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Paul Ehrenfest (18 January 1880 – 25 September 1933) was an Austrian theoretical physicist, who made major contributions to the field of
statistical mechanics In physics, statistical mechanics is a mathematical framework that applies statistical methods and probability theory to large assemblies of microscopic entities. It does not assume or postulate any natural laws, but explains the macroscopic be ...
and its relations with quantum mechanics, including the theory of phase transition and the Ehrenfest theorem. He bonded with
Albert Einstein Albert Einstein ( ; ; 14 March 1879 – 18 April 1955) was a German-born theoretical physicist, widely acknowledged to be one of the greatest and most influential physicists of all time. Einstein is best known for developing the theory ...
on a visit to Prague in 1912 and became a professor in Leiden, where he frequently hosted Einstein.


Biography

Paul Ehrenfest was born and grew up in Vienna to Jewish parents from
Loštice Loštice (, german: Loschitz) is a town in Šumperk District in the Olomouc Region of the Czech Republic. It has about 2,900 inhabitants. Administrative parts The village of Žádlovice is an administrative part of Loštice. Geography Loštice is ...
in Moravia (now part of the Czech Republic). His parents, Sigmund Ehrenfest and Johanna Jellinek, ran a grocery store. Although the family was not overly religious, Paul studied
Hebrew Hebrew (; ; ) is a Northwest Semitic language of the Afroasiatic language family. Historically, it is one of the spoken languages of the Israelites and their longest-surviving descendants, the Jews and Samaritans. It was largely preserved ...
and the history of the Jewish people. Later, he always emphasized his Jewish roots. Ehrenfest excelled in grade school but did not do well at the Akademisches Gymnasium, his best subject being mathematics. After transferring to the Franz Josef Gymnasium, his marks improved. In 1899, he passed the
final exams A final examination, annual, exam, final interview, or simply final, is a test given to students at the end of a course of study or training. Although the term can be used in the context of physical training, it most often occurs in the a ...
. He majored in chemistry at the Vienna Institute of Technology, but took courses at the University of Vienna, in particular from Ludwig Boltzmann on his kinetic theory of thermodynamics. These lectures had a profound influence: they were instrumental in developing Ehrenfest's interest in theoretical physics, defined his main area of research for years to come, and provided an example of inspired teaching. At the time, it was customary in the
German German(s) may refer to: * Germany (of or related to) **Germania (historical use) * Germans, citizens of Germany, people of German ancestry, or native speakers of the German language ** For citizens of Germany, see also German nationality law **Ge ...
-speaking world to study at more than one university, and in 1901, Ehrenfest transferred to University of Göttingen, which until 1933 was an important centre for mathematics and theoretical physics. There he met his future wife, Tatyana Afanasyeva, a young mathematician born in Kiev (then-capital of the
Kiev Governorate Kiev Governorate, r=Kievskaya guberniya; uk, Київська губернія, Kyivska huberniia (, ) was an administrative division of the Russian Empire from 1796 to 1919 and the Ukrainian Soviet Socialist Republic from 1919 to 1925. It wa ...
, Russian Empire) and educated in St Petersburg. In the spring of 1903, he met Dutch physicist H.A. Lorentz during a short trip to Leiden, Netherlands. In the meantime, he prepared a dissertation on ''Die Bewegung starrer Körper in Flüssigkeiten und die Mechanik von Hertz'' (''The Motion of Rigid Bodies in Fluids and the Mechanics of Hertz''). He obtained his Ph.D. degree on 23 June 1904 in Vienna, where he stayed from 1904 to 1905. On 21 December 1904, he married Afanasyeva, who collaborated with him in his work. They had two daughters and two sons: Tatyana ('Tanja') (1905–1984), also became a mathematician; Galinka ('Galja') (1910–1979), became an author and illustrator of children's books; Paul, Jr. ('Pavlik') (1915–1939), who also became a physicist; and Vassily ('Wassik') (1918–1933). The Ehrenfests returned to Göttingen in September 1906. They would not see Boltzmann again: on September 5, Boltzmann took his own life in Duino near Trieste. Ehrenfest published an extensive obituary in which Boltzmann's accomplishments are described.
Felix Klein Christian Felix Klein (; 25 April 1849 – 22 June 1925) was a German mathematician and mathematics educator, known for his work with group theory, complex analysis, non-Euclidean geometry, and on the associations between geometry and group ...
, dean of the Göttinger mathematicians and chief editor of the ''Enzyklopädie der mathematischen Wissenschaften'' ("Encyclopedia of Mathematical Sciences"), had counted on Boltzmann for a review about statistical mechanics. Now he asked Ehrenfest to take on this task. Together with his wife, Ehrenfest worked on it for several years; the article was not published until 1911. It is a review of the work of Boltzmann and his school, and shows a style all of its own: a sharp logical analysis of the fundamental hypotheses, clear delineation of unsolved questions, and an explanation of general principles by cleverly chosen transparent examples. In 1907, the couple moved to St. Petersburg. Ehrenfest found good friends there, in particular Soviet physicist A.F. Joffe, but felt scientifically isolated. Moreover, because he was unwilling to declare belief in any religious denomination, he could not apply for a professorship, and therefore had no prospect of securing a permanent position. Early in 1912, Ehrenfest set out on a tour of
German German(s) may refer to: * Germany (of or related to) **Germania (historical use) * Germans, citizens of Germany, people of German ancestry, or native speakers of the German language ** For citizens of Germany, see also German nationality law **Ge ...
-speaking universities in the hope of a position. He visited Berlin where he saw Max Planck, Leipzig where he saw his old friend German mathematician
Gustav Herglotz Gustav Herglotz (2 February 1881 – 22 March 1953) was a German Bohemian physicist best known for his works on the theory of relativity and seismology. Biography Gustav Ferdinand Joseph Wenzel Herglotz was born in Volary num. 28 to a public ...
, Munich where he met German theoretical physicist
Arnold Sommerfeld Arnold Johannes Wilhelm Sommerfeld, (; 5 December 1868 – 26 April 1951) was a German theoretical physicist who pioneered developments in atomic and quantum physics, and also educated and mentored many students for the new era of theoretica ...
, then Zürich and Vienna. While in Prague he met Albert Einstein for the first time, and they remained close friends thereafter. Einstein recommended Ehrenfest to succeed him in his position in Prague, but that did not work out. This was due to the fact that Ehrenfest had declared himself to be an
atheist Atheism, in the broadest sense, is an absence of belief in the existence of deities. Less broadly, atheism is a rejection of the belief that any deities exist. In an even narrower sense, atheism is specifically the position that there no d ...
. Sommerfeld offered him a position in Munich, but Ehrenfest received a better offer; at the same time there was an unexpected turn of events. H. A. Lorentz resigned his position as professor at the
University of Leiden Leiden University (abbreviated as ''LEI''; nl, Universiteit Leiden) is a public research university in Leiden, Netherlands. The university was founded as a Protestant university in 1575 by William, Prince of Orange, as a reward to the city of L ...
, and on his advice, Ehrenfest was appointed as his successor.


Academic career

In October 1912, Ehrenfest arrived in Leiden, and on 4 December, he gave his inaugural lecture, ''Zur Krise der Lichtaether-Hypothese'' (About the crisis of the light-ether hypothesis). He remained in Leiden for the rest of his career. In order to stimulate interaction and exchange among physics students, he organized a discussion group and a study association called ''De Leidsche Flesch'' ("The Leiden Bottle"). He maintained close contact with prominent physicists within the country and abroad, and invited them to visit Leiden University and give a presentation in his lecture series. Ehrenfest was an outstanding debater, quick to point out weaknesses and summarize the essentials. In his lectures, he would focus on simple models and examples to illustrate and clarify the underlying assumptions. His classes were small, and he made an effort to get to know students who made use of the reading room. Though few of them were accepted as majors in theoretical physics, he had long discussions with them almost daily. According to Einstein:
He was not merely the best teacher in our profession whom I have ever known; he was also passionately preoccupied with the development and destiny of men, especially his students. To understand others, to gain their friendship and trust, to aid anyone embroiled in outer or inner struggles, to encourage youthful talent—all this was his real element, almost more than his immersion in scientific problems. Quoted in
If Ehrenfest felt that there was little more he could teach a student, he would send the student to other centers in Europe for more training. He would also encourage students to accept positions abroad. Among his students were Johannes Burgers,
Hendrik Kramers Hendrik Anthony "Hans" Kramers (17 December 1894 – 24 April 1952) was a Dutch physicist who worked with Niels Bohr to understand how electromagnetic waves interact with matter and made important contributions to quantum mechanics and statistical ...
, Dirk Coster,
George Uhlenbeck George Eugene Uhlenbeck (December 6, 1900 – October 31, 1988) was a Dutch-American theoretical physicist. Background and education George Uhlenbeck was the son of Eugenius and Anne Beeger Uhlenbeck. He attended the Hogere Burgerschool (High S ...
and
Samuel Goudsmit Samuel Abraham Goudsmit (July 11, 1902 – December 4, 1978) was a Dutch-American physicist famous for jointly proposing the concept of electron spin with George Eugene Uhlenbeck in 1925. Life and career Goudsmit was born in The Hague, Net ...
, who became famous for jointly proposing the concept of electron spin,
Jan Tinbergen Jan Tinbergen (; ; 12 April 19039 June 1994) was a Dutch economist who was awarded the first Nobel Memorial Prize in Economic Sciences in 1969, which he shared with Ragnar Frisch for having developed and applied dynamic models for the analysis of ...
, Arend Rutgers,
Hendrik Casimir Hendrik Brugt Gerhard Casimir (15 July 1909 – 4 May 2000) was a Dutch physicist best known for his research on the two-fluid model of superconductors (together with C. J. Gorter) in 1934 and the Casimir effect (together with D. Polder) in 1 ...
, Gerhard Dieke,
Dirk Struik Dirk Jan Struik (September 30, 1894 – October 21, 2000) was a Dutch-born American (since 1934) mathematician, historian of mathematics and Marxian theoretician who spent most of his life in the U.S. Life Dirk Jan Struik was born in 1 ...
, and
Gerard Kuiper Gerard Peter Kuiper (; ; born Gerrit Pieter Kuiper; 7 December 1905 – 23 December 1973) was a Dutch astronomer, planetary scientist, selenographer, author and professor. He is the eponymous namesake of the Kuiper belt. Kuiper is ...
. His assistants included
Yuri Krutkov Yuri Alexandrovich Krutkov (russian: Юрий Александрович Крутков, 29 May 1890 – 12 September 1952) was a Russian and among the first Soviet theoretical physicists. Krutkov worked on cosmology, quantum theory, statistical me ...
, Viktor Trkal,
Adriaan Fokker Adriaan Daniël Fokker (; 17 August 1887 – 24 September 1972) was a Dutch physicist. He worked in the fields of special relativity and statistical mechanics. He was the inventor of the Fokker organ, a 31 equal temperament, 31-tone equal-temp ...
,
Paul Epstein Paul Epstein (July 24, 1871 – August 11, 1939) was a German mathematician. He was known for his contributions to number theory, in particular the Epstein zeta function. Epstein was born and brought up in Frankfurt, where his father was a p ...
, and Gregory Breit. Other young foreign scientists who spent an extended period in his laboratory included Gunnar Nordström, Enrico Fermi, Igor Tamm,
Oskar Klein Oskar Benjamin Klein (; 15 September 1894 – 5 February 1977) was a Swedish theoretical physicist. Biography Klein was born in Danderyd outside Stockholm, son of the chief rabbi of Stockholm, Gottlieb Klein from Humenné in Kingdom of Hunga ...
,
J. Robert Oppenheimer J. Robert Oppenheimer (; April 22, 1904 – February 18, 1967) was an American theoretical physicist. A professor of physics at the University of California, Berkeley, Oppenheimer was the wartime head of the Los Alamos Laboratory and is often ...
, Walter Elsasser,
Ralph Kronig Ralph Kronig (10 March 1904 – 16 November 1995) was a German physicist. He is noted for the discovery of particle spin and for his theory of X-ray absorption spectroscopy. His theories include the Kronig–Penney model, the Coster–Kronig ...
,
Werner Heisenberg Werner Karl Heisenberg () (5 December 1901 – 1 February 1976) was a German theoretical physicist and one of the main pioneers of the theory of quantum mechanics. He published his work in 1925 in a Über quantentheoretische Umdeutung kinematis ...
,
Paul Dirac Paul Adrien Maurice Dirac (; 8 August 1902 – 20 October 1984) was an English theoretical physicist who is regarded as one of the most significant physicists of the 20th century. He was the Lucasian Professor of Mathematics at the Univer ...
, and David Dennison. Ehrenfest held ambivalent views on science, technological progress, and cultural and social issues. In 1919, he became a member of the Royal Netherlands Academy of Arts and Sciences.


Final years

From the correspondence with his close friends, from May 1931, it appears that Ehrenfest suffered from severe depression. By August 1932, Einstein was so worried that he wrote to the Board of the University of Leiden, expressing deep concern and suggesting ways in which Ehrenfest's workload could be reduced. Having made arrangements for the care of his other children, on 25 September 1933, in Amsterdam, Ehrenfest fatally shot his younger son Wassik, who had
Down syndrome Down syndrome or Down's syndrome, also known as trisomy 21, is a genetic disorder caused by the presence of all or part of a third copy of chromosome 21. It is usually associated with physical growth delays, mild to moderate intellectual di ...
, then killed himself.


Research

Most of Ehrenfest's scientific papers deal with fundamentals, and seek to clarify single points. His publications are renowned for clarity, by solving paradoxes or by providing clearer descriptions, or are inspiring by posing melliferous questions. His approach to science is best illustrated by what he wrote to Robert Oppenheimer in the summer of 1928, after Oppenheimer invited himself for an extended stay in Leiden:
If you intend to mount heavy mathematical artillery again during your coming year in Europe, I would ask you not only not to come to Leiden, but if possible not even to Holland, and just because I am really so fond of you and want to keep it that way. But if, on the contrary, you want to spend at least your first few months patiently, comfortably, and joyfully in discussions that keep coming back to the same few points, chatting about a few basic questions with me and our young people—and without thinking much about publishing (!!!)—why then I welcome you with open arms!!
Characteristically, he did not like the abstraction of the new quantum theory of Heisenberg and Dirac. Ehrenfest's most important contribution from 1912 to 1933 is the theory of
adiabatic invariant A property of a physical system, such as the entropy of a gas, that stays approximately constant when changes occur slowly is called an adiabatic invariant. By this it is meant that if a system is varied between two end points, as the time for the ...
s. It is a concept derived from
classical mechanics Classical mechanics is a physical theory describing the motion of macroscopic objects, from projectiles to parts of machinery, and astronomical objects, such as spacecraft, planets, stars, and galaxies. For objects governed by classical m ...
that, on the one hand, can serve to refine certain methods of Niels Bohr's model of the atom (although initially Ehrenfest did not accept
Bohr Niels Henrik David Bohr (; 7 October 1885 – 18 November 1962) was a Danish physicist who made foundational contributions to understanding atomic structure and quantum theory, for which he received the Nobel Prize in Physics in 1922. B ...
's ideas), and on the other hand, makes a link between atomic mechanics and statistical mechanics. He made major contributions to
quantum physics Quantum mechanics is a fundamental theory in physics that provides a description of the physical properties of nature at the scale of atoms and subatomic particles. It is the foundation of all quantum physics including quantum chemistry, q ...
, including the theory of phase transitions and the Ehrenfest theorem, which states that expectation values of a quantum system follow classical mechanics. His name is also given to the Ehrenfest paradox, an apparent paradox in relativity still discussed today, to the
Ehrenfest model The Ehrenfest model (or dog–flea model) of diffusion was proposed by Tatiana and Paul Ehrenfest to explain the second law of thermodynamics. The model considers ''N'' particles in two containers. Particles independently change container at a rate ...
, and to Ehrenfest time, the time characterizing the departure of quantum dynamics for observables from classical dynamics. Ehrenfest was also interested in developing mathematical theories in economics. This interest was stimulated by his notion that there should be an analogy between thermodynamics and economic processes. While this did not result in publications, he did encourage his graduate student
Jan Tinbergen Jan Tinbergen (; ; 12 April 19039 June 1994) was a Dutch economist who was awarded the first Nobel Memorial Prize in Economic Sciences in 1969, which he shared with Ragnar Frisch for having developed and applied dynamic models for the analysis of ...
to follow up on this. Tinbergen's thesis was devoted to problems both from physics and economics, and he went on to become an economist and was awarded the first Nobel Memorial Prize in Economic Sciences in 1969.


Einstein and Bohr in Leiden

Ehrenfest was particularly close to both Einstein and Bohr. After Niels Bohr's first visit to Leiden in 1919, for Kramers' thesis defense, he wrote to Ehrenfest:
I am sitting and thinking of all what you have told me about so very many different things, and whatever I think of I feel that I have learned so much from you which will be of great importance for me; but, at the same time, I wish so much to express my feeling of happiness over your friendship and of thankfulness for the confidence and sympathy you have shown me, I find myself so utterly incapable of finding words for it.
On his invitation Einstein accepted in 1920 an appointment as extraordinary professor at the University of Leiden. This arrangement allowed Einstein to visit Leiden for a few weeks every year. At these occasions Einstein would stay at Ehrenfest's home. In 1923 Einstein stayed there for six weeks, after German ultra-nationalists in Berlin had made threats against his life. On the occasion of the 50th anniversary of Lorentz' doctorate (December 1925) Ehrenfest invited both Bohr and Einstein over to Leiden, in an attempt to reconcile their scientific differences about the emerging quantum theory. These discussions were continued at the 1927 Solvay Conference, where Ehrenfest much to his dismay had to side with Bohr's position in this great debate.


Quotes

Ehrenfest used colourful German language in his physics lectures:
Das ist der springende Punkt (translation: That is the crucial point)
Das ist wo der Frosch ins Wasser springt (That is where the frog jumps into the water)
Das ist der Patentanspruch (That is the patent claim, the essence)
Da hat Herr ... schlieslich der Ratte aus der Suppe gezogen (There Mr. ..finally pulled the rat out of the soup - when a scientist had solved a messy problem)
Je besser man's versteht um so besser steht es dort (The better one understands, the better it is written there, Ehrenfest's comment when Dirac was asked in writing for an explanation of his work, and Dirac characteristically simply reproduced exactly his previous explanation.)


Legacy

The monthly evening colloquium in physics at Leiden University, initiated by Paul Ehrenfest in 1912 at his house, still continues under the name ''Colloquium Ehrenfestii''. The Austrian
Institute for Quantum Optics and Quantum Information The Institute for Quantum Optics and Quantum Information (IQOQI) (german: Institut für Quantenoptik und Quanteninformation) is a member institute of the Austrian Academy of Sciences and was founded in November 2003, to create an Austrian resear ...
sponsors the annual ''Paul Ehrenfest Best Paper Award for Quantum Foundations''. The Dutch Physics Council sponsors the annual ''Ehrenfest-Afanassjewa thesis award''.


Bibliography

* H.B.G. Casimir: ''Haphazard Reality - Half a Century of Science.''
New York New York most commonly refers to: * New York City, the most populous city in the United States, located in the state of New York * New York (state), a state in the northeastern United States New York may also refer to: Film and television * ...
, Harper & Row, 1983. *
Martin J. Klein Martin Jesse Klein (June 25, 1924 – March 28, 2009), usually cited as M. J. Klein, was a science historian of 19th and 20th century physics. Biography Klein was born in the Bronx, New York City. He was an only child and both his parents we ...
: ''Paul Ehrenfest: The Making of a Theoretical Physicist.'' Biography of Paul Ehrenfest. Amsterdam: Elsevier. . 1985 edition:


References


External links


The Tragic Fate of Physicist Paul Ehrenfest
{{DEFAULTSORT:Ehrenfest, Paul 1880 births 1933 suicides Austrian atheists Austrian Jews 20th-century Austrian physicists Dutch Jews Dutch people of Austrian descent 20th-century Dutch physicists Jewish atheists Jewish scientists Leiden University faculty Members of the Royal Netherlands Academy of Arts and Sciences Scientists from Vienna Science teachers Murder–suicides in Europe 1933 deaths Dutch murderers Austrian murderers Suicides by firearm in the Netherlands