Search for Extraterrestrial Intelligence

The search for extraterrestrial intelligence (SETI) is a scientific endeavor that draws widespread and constant public attention. Media coverage of SETI has been plentiful since scientists first began to talk about it, and SETI scientists pay close [Page 786]attention to what journalists say about their work.

In the 19th century, astronomers first learned to record the spectra of celestial objects and found that they were made of the same stuff as things on Earth, including the elements of which life is made. Charles Darwin's theory of evolution gave further weight to the idea of life as originating as a result of natural processes. In the mid-20th century, radio astronomy was invented. Ever since, SETI scientists have been searching for signals of extraterrestrial technological origin.

The standard SETI technique is to use ground-based radio telescopes coupled with specially developed multichannel signal processors to listen for and identify signals originating from extraterrestrial technology. The two standard modes of searching for radio signals are targeting stars identified as good candidates for having planets and surveying sections of the sky for prominent signals. Other SETI methods include looking for optical (involving visible light) or infrared evidence of the presence of technology on a planet.

The scientific search for evidence of extraterrestrial (ET) intelligent life got its start with the publication of a paper in the British journal Nature on September 19, 1959, describing a concept for “searching for interstellar communications.” This paper, authored by physicists Giuseppe Cocconi and Philip Morrison (then affiliated with Cornell University), offered a scientific and technical rationale for searching for evidence of extraterrestrial intelligent life by using radio telescopes to listen for signals generated by ET technology. At that time, the scientific community knew enough about the origin, evolution, and nature of galaxies, stars, and planets and enough about radio astronomy techniques to make Cocconi and Morrison's proposal seem plausible, and a community of SETI scientists began to organize.

In 1960, astronomer Frank Drake conducted the first radio-telescope search for signals of extraterrestrial intelligent origin at the National Radio Astronomy Observatory (NRAO) in Green Bank, West Virginia. Drake's Project Ozma observed two nearby sunlike stars, Tau Ceti and Epsilon Eridani, listening for signals in a narrow band of the radio spectrum. In 1961, in collaboration with the Space Science Board of the National Research Council, Drake organized the first scientific conference on SETI, held at the NRAO. One product of this conference was the so-called Drake equation, named for its creator Frank Drake.

The Drake equation—N = N∗ fp ne fl fi fc fL—is not actually an equation. It is a heuristic tool, a technique for guesstimating how many planets in the Milky Way galaxy might host intelligent life. This technique rests on a number of assumptions, so its product, the estimated number of communicating civilizations in our galaxy, is by no means a precise quantity, with possible answers ranging from zero to millions or more.

N∗ is the number of stars in our galaxy (estimates range from 100 billion to 400 billion, including tens of billions of sunlike stars), fp is the proportion of those stars that might have planets, ne is the proportion of those planets that might be habitable, fl is the proportion of those habitable planets where life might have evolved, fi is the proportion of those planets with life where intelligent life actually has evolved, fc is the proportion of intelligent life forms on those planets that are willing and able to communicate by radio signals, and fL is the fraction of the life of those planets with intelligent radio-communicating life during which the communicating civilizations exist (or existed). Creating values for each of these variables is more a speculative than a mathematical exercise.

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