For all of history until 1877, every sound was gone the instant it was made. A performance existed only in the room where it happened and then vanished for good, kept alive at most in the memory of the people who heard it. Edison's phonograph broke that iron rule. By scratching the shape of a sound into a groove, it let a sound be kept and played again, and with that a performance could suddenly outlive the room it happened in. It is hard to overstate how strange this was: for the first time, a thing made of air and time could be held on to.

Underneath the machinery was one durable idea, the wave written down. Sound is a wave, a wiggle of pressure moving through air, and the phonograph's genius was to trace that wiggle onto something solid, turning a fleeting motion into a physical record. Every audio format since is a descendant of that one move. The groove became magnetic tape, then a stream of numbers, but each is only a new way of writing the same wave down so it can be read back later.

The numbering brought its own elegant rule. The Nyquist-Shannon sampling theorem showed that a wave can be captured perfectly by measuring it at twice its highest frequency, no more and no less. Human hearing tops out somewhere near twenty thousand cycles a second, so digital audio needs to take a little over forty thousand samples each second to catch everything the ear can catch. That is why, when the compact disc was standardised in 1982, it settled on 44,100 samples per second, a number that has been the quiet baseline of recorded sound ever since. From a scratch in a groove to a precise count of a wave, the whole project has been the same: to write down something that used to disappear.