SETI

Of all the scientific efforts to find life in space, none has potential consequences as profound as SETI, the Search for Extraterrestrial Intelligence. SETI researchers are trying to uncover other civilizations whose technical sophistication is at a human level or higher.

While science fiction routinely describes face-to-face encounters with intelligent aliens, it may be that we will never actually meet extraterrestrials. Building fast rockets capable of carrying living cargo to the stars is a formidable, perhaps even impossible, challenge. The amount of energy required to hurl a craft the size of the space shuttle at even half the speed of light is enormous—equivalent to the energy required to keep New York City running for 10,000 years.^* This is a problem of physics, not technology.

On the other hand, there are ways to reach other civilizations without interstellar travel. In 1959 Philip Morrison and Giuseppe Cocconi, two physicists at Cornell University, made a simple calculation to determine how far away a good radio receiver and a large antenna could detect our most powerful military radar transmitters. To their surprise, the answer turned out to be light-years—typical of the distances to the stars. Morrison and Cocconi realized that while interstellar rocketry was hard, interstellar communication by radio was easy. They suggested that other galactic civilizations might be discovered by simply eavesdropping on their radio traffic.

The Search for Extraterrestrial Intelligence

Within months, Frank Drake, a young radio astronomer at the National Radio Astronomy Observatory in Green Bank, West Virginia, tried to do just that. He was unaware of the work of the two Cornell physicists but had independently thought of the same idea. For several weeks in the spring of 1960, Drake pointed an 85-foot antenna (a radio telescope) at two Sun-like stars, Tau Ceti and Epsilon Eridani, tuning his receiver up and down the dial near 1,420 megahertz (MHz). This particular frequency was chosen because it is truly a universal radio channel. Hydrogen gas, which drifts and swirls through the immense spaces between the stars, naturally emits some radio noise at 1,420 MHz. Drake believed that every sophisticated society in the cosmos would know of this hydrogen hiss, and consequently it would make sense to broadcast interstellar hailing signals near this sweet spot on the dial.

Drake's Project Ozma was the first modern SETI search. By the early twenty-first century, about seventy others were undertaken. One of the most ambitious was the NASA SETI program, which ultimately became known as the High Resolution Microwave Survey. NASA got into SETI slowly, beginning in the 1970s with a technical study of the equipment and strategy required for a serious search. In the fall of 1992, sufficient equipment had been built to start the listening. However, very shortly thereafter, the U.S. Congress stopped all NASA SETI efforts. The rationale for canceling this research was to reduce federal spending during an era of large budget deficits.

SETI work continued, however, funded in the United States by private donations. Most of these projects have been radio experiments, of the type pioneered by Drake. The SETI Institute, in California, runs the most sensitive search, known as Project Phoenix. Various large radio telescopes, including the king-sized 305-meter (1,000-foot) antenna at Arecibo, Puerto Rico, have been used by Project Phoenix to carefully examine the neighborhoods of nearby, Sun-like stars. Other projects, such as the University of California, Berkeley's SERENDIP experiment, sweep the sky in an attempt to survey greater amounts of cosmic real estate. While more of the heavens are examined, the sensitivity in any given direction is lower. Some of the SERENDIP data have been distributed on the Web for processing at home with a screen saver program known as SETI@home.

Additional radio SETI experiments are being carried out in Australia (Southern SERENDIP), Argentina (META II), and Italy. Starting in the late twentieth century, another approach to SETI has gained a number of adherents: so-called optical SETI. Rather than tuning the dial in search of persistent, artificial signals, ordinary telescopes (with mirrors and lens) are outfitted with special detectors designed to find very short (less than a billionth of a second) laser pulses from distant worlds.

The Probability of Success

So far, no confirmed extraterrestrial signals—either radio or optical—have been found by SETI scientists. What are the chances that the aliens will ever be found? In 1961 Drake summarized the problem with a simple formula that predicts the number of galactic civilizations that are broadcasting now. Known as the Drake Equation, the computation is simply a product of factors bearing on the existence of intelligence. These factors include the number of galactic stars capable of supporting life, the fraction of such stars with planets, the number of planets in a solar system on which life evolves, the fraction of inhabited worlds where intelligence appears, and the lifetime of a broadcasting society. While we still do not know many of these factors, some scientists contend that the recent evidence for extrasolar planets and the growing suspicion that biology might be a common phenomenon have increased the chances for finding intelligence among the stars.

SETI scientists have made plans to greatly expand their search during the first two decades of the twenty-first century. The SETI Institute will build the Allen Telescope Array, a large grouping of small antennas that will be used for full-time searching. A world consortium of radio astronomers is considering the construction of a radio telescope a kilometer in size, a gargantuan instrument that could also be used for SETI. Optical SETI experiments are already increasing in number and sophistication.

In light of this rapid improvement in experimental technique, some scientists are optimistic that a signal will be found in the early decades of the twenty-first century. If so, the consequences would be dramatic. If we can ever successfully find and decode any message accompanying the signal, we might learn something of the knowledge and culture of other galactic beings, most likely from a society technologically far more advanced than our own. Even if we never understand or reply to an interstellar message, simply knowing that we are not the only "game" in town—let alone the most interesting game—would give us new and valuable perspective.

see also Extrasolar Planets (volume 2); Life in the Universe, Search for (volume 2); Why Human Exploration? (volume 3).

Seth Shostak

Bibliography

Cocconi, Giuseppe, and Philip Morrison. "Searching for Interstellar Communications." Nature 84 (1959):844.

Davies, Paul. Are We Alone? Philosophical Implications of the Discovery of Extraterrestrial Life. New York: Basic Books, 1996.

Dick, Steven. J. Life on Other Worlds. Cambridge, UK: Cambridge University Press,1998.

Drake, Frank D., and Dava Sobel. Is Anyone out There? New York: Delacorte Press,1992.

Goldsmith, Donald, and Tobias C. Owen. The Search for Life in the Universe. Reading, MA: Addison-Wesley, 1992.

Harrison, Albert. After Contact: The Response to Extraterrestrial Life. New York: Plenum,1997.

Regis, Edward, Jr., ed. Extraterrestrials: Science and Alien Intelligence. Cambridge, UK:Cambridge University Press, 1985.

Shostak, Seth. Sharing the Universe: Perspectives on Extraterrestrial Life. Berkeley, CA:Berkeley Hills Books, 1998.

^*At half the speed of light, travel time to Alpha Centauri, the nearest star system, would take nine years.

SETI

SETI (The Search for Extraterrestrial Intelligence) is a term that encompasses several different groups and their efforts to seek out intelligent extraterrestrial life. The driving force behind these groups is the ancient human desire to understand the origin and distribution of life throughout the Universe. As technology progresses, SETI efforts move from the study of extraterrestrial rocks and meteors towards scanning the skies for a variety of signal types.

Cornell University professor Frank Drake founded the first SETI program in late 1959. Drake reinforced his idea of scanning the skies with his famous Drake Equation. The Drake Equation predicts the abundance of intelligent life within a certain galaxy.

The second major development of SETI took shape in the late 1960s when NASA joined the program. NASA was minimally involved the project, but spawned many SETI related programs. These programs included the Microwave

Observing Project, Project Orion, the High Resolution Microwave Survey, Toward Other Planetary Systems, and more. One of the most intensive SETI related programs NASA would initiate began in 1992, but Congress cut funding for the program within a year. SETI projects now must rely on private funding, and SETI operates through the SETI Institute, a non-profit corporation.

Historically, scientists used several different methods for searching for extraterrestrial intelligence. The earliest method, and still most commonly used in present research, is the scanning of electromagnetic emissions. Radio waves are picked up by an array of radio telescopes and scanned for non-random patterns. More modern methods expand the search to other regions of the electromagnetic spectrum , including the infrared spectrum.

As of 2002, the University of California at Berkeley hosts the most widespread SETI effort in history. Berkeley projects include SETI@Home, SERENDIP, Optical SETI, and Southern SERENDIP. SETI@Home collects its data in the background of the Arecibo Radio Observatory and relays it back to the lab in Berkeley. The data is then divided into workunits and sent out to the personal computers of volunteers throughout the world. These personal computers scan the data for candidate signals. If a candidate signal appears, it is relayed back to Berkeley, where the signal is checked for data integrity. Finally, the lab removes radio interference and scans the data for final candidates. The Berkeley faction of SETI will be expanding their efforts with the Allen Telescope Array (formerly known as the One Hectare Telescope) designated specifically for this research.

Project Phoenix, also run by the SETI Institute, concentrates on obtaining signals from targeted areas within our galaxy. The focused Phoenix receiver can amass radio energy for longer periods of time and with greater sensitivity than previous SETI radio telescopes, allowing for faster and more precise analysis.

Although only a small fraction of the sky has been scanned, so far, SETI initiatives have not confirmed a signal from an extraterrestrial source that is conclusive proof of an extraterrestrial intelligence. A few strong and unexplained signals have intrigued SETI scientists; the most well known was received in 1977 at the Ohio State Radio Observatory. None of the signals have ever repeated.

SETI [ S earch for E xtra T errestrial I ntelligence], name given to a series of independent programs to detect radio signals from civilizations beyond the solar system. Modern SETI efforts can be dated from 1959 when Cornell Univ. physicists Giuseppi Cocconi and Philip Morrison pointed out the potential for using microwave radio signals to communicate with extraterrestrial civilizations. They suggested that a frequency of 1420 MHz be utilized as a communication channel since that corresponded to the signal emitted by neutral hydrogen, the most abundant element in the universe; that frequency, by international agreement, is now prohibited in all radio transmissions everywhere on and off the earth.

In 1960 the first such program, Project Ozma, led by American astronomer Frank Drake at the National Radio Astronomy Observatory in Green Bank, W.Va., focused on the nearby stars Epsilon Eridani and Tau Ceti. Since then some 50 searches, most of limited duration and concentrating on stars similar to the sun, have been conducted without success. In 1992 the High Resolution Microwave Survey (HRMS) was initiated. Using radio telescopes around the world in a planned ten-year search, HRMS envisioned a two-pronged approach. One group was to focus on solar-type stars within 100 light-years of Earth; the other was to conduct an all-sky survey. HRMS was halted by a funding cutback in 1993, but privately raised funds were used by the SETI Institute beginning in 1995 to conduct a search of nearby solar-type stars. Project Phoenix, as it is now called, monitors only microwave frequencies because there are few natural sources of emissions in that range and researchers hope that extraterrestrials would recognize that range as a quiet region of the electromagnetic spectrum suited to sending a message.

In addition to the listening efforts of the radio astronomers, other forms of contact have been attempted. Various coded messages have been broadcast in the hope that an extraterrestrial civilization might also have a SETI program. Gambling on a chance encounter with an extraterrestrial civilization, the U.S. space probes Pioneer 10 and 11 each carry an engraved plaque with a message from the earth, and Voyager 1 and 2 each have a recorded message of words and music. All four of these space probes have left the solar system and will travel in interstellar space indefinitely.

Bibliography: See also E. Ashpole, The Search for Extraterrestrial Intelligence (1990); D. Swift, SETI Pioneers: Scientists Talk about Their Search for Extraterrestrial Life (1990); F. Drake and D. Sobel, Is Anyone Out There? The Scientific Search for Extraterrestrial Intelligence (1992).

SETI Abbr. for Search for Extraterrestrial Intelligence.

SETI (ˈsɛtɪ) Astronomy search for extraterrestrial intelligence