# Astronomy:Search for extraterrestrial intelligence

Short description: Effort to find civilizations not from Earth
Screen shot of the screensaver for SETI@home, a distributed computing project in which volunteers donate idle computer power to analyze radio signals for signs of extraterrestrial intelligence.

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.[1][2][3]

Scientific investigation began shortly after the advent of radio in the early 1900s, and focused international efforts have been ongoing since the 1980s.[4] In 2015, Stephen Hawking and Russian billionaire Yuri Milner announced a project called Breakthrough Listen.[5]

## History

### Early work

There have been many earlier searches for extraterrestrial intelligence within the Solar System. In 1896, Nikola Tesla suggested that an extreme version of his wireless electrical transmission system could be used to contact beings on Mars.[6] In 1899, while conducting experiments at his Colorado Springs experimental station, he thought he had detected a signal from Mars since an odd repetitive static signal seemed to cut off when Mars set in the night sky. Analysis of Tesla's research has led to a range of explanations including: Tesla simply misunderstood the new technology he was working with,[7] that he may have been observing signals from Marconi's European radio experiments, and even speculation that he could have picked up naturally occurring radio noise caused by a moon of Jupiter (Io) moving through the magnetosphere of Jupiter.[8] In the early 1900s, Guglielmo Marconi, Lord Kelvin and David Peck Todd also stated their belief that radio could be used to contact Martians, with Marconi stating that his stations had also picked up potential Martian signals.[9]

On August 21–23, 1924, Mars entered an opposition closer to Earth than at any time in the century before or the next 80 years.[10] In the United States, a "National Radio Silence Day" was promoted during a 36-hour period from August 21–23, with all radios quiet for five minutes on the hour, every hour. At the United States Naval Observatory, a radio receiver was lifted 3 kilometres (1.9 miles) above the ground in a dirigible tuned to a wavelength between 8 and 9 km, using a "radio-camera" developed by Amherst College and Charles Francis Jenkins. The program was led by David Peck Todd with the military assistance of Admiral Edward W. Eberle (Chief of Naval Operations), with William F. Friedman (chief cryptographer of the United States Army), assigned to translate any potential Martian messages.[11][12]

A 1959 paper by Philip Morrison and Giuseppe Cocconi first pointed out the possibility of searching the microwave spectrum. It proposed frequencies and a set of initial targets.[13][14]

In 1960, Cornell University astronomer Frank Drake performed the first modern SETI experiment, named "Project Ozma" after the Queen of Oz in L. Frank Baum's fantasy books.[15] Drake used a radio telescope 26 metres (85 ft) in diameter at Green Bank, West Virginia, to examine the stars Tau Ceti and Epsilon Eridani near the 1.420 gigahertz marker frequency, a region of the radio spectrum dubbed the "water hole" due to its proximity to the hydrogen and hydroxyl radical spectral lines. A 400 kilohertz band around the marker frequency was scanned using a single-channel receiver with a bandwidth of 100 hertz. He found nothing of interest.

Soviet scientists took a strong interest in SETI during the 1960s and performed a number of searches with omnidirectional antennas in the hope of picking up powerful radio signals. Soviet astronomer Iosif Shklovsky wrote the pioneering book in the field, Universe, Life, Intelligence (1962), which was expanded upon by American astronomer Carl Sagan as the best-selling book Intelligent Life in the Universe (1966).[16]

The Wow! Signal
Credit: The Ohio State University Radio Observatory and the North American AstroPhysical Observatory (NAAPO).

In the March 1955 issue of Scientific American, John D. Kraus described an idea to scan the cosmos for natural radio signals using a flat-plane radio telescope equipped with a parabolic reflector. Within two years, his concept was approved for construction by Ohio State University. With a total of United States dollar 71,000 (equivalent to $646,319 in 2019) in grants from the National Science Foundation, construction began on an 8-hectare (20-acre) plot in Delaware, Ohio. This Ohio State University Radio Observatory telescope was called "Big Ear". Later, it began the world's first continuous SETI program, called the Ohio State University SETI program. In 1971, NASA funded a SETI study that involved Drake, Barney Oliver of Hewlett-Packard laboratories, and others. The resulting report proposed the construction of an Earth-based radio telescope array with 1,500 dishes known as "Project Cyclops". The price tag for the Cyclops array was US$10 billion. Cyclops was not built, but the report[17] formed the basis of much SETI work that followed.

The Ohio State SETI program gained fame on August 15, 1977, when Jerry Ehman, a project volunteer, witnessed a startlingly strong signal received by the telescope. He quickly circled the indication on a printout and scribbled the exclamation "Wow!" in the margin. Dubbed the Wow! signal, it is considered by some to be the best candidate for a radio signal from an artificial, extraterrestrial source ever discovered, but it has not been detected again in several additional searches.[18]

### Sentinel, META, and BETA

In 1980, Carl Sagan, Bruce Murray, and Louis Friedman founded the U.S. Planetary Society, partly as a vehicle for SETI studies.[3]

In the early 1980s, Harvard University physicist Paul Horowitz took the next step and proposed the design of a spectrum analyzer specifically intended to search for SETI transmissions. Traditional desktop spectrum analyzers were of little use for this job, as they sampled frequencies using banks of analog filters and so were restricted in the number of channels they could acquire. However, modern integrated-circuit digital signal processing (DSP) technology could be used to build autocorrelation receivers to check far more channels. This work led in 1981 to a portable spectrum analyzer named "Suitcase SETI" that had a capacity of 131,000 narrow band channels. After field tests that lasted into 1982, Suitcase SETI was put into use in 1983 with the 26-meter (85 ft) Harvard/Smithsonian radio telescope at Oak Ridge Observatory in Harvard, Massachusetts. This project was named "Sentinel" and continued into 1985.

Even 131,000 channels were not enough to search the sky in detail at a fast rate, so Suitcase SETI was followed in 1985 by Project "META", for "Megachannel Extra-Terrestrial Assay". The META spectrum analyzer had a capacity of 8.4 million channels and a channel resolution of 0.05 hertz. An important feature of META was its use of frequency Doppler shift to distinguish between signals of terrestrial and extraterrestrial origin. The project was led by Horowitz with the help of the Planetary Society, and was partly funded by movie maker Steven Spielberg. A second such effort, META II, was begun in Argentina in 1990, to search the southern sky. META II is still in operation, after an equipment upgrade in 1996.

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.[19] This forced the BETA project to cease operation.

### MOP and Project Phoenix

Sensitivity vs range for SETI radio searches. The diagonal lines show transmitters of different effective powers. The x-axis is the sensitivity of the search. The y-axis on the right is the range in light-years, and on the left is the number of Sun-like stars within this range. The vertical line labeled SS is the typical sensitivity achieved by a full sky search, such as BETA above. The vertical line labeled TS is the typical sensitivity achieved by a targeted search such as Phoenix.[20]

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;[21] however, funding was restored in 1982, after Carl Sagan talked with Proxmire and convinced him of the program's value.[21] In 1992, the U.S. government funded an operational SETI program, in the form of the NASA Microwave Observing Program (MOP). MOP was planned as a long-term effort to conduct a general survey of the sky and also carry out targeted searches of 800 specific nearby stars. MOP was to be performed by radio antennas associated with the NASA Deep Space Network, as well as the 140-foot (43 m) radio telescope of the National Radio Astronomy Observatory at Green Bank, West Virginia and the 1,000-foot (300 m) radio telescope at the Arecibo Observatory in Puerto Rico. The signals were to be analyzed by spectrum analyzers, each with a capacity of 15 million channels. These spectrum analyzers could be grouped together to obtain greater capacity. Those used in the targeted search had a bandwidth of 1 hertz per channel, while those used in the sky survey had a bandwidth of 30 hertz per channel.

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

MOP drew the attention of the United States Congress, where the program was ridiculed[22] and canceled one year after its start.[21] SETI advocates continued without government funding, and in 1995 the nonprofit SETI Institute of Mountain View, California resurrected the MOP program under the name of Project "Phoenix", backed by private sources of funding. Project Phoenix, under the direction of Jill Tarter, is a continuation of the targeted search program from MOP and studies roughly 1,000 nearby Sun-like stars. From 1995 through March 2004, Phoenix conducted observations at the 64-meter (210 ft) Parkes radio telescope in Australia , the 140-foot (43 m) radio telescope of the National Radio Astronomy Observatory in 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 GW EIRP to a distance of about 200 light-years. According to Prof. Tarter, in 2012 it costs around "$2 million per year to keep SETI research going at the SETI Institute" and approximately 10 times that to support "all kinds of SETI activity around the world".[23] ## Ongoing radio searches Microwave window as seen by a ground based system. From NASA report SP-419: SETI – the Search for Extraterrestrial Intelligence Many radio frequencies penetrate Earth's atmosphere quite well, and this led to 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. If this is typical, one way of discovering an extraterrestrial civilization might be to detect artificial radio emissions from a location outside the Solar System. 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.[24] ### Allen Telescope Array Main page: Astronomy:Allen Telescope Array 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, something like 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 would be equivalent to a single large dish more than 100 meters in diameter if completed. Presently, the array under construction has 42 dishes at the Hat Creek Radio Observatory in rural northern California.[25][26] The full array (ATA-350) is planned to consist of 350 or more offset-Gregorian radio 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 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).[27] 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.[28][29][30]

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 has 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.[31][32] Researchers in CSR are presently working on ways to reduce the threshold time limit, and to expand ATA's capabilities for detection of signals that may have embedded messages.[33]

Berkeley astronomers used the ATA to pursue several science topics, some of which might have turned up transient SETI signals,[34][35][36] 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.[37][38]

In April 2011, the ATA was forced to enter an 8-month "hibernation" due to funding shortfalls. Regular operation of the ATA was resumed on December 5, 2011.[39][40]

## Criticism

As various SETI projects have progressed, some have criticized early claims by researchers as being too "euphoric". For example, Peter Schenkel, while remaining a supporter of SETI projects, wrote in 2006 that

"[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 good Popperian criteria for falsifiability, as explained in a 2009 editorial in Nature, 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]

Nature added that SETI was "marked by a hope, bordering on faith" that aliens were aiming signals at us, that a hypothetical alien SETI project looking at Earth with "similar faith" would be "sorely disappointed" (despite our many untargeted radar and TV signals, and our few targeted Active SETI radio signals denounced by those fearing aliens), and that it had difficulties attracting even sympathetic working scientists and Government funding because it was "an effort so likely to turn up nothing".[4]

However, Nature also added, "Nonetheless, a small SETI effort is well worth supporting, especially given the enormous implications if it did succeed" and that "happily, a handful of wealthy technologists and other private donors have proved willing to provide that support".[4]

Supporters of the Rare Earth Hypothesis argue that advanced lifeforms are likely to be very rare, and that, if that is so, then SETI efforts will be futile.[147][148][149] However, the Rare Earth Hypothesis itself faces many criticisms.[149]

In 1993, Roy Mash stated that "Arguments favoring the existence of extraterrestrial intelligence nearly always contain an overt appeal to big numbers, often combined with a covert reliance on generalization from a single instance" and concluded that "the dispute between believers and skeptics is seen to boil down to a conflict of intuitions which can barely be engaged, let alone resolved, given our present state of knowledge".[150] In 2012, Milan M. Ćirković (who was then research professor at the Astronomical Observatory of Belgrade and a research associate of the Future of Humanity Institute at the University of Oxford[151]) said that Mash was unrealistically over-reliant on excessive abstraction that ignored the empirical information available to modern SETI researchers.[152]

George Basalla, Emeritus Professor of History at the University of Delaware,[153] 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",[154][155] and has in turn been criticized by Milan M. Ćirković[151] for, among other things, being unable to distinguish between "SETI believers" and "scientists engaged in SETI", who are often sceptical (especially about quick detection), such as Freeman Dyson (and, at least in their later years, Iosif Shklovsky and Sebastian von Hoerner), and for ignoring the difference between the knowledge underlying the arguments of modern scientists and those of ancient Greek thinkers.[155]

Massimo Pigliucci, Professor of Philosophy at CUNY-City College,[156] asked in 2010 whether SETI is "uncomfortably close to the status of pseudoscience" due to the lack of any clear point at which negative results cause the hypothesis of Extraterrestrial Intelligence to be abandoned,[157] before eventually concluding that SETI is "almost-science", which is described by Milan M. Ćirković[151] as Pigliucci putting SETI in "the illustrious company of string theory, interpretations of quantum mechanics, evolutionary psychology and history (of the 'synthetic' kind done recently by Jared Diamond)", while 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[150] and Basalla[154]".[158]

### Ufology

Ufologist Stanton Friedman has often criticized SETI researchers for, among other reasons, what he sees as their unscientific criticisms of Ufology,[159][160] but, unlike SETI, Ufology has generally not been embraced by academia as a scientific field of study,[161][162] and it is usually characterized as a partial[163] or total[164][165] pseudoscience. In a 2016 interview, Jill Tarter pointed out that it is still a misconception that SETI and UFOs are related.[166] She says that, "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."[166] The Galileo Project headed by Harvard astronomer Avi Loeb is one of the few scientific efforts to study UFOs or UAPs.[167] 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.[168] The Galileo Projects finds that after the publication of the UFO Report by the U.S. Intelligence the scientific community needs to "systematically, scientifically and transparently look for potential evidence of extraterrestrial technological equipment".[169]

## References

1. Schenkel, Peter (May-June 2006). "SETI Requires a Skeptical Reappraisal". Skeptical Inquirer 30 (3). Archived from the original on 2009-11-05. Retrieved June 28, 2009.
2. Moldwin, Mark (November 2004). "Why SETI is science and UFOlogy is not". Skeptical Inquirer.
3. Johnson, Steven (28 June 2017). "Greetings, E.T. (Please Don't Murder Us.)". New York Times.
4. "SETI at 50". Nature 416 (7262): 316. 2009. doi:10.1038/461316a. PMID 19759575. Bibcode2009Natur.461..316.. "Indeed, SETI is marked by a hope, bordering on faith, that not only are there civilizations broadcasting out there, but that they are somehow intent on beaming their signals at Earth. An alien SETI project relying on a similar faith in Earth would be sorely disappointed. It's true that a random mix of radar and television signals has been expanding outwards from Earth at the speed of light for the past 70 years. But there have been only a few short-lived attempts to target radio messages at other stars — with each attempt arousing concerns over alien reprisals. Understandably, many scientists who support SETI in spirit have instead pursued astronomical targets more likely to offer positive data — and tenure. Governments have also been averse to funding an effort so likely to turn up nothing.".
5. Katz, Gregory (July 20, 2015). "Searching for ET: Hawking to look for extraterrestrial intelligence". AP News.
6. Seifer, Marc J. (1996). "Martian Fever (1895–1896)". Wizard : the life and times of Nikola Tesla: biography of a genius. Secaucus, New Jersey: Carol Pub.. p. 157. ISBN 978-1-55972-329-9. OCLC 33865102.
7. Spencer, John (1991). The UFO Encyclopedia. New York: Avon Books. ISBN 978-0-380-76887-5. OCLC 26211869.
8. W. Bernard Carlson, Tesla: Inventor of the Electrical Age, Princeton University Press - 2013, pages 276-278.
9. Corum, Kenneth L.; James F. Corum (1996). Nikola Tesla and the electrical signals of planetary origin. pp. 1, 6, 14. OCLC 68193760.
10. Dick, Steven (1999). The Biological Universe: The Twentieth Century Extraterrestrial Life Debate. ISBN 978-0-521-34326-8.
11. Prepare for Contact. Letters of Note (2009-11-06). Retrieved on 2011-10-14.
12. Cocconi, Giuseppe; Philip Morrison (1959). "Searching for interstellar communications". Nature 184 (4690): 844–846. doi:10.1038/184844a0. Bibcode1959Natur.184..844C.
13. "Science: Project Ozma," Time, April 18, 1960 (web version accessed 17 September 2010)
14. Sagan, Carl; Iosif Shklovskii (1966). Intelligent Life in the Universe. ISBN 978-0-330-25125-9.
15. Robert H. Gray (2012). The Elusive WOW: Searching for Extraterrestrial Intelligence. Chicago: Palmer Square Press. ISBN 978-0-9839584-4-4.
16. Alan M. MacRobert (29 March 2009). "SETI Searches Today". Sky and Telescope.
17. Garber, S. J. (1999). "Searching for Good Science - the Cancellation of NASA's SETI Program". Journal of the British Interplanetary Society 52 (1): 3. Bibcode1999JBIS...52....3G.
18. "Ear to the Universe Is Plugged by Budget Cutters". The New York Times. October 7, 1993.
19. Siemion, Andrew (September 29, 2015). "Prepared Statement by Andrew Siemion - Hearing on Astrobiology Status Report - House Committee on Science, Space, and Technology". SpaceRef.com.
20. Welch, Jack (August 2009). "The Allen Telescope Array: The First Widefield, Panchromatic, Snapshot Radio Camera for Radio Astronomy and SETI". Proceedings of the IEEE 97 (8): 1438–1447. doi:10.1109/JPROC.2009.2017103. Bibcode2009IEEEP..97.1438W.
21. Gutierrez-Kraybill, Colby (2010). "Commensal observing with the Allen Telescope array: Software command and control". in Radziwill, Nicole M; Bridger, Alan. Proceedings of the SPIE. Software and Cyberinfrastructure for Astronomy. 7740. pp. 77400Z. doi:10.1117/12.857860. Bibcode2010SPIE.7740E..0ZG.
22. Harp, G. R. "Customized beam forming at the Allen Telescope Array." ATA Memo Series 51 (2002), available at http://www.seti.org/sites/default/files/ATA-memo-series/memo51.pdf.
23. Barott, William C. (2011). "Real-time beamforming using high-speed FPGAs at the Allen Telescope Array". Radio Science 46 (1): n/a. doi:10.1029/2010RS004442. Bibcode2011RaSc...46.1016B.
24. Harp, G. R. (2013). "Using Multiple Beams to Distinguish Radio Frequency Interference from SETI Signals". Radio Science 40 (5): n/a. doi:10.1029/2004RS003133. Bibcode2005RaSc...40.5S18H.
25. Tarter, Jill (2011). "The first SETI observations with the Allen telescope array". Acta Astronautica 68 (3–4): 340–346. doi:10.1016/j.actaastro.2009.08.014. Bibcode2011AcAau..68..340T.
26. Backus, Peter R.; Allen Telescope Array Team (2010). "The ATA Galactic Center Survey: SETI Observations in 2009". American Astronomical Society 215: 403.02. Bibcode2010AAS...21540302B.
27. Harp, Gerald R., et al. A new class of SETI beacons that contain information. Communication with Extraterrestrial Intelligence. State University of New York Press, 2011.
28. Croft, Steve (2010). "The Allen Telescope Array Twenty-Centimeter Survey—A 690 Deg2, 12 Epoch Radio Data Set. I. Catalog and Long-Duration Transient Statistics". The Astrophysical Journal 719 (1): 45–58. doi:10.1088/0004-637X/719/1/45. Bibcode2010ApJ...719...45C.
29. Siemion, Andrew P.V. (2012). "The Allen Telescope Array Fly's Eye Survey for Fast Radio Transients". The Astrophysical Journal 744 (2): 109. doi:10.1088/0004-637X/744/2/109. Bibcode2012ApJ...744..109S.
30. Siemion, Andrew (2011). "Results from the Fly's Eye Fast Radio Transient Search at the Allen Telescope Array". American Astronomical Society 217: 240.06. Bibcode2011AAS...21724006S.
31. Terdiman, Daniel. (2008-12-12) SETI's large-scale telescope scans the skies | Geek Gestalt – CNET News. News.cnet.com. Retrieved on 2011-10-14.
32. Rendering of 350 image – Photos: Searching the heavens for life – CNET News. News.cnet.com (2008-12-12). Retrieved on 2011-10-14.
33. The Great Beyond. Nature Blogs, ed (25 April 2011). "SETI scope suspends search".
34. "SERENDIP". UC Berkeley.
35. Cite error: Invalid <ref> tag; no text was provided for refs named Merali0720
36. MacMahon, David H. E.; Price, Danny C.; Lebofsky, Matthew; Siemion, Andrew P. V.; Croft, Steve; DeBoer, David; Enriquez, J. Emilio; Gajjar, Vishal et al. (2017-07-19). "The Breakthrough Listen Search for Intelligent Life: A Wideband Data Recorder System for the Robert C. Byrd Green Bank Telescope". Publications of the Astronomical Society of the Pacific 130 (986): 044502. doi:10.1088/1538-3873/aa80d2.
37. Cite error: Invalid <ref> tag; no text was provided for refs named washingtonp1
38. Brinks, Elias (11 July 2016). "China Opens the Aperture to the Cosmos". The Conversation. U.S. News & World Report.
39. Byrd, Deborah (4 June 2022). "Has China's FAST telescope detected alien intelligence?". Earth & Sky. Retrieved 15 June 2022.
40. Margot, Jean-Luc; Greenberg, Adam H.; Pinchuk, Pavlo; Shinde, Akshay; Alladi, Yashaswi; MN, Srinivas Prasad; Bowman, M. Oliver; Fisher, Callum et al. (25 April 2018). "A Search for Technosignatures from 14 Planetary Systems in the Field with the Green Bank Telescope at 1.15–1.73 GHz". The Astronomical Journal 155 (5): 209. doi:10.3847/1538-3881/aabb03. Bibcode2018AJ....155..209M.
41. de Zutter, Willy. "SETI@home – Credit Overview". BOINCstats.
42. Whitehouse, David (2004-09-02). "Astronomers deny ET signal report". BBC News.
43. Alan M. MacRobert. "SETI Searches Today". Sky and Telescope (2010?).
44. Overbye, Dennis (2020-03-23). "The Search for E.T. Goes on Hold, for Now" (in en-US). The New York Times. ISSN 0362-4331.
45. Chown, Marcus (April 1997). "The Alien Spotters". New Scientist: 28. Retrieved 2008-04-13.
46.
47. Shostak, Seth (2006-07-19). "The Future of SETI" (in en-US).
48. Townes, C. H. (1983). "At what wavelengths should we search for signals from extraterrestrial intelligence?". Proceedings of the National Academy of Sciences 80 (4): 1147–1151. doi:10.1073/pnas.80.4.1147. PMID 16593279. Bibcode1983PNAS...80.1147T.
49. Search for extraterrestrial intelligence extends to new realms. University of California. Published by PhysOrg. 20 March 2016.
50. Exers, Ronald, ed (2003). SETI 2020: A Roadmap for the Search for Extraterrestrial Intelligence. SETI Press. ISBN 978-0-9666335-3-5.
51. Steven S. Vogt et al., APF - The Lick Observatory Automated Planet Finder, 26 February 2014.
52. Tellis, Nathaniel K.; Marcy, Geoffrey W. (April 2017). "A Search for Laser Emission with Megawatt Thresholds from 5600 FGKM Stars". The Astronomical Journal 153 (6): 251. doi:10.3847/1538-3881/aa6d12. Bibcode2017AJ....153..251T.
53. Tellis, Nathaniel K.; Marcy, Geoffrey W. (12 May 2017). "A Search for Laser Emission with Megawatt Thresholds from 5600 FGKM Stars". The Astronomical Journal 153 (6): 251. doi:10.3847/1538-3881/aa6d12. Bibcode2017AJ....153..251T.
54. SETI, Institute. "Why We Need a New Type of SETI Instrument".
55. Hippke, Michael (13 April 2021). "Searching for Interstellar Quantum Communications". The Astronomical Journal 162 (1): 1. doi:10.3847/1538-3881/abf7b7. Bibcode2021AJ....162....1H. Retrieved 9 May 2021.
56. Conover, Emily (5 July 2022). "Aliens could send quantum messages to Earth, calculations suggest". Science News.
57. Berera, Arjun; Calderón-Figueroa, Jaime (28 June 2022). "Viability of quantum communication across interstellar distances". Physical Review D 105 (12): 123033. doi:10.1103/PhysRevD.105.123033. Bibcode2022PhRvD.105l3033B.
58. Freitas Jr., Robert A. (1980). "Interstellar probes – A new approach to SETI".
59. Freitas Jr., Robert A (1983). "Debunking the Myths of Interstellar Probes".
60. Freitas Jr., Robert A. (1983). "The Case for Interstellar Probes".
61. C. Rose; G. Wright (2 September 2004). "Inscribed matter as an energy efficient means of communication with an extraterrestrial civilization". Nature 431 (7004): 47–9. doi:10.1038/nature02884. PMID 15343327. Bibcode2004Natur.431...47R.
62. Woodruff T. Sullivan (2 September 2004). "Message in a bottle". Nature Magazine 431 (7004): 27–28. doi:10.1038/431027a. PMID 15343314. Bibcode2004Natur.431...27S.
63. Freitas Jr., Robert A (November 1983). "The Search for Extraterrestrial Artifacts (SETA)".
64. "NY Times Editorial". New York Times. 8 September 2004.
65. Rose, Christopher (September 2004). "Cosmic Communications".
66. Freitas Jr., Robert A; Valdes, Francisco (1980). "A Search for Natural or Artificial Objects Located at the Earth-Moon Libration Points".
67. Valdes, Francisco; Freitas Jr., Robert A (1983). "A Search for Objects near the Earth-Moon Lagrangian Points".
68. Valdes, Francisco; Freitas Jr., Robert A (1986). "A Search for the Tritium Hyperfine Line from Nearby Stars".
69. Korpela, Eric (2015). "Modeling Indications of Technology in Planetary Transit Light Curves -- Dark-side illumination". The Astrophysical Journal 809 (2): 139. doi:10.1088/0004-637X/809/2/139. Bibcode2015ApJ...809..139K.
70. Almár, Iván (2011). "SETI and astrobiology: The Rio Scale and the London Scale". Acta Astronautica 69 (9–10): 899–904. doi:10.1016/j.actaastro.2011.05.036. Bibcode2011AcAau..69..899A.
71. Freemann J. Dyson (1960). "Search for Artificial Stellar Sources of Infra-Red Radiation". Science 131 (3414): 1667–1668. doi:10.1126/science.131.3414.1667. PMID 17780673. Bibcode1960Sci...131.1667D.
72. Impossible vanishing stars could be signs of advanced alien life. Shannon Hall, New Scientist. 1 July 2016.
73. Griffith, Roger L.; Wright, Jason T.; Maldonado, Jessica; Povich, Matthew S.; Sigurđsson, Steinn; Mullan, Brendan (15 April 2015). "The Ĝ Infrared Search for Extraterrestrial Civilizations with Large Energy Supplies. III. The Reddest Extended Sources in WISE". The Astrophysical Journal Supplement Series 217 (2): 25. doi:10.1088/0067-0049/217/2/25. Bibcode2015ApJS..217...25G.
74. Duncan Forgan; Martin Elvis (2011). "Extrasolar Asteroid Mining as Forensic Evidence for Extraterrestrial Intelligence". International Journal of Astrobiology 10 (4): 307. doi:10.1017/S1473550411000127. Bibcode2011IJAsB..10..307F.
75. Extrasolar Planets: Formation, Detection and Dynamics Rudolf Dvorak, page 14 John Wiley & Sons, 2007
76. Povich, Matthew (11 August 2014). "Infrared Search for Extraterrestrial Civilizations with Large Energy Supplies". astro-ph.GA (Astrobiology Web).
77. "Alien Hairspray May Help Us Find E.T.". Space.com. 2012-11-26.
78. "How to Find ET with Infrared Light". Astronomy.com. June 2013.
79. Shostak, Seth; Astronomer, Senior (2018-04-16). "Alien satellites might offer a new way to find E.T.".
80. Zubrin, Robert (1995). "Detection of Extraterrestrial Civilizations via the Spectral Signature of Advanced Interstellar Spacecraft". in Shostak, Seth. Progress in the Search for Extraterrestrial Life. Astronomical Society of the Pacific. pp. 487–496. Bibcode1995ASPC...74..487Z.
81. Freitas, Robert (November 1983). "The Case for Interstellar Probes". Journal of the British Interplanetary Society 36: 490–495. Bibcode1983JBIS...36..490F.
82. Tough, Allen (1998). "Small Smart Interstellar Probes". Journal of the British Interplanetary Society 51: 167–174.
83. Brian C. Lacki (2015). "SETI at Planck Energy: When Particle Physicists Become Cosmic Engineers". arXiv:1503.01509 [(astro-ph.HE].
84. Jones, Eric (March 1985). ""Where is everybody?", An account of Fermi's question". Los Alamos National Laboratory.
85. Ben Zuckerman and Michael H. Hart (editors), Extraterrestrials: Where Are They? Elsevier Science & Technology Books (1982), ISBN:9780080263427
86. Stephen Webb, Where is Everybody? Fifty Solutions to Fermi's Paradox, Copernicus, 2002 edition, 978-0387955018
87. von Hoerner, Sebastian (December 8, 1961). "The Search for Signals from Other Civilizations". Science 134 (3493): 1839–43. doi:10.1126/science.134.3493.1839. ISSN 0036-8075. PMID 17831111. Bibcode1961Sci...134.1839V.
88. "Hoerner, Sebastian von (1919–2003)". Hoerner, Sebastian von (1919–2003). Retrieved 2020-07-26.
89. Shklovsky, Iosif (1987) (in ru). Вселенная. Жизнь. Разум. Moscow: Наука. pp. 250–252. Retrieved 2020-07-26.
90. Lem, Stanisław (2013). "Space Civilizations". Summa Technologiae. Minneapolis: University of Minnesota Press. ISBN 978-0816675777.
91. "Eavesdropping on the Earth". astrosociety.org. 1979.
92. "SETI Insentitive To Earth-like Signals". spacedaily.com. 1998.
93. Pope, Nick What to do if we find extraterrestrial life? Who gets notified? Do we reply? Experts are already arguing NBC News 10/18/2010
94. "The Rio Scale". International Academy of Astronautics.
95. Iván Almár (November–December 2011). "SETI and Astrobiology: The Rio Scale and the London Scale". Acta Astronautica 69 (9–10): 899–904. doi:10.1016/j.actaastro.2011.05.036. Bibcode2011AcAau..69..899A.
96.
97. Whitehouse, David (2004-09-02). "Astronomers deny ET signal report". BBC News.
98. Vance, Ashlee SETI urged to fess up over alien signals The Register 31 July 2006
99. Siegel, Lee [The Meaning of Life [1]] NASA July 6, 2001
100. Brin, David (June 2006). "Shouting at the Cosmos". Lifeboat Foundation.
101. Carrigan Jr., Robert A. (June 2006). "Do potential SETI signals need to be decontaminated?". Fermi National Accelerator Laboratory.
102. Iván Almára and H.Paul Shuch (January 2007). "The San Marino Scale: A new analytical tool for assessing transmission risk". Acta Astronautica 60 (1): 57–59. doi:10.1016/j.actaastro.2006.04.012. Bibcode2007AcAau..60...57A.
103. Zaitsev, Alexander L. (September 2007). "Sending and searching for interstellar messages". 58th International Astronautical Congress.
104. Zaitsev, Alexander L. (April 2008). "Detection probability of terrestrial radio signals by a hostile super-civilization". Journal of Radio Electronics 5. Bibcode2008arXiv0804.2754Z.
105. de Magalhaes, J. P. (2015). "A direct communication proposal to test the Zoo Hypothesis". Space Policy 38: 22–26. doi:10.1016/j.spacepol.2016.06.001.
106. Borenstein, Seth (13 February 2015). "Should We Call the Cosmos Seeking ET? Or Is That Risky?". The New York Times.
107. Ghosh, Pallab (12 February 2015). "Scientist: 'Try to contact aliens'". BBC News.
108. Shostak, Seth (28 March 2015). "Should We Keep a Low Profile in Space?". The New York Times.
109. Ward, Peter D.; Brownlee, Donald (2007). Rare Earth: Why Complex Life is Uncommon in the Universe. Springer. p. 250. ISBN 9780387218489. "Unfortunately, it is very difficult to know if SETI is an effective use of resources. If the Rare Earth Hypothesis is correct, then it clearly is a futile effort."  Revised edition (first published in 2000)
110. Denton, Peter H.; Restivo, Sal (2008). Battleground: Science and Technology [2 volumes]. Battleground. ABC-CLIO. p. 403. ISBN 9781567207439. "SETI enthusiasts believe that the human race is characterized by mediocrity rather than excellence. According to Frank Drake and his followers, this means that intelligent life is common in the universe. Peter Ward and Donald Brownlee challenge the principle of mediocrity with the rare Earth hypothesis in their book Rare Earth: Why Complex Life Is Uncommon in the Universe (2000)."
111. Losch, Andreas (2017). What is Life? On Earth and Beyond. Cambridge University Press. p. 167. ISBN 9781107175891. "In many ways, the rare-Earth hypothesis has since become somewhat of a default position in many astrobiological circles, and – since it predicts the absence of rationale for SETI – a mainstay of SETI scepticism. ... There are many criticisms rightly raised against the rare-Earth Hypothesis, but here I shall mention just one."
112. Mash, Roy (1993). "Big numbers and induction in the case for extraterrestrial intelligence". Philosophy of Science 60 (2): 204–22. doi:10.1086/289729.
113. Cirkovic, Milan M.; Ćirković, Milan M. (2012-06-21). About the author (2012). ISBN 9780521197755. Retrieved 2017-12-13. "Milan M. Ćirković is a research professor at the Astronomical Observatory of Belgrade, (Serbia) and a research associate of the Future of Humanity Institute at the University of Oxford."
114. Ćirković (2012), p166
115. Basalla, George (2006-01-19). About the author (2006). ISBN 9780198038351. Retrieved 2017-12-13.
116. George Basalla (2006). Civilized Life in the Universe: Scientists on Intelligent Extraterrestrials. Oxford University Press. p. 14. ISBN 9780198038351. Retrieved 2017-12-13. "Despite all their scientific trappings, the extraterrestrials discussed by scientists are as imaginary as the spirits and gods of religion or myth."
117. Ćirković (2012), p172, "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, ..."
118. Massimo Pigliucci (2010). Nonsense on Stilts: How to Tell Science from Bunk. University of Chicago Press. p. 34. ISBN 9780226667874. Retrieved 2017-12-13. "But in the case of SETI, negative results are what is expected most of the time, perhaps even forever, regardless of the truth of the central hypothesis. This raises the question: when will SETI researchers think that enough negatives have been accumulated to reject the hypothesis of existence of other technological civilizations? If the answer is that such hypothesis can never be rejected, regardless of the empirical results, that pushes SETI uncomfortably close to the status of pseudoscience. There is another way to look at the problem, based on an additional element (besides empirical evidence and testability) ..."
119. Ćirković (2012), p175, "However, in the second chapter, tellingly entitled 'Almost Science', the author (a distinguished philosopher, mainly involved in the philosophy of biology) devotes several subsections to the fields which are, in his opinion, neither pseudosciences, nor fully legitimate members of the scientific family. Here he puts SETI studies in the illustrious company of string theory, interpretations of quantum mechanics, evolutionary psychology and history (of the 'synthetic' kind done recently by Jared Diamond). While the club is fun to be in - and only a staunch conservative does not expect great breakthroughs to come out of one or more of these domains in the next few decades - the justification offered by Pigliucci in the case of SETI is weak, outdated, and reflecting particular philosophical prejudices similar to the ones described above in Mash and Basalla.[60]"
120. Friedman, Stanton T. (2002-05-13). "UFOs: Challenge to SETI Specialists".
121. Friedman, Stanton T. (2009-05-30). Pseudo-Science of Anti-Ufology. The UFO Chronicles. Retrieved 2017-12-17.
122. Denzler, Brenda (2003). The lure of the edge: scientific passions, religious beliefs, and the pursuit of UFOs. University of California Press. p. 69. ISBN 978-0-520-23905-0.
123. Moldwin, Mark (12 November 2004). "Why SETI Is Science and UFOlogy Is Not – A Space Science Perspective on Boundaries". Archived from the original on 2011-05-23.
124. Tuomela, Raimo (1985). Science, action, and reality. Springer. p. 234. ISBN 978-90-277-2098-6.
125. Feist, Gregory J. (2006). The psychology of science and the origins of the scientific mind. Yale University Press. p. 219. ISBN 978-0-300-11074-6.
126. Restivo, Sal P. (2005). Science, technology, and society: an encyclopedia. Oxford University Press US. p. 176. ISBN 978-0-19-514193-1.