Ralph Kronig

Ralph Kronig (Dresden, Germany, March 10, 1904 - Zeist, The Netherlands, November 16, 1995) was a renowned German/American/Dutch theoretical physicist. He is noted for his theory of x-ray absorption spectroscopy. His theories include the Kronig–Penney model, the Coster–Kronig transition and the Kramers–Kronig relations. He came close to the discovery of electron spin.

Life
The father of Ralph Kronig, Harold Theodor Kronig, born in 1866, came from a wealthy German family that owned a textile factory in Bohemia. At the age of eighteen Harold befriended an American pastor, the Reverend Potts. When Potts returned to the USA, Harold followed him. Harold first worked on the farm owned by Potts, but later became a painter. During internships in art studios in Paris &mdash; between the years 1885 and 1890 &mdash; he acquired a professional level. Around that time Harold Kronig became an American citizen. Ralph's mother, Augusta de Laer was of Dutch descent and was born on September 25, 1868 in Surabaya in the Netherlands East Indies (the present Indonesia). Augusta married her first husband in 1888, and became a widow at the age of twenty-seven.When Harold Kronig was back in Europe on a short visit, he met Augusta. They married in 1903 and settled in Dresden. One of the reasons for choosing Dresden as residence was that Harold's parents had moved there, and also because of the beauty of the city and its surroundings. Ralph Kronig remarked in later life that his childhood in Dresden was very happy.

World War I did not cause the Kronig family much hardship, although father Kronig was an American citizen and the US had declared war on Germany in April 1917. Only after the war had ended in 1918, did Harold feel that the situation in Germany was no longer safe, both economically and politically. In 1919 he decided to take his family to New York, where Ralph entered Columbia University.

Thus the education of Ralph was divided into two phases: German gymnasium (secondary school) and American university. Ralph spoke with appreciation of his German school, relating how it took him little effort to belong to the five best in his class and that he had plenty of spare time for his hobbies, among them chemical experiments for which his father supplied him with some professional-quality instruments. During these chemical experiments Ralph came to realize that to truly understand the properties of matter you had to look inside atoms and molecules and that that is exactly what physicists do. Further he discovered that to understand physics you need mathematics. At Ralph's request his father paid private lessons in mathematics for him. His father also made sure that he was well educated in general culture and the English language.

Ralph's study at Columbia proceeded smoothly; it appeared that the German gymnasium was a good preparation and he received his BA on February 28, 1923. On February 7, 1925 he received his PhD degree.

The training at Columbia was thorough but almost exclusively in classical physics. Quantum theory and recent results in atomic structure and spectra were not part of the curriculum. Exactly these topics interested Ralph most. In those days new discoveries were published almost exclusively in German. Since he was not hindered by a language barrier, he could &mdash; and did &mdash; study the German scientific journals. Ralph was not alone in his endeavors: a number of graduate students, led by I.I. Rabi (who graduated in 1926) had formed a club to discuss the latest results from Europe. Ralph benefited from these discussions and at the time of his graduation his knowledge of modern physics was far above the requirements of Columbia.

In 1928 Kronig left the US and in 1930 he settled in The Netherlands. After retirement (1969) he moved to Zürich, Switzerland, to live closer to the mountains. In the last phase of his life, when his health deteriorated, he spent more and more time in The Netherlands again, where he died in Zeist.

As a person, Kronig was somewhat reserved and formal, but his behavior towards other people was consistent and reliable. He was married twice. His first marriage, with E. Ovink, ended in divorce in 1939. There were two children from his first marriage, a daughter and a son, who died young. In 1941 he married the Danish Grethe Petersen, who until the very end was his partner and finally, his nurse. Also with Grethe he had a son and a daughter. Kronig's interest was not limited to physics only. He was well-read in many languages and loved mountain hiking.

The honors received by Kronig include: membership of the Dutch Royal Academy of Science (KNAW, in 1946), the Max Planck medal (1962), and an honorary doctorate of the University of Trondheim, Norway (1972).

Career
Kronig's career can be divided in different periods:


 * 1925-1930 Wanderjahre (roaming years) during which he repeatedly changed places. Scientifically this was his most fruitful period.
 * 1930-1939 University of Groningen (The Netherlands), first as a "conservator" then as a "lector" (associate professor).
 * 1939-1969 Professor at the Technical University of Delft (The Netherlands).
 * 1969-1991 Retired, living in Zürich (Switzerland).
 * 1991-1995 His last years of increasingly bad health mostly spent in The Netherlands.

Wanderjahre
Paul Ehrenfest met Ralph during his 1924 visit to America and advised the young physicist to visit Europe. Kronig applied for a Bayard Cutting Traveling Fellowship and after receiving it, returned to Europe (end of 1924, before his graduation). He spent most of his time working in the group of Niels Bohr in Copenhagen, but he also visited some universities in his country of birth.

It is sometimes stated that Kronig discovered particle spin, while most textbooks credit the Dutch physicists Goudsmit and Uhlenbeck for the discovery. (See Ref. for Goudsmit's version of the event.) The eminent Dutch physicist Hendrik Casimir, a contemporary of the three physicists, has the following to say about it (free translation from the Dutch):

On January 7, 1925, Kronig, fresh from the US, visits Alfred Landé (1888–1976) in Tübingen. Landé, an eminent specialist in the field of atomic spectra, kindly receives him and invites him to a discussion meeting with Pauli, who happens to be in Tübingen on a short visit. As a preparation to this meeting, Landé gives Ralph a copy of a letter from Pauli. This letter contains the essence of what later became to be called the Pauli exclusion principle. Kronig is highly impressed and gets a brilliant flash of inspiration. He sees that Pauli's scheme becomes much more transparent if one assumes that each electron possesses its own angular momentum and its own magnetic moment, or, in other words, that it rotates. During the discussions next day, Ralph gets an opportunity to put forward his idea. According to Kronig Pauli responds to this with the words Das ist ja ein ganz witziger Einfall [that's a very witty idea], but does not believe it; he sees too many difficulties. Discouraged, Kronig does not press on. Afterwards he tries in vain to interest Bohr and Heisenberg. More than half a year later Uhlenbeck and Goudsmit publish the idea with backing from Ehrenfest and unaware of Kronig's priority. For the young&mdash;twenty year old&mdash;Ralph Kronig the affair is a heavy blow, and in my opinion he never quite overcame it. What could have been a great triumph, became a historical curiosity, even though both Pauli and Bohr tried to some extent to make up for it.

In December 1925 Ralph Kronig sailed back to New York, where he joined the physics department of Columbia University (January 1926), first as lecturer, then as assistant professor.

In 1926 he published, a few months before Kramers, an outstanding paper on what is now called the Kramers-Kronig relations. At that time he corresponded with some of the founders of quantum mechanics, particularly with Wolfgang Pauli and Werner Heisenberg. The seminal Heisenberg paper of the summer of 1925 was a rewrite of a letter that Heisenberg sent to Kronig. In 1927 he and Rabi published an analytic solution for the Schrödinger equation of the symmetric top, one of the few cases where the equation can be solved analytically.

In 1928 Kronig left the US for good, heading again for Europe, now on a Rockefeller Foundation Fellowship from the International Education Board. He worked with Anthony Kramers in Utrecht and from May to September 1928 he was Wolfgang Pauli's first assistant at the Swiss Federal Institute of Technology (ETH) in Zürich. After the end of his fellowship (1929) he found a position as lecturer at the Imperial College London. While in England he gave lectures in Cambridge on molecular spectra that formed the basis of a book that for a long time was considered the standard in this area.

Groningen
In 1930, at the age of twenty-seven, Kronig's reputation as prominent theorist was firmly established. When the experimental physicist Dirk Coster wanted to get him to Groningen, he asked Wolfgang Pauli for his opinion. Pauli was widely admired, but feared for his severe judgments and harsh criticisms. Pauli sent the following testimonial (in a free translation from the German):

Considering his scientific achievements, Kronig is ready, in my opinion, to become a professor in theoretical physics. He possesses the autonomy of mind and shows the independence in his actions required for such a position. His work is on spectral theory, and lately on band spectra, where he may be called the world's foremost expert. He is also knowledgeable about the theory of metals etcetera. All his work shows great originality in picking problems and is characterized by thoroughness and reliability. It also must be emphasized that, although he did not publish about it, Kronig came close to the discovery of electron spin. Finally, I judge him, in view of his command of general modern quantum physics, to be the proper person to teach this field at university level.

At the Imperial College Kronig had an outstanding student, William George Penney (later Baron Penney of East Hendred and the head of the British nuclear bomb activities). Penney followed Kronig's lectures on perturbations on band spectra, and worked one semester in Groningen. The Kronig and Penney collaboration resulted in a famous article on the motion of electrons in a periodic potential field.

In Groningen, Kronig collaborated as lector (associate professor) much with experimentalists. For instance with D. Coster with whom he wrote an article about what is now called a Coster-Kronig transition (probability of ionization simultaneously with a transition between 2s and 2p orbitals, one of which is vacant). Other well-known work from this period is on the fine structure in X-ray absorption, XAFS, in solids and paramagnetic relaxation. Typical of his Groningen years are his significant contributions to scientific literature: a second monograph and several review articles.

Delft
In 1939 Kronig accepted an appointment as full professor in the department of Technical Physics at the Technical University in Delft. That did not mean that his interests in fundamental physics suddenly came to an end. For instance, in the 1960s he collaborated on a study of the nonlinear Compton effect, in which he and his co-workers applied exact solutions of the Dirac equation.

Nevertheless, after his appointment in Delft, a shift in his interests occurred. He focused more on education and saw it as his task to let engineering students become acquainted with the mathematical techniques of theoretical physics. Kronig's lectures were clear and precise and he became famous among students by his mastership of the blackboard. He was reluctant, though, to accept PhD students; only eight graduated under his direction during his thirty years in Delft.

After World War II Kronig became more and more committed to editorial work: together with Viktor Weisskopf, he edited Pauli's collected work. He was the main editor of the college-level textbook Leerboek der Natuurkunde. Kronig's organizational skills became apparent during his rectorship of the university (from 1959 to 1962).

Notes and references

 * Max Dresden, Obituary R. Kronig, Physics Today, 50(3) 1997, p. 97. DOI
 * Interview with Dr. R. (Ralph) Kronig by John L. Heilbron. At Kronig’s Office at the T.H. Delft, Holland, November 12, 1962