The search for extraterrestrial intelligence is the diverse efforts and scientific projects intended to detect extraterrestrial signals, or any evidence of intelligent life beyond Earth.
Arecibo Telescope in Puerto Rico with its 300 m (980 ft) dish was one of the world's largest filled-aperture (i.e., full dish) radio telescopes and conducted some SETI searches.
The Wow! Signal
Sensitivity vs range for SETI radio searches. The diagonal lines show transmitters of different effective powers. The x-axis is the sensitivity of the search. The y-axis on the right is the range in light-years, and on the left is the number of Sun-like stars within this range. The vertical line labeled SS is the typical sensitivity achieved by a full sky search, such as BETA above. The vertical line labeled TS is the typical sensitivity achieved by a targeted search such as Phoenix.
Microwave window as seen by a ground based system. From NASA report SP-419: SETI – the Search for Extraterrestrial Intelligence
Fermi paradox
Videos
The Fermi paradox is the seeming inconsistency between the lack of evidence of extraterrestrial civilizations and the apparently high likelihood of their existence.
Search for extraterrestrial intelligence
…SETI for being 'overly hopeful'. Critics argue that SETI is speculative and unfalsifiable, while supporters see it as a crucial step in addressing the Fermi paradox and understanding extraterrestrial technosignature. There have been many earlier searches for extraterrestrial intelligence within the Solar System…
Russian rocket scientist Konstantin Tsiolkovsky
Carl Sagan, seen here beside a Viking lander mockup, first mentioned the paradox in print.
Enrico Fermi (1901–1954)
Radio telescopes are often used by SETI projects.