Speculation about life in the universe has a long and interesting history. Two thousand years ago, Plutarch wrote about "collections of matter, some of which are other worlds with their own skies and races of men and beasts." The Church declared the doctrine of the "plurality of worlds" to be heretical in the 11th century, and Giordano Bruno was burned at the stake in 1609 for proposing an infinity of worlds with life on them, although in fact some of his other heresies regarding the Bible bothered the Church more than his views of life beyond Earth.
Practical SETI began in 1820, when Karl Gauss suggested planting large tracts of Siberian forest in a graphical demonstration of the Pythagorean theorem. Joseph von Littrow in 1840 wanted to ignite kerosene-filled trenches in the Sahara Desert in an assortment of geometric shapes. Neither of these proposals was ever funded. At the turn of the century, the eccentric genius Nikola Tesla built enormous 150-foot electrical coils and strained the power production capacity of Colorado Springs in order to send radio signals to extraterrestrials. Tesla thought that he had detected signals from an extraterrestrial civilization, but in fact he discovered the atmospheric phenomenon known as whistlers. Guglielmo Marconi also used his early radio technology to listen for signals from beyond Earth.
Modern SETI began with Project OZMA, which ran from 1959 to 1960. In this experiment, Frank Drake scanned across a 400-kHz band of radiation from the two nearest solar-type stars. Nothing conclusive was found. Since then, dozens of radio SETI projects have been conducted without any convincing sign of extraterrestrial intelligence. Of course, all of these experiments combined have searched only a tiny fraction of the cosmic "haystack," which is the vast number of targets and frequencies that must be scanned.
On October 12, 1992 — 500 years after Columbus' discovery of the New World — NASA began the most ambitious SETI project ever. The prosaically named Microwave Observing Project (MOP) searched more of the cosmic haystack in its first five minutes than all previous SETI experiments combined! The long time scales for sending and receiving messages led NASA to a passive strategy: to search for signals that may have been deliberately or accidentally sent our way long ago. The search strategy has two components. The first part consists of the detailed monitoring of the 1000 solar-type stars nearest the Sun. The second part involves a scan of the entire sky, to allow for the possibility of rarer but more powerful signals from across the Galaxy.
After funding was repeatedly cut by the U.S. Congress, SETI activists started to look for private sources of funding. The current plan, called Project Phoenix, is not funded by taxpayers at all. Famous corporate leaders and media moguls like Steven Jobs and Paul Allen and Steven Speilberg have provided millions of dollars for a new generation of SETI experiments. Even though Congress has been dubious, the public is entranced by the idea of making contact with intelligence far beyond the solar system. The SETI Institute, a non-profit research lab, has parceled out data from the current radio experiments to millions of people around the country. Each person runs a program that uses spare capacity on their PC to look for artificial signals in a stream of radio noise. In this way, the SETI researchers can harness the power of millions of PCs to look for the elusive evidence of extraterrestrial intelligence.
The new wave of SETI experiments depends on powerful receivers and large telescopes. Previous projects could only search a few hundred different frequency channels simultaneously. The MOP listened in on tens of millions of channels simultaneously. The improvement has been made possible by modern digital techniques and custom integrated circuits. The receivers are also extraordinarily sensitive. Astronomers achieved a radio detection of the weak signal of Pioneer 10 after it left the Solar System; the detection had a changing Doppler shift due to the Earth's rotation. The signal has the equivalent power of a single Christmas tree light at a distance of over 5 billion miles! The 1000-foot diameter radio dish at Arecibo in Puerto Rico offers even greater sensitivity. The dish is so large that it could hold 357 million boxes of corn flakes or all the beer consumed on Earth in one year. Detectors at the focus of the Arecibo dish could detect a 1/2 megawatt radio signal, equal to the strength of a modest radio station, on the other side of the Milky Way.
More ambitious SETI schemes are underway. The Allen Telescope Array (ATA) is a set a radio dishes that benefited from more than $30 million in gifts from Microsoft co-founder Paul Allen. Even with his support, expansion from the current set of 42 dishes to the goal of 350 is very uncertain. It may one day be possible to send a beacon and receiver hundreds of A.U. from the Earth and use the Sun as a gravitational lens to amplify and direct signals to distant targets. Future telescopes in space will allow the spectroscopic analysis of the reflected light from planets in other stellar systems. The direct detection of atmospheric chemistry that indicates life would cut through the web of anthropocentric arguments regarding technology and radio communication. In the meantime, researchers must think as broadly as possible about the nature of life. The search for life in the universe will become a truly scientific subject one step at a time.
Few scientific subjects generate as strong an emotional response as SETI. Debates between SETI optimists and pessimists can be acrimonious. Pragmatists argue that the scientific basis for the optimistic calculations is flimsy and that no search strategy can be logically justified. SETI has been unpopular with some politicians, who see it as a frivolous use of taxpayers' money. NASA has had considerable trouble in funding SETI, despite the fact that it never accounted for more than 0.1% of the agency's science budget. However, popular support for SETI remains strong. "The probability of success is difficult to estimate," wrote physicists Giuseppe Cocconi and Phillip Morrison in 1959 in a paper that started the modern era of SETI, "But if we never search, the chance of success is zero." Few people can resist the excitement of one of the most profound questions humans can ask: what is our place in the universe?