On Aug. 2, 1939, Albert Einstein sent a letter to President Franklin Roosevelt about the work of his colleagues, physicists Enrico Fermi and Leó Szilard. Einstein wrote that the element uranium might be turned into a new source of energy – a term we know today as a “nuclear reaction.” And, he explained to the president, with the right construction, this chain reaction could be transformed into a new, “extremely powerful” bomb.