- Are we Alone?The Search for Extraterrestrial Civilizations
For thousands of years earthlings have been asking the question "Are we alone?" Our civilization may now have the tools to answer this question.
Present radio and laser technologies are capable of transmitting and receiving messages across our Milky Way galaxy. Our civilization has been unintentionally transmitting television, radio and radar signals out into space for the past 50 years. Early television broadcasts, such as I Love Lucy, have already reached thousands of stars and they continue to propagate out at the speed of light. Earthlings have also sent a few intentional radio transmissions, such as the Arecibo message beamed to a star cluster in 1974, which contained information on our biochemical make-up and our location in the solar system .
Nearby extraterrestrial civilizations could detect our radio, radar and television leakage using large radio telescopes. Advanced civilizations might also detect life on Earth by using large optical telescopes and spectrometers to analyze the chemical constituents of Earth's atmosphere; oxygen, ozone and methane betray the presence of life.
Like our civilization, extraterrestrial civilizations might also transmit unintentional radio, radar or navigational beacons out into space. Or they might deliberately transmit radio or optical messages for the purpose of interstellar communication, either beamed toward us, toward other civilizations, or transmitted omnidirectionally. Intentional communication could contain substantial information about their culture. Such messages could be sent anticryptographically, with language lessons, pictures, and mathematics; messages might contain a civilization's literature, music, art, history, science, medicine and information about other civilizations they may have been in contact with. At about five billion years old, our solar system is middle-aged; some stars are twice our age and may harbor very advanced civilizations.
Using this rationale, several research groups are conducting searches for radio and optical signals from extraterrestrial civilizations. For a summary of current SETI programs, see Tarter  and MacRobert .
It is not possible to do a comprehensive SETI search with current technology, so there are several different search strategies. For example, the SETI Institute's Project Phoenix is conducting a targeted radio search of 1,000 nearby solar-type stars (see section frontispiece, p. 32); this search has good frequency coverage and is very sensitive, but targeted searches can cover only a small fraction of the sky. In contrast, SETI sky surveys have good sky coverage, observing billions of stars and galaxies, but they have relatively poor frequency coverage and sensitivity. Radio SETI sky surveys are ongoing in Argentina (Project META) and Australia (Southern Serendip), and at Harvard (Project BETA) and the University of California at Berkeley (SERENDIP and SETI@home). Optical searches for pulsed or continuous laser signals are ongoing at observatories in Australia and Ohio, and at Harvard, Princeton and the University of California.
Although our civilization cannot presently conduct a comprehensive search for extraterrestrial electromagnetic communication, SETI capabilities are doubling every year as telescopes and signal processing capabilities improve. If there are radio or optical signals out there, earthlings will soon discover ET.