Most high-precision tests of general relativity constrain reciprocity-even, largely local observables within single-metric frameworks. This leaves open a specific underdetermination between General Relativity (GR) and a class of two-metric disformal scalar-tensor modifications, exemplified here by the Temporal Equivalence Principle (TEP). This paper formalizes a measurement taxonomy distinguishing gauge-invariant from convention-dependent observables and identifies six recurring scope limitations in the experimental canon: (1) two-way measurement dominance; (2) local/global conflation; (3) model-dependent calibration; (4) single-path multi-messenger constraints on differential propagation that do not directly test common-mode conformal clock-sector structure; (5) theory-laden data reduction; and (6) the density-regime screening blind spot, whereby tests performed in deep potential wells probe only the screened regime where scalar-field gradients are continuously suppressed, leaving the unscreened low-density regime unexplored. These characteristics do not diminish the experimental achievements but indicate that, in many cases, the tests primarily constrain parameter space within assumed frameworks rather than systematically discriminating between alternatives. Discriminating observables—specifically loop asymmetries, spatial correlations, and density-regime screening transitions—are proposed, together with experimental configurations capable of resolving the underdetermination. These include large-area triangle holonomy tests (targeting residual synchronization holonomy Hᵣesid), interplanetary closed-loop timing, altitude-varying optical clock networks to map continuous geometric screening, and matter-wave interferometry. Website: https: //mlsmawfield. com/tep/exp/Code Availability: https: //github. com/matthewsmawfield/TEP-EXP DOI: 10. 5281/zenodo. 18109760 Keywords: temporal equivalence principle – experimental tests – equivalence principle – scalar-tensor theory – gravitational screening – synchronization holonomy – modified gravity Open Science Statement: This work is a preprint and is open to community review, ideas, and collaboration. All materials required for full reproducibility—including data downloads, analysis scripts, code, and manuscripts—are open-source. Feedback and contributions to further test these results are welcome.
Matthew Lukin Smawfield (Wed,) studied this question.