How to catch a terrorist with Network Theory?

Can Network Theory Thwart Terrorists?
By PATRICK RADDEN KEEFE
NYTimes Magazine
Published: March 12, 2006

During the last decade, mathematicians, physicists and sociologists have advanced the scientific study of networks, identifying surprising commonalities among the ways airlines route their flights, people interact at cocktail parties and crickets synchronize their chirps. In the increasingly popular language of network theory, individuals are "nodes," and relationships and interactions form the "links" binding them together; by mapping those connections, network scientists try to expose patterns that might not otherwise be apparent. Researchers are applying newly devised algorithms to vast databases — one academic team recently examined the e-mail traffic of 43,000 people at a large university and mapped their social ties. Given the difficulty of identifying elusive terror cells, it was only a matter of time before this new science was discovered by America's spies.

In its simplest form, network theory is about connecting the dots. Stanley Milgram's finding that any two Americans are connected by a mere six intermediaries — or "degrees of separation" — is one of the animating ideas behind the science of networks; the Notre Dame physicist Albert-Laszlo Barabasi studied one obvious network — the Internet — and found that any two unrelated Web pages are separated by only 19 links. After Sept. 11, Valdis Krebs, a Cleveland consultant who produces social network "maps" for corporate and nonprofit clients, decided to map the hijackers. He started with two of the plotters, Khalid al-Midhar and Nawaf Alhazmi, and, using press accounts, produced a chart of the interconnections — shared addresses, telephone numbers, even frequent-flier numbers — within the group. All of the 19 hijackers were tied to one another by just a few links, and a disproportionate number of links converged on the leader, Mohamed Atta. Shortly after posting his map online, Krebs was invited to Washington to brief intelligence contractors.

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