Only three days later, the United States dropped a second atomic bomb on Nagasaki.
He wasn't about to believe Death until he saw Katie for himself. A minute after midnight on the fourth day, if Katie wasn't standing beside him, he'd need the knowledge of where to set off the atomic bomb within him.
The answer to the question has much to do with the question of why the Germans never tried to make an atomic bomb.
The play is set in June 1945, a month before the testing of the first atomic bomb.
Coincidentally, he also assisted in building the first atomic bomb.