Science – M. Mitchell Waldrop, Apr. 12, 2018: “…Known as National Planning Scenario 1 (NPS1), that nuclear attack story line originated in the 1950s as a kind of war game, a safe way for national security officials and emergency managers to test their response plans before having to face the real thing. Sixty years later, officials are still reckoning with the consequences of a nuclear catastrophe in regular NPS1 exercises. Only now, instead of following fixed story lines and predictions assembled ahead of time, they are using computers to play what-if with an entire artificial society: an advanced type of computer simulation called an agent-based model. Today’s version of the NPS1 model includes a digital simulation of every building in the area affected by the bomb, as well as every road, power line, hospital, and even cell tower. The model includes weather data to simulate the fallout plume. And the scenario is peopled with some 730,000 agents—a synthetic population statistically identical to the real population of the affected area in factors such as age, sex, and occupation. Each agent is an autonomous subroutine that responds in reasonably human ways to other agents and the evolving disaster by switching among multiple modes of behavior—for example, panic, flight, and efforts to find family members. The point of such models is to avoid describing human affairs from the top down with fixed equations, as is traditionally done in such fields as economics and epidemiology. Instead, outcomes such as a financial crash or the spread of a disease emerge from the bottom up, through the interactions of many individuals, leading to a real-world richness and spontaneity that is otherwise hard to simulate…”