• Communications with extra-terrestrial intelligence, usually meant to mean electromagnetic communications. CETI has been an empirical reality since 1960, when the first search for interstellar radio messages was made. In 1974, the Arecibo radio telescope was used to send the first message deliberately intended for alien intelligences. In addition, a US Voyager spacecraft leaving the solar system has been equipped with messages for any finders. But the most pervasive message humans are sending out from the Earth is a barrage of ‘leaking’ radio and TV broadcasts which, since they have been transmitted for several decades, are now tens of light years out. For more considered conversation, two large radio telescopes of the type used by Earthly radio astronomers should suffice for conversations across much of a normal-sized galaxy like our own. In the 1990s, advances in digital signal processing technology (BETA and META) made it possible to build equipment to search the sky automatically in many channels to look for signals of possible artificial origin.

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