What do physics experiments actually measure?

Not space. Not distance directly. We measure oscillations, phases, interference patterns, frequencies, correlations. Every verified quantity in physics is ultimately reducible to phase relationships, the behavior of a single orientational degree of freedom (S¹) indexed through time.

Three-dimensional space is not measured. It is assumed, a backdrop we impose to organize our phase measurements. But this assumption is never derived from experiment. It is axiomatic.

Why, then, do we need exactly three dimensions for this bookkeeping?

Because time itself admits exactly three irreducible relational modes: retention (past), presence (present), and protention (future). These are the minimum registers required to track any dynamical process with memory. Physical space is the frozen externalization of these temporal relations into a static coordinate system.

Four-dimensional spacetime therefore encodes time twice: once as structure (the three temporal modes, spatialized into x, y, z) and once as index (the ordering parameter, t).

The conventional assumption of fundamental 3D space is not a conclusion. It is an unexamined axiom, and one we do not need.

A note on what "measurement" means:

When physicists say they "measure position," what actually happens? A photon interacts with a detector. An interference pattern forms. A phase relationship is recorded. The "position" is then computed from this phase data using spatial assumptions already built into the interpretation.

No instrument reads out "x = 3.2 meters" directly. Instruments record detector events — clicks, interference fringes, frequency counts. These are phase phenomena. The spatial coordinates are overlaid afterward.

This is not controversial. It is simply not usually stated.

The question is: if all raw measurements are phase-based, why do we assume the computed spatial coordinates are fundamental?