Temporal Proxies in Physics: A Structural Analysis of Time Assignment

Time plays an indispensable role in every major physical theory, yet its empirical status remains obscure. Unlike other SI base quantities, time occupies a distinctive position in measurement practice: no measurement interaction in current physics yields an outcome that instantiates time itself as a physical magnitude. Every operational assignment of temporal intervals instead relies on the behaviour of non-temporal physical processes—periodic oscillations, dynamical evolution, spacetime geometry, entropy gradients, or cosmological expansion—whose measured quantities are interpreted as temporal within a theoretical framework.This paper offers a systematic, operational analysis of how temporal structure is assigned across the principal domains of physics. By comparing time with other SI base quantities, it shows that time alone lacks an instantiating measurement process. It then examines the physical processes employed operationally as clocks in general relativity, quantum mechanics, thermodynamics, and cosmology, identifying the non-temporal quantities these processes actually measure. Because each theory must construct temporal structure indirectly from resources internal to its own ontology, the resulting temporal notions are precise and indispensable within their respective frameworks yet mutually incommensurable.The plurality of temporal concepts in physics is thus diagnosed as a structural consequence of measurement theory, arising from the absence of any instantiating temporal observable rather than from interpretive disagreement. The analysis is diagnostic rather than metaphysical: it neither affirms nor denies any particular ontology of time. Instead, it clarifies the measurement-theoretic constraint under which physics assigns temporal structure and explains why attempts at temporal unification have encountered principled limits.