Finding simplicity in complex systems
Luís M. A. Bettencourt
Theoretical Division, Los Alamos National Laboratory
Santa Fe institute
lmbettencourt@gmail.com
1
Each piece, or part, of the whole nature is always an
approximation to the complete truth, or the complete truth so
far as we know it. In fact, everything we know is only some
kind of approximation.
The Feynman Lectures on Physics (1964)
Taming Complex Systems
Measuring new and better things
genome, pathogens, medicine, ecosystems,
science & technology, computer networks,
nations, cities, organizations, individuals.
Big Data
data are enormous, varied, growing exponentially
So what?
do we have better public health?
policy?
science and technology?
more efficient markets?
Why (not)?
The problem:
familiarity, observation vs. experiments,
drowning in data, entanglement, speed, scope
The answer [this talk]:
most things don’t matter!
but
some things matter a lot:
measure
infer mechanism
act fast
act systematically
iterate
know what’s important!
approximately!
news
amanda knox
contagion processes
the early history
source: wikipedia
Hand bill from the New York City Board of Health, 1832
These miserable outcasts called that "fumigating" us, and the
term was a tame one indeed. They fumigated us to guard
themselves against the cholera, though we hailed from no
infected port. We had left the cholera far behind us all the
time. However, they must keep epidemics away somehow or
other, and fumigation is cheaper than soap.
RANT, RANT, RANT
Mark Twain 1867
Life photo archive /Google Images.
The data
soho london (1854)
Prockter, A. photo image, Feb. 23, 2006.
7th September, 1855
Cady Staley and Geo. S. Pierson, The Separate System of Sewerage, Its Theory and Construction
(New York: D. Van Nostrand, Co., 1899), p. 33.
“Whenever you can, count.”
Francis Galton
b=2
the nature of branching processes
Nk= bk Nk-1
b
is some
complicated
function
b>1
b=1
b<1
k ~ time, space,...
supercritical blows up
critical = threshold
subcritical fizzles out
in epidemics b = contact rate x infectious time
Most complex systems operate
fast and extensively
by triggering and regulating cascades
Motor Control
Epileptic Seizure
Generalized 3 Hz spike and wave discharges in a child with childhood absence epilepsy
source: wikipedia
HIV vs the human immune system
information arms races
Highly Active Antiretroviral Therapy (HAAT) or “drug cocktail”
anti-retroviral drugs
give the immune system
a hand by reducing b
for the virus
information cascades
heuristics of decision making under uncertainty
Bikhchandani, S., Hirshleifer, D., and Welch, I. (1992)
when in doubt imitate
Bettencourt (2002-3)
arXiv:cond-mat/0212267
arXiv:cond-mat/0304321
Proof:
no a priori predictability of new trends is possible
[early predictability very unreliable]
network visibility (number of incoming-links) leads to the
emergence and reinforcement of
(just as dumb) market makers
The spread of scientific ideas
USA, Japan, USSR
The power of a good idea
Quantitative modeling of the spread of ideas from epidemiological models
Bettencourt et al Physica A (2006), Scientometrics (2008), PNAS (2011)
much of what we do in society
makes sense in light of information transmission
increase contact rate & increase lifetime of information
cities
data storage
meetings
written documents
phd programs
libraries & archives
postdocs
databases
search engines
b = contact rate x lifetime of information
It’s all about b [networks]
don’t need to know it
measure it!
but what about uncertainty?
predict and measure it!
what about understanding, control?
act, predict and measure it!
Marburg Hemorrhagic Fever
Uige, Angola 2005
89% death rate
observation
model prediction
mechanism
inferred
Bettencourt (2006)
in Mathematical and Statistical Estimation Approaches in Epidemiology
Springer
good idea!
measure
predict
measure
anomaly?
iterate
swine flu pandemic
Mexico 2009
shut down Mexico City
20 million people
but with mixed results
world pandemic
Faster than the speed of life
b=1.3 for influenza (b<3)
infectious period ~ days
information technology ~ fraction of second
Neo dodging bullets
The matrix
Computers are now being used to generate news stories
about company earnings results or economic statistics as
they are released. This almost instantaneous information is
fed directly into other computers, which trade on the news.
City trusts computers to keep up with the news
Aline van Duyn, Financial Times, April 16 2007 03:00
Taming complexity is a wicked problem
but often it is enough to know what is critical
[failure modes, instabilities, cascades]
it is easier to be simple-minded but fast*
than slow and complex
* but you’ve got to learn
action mechanisms need to be further developed