Senator Barack Obama swept the Potomac primaries yesterday- winning Virginia, Maryland, and Washington DC, all with convincing margins. Which has lead the pundits to exercise full and vigorous use of the M word. So there's still this question: does political momentum actually exist? And if it does, how does it operate? Columbia University sociologist Duncan Watts has spent years examining how trends propagate, and he's developed some groundbreaking ideas that are upsetting the traditional ways we've understood trends and marketing.

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