Per Bak was born in Denmark in 1948 and received a PhD from the Technical University of Denmark in 1974. He then joined the Brookhaven lab, where he began studying phase transitions in condensed-matter systems. Bak, Tang and Wiesenfeld developed the concept of "self-organized criticality" in 1987 while studying the theoretical behaviour of a sand pile, in which grains of sand are sprinkled onto the pile, one at a time. As the pile grows, its sides become steeper, eventually reaching a critical state when just one more grain would trigger an avalanche.

Bak and his co-workers realised that it was impossible to predict if a particular grain would cause an avalanche. The size of these avalanches is, however, distributed according to a "power law", and they coined the phrase "self-organized criticality" to describe the pile's natural growth to a critical state. Their work showed that many phenomena in Nature are so complicated that their large-scale behaviour cannot be predicted from their microscopic origin. Self-organized criticality has since been applied to many other natural systems, including the size of earthquakes, the spreading of forest fires, the fluctuations of stock-market prices, and X-rays emitted by solar flares.

Bak, who was based at Imperial College London since 2000, wrote the ambitiously titled How Nature Works in 1996. He also wrote "Why Nature is complex" with his second wife Maya Paczuski in the December 1993 issue of Physics World. Bak died on 16 October in Copenhagen.