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Position and momentum of the massive model have no joint sharp observable
The incompatibility of the j-th position and j-th momentum of the massive spin-s particle model: their spectral projection-valued measures do not commute, so the pair possesses no joint sharp observable with the two spectral measures as marginals. The statement is a conditional consequence of P2, P5 and P6 and the cited mathematics; it asserts no approximate joint measurement, no apparatus, no error-disturbance relation and no empirical result, and it concerns the model's assigned sharp observables only.
Statement
Adopt the observable and probability postulate P2, the kinematics and massive-particle model postulate P5 and the internal-factor assignment P6, read for the spin- model of The massive spin-s particle model of the framework: state space, operators, units and frame readings with mass parameter and spin label (Observable and probability postulate P2: self-adjoint observables, spectral probability and finite unsharp outcomes, Kinematics and massive-particle model postulate P5: Newtonian frames and one-particle operators, Identical-particle and spin assignments P6: permutation sectors and finite-spin representations), and assume the Axiom of Countable Choice and the Axiom of Choice (The Axiom of Countable Choice (), The Axiom of Choice). Fix a coordinate index . Then:
(a) the spectral projection-valued measures and of the model's -th position and -th momentum do not strongly commute: there are Borel sets with ;
(b) consequently the pair is not compatible: there is no regular projection-valued measure on the Borel sets of whose marginals are and , and in particular the framework assigns to the pair no joint sharp outcome law in any preparation;
(c) Scope. The statement is conditional on P2, P5, P6 and the cited mathematics and concerns the model's assigned sharp operators only. It does not assert that no approximate or unsharp joint measurement exists, gives no error-disturbance relation, and records no apparatus or empirical result.
Facts & Assumptions
Given: The postulates P2, P5, P6 read through The massive spin-s particle model of the framework: state space, operators, units and frame readings; the Axiom of Countable Choice and the Axiom of Choice; the model operators with their maximal domains; real ; a nonzero class .
Model operators and translations. and are self-adjoint with the maximal componentwise domains and , and the componentwise translation satisfies , and , the exponential being the spectral-calculus flow of (Componentwise position and momentum operators on the component-valued wave-function space clauses (a), (c) and (d); The massive spin-s particle model of the framework: state space, operators, units and frame readings clause (b)).
Bounded Borel calculus. If two self-adjoint operators have strongly commuting spectral projection-valued measures, then for every bounded Borel one has and for every (Bounded Borel calculus of strongly commuting self-adjoint operators clause (b)).
PVM multiplicativity and compatibility vocabulary. Every projection-valued measure satisfies for measurable (Projection valued measure). A joint sharp observable and compatibility mean, respectively, a regular PVM on with the two spectral marginals and the existence of such a PVM, as defined in the Statement of Compatible sharp observables, their joint law and the ideal joint instrument; only this terminology is used here, not that theorem's deductions under P1-P3.
The exponential function. is a bounded Borel function of with values of modulus , so it is admissible in the bounded Borel functional calculus of a self-adjoint operator (Spectral theorem for unbounded self-adjoint operators (PVM form), Projection valued measure).
Proof
Given: The data of the statement and [F1]-[F4].
[Supposition and its consequence] Assume for contradiction that and strongly commute, and fix . By [F4] the function is bounded Borel, and by [F1] its operator is . Applying [F2] with , gives and
[The contradiction] Let be nonzero, which exists because is dense, and put ; by the domain invariance of [F1], . The conjugation identity of [F1] gives while step 1.1 gives . Hence , that is , which contradicts and . Therefore and do not strongly commute, and there are Borel sets on which the two projections fail to commute. This proves (a).
[No joint sharp observable] If a regular PVM on had the two spectral marginals, then [F3], with and , would give for all Borel . This contradicts step 2.1. Thus no such PVM exists and, by the terminology in [F3], no joint sharp outcome law of the pair is assigned by the framework to any preparation. This proves (b). The argument uses only the stated postulates and mathematical PVM properties; it asserts no approximate or unsharp joint measurement, error-disturbance relation, apparatus model or empirical result, as required by (c).
Depends on
The Axiom of Choice
The Axiom of Countable Choice ()
Hilbert space
Dimensions, SI unit conventions and quantity types of the framework
The massive spin-s particle model of the framework: state space, operators, units and frame readings
Assigned sharp observables and the outcome law of the framework
Strong spectral commutation of self-adjoint operators
Projection valued measure
Componentwise position and momentum operators on the component-valued wave-function space
Bounded Borel calculus of strongly commuting self-adjoint operators
Identical-particle and spin assignments P6: permutation sectors and finite-spin representations
Kinematics and massive-particle model postulate P5: Newtonian frames and one-particle operators
Observable and probability postulate P2: self-adjoint observables, spectral probability and finite unsharp outcomes
Compatible sharp observables, their joint law and the ideal joint instrument
Spectral theorem for unbounded self-adjoint operators (PVM form)
Dependency tree · 2 levels
Sources
- D. Tong, Quantum Mechanics (author-hosted lecture set), Section 3.4, printed pages 81-84 (PDF pages 84-87): the canonical commutation relation, the shared-eigenfunction claim for commuting operators and its non-degeneracy caveat, and the physical reading of simultaneous values
- B. Zwiebach, MIT 8.05 Quantum Physics II (Fall 2013), Chapter 5 'Uncertainty Principle and Compatible Observables', Sections 7-8, printed pages 16-19: simultaneous diagonalization of commuting Hermitian operators
- G. Teschl, Mathematical Methods in Quantum Mechanics, 1st edition 2009, Section 8.1, printed pages 173-175 (PDF pages 185-187): the momentum operator as the generator of translations and the algebraic position-momentum calculus