New Scientist explains that the recipients, if there are any, could answer in about 25 years – enough time for a radio message to travel the 12 light years to GJ 273b, a habitable planet in Canis Major, and back:
This missive to the stars was sent on the anniversary of the “Arecibo message,” a radio transmission beamed toward a distant star cluster in 1974 from the Arecibo radio telescope in Puerto Rico. The Arecibo message contained information on the planets of our solar system, the structure of DNA, a cartoon-like picture of what a human being looks like, and other basic information about the earth and its inhabitants.
This new message – beamed from an antenna in Norway over roughly eight hours over a three-day period in October – is simpler and may be more readily understood, Vakoch says.
It begins with information about counting, arithmetic, geometry, and trigonometry, and includes a description of the radio waves that carry the message, as well as a tutorial on clocks and timekeeping, to see if any potential inhabitants of GJ 273b have an understanding of time similar to our own.
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Clock instructions. Be really funny if hostile aliens destroy our planet and everything we’ve ever known over, like, “The big hand is on the 12.”