Поиск внеземных цивилизаций: история и методы.

Effort to find civilizations not from Earth

The search for extraterrestrial intelligence (SETI) is a collective term for scientific searches for intelligent extraterrestrial life, for example, monitoring electromagnetic radiation for signs of transmissions from civilizations on other planets. Scientific investigation began shortly after the advent of radio in the early 1900s, and focused international efforts have been ongoing since the 1980s.

In 1980, Carl Sagan, Bruce Murray, and Louis Friedman founded the U. S. Planetary Society, partly as a vehicle for SETI studies. 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 (FFT) engines, each capable of performing a 222 point complex FFTs in two seconds, and 21 general purpose personal computers equipped with custom 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 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 Earth's 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. This forced the BETA project to cease operation.

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; however, funding was restored in 1982, after Carl Sagan talked with Proxmire and convinced him of the program's value. and canceled one year after its start. Project Phoenix, under the direction of Jill Tarter, was a continuation of the targeted search program from MOP and studied roughly 1,000 nearby Sun like stars until approximately 2015. From 1995 through March 2004, Phoenix conducted observations at the Parkes radio telescope in Australia, the 140 feet (43 m) radio telescope of the National Radio Astronomy Observatory in Green Bank, West Virginia, and the 1000 feet (305 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 GW EIRP to a distance of about 200 light years.

Many radio frequencies penetrate Earth's atmosphere quite well, and this led to radio telescopes 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 bandwidths. Earth has been sending radio waves from broadcasts into space for over 100 years. These signals have reached over 1,000 stars, most notably Vega, Aldebaran, Barnard's Star, Sirius, and 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 ionosphere. Many international radio telescopes are currently being used for radio SETI searches, including the Low Frequency Array (LOFAR) in Europe, the Murchison Widefield Array (MWA) in Australia, and the Lovell Telescope in the United Kingdom.

The SETI Institute collaborated with the Radio Astronomy Laboratory at the 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, 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. The full array (ATA 350) is planned to consist of 350 or more offset Gregorian radio dishes, each 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 interferometer output at the same time. Typically, the ATA snapshot imager (used for astronomical surveys and SETI) is run in parallel with a beamforming system (used primarily for SETI). 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. 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. 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. Berkeley astronomers used the ATA to pursue several science topics, some of which might have transient SETI signals, 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. In April 2011, the ATA entered an 8 month "hibernation" due to funding shortfalls. Regular operation of the ATA resumed on December 5, 2011. In 2012, the ATA was revitalized with a $3.6 million donation by Franklin Antonio, co founder and Chief Scientist of QUALCOMM Incorporated. 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 of astronomical radio sources, such as heretofore unexplained non repeating, possibly extragalactic, pulses known as fast radio bursts or FRBs.

SERENDIP (Search for Extraterrestrial Radio Emissions from Nearby Developed Intelligent Populations) is a SETI program launched in 1979 by the Berkeley SETI Research Center. SERENDIP takes advantage of ongoing "mainstream" 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 Arecibo 305m telescope. Rather than having its own observation program, SERENDIP analyzes deep space radio telescope data that it obtains while other astronomers are using the telescopes. The most recently deployed SERENDIP spectrometer, SERENDIP VI, was installed at both the Arecibo Telescope and the Green Bank Telescope in 2014–2015.

Breakthrough Listen is a ten year initiative with $100 million funding begun in July 2015 to actively search for intelligent extraterrestrial communications in the universe, in a substantially expanded way, using resources that had not previously been extensively used for the purpose. located in the Astronomy Department at the University of California, Berkeley. Announced in July 2015, the project is observing for thousands of hours every year on two major radio telescopes, the Green Bank Observatory in West Virginia, and the Parkes Observatory in Australia. Previously, only about 24 to 36 hours of telescope time per year were used in the search for alien life. Some of the data are also analyzed by volunteers in the SETI@home volunteer computing network.

China's 500 meter Aperture Spherical Telescope (FAST) lists detecting interstellar communication signals as part of its science mission. It is funded by the 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. FAST consists of a fixed diameter spherical dish constructed in a natural depression sinkhole caused by karst processes in the region. It is the world's largest filled aperture radio telescope. 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. On 14 June 2022, astronomers, working with China's 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. 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.".

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 Kepler field, TRAPPIST 1, and solar type stars. 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.

The SETI@home project used volunteer computing to analyze signals acquired by the SERENDIP project. SETI@home was conceived by David Gedye along with Craig Kasnoff and is a popular volunteer computing project that was launched by the Berkeley SETI Research Center at the University of California, Berkeley, in May 1999. It was originally funded by The Planetary Society and Paramount Pictures, and later by the state of California. The project is run by director David P. Anderson and chief scientist Dan Werthimer. Any individual could become involved with SETI research by downloading the 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 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 servers 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 teraFLOPS. 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. 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. 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.

SETI Network was the only fully operational private search system. 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 standard 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 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.

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 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 in diameter into research grade radio telescopes of modest sensitivity. 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. 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 10−23 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. 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.

While most SETI sky searches have studied the radio spectrum, some SETI researchers have considered the possibility that alien civilizations might be using powerful lasers for interstellar communications at optical wavelengths. 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, 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. 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. Optical SETI supporters have conducted paper studies 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 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 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 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 SERENDIP and SETI@home, is also conducting optical SETI searches and collaborates with the NIROSETI program. The optical SETI program at Breakthrough Listen was initially directed by 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 Automated Planet Finder 2.4 m telescope at the Lick Observatory, situated on the summit of Mount Hamilton, east of San Jose, California. 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 '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. 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.

In a 2021 preprint, an astronomer described for the first time how one could search for quantum communication transmissions sent by ETI using existing telescope and receiver technology. He also provides arguments for why future searches of ETI should also target interstellar quantum communication networks. 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 technosignatures. They may, for example, use X ray photons for remotely established quantum communication and quantum teleportation as the communication mode.

The possibility of using interstellar messenger probes in the search for extraterrestrial intelligence was first suggested by Ronald N. Bracewell in 1960 (see Bracewell probe), and the technical feasibility of this approach was demonstrated by the British Interplanetary Society's starship study Project Daedalus in 1978. Starting in 1979, Robert Freitas advanced arguments for the proposition that physical space probes are a superior mode of interstellar communication to radio signals (see 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. 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. That said, for simple messages such as "hello," radio SETI could be far more efficient. If energy requirement is used as a proxy for technical difficulty, then a solarcentric Search for Extraterrestrial Artifacts (SETA) may be a useful supplement to traditional radio or optical searches. Much like the "preferred frequency" concept in SETI radio beacon theory, the Earth Moon or Sun Earth libration orbits The authors conducted a second, more comprehensive photographic search for probes in 1982 that examined the five Earth Moon 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 , and also 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 /, and 14–16th magnitude for Sun Earth
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 SETI 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 nuclear fusion energy production by extraterrestrial civilizations. The wideband and narrowband channel observations achieved sensitivities of 5–14 W/m2/channel and 0.7–2 W/m2/channel, respectively, but no detections were made. Others have speculated, that we might find traces of past civilizations in our very own Solar System, on planets like Venus or Mars, although the traces would be found most likely underground.

Technosignatures, including all signs of technology, are a recent avenue in the search for extraterrestrial intelligence. and space mirrors or space shaders to the atmospheric contamination created by an industrial civilization, or city lights on extrasolar planets, and may be detectable in the future with large hypertelescopes. Technosignatures can be divided into three broad categories: astroengineering projects, signals of planetary origin, and spacecraft within and outside the Solar System. An astroengineering installation such as a 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, or by the star's apparent disappearance in the visible spectrum over several years. 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. 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 transits across the star abruptly end with the thruster in front. Asteroid mining within the Solar System is also a detectable technosignature of the first kind. Individual extrasolar planets can be analyzed for signs of technology. 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. In addition, the excess infrared radiation and chemicals produced by various industrial processes or terraforming efforts 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, 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 exoplanets, particularly such in geostationary orbit, might be detectable even with today's technology and data, and would allow, similar to fossils on Earth, to find traces of extrasolar life from long ago. Extraterrestrial craft are another target in the search for technosignatures. Magnetic sail interstellar spacecraft should be detectable over thousands of light years of distance through the synchrotron radiation they would produce through interaction with the interstellar medium; other interstellar spacecraft designs may be detectable at more modest distances. In addition, robotic probes within the Solar System are also being sought with optical and radio searches. For a sufficiently advanced civilization, hyper energetic neutrinos from Planck scale accelerators should be detectable at a distance of many Mpc.

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, which has been leaking radio and TV signals for less than 100 years. For 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.

The 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, technology, and international policy. The SPSG meets in conjunction with the 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 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." However, the protocols mentioned apply only to radio SETI rather than for METI (Active SETI). 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 Jill Tarter presented a paper to The SETI Permanent Study Group in Rio de Janeiro, Brazil which proposed a scale (modelled after the Torino scale) which is an ordinal scale between zero and ten that quantifies the impact of any public announcement regarding evidence of extraterrestrial intelligence; the Rio scale has since inspired the 2005 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). The Rio scale itself was revised in 2018. The SETI Institute does not officially recognize the Wow! signal as 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''. The SETI Institute has also publicly denied that the candidate signal Radio source SHGb02+14a is of extraterrestrial origin. 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 Steven M. Greer, 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 Bruce Jakosky 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 religions.

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.

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 globular cluster M13, which is 25,000 light years from Earth. Further IRMs Cosmic Call, Teen Age Message, Cosmic Call 2, and A Message From Earth were transmitted in 1999, 2001, 2003 and 2008 from the Evpatoria Planetary Radar.

Whether or not to attempt to contact extraterrestrials has attracted significant academic debate in the fields of space ethics and space policy. Physicist Stephen Hawking, in his book 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". In one response to Hawking, in September 2016, astronomer Seth Shostak sought to allay such concerns. Astronomer 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.

Ufologist Stanton Friedman has often criticized SETI researchers for, among other reasons, what he sees as their unscientific criticisms of Ufology, but, unlike SETI, Ufology has generally not been embraced by academia as a scientific field of study, and it is usually characterized as a partial or total pseudoscience. In a 2016 interview, Jill Tarter pointed out that it is still a misconception that SETI and UFOs are related. 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." 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. The Galileo Project's position after the publication of the 2021 UFO Report by the U. S. Intelligence community is that the scientific community needs to "systematically, scientifically and transparently look for potential evidence of extraterrestrial technological equipment".