SETI

SETI

Type	Program or study
Also known as	Search for Extraterrestrial Intelligence
Began	1960 (Project Ozma)
Method	Radio and optical telescopes
Famous candidate	The Wow! signal (1977)

SETI, the Search for Extraterrestrial Intelligence, is the collection of scientific projects that try to detect evidence of intelligent life beyond Earth — usually radio or laser signals, but increasingly any "technosignature" of alien technology.^[1] It is a separate field from ufology: SETI scientists look for evidence across interstellar distances with telescopes, not for craft visiting Earth. After more than sixty years of searching, no confirmed signal has been found.^[1]

Supporters see SETI as the most direct way to tackle the Fermi paradox; critics have called it speculative and "overly hopeful".^[1]

Before modern SETI

The idea of listening for other worlds is as old as radio. In 1896 Nikola Tesla suggested that his wireless transmission system could be used to contact beings on Mars, and in 1899, at his Colorado Springs station, he thought he had picked up a repeating signal from Mars; later explanations include a misunderstanding of the new technology, signals from Marconi's experiments, or natural radio noise from Jupiter's moon Io.^[1] In the early 1900s Guglielmo Marconi, Lord Kelvin and David Peck Todd also believed radio could reach Martians.^[1]

When Mars came unusually close to Earth on 21–23 August 1924, the United States promoted a "National Radio Silence Day": radios were to fall quiet for five minutes on the hour, every hour, for 36 hours, while the US Naval Observatory listened with a receiver lifted 3 km into the air by a dirigible.^[1]

The first searches

Modern SETI began with a 1959 paper by the physicists Giuseppe Cocconi and Philip Morrison, who pointed out that the microwave band could carry signals between the stars and proposed frequencies and target stars.^[1] They suggested listening near 1,420 MHz, the 21 cm radio line emitted naturally by hydrogen — a frequency any radio astronomer anywhere would know.^[2]

In 1960 the Cornell astronomer Frank Drake carried out Project Ozma, named after the princess of L. Frank Baum's land of Oz. Using an 85-foot (26 m) radio telescope at Green Bank, West Virginia, he listened to two nearby Sun-like stars, Tau Ceti and Epsilon Eridani, for some 150 hours over four months. A false signal on 8 April 1960 turned out to come from a high-flying aircraft; nothing else was found.^[2] A follow-up, Ozma II, monitored 670 nearby stars with a 300-foot telescope from 1972 to 1976.^[2] In 1961 Drake wrote the Drake equation to frame how many detectable civilisations might exist.^[1]

Soviet scientists took a strong interest in the 1960s, using omnidirectional antennas to listen for powerful signals. Iosif Shklovsky's book Universe, Life, Intelligence (1962) was expanded by Carl Sagan into the best-seller Intelligent Life in the Universe (1966).^[1]

Big Ear and the Wow! signal

Ohio State University's "Big Ear" radio telescope, built on a 20-acre site in Delaware, Ohio, with $71,000 from the National Science Foundation, ran the world's first continuous SETI programme.^[1] On 15 August 1977 volunteer Jerry Ehman found a startlingly strong signal on its printout, circled it and wrote "Wow!" in the margin. It has never been detected again.^[1] See Wow! signal.

Project Cyclops

In 1971 a NASA-funded study involving Drake and Barney Oliver of Hewlett-Packard proposed "Project Cyclops", an array of 1,500 radio dishes costing $10 billion. It was never built, but its report shaped much of the SETI work that followed.^[1]

NASA and the private era

NASA's SETI work was attacked in 1978 by Senator William Proxmire, and Congress removed its funding in 1981. Carl Sagan persuaded Proxmire of its value and funding returned in 1982.^[1] In 1992 NASA launched its Microwave Observing Program, but Congress cancelled it a year later.^[1][9] SETI continued with private money: in 1995 the nonprofit SETI Institute of Mountain View, California, revived the programme as "Project Phoenix".^[1]

The SETI Institute was founded in 1984 by Thomas Pierson and the astronomer Jill Tarter, with support over the years from Sagan, Drake, Bernard Oliver, David Packard, William Hewlett, Gordon Moore and Paul Allen.^[5] Its Carl Sagan Center has more than 80 scientists working on astronomy, exoplanets, planetary exploration, astrobiology and SETI.^[5]

Other private searches followed:^[1]

• The Planetary Society, founded in 1980 by Sagan, Bruce Murray and Louis Friedman, partly as a vehicle for SETI.
• META and BETA. Harvard physicist Paul Horowitz built ever-larger spectrum analysers: "Suitcase SETI", then META (1985) with 8.4 million channels, then BETA (1995), the "Billion-channel Extraterrestrial Assay".
• SERENDIP, launched by Berkeley in 1979, "piggy-backs" on other astronomers' observations at large telescopes such as Green Bank and, formerly, Arecibo.
• The SETI League, founded in 1994 after NASA's programme was cancelled, links amateur radio astronomers using converted satellite-TV dishes in "Project Argus", an attempt at real-time coverage of the whole sky.

The Allen Telescope Array

The SETI Institute and Berkeley developed the Allen Telescope Array in northern California, named after its benefactor, the Microsoft co-founder Paul Allen. Planned as 350 dishes each 6.1 m across, it has been operated with 42. From 2007 to 2015 it identified hundreds of millions of technological signals, all of them assigned to noise or radio-frequency interference from satellites and Earth-based transmitters.^[1][6] Funding shortfalls forced an eight-month "hibernation" in 2011; a $3.6 million donation from the Qualcomm co-founder Franklin Antonio in 2012 paid for more sensitive receivers.^[1]

SETI@home

From May 1999 to March 2020, SETI@home let millions of people donate spare computer time to analyse data from the Arecibo radio telescope gathered by SERENDIP. Originally funded by The Planetary Society and Paramount Pictures, it covered more than 67 per cent of the sky visible from Arecibo with at least three scans by 2010. It stopped sending out new work on 31 March 2020, when it had 91,454 active users.^[1][3]

Breakthrough Listen

Breakthrough Listen, announced in July 2015 by the investor Yuri Milner and Stephen Hawking, is a ten-year, $100 million search described as the most comprehensive to date. It buys thousands of hours a year on the Green Bank Telescope in West Virginia and the Parkes telescope in Australia — compared with 24 to 36 hours a year previously — and uses the Automated Planet Finder at Lick Observatory to look for laser signals.^[1][4] Its targets include the million nearest stars and the centres of 100 galaxies, and its data are published openly.^[4] The radio telescopes could detect a transmitter as powerful as an ordinary aircraft radar around the 1,000 nearest stars.^[4]

Its results so far:^[4][7]

• April 2017. The first results, from 692 nearby stars, found 11 significant events, all consistent with radio-frequency interference.
• December 2017. It listened to the interstellar object ʻOumuamua for eight hours across 1.1–11.6 GHz and detected nothing.
• 2020. Preliminary results for stars within 150 parsecs found no artificial transmitters comparable to Arecibo.
• BLC1. In December 2020 it reported "Breakthrough Listen Candidate 1", a narrow signal at 982.002 MHz picked up by Parkes in April–May 2019 from the direction of Proxima Centauri, the nearest star. It was never detected again, and in October 2021 two studies concluded it was most likely terrestrial interference.

Breakthrough Listen has also worked with NASA's TESS planet-hunting team since 2019 to scan thousands of newly found planets for technosignatures.^[4]

Other searches

• China's FAST. The 500-metre FAST telescope is the first radio observatory built with SETI as a core goal; it can search to 28 light-years and reach 1,400 stars. In June 2022 astronomers working with it reported a possible artificial signal, but within days Dan Werthimer, chief scientist for several SETI projects, said the signals were radio interference.^[1]
• Optical SETI. Since a 1961 paper by Charles Townes, inventor of the laser, some researchers have searched for brief laser pulses. Horowitz's group at Harvard built an all-sky optical survey telescope, and the SETI Institute's "Laser SETI" cameras watch the whole night sky for millisecond flashes.^[1]
• Technosignatures. Searches now also look for megastructures such as Dyson spheres (a survey of some 100,000 galaxies found none), industrial pollution or city lights on exoplanets, and alien probes in the Solar System. Photographic searches begun in 1979 by Robert Freitas and Valdes of the Earth–Moon "parking orbits" where a probe might wait found nothing.^[1][8]

Sending messages

SETI is mostly about listening, but humans have also transmitted. On 16 November 1974 the Arecibo telescope broadcast a 1,679-bit message, designed by Drake, Sagan and others, toward the star cluster M13, about 25,000 light-years away; it was meant more as a demonstration of technology than a real attempt at conversation.^[10] In 2015 many SETI scientists signed a statement that "a worldwide scientific, political and humanitarian discussion must occur before any message is sent".^[5]

SETI and UFOs

Most SETI scientists are sceptical of UFO reports. Carl Sagan, a leading supporter of SETI, argued that extraordinary claims require extraordinary evidence and that UFO cases had not met that standard.^[1] The fields have drawn closer in recent years, as astronomers have begun to look for physical objects as well as signals: Harvard's Galileo Project, led by Avi Loeb, built observatories to record unidentified aerial objects and studies interstellar visitors such as ʻOumuamua and 3I/ATLAS.^[1]

The silence

No confirmed signal has been found in more than six decades. Scientists still debate what the silence means: it lies at the heart of the Fermi paradox — if civilisations are common, why have we not heard from any? Searches so far have covered only a tiny fraction of the stars, frequencies and moments that could carry a signal, so supporters argue that the silence says little yet.^[1]

See also: Drake equation, Fermi paradox, Wow! signal, Are aliens real?.

References

Facts in this article are drawn in part from the Wikipedia articles “Search for extraterrestrial intelligence”, “Project Ozma”, “SETI@home”, “Breakthrough Listen”, “SETI Institute”, “Allen Telescope Array”, “BLC1”, “Technosignature”, “Arecibo message”, available under the CC BY-SA 4.0 licence.