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ExampleConstruction: AI-generatedVerification: AI-generatedPipeline-generatedprecheck passverified 2026-10-22 (gpt-6.1-sol)
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Static supertranslation shear without Bondi news

Physical scope

A formal boundary-data example with an exact flat seed and finite shear action; it distinguishes shear from news and does not claim a full transformed vacuum metric or observation.

Example

Let qAB be the unit round metric on S2, DA its Levi-Civita derivative, and dΩ its area form. On I=R and r>r0>0, with r0 a length, the exact flat Bondi metric is g0=−du2−2du dr+r2qABdxAdxB; its shear, news and mass aspect are CAB=NAB=m=0. Let mB(u)=14π∫S2m(u,x) dΩ. Let Y(x)=12(3x32−1) on S2, set f=aY for a nonzero length a, and use the finite active boundary action CAB(f)(u,x)=CAB(u+f(x),x)−2DADBf+qABDCDCf, NAB(f)(u,x)=NAB(u+f(x),x). It gives CAB(f)=−2aTAB, where TAB=DADBY−12qABD2Y, and NAB(f)=0. Treat these as formal boundary data with m=0 and impose the local coefficient constraint ∂um=14DADBNAB−18NABNAB. Then mB=0, dmB/du=0, and ΔCAB=0 on every retarded-time interval, where ΔCAB=CAB(u2,x)−CAB(u1,x) for u1<u2. No full finite-transformed vacuum metric is asserted. A nonzero shear value alone therefore does not imply nonzero news or a nonzero shear difference.

Facts & Assumptions

Given: The exact flat Bondi end and boundary data in the Example, the finite active boundary shear action of Bondi supertranslations and their action on the shear, and the stipulated local coefficient constraint whose vacuum-expansion version is established in Bondi mass-aspect constraint coefficient.

[F1]

The angular metric q is positive definite, D is its Levi-Civita derivative, and the news is NAB=∂uCAB (A smooth vacuum Bondi expansion).

[F2]

In the fixed Bondi frame the finite active boundary action is CAB(f)(u,x)=CAB(u+f(x),x)−2DADBf+qABD2f and NAB(f)(u,x)=NAB(u+f(x),x) on its stated domain (Bondi supertranslations and their action on the shear).

[F3]

The local mass-aspect equation ∂um=14DADBNAB−18NABNAB is imposed on the formal boundary data; it is the coefficient proved under the stronger smooth-vacuum hypotheses of Bondi mass-aspect constraint coefficient.

[A1]

Countable Choice ACω is the inherited smooth Lorentzian sphere-bundle convention; full Choice is not used (The Axiom of Countable Choice (ACω)).

Verification

Given: [F1]–[F3] and [A1].

1.1A1F1givenalgebra

The restriction Y=12(3x32−1) is smooth on the sphere because it is the restriction of a polynomial. Put TAB:=DADBY−12qABD2Y. Its trace is zero. At θ=π/2, DθDθY=3, DϕDϕY=0 and D2Y=3, so Tθθ=3/2; hence T is nonzero.

2.1F1F2step 1.1algebra

With t=u+r, the metric g0=−du2−2du dr+r2q becomes −dt2+dr2+r2q, the Minkowski metric in spherical spatial coordinates. Thus its Bondi data are exactly CAB=NAB=m=0. Since f=aY is independent of u, [F2] gives CAB(f)=−2aDADBY+aqABD2Y=−2aTAB and NAB(f)=0. We use only these as formal boundary data; we do not assert that they are the coefficients of a finite transformed vacuum metric.

3.1F3step 2.1algebra∎

Set m(u,x)=0 for the formal data. With NAB=0, [F3] is satisfied and mB(u)=14π∫S2m dΩ=0, so dmB/du=0. The shear CAB(f)=−2aTAB is independent of u, hence ΔCAB=0 on every interval. These conclusions follow directly from the stipulated data and constraint; neither vacuum mass-loss nor the memory proposition is invoked.

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