Emil Cohn (physicist)

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Emil Georg Cohn (born September 28, 1854 in Neustrelitz ; † January 28, 1944 in Ringgenberg , Canton of Bern , Switzerland ) was a German physicist .

Life

Emil Cohn was a son of the Neustrelitz lawyer August Cohn (* 1826) and his wife Charlotte, née Hahn (1835-1924). As a 17-year-old Emil Cohn first started to study law at the University of Leipzig . However, he soon turned to the natural sciences and continued his studies at the universities of Heidelberg and Strasbourg. In Strasbourg he received his doctorate in 1879. phil. nat. From 1881 to 1884 he was August Kundt's assistant at the Physics Institute. On February 5, 1884, he completed his habilitation in theoretical physics and was admitted as a private lecturer. From 1884 to 1918 he was a faculty member at the University of Strasbourg . On September 27, 1884, he was appointed assistant professor. At first he also dealt with experimental physics, but then turned completely to theoretical physics. In 1918 he was appointed associate professor.

After the end of the First World War and the occupation of Alsace-Lorraine by France, Cohn and his family were expelled from Strasbourg (on Christmas Eve 1918). In April 1919 he was appointed full honorary professor at the University of Rostock . From June 1920 he held lectures on theoretical physics as a full honorary professor at the University of Freiburg im Breisgau . In 1935 he retired in Heidelberg, where he lived until 1939. In 1938, under the impression of the arbitrariness of the Nazi regime, together with Richard Gans , Leo Graetz , George Jaffé , Walter Kaufmann and other physicists of “Israelite descent” to Peter Debye, he demonstratively declared his departure from the German Physical Society (DPG).

Cohn was baptized as a Protestant and married to Marie Goldschmidt (1864–1950), with whom he had two daughters. Because of his "Israelite descent" he was forced to emigrate to Switzerland in 1939 under the pressure of the Nazi dictatorship. He lived there first in Hasliberg -Hohfluh, then from 1942 in Ringgenberg on Lake Brienz .

Cohn's younger brother Carl Cohn (1857–1931) was a successful overseas merchant from Hamburg , who served as Hamburg's senator from 1921–1929 .

plant

At the turn of the 19th and 20th centuries, Cohn was one of the most respected experts in the field of theoretical electrodynamics. He was dissatisfied with the Lorentzian electrodynamics of Hendrik Antoon Lorentz for moving bodies and proposed his own theory. His alternative theory, which was based on a modification of Maxwell's macroscopic field equations without taking into account the atomic structure of matter, was in agreement with all relevant electrodynamic and optical experiments at the time, including the Michelson-Morley experiment . He discarded the Lorentz contraction , but took over the concept of local time from Lorentz. Cohn's theory was further developed by Richard Gans in 1905 and applied to electron theory.  

In 1900 he used the term "Lorentz transformation", where Henri Poincaré finally 1905 until today common term Lorentz transformation used. Some of his insights anticipated certain aspects of Albert Einstein's Special Theory of Relativity (SRT), for example the renunciation of the concept of the “ light ether ”, which he replaced with the fixed stars as a reference system (1901). Cohn restricted that it might be heuristically useful, for reasons of clarity, to imagine the fixed system as resting in a substantial “ether”, but this is only visual language that does not add anything to his theory. And like Poincaré, he interpreted Lorentz's local time in 1904 as the time that arises under the assumption of a constant speed of light - whereby Cohn coined the picture according to which light propagates in all systems as a spherical wave. Even before Einstein, he illustrated effects such as Lorentz contraction and time dilation with measuring rods and clocks. He also noted that, at least on the basis of Lorentz's theory, the distinction between universal and local time seems artificial, since no experimental distinction between them can be made in this theory. However, Cohn himself believed that Lorentz's assumption is only valid for optical effects anyway, while according to his own theory, mechanical clocks could display the "absolute" time.

However, due to internal discrepancies, Cohn's theory could not prevail against the theories of Lorentz and Einstein. His theory contradicted the principle of reaction as well as the principle of relativity . Cohn also believed that the speed in relation to the fixed star background was an absolute speed. In addition, his theory predicted different results for the Michelson-Morley experiment depending on direction in certain media. Cohn himself later (1911) accepted the "Relativity Principle of Lorentz and Einstein" and wrote a description of the SRT, which Einstein agreed with.

See also

Publications

  • Emil Cohn: Faraday and Maxwell . In: Deutsches Museum - Treatises and Reports . 4, No. 1, 1932.

literature

Web links

Wikisource: Emil Cohn  - Sources and full texts

Individual evidence

  1. ^ Fritz Emde : Obituary for Emil Cohn; Archive of Electrical Transmission (AEÜ) 1 (1/2), 1947, pp. 81–83 (with portrait of Emil Cohn)
  2. ^ AI Miller: Albert Einstein's special theory of relativity. Emergence (1905) and early interpretation (1905-1911) . Addison-Wesley, Reading 1981, ISBN 0-201-04679-2 , pp. 191-182.
  3. ^ O. Darrigol: Emil Cohn's electrodynamics of moving bodies . In: American Journal of Physics . 63, No. 10, 1995, pp. 908-915.
  4. ^ M. Janssen, J. Stachel: The Optics and Electrodynamics of Moving Bodies . In: Max Planck Institute for the History of Science . 2004.
personal data
SURNAME Cohn, Emil
ALTERNATIVE NAMES Cohn, Emil Georg
BRIEF DESCRIPTION German physicist
DATE OF BIRTH September 28, 1854
PLACE OF BIRTH Neustrelitz
DATE OF DEATH January 28, 1944
Place of death Ringgenberg , Canton of Bern, Switzerland