Length matters: The Einstein–Swann correspondence and the constructive approach to the special theory of relativity

I discuss a rarely mentioned correspondence between Einstein and Swann on the constructive approach to the special theory of relativity, in which Einstein points out that the attempts to construct a dynamical explanation of relativistic kinematical effects require postulating a fundamental length scale in the level of the dynamics. I use this correspondence to shed light on several issues under dispute in current philosophy of spacetime that were highlighted recently in Harvey Brown's monograph Physical Relativity, namely, Einstein's view on the distinction between principle and constructive theories, and the consequences of pursuing the constructive approach in the context of spacetime theories.