On Nov. 16, 1974, a few hundred astronomers, government officials and other dignitaries gathered in the tropical forests of Puerto Rico’s northwest interior, a four-hour drive from San Juan. The occasion was a rechristening of the Arecibo Observatory, at the time the largest radio telescope in the world. The mammoth structure — an immense concrete-and-aluminum saucer as wide as the Eiffel Tower is tall, planted implausibly inside a limestone sinkhole in the middle of a mountainous jungle — had been upgraded to ensure its ability to survive the volatile hurricane season and to increase its precision tenfold.
To celebrate the reopening, the astronomers who maintained the observatory decided to take the most sensitive device yet constructed for listening to the cosmos and transform it, briefly, into a machine for talking back. After a series of speeches, the assembled crowd sat in silence at the edge of the telescope while the public-address system blasted nearly three minutes of two-tone noise through the muggy afternoon heat. To the listeners, the pattern was indecipherable, but somehow the experience of hearing those two notes oscillating in the air moved many in the crowd to tears.
That 168 seconds of noise, now known as the Arecibo message, was the brainchild of the astronomer Frank Drake, then the director of the organization that oversaw the Arecibo facility. The broadcast marked the first time a human being had intentionally transmitted a message targeting another solar system. The engineers had translated the missive into sound, so that the assembled group would have something to experience during the transmission. But its true medium was the silent, invisible pulse of radio waves, traveling at the speed of light.
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