Unruh Radiation Does Not License Vacuum Substance Ontology - A Short Interpretive No-Go on Observer-Relative Particle Content

The Unruh effect shows that a uniformly accelerated observer assigns thermal particle content to the Minkowski vacuum, with temperature proportional to proper acceleration. This note states a narrow interpretive no-go result: Unruh radiation does not license vacuum substance ontology. The effect does not show that the inertial vacuum contains a hidden bath of localized particles. It shows that particle content is observer-relative when the field is decomposed with respect to inequivalent time descriptions. The accelerated observer uses Rindler time rather than inertial Minkowski time. The Minkowski vacuum, restricted to the right Rindler wedge and decomposed in Rindler modes, is a thermal state at the Unruh temperature, and an accelerated detector coupled to the field responds accordingly. The detector response is real. What is not licensed is the further inference that the inertial vacuum contains a frame-independent inventory of particle substance. The paper proves five propositions: (1) the Unruh effect depends on the inequivalence between inertial and uniformly accelerated mode decompositions; (2) the thermal response of an accelerated detector does not establish that localized particles were already present in the inertial vacuum; (3) detector response associated with one observer class does not, by itself, fix a frame-independent particle ontology; (4) uniform acceleration changes the observer's description and detector response, but does not reveal a hidden substance in the vacuum; (5) the Unruh effect establishes observer-relative particle content, not observer-independent vacuum substance. The claim is conservative. It does not deny the reality of the Unruh effect, the physical significance of acceleration, or the soundness of the standard derivation. It denies only the stronger ontological inference from observer-relative detector response to observer-independent vacuum substance. The note also situates the result against Hawking radiation. The Unruh effect displays the core interpretive point — thermal particle content arising from observer-relative field decomposition — without the complications of black-hole geometry, singularity, or horizon formation by collapse. The vacuum is not a hidden particle reservoir. It is a field state, and its particle content depends on the decomposition. This is a short interpretive note (5 pages) intended for readers in foundations of physics, quantum field theory in curved and non-inertial spacetimes, and philosophy of physics.