Criticality

Critical Mass, Length, Temperature Many systems and processes—whether in nature or society—can appear unchanging until they reach a certain transition point, after which their behavior shifts dramatically. Think of disease outbreaks, the spread of information in a network, or battery failures in electric vehicles. Let’s go through some examples. Disease Outbreaks Consider a disease outbreak such as COVID-19 in 2020. The goal of many governments was not just to reduce infections slightly, but to bring the average number of new infections per case—known as the reproduction number, or $R_0$—below the critical threshold of 1. When $R_0 < 1$, each infected person, on average, transmits the disease to fewer than one other person, and the outbreak dies out. However, if $R_0 > 1$ even by a small margin, the number of infections can grow exponentially. ...