This book establishes the detection theory of the Energy Quantum Theory (EQT), systematically proposing that observation is not passive recording but active participation in the energy quantum density field \rho(\mathbf{r}, t, f). Building on the three core postulates—(Postulate I: the Frequency Partitioning Postulate), (Postulate II: the Gradient Flow Postulate), and (Postulate III: the Non-cumulative Emergence Postulate)—it demonstrates that all measurement arises from co-evolutionary dynamics between detector and target, where the detector itself is a localized condensed state of \rho. This work introduces four foundational principles: - (Principle 1: Co-Frequency Resonance): Invisibility stems not from absence but from frequency detuning—detection requires |f - f_0| < \gamma; - (Principle 2: Dual Threshold Gating): Effective measurement demands both frequency resonance and amplitude sufficiency (\delta\rho \rho_{\text{th}} = \Gamma/k) to trigger nonlinear amplification; - (Principle 3: Triadic State Response): Physical reality manifests in three dynamical modes—free (propagating), bound (localized), and condensed (rigid)—and complete observation must resolve all three; - (Principle 4: Re-interpretive Verification): New physics emerges not from new experiments alone, but from re-reading old data through a new ontological lens. Rooted in the frequency-first foundation of Planck’s Unfinished Radiance (KAIPUR), this monograph provides the observational closure of the EQT edifice. It derives concrete, falsifiable predictions: spatial correlation between JWST star-forming galaxies and the LISA stochastic gravitational-wave background; absence of cold gas in high-z quenched galaxies due to phase-decoherence quenching; and mid-frequency “breathing” of planetary systems as local expressions of cosmic rhythm. As the ninth and final volume of the EQT corpus, this work completes the paradigm shift from an “entity ontology” to EQT’s “rhythmic process ontology.” It transforms detection from a technical appendage into the epistemic core of a unified physics—where to observe is to participate, and to participate is to co-create. The Energy Quantum Theory (EQT) Part I: Physical Theory (Books 1–4) 1. Energy Quantum Theory (KAIEQT) 2. Gravitational Unification (KAIGUA) 3. The Quantum Energy Universe (KAITQE) 4. The Universe of Gradients (KAITUO-6) Part II: Philosophical and Interdisciplinary Extensions (Books 5–7) 5. Energy, Process, and the Fate of the Cosmos (KAIEPA) 6. Gradient Rhythm (KAIGRU) 7. Gradient Is Mind-Matter (KAIGIM) Part III: Theoretical Foundation 8. Planck's Unfinished Radiance (KAIPUR) Part IV: Observational Verification 9. Observation as Participation (KAITDP)