2 Before World War II: Research and teaching by single scientists 2.1 The early years of general relativity 2.1.1 Progress during World War I: 1915 to 1918 After a long maturing period, general relativity was completed in November 1915 with field equations suggested by the physicist Albert Einstein [7] and the mathematician David Hilbert [8]. (For the priority debate, cf. [9], [10], [11], [12], [13].) Unlike in the case of special relativity, the physicists around Einstein in Berlin were slow in accepting the new theory. In a letter to Arnold Sommerfeld, he expressed perfect satisfaction about his achievement, but held: "[..] but none of the peers has recognized up to now the depth and necessity of this path." In particular, Max Planck and Max von Laue were not open to his considerations of principle [14]. Nevertheless, already in 1916 and 1917 three of the best known and most useful exact solutions of Einstein's field equations were found. The first, describing an isolated finite spherically symmetric mass-distribution at rest, was communicated to Einstein already in December 1915 by his colleague in Berlin, the astronomer Karl Schwarzschild (1873-1916), then in Russia with the German army. The result was published in 1916 [15], [17], [16]. In the same year, a student of Hendrik Antoon Lorentz in Leiden, Johannes Droste (1886-1963), in his dissertation independently presented this same (Schwarzschild) solution [20]. 4The aircraft-designer and professor at the Technical University in Berlin-Charlottenburg, Hans Reissner (1874-1967), followed with an exact solution for a time-independent, isolated, electrically charged spherically symmetric mass [21]. It was rediscovered in 1918 by Gunnar Nordström [22]. The third important solution, the de Sitter-solution, is described further below. Einstein himself propagated his new gravitational theory in an article [23] and in a book for everbody (with some physics education) [24]; in a number of papers he also worked out consequences of the theory, among others for cosmology and gravitational waves [27]. Also in 1916, the astronomer Erwin