What does the performer do in John Cage's piece 4'33"?

Answer

The 'music' is the ambient sound the audience hears.

Context

Pianist David Tudor gave the first performance on 29 August 1952. Britannica says he timed the three movements with a stopwatch and signalled each by opening or closing the keyboard lid, and that Cage's only instruction for each movement was the word tacet, meaning be silent. The piece is usually reported to have been played at a concert hall in Woodstock, New York, in three parts lasting 33 seconds, 2 minutes 40 seconds and 1 minute 20 seconds; these figures are as commonly reported.

The idea followed Cage's visit to a soundproofed anechoic chamber at Harvard in 1951, where he reported hearing two sounds, which he later explained as his nervous system and his blood circulation. That experience is usually cited as his reason for concluding that complete silence does not exist, so the work asks listeners to notice what is already sounding.

MoMA holds the 1952 to 1953 score in proportional notation, in which the length of each movement was shown by spacing on the page. Later versions simply instruct the performer to be silent for a stated time, and the work can be performed on any instrument or ensemble.

The premiere puzzled many listeners and some walked out. Over time the piece came to be read as showing that coughs, creaking seats and footsteps are part of any concert. It remains controversial as a question of what counts as music.