Presumably they're using euclidean distance to scale down the brightness of long-distance links -- to ensure that long-distance relationships don't overpower sh

Presumably they're using euclidean distance to scale down the brightness of long-distance links -- to ensure that long-distance relationships don't overpower short ones in the visualization. Otherwise, a pair of friends thousands of miles apart would be contributing the same "energy" to the image as a thousand pairs a few miles apart... You'd never be able to see local detail in the areas heavily crossed by global connections (e.g. the Caribbean).