A Universal Principle of Rest Defined by Equal One-Way Light Intervals

The 1905 derivation of the relativity of simultaneity requires the joint affirmation of two incompatible claims: that a rod moves relative to the light-propagation structure (so that the unequal intervals L/(c+v) and L/(c-v) appear), and that the resulting differentiation is evidence that no such relative motion exists. We replace the stipulation that one-way light intervals are equal inside every inertial frame with the measurable condition that those intervals are equal only for bodies at rest relative to the light-propagation events. The condition TAB =TBA is adopted as a universal standard of rest. Local proper-time records and the round-trip identity are retained. Time-dilation factors and mass-energy equivalence are recovered from the kinematics of light relative to moving clocks without imposing frame-dependent simultaneity or the Lorentz transformation. The construction is incompatible with full Lorentz invariance; its discriminating prediction is a first-order differential in local intervals that vanishes only for the body satisfying the rest condition. The kinematic structure is consistent with a dynamical extension in which proper time is the accumulated record of a single condensation rate.