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Charged dust balance equivalence and advected charge ratio
Statement
Let be an open subset of the oriented, time-oriented Lorentzian domain of Smooth formal matter, EM tensors and observer quantities. Let with , and let be future timelike with . Set , , and . Assume the formal matter-exchange equation . Then this equation is equivalent on to
On , define . Under the continuity equation, charge-current conservation is equivalent to . Thus the equation constrains the derivative of along the dust flow; it does not impose spatial constancy of . The claim is a pointwise conditional identity and supplies no coupled-field existence theorem or Maxwell equation.
Facts & Assumptions
Given: with the regularity and sign conditions in the Statement, together with the formal exchange equation.
Smooth formal matter, EM tensors and observer quantities: , , , and the exchange equation is .
Existence and uniqueness of the metric-selected connection of a nondegenerate metric, and its extension to tensor fields: is the metric-compatible torsion-free connection; its induced tensor derivative commutes with contractions and annihilates and .
Product connection on tensor and hom bundles: the induced tensor connection obeys the Leibniz rule on tensor products and on scalar multiples of tensor fields.
Continuity and derivatives of positive-base real powers: is on with derivative .
The chain rule for total derivatives: and Euclidean maps are closed under componentwise algebra and composition: compositions of coordinate maps are , and their total derivatives obey the chain rule.
Sums, scalar multiples, products and quotients: , , , and when : products of differentiable real functions obey the product rule.
on . This is the exact domain on which division by in the equivalence and the definition of are permitted.
Proof
The product rule gives . Differentiating along and using metric compatibility gives . Antisymmetry of gives .
Contract the exchange equation with . By step 1.1, its left side is and its right side is . Since , continuity follows. Substituting it into the uncontracted equation from step 1.1 leaves .
Conversely, substituting continuity and the force equation into gives , recovering the exchange equation. This proves the equivalence without dividing at zero density.
Because , the scalar function from [F4] composes with the coordinate representation of to a function by [F5]. Differentiating with the product rule in [F6] gives , so is a well-defined scalar field. Since , the product rule gives . Under continuity this is ; by [A1] it vanishes exactly when . This condition is directional along and contains no transverse derivative.
Remarks
- The hypotheses do not impose a constant charge-to-mass ratio. Example Advected nonconstant charge ratio without Maxwell closure ↗ gives an explicit nonconstant ratio satisfying this matter identity while failing sourced Maxwell closure.
- On , the proof makes no division and does not infer a dust velocity or force law from the exchange equation.
Depends on
Smooth formal matter, EM tensors and observer quantities
Existence and uniqueness of the metric-selected connection of a nondegenerate metric, and its extension to tensor fields
Product connection on tensor and hom bundles
Continuity and derivatives of positive-base real powers
The chain rule for total derivatives:
Sums, scalar multiples, products and quotients: , , , and when
Euclidean maps are closed under componentwise algebra and composition
Dependency tree · 2 levels
Sources
- Sean M. Carroll, Lecture Notes on General Relativity, arXiv:gr-qc/9712019v1 (1997), §1.4.1, equations (1.104)–(1.106), PDF pp. 34–35 (printed pp. 28–29)
- David Tong, General Relativity, full author-hosted lecture notes, 2019, §§4.5.3–4.5.4, equations (4.52)–(4.55), PDF pp. 190–194 (printed pp. 183–187)
- Reinhard Meinel and Moritz Hütten, On the black hole limit of electrically counterpoised dust configurations, arXiv:1105.3807v3 (2011), §2, equations (1)–(7), PDF pp. 2–3