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Per
Bak died on October 16, 2002, at the age of 54.
Per was one of the founders and most influential
contributors to the study of complex systems. Per
made many contributions to science, but the best
known was a general theory of self-organization,
which he called, "self-organized criticality".
His ideas and discoveries have had an influence
over how people think about a broad range of phenomena,
from physics to biology, neurosciences, cosmology,
earth sciences, economics and beyond. As a scientist
and as a person he was an inspiration and a challenge
to those of us who knew him. He was, for me that
rare scientist who, though not in my field, could
at any moment surprise me by saying something that
would make me realize I had to rethink something
I had thought I understood.
Per's
theory of self-organized criticality was formulated
in a paper he wrote in 1987 with two younger colleagues,
Chao Tang and Kurt Wiesenfeld. This was one of the
most highly cited and influential physics papers
of the last two decades. It presented a general
mechanism by which systems which are out of thermal
equilibrium may evolve to a fractal, or scale invariant,
distribution. These distributions are characteristic
of many non-equilibrium systems, but before their
paper no one had understood why. This idea, and
the methodology it spawned, has been applied to
understand the patterns that form spontaneously
in many different systems, including earthquakes,
forest fires, traffic jams, economic markets, and
biological phenomena ranging from natural selection
to the distribution of species of trees in a forest.
It has also been applied to astrophysical phenomena
ranging from x ray busts to the distribution of
galaxies in the universe. Many of these applications
were pioneered by Per himself, in collaboration
with specialists in these fields.
Per's
career proves that a scientist can indeed be a public
intellectual, for the influence of his work extends
far beyond science. Al Gore mentioned self-organized
criticality in his book, "Earth in the Balance". I've
heard executives of powerful software companies say
they did not understand why their business strategies
succeeded until they read Per's popular book, How
Nature Works. At a meeting in Santa Fe I heard
Lt. General (Retired) Paul Van Riper, one of the planners
of the Gulf War, say that he learned so much from
that book that could be applied to military strategy
that he made it required reading for all U.S. Marine
officers in training.
There was no one better than Per at grasping the heart
of a complicated phenomena, and then realizing his
insight by the invention of a simple model. Per's
models were so elegant in their elimination of extraneous
features that some experts were unable to understand
why they applied at all to their subject. You could
explain any of his models to a child, but to understand
how they worked you had to be able to be able to strip
everything away from a phenomena but the mechanisms
by which a pattern forms and propagates. A tree on
fire is like a person with the flu, is like a galaxy
forming stars, is like a gene turning on, is like
a trader making a bid...and each is represented by
a single bit: all that matters is how many neighbors
have to be on for it to light up. I remember him telling
me once how excited he had been when he had accidentally
run into Stephen J Gould. He introduced himself and
very excitedly said that he had invented a theory
that could explain Gould and Eldridges notion of evolution
by punctuated equilibrium. Gould replied that he wasnt
interested because "punctuated equilibrium is already
a theory". Per was very disappointed. Several years
later I was fortunate myself to meet Gould and I was
glad to be able to report back to Per that I had spent
a dinner explaining his theory to Gould and he had
finally got it.
Spending time with Per was an exhilarating but sometimes
daunting experience. Per's energy exceeded that of
most people, he was full of ideas, and completely
direct in expressing his opinions. If he liked an
idea he was childlike in his enthusiasm; he praised
it unreasonably, no matter whether it was his or someone
elses. But if he thought something was stupid, he
said so bluntly, no matter his relation with the speaker,
or the consequences.
Per was perhaps the most fearless scientist I ever
met. He more than once began a talk at a conference
of neuroscientists or cosmologists with the statement
that after six months of thinking about their field,
he realized that everything done in the last 20 years
was wrong. Having said that he would explain where
he saw the mistake and propose a simple theory that
reversed it, which he illustrated by a simple computer
model. One such talk I heard, about how the brain
works, was titled, "Learning from mistakes." Certainly
some people went away unhappy, but I am convinced
this was more childlike simplicity than arrogance.
It would not have occurred to him that there was any
other way to be: science is hard and we have to say
what we think.
What Per understood better than any other scientist
Ive known is that doing good science takes courage,
stubbornness and a willingness to take big risks in
the hope of making big advances. He argued stubbornly
when he felt that someone did not understand one of
his ideas, but he did not seem to mind if one of his
theories was shown wrong or improved upon.
What Per certainly had no patience for was anything
that smacked of insincerity or hypocrisy. If there
was not a good reason to do something, he didnt waste
his time, nor did he see why anyone else should. He
was the kind of professor that university administrators
most fear: a renowned scientist who simply said no
when asked to take time and energy away from science
in order to do something that interested him less
at that moment.
The philosopher Roberto Mangabeira Unger explains
some of the problems faced by contemporary scientists
by saying that "we are something relatively infinite
caught within finite realities: the body, society
and culture." Certainly this is how I remember Per.
