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BMT orthogonality and conserved spin length
Conditional constraint preservation and restricted-ansatz coefficient matching for the adopted classical BMT polarization model on a prescribed external field. No measured value of the gyromagnetic ratio, quantum-spin derivation, or empirical validation is asserted.
Statement
Assume of The Axiom of Countable Choice () for the canonical tangent-bundle and connection suppliers used by the BMT transport and charged-action equivalence. No full Axiom of Choice is assumed.
Adopt the charged-body quantities and SI tensor convention of Charged and spinning test-body quantities and The electromagnetic two-form, its observer decomposition and the four-current, the polarization transport postulate Classical BMT polarization model, and its translational equation obtained from Stationary charged action and Lorentz force. On a local potential domain, assume the finite curve segments are stationary for the adopted action. Let be a future timelike proper-time curve, , , with prescribed continuous alternating field along it. Let , , , and an initial spin vector satisfy . Put , , and . The adopted equations and action result give
Then and are constant along every solution. In the restricted ansatz , require preservation of orthogonality for every initially orthogonal datum and every allowed alternating field. For this gives . If, in addition, the pointwise projected rest-space matching condition of BMT transport constraint and restricted-ansatz algebra holds with target coefficient , that lemma gives , and hence . This coefficient conclusion is only within that ansatz and the stated joint conditions. If , the rest-space matching condition does not identify .
Facts & Assumptions
The Axiom of Countable Choice (): assume only countable choice for the canonical tangent-bundle smooth structure and inherited BMT/action-equivalence supplier hypotheses.
Given: The adopted classical BMT model, , constant , a future timelike proper-time curve, a prescribed alternating field, and an initially orthogonal spin vector.
The adopted BMT transport and Lorentz-force translation equations, with and . Classical BMT polarization model
The trajectory is stationary under the adopted action on a local potential domain, and the initial spin satisfies .
The SI field tensor has and in the named inertial convention; the charged-body definition uses the same convention. The electromagnetic two-form, its observer decomposition and the four-current; Charged and spinning test-body quantities
For the restricted two-term transport ansatz, metric compatibility and skew-adjointness give ; if , orthogonality and spin norm are preserved, and the pointwise projected rest-space matching condition defined in the supplier determines when . BMT transport constraint and restricted-ansatz algebra
Under [A3], stationarity of the adopted local charged-particle action implies on the stated finite fixed-endpoint potential-domain model. Stationary charged action and Lorentz force
Proof
In the lemma [F2], set and . The postulate [A1] has exactly this displayed transport equation because , and [F3] gives the matching translation equation . Thus in the lemma; its derivative identity gives . The initial value is zero by [A2], hence orthogonality is preserved.
With from step 1.1 and [A2], the same lemma gives : the term vanishes by skew-adjointness, and the term proportional to vanishes by orthogonality. Thus is constant.
In the general ansatz, [F2] shows that for , preservation for every initially orthogonal datum and every allowed field requires ; an orthogonal rest spin with an alternating field satisfying gives an allowed instance with . The pointwise spatial projection in [F2] gives under the stipulated rest-space matching condition for every ; substituting into gives . If , both transport equations vanish and impose no coefficient condition. This establishes uniqueness only under the joint conditions and within the stated ansatz.
Remarks
This is a conditional deduction inside the adopted classical transport model. The original BMT treatment is flat and homogeneous-field; it does not prove the separately adopted curved or varying-field law, and this theorem makes no quantum or empirical claim.