Projects to look for such signals are known as the search for extraterrestrial intelligence (SETI). The first modern SETI experiment was American astronomer Frank Drake’s Project Ozma, which took place in 1960. Drake used a radio telescope (essentially a large antenna) in an attempt to uncover signals from nearby Sun-like stars. In 1961 Drake proposed what is now known as the Drake equation, which estimates the number of signaling worlds in the Milky Way Galaxy. This number is the product of terms that define the frequency of habitable planets, the fraction of habitable planets upon which intelligent life will arise, and the length of time sophisticated societies will transmit signals. Because many of these terms are unknown, the Drake equation is more useful in defining the problems of detecting extraterrestrial intelligence than in predicting when, if ever, this will happen.

By the mid-1970s the technology used in SETI programs had advanced enough for the National Aeronautics and Space Administration to begin SETI projects, but concerns about wasteful government spending led Congress to end these programs in 1993. However, SETI projects funded by private donors (in the United States) continued. One such search was Project Phoenix, which began in 1995 and ended in 2004. Phoenix scrutinized approximately 1,000 nearby star systems (within 150 light-years of Earth), most of which were similar in size and brightness to the Sun. The search was conducted on several radio telescopes, including the 305-metre (1,000-foot) radio telescope at the Arecibo Observatory in Puerto Rico, and was run by the SETI Institute of Mountain View, California.

Other radio SETI experiments, such as Project SERENDIP V (begun in 2009 by the University of California at Berkeley) and Australia’s Southern SERENDIP (begun in 1998 by the University of Western Sydney at Macarthur), scan large tracts of the sky and make no assumption about the directions from which signals might come. The former uses the Green Bank Telescope and, until its collapse in 2020, the Arecibo telescope, and the latter (which ended in 2005) was carried out with the 64-metre (210-foot) telescope near Parkes, New South Wales. Such sky surveys are generally less sensitive than targeted searches of individual stars, but they are able to “piggyback” onto telescopes that are already engaged in making conventional astronomical observations, thus securing a large amount of search time. In contrast, targeted searches such as Project Phoenix require exclusive telescope access.

In 2007 a new instrument, jointly built by the SETI Institute and the University of California at Berkeley and designed for round-the-clock SETI observations, began operation in northeastern California. The Allen Telescope Array (ATA, named after its principal funder, American technologist Paul Allen) has 42 small (6 metres [20 feet] in diameter) antennas. When complete, the ATA will have 350 antennas and be hundreds of times faster than previous experiments in the search for transmissions from other worlds.

Beginning in 2016, the Breakthrough Listen project began a 10-year survey of the one million closest stars, the nearest 100 galaxies, the plane of the Milky Way Galaxy, and the galactic centre using the Parkes telescope and the 100-metre (328-foot) telescope at the National Radio Astronomy Observatory in Green Bank, West Virginia. That same year the largest single-dish radio telescope in the world, the Five-hundred-meter Aperture Spherical Radio Telescope in China, began operation and had searching for extraterrestrial intelligence as one of its objectives.

Since 1999 some of the data collected by Project SERENDIP (and since 2016, Breakthrough Listen) has been distributed on the Web for use by volunteers who have downloaded a free screen saver, SETI@home. The screen saver searches the data for signals and sends its results back to Berkeley. Because the screen saver is used by several million people, enormous computational power is available to look for a variety of signal types. Results from the home processing are compared with subsequent observations to see if detected signals appear more than once, suggesting that they may warrant further confirmation study.

Nearly all radio SETI searches have used receivers tuned to the microwave band near 1,420 megahertz. This is the frequency of natural emission from hydrogen and is a spot on the radio dial that would be known by any technically competent civilization. The experiments hunt for narrowband signals (typically 1 hertz wide or less) that would be distinct from the broadband radio emissions naturally produced by objects such as pulsars and interstellar gas. Receivers used for SETI contain sophisticated digital devices that can simultaneously measure radio energy in many millions of narrowband channels.

Optical SETI

SETI searches for light pulses are also under way at a number of institutions, including the University of California at Berkeley as well as Lick Observatory and Harvard University. The Berkeley and Lick experiments investigate nearby star systems, and the Harvard effort scans all the sky that is visible from Massachusetts. Sensitive photomultiplier tubes are affixed to conventional mirror telescopes and are configured to look for flashes of light lasting a nanosecond (a billionth of a second) or less. Such flashes could be produced by extraterrestrial societies using high-powered pulsed lasers in a deliberate effort to signal other worlds. By concentrating the energy of the laser into a brief pulse, the transmitting civilization could ensure that the signal momentarily outshines the natural light from its own sun.

Results and two-way communication

No confirmed extraterrestrial signals have yet been found by SETI experiments. Early searches, which were unable to quickly determine whether an emission was terrestrial or extraterrestrial in origin, would frequently find candidate signals. The most famous of these was the so-called “Wow” signal, measured by a SETI experiment at Ohio State University in 1977. Subsequent observations failed to find this signal again, and so the Wow signal, as well as other similar detections, is not considered a good candidate for being extraterrestrial.

Most SETI experiments do not transmit signals into space. Because the distance even to nearby extraterrestrial intelligence could be hundreds or thousands of light-years, two-way communication would be tedious. For this reason, SETI experiments focus on finding signals that could have been deliberately transmitted or could be the result of inadvertent emission from extraterrestrial civilizations.