Steve Reich started it all with some tape compositions, like "Come Out", and "It's Gonna Rain". What he did was play the same tape on two different machines, and then, by hand (!), gradually slowing one of the tapes so that it went out of phase with the other. This presented the possibility of live instruments doing the same kind of thing. You might enjoy this video --- https://www.youtube.com/watch?v=7P_9hDzG1i0&t=371s --- which allows you to both see and hear one of these pieces. He went on to compose some very elaborate compositions --- "Drumming" is a great one.
There are other approaches to the idea of minimalism: like, doing very little. Lamont Young is a conspicuous example (https://www.youtube.com/watch?v=VtMnDo7NItU); Alvin Lucier is another (see how you survive this: https://www.youtube.com/watch?v=fAxHlLK3Oyk&t=2s). Brian Eno composed a prettier way to do very little --- https://www.youtube.com/watch?v=vNwYtllyt3Q.
It is surprising to find that this idea had already occurred, in the curious mind of Erik Satie (1866-1925), who composed something called "Furniture Music", music to be ignored; and the notorious Vexations --- here's a 9 hour version: https://www.youtube.com/watch?v=gImDzmNuEDA
Had enough? I could go on, citing some medieval examples. Contact me on Wyzant if you'd like to explore this further.
