The Equivalence Principle and Kinematical Structure in the ADM Framework
The relation between uniformly accelerated laboratories and laboratories supported in a gravitational field lies at the conceptual core of the Equivalence Principle, yet its precise kinematical content beyond strictly local considerations remains subtle. In this work we develop a unified metric description of these configurations using the standard Arnowitt-Deser-Misner (ADM) formulation of General Relativity, which provides an explicit decomposition of spacetime into spatial hypersurfaces and their temporal evolution. In this setting the ADM shift vector is interpreted as a physical quantity encoding the kinematical relation between spatial slices and their temporal embedding associated with a chosen foliation. This interpretation allows uniformly accelerated laboratories and laboratories supported in a gravitational field to be described within a common structural framework, showing that configurations experiencing identical proper acceleration share an equivalent local shift structure. This viewpoint clarifies the apparent asymmetry between the spatial displacement and energetic cost associated with accelerated motion and their apparent absence in phenomenological descriptions of observers supported in a gravitational field. The formulation remains fully equivalent to standard General Relativity at the level of the field equations and constraints while making explicit kinematical features that are usually implicit in its geometric description. Consequences of this interpretation include a relational account of gravitational time dilation and the emergence of observer-dependent horizons.