Search for extraterrestrial intelligence

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 that planet since an odd repetitive static signal seemed to cut off when Mars set in the night sky. Analysis of Tesla's research has ranged from suggestions that Tesla detected nothing and that he simply misunderstood the new technology he was working with,[7] to claims that Tesla may have been observing signals from Marconi's European radio experiments and even that he could have picked up naturally occurring Jovian plasma torus signals.[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, and 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 US$71,000 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, Bernard M. Oliver of Hewlett-Packard Corporation, 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]

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.

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 Observatory in Puerto Rico with its 300 m (980 ft) dish, one of the world's largest filled-aperture (i.e. full dish) radio telescope, conducts 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]

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 the beam forming 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 2007-2015, 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]

In 2012, new life was breathed into the ATA thanks to a $3.6M philanthropic donation by Franklin Antonio, Co-Founder and Chief Scientist of QUALCOMM Incorporated.[41] This gift supports upgrades of all the receivers on the ATA dishes to have dramatically (2x - 10x from 1–8 GHz) greater sensitivity than before and supporting sensitive observations over a wider frequency range from 1–18 GHz, though initially the radio frequency electronics go to only 12 GHz. As of July, 2013 the first of these receivers was installed and proven. Full installation on all 42 antennas is expected in June, 2014. ATA is especially 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.[42] 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 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 V.v, was installed at the Arecibo Observatory in June 2009 and is currently operational. The digital back-end instrument is an FPGA-based 128 million-channel digital spectrometer covering 200 MHz of bandwidth. It takes data commensally with the seven-beam Arecibo L-band Feed Array[43] (ALFA). The program has found around 400 suspicious signals, but there is not enough data to prove that they belong to extraterrestrial intelligence.[44]

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.[45][46][47][3] It has been described as the most comprehensive search for alien communications to date.[46] The science program for Breakthrough Listen is based at Berkeley SETI Research Center,[48][49] located in the Astronomy Department[50] 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.[51] Previously, only about 24 to 36 hours of telescope per year were used in the search for alien life.[46] Furthermore, the Automated Planet Finder at Lick Observatory is searching for optical signals coming from laser transmissions. The massive data rates from the radio telescopes (24 GB/s at Green Bank) necessitated the construction of dedicated hardware at the telescopes to perform the bulk of the analysis.[52] Some of the data are also analyzed by volunteers in the SETI@home distributed computing network.[51] Founder of modern SETI Frank Drake is one of the scientists on the project's advisory committee.[53][45][46]

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 for technosignatures by Breakthrough Listen partner facilities across the globe. Data from TESS monitoring of stars will also be searched for anomalies.[54]

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.[55] FAST consists of a fixed 500 m (1,600 ft) 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.[56] According to its website, FAST could search out to 28 light-years, and would be able to reach 1400 stars. If the transmitter's radiated power is increased to 1000,000 MW, FAST would be able to reach one million stars. This is compared to the Arecibo 305 meter telescope detection distance of 18 light-years.[57]

Since 2016, 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.[58] The search is sensitive to Arecibo-class transmitters located within 420 ly of Earth and to transmitters that are 1000 times more powerful than Arecibo located within 13,000 ly of Earth.[59]

The SETI@home project uses distributed 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 distributed 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 can 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 runs 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 is complete, the results are 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 give SETI@home an average computational power of 617 teraFLOPS.[60] 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.[61][62]

As of 2010, after 10 years of data collection, SETI@home has 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.[63]

SETI Network is the only operational private search system.

The SETI Net station consists of off-the-shelf, consumer-grade electronics to minimize cost and to allow this design to be replicated as simply as possible. It has a 3-meter parabolic antenna that can be directed in azimuth and elevation, an LNA that covers 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 can be pointed and locked to one sky location, enabling the system to integrate on it for long periods. Currently the Wow! signal area is being monitored when it is above the horizon. All search data are collected and made 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 has developed several software packages for the amateur SETI community. It has 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.

It can be reached at https://www.seti.net

Founded in 1994 in response to the United States Congress cancellation of the NASA SETI program, The SETI League, Inc. 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 3 to 5 m (10–16 ft) in diameter into research-grade radio telescopes of modest sensitivity.[64] 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.[65] 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.[66]

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.[67]

A significant problem is the vastness of space. Despite piggybacking on the world's most sensitive radio telescope, Charles Stuart Bowyer said, the instrument could not detect random radio noise emanating from a civilization like ours, which has been leaking radio and TV signals[109] 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.[110]

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.

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, allays such concerns.[119] 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 does think 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.[120]

The concern over METI was raised by the science journal Nature in an editorial in October 2006, which commented on a recent meeting of the International Academy of Astronautics SETI study group. The editor said, "It is not obvious that all extraterrestrial civilizations will be benign, or that contact with even a benign one would not have serious repercussions" (Nature Vol 443 12 October 06 p 606). Astronomer and science fiction author David Brin has expressed similar concerns.[121]

Richard Carrigan, a particle physicist at the Fermi National Accelerator Laboratory near Chicago, Illinois, suggested that passive SETI could also be dangerous and that a signal released onto the Internet could act as a computer virus.[122] Computer security expert Bruce Schneier dismissed this possibility as a "bizarre movie-plot threat".[123]

To lend a quantitative basis to discussions of the risks of transmitting deliberate messages from Earth, the SETI Permanent Study Group of the International Academy of Astronautics adopted in 2007 a new analytical tool, the San Marino Scale.[124] Developed by Prof. Ivan Almar and Prof. H. Paul Shuch, the scale evaluates the significance of transmissions from Earth as a function of signal intensity and information content. Its adoption suggests that not all such transmissions are equal, and each must be evaluated separately before establishing blanket international policy regarding active SETI.

However, some scientists consider these fears about the dangers of METI as panic and irrational superstition; see, for example, Alexander L. Zaitsev's papers.[125][126] Biologist João Pedro de Magalhães also proposed in 2015 transmitting an invitation message to any extraterrestrial intelligences watching us already in the context of the Zoo Hypothesis and inviting them to respond, arguing this would not put us in any more danger than we are already if the Zoo Hypothesis is correct.[127]

On 13 February 2015, scientists (including Geoffrey Marcy, Seth Shostak, Frank Drake, Elon Musk and David Brin) at a convention of the American Association for the Advancement of Science, discussed Active SETI and whether transmitting a message to possible intelligent extraterrestrials in the Cosmos was a good idea;[128][129] one result was a statement, signed by many, that a "worldwide scientific, political and humanitarian discussion must occur before any message is sent".[130] On 28 March 2015, a related essay was written by Seth Shostak and published in The New York Times.[131]

The Breakthrough Message program is an open competition announced in July 2015 to design a digital message that could be transmitted from Earth to an extraterrestrial civilization, with a US$1,000,000 prize pool. The message should be "representative of humanity and planet Earth". The program pledges "not to transmit any message until there has been a wide-ranging debate at high levels of science and politics on the risks and rewards of contacting advanced civilizations".[132][3]