How do fads spread? How many power substations need to fail for a grid to collapse? When does the stock market crash? Can we predict any of these before they happen?

The unsatisfying short answer: it’s complicated.

These seemingly disparate questions are actually one in the same and they form the basis of a physical model known as the Global Cascades Model. The earliest graph-theoretic models of this was proposed by the social scientist Duncan Watts in 2002.