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Complete finite-slab production and outward-flux balance
Statement
Assume the background of Finite spacelike slab and normal flux charge. Let , , the current , its slice charge and its net outward side flux be as in Finite spacelike slab and normal flux charge. Use the oriented metric volume form of The metric volume form, current flux through 3-faces and finite-chain charges clause (a). Put , using the divergence of Metric divergence density identity for a C¹ current. Then the exact finite-slab balance is Here the four-volume integral is the Riemann integral on the bounded Jordan slab , whose compact closure is the supplied coordinate box. If everywhere on , then . In particular, nonnegative local production implies when the net outward side flux is zero. If , the identity instead reads , so zero production alone does not imply constant slice charge.
Facts & Assumptions
Given: The convention, a positively oriented Lorentzian coordinate slab with nondegenerate rectangular factors and compact closure , spacelike slices, its future unit normal and induced volume, and a current on a neighborhood of .
The Countable Choice assumption is the tangent-bundle background recorded in The Axiom of Countable Choice () and Smooth time-oriented Lorentzian four-manifolds: the geometric objects of the model; no additional choice is used.
The definition gives the coordinate slice charge and net side flux as the sum of upper-minus-lower face integrals (Finite spacelike slab and normal flux charge).
On each slice, the normal charge equals its coordinate density integral (Normal slice charge equals its coordinate density integral).
For a current, (Metric divergence density identity for a C¹ current).
The sets , and each are bounded Jordan products, and every continuous density on their compact coordinate closures is Riemann integrable on them. In particular, the two endpoints of each closed coordinate interval have content zero, by the one-dimensional case of Finite coordinate boxes and their faces are compact Jordan sets.
For an integrable function on a bounded Jordan set whose coordinate sections are integrable outside a content-zero set, its integral equals the iterated section integral; in particular this applies to continuous functions on a product rectangle (Fubini over a bounded Jordan set when all but a content-zero family of sections are integrable).
If is differentiable on a closed interval and is integrable, then its integral is , including the one-sided endpoint values (The second fundamental theorem: if is differentiable on with and is integrable, then ).
The integral over a Jordan set is the Riemann integral of its zero extension, and on a rectangle an integrable function's integral is the supremum of lower Darboux sums; lower sums use the cell infima times nonnegative rectangle volumes (The Riemann integral of a bounded function over a bounded Jordan measurable set, The lower and upper Darboux integrals over a nondegenerate rectangle in , Lower and upper Darboux sums over a grid partition in ).
In a positively oriented chart, (The metric volume form, current flux through 3-faces and finite-chain charges clause (a)).
Proof
By [F3], on the coordinate box. Since and are smooth and is , this integrand and all face integrands are continuous; [F4] makes their compact-box integrals finite.
By [F2], . Fix . Apply [F6] to the function on , then integrate over using [F5]. This gives
For each fixed and the other three coordinates, the section of in the coordinate is . By [F7], its integral is defined using the zero extension to . This extension differs from the continuous derivative only at the two endpoints, which have content zero by [F4]; since the functions are bounded, covers of those points by intervals of arbitrarily small total length show from the Darboux definition that the two integrals agree. Apply [F6] on , then integrate the endpoint difference over the remaining face coordinates by [F5]. This gives By [F1], summing these three terms gives exactly , with outward orientation and no choice of a unit normal on any side face.
To verify the nonimplication by a current independent of any consuming counterexample, use with , future direction , the standard positive coordinate orientation, and . Then , and , so these are the slab data of [F1]. For real constants , set . This is smooth on all of ; on the compact slab closure, and , so it is even future timelike there and on a neighborhood of the closure. Directly from [F1]–[F3] and the elementary rectangular integrals [F5]–[F6], Indeed only the side pair contributes, and its upper-minus-lower density is on a face parameter rectangle of volume one. These evaluations do not use the balance conclusion or any later example.
Add the temporal identity of step 1.2 and the three spatial identities of step 1.3. Additivity of the finite Riemann integral and [F5] give By step 1.1 and the coordinate expression in [F8], the left side is , proving the equality.
In the family of step 1.4 take and . Then , and . This admissible smooth current has zero production and strictly decreasing slice charge, proving that zero production alone does not imply charge constancy.
If , then because . The zero extension of from to a containing rectangle is nonnegative; each of its lower Darboux sums is nonnegative by [F7], so . The equality in step 2.1 yields ; setting the net side flux to zero gives the stated slice-charge inequality. If , step 2.1 gives .
Taking instead gives positive production whose integral exceeds the positive outward flux , with positive charge change . Thus zero side flux is sufficient under nonnegative production but is not necessary for nondecreasing charge. The choice , also gives zero production, negative net side flux and positive charge change. These explicit cases establish both possibilities without treating a later consuming counterexample as a premise.
Remarks
- This is an equality on the stated finite rectangular coordinate slab. It asserts no arbitrary-domain Stokes theorem, infinite exhaustion, boundary at infinity, or convergence result.
- For nonnegative production, zero net outward side flux is sufficient for endpoint charge monotonicity. The exact criterion is that integrated production is at least the net outward side flux, as illustrated in proof steps 1.4, 2.2 and 4.1. The theorem places no causal-character restriction on the side faces; their flux is defined by the three-form/coordinate face integral, so null side faces need no unit normal.
- This is a pure mathematical identity. A physical entropy interpretation, units, and a local second-law premise must be supplied separately.
Depends on
The Axiom of Countable Choice ()
Smooth time-oriented Lorentzian four-manifolds: the geometric objects of the model
The metric volume form, current flux through 3-faces and finite-chain charges
Finite spacelike slab and normal flux charge
Metric divergence density identity for a C¹ current
Finite coordinate boxes and their faces are compact Jordan sets
Normal slice charge equals its coordinate density integral
The Riemann integral of a bounded function over a bounded Jordan measurable set
The lower and upper Darboux integrals over a nondegenerate rectangle in
Lower and upper Darboux sums over a grid partition in
The second fundamental theorem: if is differentiable on with and is integrable, then
Fubini over a bounded Jordan set when all but a content-zero family of sections are integrable
Dependency tree · 2 levels
Sources
- Harvey Reall, Part III General Relativity, University of Cambridge lecture notes, 12 October 2022, §§8.6–8.7, printed pp. 108–110 (PDF pp. 109–111)
- David Tong, General Relativity, University of Cambridge Part III Mathematical Tripos lecture notes, Michaelmas Term 2019, §3.2.4 and §4.5.5, printed pp. 110–112 and 187–189 (PDF pp. 117–119 and 194–196)