The Observer Expelled: Ensemble Interpretation, Objective Decoherence, and the Quantum–Classical Boundary in Physical and Biological Systems

The measurement problem in quantum mechanics persists not because of a deficiency in the mathematical formalism, but because of an unexamined philosophical commitment inherited from Mach via Bohr: the observer is treated as constitutive of physical reality. This paper traces the historical origin of this commitment — from Mach's principle of observability through Bohr's complementarity to von Neumann's 1932 formal regress argument — and argues that it was a philosophical choice, not a physical discovery. The ensemble interpretation (Blokhintsev, Ballentine), combined with Zurek's decoherence programme, supplies a complete observer-free account of measurement. Blokhintsev's decomposition 𝔐 = M + (A+D) is developed and mapped exactly onto the NV-centre spin system in diamond, where the quantum-classical boundary is directly measurable via T2. Rovelli's relational quantum mechanics is examined alongside Uemov's earlier and more consistent relational ontology (1963): both treat properties as relational, but Rovelli makes facts observer-relative while Uemov makes relations objective — a distinction with decisive consequences for whether the resulting theory is realist or idealist. An extension to biological systems — NV magnetometry of haemoglobin oxygenation in tumour tissue — is proposed as a second experimental programme.