Albert Einstein is best known for his theory of relativity and specifically mass=energy equivalence. Einstein's many contributions to physics include his special theory of relativity,
which reconciled mechanics with electromagnetism, and his general theory of relativity, which extended the principle of relativity to non-uniform motion, creating a new theory of gravitation. His other contributions include relativistic cosmology, capillary action, critical opalescence, classical problems of statistical mechanics and their application to quantum theory, an explanation of the Brownian movement of molecules, atomic transition probabilities, the quantum theory of a monatomic gas, thermal properties of light with low radiation density (which lay the foundation for the photon theory), a theory of radiation including stimulated emission, the conception of a unified field theory, and the geometrization of physics.
The Manhattan ProjectConcerned scientists, many of them refugees from European anti-Semitism in the U.S. recognized the danger of German scientists developing an atomic bomb based on the newly-discovered phenomena of nuclear fission. In 1939,the Hungarian emigre Leo Szilard, having failed to arouse U.S. government interest on his own, worked with Einstein to write a letter to U.S. President Franklin Delano Roosevelt, which Einstein signed, urging U.S. development of such a weapon. In August 1939, Roosevelt received the Einstein-Szilard letter and authorized secret research into the harnessing of nuclear fission for military purposes.
The Cold War eraWhen he was a visible figure working against the rise of Nazism, Einstein sought help and developed working relationships with the West and what was to become the soviet bloc. After World War II, enmity between the former allies became a very serious issue for people with international resumes. To make things worse, during the first days of McCarthyism Einstein was writing about a single world government; it was at this time that he wrote, " I do not know that the third World War will be fought , but I can tell you what they will use in the Fourth --rocks!" In a 1949 Monthly Review article entitled "Why Socialism?" Albert Einstein described chaotic capitalist society, a source of evil to be overcome, as the "predatory phase of human development". With Albert Schweitzer and Bertrand Russell, Einstein lobbied to stop nuclear testing and future bombs. Days before his death, Einstein signed the Russell-Einstein Manifesto, which led to the Pugwash Conferences on Science and World Affairs.
