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Albert Einstein autograph letter signed, an unusual letter of reference for fellow physicist Dr. Paul Sophus Epstein. August 14, 1921, 1 p. Expected paper folds. An extra wrinkle near the top edge, and two tiny chips along the top at center are mentioned just for accuracy. Else near fine. 7.5" x 11".

Albert Einstein scientific letter discussing “advanced and modern quantum theory …the splitting of spectral lines in electrical field (Stark effect)…the Rutherford-Bohr atomic theory”. All these relate to the Theory of Relativity and contribute to the science of the atomic and hydrogen bombs.

Einstein writes in German, translated in full:

''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 establishes 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.''

The Stark effect is the shifting and splitting of spectral lines of atoms and molecules due to the presence of an external electric field. It is the electric-field analogue of the Zeeman effect, where a spectral line is split into several components due to the presence of the magnetic field. Although initially coined for the static case, it is also used in the wider context to describe the effect of time-dependent electric fields. In particular, the Stark effect is responsible for the pressure broadening (Stark broadening) of spectral lines by charged particles in plasmas. For most spectral lines, the Stark effect is either linear (proportional to the applied electric field) or quadratic with a high accuracy.

The Stark effect can be observed both for emission and absorption lines. The latter is sometimes called the inverse Stark effect, but this term is no longer used in the modern literature.

In atomic physics, the Rutherford–Bohr model or Bohr model, presented by Niels Bohr and Ernest Rutherford in 1913, is a system consisting of a small, dense nucleus surrounded by orbiting electrons—similar to the structure of the Solar System, but with attraction provided by electrostatic forces in place of gravity. After the cubic model (1902), the plum-pudding model (1904), the Saturnian model (1904), and the Rutherford model (1911) came the Rutherford–Bohr model or just Bohr model for short (1913). The improvement to the Rutherford model is mostly a quantum physical interpretation of it. The model's key success lay in explaining the Rydberg formula for the spectral emission lines of atomic hydrogen. While the Rydberg formula had been known experimentally, it did not gain a theoretical underpinning until the Bohr model was introduced. Not only did the Bohr model explain the reason for the structure of the Rydberg formula, it also provided a justification for its empirical results in terms of fundamental physical constants.

The Bohr model is a relatively primitive model of the hydrogen atom, compared to the valence shell atom model. As a theory, it can be derived as a first-order approximation of the hydrogen atom using the broader and much more accurate quantum mechanics and thus may be considered to be an obsolete scientific theory. However, because of its simplicity, and its correct results for selected systems (see below for application), the Bohr model is still commonly taught to introduce students to quantum mechanics or energy level diagrams before moving on to the more accurate, but more complex, valence shell atom. A related model was originally proposed by Arthur Erich Haas in 1910 but was rejected. The quantum theory of the period between Planck's discovery of the quantum (1900) and the advent of a mature quantum mechanics(1925) is often referred to as the old quantum theory.

Paul Sophus Epstein (Warsaw, then part of Imperial Russia, now Poland, March 20, 1883 – Pasadena, February 8, 1966) was a Russian-American mathematical physicist. He was known for his contributions to the development of quantum mechanics, part of a group that included Lorentz, Einstein, Minkowski, Thomson, Rutherford, Sommerfeld, Röntgen, von Laue, Bohr, de Broglie, Ehrenfest and Schwarzschild.

Paul Epstein's parents, Siegmund Simon Epstein and Sarah Sophia (Lurie) Epstein were of a middle class Jewish family. He said that his mother recognized his potential at the age of four years and predicted that he would be a mathematician. He went to the Hochschule in Minsk, and from 1901-1905 studied mathematics and physics at the Imperial University of Moscow under Pyotr Nikolaevich Lebedev. In 1909 he graduated, and became a Privatdozent at the University of Moscow. In 1910 he went to Munich, Germany, to do research under Arnold Sommerfeld, who was his advisor, and Epstein was granted a Ph.D. on a problem in the theory of diffraction of electromagnetic waves.[1] from the Technische Universität München, in 1914.[2] At the outbreak of World War I he was in Munich, and considered an "enemy alien". Thanks to Sommerfeld's intervention he was allowed to stay in Munich as a private citizen, and could continue with his research. By that time Epstein became interested in the quantum theory of atomic structure. In 1916, he published a seminal paper explaining the Stark effect using the Bohr Sommerfeld ("old") quantum theory.[3] After the war he went to Leiden, to become Hendrik Lorentz' and Paul Ehrenfest's assistant. In 1921 he was recruited by Robert Millikan to come to the California Institute of Technology, where he remained for the rest of his career. In 1922 he published 3 papers on Bohr's quantum mechanics in the Zeitschrift für Physikand one in the Physical Review. In 1926 he published the paper "The Stark Effect from the Point of View of Schrodinger's Quantum Theory" in the Physical Review, as well as "The New Quantum Theory and the Zeeman Effect" (1926), and "The Magnetic Dipole in Undulatory Mechanics" (1927).

In 1920, Epstein calculated the effect of Polflucht as a possible causing force of Wegener's continental drift. His value was about 10^-6 of the gravitational force. Some years later other geophysicists could show that the Polflucht force is far too weak to cause plate tectonics. The toughness of the sublayers of the Earth's crust is much stronger than assumed by Epstein.  In 1930, he was elected a member of the National Academy of Sciences.

In 1930, Epstein married Alice Emelie Ryckman, and the couple had a daughter Sari (Mittelbach). After World War II, concerned that some young American intellectuals were becoming attracted to communism, he joined the American Committee for Cultural Freedom and in 1951 he served as one of the three US delegates to the seminar the Congress held in Strasbourg.

As well as quantum theory, Epstein also published papers in other fields. Examples include ""Zur Theorie des Radiometers" (1929), "Reflection of Waves in an Inhomogeneous Absorbing Medium" (1930) and "On the Air Resistance of Projectiles" (1931). Other subjects that he worked on were the settling of gases in the atmosphere, the theory of vibrations of shells and plates and the absorption of sound in fogs and suspensions. He retired as Emeritus Professor at Caltech in 1953 and continued to serve as a consultant for a number of industrial companies. One of the significant reports he wrote during this period was "Theory of Wave Propagation in a Gyromagnetic Medium" (1956).

Epstein had other interests as well. He was very interested in Freudian psychology and was one of the founding members of a Psychoanalytic Study Group (together with Thomas Libbin) that later merged into the Los Angeles Institute for Psychoanalysis. In the 1930s he published two articles in the monthly literary and scientific magazine Reflex - "The Frontiers of Science" and "Uses and Abuses of Nationalism". Although he was not an active Zionist, Epstein served as a member of the American National Society of Friends of the Hebrew University and was very friendly with the prominent Israeli mathematician Abraham Fraenkel. The Epstein Memorial Fund was established through donations from more than fifty of his former students and a bronze bust of Epstein stands in the Physics and Mathematics section of the Millikan library at Caltech.

This item comes with a Certificate from John Reznikoff, a premier authenticator for both major 3rd party authentication services, PSA and JSA (James Spence Authentications), as well as numerous auction houses.

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