THE atomic bomb the United States dropped on Hiroshima on Aug. 6, 1945 killed tens of thousands and flattened the Japanese city in an instant. "Little Boy," as it was known, was the endpoint of years of research, wrangling a physics theory into a mechanism that would release the energy that binds together atoms. The concept was simple: driving together enough uranium or plutonium at high enough speeds would create a "critical mass" so quickly that it would start an uncontrolled, nearly instantaneous chain reaction of neutrons knocking apart atomic nuclei. Click https://tmsnrt.rs/2Pk2iKB to see an interactive graphic about developing…