Oskar Benjamin Klein was born on 15 September 1894 in Stockholm, Sweden. He was the son of Gottlieb Klein, a Hungarian rabbi who had moved to Stockholm the year before Oskar’s birth.

An early fascination for science

Already during his childhood, Klein showed keen interest in science [1,2]. He distinctly recalled looking at the stars with his mother’s opera glasses, and finally being able to see Sirius at the age of fourteen: “I wasn’t permitted to stay out very long, so it took quite a time before I saw Sirius. And I remember that was a very great event. We came home from some party and then I saw Sirius” [1].

In 1910, Klein joined his father at the house of Svante Arrhenius, renowned physical chemist and Sweden’s first Nobel Prize winner. Arrhenius invited Klein to join him at his lab. Because Klein was still in school, he did the lab work entirely in his spare time. His lab work ultimately resulted in Klein’s first publication [1]. Klein—somewhat reluctantly, according to his own words [1]—skipped two grades in the Swedish gymnasium and obtained his bachelor degree (“fil. Kand.”) in 1914. In an interview from 1962, Klein recalled that he didn’t have much science training, but his curiosity propelled him further [1].

After his graduation, Klein wanted to go work with Jean Perrin in Paris. However, two weeks after Klein arrived in Bretagne to learn French in preparation, World War I broke out. Klein returned to Arrhenius’ lab in Sweden. He completed his military service between June 1915 and September 1916. By the end of 1917, Klein graduated with a master’s degree (“licentiat”) [1].

First contacts with Bohr's research group in Copenhagen

Around that time, Hans Kramers, Bohr’s research assistant, visited Stockholm. This proved to be a turning point for Klein. Klein arrived in May 1918 in Copenhagen to work with Bohr and Kramers. Initially, he primarily interacted with Kramers: “Kramers was my teacher in these new things; Hamilton-Jacobi, the correspondence principle and beginnings of the Copenhagen philosophy” [1]. Later on, Klein became closer to Bohr. During long walks, Bohr and Klein discussed philosophy and physics—including Bohr’s early thoughts about the correspondence principle.

In 1922, Klein briefly returned to Stockholm to defend his doctoral dissertation [3]. After his PhD defense, the recently married Klein tried to find a permanent position in Sweden, but to no avail. He turned to Bohr for help. Bohr recommended him for a position in at the University of Michigan in Ann Arbor, Michigan, USA. Klein and his wife Gerda Agnete Koch move to the USA in 1923.

In Michigan, Klein received a stable position but felt isolated. He was far removed from the exciting work happening in Copenhagen at the time, and he found it hard to connect to the work of his colleagues in Ann Arbour [4]. Nonetheless, it was during this time that Klein laid the groundwork for some of his most important contributions to physics. While teaching a course on electromagnetism in 1924, Klein was inspired him to start working a unified field theory and developed a formalism in five dimensions [2]. In 1925, Klein returned to Europe.

Klein's most productive period: 1926-1931

After a delay due to illness, Klein began arguably his most productive years in physics. Between 1926 and 1929, he made significant contributions to especially quantum theory and general relativity [3]. For one, Klein reconvened his work on a five-dimensional unified field theory—even though he had lost some of his early calculations during the move [5]. In Europe, Klein learned that his work was to some extent reproducing the work of Kaluza from 1919 [Klein bio]. But Klein also realized that he had taken the idea one step further by giving a quantum interpretation [5]. He published it in 1926 in a short piece for Nature. Although initially popular, it seems that Klein gave up the idea rather quickly. He recalled that “Pauli and I drank some wine on the death of the fifth dimension in 1928” [5]. Nonetheless, the Kaluza-Klein formalism would be picked up again in the 1970s in context of supergravity research [6, p. 409-410].

Also in 1926, Klein published a paper containing the first relativistic version of the Schroedinger wave equation, around the same time as Gordon’s similar result—leading Dirac to baptize the equation the Klein-Gordon equation  (much to the chagrin of Schroedinger, apparently, who claimed to have discovered the same result but never published it [5]). The Klein-Gordon equation did not extend to particles with spin, however—this required the Dirac equation. But using the Dirac equation, Klein collaborated with Yoshio Nishina to develop an equation for the scattering of radiation from a target electron. The Klein-Nishina formula was published in 1928. Also between 1926 and 1929 Klein worked out the Klein paradox and the “second quantization” with Jordan [3].

During this time, Klein was also one of Bohr’s closest collaborators. They collaborated on the correspondence principle (the idea that the behavior of quantum systems reproduces the behavior of classical systems in the limit of large quantum numbers), complementarity and the Heisenberg uncertainty relations [4, 6 p. 208, 7]. In 1929, Klein used Bohr’s correspondence principle in his quantum treatment of molecules with rotational symmetry [3].

Permanent employment in Stockholm

In 1931, Klein was appointed as lecturer at Stockholms Hogskola (now known as Stockholm University). He would maintain this position until his retirement. Although now more focused on teaching, Klein made several more crucial contributions to physics. He worked with Rydberg on molecular spectra in the early 1930s, which ultimately led to the Rydberg-Klein-Rees method [3]. He also made contributions to gauge theory [3], and from 1945, started an entirely new research line in astrophysics and cosmology [3]. Klein retired in 1962. He died on 5 February 1977 in Stockholm, Sweden.

Bibliography

[1] Interview of Oskar Klein by Thomas S. Kuhn and John L. Heilbron on 1962 September 25, Niels Bohr Library & Archives, American Institute of Physics, College Park, MD USA, www.aip.org/history-programs/niels-bohr-library/oral-histories/4709-1

[2] Klein, O. (2014). From my life of physics. In: The Oskar Klein Memorial Lectures: 1988-1999.  Ekspong, G. (Ed.). World Scientific. (pp. 105-118).

[3] Fischer-Hjalmars, I., & Laurent, B. (2014). Oskar Klein. In: The Oskar Klein Memorial Lectures: 1988-1999.  Ekspong, G. (Ed.). World Scientific. (pp. 7-15).

[4] Interview of Oskar Klein by T. S. Kuhn and J. L. Heilbron on 1963 July 15, Niels Bohr Library & Archives, American Institute of Physics, College Park, MD USA, www.aip.org/history-programs/niels-bohr-library/oral-histories/4709-5

[5] Interview of Oskar Klein by J. L. Heilbron and L. Rosenfeld on 1963 February 25, Niels Bohr Library & Archives, American Institute of Physics, College Park, MD USA, www.aip.org/history-programs/niels-bohr-library/oral-histories/4709-3

[6] Kragh, H. (1999). Quantum Generations: A History of Physics in the Twentieth Century. Princeton University Press.

[7] Interview of Oskar Klein by T. S. Kuhn and J. L. Heilbron on 1963 July 16, Niels Bohr Library & Archives, American Institute of Physics, College Park, MD USA, www.aip.org/history-programs/niels-bohr-library/oral-histories/4709-6

Further reading

Ekspong, G. (Ed.). (2014). The Oskar Klein Memorial Lectures: 1988-1999. World Scientific.

This volume is a collection of three previous volumes bringing together the Oskar Klein memorial lectures between 1988 and 1999. The volume includes a short biography of Oskar Klein by Inga Fischer-Hjalmars and Bertel Laurent, two of his students, and a transcript of an autobiographical address by Oskar Klein. It also includes English translations of some of Oskar Klein’s most important scientific publications.

Ian T. Durham wrote up a biography of Oskar Klein, including a summary of his most important contributions to theoretical physics.