Local Versus Global Time in Early Relativity Theory

In his groundbreaking 1905 paper on special relativity, Einstein distinguished between local and global time in inertial systems, introducing his famous definition of distant simultaneity to give physical content to the notion of global time. Over the following decade, Einstein attempted to generalize this analysis of relativistic time to include accelerated frames of reference, which, according to the principle of equivalence, should also account for time in the presence of gravity. Characteristically, Einstein’s early methodology focused on simple, intuitively accessible physical situations, often exhibiting a high degree of symmetry. In this paper, we show that while Einstein’s heuristic approach led to significant insights into the nature of relativistic time, it also posed challenges for him in making the final conceptual leap to global time as a conventionally chosen fourth coordinate. We also address some misunderstandings concerning Einstein’s early work on time and the gravitational redshift.