The makers of the Voyager Golden Record could not assume an alien would understand minutes or seconds, so the playback speed engraved on its cover is defined against a single, unchanging frequency of the hydrogen atom — a clock written into the physics of the universe.

The two Voyager spacecraft, launched in 1977, each carry a gold-plated copper phonograph record. The record holds music, spoken greetings and encoded images. To be of any use to a finder, it has to be played at the right speed, and that posed a specific problem for the people who made it. A second is [...] The post The makers of the Voyager Golden Record could not assume an alien would understand minutes or seconds, so the playback speed engraved on its cover is defined against a single, unchang