In the search for extraterrestrial intelligence (SETI), researchers differentiate between advanced and non-advanced civilizations. An advanced civilization is defined as one with the ability to conduct interstellar communication — to send electromagnetic signals through space. Using this definition, human civilization only became advanced within the past century, when local communication such as radio and television broadcasts began to be transmitted to interstellar space.

SETI researchers are looking for broadcast signals that could only be produced by intelligent alien civilizations. Because our technology is not advanced enough to pick up weak signals, a strong, deliberate beacon created by extraterrestrials with the purpose of getting our attention is our best hope for finding intelligent alien life. Using some of the largest telescopes in the world, researchers look in the radio and optical bands of the electromagnetic spectrum for unusual signals, such as pulses, that have characteristics that are unlikely to be produced by nature.

The first modern SETI search was conducted in 1960 and led to many more projects. However, federal funding for SETI was eliminated in the 1990s. Since then, SETI has continued in the form of privately funded programs. The SETI Institute, a nonprofit scientific and educational institution, is one of the most respected groups. Other organizations, universities, amateur astronomers, and even home computer users also contribute to the search. In the SETI@home project, over five million people with personal computers have helped to analyze data via the Internet.

SETI is an exercise in optimism and hope. When trying to estimate the number of advanced civilizations within the Milky Way, researchers use an equation that considers the number of habitable planets, the fraction of these that support life, the fraction that developed intelligent communication, and the fraction that exist currently. Unfortunately, scientists do not know any of the numbers needed to complete this equation. Although it seems reasonable to many that there may be intelligent life out there, the likelihood of us finding it is slim. When researchers target individual stars and scan the sky for artificial signals, they know they are only looking at a very small sample of the universe. With perhaps a few hundred billion stars in our galaxy and couple hundred billion galaxies in the universe, the search has barely begun.

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