Pre-1940's

• 1895

  Wilhelm Roentgen discovers x-rays. The world immediately appreciates their medical potential. Within five years, for example, the British Army is using a mobile x-ray unit to locate bullets and shrapnel in wounded soldiers in the Sudan.

• 1898

  Marie Curie discovers the radioactive elements radium and polonium.

• 1905

  Albert Einstein develops theory about the relationship of mass and energy.

• 1911

  Georg von Hevesy conceives the idea of using radioactive tracers. This idea is later applied to, among other things, medical diagnosis. Von Hevesy wins the Nobel Prize in 1943.

• 1927

  Herman Blumgart, a Boston physician, first uses radioactive tracers to diagnose heart disease.

• December 1938

  Two German scientists, Otto Hahn and Fritz Strassman, demonstrate nuclear fission.

• August 1939

  Albert Einstein sends a letter to President Roosevelt informing him of German atomic research and the potential for a bomb. This letter prompts Roosevelt to form a special committee to investigate the military implications of atomic research.