On a mid-July day in 1939, Albert Einstein, still in his slippers, opened the door of his summer cottage in Peconic on the fishtail end of Long Island. There stood his former student and onetime partner in an electromagnetic refrigerator pump, the Hungarian physicist Leo Szilard, and next to him a fellow Hungarian (and fellow physicist), Eugene Wigner. The two had not come to Long Island for a day at the beach with the most famous scientist in the world but on an urgent mission.