Per Bak, Physicist of Sudden Change, Dies at 54By GEORGE JOHNSON
Per Bak, an intellectually pugnacious physicist who sought to understand
how complexity arises in the world, died on Oct. 16 in Copenhagen. He was
The cause was complications of a stem-cell transplant, given as
treatment for a serious blood disorder, friends and family members said.
A large, physically imposing man, Dr. Bak delighted in prodding colleagues
to confront what to him was the deepest mystery of all: how a universe made
from simple fundamental particles produces such intricate order.
"It's a fantastic question," he said in a recent radio interview. "How can
we start with quarks and gluons and get humans and astrophysics and earthquakes?"
The answer, he believed, lies in a concept called self-organized criticality.
He frequently illustrated the idea with the image of a sand pile, like one
that forms at the bottom of an hourglass.
As grains of sand trickle
onto the cone-shaped hill, the structure grows larger and larger until it
reaches a point — a state of criticality — where it can grow no more. Each
additional grain sets off a landslide, paring the pile back down.
What fascinated Dr. Bak was that it is impossible to predict whether a particular
grain will cause a tiny, barely perceptible shudder or a catastrophic avalanche.
The probability can be described by what mathematicians call a power law:
there are many small disturbances and relatively few giant ones. But each
individual event comes as a surprise.
Dr. Bak proposed that other
complex phenomena, from real earthquakes and mass extinctions to stock market
fluctuations and traffic jams, follow the same pattern — a possibility that
Al Gore found so captivating that he mentioned it in the conclusion of his
book "Earth in the Balance."
Faced with many skeptics, Dr. Bak pursued
the implications of his theory at a number of institutions, including the
Brookhaven National Laboratory in Upton, N.Y., the Santa Fe Institute in
New Mexico, the Niels Bohr Institute in Copenhagen and the Imperial College
in London, where he became a professor in 2000.
He took his ideas to a general readership in 1996 with his ambitiously titled book "How Nature Works."
"He was the most American of Danes," said Dr. Predrag Cvitanovic, a professor
of physics at the Georgia Institute of Technology in Atlanta. "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."
Per Bak was born in Bronderslev,
Denmark. After studying at the Technical University of Denmark, he received
a doctorate in 1974 and went to work at the Brookhaven lab.
At a time
when studying elementary particles was the glamour job of physics, Dr. Bak
theorized instead about condensed matter — how hordes of particles interact
to produce phenomena like magnetism or crystallization. He specialized in
phase transitions, like those occurring when an insulator suddenly becomes
a conductor of electricity or when water freezes into ice.
him to the more general question of how order emerges from disorder and how,
in turn, the most enduring structures can unexpectedly collapse.
In 1987 he and two postdoctoral researchers, Dr. Chao Tang and Dr. Kurt Wiesenfeld,
presented their ideas on self-organized criticality in a much-cited paper
in Physical Review Letters.
Dr. Tang said his mentor's irreverent
style was already evident. "He certainly was one of the most original people
in science," Dr. Tang said, "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
Over the next few years, "Bak's sand pile," as many called it,
became a leading contender for those seeking a grand theory that might explain
the nature of complex systems.
Other scientists though, criticized
him for taking the idea too far, engaging in what one colleague called an
"impressionistic" physics that glossed over crucial distinctions. The title
of a typical Bak lecture was "Forest Fires, Measles and the Structure of
But in a field that is still seeking its bearings, self-organized criticality has secured an influential foothold.
In May 2001, Dr. Bak learned that he had myelodysplastic syndrome, sometimes
called smoldering leukemia. It was the second of his personal tragedies.
His first wife, Elizabeth, died in 1985.
Dr. Bak is survived by his
second wife, Dr. Maya Paczuski, a physicist at the Imperial College; their
son, Daniel; three children from his earlier marriage, Jakob, Tine and Thomas;
his parents; and a brother in Copenhagen.
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