Decoherence and einselection do not specify, among the field operators of the Standard Model, what realises the pointer basis for Born-rule probabilities on a physical observer. Since observers in relativistic quantum theory are themselves composite fermionic matter, we identify the physical Dirac matter current as the pointer observable on the class of composite detectors built from Standard Model fermions, defined by pure-dephasing environment coupling; strict-timelike smeared current; macroscopic record stability; and the pointer, macroscopicity, and einselection conditions. We read the matter-current expectation as physical-matter density rather than a probability average over outcomes. In this reading, we recover the Born form of pointer-outcome probabilities from non-contextual probability assignment on the joint projection lattice and the Born state-probability link identifying the density operator with the unitarily evolved preparation. Independent and identically distributed trials then recover the Born frequency law under the classical strong law. The branch-conditioned matter-current expectations serve as local beables in Bell’s sense once decoherence has selected the branch structure, and the wavefunction describes correlations in the matter current, with its independent physical status left open. The construction excludes antimatter-dominated, particle-producing, non-Markovian, and rotating or spinful regimes, and single-outcome selection across decoherent branches lies outside standard unitary dynamics and is not derived here.
Decoherence and einselection do not specify, among the field operators of the Standard Model, what realises the pointer basis for Born-rule probabilities on a physical observer. Since observers in relativistic quantum theory are themselves composite fermionic matter, we identify the physical Dirac matter current as the pointer observable on the class of composite detectors built from Standard Model fermions, defined by pure-dephasing environment coupling; strict-timelike smeared current; macroscopic record stability; and the pointer, macroscopicity, and einselection conditions. We read the matter-current expectation as physical-matter density rather than a probability average over outcomes. In this reading, we recover the Born form of pointer-outcome probabilities from non-contextual probability assignment on the joint projection lattice and the Born state-probability link identifying the density operator with the unitarily evolved preparation. Independent and identically distributed trials then recover the Born frequency law under the classical strong law. The branch-conditioned matter-current expectations serve as local beables in Bell’s sense once decoherence has selected the branch structure, and the wavefunction describes correlations in the matter current, with its independent physical status left open. The construction excludes antimatter-dominated, particle-producing, non-Markovian, and rotating or spinful regimes, and single-outcome selection across decoherent branches lies outside standard unitary dynamics and is not derived here.
The standard measurement postulate of quantum mechanics stipulates an instantaneous, non-local update of the global wavefunction upon measurement of an entangled subsystem. This postulate has been a persistent source of foundational tension, both with the locality structure of relativistic physics and with the dynamical character of the decoherence program. We separate the measurement postulate into three logically independent claims: definite outcome realization, Born-rule probability assignment, and instantaneous projection for the global wavefunction. We argue that the third claim can be eliminated at the axiomatic level and propose a modified measurement postulate, Postulate M, that replaces it. Under Postulate M, a single pointer outcome is realized locally when environmental decoherence has einselected a pointer basis at the measurement site; the corresponding selection acts on the global wavefunction as a conditional restriction to a branch rather than as a physical operation on spacelike-separated regions. We show that Postulate M recovers the standard measurement postulate as a limiting idealization for vanishing decoherence time, derives Lüders’ rule for sequential measurements, and yields no-signaling as an automatic structural consequence — not only at the level of measurement statistics but at the level of physical states themselves. Two explicit illustrations in the EPR scenario — with one and with both particles coupled to local spin environments — demonstrate the framework concretely and exhibit its frame-independent, manifestly local character. The framework is built on standard decoherence theory, compatible with multiple candidate mechanisms for single-outcome realization including gravitationally-induced collapse proposals, and naturally situated within a ψ-ontic reading of the quantum state. It represents, we argue, a reformulation of quantum mechanics as a local, state-realist theory — preserving Einstein’s aspiration for an objective physical world in the only form consistent with the empirical content of quantum mechanics, namely realism about the wavefunction rather than about pre-measurement observable values.
The textbook projection postulate of quantum mechanics stands in tension with the symmetries of relativistic spacetime: it is stated as an update applied at a single instant to the entire composite state, which presupposes a preferred simultaneity surface, and its description of the distant partner of an entangled pair invites the reading that measurement physically changes that partner. We argue that this tension arises from one removable ingredient of the axiom, and we make precise what that ingredient is and is not. Separating the postulate into three logically independent claims—definite-outcome realization, Born-rule probability, and a globally-timed projection—we show that the third can be replaced by a locally triggered rule: branch selection upon completed local decoherence, in the einselected pointer basis (Postulate M). The measurement operator itself is not modified: the update at a site A is of the local form PiA⊗IB under both the standard postulate and Postulate M, and the two rules yield identical post-measurement states and probabilities, the standard rule being recovered as the vanishing-decoherence-time limit. Postulate M is therefore a reinterpretation of standard quantum mechanics rather than a modification of it; what it changes is the trigger of the update, from a global instant to a local decoherence event, so that the axiom can be stated without reference to any foliation. Parameter independence holds exactly, as it does in the standard theory; the final branch structure and joint outcome statistics are foliation-independent; and the exchange symmetry of the entangled pair is respected. We illustrate the framework in the Einstein–Podolsky–Rosen (EPR) scenario with one and with both particles coupled to local spin environments. The Born-rule probabilities in the pointer basis are exactly conserved throughout decoherence, while the maximal CHSH (Clauser–Horne–Shimony–Holt) value, obtained from the Horodecki criterion, is Smax=21+∣z(t)∣2 in terms of the decoherence factor z(t), interpolating between the Tsirelson bound and the classical bound; this corrects the constant-22 claim of Ref. [10]. The single symmetry that quantum mechanics does break, outcome independence, is located entirely in the nonseparability of the global state, never in the measurement operator: it transmits no signal and selects no frame. The single-outcome question—which branch is realized—is left open.
We derive the Born rule as a consequence of two physical processes: environment-induced decoherence, which selects the pointer-basis decomposition of the density matrix, and a contraction map on projective Hilbert space, which assigns each pointer state to a definite outcome basin. The contraction map partitions projective space into basins of attraction whose boundaries are determined by the measurement projectors. We prove that the decoherence-selected decomposition of the reduced density matrix, combined with the basin assignment, reproduces the Born probabilities p_i = ⟨ψ|Π_i|ψ⟩ exactly for projective measurements in any finite dimension, including dimension 2 where Gleason's theorem does not apply. For generalized measurements (POVMs), the result extends via Naimark dilation. The derivation resolves the "probability problem" of decoherence-based approaches: standard decoherence produces the correct diagonal density matrix but leaves open why diagonal entries should be interpreted as outcome probabilities. Our answer is geometric — each diagonal component corresponds to a pointer state lying deep inside a definite basin of the contraction map, and the mixture weights become outcome probabilities through the basin assignment. The derivation is geometric rather than decision-theoretic, does not assume the probability interpretation of the density matrix, requires no ensemble averaging, and applies to single systems. Perturbations to the contraction map produce bounded corrections to the Born probabilities, providing a falsifiable prediction: any measured deviation from Born statistics constrains the perturbation strength. This work connects quantum measurement theory to the geometry of projective Hilbert space and contraction dynamics, offering a new perspective on the origin of quantum probabilities that complements existing approaches by Gleason, Zurek, and Deutsch-Wallace.
We derive the Born rule as a consequence of two physical processes: environment-induced decoherence, which selects the pointer-basis decomposition of the density matrix, and a contraction map on projective Hilbert space, which assigns each pointer state to a definite outcome basin. The contraction map partitions projective space into basins of attraction whose boundaries are determined by the measurement projectors. We prove that the decoherence-selected decomposition of the reduced density matrix, combined with the basin assignment, reproduces the Born probabilities p_i = ⟨ψ|Π_i|ψ⟩ exactly for projective measurements in any finite dimension, including dimension 2 where Gleason's theorem does not apply. For generalized measurements (POVMs), the result extends via Naimark dilation. The derivation resolves the "probability problem" of decoherence-based approaches: standard decoherence produces the correct diagonal density matrix but leaves open why diagonal entries should be interpreted as outcome probabilities. Our answer is geometric — each diagonal component corresponds to a pointer state lying deep inside a definite basin of the contraction map, and the mixture weights become outcome probabilities through the basin assignment. The derivation is geometric rather than decision-theoretic, does not assume the probability interpretation of the density matrix, requires no ensemble averaging, and applies to single systems. Perturbations to the contraction map produce bounded corrections to the Born probabilities, providing a falsifiable prediction: any measured deviation from Born statistics constrains the perturbation strength. This work connects quantum measurement theory to the geometry of projective Hilbert space and contraction dynamics, offering a new perspective on the origin of quantum probabilities that complements existing approaches by Gleason, Zurek, and Deutsch-Wallace.
We derive the Born rule as a consequence of two physical processes: environment-induced decoherence, which selects the pointer-basis decomposition of the density matrix, and a contraction map on projective Hilbert space, which assigns each pointer state to a definite outcome basin. The contraction map partitions projective space into basins of attraction whose boundaries are determined by the measurement projectors. We prove that the decoherence-selected decomposition of the reduced density matrix, combined with the basin assignment, reproduces the Born probabilities p_i = ⟨ψ|Π_i|ψ⟩ exactly for projective measurements in any finite dimension, including dimension 2 where Gleason's theorem does not apply. For generalized measurements (POVMs), the result extends via Naimark dilation. The derivation resolves the "probability problem" of decoherence-based approaches: standard decoherence produces the correct diagonal density matrix but leaves open why diagonal entries should be interpreted as outcome probabilities. Our answer is geometric — each diagonal component corresponds to a pointer state lying deep inside a definite basin of the contraction map, and the mixture weights become outcome probabilities through the basin assignment. The derivation is geometric rather than decision-theoretic, does not assume the probability interpretation of the density matrix, requires no ensemble averaging, and applies to single systems. Perturbations to the contraction map produce bounded corrections to the Born probabilities, providing a falsifiable prediction: any measured deviation from Born statistics constrains the perturbation strength. This work connects quantum measurement theory to the geometry of projective Hilbert space and contraction dynamics, offering a new perspective on the origin of quantum probabilities that complements existing approaches by Gleason, Zurek, and Deutsch-Wallace.
We develop a claim-graded theory of quantum records and history measures from completely positive instruments. Class operators and Hilbert-valued event measures generate strongly positive decoherence functionals and a canonical history Hilbert space. In the declared linear-instrument class, event probabilities are affine in the state and additive in effects, hence have Born trace form; an independent physical exponent coordinate is incompatible with this structure. Outcome probabilities factor through effects, whereas conditional post-record states factor through complete CP operations. A single Stinespring dilation yields both the unconditional channel and the conditional states, so no second collapse map is required. In a gap-protected record parent, the fixed algebra retains outcome and CP-operation data while erasing implementation gauge, and its center supplies objective event propositions; nevertheless, deterministic unconditional CPTP evolution cannot select one pure central branch from a mixed central state while fixing all pure branches. Explicit jump–diffusion record dilations realize the same reduced dephasing semigroup with inequivalent path laws, proving that the reduced channel does not select a unique physical unravelling. Competing admissible information and work utilities likewise select inequivalent readouts. Bounded marked-Poisson parents, compensated interaction-picture measures, modularly damped Wick classes, and Boolean–Möbius/Borel-resummed local nets provide projective strongly positive histories in named finite and quasi-local classes. By contrast, global extensive event counts, positive-Poisson continuum uniformization, raw all-Fock absolute semivariation, and fixed final energy damping obey class-level No-Go theorems. The paper establishes no physical -flow, no unique unravelling, no universal utility, no deterministic collapse theorem, and no completed interacting relativistic QFT history measure.