It began as a light-hearted discussion over a beer, but ended in a profound revelation. Paul Ormerod, an economist at Volterra Consulting in London, was talking about his latest work on the science of networks, such as the internet and groups of friends. "That's interesting," said his friend Andrew Roach, a medieval historian at the University of Glasgow. "That sounds rather like what I'm working on." It was quite a claim for Roach to make. He studies the persecution of medieval heretics -people in Catholic countries who rejected, among other things, the unquestionable authority of the pope. But his comment sparked the pair into exploring the issue further, and it seems that Roach was right. In the 13th century, Catholic inquisitors halted the spread of heresy by exploiting insights that look remarkably similar to the science we now use to describe networks as diverse as social structures, the spread of disease and the web.
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