At 8:16 a.m. on August 6, 1945, a fission weapon containing sixty-four kilograms of uranium detonated 580 meters above the Japanese city of Hiroshima, and Einstein’s equation proved mercilessly accurate. The bomb itself was extremely inefficient: just one kilogram of the uranium underwent fission, and only seven hundred milligrams of mass—the weight of a butterfly—was converted into energy. But it was enough to obliterate an entire city in a fraction of a second. Some seventyeight thousand people died instantly, or immediately afterward—vaporized, crushed, or incinerated in the firestorm that followed the blast wave. But by the end of the year, another twenty-five thousand men, women, and children would also sicken and die from their exposure to the radiation liberated by the world’s first atom bomb attack.