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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 ()). Fix , , a real bounded measurable , and let with domain be the free-plus-bounded-potential operator of Real multiplication operators and free plus bounded-potential Schrödinger operators clause (c); write for . For and define the boost operator Then:
(a) for every , is unitary with , and maps and onto themselves;
(b) for every and every the curve lies in , is differentiable into on , and satisfies and where is multiplication by . It is the unique such curve: if is continuous into , differentiable into and satisfies the same equation with , then ;
(c) the two-parameter family consists of unitaries preserving , satisfies , , and is the solution of (b) at time for every ;
(d) free propagator covariance. With (that is ) one has for all .
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: , , a real bounded measurable , the boost with phase , translation , the operator , and .
is bounded self-adjoint; is self-adjoint with , and is a strongly continuous unitary group with , commuting with on , and derivative exactly on (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).
Translations and unimodular multiplications: is unitary and preserves , , with (Self-adjointness, Fourier representation and translation identities of position and momentum clause (c)); the phase family is unitary, preserves and , and has -derivative on (Gauge multiplication preserves H2 and transports the magnetic momentum applied with , whose hypotheses hold because is affine in and ).
For a smooth scalar , define and . The complex exponential addition law holds (, and the complex exponential extends the real exponential). For fixed , put ; the gauge-domain theorem F2, clause (b), gives the first derivative identity as an class, which is classical for smooth ; differentiating it with the product rule Sums, scalar multiples, products and quotients: , , , and when gives , and, since is constant in , . Summing these coordinate identities yields the gradient and Laplacian formulas used below.
Density extends the identities of [F3] to the Sobolev classes. First, for fixed the translation-difference-quotient identity in as : for smooth this is the classical chain rule, , and the bound for smooth together with the density of in extends the identity to all (the translation is isometric by [F2]). Second, for and , one has as vector classes; and for also as multiplier classes. Both follow from the smooth case in [F3] by density of in and (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.
on its domain, so for ; and with the analogous derivative term (Self-adjointness, Fourier representation and translation identities of position and momentum, Position and momentum operators on Euclidean L2, [F2]).
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 of Cauchy's: for continuous on with and differentiable on there is with ; restrictions of a differentiable curve to compact intervals satisfy these continuity hypotheses.
Proof
[Unitarity and domain preservation] By [F2], is a product of two unitary operators preserving and , hence unitary on and preserving both Sobolev spaces; its inverse is , so and (a) holds.
[Derivative identities] Put . Direct computation from the definition gives , so for fixed the difference quotient is . The bracket tends to in : 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 , which gives . Hence . By the transported-gradient identity of [F4], , so the identity also reads ; replacing by , which gives directly from the definition, yields for fixed .
[Transported potential] : indeed by the change of variables , and conjugation by a unimodular multiplication fixes multiplication by a real function.
[Transported Laplacian] Let and . Applying [F4] twice, and ; consequently .
[Forward equation and initial data] Let and , put , and . By [F1] the group preserves , commutes with there and is strongly continuous both on and on with graph norm: apply strong continuity to and . That graph norm is equivalent to the norm, since the free operator has this equivalence and the bounded perturbation satisfies and . Hence is continuous into and differentiable into with . The boosts are strongly continuous on : 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 . Thus is continuous into . For differentiability, decompose its quotient into ; strong continuity on , unitarity and step 1.2 give , where . By steps 1.3-1.4, . Therefore ; both extra terms cancel. At the actual initial time, , with no assumption that . This proves existence and the initial condition in (b).
[Uniqueness with the transformed initial datum] Let be another solution in the stated class, and put . The continuity argument of step 2.1 applies also to the inverse boost. For its derivative, decompose the difference quotient just as in step 2.1, using the inverse derivative from step 1.2; this gives . The inverse transported identities of steps 1.3-1.4 give . Substituting the equation for cancels the scalar and first-order terms, so and . Set and . The quotient decomposition converges in : the first term by strong continuity and unitarity, the second by [F1] on the fixed vector . Hence . 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 , and . Multiplying by proves . This proves the uniqueness part of (b) with its actual initial datum.
[Propagator properties] Each is unitary and preserves by (a) and [F1]; ; and the composition law follows from and the group law of : . By step 2.1, solves the moving-potential equation with initial value , and step 3.1 makes it unique. This is (c).
[Free covariance] Let , so , and the equation of (b) is the free equation. By step 4.1 with , the propagator for the free equation is , and it equals because the free equation has at most one -valued solution for each initial datum (step 3.1 with ). Multiplying the identity on the right by gives (d). This equality first holds on and extends to all because both sides are bounded unitaries and is dense in by [F4].
Depends on
Position and momentum operators on Euclidean L2
Self-adjointness, Fourier representation and translation identities of position and momentum
Gauge multiplication preserves H2 and transports the magnetic momentum
Real multiplication operators and free plus bounded-potential Schrödinger operators
A self-adjoint operator generates a strongly continuous unitary group
Integral of a measurable function against a projection-valued measure
Sums, scalar multiples, products and quotients: , , , and when
The chain rule for total derivatives:
, and the complex exponential extends the real exponential
Compactly supported smooth functions are dense in W^{k,p}(R^n)
Integer-order Sobolev spaces and their norms
Hilbert space
The Axiom of Countable Choice ()
The Axiom of Choice
The mean value theorem, as the case of Cauchy's: for continuous on with and differentiable on there is with
Dependency tree · 2 levels
Sources
- C. Torre, Galilean Relativity and the Path Integral Formalism in Quantum Mechanics, arXiv:2306.13487 (2023), Section 4 equations (4.23)-(4.24), pages 12-13: the boost phase and the transformed initial wave function; Section 6 equation (6.2), page 17: boost action on wave functions
- G. Teschl, Mathematical Methods in Quantum Mechanics, 1st edition 2009, Section 7.2 printed pages 167-168 (PDF pages 179-180): the free operator on H2 and its Fourier multiplication representation; Section 7.3 printed pages 169-170 (PDF pages 181-182): free time evolution; Section 8.1 printed pages 173-174 (PDF pages 185-186): position and momentum, translations and Fourier unitary equivalence