Erich Justus Kretschmann (14 July 1887 – 1973) was a German physicist. Life Kretschmann was born in Berlin. He obtained his PhD 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. Work 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 D. 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. Einstein wrote concerning Kretschmann's objection: "Although it is true that every empirical law can be put in a generally covariant form, yet the principle of relativity possesses a great heuristic power....Of two theoretical systems, both of which agree with experience, the one is to be preferred which, from the point of view of the absolute differential calculus is the simpler and more transparent. Let one express Newtonian mechanics four-dimensionally in the form of generally covariant equations and one will surely be convinced that the principle of relativity excludes this theory from the practical, though not the theoretical, viewpoint." (Einstein, Albert, 1918. "Principielles zur allgemeinen Relativitätstheorie," Annalen der Physik, vol. 55, esp. p. 242) Einstein suggested that Newtonian theory would be impossibly complex if put