Inevitably, Einstein’s fame and the great success of his theories created a backlash. The rising Nazi movement found a convenient target in relativity, branding it “Jewish physics” and sponsoring conferences and book burnings to denounce Einstein and his theories. The Nazis enlisted other physicists, including Nobel laureates Philipp Lenard and Johannes Stark, to denounce Einstein. One Hundred Authors Against Einstein was published in 1931. When asked to comment on this denunciation of relativity by so many scientists, Einstein replied that to defeat relativity one did not need the word of 100 scientists, just one fact.

In December 1932 Einstein decided to leave Germany forever (he would never go back). It became obvious to Einstein that his life was in danger. A Nazi organization published a magazine with Einstein’s picture and the caption “Not Yet Hanged” on the cover. There was even a price on his head. So great was the threat that Einstein split with his pacifist friends and said that it was justified to defend yourself with arms against Nazi aggression. To Einstein, pacifism was not an absolute concept but one that had to be reexamined depending on the magnitude of the threat.

Einstein settled at the newly formed Institute for Advanced Study at Princeton, New Jersey, which soon became a mecca for physicists from around the world. Newspaper articles declared that the “pope of physics” had left Germany and that Princeton had become the new Vatican.

Personal sorrow, World War II, and the atomic bomb

The 1930s were hard years for Einstein. His son Eduard was diagnosed with schizophrenia and suffered a mental breakdown in 1930. (Eduard would be institutionalized for the rest of his life.) Einstein’s close friend, physicist Paul Ehrenfest, who helped in the development of general relativity, committed suicide in 1933. And Einstein’s beloved wife, Elsa, died in 1936.

To his horror, during the late 1930s, physicists began seriously to consider whether his equation E = mc² might make an atomic bomb possible. In 1920 Einstein himself had considered but eventually dismissed the possibility. However, he left it open if a method could be found to magnify the power of the atom. Then in 1938–39 Otto Hahn, Fritz Strassmann, Lise Meitner, and Otto Frisch showed that vast amounts of energy could be unleashed by the splitting of the uranium atom. The news electrified the physics community.

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In July 1939 physicist Leo Szilard convinced Einstein that he should send a letter to U.S. President Franklin D. Roosevelt urging him to develop an atomic bomb. With Einstein’s guidance, Szilard drafted a letter on August 2 that Einstein signed, and the document was delivered to Roosevelt by one of his economic advisers, Alexander Sachs, on October 11. Roosevelt wrote back on October 19, informing Einstein that he had organized the Uranium Committee to study the issue.

Einstein was granted permanent residency in the United States in 1935 and became an American citizen in 1940, although he chose to retain his Swiss citizenship. During the war Einstein’s colleagues were asked to journey to the desert town of Los Alamos, New Mexico, to develop the first atomic bomb for the Manhattan Project. Einstein, the man whose equation had set the whole effort into motion, was never asked to participate. Voluminous declassified Federal Bureau of Investigation (FBI) files, numbering several thousand, reveal the reason: The U.S. government feared 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 was asked to help the U.S. Navy evaluate designs for future weapons systems. Einstein also helped the war effort by auctioning off priceless personal manuscripts. In particular, a handwritten copy of his 1905 paper on special relativity was sold for $6.5 million. It is now located in the Library of Congress.

Einstein was on vacation when he heard the news that an atomic bomb had been dropped on Japan. Almost immediately he was part of an international effort to try to bring the atomic bomb under control, forming the Emergency Committee of Atomic Scientists.

The physics community split on the question of whether to build a hydrogen bomb. J. Robert Oppenheimer, the director of the atomic bomb project, was stripped of his security clearance for having suspected leftist associations. Einstein backed Oppenheimer and opposed the development of the hydrogen bomb, instead calling for international controls on the spread of nuclear technology. Einstein also was increasingly drawn to antiwar activities and to advancing the civil rights of African Americans.

In 1952 David Ben-Gurion, Israel’s premier, offered Einstein the post of president of Israel. Einstein, a prominent figure in the Zionist movement, respectfully declined.

Increasing professional isolation and death

Although Einstein continued to pioneer many key developments in the theory of general relativity—such as wormholes, higher dimensions, the possibility of time travel, the existence of black holes, and the creation of the universe—he was increasingly isolated from the rest of the physics community. Because of the huge strides made by quantum theory in unraveling the secrets of atoms and molecules, the majority of physicists were working on the quantum theory, not relativity. In fact, Einstein would engage in a series of historic private debates with Niels Bohr, originator of the Bohr atomic model. Through a series of sophisticated “thought experiments,” Einstein tried to find logical inconsistencies in the quantum theory, particularly its lack of a deterministic mechanism. Einstein would often say that “God does not play dice with the universe.”

In 1935 Einstein’s most celebrated attack on the quantum theory led to the EPR (Einstein-Podolsky-Rosen) thought experiment. According to quantum theory, under certain circumstances two electrons separated by huge distances would have their properties linked, as if by an umbilical cord. Under these circumstances, if the properties of the first electron were measured, the state of the second electron would be known instantly—faster than the speed of light. This conclusion, Einstein claimed, clearly violated relativity. (Experiments conducted since then have confirmed that the quantum theory, rather than Einstein, was correct about the EPR experiment. In essence, what Einstein had actually shown was that quantum mechanics is nonlocal—i.e., random information can travel faster than light. This does not violate relativity, because the information is random and therefore useless.)

The other reason for Einstein’s increasing detachment from his colleagues was his obsession, beginning in 1925, with discovering 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. In his later years he stopped opposing the quantum theory and tried to incorporate it, along with light and gravity, into a larger unified field theory. Gradually Einstein became set in his ways. He rarely traveled far, 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. In 1950 he published an article on his theory in Scientific American, but because it neglected the still-mysterious strong force, it was necessarily incomplete. When he died five years later of an aortic aneurysm, it was still unfinished.