Here is a modern version of a Mensuration Canon, where the same melody is played simultaneously at different speeds by augmenting or diminishing note values proportionally. In this case, many different rates at the same time. Otherwise, it's just a descending a natural minor scale, played on alternating short and long note values.
Arvo Pärt - Cantus in Memoriam Benjamin Britten
Estonian National Symphony Orchestra Paavo Järvi
^ I should add that while this concept sounds simple, the effect created here is like nothing you've ever heard. If you listen straight through and make it to the end, you just might be in a puddle of emotions. It's gut-wrenching.
cc: @White_Rabbit
This piece was famously used in the movie Hiroshima, underscoring Little Boy was being loaded onto the Enola Gay.