Established physics describes with precision how mass behaves, but not what it is. We propose PostulateQ, a semi-classical framework offering a microscopic, geometric interpretation: mass is the name for null energy (an electromagnetic wave) closed upon itself into a persistent, self-balanced loop. Radiation and matter become two topologies — open and closed — of the same conserved energy, connected by a single accrual rule dχQ = ל (ρE /r) ds, with ל = G/c4 = 1/FP the reciprocal of the Planck force — Einstein’s own gravitational coupling — fixed by dimensional analysis and closure, introducing no adjustable constant. A curved photon accrues a closure parameter χQ that gravitates while its invariant mass stays exactly zero; only a closed loop recovers m = E/c2. General Relativity is taken as given and reinterpreted microscopically — as statistical mechanics interprets thermodynamics — not replaced; closed-loop solutions are Wheeler’s geons (“mass without mass”). The frameworkintroduces no new empirical predictions, reproducing general relativity to present precision.Its contribution is a change of perspective in which mass appears as a derived label and the boundary between light and matter becomes a continuous coordinate. Open problems, including stability and mass selection, are stated explicitly.