Jürgen Ehlers (physicist)

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Jürgen Ehlers on the occasion of the awarding of the medal by the Charles University in Prague

Jürgen Ehlers (born December 29, 1929 in Hamburg ; † May 20, 2008 in Potsdam ) was a German physicist who made important contributions to Einstein's general theory of relativity . During his academic training and afterwards, he first worked at Pascual Jordan in Hamburg . He was then a lecturer and professor before he was appointed to the Max Planck Institute for Astrophysics in Munich .

Ehlers is known for his classifications of the exact solutions of Einstein's field equations , for the Ehlers-Geren-Sachs theorem , for a space-time-oriented representation of the phenomenon of gravitational lenses, and for his work on the connection between general relativity and Newton's theory of gravity. In addition, Ehlers tried to explain the sciences in a generally understandable way and was interested in the history and philosophy of physics.

life and work

Jürgen Ehlers was born in Hamburg and went to school between 1936 and 1949 before studying physics, mathematics and philosophy at the University of Hamburg from 1949 to 1955 . In the winter semester he passed the examination for the teaching profession, but he then went to a research group. In 1958 he received his doctorate under Pascual Jordan on the construction and characterization of Einstein's equations . The group around Jordan was originally busy with the modification of Einstein's theory to a scalar tensor theory, by Ehlers the emphasis was placed again on the interpretation of the original theory of Einstein . Other members of the group were Wolfgang Kundt , Engelbert Schücking , Otto Heckmann , Rainer K. Sachs and Manfred Trümper . In 1959 he was Jordan’s research assistant in Hamburg.

Ehlers completed his habilitation in Hamburg in 1961. After teaching and research activities in Kiel (teaching assignment 1961), Syracuse University (Research Associate 1962/63) and Hamburg (lecturer 1963/64), he accepted invitations to the USA, where he was a visiting associate professor at Graduate Research from 1964 to 1965 Center of the Southwest in Dallas and from 1965 to 1971 at the University of Texas at Austin worked, first as an associate professor and from 1967 as a professor of physics. At the same time, he was visiting professor at the University of Würzburg and the University of Bonn in 1969/70 .

Munich

In 1971 Ehlers was offered a position at the Max Planck Institute for Physics and Astrophysics (MPIAP) in Munich after the director of the institute at the time, Ludwig Biermann , became aware of him. Ehlers accepted the offer and at the same time received an honorary professorship at the Ludwig Maximilians University . At the MPIAP he headed the gravitation theory working group as a Scientific Member until 1995 . During his time as director he worked at the institute with numerous researchers, including a. Bernd Schmidt , Demetrios Christodoulou , Gary Gibbons , John Stewart and Brandon Carter together. Also Reinhard Breuer , who later became editor of the popular science Scientific American , was a PhD at that time a researcher at Ehlers.

Potsdam

Soon after the reunification of Germany in 1990, Ehlers successfully campaigned for a Max Planck Institute to investigate Albert Einstein's research approaches. In 1995 he became the first director of the newly founded Max Planck Institute for Gravitational Physics in Potsdam-Golm, where he worked until his retirement in 1998. Here he also headed the research group “Fundamentals and Mathematics of General Relativity”. He also founded a second group around Bernard Schutz that dealt with gravitational waves . After his retirement , Ehlers continued to work at the institute until his death.

Scientific work

Ehlers dealt with questions of general relativity and cosmology , including the theory of gravitational waves . In his doctoral thesis, he examined the exact solutions of Einstein's equations for universes that are simple enough to allow explicit solutions. However, since the generally relativistic laws are covariant and therefore independent of the coordinate systems chosen , exactly the same model universes could be defined in different descriptions. Ehlers looked for ways to characterize these descriptions invariant , so that they no longer depend on the chosen coordinate systems and their equality becomes apparent. To do this, he examined the inherent geometric properties of the known exact solutions of Einstein's equations.

Gravitational waves and black holes

Ehlers later published work that pursued the ideas of his doctoral thesis. With Pascual Jordan and Wolfgang Kundt , he dealt with the systematic recording and characteristics of the exact solutions to Einstein's equations of general relativity. The authors used sophisticated methods of differential geometry , among other things. a. the Petrov classification (after Alexei Petrow ) of the Weyl tensors (i.e. those parts of the Riemann tensor describing the space-time curvature that is not restricted by the Einstein equations), isometric groups and conformal transformations . This resulted in the first definition and classification of a class of particularly simple gravitational waves, the pp waves . With Rainer K. Sachs and Manfred Trümper he dealt with vacuum solutions for gravitational radiation , which have special algebraic properties and were expressed in the Bispinor formalism . In his investigations into exact solutions, Ehlers also systematically demonstrated the geometric properties of light bundles ( congruences ) with expansion and general deformation ( torsion and shear ). The mathematical aids that were developed later helped Roy Kerr in discovering the Kerr solution for rotating black holes , an important exact solution to Einstein's equations. In the last work of this phase Ehlers dealt with the general-relativistic treatment of continuous media .

Another part of Jürgen Ehler's doctoral thesis also turned out to be fruitful: he dealt with the proof of some properties of the surface of black holes that later led to the concept of the event horizon . An important consequence of these properties was that the gravitational field within the surface cannot be static, but has to change over time.

Ehlers group

Ehlers found a dual symmetry between various components of the metric of a stationary spacetime that maps solutions of Einstein's equations to other solutions. This duality was later generalized to an SL (2) symmetry called the Ehlers group. Further generalizations then led to the Geroch group , which consists of two non-commuting subgroups, one of which is the Ehlers group. From this the idea of ​​“hidden symmetries” developed, which play an important role in Kaluza-Klein theory extensions of general relativity and its generalizations, such as eleven-dimensional supergravity .

Basic concepts of general relativity

During his research work, Ehlers repeatedly took care of the fundamentals of general relativity. In the 1960s he worked with Felix Pirani and Alfred Schild on a constructive-axiomatic approach to general relativity, looking for a way to derive Einstein's theory from a minimal number of objects and axioms. These include A. Events , rays of light, particles and free falling particles. In the beginning, space-time, according to its approach, is a set of events without any further structuring. By assuming the fundamental properties of light and free-falling particles introduced as axioms, the differential topology , conformal structure and finally the metric structure of spacetime are constructed. The construction process is based on idealized measurements. In the final step, Einstein's equations are derived using the weakest additional axioms. This creates a formulation of the equations in which the assumptions that lead to the general theory of relativity are clearly defined.

In the 1970s, Ehlers and Ekkart Rudolph dealt with the general relativistic treatment of the rigid body . Although had Max Born , in 1909 given a definition of a rigid body that is compatible with the relativistic physics, but this definition is based on assumptions that no longer met the general-relativistic spacetime and are too restrictive. Ehlers and Rudolph generalized Born's definition to a more easily applicable definition, which they called “pseudo-strength” and which is a better approximation of the concept of strength used in classical physics.

The Ehlers-Geren-Sachs theorem, which he proved in 1968 with P. Geren and Rainer K. Sachs, concludes from the observation of the isotropy of the cosmic background radiation for all free-falling observers that the cosmology is described by homogeneous and isotropic FLRW metrics .

Gravitational lenses

Representation of a gravitational lens, with the help of which light is deflected by gravitational force.

In the 1980s, Ehlers and Peter Schneider turned to research into the fundamentals of gravitational lenses . One result was a study published jointly with Emilio Falco in 1992 , which for the first time provided a systematic presentation of both the theoretical results and the observational data. From the point of view of astronomy, gravitational lenses are often introduced as a quasi-Newtonian approximation, in which the gravitational field is small and the deflection is infinitesimal - this is also sufficient for most observations. In their publication, however, the researchers developed a complete and consistent description of gravitational lenses using purely general relativistic means.

Frame theory and Newton's gravitational model

The derivation of Isaac Newton's theory of gravity from the general theory of relativity was already presented by Einstein at the beginning and made predictions of observable phenomena such as the perihelion of Mercury . Later work by Élie Cartan , Kurt Friedrichs and others showed a more concrete way how Newton's theory can be understood from general relativity ( Newton-Cartan theory ): for this only one specific parameter λ had to go to 0. Ehlers continued this work and generalized it to frame theory, a mathematically precise way of constructing the Newton-Cartan limit not only for the physical laws of general relativity, but also for all solutions to Einstein's equations. The Newtonian limit case for a black hole according to the Schwarzschild solution is then a simple particle with no expansion. In addition, interesting exact solutions for Friedmann-Lemaître spacetimes and the Gödel universe can be given.

Ehlers also took part in discussions (with Peter Havas, among others ) about the retroactive effects of gravitational radiation and examined how this situation can be systematically described in a general relativistic theory. He found that the usual quadrupole formula for the energy flow for systems such as binary pulsars (for example in the much investigated classical system PSR 1913 + 16 , on which the detection of gravitational waves was possible) had not been strictly solved and a correct representation of higher terms Should include order.

The work on the Newtonian limit values ​​of the general relativity theory gave Ehlers and his colleague Thomas Buchert the idea of ​​a systematic investigation of disturbances and inhomogeneities in a Newtonian cosmos. Buchert drew from this the basis of his general relativistic generalization of inhomogeneities, with an interpretation of the cosmological constant - or in modern terminology dark energy - as a non-linear sequence of inhomogeneities in general relativistic cosmology.

History and Philosophy of Physics

Jürgen Ehlers was interested not only in the hard topics of research, but also in the history of physics and, in a broader sense, the history of science. Until his death he worked on a project of the Max Planck Institute for the History of Science on the history of quantum physics , examining Pascual Jordan's important work from 1925 to 1928.

Memberships and honors

family

Ehlers was married to Anita Ehlers. She studied physics and mathematics in Hamburg, where she met her husband. She worked as a teacher in Munich and became known for translating popular science books. Her own love for music led her to work on the subject of "Einstein and Music". Her book “Dear Hertz! Physicists and mathematicians in anecdotes ”.

Fonts

  • With Gerhard Börner, Heinrich Meyer (Ed.): From the Big Bang to the Complex Universe: The Cosmology of the Present. Piper, 1993.
  • With Gerhard Börner (ed.): Gravitation. Spectrum, 1996.
  • (Ed.): Relativity theory and astrophysics. American Mathematical Society, 1967.
  • (Ed.): Isolated gravitational systems in general relativity. E. Fermi School, Varenna 1977, North Holland 1979.
  • With Claus Lämmerzahl (Ed.): Special relativity: will it survive the next 101 years? Springer 2006 (Heraeus Seminar).
  • With H. Friedrich (Ed.): Canonical gravity- from classical to quantum. Springer 1994 (Heraeus Seminar).
  • With Peter Schneider, EE Falco: Gravitational Lenses. Springer 1992, 1999.
  • With Wolfgang Kundt : Exact solutions of gravitational field equations. In: L. Witten (Ed.): Gravitation - introduction to current research. Wiley 1962-
  • Survey of General Relativity. In: Werner Israel (Ed.): Relativity, Astrophysics and Cosmology. Reidel 1973.
  • Contributions to the relativistic dynamics of continuous media , treatises of mathematic and natural science. Class, Academy of Sciences Mainz, No. 11, 1961, pp. 782-837, reprinted in: George FR Ellis , Malcolm AH MacCallum, Andrzej Krasinski (eds.) Golden Oldies in General Relativity. Hidden Gems , Springer Verlag 2013.
  • With Pascual Jordan , Wolfgang Kundt Strict solutions of the field equations of general relativity , treatises of mathematic and natural science. Class, Academy of Sciences, Mainz, No. 2, 1960, pp. 21-105.
  • With Felix Pirani , Alfred Schild The geometry of free fall and light propagation , in O´Raifeartaigh (Ed.) General Relativity. Papers in honor of JL Synge , Oxford, Clarendon Press 1972, pp. 63-84.

literature

  • Bernd G. Schmidt (Ed.): Einstein's field equations and their physical implications. Selected essays in honor of Jürgen Ehlers. Springer 2000.

Web links

Individual evidence

  1. ^ George Ellis, Andrzej Krasiński: Editors' comment. In: General Relativity and Gravitation. Vol. 39, 2007, pp. 1941-1942.
  2. ^ Rüdiger Braun: Where time and space end. The co-founder of the Max Planck Institute for Gravitational Physics in Golm, Jürgen Ehlers, passed away unexpectedly. In: Märkische Allgemeine Zeitung. May 27, 2008.
  3. ↑ Bernd Schmidt: Foreword. In: Einstein's Field Equations and their Physical Implications. Selected Essays in Honor of Jürgen Ehlers. Springer, Berlin 2000, ISBN 3-540-67073-4 , pp. 1–126.
personal data
SURNAME Ehlers, Jürgen
BRIEF DESCRIPTION German physicist
DATE OF BIRTH December 29, 1929
PLACE OF BIRTH Hamburg
DATE OF DEATH May 20, 2008
Place of death Potsdam