Scope and empirical status of the measurement-and-instruments page

Adopted, not derived. Every measurement statement of this page is conditional on the adopted ideal conditional-measurement postulate P3 (Ideal conditional measurement postulate P3: the projective Lueders update and discarded outcomes), read in the vocabulary of Declared ideal measurements and instruments of the framework. The Lueders update is not derived from the Born outcome law of P2, and the Born outcome law is not derived from the collapse rule: the two are separate adopted clauses, and the theorems of this page deduce conditional consequences of them and of the cited mathematics. P3 itself records that its declared class is an idealization.

Declared apparatus models only. Consequences of a declared ideal sharp measurement: repeatability and discarded ensembles states the outcome law, repeatability, the discarded-outcome ensemble and coherence loss for a declared ideal sharp measurement; The Lueders update on a degenerate eigenspace preserves intra-eigenspace coherence shows that the update by an eigenspace projection preserves coherence inside a degenerate eigenspace; Outcome effects do not determine the conditional update, and general instruments need not be repeatable demonstrates that the same effects can have different conditional updates; A declared finite effect family is realised by a dilated ideal projective measurement realizes a declared finite effect family as the outcome statistics of one ideal dilated projective model and computes the square-root instrument induced by that dilation; Declared countable instruments: normalization, discarded ensembles and the repeatability criterion develops declared countable instruments and their repeatability criterion. None of these items asserts that a particular laboratory device realizes the declared measurement, filter or instrument, and none asserts a preferred or unique recorded outcome; the identification of a device with a declared model is a tested identification with uncertainty under P7 (Preparation and apparatus postulate P7: declared preparations, isolation and sampling assumptions).

What is excluded from the declared sharp class. A measurement of an exact value of a quantity with continuous spectrum is not represented here by normalizing a delta symbol (clauses (iii) of P2 and (v) of P3); a destructive or absorbing detector is not a projective measurement of the incoming system and must be modeled by its own instrument, one instance being the finite instrument of the companion page; and a unitary system-apparatus interaction produces conditional correlations without, by itself, selecting a recorded outcome (clause (v) of P3).

No empirical content. No item of this page records a measurement, an apparatus calibration, a count, an uncertainty or a comparison with data. The probabilities computed in this page are model probabilities for declared preparations and declared apparatus models; agreement or disagreement with recorded data would test the combined model and apparatus assumptions and is not asserted anywhere here.

Deferred measurement topics. Joint and compatible sharp measurements, their order independence and their joint laws are developed with the uncertainty-and-compatibility content of the category. The spatial Stern-Gerlach force model, the magnetic-field geometry and the detector-position analysis belong to the magnetic-and-detection material and are not derived here. Measurement-model boundaries for Bell-type comparisons and finite-sample statistical qualifications belong to the later Bell-models page. The reported Stern-Gerlach experiments of 1922 and other primary experimental reports remain candidate experiment items that are not asserted by this page.