In Princeton Einstein set a pattern that was to vary little for more than 20 years. He lived with his wife in a simple, two-story frame house and most mornings walked a mile or so to the Institute, where he worked on his unified-field theory and talked with colleagues. For relaxation he played his violin and sailed on a local lake. Only rarely did he travel, even to New York. In a letter to Queen Elisabeth of Belgium, he described his new refuge as a "wonderful little spot, . . . a quaint and ceremonious village of puny demigods on stilts." Eventually he acquired American citizenship, but he always continued to think of himself as a European. Pursuing his own line of theoretical research outside the mainstream of physics, he took on an air of fixed serenity. "Among my European friends, I am now called Der grosse Schweiger ("The Great Stone Face"), a title I well deserve," he said. Even his wife's death late in 1936 did not disturb his outward calm. "It seemed that the difference between life and death for Einstein consisted only in the difference between being able and not being able to do physics," wrote Leopold Infeld, the Polish physicist who arrived in Princeton at this time.
Niels Bohr, the great Danish atomic physicist, brought news to Einstein in 1939 that the German refugee physicist Lise Meitner had split the uranium atom, with a slight loss of total mass that had been converted into energy. Meitner's experiments, performed in Copenhagen, had been inspired by similar, though less precise, experiments done months earlier in Berlin by two German chemists, Otto Hahn and Fritz Strassmann. Bohr speculated that, if a controlled chain-reaction splitting of uranium atoms could be accomplished, a mammoth explosion would result. Einstein was skeptical, but laboratory experiments in the United States showed the feasibility of the idea. With a European war regarded as imminent and fears that Nazi scientists might build such a "bomb" first, Einstein was persuaded by colleagues to write a letter to President Franklin D. Roosevelt urging "watchfulness and, if necessary, quick action" on the part of the United States in atomic-bomb research. This recommendation marked the beginning of the Manhattan Project.
Although he took no part in the work at Los Alamos, New Mexico, and did not learn that a nuclear-fission bomb had been made until Hiroshima was razed in 1945, Einstein's name was emphatically associated with the advent of the atomic age. He readily joined those scientists seeking ways to prevent any future use of the bomb, his particular and urgent plea being the establishment of a world government under a constitution drafted by the United States, Britain, and Russia. With the spur of the atomic fear that haunted the world, he said "we must not be merely willing, but actively eager to submit ourselves to the binding authority necessary for world security." Once more, Einstein's name surged through the newspapers. Letters and statements tumbled out of his Princeton study, and in the public eye Einstein the physicist dissolved into Einstein the world citizen, a kind "grand old man" devoting his last years to bringing harmony to the world.
The rejection of his ideals by statesmen and politicians did not break him, because his prime obsession still remained with physics. "I cannot tear myself away from my work," he wrote at the time. "It has me inexorably in its clutches." In proof of this came his new version of the unified field in 1950, a most meticulous mathematical essay that was immediately but politely criticized by most physicists as untenable.
Compared with his renown of a generation earlier, Einstein was virtually neglected and said himself that he felt almost like a stranger in the world. His health deteriorated to the extent that he could no longer play the violin or sail his boat. Many years earlier, chronic abdominal pains had forced him to give up smoking his pipe and to watch his diet carefully.On April 18, 1955, Einstein died in his sleep at Princeton Hospital. On his desk lay his last incomplete statement, written to honour Israeli Independence Day. It read in part: "What I seek to accomplish is simply to serve with my feeble capacity truth and justice at the risk of pleasing no one." His contribution to man's understanding of the universe was matchless, and he is established for all time as a giant of science. Broadly speaking, his crusades in human affairs seem to have had no lasting impact. Einstein perhaps anticipated such an assessment of his life when he said, "Politics are for the moment. An equation is for eternity."