Per Bak


Per Bak was a Danish theoretical physicist who coauthored the 1987 academic paper that coined the term "self-organized criticality."

Life and work

After receiving his Ph.D. from the Technical University of Denmark in 1974, Bak worked at Brookhaven [National Laboratory]. He specialized in phase transitions, such as those occurring when an insulator suddenly becomes a conductor or when water freezes.
In 1987, he and two postdoctoral researchers, Chao Tang and Kurt Wiesenfeld, published an article in Physical Review Letters setting a new concept they called self-organized criticality. The first discovered example of a dynamical system displaying such self-organized criticality, the Bak-Tang-Wiesenfeld sandpile model, was named after them. This followed publication in 1986 in the Journal of Geophysical Research of a very similar model of earthquakes by Jonathan I. Katz of Washington University in St. Louis.
Faced with many skeptics, Bak pursued the implications of his theory at a number of institutions, including the Brookhaven National Laboratory, the Santa Fe Institute, the Niels Bohr Institute in Copenhagen, and Imperial College London, where he became a professor in 2000.
In 1996, he took his ideas to a broader audience with his ambitiously titled book, How Nature Works. In 2001, Bak learned that he had myelodysplastic syndrome and died from complications of a stem-cell transplant.
Bak is survived by his second wife, Maya Paczuski, a fellow physicist and current professor at the University of Calgary, with whom he has coauthored papers, and his four children.

Selected publications

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