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{{Infobox scientist
| name = Emil Cohn
| caption = Emil Georg Cohn
| birth_date = {{Birth date|1854|8|28|mf=y}}
| birth_place = [[Neustrelitz]], [[Mecklenburg]]
| death_date = {{death date and age|1944|1|28|1854|8|28|mf=y}}
| death_place = [[Ringgenberg]], [[Switzerland]]
| field = [[Physicist]]
| work_institution = [[University of Freiburg]]
| alma_mater = [[University of Strasbourg]]
| doctoral_advisor = [[August Kundt]]
| known_for = theoretical [[electromagnetism]]
}}
'''Emil Georg Cohn''' (28 September 1854 – 28 January 1944), was a [[:Category:German physicists|German physicist]].

==Life==
Cohn was born in [[Neustrelitz]], [[Mecklenburg]] on 28 September 1854. He was the son of August Cohn, a [[lawyer]], and Charlotte Cohn. At the age of 17, Cohn began 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 [[Habilitation|habilitated]] in [[theoretical physics]] and was admitted as a private lecturer. From 1884 to 1918, he was a faculty member of the University of 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 from Strasbourg on the [[Christmas Eve]] of 1918. 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 (physicist)|Walter Kaufmann]], in protest at the despotism of the [[Nazi 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 to emigrate to Switzerland because of the pressure under the Nazi regime. 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.<ref>
{{cite journal
|author=[[Fritz Emde]]
|year=1947
|title=Nachruf auf Emil Cohn
|journal=[[Archiv der Elektrischen Übertragung]]
|volume=1 |issue=1–2 |pages=81–83
|doi=
}}</ref>

==Work==
:{{See|History of special relativity}}
At the beginning of the 20th century, Cohn was one of the most respectable experts in the area of theoretical [[electrodynamics]]. He was unsatisfied with the [[Lorentz ether theory|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 known at that time (1900–1904), including the [[Michelson-Morley experiment]] (MMX) of 1887. Cohn's electrodynamics of moving bodies was based on the assumption that light travels within the Earth's atmosphere with a constant velocity - however, his theory suffered from internal failures. While the theory predicted the negative result of MMX within air, a positive result would be expected within vacuum. Another weak point stems from the fact, that his concept was formulated without the use of atoms and electrons. So after 1905 his theory was superseded by [[Hendrik Lorentz]]'s and [[Albert Einstein]]'s.<ref>
{{cite journal
|last=Darrigol, O.
|year=1995
|title=Emil Cohn's electrodynamics of moving bodies
|journal=[[American Journal of Physics]]
|volume=63 |issue=10 |pages=908–915
|doi=10.1119/1.18032
|first1=Olivier
|bibcode = 1995AmJPh..63..908D }}</ref>
<ref>{{Cite book
|author=Darrigol, Olivier
|year=2000
|title= Electrodynamics from Ampére to Einstein
|place=Oxford
|publisher=Clarendon Press
|isbn=0-19-850594-9}}</ref>
<ref>
{{cite journal
|author=Janssen, M. and Stachel, J.
|year=2004
|title=The Optics and Electrodynamics of Moving Bodies
|url=http://www.mpiwg-berlin.mpg.de/Preprints/P265.PDF
}}</ref><ref>{{cite book
|last=Miller, A.I.
|year=1981
|title= Albert Einstein’s special theory of relativity. Emergence (1905) and early interpretation (1905–1911)
|publisher=[[Addison–Wesley]]
|isbn=0-201-04679-2
|series=191-182
}}</ref>

Based on the [[Principle of Economy]] he eliminated the known concept of [[luminiferous aether]] (but also the concept of atoms) and argued that one can simply call it vacuum. He also maintained that one can use a frame of reference in which the fixed stars are at rest. As a heuristic concept this can be described as a material "aether", but in Cohn's opinion this would be only "metaphorical" and would not affect the consequences of his theory. Some of his insights entailed certain aspects of [[Albert Einstein]]'s [[special relativity]], in particular some aspects of the interpretation of the [[Lorentz transformation]]. The transformation equations ''x'=x-vt'' and ''t'=t-vx/c²'' which were introduced by Lorentz in 1895 were called by Cohn the "Lorentzian Transformation" in 1900. In 1905 this name (for transformations valid to ''all'' orders in v/c) was altered by [[Henri Poincaré]] into the commonly used expression "Lorentz transformation". In 1904 Cohn gave a physical interpretation of Lorentz's concept of [[Lorentz ether theory#Local time|local time]] – he argued that this effect is a consequence of the assumption that light propagates in spherical waves with constant velocity in all directions (a similar definition was already given by Poincaré in 1900).

{{Quote|Everywhere, where the propagation of radiation is not the object of measurement, we define identical moments of time at different points of Earth's surface, by treating the propagation of light as ''timeless''. In optics, however, we ''define'' these identical moments of time by assuming, that the propagation takes place in ''spherical'' waves for every relatively resting and isotropic medium. This means: the "time" which actually serves us for the representation of terrestrial processes, is the "''local time''" <math>t'</math>, for which the equations I'b to IVb hold, – not the "''general time''" <math>t</math>.<ref>Cohn, 1904 II, p. 1408.</ref>}}

He also illustrated the effects of [[length contraction]] and [[time dilation]] by using moving rods and clocks.

{{Quote|1=<math>x_0\ y_0\ z_0</math> are those measuring numbers being read at an "initially correct" measuring-rod (initially = when at rest), after it was introduced into the system and was accordingly deformed. [...] <math>t_0</math> are those time intervals indicated by an "initially correctly ticking" clock, after it was inserted into the system and accordingly has changed its rate.<ref>Cohn, 1904 I, p. 1299.</ref>}}

He critically remarked that the distinction between "true time" and "local time" in Lorentz's theory is artificial, because it cannot be verified by experiment. However, Cohn himself believed that the validity of Lorentz's theory is limited to optical phenomena, whereas in his own theory it is possible that mechanical clocks might indicate the "true" time.<ref>Darrigol (2000), p. 368</ref><ref>Janssen/Stachel (2004), pp. 31-32</ref> Later in 1911 (after his own theory was disproved), Cohn accepted the [[relativity principle]] of "Lorentz and Einstein" and wrote a summary on special relativity, which was applauded by Einstein.<ref>Miller (1981), p. 182</ref>

==Sources==
{{reflist}}

==Publications==
{{Wikisourceauthor}}
*{{cite book
|last=Cohn |first=E.
|year=1900
|title=Das Elektromagnetische Feld - Vorlesungen über die Maxwell'sche Theorie
|location=Leipzig
|publisher=[[S. Hirzel]]
|url=http://www.archive.org/details/DasElektromagnetischeFeld
}}; Second edition Berlin 1927: [http://www.archive.org/details/daselectromagnet017159mbp Das elektromagnetische Feld - Ein Lehrbuch]
*{{cite journal
|last=Cohn |first=E.
|year=1900
|title=[[wikilivres:Über die Gleichungen der Elektrodynamik für bewegte Körper|Über die Gleichungen der Electrodynamik für bewegte Körper]]
|journal=Recueil de travaux offerts par les auteurs à H. A. Lorentz à l’occasion du 25ème anniversaire de son doctorat le 11 décembre 1900, Archives néerlandaises
|volume=5 |pages=516–523
}}
*{{cite journal
|last=Cohn |first=E.
|year=1901
|title=[[wikilivres:Über die Gleichungen des elektromagnetischen Feldes für bewegte Körper|Über die Gleichungen des elektromagnetischen Feldes für bewegte Körper]]
|journal=[[Göttinger Nachrichten]]
|volume= |pages=74–99
}}
*{{cite journal
|last=Cohn |first=E.
|year=1901
|title=Ueber die Gleichungen des elektromagnetischen Feldes für bewegte Körper
|journal=[[Annalen der Physik]]
|volume=312 |issue=1 |pages=29–56
|bibcode=1901AnP...312...29C
|doi=10.1002/andp.19013120103
}}
*{{cite journal
|last=Cohn |first=E.
|year=1904
|title=[[wikilivres:Zur Elektrodynamik bewegter Systeme I|Zur Elektrodynamik bewegter Systeme I]]
|journal=[[Sitzungsberichte der Königlich Preussischen Akademie der Wissenschaften]]
|volume=1904 |issue=40 |pages=1294–1303
}}
:*Wikisource translation: [[s:Translation:On the Electrodynamics of Moving Systems I|On the Electrodynamics of Moving Systems I]]
*{{cite journal
|last=Cohn |first=E.
|year=1904
|title=[[wikilivres:Zur Elektrodynamik bewegter Systeme II|Zur Elektrodynamik bewegter Systeme II]]
|journal=[[Sitzungsberichte der Königlich Preussischen Akademie der Wissenschaften]]
|volume=1904 |issue=43 |pages=1404–1416
}}
:*Wikisource translation: [[s:Translation:On the Electrodynamics of Moving Systems II|On the Electrodynamics of Moving Systems II]]
*{{cite journal
|last=Cohn |first=E
|year=1904
|title=Antikritisches zu Hrn. Wiens "Differentialgleichungen der Elektrodynamic für bewegte Körper"
|journal=[[Annalen der Physik]]
|volume=14 |issue=6 |pages=208
|bibcode = 1904AnP...319..208C
|doi=10.1002/andp.19043190619
}}
*"[http://www.archive.org/details/physikalischesb00cohngoog 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.).
*"Faraday und Maxwell", ''Deutsches Museum - Abhandlungen und Berichte'' '''4''' (1), Berlin 1932 (29 S.).

{{Authority control|VIAF=333294}}
{{Persondata 
| NAME = Cohn, Emil
| ALTERNATIVE NAMES =
| SHORT DESCRIPTION = German physicist
| DATE OF BIRTH = 28 September 1854
| PLACE OF BIRTH =
| DATE OF DEATH = 28 January 1944
| PLACE OF DEATH =
}}
{{DEFAULTSORT:Cohn, Emil}}
[[Category:German physicists]]
[[Category:1854 births]]
[[Category:1944 deaths]]

Reversible diffusion - Revision history

en>APerson: Repairing links to disambiguation pages - You can help!

en>Sodin: added Category:Probability theorems using HotCat