Past exam of the short options paper shared by Prelims and Parts A and B of the physics course of the University of Oxford / 2016 / S27 / 5 / Solution

A constructive theory attempts to build observed phenomena from a specified underlying mechanism or constitution. Kinetic theory, for example, explains gas pressure and thermal behavior through molecular motions and interactions. A principle theory begins from general constraints that processes must satisfy and derives restrictions on possible descriptions, rather than first proposing a detailed microscopic mechanism. Thermodynamics is an example through its conservation and transformation principles.

Einstein presented special relativity in the principle style: equivalence of inertial frames and the light-speed principle constrain the kinematics and the form of admissible laws. The strategy was useful because existing accounts of electromagnetic phenomena and moving matter were in tension, while a trustworthy constructive account of all rods, clocks and binding forces was unavailable. It allowed broad conclusions without assuming a detailed ether mechanism whose observable effects had not been established.

The distinction is methodological rather than a division between empirical and nonempirical theories. Principles need empirical motivation and coherent interpretation, while constructive models also rely on general assumptions. Nor does a successful principle theory make questions about the dynamics of rods and clocks illegitimate. A constructive account may explain how particular material systems realize the constraints, provided it reproduces the same operational relations.

Einstein's reason for introducing the distinction was therefore partly to explain the character and limitations of the relativistic strategy. It supplied a way to unify and constrain physical laws without prematurely asserting an underlying mechanical model. It was not a proof that constructive explanations can never be valuable or that relativistic effects are only coordinate conventions.^[ref]