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Exact spherical horizon charge

Statement

Fix m>0 and real qe,qm with qe2+qm2=m2, and take the ingoing extremal RN metric and coordinates specified by the spherical test-scalar definition. Let ψ be any admissible neutral spherical C3 solution of □gψ=0, set ϕ=rψ, and define its spherical horizon charge by

Hψ(v):=∂rϕ(v,m).

Then Hψ is constant for all v∈R. If this constant is nonzero, the regular ingoing-chart transverse derivative ∂rϕ(v,m) is nonzero and does not tend to zero along the horizon. If [ψ] is the scalar-field unit, then [Hψ]=[ψ]. This is a local fixed-background statement and asserts no global decay or nonlinear instability.

Facts & Assumptions

Given: m>0, qe2+qm2=m2, ACω, and an admissible real spherical C3 neutral solution ψ of □gψ=0 on the selected ingoing RN model.

[F1]

Spherical scalar waves in regular extremal RN coordinates: the background is g=−D dv2+2 dv dr+r2γS2 with D=(1−m/r)2 and fixed Maxwell field Fin=−qer−2dv∧dr+qmωS2. The vector field ξ=∂v generates the null level r=m with zero signed surface gravity. The reduced C3 profile ψ(v,r) lifts to ψ~(v,r,ω)=ψ(v,r), ϕ=rψ, and the wave operator on this lift is the trace of its C1 Hessian, equivalently w−1∂a(wgab∂bψ~); it is C1. All radial derivatives are ingoing-chart derivatives.

[F2]

The derivative of the metric density and the chart formula for the divergence of a vector field: for a smooth vector field J, ∇aJa=w−1∂a(wJa), where w=∣det⁡(gab)∣. Its coordinate proof uses only first derivatives of J and the product rule, so the same formula holds for C1 vector fields. Since ψ is C3 and g is smooth, Ja=gab∂bψ~ is C2 and the extension applies.

[F3]

Existence and uniqueness of the metric-selected connection of a nondegenerate metric, and its extension to tensor fields: the metric-selected connection is metric compatible and torsion free; in particular ∇agbc=0 and its tensor derivatives have the stated coordinate formula.

[F4]

A function continuous on an interval I whose derivative vanishes at every interior point of I is constant on I; consequently two such functions with the same derivative differ by a constant: a continuous real function on an order-convex interval whose derivative vanishes at every interior point is constant on that interval.

[A1]

The Axiom of Countable Choice (ACω): the tangent-bundle smooth structure used by [F1] and [F3] assumes ACω. This is the only choice assumption inherited here; no full axiom of choice is used.

Proof

1.1givenF1F2F3A1algebra

In a local sphere chart with angular metric components γAB, the ingoing (v,r) metric block and its inverse are (−D110),(011D). The first block has determinant −1, so the full metric density is w=r2det⁡(γAB). Put Ja:=gab∂bψ~. The spherical lift gives Jv=ψr, Jr=ψv+Dψr, and JA=0; this vector field is C2, so [F2] applies. By [F1], [F3] and metric compatibility, □gψ~=∇aJa. Applying the density formula and cancelling the angular density, which is independent of v,r, gives 0=□gψ~=1r2[∂v(r2ψr)+∂r ⁣(r2(ψv+Dψr))]. Expanding the derivatives and using equality of the continuous mixed partials yields 2r2ψvr+2rψv+∂r(r2Dψr)=0.

2.1step 1.1F1algebra

Since ϕ=rψ, one has 2r2ψvr+2rψv=2rϕvr and r2Dψr=D(rϕr−ϕ). Differentiating the latter expression gives ∂r(r2Dψr)=rDϕrr+rD′ϕr−D′ϕ. Divide the equation of step 1.1 by r>0 to obtain 2ϕvr+Dϕrr+D′ϕr−D′rϕ=0.

3.1step 2.1F1algebra

From D=(1−m/r)2 it follows by direct differentiation that D(m)=D′(m)=0. Substituting r=m into step 2.1 leaves 2ϕvr(v,m)=0. The assumed C3 regularity makes Hψ(v)=ϕr(v,m) continuous and differentiable with Hψ′(v)=ϕrv(v,m)=ϕvr(v,m)=0.

4.1F1F4step 3.1algebra∎

The real line is an order-convex interval, so [F4] applied to Hψ and step 3.1 shows that Hψ(v)=Hψ(v0) for every pair v,v0∈R. If this common value is nonzero, the defining equality ∂rϕ(v,m)=Hψ(v) shows that the ingoing transverse derivative is the same nonzero number for every v and therefore cannot tend to zero.

Remarks

The charge is the derivative of ϕ=rψ in the regular ingoing chart. It is not the derivative of ψ alone and is not computed in a singular static chart. In Aretakis Part I's convention H0=∂rψ+ψ/m, this normalization is Hψ=mH0, since ∂r(rψ)∣r=m=ψ+m∂rψ. The result is proved directly from the local wave equation; no decay estimate or initial-data theorem is assumed.

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