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Finding simplicity in complex systems

Luís M. A. BettencourtTheoretical Division, Los Alamos National LaboratorySanta Fe institute

lmbettencourt@gmail.com

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 thingsgenome, 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: know what’s important!

measureinfer mechanism act fast act systematicallyiterate

approximately!

newsamanda knox

contagion processesthe early history

source: wikipediaHand bill from the New York City Board of Health, 1832

Life photo archive /Google Images.

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

The datasoho 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

the nature of branching processes

Most complex systems operate fast and extensively

by triggering and regulating cascades

Nk= bk Nk-1 k ~ time, space,...

b>1 supercritical blows up b=1 critical = thresholdb<1 subcritical fizzles out

b is some

complicated

function

b=2

in epidemics b = contact rate x infectious time

Motor Control

Generalized 3 Hz spike and wave discharges in a child with childhood absence epilepsysource: wikipedia

Epileptic Seizure

HIV vs the human immune systeminformation arms races

Highly Active Antiretroviral Therapy (HAAT) or “drug cocktail”

give the immune system a hand by

reducing b for the virus

anti-retroviral drugs

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

information cascadesheuristics of decision making under uncertainty

when in doubt imitate

Bikhchandani, S., Hirshleifer, D., and Welch, I. (1992)

Bettencourt (2002-3) arXiv:cond-mat/0212267arXiv:cond-mat/0304321

The spread of scientific ideasUSA, Japan, USSR

The power of a good ideaQuantitative modeling of the spread of ideas from epidemiological modelsBettencourt 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 &

citiesmeetings

phd programspostdocs

search engines

increase lifetime of information

data storagewritten documentslibraries & archives

databases

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 FeverUige, Angola 2005

89% death rate

observation model prediction inferred mechanism

Bettencourt (2006) in Mathematical and Statistical Estimation Approaches in EpidemiologySpringer

good idea!

measurepredictmeasureanomaly?iterate

swine flu pandemicMexico 2009

shut down Mexico City20 million people

but with mixed resultsworld pandemic

Neo dodging bulletsThe matrix

b=1.3 for influenza (b<3)

infectious period ~ days

information technology ~ fraction of second

Faster than the speed of life

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