This paper presents the first experimental challenge proposed by the USP Field Theory, offering a fundamental reinterpretation of gravity. Instead of describing gravity as the curvature of spacetime, USP Field Theory defines it as the gradient of a universal spin tension field (∇T) originating from ultra-sub particles (USPs), the fundamental units of spin. We propose a simple gravimetric experiment using materials of identical mass but varying atomic-core density. The goal is to detect measurable differences in gravitational pull, which would support USP’s spin-tension model over General Relativity’s geometric framework. This paper includes a direct equation comparison, conceptual framework, and a clear test procedure — opening the door for empirical validation of USP Field Theory.