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Network theory: a key to unraveling how nature works
One evening in 1994, three college students in Pennsylvania were watching Bacon in the eminently forgettable basketball movie The Air Up There. They started thinking about all the movies Bacon had starred in, and all the actors he had worked with, and all the actors those actors had worked with. The students came up with a game they called Six Degrees of Kevin Bacon, counting the steps from Bacon to any actor in Hollywood. In general, it takes remarkably few steps to reach him. Even Charlie Chaplin, who made most of his movies decades before Bacon was born, was only three steps away. (Chaplin starred with Barry Norton in Monsieur Verdoux, Norton starred with Robert Wagner in What Price Glory, and Wagner and Bacon worked together in Wild Things.)
Six Degrees of Kevin Bacon became a 1990s sensation, complete with talk show appearances and a book deal. University of Virginia computer scientists got on the bandwagon as well, building a web site called The Oracle of Bacon. You can use it to analyze the connections between all 1.6 million actors listed in the Internet Movie Database. It reveals that all actors in Hollywood are connected to Bacon on average by just 2.95 steps.
But it turns out that Bacon is not so remarkable. Over 1,000 actors are linked to the rest of Hollywood by less then three links. And even the most obscure actors can be linked to everyone else in Hollywood by less than ten steps. Six Degrees of Kevin Bacon says less about Bacon, in other words, than it does about the organization of the Hollywood network. If you were to simply join 1.6 million actors to each other at random, it would take many more links to connect any two of them than in real life.
Scientists hope to use network theory to help them prevent the collapse of ecosystems.forming a small-world network. The World Wide Web is organized in the same way. So is the anatomy of the human brain. What’s more, all these networks get their small-world organization because they follow similar rules. In each network, most nodes are linked to only a few other nodes. But a small fraction of nodes have lots of links. These hubs shorten the paths between all the nodes in the entire network.
Over the past two decades, Barabasi and other researchers have developed a sophisticated theory of networks that helps make sense of what look at first like hopelessly intricate tangles. Network theory is allowing scientists to understand how networks produce unexpected kinds of behavior you wouldn’t be able to predict from looking at individual parts, from the remarkable robustness of the Internet to the sudden crash of financial markets.
In the past few years, ecologists have begun to apply network theory to nature. By mapping the connections between species, they are discovering some of the rules by which all ecological networks are organized, and how these rules help foster biodiversity. They’re also studying how biological invasions, overfishing, and other threats are reorganizing these networks, and possibly putting them at risk of collapse. By discovering early warning signs of networks in trouble, scientists hope to be able to predict these collapses and prevent them from occurring.
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