Vacuum Is Not Frame-Independent - A Short Interpretive No-Go on Absolute Emptiness in Quantum Field Theory

The vacuum of quantum field theory is often described as empty space. This note states a narrow interpretive no-go result: vacuum is not a frame-independent inventory of absence. In quantum field theory, a vacuum state is defined as the state annihilated by a chosen set of annihilation operators. Those operators are fixed only after a field has been decomposed into modes, and the positive-frequency splitting that defines the decomposition depends on a time parameter. Observers or regimes with inequivalent time descriptions can therefore disagree about whether a given field state is vacuum or thermally populated. The Unruh and Hawking effects make this structure explicit: the disagreement is not an anomaly but a consequence of how particle content is defined. The paper proves five propositions: (1) vacuum requires a mode decomposition; (2) a positive-frequency mode decomposition requires a time description; (3) inequivalent time descriptions can produce inequivalent vacua via Bogoliubov transformations with nonzero β coefficients; (4) vacuum disagreement between observers using inequivalent decompositions is not a logical contradiction; (5) a vacuum assignment does not, by itself, license frame-independent emptiness. The claim is conservative. It does not deny the usefulness of vacuum states, the success of quantum field theory, or the physical reality of detector responses in accelerated or curved-spacetime contexts. It denies only the stronger ontological inference that a vacuum assignment establishes an observer-independent fact that no particles are present. Vacuum is a frame-relative field-state assignment, not an absolute ontology of nothingness. This is a short interpretive note (5 pages) intended for readers in foundations of physics, quantum field theory in curved spacetime, and philosophy of physics.