 Solution

Einstein's distinctive achievement in 1905 was a reorganization of the physical interpretation and foundations of the theory, not the invention of every transformation formula or every previously known optical effect. He took the equivalence of inertial descriptions and a light-propagation postulate as principles from which to construct a general kinematics of rods, clocks and electromagnetic processes, without assigning explanatory work to an observable state of rest of the ether.^[ref]

The nineteenth-century comparison matters. Hendrik Antoon Lorentz's 1895 theory explicitly describes matter moving through a stationary ether and introduces transformed quantities to relate moving and stationary descriptions. Such work supplied genuine physical insight and important mathematical antecedents; it should not be dismissed as an absence of relativity reasoning.^[ref] His 1904 paper, a useful immediate pre-Einstein comparison rather than a nineteenth-century publication, begins from electron theory and adds assumptions about moving material systems. It already seeks a systematic explanation of the failure to detect motion, rather than an unrelated hypothesis for every experiment.^[ref]

The conceptual change can be seen by asking what a moving clock's “local time” represents. In an ether-based explanation there may remain a privileged time relative to which the moving apparatus is dynamically distorted. In Einstein's formulation the times assigned by identically constructed, appropriately synchronized clocks in each inertial system enter the physical description on the same footing. One should not infer a physically privileged inertial system simply from calling one set of coordinates stationary.

This also changes the scope of the explanation. A special electron model might account for the behaviour of one class of instruments. A relativity principle constrains any acceptable dynamics of measuring devices and matter: if one kind of clock revealed absolute uniform motion, the principle would fail. The result is a framework within which more detailed constructive theories can be developed, rather than a substitute for all such theories.

A careful assessment therefore credits both continuity and innovation. Transformation mathematics and contraction hypotheses had antecedents; the operational treatment of distant time, the equal standing of inertial descriptions, and the use of general principles to unify kinematics and electrodynamics form the characteristic 1905 synthesis. It does not follow that an empirically indistinguishable preferred-frame ontology is logically impossible. What is removed is its necessity in this physical formulation. Nor should the separate 1905 mass–energy paper be silently treated as a section of the June relativity paper.