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Electrovacuum tensor Cauchy system — Examples

1 · Prerequisites

2 · Summary

These examples check three separate interfaces of the tensor formulation. Zero field on Minkowski spacetime is an exact coupled solution; a nonzero Maxwell test field on a fixed flat metric generally fails the Einstein constraint; and a closed magnetic field on a torus can have a nonzero period, allowing local potentials but obstructing a single-valued global potential. The magnetic-period example is explicitly a Maxwell-sector example on a prescribed background, not a coupled flat solution.

3 · Logical flowchart

4 · Definitions, postulates, theorems and proofs

None yet.

5 · Examples, experiments and counterexamples

ExampleConstruction: AI-generatedVerification: AI-generatedprecheck pendingjudge pass (gpt-6-luna)Open item page →

Flat zero-field data and Minkowski development

Example

Take M=R1,3 with coordinates (x0,x1,x2,x3), Minkowski metric g=−(dx0)2+(dx1)2+(dx2)2+(dx3)2, the standard orientation and future direction, and the hypersurface Σ={x0=0}. Set F=0 and Λ=0, with the fixed positive parameters κ,μ0 of Electrovacuum tensor Cauchy system. The induced data are h=δ, K=0, and e=b=0. This is a smooth exact solution and its data satisfy all four electrovacuum constraints.

Facts & Assumptions

Given: The source-free tensor system and initial-data conventions of Electrovacuum tensor Cauchy system and Constrained electrovacuum initial data.

[F1]

Electrovacuum tensor Cauchy system: the equations are G+Λg=κμ0−1Q(g,F), dF=0, and d(⋆gF)=0.

[F2]

Constrained electrovacuum initial data: the induced data are tested by the two Maxwell divergences, the Hamiltonian constraint, and the momentum constraint with K=+12Lnh.

Verification

Given: g=η, F=0, Λ=0, and κ,μ0>0.

1.1F1algebra

In the displayed global coordinates the metric coefficients are constant, so every Christoffel symbol and every curvature component is zero; hence Gab=0.

2.1F1step 1.1algebra

Since F=0, one has dF=0, d(⋆gF)=0, and Qab=TabEM=0. Thus G+Λg=κTEM holds identically, proving that (g,F) is an exact electrovacuum solution.

3.1F2step 1.1algebra∎

The slice has future unit normal n=∂0, induced metric h=δ, second fundamental form K=0, and observer fields e=b=0. Its scalar curvature and all four constraint expressions vanish, so the induced data satisfy Constrained electrovacuum initial data.

CounterexampleConstruction: AI-generatedVerification: AI-generatedprecheck pendingjudge pass (gpt-6-luna)Open item page →

Nonzero flat test field is not a coupled flat solution

Statement refuted

For Λ=0, every smooth source-free Maxwell field on a prescribed Minkowski metric is an electrovacuum solution with that same flat metric.

Facts & Assumptions

Given: The tensor system and constraint conventions of Electrovacuum tensor Cauchy system and Constrained electrovacuum initial data, with κ>0, μ0>0, Λ=0, and B0≠0.

[F1]

Electrovacuum tensor Cauchy system: a coupled solution must satisfy both Maxwell equations and G+Λg=κTEM.

[F2]

Maxwell stress algebra and covariant conservation: for e=0 and b=(0,0,B0), TEM(n,n)=B02/(2μ0).

[F3]

Gauss-Codazzi and Maxwell initial constraints: any smooth coupled solution induces data satisfying the Hamiltonian constraint.

Counterexample

Given: The system and constraint conventions above; κ,μ0>0, Λ=0, and a fixed B0≠0 (tesla in the SI interpretation).

1.1F1algebra

Take M=R4 with length coordinates (x0,x1,x2,x3), metric g=−(dx0)2+(dx1)2+(dx2)2+(dx3)2, orientation dx0∧dx1∧dx2∧dx3, and future field N=∂x0; in the SI interpretation x0=ct. Set F=B0 dx1∧dx2. The metric coefficients and those of F and ⋆gF are constant, so the curvature vanishes and dF=d(⋆gF)=0. Hence this is a smooth source-free Maxwell field on the prescribed Minkowski background.

2.1F1F2F3algebra∎

On the slice x0=0, the data are h=δ, K=0, e=0, and b=(0,0,B0). The Hamiltonian constraint from [F3] would require 0=(κ/μ0)B02, because R(h)=0 and Λ=0. This is impossible since κ,μ0>0 and B0≠0. Hence the given flat Maxwell test field is not a coupled electrovacuum solution with that metric.

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Magnetic period obstructs a global potential

Example

Let M=Rx0,x32×TL2, where TL2=(R/LZ)2 carries its standard global translation-invariant one-forms dx1,dx2, and give M the flat product Lorentz metric

g=−(dx0)2+(dx1)2+(dx2)2+(dx3)2.

Take future direction ∂x0 and orient M by dx0∧dx1∧dx2∧dx3. In the SI interpretation use the length-coordinate convention x0=ct of Electrovacuum tensor Cauchy system, with c>0 and x0,x3 and local lifts of x1,x2 measured in metres; L>0 is measured in metres. For a fixed B0≠0 tesla, define the smooth two-form F=B0 dx1∧dx2. Then F and its Hodge dual have constant coefficients, so dF=d(⋆gF)=0 on the flat background. On the oriented spatial torus S:={x0=x∗0, x3=x∗3}≅TL2, oriented by dx1∧dx2, its period is ∫SF=B0L2≠0. The field has local potentials but no single-valued global one-form potential on M. This example concerns the Maxwell field-strength sector on a prescribed background; it is not asserted to solve the coupled Einstein equation with that flat metric.

Facts & Assumptions

Given: The exterior-calculus and source-free Maxwell conventions of Electrovacuum tensor Cauchy system and the local potential conclusion of Local potential homotopy and Lorenz gauge reachability.

[F1]

Electrovacuum tensor Cauchy system: the source-free Maxwell equations are dF=0 and d(⋆gF)=0.

[F2]

Local potential homotopy and Lorenz gauge reachability: every smooth closed two-form has a potential on a star-shaped coordinate neighborhood.

Verification

Given: L>0, B0≠0, flat product coordinates, and F=B0 dx1∧dx2.

1.1F1algebra

The global invariant one-forms dx1,dx2 on TL2 are closed and B0 is constant, so dF=0. The flat Hodge dual has constant coefficients and is closed as well. On S at fixed x0,x3, direct integration over a fundamental square gives ∫SF=∫[0,L]2B0 dx1dx2=B0L2≠0.

2.1step 1.1F2algebra∎

On each star-shaped coordinate neighborhood, [F2] supplies a local potential. If a global smooth one-form A on M satisfied F=dA, its components A1,A2 are periodic in the torus variables. At fixed x∗0,x∗3, direct integration over a fundamental square and periodicity give ∫SF=∫[0,L]2(∂1A2−∂2A1) dx1dx2=∫0L(A2(L,x2)−A2(0,x2)) dx2−∫0L(A1(x1,L)−A1(x1,0)) dx1=0. This contradicts the nonzero period in step 1.1, so no global potential exists although local potentials do.