Emil Georg Cohn (28 September 1854 – 28 January 1944), was a German physicist.

Life

Emil Cohn was born in Neustrelitz, Mecklenburg, was the son of August Cohn (* 1826, lawyer for Neustrelitz) and Charlotte Cohn (1835 - 1924). At the age of 17, Emil Cohn had begun to study jurisprudence at the University of Leipzig. However, At the Ruprecht Karl University of Heidelberg and the University of Strasbourg he began to study physics. In Strasbourg, he graduated in 1879. From 1881 to 1884, he was an assistant of August Kundt at the physical institute. In 1884 he habilitated in theoretical physics and was admitted as a private-lecturer. From 1884 to 1918, he was a faculty-member of the university Strasbourg and was nominated as an assistant professor in 27 September 1884. He dealt with experimental-physics at first, and then turned completely to theoretical physics. In 1918 he was nominated as an extraordinary professor.

After the end of World War I and the occupation of Alsace-Lorraine by France, Cohn and his family were expelled (on the Christmas Eve 1918) from Strasbourg. In April 1919, he was nominated as a professor at the University of Rostock. From June 1920, he gave lectures about theoretical physics at the University of Freiburg. In 1935 he retired in Heidelberg where he lived until 1939. He resigned from the Deutsche Physikalische Gesellschaft (DPG) together with other physicists like Richard Gans, Leo Graetz, George Jaffé, Walter Kaufmann, in protest at the despotism of the NS-regime.

Cohn was a baptized Protestant and was married with Marie Goldschmidt (1864 - 1950), with whom he had two daughters. Because of his Jewish descent he found himself forced in 1939 under the pressure of the NS-regime to emigrate to Switzerland. He lived in Hasliberg-Hohfluh at first, and from 1942 in Ringgenberg, Switzerland, where he died at the age of 90.

Cohn’s younger brother Carl Cohn (1857-1931) was a successful overseas-merchant from Hamburg, who worked from 1921 until 1929 as a senator in Hamburg.

Work

At the change of the 19. to the 20. Century, Emil Cohn was one of the most respectable experts in the area of theoretical electrodynamics. He was unsatisfied with the Lorentzian theory of electrodynamics for moving bodies and proposed an independent theory. His alternative theory, which was based on a modification of the Maxwell field-equations, was compatible to all relevant electrodynamic and optical experiments, incl. the Michelson-Morley experiment.

Some of his insights entailed certain aspects of special relativity, in particular some aspects of the interpretation of the Lorentz transformation by. After Henri Poincaré but before Albert Einstein, he interpreted Lorentz's local time as the result of clock synchronisation by light signals (1904). However, he believed that mechanical clocks still show the "true" time. He also eliminated the luminiferous aether and replaced it like Ernst Mach by the fixed stars (1901). Because of internal failures of his theory (like different light speeds in different directions) his theory was superseded by Lorentz's and Einstein's.

Sources

Publications

• Das Elektromagnetische Feld - Vorlesungen über die Maxwell'sche Theorie, Leipzig 1900 (576 S.).
• (1901). "Über die Gleichungen des elektromagnetischen Feldes für bewegte Körper". Göttinger Nachrichten 74-99.
• Cohn, E. (1902) „Ueber die Gleichungen des elektromagnetischen Feldes für bewegte Körper“ / [|"On the equations of the electromagnetic field for moving bodies"] ; Annalen der Physik (Series 4), 7(1), 29-56; (but now as digitized German text, with parallel English translation using modern notation, plus extra references and footnotes).
• (1904). "Zur Elektrodynamik bewegter Systeme". Sitzungsberichte der Königlich Preußische Akademie der Wissenschaften 40 1294–1303, 1404-1416.
• Cohn, E. (1904) „Antikritisches zu Hrn. Wiens "Differentialgleichungen der Elektrodynamic für bewegte Körper"“ / |— 'Reply to the criticism in W.Wien's "Differential equations of the electrodynamics for moving bodies"'; Annalen der Physik (Series 4), 14(6), 208 — (but now as digitized German text, with parallel English translation, plus extra references and footnotes; — here attached to Cohn (1902) see above).
• „Physikalisches über Raum und Zeit“, Himmel und Erde XIII, 117 - 136 (1911); auch als Broschüre veröffentlicht: Physikalisches über Raum und Zeit, Berlin/Leipzig 1920, 4. Auflage (30 S.).
• Das elektromagnetische Feld - Ein Lehrbuch, 2. Auflage Berlin 1927 (366 S.).
• „Faraday und Maxwell“, Deutsches Museum - Abhandlungen und Berichte 4 (1), Berlin 1932 (29 S.).

See also

• History of special relativity