Physics

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. ...

Do you trust your calculations? This book review is about The Los Alamos Primer, the first lectures on how to build an atomic bomb, by Robert Serber. The review focuses on the basic calculations and physics knowledge of the Manhattan Project scientists. Since the Primer is a collection of 1943 lecture notes combined with Serber’s hindsight commentary, it corrects some of the original calculations and measurements by comparing them with modern values. This makes it fascinating to see how close—or off—the physicists were at the time. For anyone interested in the topic, I highly recommend reading both the Primer and The Making of the Atomic Bomb by Richard Rhodes, one of my all-time favorite books. ...