Jürgen Ehlers (German: [ˈjʏʁɡn̩ ˈʔeːlɐs]; 29 December 1929 – 20 May 2008) was a German physicist who contributed to the understanding of Albert Einstein's theory of general relativity. From graduate and postgraduate work in Pascual Jordan's relativity research group at Hamburg University, he held various posts as a lecturer and, later, as a professor before joining the Max Planck Institute for Astrophysics in Munich as a director. In 1995, he became the founding director of the newly created Max Planck Institute for Gravitational Physics in Potsdam, Germany. Ehlers' research focused on the foundations of general relativity as well as on the theory's applications to astrophysics. He formulated a suitable classification of exact solutions to Einstein's field equations and proved the Ehlers–Geren–Sachs theorem that justifies the application of simple, general-relativistic model universes to modern cosmology. He created a spacetime-oriented description of gravitational lensing and clarified the relationship between models formulated within the framework of general relativity and those of Newtonian gravity. In addition, Ehlers had a keen interest in both the history and philosophy of physics and was an ardent populariser of science.
Biography
Early life Jürgen Ehlers was born in Hamburg on 29 December 1929. He attended public schools from 1936 to 1949, and then went on to study physics, mathematics and philosophy at Hamburg University from 1949 to 1955. In the winter term of 1955–56, he passed the high school teacher's examination (Staatsexamen), but instead of becoming a teacher undertook graduate research with Pascual Jordan, who acted as his thesis advisor. Ehlers' doctoral work was on the construction and characterization of solutions of the Einstein field equations. He earned his doctorate in physics from Hamburg University in 1958. Prior to Ehlers' arrival, the main research of Jordan's group had been dedicated to a scalar-tensor modification of general relativity that later became known as Jordan–Brans–Dicke theory. This theory differs from general relativity in that the gravitational constant is replaced by a variable field. Ehlers was instrumental in changing the group's focus to the structure and interpretation of Einstein's original theory. Other members of the group included Wolfgang Kundt, Rainer K. Sachs and Manfred Trümper. The group had a close working relationship with Otto Heckmann and his student Engelbert Schücking at Hamburger Sternwarte, the city's observatory. Guests at the group's colloquium included Wolfgang Pauli, Joshua Goldberg and Peter Bergmann. In 1961, as Jordan's assistant, Ehlers earned his habilitation, qualifying him for a German professorship. He then held teaching and research positions in Germany and in the US, namely at the University of Kiel, Syracuse University and Hamburg University. From 1964 to 1965, he was at the Graduate Research Center of the Southwest in Dallas. From 1965 to 1971, he held various positions in Alfred Schild's group at the University of Texas at Austin, starting as an associate professor and, in 1967, obtaining a position as full professor. During that time, he held visiting professorships at the universities of Würzburg and Bonn.
Munich In 1970, Ehlers received an offer to join the Max Planck Institute for Physics and Astrophysics in Munich as the director of its gravitational theory department. Ehlers had been suggested by Ludwig Biermann, the institute's director at the time. When Ehlers joined the institute in 1971, he also became an adjunct professor at LMU Munich. In March 1991, the institute split into the Max Planck Institute for Physics and the Max Planck Institute for Astrophysics, where Ehlers' department found a home. Over the 24 years of his tenure, his research group was home to, among others, Gary Gibbons, John Stewart and Bernd Schmidt, as well as visiting scientists including Abhay Ashtekar, Demetrios Christodoulou and Brandon Carter. One of Ehlers' postdoctoral students in Munich was Reinhard Breuer, who later became editor-in-chief of Spektrum der Wissenschaft, the German edition of the popular-science journal Scientific American.
Potsdam When German science institutions reorganized after German reunification in 1990, Ehlers lobbied for the establishment of an institute of the Max Planck Society dedicated to research on gravitational theory. On 9 June 1994, the Society decided to open the Max Planck Institute for Gravitational Physics in Potsdam. The institute started operations on 1 April 1995, with Ehlers as its founding director and as the leader of its department for the foundations and mathematics of general relativity. Ehlers then oversaw the founding of a second institute department devoted to gravitational wave research and headed by Bernard F. Schutz. On 31 December 1998, Ehlers retired to become founding director emeritus. Ehlers continued to work at the institute until his death on 20 May 2008. He left behind his wife Anita Ehlers, his four children, Martin, Kathrin, David, and Max, as well as five grandchildren.
Research Ehlers' research was in the field of general relativity. In particular, he made contributions to cosmology, the theory of gravitational lenses and gravitational waves. His principal concern was to clarify general relativity's mathematical structure and its consequences, separating rigorous proofs from heuristic conjectures.
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