Detector-Level Evidence for SpacePhase: What LIGO Teaches Beyond Strain

Gravitational-wave observatories are commonly described as instruments that confirmpredictions of General Relativity. This paper argues that detector-level behavior revealsfar more: LIGO is already probing the physical properties of space-phase itself. Beyondstrain measurement, the quadrupolar response, phase coherence, memory effects, absenceof drag, frequency-dependent coupling, and geographically correlated signalsdemonstrate that detectors are embedded within a responding substrate. Interpreting thesebehaviors within the SP3 framework clarifies what gravitational-wave experiments areactually measuring and why geometry alone is an incomplete physical description.