Jürgen Ehlers

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Jürgen Ehlers
At the award ceremony for the Charles University Medal in Potsdam, September 2007
Born	November 29, 1929(1929-11-29) Hamburg
Died	May 20, 2008 (aged 78) Potsdam, Germany
Residence	Germany
Nationality	German
Fields	Physics
Institutions	University of Hamburg Max Planck Institute for Astrophysics Max Planck Institute for Gravitational Physics
Alma mater	University of Hamburg
Doctoral advisor	Pascual Jordan
Doctoral students	Thomas Buchert
Known for	General relativity Mathematical physics
Notable awards	Max Planck Medal (2002)

Jürgen Ehlers (December 29, 1929–May 20, 2008) was a German physicist who made notable contributions to the current understanding of Albert Einstein's theory of general relativity. In particular, Ehlers' research focused on the theory's foundations and on its applications to gravitational waves, gravitational lenses, and cosmology. At the time of his death, he was the founding director emeritus of the Max Planck Institute for Gravitational Physics in Potsdam, Germany.

Contents 1 Biography 1.1 Early career 1.2 Munich 1.3 Potsdam 1.4 Honours and awards 2 Work 2.1 Gravitational lensing 2.2 History of science 2.3 Science communication 3 Selected publications 4 References 5 External links

[edit] Biography

[edit] Early career

From 1949 to 1955, Ehlers studied physics, mathematics, and philosophy at Hamburg University. His graduate research on the construction and characterization of solutions of the Einstein field equations earned him a doctorate in 1958; his doctoral advisor was Pascual Jordan. In 1961, having become Jordan's assistant, Ehlers obtained his "Habilitation," the advanced degree that, by the rules of German academia, qualifies the bearer for a professorship. He then held teaching and research positions at the University of Kiel, Syracuse University, and Hamburg University, before once more moving to the United States: from 1964 to 1965, he was at the Graduate Research Center of the Southwest in Dallas, and from 1965 to 1971 at the University of Texas in Austin, first as an associate professor and, from 1967 on, as a full professor of physics. During that time, he also held visiting professorships at the universities of Würzburg and Bonn.^[1]

[edit] Munich

In 1971, Ehlers received an offer to join the Max Planck Institute for Physics and Astrophysics in Munich, as the director of its gravitational theory department, as well as an adjunct professor at the University of Munich. Later, the department would become part of the newly formed Max Planck Institute for Astrophysics. Over the following 24 years, 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.^[2]

[edit] Potsdam

As German science institutions reorganized after German re-unification in 1990, Ehlers lobbied for the establishment of an institute of the Max Planck Society dedicated to research on Einstein's theories of gravity. He was successful, and became the founding director of the Max Planck Institute for Gravitational Physics in Potsdam in 1995, as well as the leader of its department for the foundations and mathematics of general relativity. He also oversaw the funding of a second institute department devoted to gravitational wave research and headed by Bernard F. Schutz. In the beginning of 1999, Ehlers retired to become founding director emeritus.^[3]

[edit] Honours and awards

In the course of his career, Ehlers received various awards and honours. He became a member of the Berlin-Brandenburgische Akademie der Wissenschaften, the Akademie der Wissenschaften und der Literatur, Mainz (1972), the Leopoldina in Halle (1975), and the Bavarian Academy of Sciences and Humanities in Munich (1979).^[4] From 1995 to 1998, he served as president of the International Society on General Relativity and Gravitation.^[5] He also received the 2002 Max Planck Medal of the German Physical Society, the Volta Gold Medal of Pavia University (2005), and the medal of the Faculty of Natural Sciences of Charles University, Prague (2007).^[6]

[edit] Work

Ehlers' research was in the field of general relativity. In particular, he made important contributions to cosmology, the theory of gravitational lenses and gravitational waves. His principal concern was to clarify the theory's mathematical structure and its consequences, separating rigorous proofs from heuristic conjectures.^[7]

In the research that led to his doctoral thesis, he proved important properties of the surface around black holes that would later be identified as a black hole's horizon, in particular that the gravitational field inside cannot be static, but must change with time. During the 1960s, Ehlers published a series of seminal papers, all but one in collaboration with colleagues from the Hamburg group: a systematic exposition of the properties and characteristics of exact solutions to Einstein's field equations, including the definition of pp-waves (with Jordan and Wolfgang Kundt),^[8] two treatises on gravitational radiation (one with Rainer Sachs, one with Manfred Trümper), and a seminal paper on the general-relativistic treatment of the mechanics of continuous media.^[9]

Ehlers' doctoral thesis also introduced part of what is now known as the Ehlers group, a (finite-dimensional) transformation group of stationary vacuum spacetimes which maps solutions of Einstein's field equations to other solutions. It has since found a number of applications, from use as a tool in the discovery of previously unknown solutions to a proof that solutions in the stationary axi-symmetric case form an integrable system.^[10]

Interested in the foundations of relativity, Ehlers also took part (in 1972) in the derivation of the theory from assumptions about falling bodies and about the propagation of light alone, together with Felix Pirani and Alfred Schild.

Ehlers also developed a mathematically precise way of expressing the Newtonian limit of general relativity, recovering Newton's classical description of gravity from the geometrical space-time description of Einstein's theory, and took part in the discussion of how the back-reaction from gravitational radiation onto a radiating system could be systematically described in a non-linear theory such as general relativity.^[11]

[edit] Gravitational lensing

With Peter Schneider, Ehlers embarked on an in-depth study of the foundations of gravitational lensing; one result of this work was an 1992 monograph co-authored with both Schneider and Emilio Falco: the first systematic exposition of the field that included both the theoretical foundations and observational results. From the viewpoint of astronomy, gravitational lensing is often described using a quasi-Newtonian approximation — assuming the gravitational field to be small and the deflection angles to be minute — which is perfectly sufficient for most situations of astrophysical relevance. In contrast, the monograph developed a thorough and complete description of gravitational lensing from a fully relativistic space-time perspective; this feature of the book has played a major part in the book's long-term positive reception.^[12] In the following years, Ehlers continued his research on the propagation of bundles of light in arbitrary spacetimes.^[13]

[edit] History of science

With a keen interest in the historical background of his own science, Ehlers also did research in the history of science. Up until his death, he collaborated in a project on the history of quantum theory at the Max Planck Institute for the History of Science in Berlin.^[14]

[edit] Science communication

Ehlers showed a keen interest in communicating his area of research to a general audience. A frequent public lecturer, he has also authored a number of popular-science articles, edited a compilation of articles on gravity from the German edition of Scientific American.^[15] and contributed to general-audience journals such as Bild der Wissenschaft.^[16]

[edit] Selected publications

• Börner, G. & Ehlers, J., eds. (1996), Gravitation, Spektrum Akademischer Verlag, ISBN 3-86025-362-X 
• Ehlers, Jürgen (1973), “Survey of general relativity theory”, in Israel, Werner, Relativity, Astrophysics and Cosmology, D. Reidel, pp. 1–125, ISBN 90-277-0369-8 
• Schneider, P.; Ehlers, J. & Falco, E. E. (1992), Gravitational lenses, Springer, ISBN 3540665064 

[edit] References

[edit] External links

Jürgen Ehlers in the German National Library catalogue

Persondata
NAME	Ehlers, Jürgen
ALTERNATIVE NAMES
SHORT DESCRIPTION	German physicist, general relativity
DATE OF BIRTH	December 20, 1929
PLACE OF BIRTH	Hamburg, Germany
DATE OF DEATH	May 20, 2008
PLACE OF DEATH

This article incorporates text translated from the corresponding German Wikipedia article as of May 21, 2008.