Abstract

The quest for a ‘unified field theory’, which aims to integrate gravitational and electromagnetic fields into a single field structure, spanned most of Einstein’s professional life from 1919 until his death in 1955. It is seldom noted that Hans Reichenbach was possibly the only philosopher who could navigate the technical intricacies of the various unification attempts. By analyzing published writings and private correspondences, this paper aims to provide an overview of the Einstein-Reichenbach relationship from the point of view of their evolving attitudes toward the program of unifying electricity and gravitation. The paper concludes that the Einstein-Reichenbach relationship is more complex than usually portrayed. Reichenbach was not only the indefatigable ‘defender’ of relativity theory but also the caustic ‘attacker’ of Einstein’s and others’ attempts at unified field theory. Over the years, Reichenbach managed to provide the first, and possibly only, overall philosophical reflection on the unified field theory program. Thereby, Reichenbach was responsible for bringing to the debate, often for the first time, some of the central issues of the philosophy of space-time physics: (a) the relation between a theory’s abstract geometrical structures (metric, affine connection) and the behavior of physical probes (rods and clocks, free particles, and so on); (b) the question of whether such association should be regarded as a geometrization of physics or a physicalization of geometry; (c) the interplay between geometrization and unification in the context of a field theory.