December 8, 1948|
|Died||October 16, 2002
|Institutions||Brookhaven National Laboratory
University of Copenhagen
Santa Fe Institute
Niels Bohr Institute
Imperial College London
|Alma mater||Technical University of Denmark
Risø National Laboratory
|Doctoral students||Sergei Maslov
|Known for||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 that context, he also did important work on complicated spatially modulated (magnetic) structures in solids. This research led him to the more general question of how organization emerges from disorder.
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.
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 entitled book, How Nature Works. In 2001, Bak learned that he had myelodysplastic syndrome and died from it the following year. Pak was survived by his second wife, Maya Paczuski, a fellow physicist at Imperial College with whom he has coauthored papers, and his four children.
Others about Per Bak 
- "He was the most American of Danes," said Predrag Cvitanović. "Danes eschew confrontation, but he was arrogant and loved to fight with his colleagues in academia. We all have stories of how we first met him, usually remembered by some outrageous statement or insult."
- A sample of Prof. Bak's statements at conferences: After a young and hopeful researcher had presented his recent work, Prof. Bak stood up and almost screamed: "Perhaps I'm the only crazy person in here, but I understand zero - I mean ZERO - of what you said!". Another young scholar was met with the gratifying question: "Excuse me, but what is actually non-trivial about what you did?"
- Chao Tang mentions his mentor's irreverent style, "He certainly was one of the most original people in science, and also one of the very few who truly doesn't care what other people think about what he is doing. He was sort of on his own."
Selected publications 
- 1982, "Commensurate phases, incommensurate phases, and the devil's staircase", in: Reports on Progress in Physics, Vol. 45, pp. 587–629;doi:10.1088/0034-4885/45/6/001 (1004 citations as of April 2012)
- 1987, "Self-organized criticality: an explanation of 1/f noise", with Chao Tang and Kurt Wiesenfeld, in: Physical Review Letters, Vol. 59, pp. 381–384;doi:10.1103/PhysRevLett.59.381 (4957 citations as of April 2012)
- 1996, How Nature Works: The Science of Self-Organized Criticality, New York: Copernicus. ISBN 0-387-94791-4 (2744 citations as of April 2012)
- Obituary in Nature.
- Sand Piles and Cancer. Short article about Per Bak by Azra Raza, M.D., in 3 Quarks Daily