• The activity of looking for celestial signals of artificial origin, usually in radio wavelengths, and for other signs of intelligent life beyond the Earth, such as META. Most searches concentrate on the region of the radio spectrum around 1–10 gigahertz, where signals can travel well both in space and in the Earth’s atmosphere and where numerous key spectral lines, such as the 21cm hydrogen line, are to be found which might (the theory goes) suggest themselves to other species as markers near which transmissions might be sent. The US Congress has traditionally targeted SETI experiments for high-profile budget cuts, leaving it to be resurrected by private donors and scientific societies.

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