In December, 1932, Einstein decided to leave Germany forever. He took a position a the newly formed Institute for Advanced Study at Princeton, New Jersey, which soon became a Mecca for physicists from around the world. It was here that he would spend the rest of his career trying to develop a unified field theory—an all-embracing theory that would unify the forces of the universe, and thereby the laws of physics, into one framework—and refute the accepted interpretation of quantum physics. Other European scientists also fled various countries threatened by Nazi takeover and came to the United States. Some of these scientists knew of Nazi plans to develop an atomic weapon. For a time, their warnings to Washington, D.C. went unheeded.
In the summer of 1939, Einstein, along with another scientist, Leo Szilard, was persuaded to write a letter to President Franklin D. Roosevelt to alert him of the possibility of a Nazi bomb. President Roosevelt could not risk the possibility that Germany might develop an atomic bomb first. The letter is believed to be the key factor that motivated the United States to investigate the development of nuclear weapons. Roosevelt invited Einstein to meet with him and soon after the United States initiated the Manhattan Project.
Not long after he began his career at the Institute in New Jersey, Albert Einstein expressed an appreciation for the "meritocracy" of the United States and the right people had to think what they pleased—something he didn’t enjoy as a young man in Europe. In 1935, Albert Einstein was granted permanent residency in the United States and became an American citizen in 1940. As the Manhattan Project moved from drawing board to testing and development at Los Alamos, New Mexico, many of his colleagues were asked to develop the first atomic bomb, but Einstein was not one of them. According to several researchers who examined FBI files over the years, the reason was the U.S. government didn't trust Einstein's lifelong association with peace and socialist organizations. FBI director J. Edgar Hoover went so far as to recommend that Einstein be kept out of America by the Alien Exclusion Act, but he was overruled by the U.S. State Department. Instead, during the war, Einstein helped the U.S. Navy evaluate designs for future weapons systems and contributed to the war effort by auctioning off priceless personal manuscripts. One example was a handwritten copy of his 1905 paper on special relativity which sold for $6.5 million, and is now located in the Library of Congress.
On August 6, 1945, while on vacation, Einstein heard the news that an atomic bomb had been dropped on Hiroshima, Japan. He soon became involved in an international effort to try to bring the atomic bomb under control, and in 1946, he formed the Emergency Committee of Atomic Scientists with physicist Leo Szilard. In 1947, in an article that he wrote forThe Atlantic Monthly, Einstein argued that the United States should not try to monopolize the atomic bomb, but instead should supply the United Nations with nuclear weapons for the sole purpose of maintaining a deterrent. At this time, Einstein also became a member of the National Association for the Advancement of Colored People. He corresponded with civil rights activist W.E.B. Du Bois and actively campaigned for the rights of African Americans.
After the war, Einstein continued to work on many key aspects of the theory of general relativity, such as wormholes, the possibility of time travel, the existence of black holes, and the creation of the universe. However, he became increasingly isolated from the rest of the physics community. With the huge developments in unraveling the secrets of atoms and molecules, spurred on by the development to the atomic bomb, the majority of scientists were working on the quantum theory, not relativity. Another reason for Einstein's detachment from his colleagues was his obsession with discovering his unified field theory. In the 1930s, Einstein engaged in a series of historic private debates with Niels Bohr, the originator of the Bohr atomic model. In a series of "thought experiments," Einstein tried to find logical inconsistencies in the quantum theory, but was unsuccessful. However, in his later years, he stopped opposing quantum theory and tried to incorporate it, along with light and gravity, into the larger unified field theory he was developing.
In the last decade of his life, Einstein withdrew from public life, rarely traveling far and confining himself to long walks around Princeton with close associates, whom he engaged in deep conversations about politics, religion, physics and his unified field theory.