1938: The first nuclear fission of uranium is achieved by German scientists. This is the physical reaction that leads to the nuclear bomb.

1945: Nuclear bombs are dropped on the Japanese cities Hiroshima and Nagasaki, hastening the end of World War II by killing nearly two hundred thousand people.

Today: After decades of international fear about the devastation that nuclear weapons can cause, no other nuclear bombs have been used during wartime.

1938: Cancer is barely understood. The first cancer- causing agents, known as "carcinogens," have been isolated in England just five years before.

Today: Cancer is the number two cause of death in the United States, but millions of dollars are spent on research each year, and much progress has been made in understanding causes and treatments.

1938: The first steps in photocopy technology are made, as inventor Chester Carlson develops a method to reproduce an image...