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Smooth local continuity equation and its integrated forms

Statement

Assume the Axiom of Countable Choice (The Axiom of Countable Choice (ACω)). Fix ℏ>0, m>0, let Ω⊆R3 and I⊆R be nonempty open sets, let V:Ω→R be continuous and let ψ:Ω×I→C be such that ψ, ∂tψ and all spatial partial derivatives ∂jψ, ∂k∂jψ (j,k∈{1,2,3}) exist and are continuous on Ω×I, and iℏ ∂tψ(x,t)=−ℏ22mΔψ(x,t)+V(x)ψ(x,t)for every (x,t)∈Ω×I, where ∇ψ:=(∂1ψ,∂2ψ,∂3ψ) is the spatial gradient and Δψ:=∑j=13∂j∂jψ is the classical Laplacian on R3 (The Laplacian of a C2 function and of a C2 vector field, Directional derivatives and partial derivatives of a map U⊆Rm→Rn). Define the current j(x,t):=ℏmIm⁡(ψ(x,t)‾ ∇ψ(x,t))∈R3. Then:

(a) pointwise continuity equation. For every (x,t)∈Ω×I, ∂t∣ψ∣2(x,t)+div⁡j(x,t)=0, where the divergence is the classical one of the C1 field x↦j(x,t) (Divergence and curl are linear and satisfy the scalar product rules);

(b) integrated balance. If D⊆Ω is a bounded open set with C1 boundary and D‾⊆Ω (Divergence on a bounded C1 Euclidean domain), then t↦∫D∣ψ(x,t)∣2dx is differentiable on I with ddt∫D∣ψ(x,t)∣2dx=−∫∂Dj(x,t)⋅n(x) dS, n the outward unit normal;

(c) global conservation under domination. If Ω=R3, if there is g∈L1(R3) with ∣ψ(x,t)∣2≤g(x) for almost every x and every t∈I, and if lim⁡R→∞ sup⁡t∈I ∫{∣x∣=R}∣j(x,t)∣ dS=0, then, for all s,t∈I such that the closed interval joining s and t is contained in I, ∥ψ(⋅,s)∥L2(R3)2=∥ψ(⋅,t)∥L2(R3)2;

(d) norm conservation from unitary dynamics. Assume additionally the Axiom of Choice (The Axiom of Choice). If K is a self-adjoint operator on a complex Hilbert space H and ψ(t):=e−itK/ℏψ0 for some ψ0∈H, then ∥ψ(t)∥=∥ψ0∥ for every t∈R (A self-adjoint operator generates a strongly continuous unitary group, Integral of a measurable function against a projection-valued measure).

Assertion (d) is independent of the smooth local model of (a)-(c); no interpretation of ∣ψ∣2 as a measured counting rate is asserted.

Facts & Assumptions

Given: ℏ,m>0, open Ω⊆R3, nonempty open time set I, continuous real V, a C2 solution ψ of the displayed equation with continuous ∂tψ, and the current j.

[F1]

The real one-variable product and sum rules are supplied by Sums, scalar multiples, products and quotients: (f+g)′(c)=f′(c)+g′(c), (αf)′(c)=αf′(c), (fg)′(c)=f′(c)g(c)+f(c)g′(c), and (f/g)′(c)=(f′(c)g(c)−f(c)g′(c))/g(c)2 when g(c)≠0. Writing ψ=u+iv with real u,v and restricting to a time or spatial coordinate line gives ∂t∣ψ∣2=∂t(u2+v2)=2u∂tu+2v∂tv=2Re⁡(ψ‾ ∂tψ) and likewise for ∂j∣ψ∣2. Also j=ℏm(u∇v−v∇u); the real scalar-vector product rule of Divergence and curl are linear and satisfy the scalar product rules gives div⁡j=ℏm(uΔv−vΔu)=ℏmIm⁡(ψ‾Δψ), since the two gradient inner products cancel.

[F2]

The conjugated equation: ∂tψ=iℏ(ℏ22mΔψ−Vψ), since V is real-valued.

[F3]

Divergence theorem: for a bounded C1 domain D and F∈C1(D‾;R3), ∫Ddiv⁡F dx=∫∂DF⋅n dS (Divergence on a bounded C1 Euclidean domain).

[F4]

Differentiation under the integral sign on an open interval J⊆R: if f:D×J→C has integrable spatial sections, differentiable time sections outside one measurable null set, measurable spatial sections of its time derivative, and ∣∂tf(x,t)∣≤G(x) outside one measurable null set for every t∈J, with one nonnegative G∈L1(D), then t↦∫Df(x,t)dx is differentiable on J with derivative ∫D∂tf(x,t)dx (Differentiation under the integral sign).

[F5]

Dominated convergence, and the fact that an integral over BR of an L1 class converges to the integral over R3 as R→∞ (Dominated convergence).

[F6]

A self-adjoint K with spectral PVM E has e−itK/ℏ=∫e−itλ/ℏdE(λ), a strongly continuous unitary group: each e−itK/ℏ is unitary, hence norm preserving (A self-adjoint operator generates a strongly continuous unitary group, Integral of a measurable function against a projection-valued measure).

[F7]

If a real function F is continuous on [s,t], differentiable on (s,t), and ∣F′(τ)∣≤M there, the mean value theorem gives ∣F(t)−F(s)∣≤M(t−s) (The mean value theorem, as the case g(x)=x of Cauchy's: for f continuous on [a,b] with a<b and differentiable on (a,b) there is c∈(a,b) with f(b)−f(a)=f′(c)(b−a)).

Proof

1.1givenF1F2algebra

[The pointwise identity] At a point (x,t)∈Ω×I, [F2] gives ∂tψ=iℏ(ℏ22mΔψ−Vψ) and hence, by [F1], ∂t∣ψ∣2=2Re⁡(ψ‾ ∂tψ)=2Re⁡[iℏ(ℏ22mψ‾Δψ−V∣ψ∣2)]=ℏmRe⁡(i ψ‾Δψ)=−ℏmIm⁡(ψ‾Δψ), because the term −iℏV∣ψ∣2 is purely imaginary and V is real. Comparing with [F1]'s expression for div⁡j gives ∂t∣ψ∣2+div⁡j=0, which is (a).

2.1givenstep 1.1F3F4algebra

[The integrated balance] Let D be as in (b). If D=∅, then ∂D=∅, both integrals are zero and (b) holds. Suppose D≠∅ and fix t0∈I. Openness of I gives ε>0 with [t0−ε,t0+ε]⊆I; put J:=(t0−ε,t0+ε). By step 1.1, ∂t∣ψ∣2=−div⁡j, continuous on the nonempty compact set D‾×[t0−ε,t0+ε], so M:=max⁡D‾×[t0−ε,t0+ε]∣div⁡j∣<∞. Each section ∣ψ(⋅,t)∣2 is bounded on D‾ and integrable on the bounded set D; its time derivative exists everywhere and has continuous, hence measurable, spatial sections. The fixed majorant G(x):=M on D is integrable and bounds the derivative for every t∈J, with no exceptional set. Thus [F4] applies to D×J, giving differentiability at t0 and derivative −∫Ddiv⁡j(⋅,t0). By [F3] applied to the C1 field j(⋅,t0), this equals −∫∂Dj(⋅,t0)⋅n dS. Since t0 was arbitrary, (b) holds on the possibly disconnected open set I.

3.1givenstep 2.1F5F7algebra

[Global conservation] Assume (c), and let s,t∈I with s<t and [s,t]⊆I. For R>0 with KR:={x:∣x∣<R} we have by step 2.1 applied to D=KR (for R so large that KR‾⊆R3, which always holds) ddτ∫KR∣ψ∣2=−∫∂KRj⋅n dS, so [F7] gives ∣∫KR∣ψ(x,t)∣2dx−∫KR∣ψ(x,s)∣2dx∣≤(t−s)sup⁡τ∈[s,t]∫∂KR∣j∣ dS, and the right side tends to 0 as R→∞ by the hypothesis on the flux. By [F5], ∫KR∣ψ(⋅,τ)∣2→∥ψ(⋅,τ)∥2 for τ=t and τ=s, using ∣ψ∣2≤g∈L1. Letting R→∞ in the inequality gives ∣∥ψ(⋅,t)∥2−∥ψ(⋅,s)∥2∣=0. Thus the norms agree for every pair with the closed interval between them contained in I, proving (c).

4.1givenF6algebra∎

[Unitary dynamics] By [F6], ψ(t)=e−itK/ℏψ0 with e−itK/ℏ unitary, so ∥ψ(t)∥=∥ψ0∥ for every t. This is (d) and completes the proof.

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