U.S. Department of Energy · Public domain

Conceived the nuclear chain reaction in 1933 and patented it three years later, then in 1939 drafted the letter that carried Albert Einstein's signature to Franklin Roosevelt and set the Manhattan Project in motion. Having helped start the bomb program, he tried to stop its use on people: his 1945 petition urged Truman to demonstrate the weapon rather than drop it on civilians. He also filed some of the earliest patents for the electron microscope and the cyclotron, and was among the first to connect thermodynamics with information theory.