l x v i I N T R O D U C T I O N T O V O L U M E 1 6 constructing a metric-compatible Levi-Civita connection from the metric, as well as the associated notion of parallel transport, the tetrad field allowed the definition of another connection with its notion of parallelism. In contrast to the Levi-Civita connection, the teleparallel connection is asymmetric and describes a geometry that has vanishing Riemann curvature. Instead, it is characterized by the nonvanishing of a tensorial quantity constructed from the teleparallel connection that is now known as the torsion tensor. Taking the mathematical expression of torsion, a third-rank tensor, Einstein tentatively identified its contraction with the electromagnetic four-potential. And settling on what seemed to be the simplest in- variant to be taken as a basis for a tentative field theory, Einstein succeeded in de- riving, to first approximation in the field components, both the gravitational field equations of general relativity as well as an equivalent version of the Maxwell equations. Again, a brief note on this work was presented by Planck to the Academy, on 14 June 1928 (see Abs. 593), and was published in July under the title “New Pos- sibility for a Unified Field Theory of Gravitation and Electricity” (Doc. 219). These two notes mark the beginning of a search for a unified field theory in this teleparallel framework that would preoccupy Einstein for the next two or three years, beyond the end of the time period covered by this volume. The elaboration of the implications and consequences of the framework of dis- tant parallelism for a unified theory was explored by Einstein in intense coopera- tion with a number of correspondents and coworkers who either reacted to the new approach or carried out calculations at Einstein’s request. Early correspondents include a student named Curt Schwarz (Doc. 229), as well as Einstein’s longtime collaborator Jakob Grommer. The latter soon pointed out to Einstein some difficul- ties for the physical interpretation of the geometric framework caused by identify- ing the contracted torsion tensor with the potential vector (Doc. 232). Most critical of these difficulties was Grommer’s warning that no solution of the full equations may exist for every solution of the linearized equation (Doc. 91). The most intense correspondence on the teleparallel approach in this phase of the project took place with the mathematician Chaim Herman Müntz. Müntz was born in Lodz, Poland, in 1884.[44] His family was bourgeois and Jewish, though not religious. After studying mathematics, sciences, and philosophy in Berlin under Georg Frobenius and Hermann Amandus Schwarz, among others, Müntz obtained his Ph.D. with a dissertation on the boundary-value problem of partial differential equations of minimal surfaces. At the time of the collaboration with Einstein, Müntz was living again in Berlin and, in spite of having published a number of re- search papers in various mathematical fields, had not obtained a Habilitation and