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**"PROF. EPSTEIN IS CERTAINLY ONE OF THE MOST PROMINENT LIVING THEORETICAL PHYSICISTS OF THE GERMAN-SPEAKING WORLD": VERY RARE AND FASCINATING AUTOGRAPH LETTER OF RECOMMENDATION WRITTEN AND SIGNED BY EINSTEIN, LAUDING THE ACCOMPLISHMENTS OF HIS FRIEND AND COLLEAGUE, RUSSIAN-AMERICAN MATHEMATICAL PHYSICIST PAUL EPSTEIN, WITH DISCUSSION OF QUANTUM PHYSICS AND ATOMIC THEORY**

EINSTEIN, Albert. **Autograph letter signed. **No place, August 14, 1921. Single sheet of cream lined paper, measuring 7-1/2 by 11 inches; p. 1. $35,000.

*Rare heartfelt autograph letter of recommendation written and signed by Einstein in German, enthusiastically recommending his friend and colleague, physicist Prof. Dr. Paul Epstein, for an academic position.*

The autograph letter, dated "14 VIII 21," written entirely in Einstein's hand reads (translated from the original German): "Prof. Dr. Epstein is certainly one of the most prominent living theoretical physicists of the German-speaking world. Without a doubt, he would have been appointed to a German professorship a long time ago, had his Russian nationality not stood in the way. Among Epstein's numerous original scientific papers, two findings, which advanced the modern quantum theory in crucial ways, should be noted. After Mr. Sommerfeld, as the first physicist who, on the basis of special hypotheses, had applied the quantum theory to a certain mechanical system of more than one degree of freedom, Mr. Epstein discovered an important generalization of the quantum principle, which established the application of the quantum theory for all quasi-periodic mechanical systems. Based on that general application of the quantum principle, he then provided an analysis of the splitting of spectral lines in the electrical field (Stark effect), the accordance of which with the experiment provides one of the strongest supports for the Rutherford-Bohr atomic theory. I would like to add that I have also come to appreciate Mr. Epstein in personal interactions as a human being, and that I had the pleasure of attending several scientific lectures given by him, which enabled me to convince myself of his competence in delivering clearly understandable oral exposition. / A. Einstein."

Einstein and Epstein were friends and longtime correspondents who shared an interest in physics, Judaism, and the founding of Israel. Paul Epstein was a Russian-American mathematical physicist. He remains best known for his contributions to the development of quantum mechanics. Indeed, he was one of a select group that included Lorentz, Einstein, Minkowski, Thomson, Rutherford, Sommerfeld, Röntgen, von Laue, Bohr, de Broglie, Ehrenfest, and Schwarzschild. Born in Warsaw, then part of Imperial Russia, Epstein was brought up solidly middle class. He later stated that his mother recognized his potential at the age of four and predicted his future as a mathematician. Epstein studied mathematics and physics for his entire university career, eventually earning a degree from the Imperial University of Moscow. He then went on to earn a Ph.D. at the Technical University of Munich in 1914, concentrating on a problem in the theory of diffraction of electromagnetic waves. However, the outbreak of World War I rendered Epstein an enemy alien in Germany. Sommerfeld intervened on his behalf and he was allowed to stay as a private citizen and continue his research. In 1916, Epstein published an important paper explaining the Stark Effect using the Bohr-Sommerfeld quantum theory. After the war, Epstein went to Leiden and worked as an assistant for Lorentz and Ehrenfest. In 1921—the year this letter was written—Epstein was recruited by Robert Millikan to join the physicists at the California Institute of Technology. Epstein accepted the position and stayed there for the rest of his career, publishing extensively on quantum theory. Epstein was something of polymath and worked in numerous areas outside of quantum theory including work on air resistance, the settling of gasses, the theory of vibration, and the absorption of sound. He was an avid supported of Freudian psychoanalysis (including as one of the founding members of the Psychoanalytic Study Group that later merged with the Los Angeles Institute for Psychoanalysis). Epstein was also notably anti-communist and worried about the threat of nationalism.

The areas of study mentioned in Einstein's letter of recommendation all came together to help form the science behind atomic and hydrogen bombs, though neither Einstein nor Epstein anticipated quite where the science was headed in 1921. The letter mentions the Stark effect, which is the shifting and splitting of spectral lines of atoms and molecules due to the presence of an external electric field. It is analogous to the Zeeman effect (in which a magnetic field is the influence). The Rutherford-Bohr model, presented in 1913, is a system consisting of a small, dense nucleus surrounded by orbiting electrons—somewhat like the Solar System, but with electrostatic forces instead of gravity. The Bohr model came to be recognized as a relatively primitive model of the hydrogen atom compared to the valence shell atom model. However, because of its simplicity and the correct results it generates for certain systems, it is still commonly used to introduce students to quantum mechanics.

Overall, this letter provides valuable insight into the scientific world during the height of Einstein's international career, right when he first began traveling abroad and meeting fellow scientists internationally. The letter reflects Einstein's importance in the community and is a testament to Epstein's ability as a physicist.

Original mailing creases. Fine condition.