Erich Kretschmann

Erich Justus Kretschmann (14 July 1887 in Berlin – 1973) was a German physicist.

He obtained his D.Phil. at Berlin University in 1914 with his dissertation entitled "Eine Theorie der Schwerkraft im Rahmen der ursprünglichen Einsteinschen Relativitätstheorie (A theory of gravity in the framework of the original Einstein theory of relativity)". His advisors were Max Planck and Heinrich Rubens. After working as a Gymnasium (school) teacher, he became Privatdozent for theoretical physics at the University of Königsberg in 1920, where he eventually became professor extraordinarius in 1926. From 1946 until 1952, Kretschmann was professor for theoretical physics and director of the institute for theoretical physics at the University of Halle-Wittenberg.

In his 1915 papers, he introduced the Kretschmann scalar. In his 1915 papers he also introduced, though not in name, the point coincidence argument in relativity. Similar ideas appeared in Einstein's writings on general relativity. Historians Don Howard and John Norton suggest that Einstein may have failed to adequately acknowledge Kretschmann's contribution. Kretschmann's use of the argument was more topological while Einstein's use involved physical measurements.

Kretschmann is most famous for his 1917 claim that Einstein's use of the principle of covariance in General Relativity is vacuous. Kretschmann claimed that the demand that a theory be put in generally covariant form does not limit or restrict the range of acceptable theories, but is simply a challenge to the mathematician's ingenuity. According to Kretschmann, any theory can be put in generally covariant form. Einstein responded that even if general covariance is not a purely formal limitation on acceptable theories, it plays "an important heuristic role" in the formulation of General Relativity.

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