D O C U M E N T 2 5 9 A P R I L 1 9 2 6 4 3 7 [4]Weyl 1917 contains the first discussion of the general class of static axialsymmetric solutions to the Einstein field equations, including a first discussion of the special case of an axialsymmetric dis- tribution of point masses. Levi-Civita 1917, 1919 then investigated the class of static solutions and argued that Weyl’s investigation had been correct but incomplete because Weyl restricted his attention to members of this class of metrics that left the canonical form of the line element, defined with respect to cylinder coordinates in Weyl 1917, unchanged. Weyl 1919 answered Levi-Civita that the latter had been interested in solutions to the vacuum field equations, whereas Weyl wanted to derive the gravitational field of axialsymmetric mass distributions. Thus, Weyl assumed the energy-momen- tum tensor not to vanish at the locations of the respective masses. In Weyl 1919 he further argued that such systems could only be static if the gravitational attraction of the masses was counteracted by nonvanishing stresses between the masses. Specifically, he assumed that the components and of the energy-momentum tensor density between the masses had to cancel each other out in order to keep the mass distribution in equilibrium. He used Levi-Civita’s results to calculate these stresses explicitly. Bach and Weyl 1922 then gave the most comprehensive discussion of the static axialsym- metric solutions to the Einstein field equations. Bach discussed the field of two axialsymmetric and approximately spherical bodies in detail, and argued that the solution had a singularity along the axis connecting the two bodies. Weyl investigated the static two-body problem further and found that under the special assumption that the azimutal tension between the two bodies vanishes, an assump- tion he had used in his 1917 paper, a singularity in the metric between the bodies can be avoided, but that there was still a singularity in the first derivatives of the metric. For a detailed discussion of the interpretation of static axialsymmetric solutions to the Einstein equations, see Bonnor 1992, sec. 3 for mathematical details, see Stephani et al. 2009, sec. 20.2, and Griffiths and Podolský 2009, sec.10. [5]See Doc. 245. 259. To Emil Rupp [Berlin,] 18. IV. 26. Sehr geehrter Herr Rupp! Es freut mich sehr, dass Sie die Untersuchung mit mir zusammen machen wollen.[1] Ich bitte Sie nun sehr, mich über die geplanten Anordnungen und Versu- che informieren zu wollen, auch über Resultate, zumal mir noch neue Gesichts- punkte eingefallen sind, die beachtet werden müssen. Wenn es nicht gerade das Heidelberger Laboratorium wäre,[2] würde ich hinkommen, denn die Ergebnisse werden recht wichtig werden für die Theorie der Strahlung. Also schreiben Sie bald Näheres Ihrem A. Einstein. ALSX (CaSdU). [70 703]. There are perforations for a loose-leaf binder at the left margin of the doc- ument. [1]See Doc. 252. [2]The head of the laboratory was Philipp Lenard, antirelativist and anti-Semite, who was not well- disposed to Einstein. T1 1 T22