CollegeandCharles Francis Jenkins.The program was led by David Peck Todd with the military assistance of AdmiralEdward W. Eberle(Chief of Naval Operations), withWilliam F. Friedman(chiefcryptographerof the United States Army), assigned to translate any potential Martian messages.[12][13]

A 1959 paper byPhilip MorrisonandGiuseppe Cocconifirst pointed out the possibility of searching themicrowavespectrum. It proposedfrequenciesand a set of initial targets.[14][15]

In 1960,Cornell UniversityastronomerFrank Drakeperformed the first modern SETI experiment, named "Project Ozma"after theQueen of OzinL. Frank Baum's fantasy books.[16]Drake used a radio telescope 26 metres (85 ft) in diameter atGreen Bank, West Virginia,to examine the starsTau CetiandEpsilon Eridaninear the 1.420gigahertzmarker frequency, a region of the radio spectrum dubbed the "water hole"due to its proximity to thehydrogenandhydroxyl radicalspectral 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.

Sovietscientists took a strong interest in SETI during the 1960s and performed a number of searches withomnidirectional antennasin the hope of picking up powerful radio signals. Soviet astronomerIosif Shklovskywrote the pioneering book in the field,Universe, Life, Intelligence(1962), which was expanded upon by American astronomerCarl Saganas the best-selling bookIntelligent Life in the Universe(1966).[17]

In the March 1955 issue ofScientific American,John D. Krausdescribed an idea to scan thecosmosfor natural radio signals using a flat-plane radio telescope equipped with a parabolicreflector.Within two years, his concept was approved for construction byOhio State University.With a total ofUS$71,000 (equivalent to $770,232 in 2023) in grants from theNational Science Foundation,construction began on an 8-hectare (20-acre) plot inDelaware, 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,NASAfunded a SETI study that involved Drake,Barney OliverofHewlett-Packard Laboratories,and others. The resulting report proposed the construction of an Earth-based radio telescope array with 1,500 dishes known as "Project Cyclops".The price tag for the Cyclops array was US$10 billion. Cyclops was not built, but the report[18]formed the basis of much SETI work that followed.

The Wow! Signal

The Ohio State SETI program gained fame on August 15, 1977, whenJerry 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 theWow! signal,it is considered by some to be the best candidate for a radio signal from an artificial,extraterrestrialsource ever discovered, but it has not been detected again in several additional searches.[19]

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 inThe New York Times.[20]

Sentinel, META, and BETA

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In 1980,Carl Sagan,Bruce Murray,andLouis Friedmanfounded the U.S.Planetary Society,partly as a vehicle for SETI studies.[3]

In the early 1980s,Harvard UniversityphysicistPaul Horowitztook the next step and proposed the design of aspectrum analyzerspecifically 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-circuitdigital signal processing(DSP) technology could be used to buildautocorrelationreceivers 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 26-meter (85 ft) Harvard/Smithsonian radio telescope atOak Ridge ObservatoryinHarvard, 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 frequencyDoppler shiftto 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 makerSteven Spielberg.A second such effort, META II, was begun inArgentinain 1990, to search the southern sky, receiving an equipment upgrade in 1996–1997.[21][22]

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 dedicatedfast Fourier transform(FFT) engines, each capable of performing a 222-pointcomplexFFTs in two seconds, and 21 general-purposepersonal computersequipped with customdigital signal processingboards. This allowed BETA to receive 250 million simultaneous channels with a resolution of 0.5 hertz per channel. It scanned through the microwavespectrumfrom 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 withEarth'ssiderealrotation 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.[23]This forced the BETA project to cease operation.

MOP and Project Phoenix

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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 inlight-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.[24]

In 1978, the NASA SETI program had been heavily criticized by SenatorWilliam Proxmire,and funding for SETI research was removed from the NASA budget by Congress in 1981;[25]however, funding was restored in 1982, afterCarl Sagantalked with Proxmire and convinced him of the program's value.[25]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 theNASA Deep Space Network,as well as the 140-foot (43 m) radio telescope of theNational Radio Astronomy Observatoryat Green Bank, West Virginia and the 1,000-foot (300 m)radio telescopeat theArecibo Observatoryin 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.

Arecibo Telescopein Puerto Rico with its 300 m (980 ft) dish was one of the world's largest filled-aperture (i.e. full dish) radio telescopes and conducted some SETI searches.

MOP drew the attention of theUnited States Congress,where the program met opposition[26]and canceled one year after its start.[25]SETI advocates continued without government funding, and in 1995 the nonprofitSETI InstituteofMountain View, Californiaresurrected 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.[27]Project Phoenix,under the direction ofJill Tarter,was a continuation of the targeted search program from MOP and studied roughly 1,000 nearbySun-like stars until approximately 2015.[28]From 1995 through March 2004, Phoenix conducted observations at the 64-meter (210 ft)Parkes radio telescopeinAustralia,the 140-foot (43 m) radio telescope of theNational Radio Astronomy Observatoryin Green Bank, West Virginia, and the 1,000-foot (300 m) 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 MHz. The search was sensitive enough to pick up transmitters with 1 GWEIRPto a distance of about 200light-years.

Ongoing radio searches

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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 toradio telescopesthat 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 narrowbandwidths.Earth has been sending radio waves from broadcasts into space for over 100 years.[29]These signals have reached over 1,000 stars, most notablyVega,Aldebaran,Barnard's Star,Sirius,andProxima 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'sionosphere.

Many international radio telescopes are currently[when?]being used for radio SETI searches, including theLow Frequency Array(LOFAR) in Europe, theMurchison Widefield Array(MWA) in Australia, and theLovell Telescopein the United Kingdom.[30]

Allen Telescope Array

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Main article:Allen Telescope Array

The SETI Institute collaborated with theRadio Astronomy Laboratoryat theBerkeley SETI Research Centerto 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,Paul Allen.Its sensitivity is designed to be equivalent to a single large dish more than 100 meters in diameter, if fully completed. Presently[when?],the array has 42 operational dishes at theHat Creek Radio Observatoryin rural northern California.[31][32]

The full array (ATA-350) is planned to consist of 350 or more offset-Gregorianradio dishes, each 6.1 meters (20 feet) 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 theinterferometeroutput at the same time. Typically, the ATA snapshot imager (used for astronomical surveys and SETI) is run in parallel with abeamformingsystem (used primarily for SETI).[33]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.[34][35][36]

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.[37][38]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.[39]

Berkeley astronomers used the ATA to pursue several science topics, some of which might have transient SETI signals,[40][41][42]until 2011, when the collaboration between the University of California, Berkeley and the SETI Institute was terminated.

CNETpublished an article and pictures about the Allen Telescope Array (ATA) on December 12, 2008.[43][44]

In April 2011, the ATA entered an 8-month "hibernation" due to funding shortfalls. Regular operation of the ATA resumed on December 5, 2011.[45][46]

In 2012, the ATA was revitalized with a $3.6 million donation byFranklin Antonio,co-founder and Chief Scientist of QUALCOMM Incorporated.[47]This gift supported upgrades of all the receivers on the ATA dishes to have (2× to 10× over the range 1–8 GHz) greater sensitivity than before and supporting observations over a wider frequency range from 1–18 GHz, though initially the radio frequency electronics only go to 12 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 ofastronomical radio sources,such as heretofore unexplained non-repeating, possibly extragalactic, pulses known asfast radio burstsor FRBs.[48][49]

SERENDIP

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Main article:SERENDIP

SERENDIP (Search for Extraterrestrial Radio Emissions from Nearby Developed Intelligent Populations) is a SETI program launched in 1979 by theBerkeley SETI Research Center.[50][51]SERENDIP takes advantage of ongoing "mainstream"radio telescopeobservations as a "piggy-back"or"commensal"program, using large radio telescopes including the NRAO 90m telescope at Green Bank and, formerly, theArecibo 305m telescope.Rather than having its own observation program, SERENDIP analyzesdeep spaceradio telescope data that it obtains while otherastronomersare using the telescopes. The most recently deployed SERENDIP spectrometer, SERENDIP VI, was installed at both theArecibo Telescopeand theGreen Bank Telescopein 2014–2015.[52]

Breakthrough Listen

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Main article:Breakthrough Listen

Breakthrough Listenis a ten-year initiative with $100 million funding begun in July 2015 to actively search for intelligent extraterrestrial communications in theuniverse,in a substantially expanded way, using resources that had not previously been extensively used for the purpose.[53][54][55][3]It has been described as the most comprehensive search for alien communications to date.[54]The science program for Breakthrough Listen is based atBerkeley SETI Research Center,[56][57]located in the Astronomy Department[58]at theUniversity of California, Berkeley.

Announced in July 2015, the project is observing for thousands of hours every year on two major radio telescopes, theGreen Bank Observatoryin West Virginia, and theParkes ObservatoryinAustralia.[59]Previously, only about 24 to 36 hours of telescope time per year were used in the search for alien life.[54]Furthermore, theAutomated Planet FinderatLick Observatoryis 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.[60]Some of the data are also analyzed by volunteers in theSETI@homevolunteer computing network.[59]Founder of modern SETIFrank Drakewas one of the scientists on the project's advisory committee.[61][53][54]

In October 2019, Breakthrough Listen started a collaboration with scientists from the TESS team (Transiting Exoplanet Survey Satellite) to look for signs of advanced extraterrestrial life. Thousands of new planets found by TESS will be scanned fortechnosignaturesby Breakthrough Listen partner facilities across the globe. Data from TESS monitoring of stars will also be searched for anomalies.[62]

FAST

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Main article:Five-hundred-meter Aperture Spherical Telescope

China's 500 meter Aperture Spherical Telescope (FAST) listsdetecting interstellar communication signalsas part of its science mission. It is funded by theNational 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.[63]FAST consists of a fixed 500 m (1,600 ft) diameter spherical dish constructed in a natural depression sinkhole caused bykarst processesin the region. It is the world's largest filled-aperture radio telescope.[64] 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.[65]

On 14 June 2022, astronomers, working with China'sFAST 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.[66]More recently, on 18 June 2022,Dan Werthimer,chief scientist for severalSETI-related projects, reportedly noted, "These signals are from radio interference; they are due to radio pollution from earthlings, not from E.T.".[67]

UCLA

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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 theKeplerfield,TRAPPIST-1,and solar-type stars.[68]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.[69]

Community SETI projects

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Screen shot of the screensaver forSETI@home,a formervolunteer computingproject in which volunteers donated idle computer power to analyze radio signals for signs of extraterrestrial intelligence.

SETI@home

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Main article:SETI@home

The SETI@home project usedvolunteer computingto analyze signals acquired by theSERENDIPproject.

SETI@home was conceived by David Gedye along with Craig Kasnoff and is a popularvolunteer computingproject that was launched by theBerkeley SETI Research Centerat theUniversity of California, Berkeley,in May 1999. It was originally funded byThe Planetary SocietyandParamount Pictures,and later by the state ofCalifornia.The project is run by directorDavid P. Andersonand chief scientistDan Werthimer.Any individual could become involved with SETI research by downloading theBerkeley 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 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@homeserversat 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 617teraFLOPS.[70]In 2004radio source SHGb02+14aset 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 withinrandom chance.[71][72]

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.[73]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.[74]

SETI Net

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SETI Network was the only fully operational private search system.[75]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 MHz of the 1420 MHz spectrum, a receiver to reproduce the wideband audio, and a standardpersonal computeras 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. TheWow! signalarea 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

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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 emeritusH. 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 3 to 5 m (10–16 ft) in diameter into research-grade radio telescopes of modest sensitivity.[76]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.[77]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 10−23Watts/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.[78]

The name "Argus" derives from themythical Greek guard-beastwho 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.[79]

Optical experiments

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While most SETI sky searches have studied the radio spectrum, some SETI researchers have considered the possibility that alien civilizations might be using powerfullasersfor interstellar communications at optical wavelengths.[80][81][82]The idea was first suggested by R. N. Schwartz andCharles Hard Townesin a 1961 paper published in the journalNaturetitled "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 journalProceedings of the National Academy of Sciences,[83]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.[84]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.[85][86]

Optical SETI supporters have conducted paper studies[87]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 155-centimeter (61-inch) 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 withPrinceton Universityto 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 ProfessorPaul Horowitzbuilt 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 theOak Ridge ObservatoryinHarvard, Massachusetts.

The University of California, Berkeley, home ofSERENDIPandSETI@home,is also conducting optical SETI searches and collaborates with theNIROSETIprogram. The optical SETI program at Breakthrough Listen was initially directed byGeoffrey Marcy,an extrasolar planet hunter, and it involves examination of records of spectra taken duringextrasolar planethunts for a continuous, rather than pulsed, laser signal. This survey uses theAutomated Planet Finder2.4-m telescope at theLick Observatory,situated on the summit of Mount Hamilton, east of San Jose, California.[88]The other Berkeley optical SETI effort is being pursued by the Harvard-Smithsonian group and is being directed byDan Werthimerof Berkeley, who built the laser detector for the Harvard-Smithsonian group. This survey uses a 76-centimeter (30-inch) automated telescope atLeuschner Observatoryand an older laser detector built by Werthimer.

The SETI Institute also runs a program called '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.[89][90]

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.[91][82]

In May 2017, astronomers reported studies related to laser light emissions from stars, as a way of detecting technology-related signals from analien civilization.The reported studies includedTabby's Star(designated KIC 8462852 in theKepler Input Catalog), an oddly dimming star in which its unusual starlight fluctuations may be the result of interference by an artificial megastructure, such as aDyson swarm,made by such a civilization. No evidence was found for technology-related signals from KIC 8462852 in the studies.[92][93][94]

Quantum communications

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In a 2021preprint,astronomer Michael Hipke described for the first time how one could search forquantum communicationtransmissions sent byETIusing existing telescope and receiver technology. He also provides arguments for why future searches of ETI should also target interstellar quantum communication networks.[95][96]

A 2022 paper by Arjun Berera and Jaime Calderón-Figueroa noted that interstellar quantum communication by other civilizations could be possible and may be advantageous, identifying some potential challenges and factors for detectingtechnosignatures.They may, for example, use X-ray photons for remotely establishedquantum communicationandquantum teleportationas the communication mode.[97][98]

Search for extraterrestrial artifacts

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The possibility of using interstellar messenger probes in the search for extraterrestrial intelligence was first suggested byRonald N. Bracewellin 1960 (seeBracewell probe), and the technical feasibility of this approach was demonstrated by the British Interplanetary Society's starship studyProject Daedalusin 1978. Starting in 1979,Robert Freitasadvanced arguments[99][100][101]for the proposition that physical space-probes are a superior mode of interstellar communication to radio signals (seeVoyager Golden Record).

In recognition that any sufficiently advanced interstellar probe in the vicinity of Earth could easily monitor the terrestrialInternet,'Invitation to ETI' was established byAllen Toughin 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.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.[102]That said, for simple messages such as "hello," radio SETI could be far more efficient.[103]If energy requirement is used as a proxy for technical difficulty, then a solarcentric Search for Extraterrestrial Artifacts (SETA)[104]may be a useful supplement to traditional radio or optical searches.[105][106]

Much like the "preferred frequency" concept in SETI radio beacon theory, the Earth-Moon or Sun-Earthlibrationorbits[107]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 pointsL4andL5,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.[107]The authors conducted a second, more comprehensive photographic search for probes in 1982[108]that examined the five Earth-MoonLagrangian positionsand included the solar-synchronized positions in the stable L4/L5 libration orbits, the potentially stable nonplanar orbits near L1/L2, Earth-MoonL3,and alsoL2in the Sun-Earth system. Again no extraterrestrial probes were found to limiting magnitudes of 17–19th magnitude near L3/L4/L5, 10–18th magnitude forL1/L2,and 14–16th magnitude for Sun-EarthL2.

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 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 SETIwater holeregion of the terrestrial microwave window, and (3) in addition to beacon signals, tritium hyperfine emission may occur as a byproduct of extensivenuclear fusionenergy production by extraterrestrial civilizations. The wideband- and narrowband-channel observations achieved sensitivities of 5–14×10−21W/m2/channel and 0.7–2×10−24W/m2/channel, respectively, but no detections were made.[109]

Others have speculated, that we might find traces of past civilizations in our very own Solar System, on planets likeVenusorMars,although the traces would be found most likely underground.[110][111]

Technosignatures

[edit]
See also:TechnosignatureandMegascale engineering

Technosignatures, including all signs of technology, are a recent avenue in the search for extraterrestrial intelligence.[112][3]Technosignatures may originate from various sources, from megastructures such asDyson spheres[113][114]andspace mirrorsorspace shaders[115]to the atmospheric contamination created by an industrial civilization,[116]or city lights on extrasolar planets, and may be detectable in the future with largehypertelescopes.[117]

Technosignatures can be divided into three broad categories:astroengineeringprojects, signals of planetary origin, and spacecraft within and outside theSolar System.

An astroengineering installation such as aDyson 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 asolar analogstar,[118]or by the star's apparent disappearance in the visible spectrum over several years.[119]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.[120][121]

Another hypothetical form of astroengineering, theShkadov thruster,moves its host star by reflecting some of the star's light back on itself, and would be detected by observing if itstransitsacross the star abruptly end with the thruster in front.[122]Asteroid miningwithin the Solar System is also a detectable technosignature of the first kind.[123]

Individual extrasolar planets can be analyzed for signs of technology.Avi Loebof theCenter for Astrophysics | Harvard & Smithsonianhas 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.[124][125]In addition, the excess infrared radiation[117][126]and chemicals[127][128]produced by various industrial processes orterraformingefforts[129]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,[130]a civilization heat signature should be within a "comfortable" temperature range, like terrestrialurban 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 asartificial satellitesaroundexoplanets,particularly such ingeostationary orbit,might be detectable even with today's technology and data, and would allow, similar tofossilson Earth, to find traces of extrasolar life from long ago.[131]

Extraterrestrial craft are another target in the search for technosignatures.Magnetic sailinterstellar spacecraftshould be detectable over thousands of light-years of distance through thesynchrotron radiationthey would produce through interaction with theinterstellar medium;other interstellar spacecraft designs may be detectable at more modest distances.[132]In addition, robotic probes within the Solar System are also being sought with optical and radio searches.[133][134]

For a sufficiently advanced civilization, hyper energetic neutrinos from Planck scale accelerators should be detectable at a distance of many Mpc.[135]

Advances for Bio and Technosignature Detection

[edit]

A notable advancement in technosignature detection is the development of an algorithm for signal reconstruction in zero-knowledge one-way communication channels.[136]This algorithm decodes signals from unknown sources without prior knowledge of the encoding scheme, using principles fromAlgorithmic Information Theoryto identify the geometric and topological dimensions of the encoding space. It successfully reconstructed the Arecibo message despite significant noise. The work establishes a connection between syntax and semantics inSETIand technosignature detection, enhancing fields like cryptography andInformation Theory.[137]

Based onfractaltheory and theWeierstrass function,a known fractal, another method authored by the same group called fractal messaging offers a framework for space-time scale-free communication. This method leverages properties of self-similarity and scale invariance, enabling spatio-temporal scale-independent and parallel infinite-frequency communication. It also embodies the concept of sending a self-encoding/self-decoding signal as a mathematical formula, equivalent to self-executable computer code that unfolds to read a message at all possible time scales and in all possible channels simultaneously.[138]

Fermi paradox

[edit]
Main article:Fermi paradox

Italian physicistEnrico Fermisuggested 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?"[139]

The Fermi paradox is commonly understood as asking why extraterrestrials have not visited Earth,[140]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:

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.

There are multiple explanations proposed for the Fermi paradox,[141]ranging from analyses suggesting that intelligent life is rare (the "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 astronomerSebastian von Hoernersuggested[142]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 theMilky Wayis 1,000 light years.[143][144][145]

Science writerTimothy Ferrishas 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

[edit]

A significant problem is the vastness of space. Despite piggybacking on the world's most sensitive radio telescope, astronomer and initiator of SERENDIPCharles Stuart Bowyernoted the then world's largest instrument could not detect random radio noise emanating from a civilization like ours,[citation needed]which has been leaking radio and TV signals for less than 100 years.[146]ForSERENDIPand 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.[147]

Post-detection disclosure protocol

[edit]

TheInternational Academy of Astronautics(IAA) has a long-standing SETI Permanent Study Group (SPSG, formerly called the IAA SETI Committee), which addresses matters of SETIscience,technology,andinternational policy.The SPSG meets in conjunction with theInternational 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, ProfessorPaul 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."[148]

However, the protocols mentioned apply only to radio SETI rather than for METI (Active SETI).[149]The intention for METI is covered under the SETI charter "Declaration of Principles Concerning Sending Communications with Extraterrestrial Intelligence".

In October 2000 astronomersIván AlmárandJill Tarterpresented a paper to The SETI Permanent Study Group inRio de Janeiro, Brazilwhich proposed a scale (modelled after theTorino scale) which is an ordinal scale between zero and ten that quantifies the impact of any public announcement regarding evidence of extraterrestrial intelligence;[150]theRio scalehas since inspired the 2005San Marino Scale(in regard to the risks of transmissions from Earth) and the 2010 London Scale (in regard to the detection of extraterrestrial life).[151]The Rio scale itself was revised in 2018.[152]

The SETI Institute does not officially recognize theWow! signalas of extraterrestrial origin as it was unable to be verified, although in a 2020 tweet the organization stated that ''an astronomer might have pinpointed the host star''.[153]The SETI Institute has also publicly denied that the candidate signalRadio source SHGb02+14ais of extraterrestrial origin.[154][155]Although other volunteering projects such asZooniversecredit users for discoveries, there is currently no crediting or early notification bySETI@Homefollowing the discovery of a signal.

Some people, includingSteven M. Greer,[156]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 asBruce Jakosky[157]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'sreligions.

Active SETI

[edit]
Main article: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

[edit]

In November 1974, a largely symbolic attempt was made at the Arecibo Observatory to send a message to other worlds. Known as theArecibo Message,it was sent towards theglobular clusterM13,which is 25,000 light-years from Earth. Further IRMsCosmic Call,Teen Age Message,Cosmic Call 2,andA Message From Earthwere transmitted in 1999, 2001, 2003 and 2008 from theEvpatoriaPlanetary Radar.

Debate

[edit]
See also:Active SETI § Controversy
Example of a high-resolution pictorial message to potential ETI. These messages usually contain information about the location of the solar system in theMilky Way.

Whether or not to attempt to contact extraterrestrials has attracted significant academic debate in the fields ofspace ethicsandspace policy.[158][159][160]PhysicistStephen Hawking,in his bookA 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".[161]In one response to Hawking, in September 2016, astronomerSeth Shostaksought to allay such concerns.[162]AstronomerJill Tarteralso 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.[163]

Criticism

[edit]
For criticism ofActive SETI,seeActive SETI § Controversy.

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:

[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[...] 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.[1]

Critics claim that the existence of extraterrestrial intelligence has no goodPopperiancriteria forfalsifiability,as explained in a 2009editorialinNature,which said:

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.[4]

Natureadded 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".[4]

However,Naturealso 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".[4]

Supporters of theRare Earth Hypothesisargue that advanced lifeforms are likely to be very rare, and that, if that is so, then SETI efforts will be futile.[164][165][166]However, the Rare Earth Hypothesis itself facesmany criticisms.[166]

In 1993,Roy Mashstated 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".[167]In response, in 2012,Milan M. Ćirković,then research professor at theAstronomical Observatory of Belgradeand a research associate of theFuture of Humanity Instituteat theUniversity of Oxford,[168]said that Mash was unrealistically over-reliant on excessive abstraction that ignored theempiricalinformation available to modern SETI researchers.[169]

George Basalla,EmeritusProfessor of History at theUniversity of Delaware,[170]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",[171][172]and was in turn criticized byMilan M. Ćirković[168]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 asFreeman Dysonand, 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.[172]

Massimo Pigliucci,Professor ofPhilosophyatCUNY–City College,[173]asked in 2010 whether SETI is "uncomfortably close to the status ofpseudoscience"due to the lack of any clear point at which negative results cause thehypothesisof Extraterrestrial Intelligence to be abandoned,[174]before eventually concluding that SETI is "almost-science", which is described by Milan M. Ćirković[168]as Pigliucci putting SETI in "the illustrious company ofstring theory,interpretations of quantum mechanics,evolutionary psychologyand history (of the 'synthetic' kind done recently byJared 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[167]and Basalla[171]".[175]

Richard Carrigan,aparticle physicistat theFermi National Accelerator LaboratorynearChicago, Illinois,suggested that passive SETI could also be dangerous and that a signal released onto the Internet could act as acomputer virus.[176]Computer security expertBruce Schneierdismissed this possibility as a "bizarre movie-plot threat".[177]

Ufology

[edit]

UfologistStanton Friedmanhas often criticized SETI researchers for, among other reasons, what he sees as their unscientific criticisms of Ufology,[178][179]but, unlike SETI, Ufology has generally not been embraced by academia as a scientific field of study,[180][181]and it is usually characterized as a partial[182]or total[183][184]pseudoscience.In a 2016 interview,Jill Tarterpointed out that it is still a misconception that SETI and UFOs are related.[185]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."[185]The Galileo Projectheaded by Harvard astronomerAvi Loebis one of the few scientific efforts to study UFOs or UAPs.[186]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.[187]The Galileo Project's position after the publication of the 2021UFO Report by the U.S. Intelligence communityis that thescientific communityneeds to "systematically, scientifically and transparently look for potential evidence of extraterrestrial technological equipment".[188]

See also

[edit]

References

[edit]
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  172. ^abĆirković (2012),p. 172Archived2023-01-16 at theWayback Machine,"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,..."
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