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How does SETI search for signs of extraterrestrial intelligence?

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artist's conception of Kepler-186f — extraterrestrial intelligence An artist's conception depicts Kepler-186f, the first Earth-sized extrasolar planet found in its star's habitable zone.

The search for extraterrestrial intelligence (SETI) involves scanning the electromagnetic spectrum for nonrandom patterns that may indicate the presence of technologically advanced beings. These efforts generally fall into two categories:1

Radio Searches

Most SETI projects use radio telescopes to hunt for narrowband signals, typically 1 hertz wide or less, which would be distinct from natural broadband emissions like those from pulsars. Many experiments focus on the 1,420-megahertz frequency, as this is the natural emission frequency of hydrogen and would likely be known to any advanced civilization. Approaches include:2
* Targeted searches: Focusing on specific nearby Sun-like stars, such as in Project Phoenix.21
* Sky surveys: Scanning large areas of the sky without assuming a specific source direction, such as the SERENDIP projects.21

Optical SETI

Researchers also search for brief, high-intensity light pulses using telescopes equipped with sensitive photomultiplier tubes. These pulses, lasting a nanosecond or less, could be produced by extraterrestrial societies using high-powered lasers to signal other worlds.3