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Goodbye to Per

Yi-Cheng Zhang

posted on 29 October 2002

Last week Per Bak passed away, after a long heroic battle against a
nasty disease.
For many people in the interdisciplinary physics, especially in
Econophysics, the loss
is irreplaceable: for Per is unique in his penetrating insight, broad
scope and a flair for
simple beauty in physics and science. Friends and colleagues will
remember and write of his
long successful career and contribution, I shall limit myself to Per
style of doing
research, which might inspire us to follow his suit.

The most acclaimed work lately was the BTW model, from which Per
helped to make the
"self-organized criticality" (SOC) one of the leading paradigms, well
beyond science itself.
Al Gore's book has a whole section devoted to Per's contribution;
financial economists are
finally waking up to realize crashes are just manifestations of SOC
phenomena; Geosciences
now can have a theory basis for the earth-quake mechanisms. For many,
Per appears to be a
visionary in science, but rarely do detailed math-work that
traditionally appreciated in
physics community (not just economists love math formality as they are
accused often!).
Once in a private discussion I suggested his style of research being
"Impressionist", and he
said he felt flattered with the definition. Impressionist painters paint
the world with broad
brushes, appear to care little details---no lines are direct no forms
are regular. But all the
well known impressionists a century ago had very solid training in
"normal" skills, as a matter
of fact often they were best of the pack. They were able to let their
feelings freely flow,
not to worry the tools, the fixed rules, so that they could reach such a
sublime expression
of art. They could do the same things as portrait painters, but they
have been there, done that.
Their power draws strength from their basic trainings, without
explicitly using them.

When I first met Per in Brookhaven in 1984, I was interested in his
work on
commensurate-incommensurate phase transition. In his PRL paper with
Bruisma there is a
calculation for the devil's staircase sprectrum that bothered me for a
few weeks without
being able to reproduce it. We normally assume when co-authoring with
Per, he gave ideas
and the others worked out details. But for this calculation we need some
fancy results from
Number Theory, that most of physicists never heard of. I went to a lot
of people,
including Bruisma, without success. I finally busted Per, on a
blackboard he went
lecturing me on Number theory, from basics to the subtlties for an hour,
in my total
amazement. This is at odds with his recent style, his public talks
rarely have a handful
of equations, giving the impression of sloppiness, the trademark of
But impressions can be misleading. Per rarely make important mistakes,
on crucial questions
he is faster than anyone to spot the potential and most interesting
directions. For him
physics is passion and love, he developed such an intuitive feel about
physics that many of
us consulted him regularly for important dicisions.

Per is extremely sharp in distinguishing what is essential and what is
unnecessary details.
His patience is in short supply with formality and pretension. His
personal life is also a
mirror image of the research style, upholding principles and little care
the boring details. His impact on the econophysics community is yet to
be felt and we have
still much to learn from this unique individual.

YC Zhang