Search for extraterrestrial intelligence
The search for extraterrestrial intelligence is the diverse efforts and scientific projects intended to detect extraterrestrial signals, or any evidence of intelligent life beyond Earth.
Researchers use methods such as monitoring electromagnetic radiation, searching for optical signals, and investigating potential extraterrestrial artifacts for any signs of transmission from civilizations present on other planets. Some initiatives have also attempted to send messages to hypothetical alien civilizations, such as NASA's Golden Record.
Modern SETI research began in the early 20th century after the advent of radio, expanding with projects like Project Ozma, the Wow! signal detection, and the Breakthrough Listen initiative; a $100 million, 10-year attempt to detect signals from nearby stars, announced in 2015 by Stephen Hawking and Yuri Milner. Since the 1980s, international efforts have been ongoing, with community led projects such as SETI@home and Project Argus, engaging in analyzing data. While SETI remains a respected scientific field, it often gets compared to conspiracy theory, UFO research, bringing unwarranted skepticism from the public, despite its reliance on rigorous scientific methods and verifiable data and research. Similar studies on unidentified aerial phenomena such as Avi Loeb's Galileo Project have brought further attention to SETI research.
Despite decades of searching, no confirmed evidence of alien intelligence has been found, bringing criticism onto SETI for being 'overly hopeful'. Critics argue that SETI is speculative and unfalsifiable, while supporters see it as a crucial step in addressing the Fermi paradox and understanding extraterrestrial technosignature.
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. 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,
- 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 moving through the magnetosphere of Jupiter.
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. 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 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, with William F. Friedman, assigned to translate any potential Martian messages.
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.
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. Drake used a radio telescope 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, which was expanded upon by American astronomer Carl Sagan as the best-selling book Intelligent Life in the Universe.
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 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 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.
On 24 May 2023, a test extraterrestrial signal, in the form of a "coded radio signal from Mars", was transmitted to radio telescopes on Earth, according to a report in The New York Times.
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.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 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 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, receiving an equipment upgrade in 1996–1997.
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 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.
MOP and Project Phoenix
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. In 1992, the U.S. government funded an operational SETI program, in the form of the NASA Microwave Observing Program. 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 radio telescope of the National Radio Astronomy Observatory at Green Bank, West Virginia and the 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.MOP drew the attention of the United States Congress, where the program met opposition and canceled one year after its start. 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. In 2012 it cost around $2 million per year to maintain SETI research at the SETI Institute and around 10 times that to support different SETI activities globally. 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 radio telescope of the National Radio Astronomy Observatory in Green Bank, West Virginia, and the 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.