Legendary avant-garde composer John Cage will forever be known (regrettably) for his infamous “silent” composition 4’33”. Consisting of three movements, the piece can be performed by any amount of performers on any instruments; the only instruction Cage provides is to not actually play. It was originally performed in 1952 by pianist David Tudor, who signaled the start and end of each movement by opening and closing the lid of his piano. Audiences were confused, irritated, and even outraged by what probably seemed like an elaborate joke.
But the thing about 4’33” that is hardly ever understood is that it’s really not about the absence of playing. Instead, Cage was attempting to make sound that would normally be extraneous diegetic to the performance. Hence, the sonic content of Tudor’s original performance was not silence; it was the shuffling and whispered confusions of the audience members, the creaking of the piano stool on the stage, and other ambient noises that would normally be drowned out.
Despite how you may feel about this controversial composition, it represents a popular example of an important concept in modern classical and experimental music: indeterminacy. No two performances of 4’33” are exactly the same, because the environmental sounds heard throughout the duration of the piece will always be different. Though this was not Cage’s central goal with 4’33”, it was something he and many of his peers worked with throughout their careers. Indeterminacy, or incorporating some element of chance into a musical composition, challenged the definition of what a work truly is and offered new opportunities for unconventional composing.
Works that fall under the umbrella of indeterminacy are often completely unrelated stylistically. In general, most versions of Terry Riley’s legendary minimalist composition In C, which is comprised of 53 segments that are played in an order determined by the individual performers, differ greatly from Cornelius Cardew’s The Great Learning, a choral work based on the text of the same name by Confucius. Even more interesting is the possibility for two recordings of the same piece to sound entirely different! Take, for example, the Bang on a Can All-Stars’ recording of In C, a tension filled performance consisting mainly of strings, vs. the Africa Express version, which is much looser and happier due to the use of a greater number of performers and ethnic instruments.
Also related to this idea is the concept of graphic notation. Composers, instead of using conventional notes and staves, represent the movements within a piece with pictures, lines, shapes, or other visual elements. While this can introduce more control into a performance than purely indeterminate notation, there is still an element of uncertainty, as the sounds that are produced are based on how each performer interprets the abstract score. Examples include Cardew’s legendary 173-page work Treatise, John Wiese’s Tet compositions, and this awesome visual accompaniment to Ligeti’s “Artikulation.”
Note: Most of this information was gleaned from Michael Nyman’s book Experimental Music.
Noise Not Music 
