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In 1920 Einstein was appointed to a lifelong honorary visiting professorship at the University of Leiden. During 1921-1922 Einstein, accompanied by Chaim Weizmann, the future president of the state of Israel, undertook exten­sive worldwide travels in the cause of Zionism. In Germany the attacks on Einstein began. Philipp Lenard and Johannes Stark, both Nobel Prize-winning physicists, began charac­terizing Einstein's theory of relativity as "Jewish phys­ics."This callousness and brutality increased until Einstein resigned from the Prussian Academy of Science in 1933. (He was, however, expelled from the Bavarian Academy of Science.

Career in America

On several occasions Einstein had visited the California Institute of Technology, and on his last trip to the United States Abraham Flexner offered Einstein-on Einstein's terms-a position in the newly conceived and funded Insti­tute for Advanced Studies in Princeton. He went there in 1933.

Einstein played a key role (1939) in mobilizing the resources necessary to construct the atomic bomb by sign­ing a famous letter to President Franklin D. Roosevelt which had been drafted by Leo Szilard and E.P. Wigner. When Einstein's famous equation E= me! was finally demon­strated in the most awesome and terrifying way by using the bomb to destroy Hiroshima in 1945, Einstein, the pacifist and humanitarian, was deeply shocked and distressed; for a long time he could only utter "Horrible, horrible." On April 18, 1955, Einstein died in Princeton.

Theory of Brownian Motion

From numerous references in Einstein's writings It IS evident that, of all areas in physics, thermodynamics made the deepest impression on him. During 1902-1904 Einstein reworked the foundations of thermodynamics and statistical mechanics; this, work formed the immediate background to his revolutionary papers of 1905, one of which was on Brownian -motion.

In Brownian motion (first observed in 1827 by the Scottish botanist Robert Brown), small particles suspended in a viscous liquid such as water undergo a rapid, irregular motion. Einstein, unaware of Brown's earlier observations, concluded from his theoretical studies that such a motion must exist. Guided by the thought that if the liquid in which the particles are suspended consists of atoms or molecules they should collide with the particles and set them into motion, he found that while the particle's motion is irregu­lar, fluctuating back and forth, it will in time nevertheless experience a net forward displacement. Ei nstei n proved that -this net forward displacement of the suspended particles is directly related to the number of molecules per gram atomic weight. This point created a good deal of skepticism toward Einstein's theory at the time he developed it (1905-1906), but when it was fully confirmed many of the skeptics were converted. Brownian motion is to this day regarded as one of the most direct proofs of the existence of atoms.

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