Albert Einstein and Niels Bohr smoking, c 1920.

Physicists Einstein (1879-1955) (right), and Bohr (1885-1962) both worked on quantum theory. Einstein developed his theory of relativity between 1900 and 1916 and received the Nobel Prize for Physics in 1921. Because of his political stand against the Nazis, Einstein had to leave Germany during the Nazi period, and went to live in the United States of America, taking citizenship. Bohr worked on the electronic structure of atoms, developing the 'correspondence principle' (1916) and the 'complementary principle' (1927). By 1939, he was working on nuclear fission with G V Wheeler. After the war, Bohr joined the atomic bomb programme in the USA but also organised the first Peace Conference in Geneva. Bohr was awarded the Nobel Prize in 1922.