Emil Cohn
Encyclopedia
Emil Georg Cohn was a German physicist.
, 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 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
, 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.
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 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.
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 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).
He also illustrated the effects of length contraction
and time dilation
by using moving rods and clocks.
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, while in his own theory it is possible that mechanical clocks might indicate the "true" time. 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.
Life
Cohn was born in NeustrelitzNeustrelitz
Neustrelitz is a town in the Mecklenburgische Seenplatte district in the state of Mecklenburg-Vorpommern, Germany. It is situated on the shore of the Zierker See in the Mecklenburg Lake District. From 1738 until 1918 it was the capital of the duchy of Mecklenburg-Strelitz...
, Mecklenburg
Mecklenburg
Mecklenburg is a historical region in northern Germany comprising the western and larger part of the federal-state Mecklenburg-Vorpommern...
on 28 September 1854. He was the son of August Cohn, a lawyer
Lawyer
A lawyer, according to Black's Law Dictionary, is "a person learned in the law; as an attorney, counsel or solicitor; a person who is practicing law." Law is the system of rules of conduct established by the sovereign government of a society to correct wrongs, maintain the stability of political...
, and Charlotte Cohn. At the age of 17, Cohn began to study jurisprudence
Jurisprudence
Jurisprudence is the theory and philosophy of law. Scholars of jurisprudence, or legal theorists , hope to obtain a deeper understanding of the nature of law, of legal reasoning, legal systems and of legal institutions...
at the University of Leipzig
University of Leipzig
The University of Leipzig , located in Leipzig in the Free State of Saxony, Germany, is one of the oldest universities in the world and the second-oldest university in Germany...
. However, at the Ruprecht Karl University of Heidelberg
Ruprecht Karl University of Heidelberg
The Ruprecht-Karls-Universität Heidelberg is a public research university located in Heidelberg, Baden-Württemberg, Germany. Founded in 1386, it is the oldest university in Germany and was the third university established in the Holy Roman Empire. Heidelberg has been a coeducational institution...
and the University of Strasbourg
University of Strasbourg
The University of Strasbourg in Strasbourg, Alsace, France, is the largest university in France, with about 43,000 students and over 4,000 researchers....
he began to study physics
Physics
Physics is a natural science that involves the study of matter and its motion through spacetime, along with related concepts such as energy and force. More broadly, it is the general analysis of nature, conducted in order to understand how the universe behaves.Physics is one of the oldest academic...
. In Strasbourg, he graduated in 1879. From 1881 to 1884, he was an assistant of August Kundt
August Kundt
August Adolf Eduard Eberhard Kundt was a German physicist.-Biography:Kundt was born at Schwerin in Mecklenburg. He began his scientific studies at Leipzig, but afterwards went to Berlin University. At first he devoted himself to astronomy, but coming under the influence of H. G...
at the physical institute. In 1884 he habilitated
Habilitation
Habilitation is the highest academic qualification a scholar can achieve by his or her own pursuit in several European and Asian countries. Earned after obtaining a research doctorate, such as a PhD, habilitation requires the candidate to write a professorial thesis based on independent...
in theoretical physics
Theoretical physics
Theoretical physics is a branch of physics which employs mathematical models and abstractions of physics to rationalize, explain and predict natural phenomena...
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
Experimental physics
Within the field of physics, experimental physics is the category of disciplines and sub-disciplines concerned with the observation of physical phenomena in order to gather data about the universe...
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
World War I
World War I , which was predominantly called the World War or the Great War from its occurrence until 1939, and the First World War or World War I thereafter, was a major war centred in Europe that began on 28 July 1914 and lasted until 11 November 1918...
and the occupation of Alsace-Lorraine
Alsace-Lorraine
The Imperial Territory of Alsace-Lorraine was a territory created by the German Empire in 1871 after it annexed most of Alsace and the Moselle region of Lorraine following its victory in the Franco-Prussian War. The Alsatian part lay in the Rhine Valley on the west bank of the Rhine River and east...
by France
France
The French Republic , The French Republic , The French Republic , (commonly known as France , is a unitary semi-presidential republic in Western Europe with several overseas territories and islands located on other continents and in the Indian, Pacific, and Atlantic oceans. Metropolitan France...
, Cohn and his family were expelled from Strasbourg on the Christmas Eve
Christmas Eve
Christmas Eve refers to the evening or entire day preceding Christmas Day, a widely celebrated festival commemorating the birth of Jesus of Nazareth that takes place on December 25...
of 1918. In April 1919, he was nominated as a professor at the University of Rostock
University of Rostock
The University of Rostock is the university of the city Rostock, in the German state of Mecklenburg-Vorpommern.Founded in 1419, it is the oldest and largest university in continental northern Europe and the Baltic Sea area...
. From June 1920, he gave lectures about theoretical physics at the University of Freiburg
University of Freiburg
The University of Freiburg , sometimes referred to in English as the Albert Ludwig University of Freiburg, is a public research university located in Freiburg im Breisgau, Baden-Württemberg, Germany.The university was founded in 1457 by the Habsburg dynasty as the...
. In 1935 he retired in Heidelberg where he lived until 1939. He resigned from the Deutsche Physikalische Gesellschaft
Deutsche Physikalische Gesellschaft
The Deutsche Physikalische Gesellschaft is the world's largest organization of physicists. The DPG's worldwide membership is cited as 60,000, as of 2011...
(DPG) together with other physicists like Richard Gans
Richard Gans
Richard Martin Gans , German of Jewish origin, born in Hamburg, was the physicist who founded the Physics Institute of the National University of La Plata, Argentina...
, Leo Graetz
Leo Graetz
Leo Graetz was a German physicist. He was born in Wrocław , Poland as the son of the historian Heinrich Graetz.Graetz was one of the first to investigate the propagation of electromagnetic energy...
, George Jaffé, Walter Kaufmann
Walter Kaufmann (physicist)
Walter Kaufmann was a German physicist. He is most well known for his first experimental proof of the velocity dependence of mass, which was an important contribution to the development of modern physics, including special relativity.-Life:In 1890/91 he studied mechanical engineering at the...
, 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
Ringgenberg
Ringgenberg is a municipality in the Interlaken-Oberhasli administrative district in the canton of Bern in Switzerland. It is located on the northern shores of Lake Brienz. It has a small church that was built on the ruins of a castle in the 17th century....
, Switzerland
Switzerland
Switzerland name of one of the Swiss cantons. ; ; ; or ), in its full name the Swiss Confederation , is a federal republic consisting of 26 cantons, with Bern as the seat of the federal authorities. The country is situated in Western Europe,Or Central Europe depending on the definition....
, where he died at the age of 90.
Cohn's younger brother, Carl Cohn (1857–1931) was a successful overseas merchant from Hamburg
Hamburg
-History:The first historic name for the city was, according to Claudius Ptolemy's reports, Treva.But the city takes its modern name, Hamburg, from the first permanent building on the site, a castle whose construction was ordered by the Emperor Charlemagne in AD 808...
, who worked from 1921 until 1929 as a senator in Hamburg.
Work
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 Lorentzian theory of electrodynamics
Lorentz ether theory
What is now often called Lorentz Ether theory has its roots in Hendrik Lorentz's "Theory of electrons", which was the final point in the development of the classical aether theories at the end of the 19th and at the beginning of the 20th century....
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
Michelson-Morley experiment
The Michelson–Morley experiment was performed in 1887 by Albert Michelson and Edward Morley at what is now Case Western Reserve University in Cleveland, Ohio. Its results are generally considered to be the first strong evidence against the theory of a luminiferous ether and in favor of special...
(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
Hendrik Lorentz
Hendrik Antoon Lorentz was a Dutch physicist who shared the 1902 Nobel Prize in Physics with Pieter Zeeman for the discovery and theoretical explanation of the Zeeman effect...
's and Albert Einstein
Albert Einstein
Albert Einstein was a German-born theoretical physicist who developed the theory of general relativity, effecting a revolution in physics. For this achievement, Einstein is often regarded as the father of modern physics and one of the most prolific intellects in human history...
's.
Based on the Principle of Economy, he eliminated the known concept of luminiferous aether
Luminiferous aether
In the late 19th century, luminiferous aether or ether, meaning light-bearing aether, was the term used to describe a medium for the propagation of light....
(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
Albert Einstein
Albert Einstein was a German-born theoretical physicist who developed the theory of general relativity, effecting a revolution in physics. For this achievement, Einstein is often regarded as the father of modern physics and one of the most prolific intellects in human history...
's special relativity
Special relativity
Special relativity is the physical theory of measurement in an inertial frame of reference proposed in 1905 by Albert Einstein in the paper "On the Electrodynamics of Moving Bodies".It generalizes Galileo's...
, in particular some aspects of the interpretation of the Lorentz transformation
Lorentz transformation
In physics, the Lorentz transformation or Lorentz-Fitzgerald transformation describes how, according to the theory of special relativity, two observers' varying measurements of space and time can be converted into each other's frames of reference. It is named after the Dutch physicist Hendrik...
. 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
Henri Poincaré
Jules Henri Poincaré was a French mathematician, theoretical physicist, engineer, and a philosopher of science...
into the commonly used expression "Lorentz transformation". In 1904 Cohn gave a physical interpretation of Lorentz's concept of 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).
He also illustrated the effects of length contraction
Length contraction
In physics, length contraction – according to Hendrik Lorentz – is the physical phenomenon of a decrease in length detected by an observer of objects that travel at any non-zero velocity relative to that observer...
and time dilation
Time dilation
In the theory of relativity, time dilation is an observed difference of elapsed time between two events as measured by observers either moving relative to each other or differently situated from gravitational masses. An accurate clock at rest with respect to one observer may be measured to tick at...
by using moving rods and clocks.
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, while in his own theory it is possible that mechanical clocks might indicate the "true" time. 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.
Publications
; Second edition Berlin 1927: Das elektromagnetische Feld - Ein Lehrbuch- Wikisource translation: On the Electrodynamics of Moving Systems I
- Wikisource translation: On the Electrodynamics of Moving Systems II
- "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.).