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 transition and the Kramers–Kronig relations.
Background Ralph Kronig (later Ralph de Laer Kronig) was born on 10 March 1904 to German parents (Harold Theodor Kronig, Augusta de Laer) in Dresden, Germany. He died in Zeist on 16 November 1995 at the age of 91. Kronig received his primary and high-school education in Dresden and went to New York City to study at Columbia University where he received his PhD in 1925 and subsequently became instructor (1925) and assistant professor (1927). Early in Kronig's career he met Paul Ehrenfest who, while visiting America in 1924, advised Ralph Kronig to revisit Europe, which Kronig did later the same year. He paid visits to the important centers for theoretical-physics research in Germany and Copenhagen. It was a time of great expansion in the development of quantum mechanics, and this development was mostly happening in Europe at the time. In January 1925, when Kronig was still a Columbia University PhD student, he first proposed the electron spin after hearing Pauli in Tübingen. Heisenberg and Pauli immediately hated the idea. They had just ruled out all imaginable actions from quantum mechanics. Pauli especially ridiculed the idea of spin, saying that "it is indeed very clever but of course has nothing to do with reality". Faced with such criticism, Kronig decided not to publish his theory and the idea of electron spin had to wait for others to take the credit. Ralph Kronig had come up with the idea of electron spin several months before George Uhlenbeck and Samuel Goudsmit. Most textbooks credit these two Dutch physicists with the discovery. Ralph Kronig did not hold a grudge against Pauli for this turn of events. In fact, Kronig and Pauli remained friends for many years into the future. They exchanged many ideas in physics through letters. But it remains an historic fact that Kronig had told Pauli about electron spin before Pauli had published his paper showing that two electrons can inhabit the same orbital (W. Pauli, "On the Connexion between the Completion of Electron Groups in an Atom with the Complex Structure of Spectra", Z. Physik 31, 765ff, 1925). Months later when Uhlenbeck and Goudsmit came up with particle spin, it seemed to verify Pauli's paper. Together with Isidor Isaac Rabi, Kronig gave the first solution (1927) of the Schrödinger equation for the rigid symmetric top. Werner Heisenberg in developing quantum mechanics involved Kronig in his seminal ideas of the theory. In the beginning of May 1925, Heisenberg wrote three times to Ralph Kronig, with whom he had cooperated a little earlier in Copenhagen on the spectral theory of multi-electron atoms. In the second letter, dated 5 May, Heisenberg wrote down in some detailed equations expressing the transition to his matrix mechanics. In 1927, Kronig returned to Europe for good and worked in different prominent centres of research: Copenhagen, London, Zürich (where for a year he was Pauli's assistant). Around 1930 he settled in the Netherlands: first in Utrecht, then in Groningen, first as Dirk Coster's assistant, and from 1931 as an associate professor, and since 1939 as a full professor at the Delft University of Technology where he stayed until his retirement in 1969. Between 1959 and 1962 he was the rector of the university. He was recognized internationally by then as a renowned theorist who corresponded with the leading characters of that time and made interesting contributions to quantum mechanics and the application of it particularly on the physics of molecules and molecular spectra, an area on which he was the expert of those days. The Max Planck medal was awarded to Ralph Kronig in 1962. Kronig was elected a member of the Royal Netherlands Academy of Arts and Sciences in 1946, in 1969 he became a foreign member. Among Ralph Kronig's substantial correspondence are many letters to and from the 20th century's greatest physicists that should be preserved for posterity and Kronig himself published many in books. Showing Kronig's great respect for Pauli, in one letter Ralph Kronig said regarding Pauli and the slim number of actual publications made by Pauli considering the extent of his work [translated from the German]:
... his [Pauli's] publications contain however, which is understandable due to Pauli's unusually critical attitude, only a small part of the work really carried out by him. Pauli briefs in his papers on the finished results, but not on the long, often laborious way, which had led to them, and also not on incomplete attempts. A part of his work is only carried in a satisfying way in his extensive exchange of letters Stumm von Bordwehr (1989) gives a detailed description of the life and accomplishments of Kronig, even recounting how his name was changed to Ralph de Laer Kronig.
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