In 1951, the composer John Cage walked into the anechoic chamber at Harvard University expecting to experience something no human had ever really experienced before: absolute silence. The chamber's walls were lined with sound-absorbing wedges so effective that the room measured below the threshold of human hearing — quieter than the inside of a soundproof vault, quieter than the deepest cave. Cage closed the door behind him and waited.

He heard two sounds. One high, one low. Puzzled, he asked the engineer on duty what was wrong with the chamber. The engineer's answer rewired Cage's entire philosophy of music: "The high one was your nervous system in operation. The low one was your blood in circulation."

This was the moment that produced 4′33″ — Cage's infamous 1952 composition in which a performer sits at a piano (or holds any instrument) for exactly four minutes and thirty-three seconds and plays nothing at all. It is often described, dismissively, as "the silent piece." But Cage insisted it was the opposite. Having proven to himself in that Harvard chamber that silence does not exist as long as you are alive to perceive it, the piece is actually a frame around whatever ambient sound happens to occur during its performance: coughs, shuffling programs, rain on the roof, traffic outside, your own breathing.

The score is meticulous. It is written in three movements (the famous premiere was 30″, 2′23″, and 1′40″), marked tacet — the classical musician's instruction to remain silent. David Tudor premiered it at Maverick Concert Hall in Woodstock by sitting at the piano and closing the keyboard lid to mark the start of each movement, opening it between them. Some audience members walked out. Others, Cage later said, missed the entire point — which was that they had been listening to music the whole time without realizing it.

A few rabbit-hole-worthy connections:

• The piece has been recorded many times, and the recordings are not identical — each captures a different soundscape of "nothing."
• In 2002, the estate of Cage sued composer Mike Batt for including a 60-second silent track titled "A One Minute Silence" on an album. The case settled out of court for an undisclosed six-figure sum, raising the genuinely thorny question of who owns silence.
• The piece was inspired in part by Robert Rauschenberg's White Paintings — blank canvases that Cage said acted as "airports for the lights, shadows, and particles" passing across them. 4′33″ does the same thing for sound.
• Cage was a serious amateur mycologist and once won an Italian game show by answering questions about mushrooms. The prize money funded his music.

The deepest irony? The world's quietest anechoic chamber today, at Microsoft's Building 87, measures −20.6 dBA — a sound level so low it is below the Brownian motion of air molecules. Visitors regularly report hallucinations after 20 minutes inside. Cage's revelation, it turns out, scales: the quieter the room, the louder you become.