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Galilean boost covariance of the Schrodinger model with bounded potential

Statement

Assume the Axiom of Choice and the Axiom of Countable Choice (The Axiom of Choice, The Axiom of Countable Choice (ACω)). Fix ℏ>0, m>0, a real bounded measurable V:R3→R, and let K:=12m∑jPj2+MV with domain W2,2(R3;C) be the free-plus-bounded-potential operator of Real multiplication operators and free plus bounded-potential Schrödinger operators clause (c); write Vu(x,t):=V(x−ut) for u∈R3. For u∈R3 and t∈R define the boost operator (Bu(t)ψ)(x):=e i(mℏu⋅x−m∣u∣22ℏt)ψ(x−ut),ψ∈L2(R3;C). Then:

(a) for every t, Bu(t) is unitary with Bu(t)−1=Bu(t)∗, and Bu(t) maps W1,2 and W2,2 onto themselves;

(b) for every u0∈W2,2 and every s∈R the curve v(t):=Bu(t) e−i(t−s)K/ℏBu(s)−1u0 lies in W2,2, is differentiable into L2 on R, and satisfies v(s)=u0 and iℏ v′(t)=(12m∑jPj2+MVu(⋅,t))v(t)(t∈R), where MVu(⋅,t) is multiplication by x↦V(x−ut). It is the unique such curve: if w:R→W2,2 is continuous into W2,2, differentiable into L2 and satisfies the same equation with w(s)=u0, then w=v;

(c) the two-parameter family Uu(t,s):=Bu(t) e−i(t−s)K/ℏ Bu(s)−1 consists of unitaries preserving W2,2, satisfies Uu(t,t)=I, Uu(t,r)Uu(r,s)=Uu(t,s), and v(t)=Uu(t,s)v0 is the solution of (b) at time s for every v0∈W2,2;

(d) free propagator covariance. With K0:=12m∑jPj2 (that is V=0) one has Bu(t)e−i(t−s)K0/ℏ=e−i(t−s)K0/ℏBu(s) for all t,s.

All assertions concern the displayed unitary family and the Schrodinger-type equation; no Lorentz structure, no photon limit and no worldline of an unmeasured particle is asserted.

Facts & Assumptions

Given: ℏ,m>0, u∈R3, a real bounded measurable V, the boost Bu(t)=Meiφ(⋅,t)Tut with phase φ(x,t):=mℏu⋅x−m∣u∣22ℏt, translation Taf(x)=f(x−a), the operator K=12m∑jPj2+MV, and A:=12m∑jPj2+MVu(⋅,t).

[F1]

MV is bounded self-adjoint; K is self-adjoint with D(K)=W2,2, and e−itK/ℏ is a strongly continuous unitary group with e−itK/ℏD(K)=D(K), commuting with K on D(K), and derivative −iℏKx exactly on D(K) (Real multiplication operators and free plus bounded-potential Schrödinger operators clause (c), A self-adjoint operator generates a strongly continuous unitary group, Integral of a measurable function against a projection-valued measure).

[F2]

Translations and unimodular multiplications: Ta is unitary and preserves W1,2, W2,2, with Dj(Taf)=Ta(Djf) (Self-adjointness, Fourier representation and translation identities of position and momentum clause (c)); the phase family Meiφ(⋅,t) is unitary, preserves W1,2 and W2,2, and has t-derivative i(∂tφ)Meiφ(⋅,t)=−im∣u∣22ℏMeiφ(⋅,t) on L2 (Gauge multiplication preserves H2 and transports the magnetic momentum applied with (q,ℏ,χ)=(1,1,φ), whose hypotheses hold because φ is affine in x and ∂tφ=−m∣u∣22ℏ).

[F3]

For a smooth scalar f:R3→C, define ∇f:=(∂1f,∂2f,∂3f) and Δf:=∑j=13∂j2f. The complex exponential addition law exp⁡(z+w)=exp⁡zexp⁡w holds (exp⁡(z+w)=exp⁡z exp⁡w, and the complex exponential extends the real exponential). For fixed t, put αj:=∂jφ=muj/ℏ; the gauge-domain theorem F2, clause (b), gives the first derivative identity as an L2 class, which is classical for smooth ψ; differentiating it with the product rule 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 gives ∂j(eiφψ)=eiφ(∂jψ+iαjψ), and, since αj is constant in x, ∂j2(eiφψ)=eiφ(∂j2ψ+2iαj∂jψ−αj2ψ). Summing these coordinate identities yields the gradient and Laplacian formulas used below.

[F4]

Density extends the identities of [F3] to the Sobolev classes. First, for fixed ψ∈W1,2 the translation-difference-quotient identity 1h(Tuhψ−ψ)→−u⋅∇ψ in L2 as h→0: for smooth ψ this is the classical chain rule, 1h(Tuhψ−ψ)(x)=−1h∫0h(u⋅∇ψ)(x−su) ds, and the bound ∥1h(Tuhν−ν)∥≤∥u⋅∇ν∥ for smooth ν together with the density of Cc∞ in W1,2 extends the identity to all ψ∈W1,2 (the translation is isometric by [F2]). Second, for ρ∈W1,2 and w:=Buρ, one has Bu(∇ρ)=∇w−imℏu w as L2 vector classes; and for ρ∈W2,2 also Bu(Δρ)=Δw−2imℏu⋅∇w−m2∣u∣2ℏ2w as multiplier classes. Both follow from the smooth case in [F3] by density of Cc∞ in W1,2 and W2,2 (Integer-order Sobolev spaces and their norms, Compactly supported smooth functions are dense in W^{k,p}(R^n)) and continuity of the maps involved in the relevant norms.

[F5]

Pj=−iℏDj on its domain, so u⋅Pψ=−iℏ u⋅∇ψ for ∇ψ∈L2; and Bu(t)−1=T−utMe−iφ(⋅,t) with the analogous derivative i(∂tφ) term (Self-adjointness, Fourier representation and translation identities of position and momentum, Position and momentum operators on Euclidean L2, [F2]).

[F6]

A real function continuous on a closed interval and differentiable in its interior with zero derivative is constant, by 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); restrictions of a differentiable curve to compact intervals satisfy these continuity hypotheses.

Proof

1.1givenF2

[Unitarity and domain preservation] By [F2], Bu(t) is a product of two unitary operators preserving W1,2 and W2,2, hence unitary on L2 and preserving both Sobolev spaces; its inverse is T−utMe−iφ, so Bu(t)−1=Bu(t)∗ and (a) holds.

1.2givenF2F3F4algebra

[Derivative identities] Put λ:=m∣u∣22ℏ. Direct computation from the definition gives Bu(t+h)=Bu(t) e−iλhTuh, so for fixed ψ∈W1,2 the difference quotient is 1h(Bu(t+h)ψ−Bu(t)ψ)=Bu(t)[e−iλhTuhψ−ψh+e−iλh−1hψ]. The bracket tends to −u⋅∇ψ−iλψ in L2: the first term by the translation-difference-quotient identity of [F4] (classical chain rule [F3] extended by density), the second by [F2] instantiated at the affine phase φ′(x,t):=−λt, which gives ddte−iλt=−im∣u∣22ℏe−iλt. Hence ddt[Bu(t)ψ]=−iλBu(t)ψ−Bu(t)(u⋅∇ψ). By the transported-gradient identity of [F4], Bu(t)(u⋅∇ψ)=u⋅∇(Bu(t)ψ)−2iλBu(t)ψ, so the identity also reads ddt[Bu(t)ψ]=iλBu(t)ψ−u⋅∇(Bu(t)ψ); replacing u by −u, which gives B−u(t)=Bu(t)−1 directly from the definition, yields ddt[Bu(t)−1η]=iλBu(t)−1η+u⋅∇(Bu(t)−1η) for fixed η∈W1,2.

1.3givenF2algebra

[Transported potential] Bu(t)MVBu(t)−1=MV(⋅−ut): indeed TutMVT−ut=MV(⋅−ut) by the change of variables y=x−ut, and conjugation by a unimodular multiplication fixes multiplication by a real function.

1.4givenF4algebra

[Transported Laplacian] Let ρ∈W2,2 and w:=Bu(t)ρ. Applying [F4] twice, Bu(∇ρ)=∇w−imℏu w and Bu(Δρ)=Δw−2imℏu⋅∇w−m2∣u∣2ℏ2w; consequently Bu(t)(−ℏ22mΔρ)=−ℏ22mΔw+iℏ u⋅∇w+m∣u∣22w.

2.1givenstep 1.2step 1.3step 1.4F1F2F4F5algebra

[Forward equation and initial data] Let u0∈W2,2 and s∈R, put w0:=Bu(s)−1u0, w(t):=e−i(t−s)K/ℏw0 and v(t):=Bu(t)w(t). By [F1] the group preserves D(K), commutes with K there and is strongly continuous both on L2 and on D(K) with graph norm: apply strong continuity to w0 and Kw0. That graph norm is equivalent to the W2,2 norm, since the free operator has this equivalence and the bounded perturbation satisfies ∥K0f∥≤∥Kf∥+∥V∥∞∥f∥ and ∥Kf∥≤∥K0f∥+∥V∥∞∥f∥. Hence w is continuous into W2,2 and differentiable into L2 with w′=−iKw/ℏ. The boosts are strongly continuous on W2,2: translations are isometries in each weak-derivative norm and are continuous by smooth density, while derivatives of the affine-phase multiplier are constant multiples of that multiplier; its time dependence is a scalar phase. They are locally uniformly bounded on W2,2. Thus v is continuous into W2,2. For differentiability, decompose its quotient into Bu(t+h)(w(t+h)−w(t))/h+(Bu(t+h)−Bu(t))w(t)/h; strong continuity on L2, unitarity and step 1.2 give v′=Bu(t)w′+iλv−u⋅∇v, where λ=m∣u∣2/(2ℏ). By steps 1.3-1.4, Bu(t)Kw=−ℏ2Δv/(2m)+iℏu⋅∇v+m∣u∣2v/2+MVu(⋅,t)v. Therefore iℏv′=−m∣u∣2v/2−iℏu⋅∇v+Bu(t)Kw=(−ℏ2Δ/(2m)+MVu(⋅,t))v; both extra terms cancel. At the actual initial time, v(s)=Bu(s)w0=u0, with no assumption that s=0. This proves existence and the initial condition in (b).

3.1givenstep 1.2step 1.3step 1.4step 2.1F1F5algebraF6

[Uniqueness with the transformed initial datum] Let w be another solution in the stated class, and put ρ(t):=Bu(t)−1w(t). The continuity argument of step 2.1 applies also to the inverse boost. For its L2 derivative, decompose the difference quotient just as in step 2.1, using the inverse derivative from step 1.2; this gives ρ′=iλρ+u⋅∇ρ+Bu(t)−1w′. The inverse transported identities of steps 1.3-1.4 give Bu(t)−1(−ℏ2Δ/(2m)+MVu(⋅,t))Bu(t)ρ=Kρ−iℏu⋅∇ρ+m∣u∣2ρ/2. Substituting the equation for w cancels the scalar and first-order terms, so iℏρ′=Kρ and ρ(s)=Bu(s)−1u0. Set G(t):=ei(t−s)K/ℏ and F(t):=G(t)ρ(t). The quotient decomposition G(t+h)(ρ(t+h)−ρ(t))/h+(G(t+h)−G(t))ρ(t)/h converges in L2: the first term by strong continuity and unitarity, the second by [F1] on the fixed vector ρ(t)∈D(K). Hence F′=G(t)(ρ′+iKρ/ℏ)=0. A differentiable Hilbert-space curve with zero derivative is constant, by applying the real scalar mean-value theorem to every bounded real linear functional. Thus F(t)=F(s)=Bu(s)−1u0, and ρ(t)=e−i(t−s)K/ℏBu(s)−1u0. Multiplying by Bu(t) proves w(t)=Bu(t)e−i(t−s)K/ℏBu(s)−1u0=v(t). This proves the uniqueness part of (b) with its actual initial datum.

4.1givenstep 2.1step 3.1F1algebra

[Propagator properties] Each Uu(t,s) is unitary and preserves W2,2 by (a) and [F1]; Uu(t,t)=Bu(t)Bu(t)−1=I; and the composition law follows from Bu(r)−1Bu(r)=I and the group law of e−iτK/ℏ: Uu(t,r)Uu(r,s)=Bu(t)e−i(t−r)K/ℏe−i(r−s)K/ℏBu(s)−1=Uu(t,s). By step 2.1, t↦Uu(t,s)v0 solves the moving-potential equation with initial value v0, and step 3.1 makes it unique. This is (c).

5.1step 3.1step 4.1F1F4algebra∎

[Free covariance] Let V=0, so Vu=0, K=K0 and the equation of (b) is the free equation. By step 4.1 with V=0, the propagator for the free equation is Bu(t)e−i(t−s)K0/ℏBu(s)−1, and it equals e−i(t−s)K0/ℏ because the free equation has at most one W2,2-valued solution for each initial datum (step 3.1 with V=0). Multiplying the identity Bu(t)e−i(t−s)K0/ℏBu(s)−1=e−i(t−s)K0/ℏ on the right by Bu(s) gives (d). This equality first holds on W2,2 and extends to all L2 because both sides are bounded unitaries and Cc∞⊂W2,2 is dense in L2 by [F4].

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