nLab U(ℋ)

Contents

Context

Group Theory

Operator algebra

algebraic quantum field theory (perturbative, on curved spacetimes, homotopical)

Introduction

Concepts

field theory:

Lagrangian field theory

quantization

quantum mechanical system, quantum probability

free field quantization

gauge theories

interacting field quantization

renormalization

Theorems

States and observables

Operator algebra

Local QFT

Perturbative QFT

Contents

Idea

The unitary group of unitary operators on an infinite-dimensional separable complex Hilbert space \mathcal{H} is traditionally denoted U()U(\mathcal{H}). It does not quite have an established prose name, but is often referred to by some combination of the words “unitary group” and “strong topology”.

Its quotient by the circle subgroup U(1) is the corresponding projective unitary group PU(ℋ).

Definition

Throughout, let \mathcal{H} be any separably infinite-dimensional complex Hilbert space.

Definition

The set B()B(\mathcal{H}) of bounded operators carries the following topologies, characterized by the conditions under which a sequence {T kB()} k\{ T_k \,\in\, B(\mathcal{H}) \}_{k \in \mathbb{N}} converges to a fixed operator TB()T \,\in\, B(\mathcal{H}):

The limit of this sequence is limkT k=T\underset{k \to \infty}{lim} T_k \;=\; T

(This list follows Espinoza & Uribe 2014, p. 2.)

Properties

Proposition

The norm topology makes U()U(\mathcal{H}) a topological group, in fact a Banach Lie group. Further, it is metrizable, hence paracompact.

(For it being a Banach Lie group cf. Schottenloher 2013 p. 4/Sec. 3. For paracompactness/metrizability cf. Nikolaus, Sachse & Wockel 2013 Sec. 3)

Proposition

On U()U(\mathcal{H}) the weak operator topology and strong operator topology agree and make it a topological group.

(Hilgert & Neeb 1993, Cor. 94; Schottenloher 2013, Prop. 1; Espinoza & Uribe 2014, Lem. 1.5)

Proposition

The compact-open topology coincides with the strong operator topology

(Schottenloher 2013 Prop. 2; Espinoza & Uribe 2014, Lem. 1.8)

Proposition

The norm topology on U()U(\mathcal{H}) (from Prop. ) is strictly finer than the (weak or strong) operator topology (from Prop. ).

(e.g. Espinoza & Uribe 2014, p. 5-6, see also Schottenloher 2013, p. 4)

In summary:

(1)U() normU() strongAA=AAU() weakAA=AAU() comp.open U(\mathcal{H})_{norm} \xrightarrow{ \;\;\; \neq \;\;\; } U(\mathcal{H})_{strong} \overset{ \phantom{AA} = \phantom{AA} }{\leftrightarrow} U(\mathcal{H})_{weak} \overset{ \phantom{AA} = \phantom{AA} }{\leftrightarrow} U(\mathcal{H})_{comp.open}

(also Espinoza & Uribe 2014, Thm. 1.2)

Proposition

Equipped with the strong topology (Prop. ), U()U(\mathcal{H}) is completely metrizable.

(Neeb 1997, Prop. II.1, Schottenloher 2013 Prop. 3, Espinoza & Uribe 2014, Lem. 1.6)

Proposition

Equipped with the strong topology (Prop. ), U()U(\mathcal{H}) is not locally compact.

(Grigorchuk & de la Harpe 2014, Sec. 5)

Theorem

(Kuiper’s theorem)
The topological unitary group U ( ) \mathrm{U}(\mathcal{H}) in either the

or the

is contractible in that there is a left homotopy between the identity id:U()U()id \;\colon\; U(\mathcal{H}) \to U(\mathcal{H}) and the constant function const e:U()U()const_{\mathrm{e}} \;\colon\; U(\mathcal{H}) \to U(\mathcal{H}).

See the references at Kuiper's theorem.

Proposition

The U(1)-quotient space coprojection of U(ℋ) over PU(ℋ) – both in their strong operator topology – is a circle-principal bundle:

S 1 U() PU()U()/S 1 \array{ S^1 &\hookrightarrow& \mathrm{U}(\mathcal{H}) \\ && \big\downarrow \\ && PU(\mathcal{H}) \mathrlap{ \; \simeq \; \mathrm{U}(\mathcal{H})/S^1 } }

(Simms 1970, Thm. 1)
Remark

Prop. means in particular that U()PU()\mathrm{U}(\mathcal{H}) \xrightarrow{\;} PU(\mathcal{H}) is locally trivial, hence that the coset space coprojection U()U()/S 1\mathrm{U}(\mathcal{H}) \xrightarrow{\;} \mathrm{U}(\mathcal{H})/S^1 admits local sections. See also at coset space coprojection admitting local sections.

References

Early discussion of U()U(\mathscr{H}) motivated from quantum mechanics and in relation to PU(ℋ):

Beware of the false claim in Simms 1968 p. 10 that U(ℋ) is not a topological group in the strong operator topology, corrected by Schottenloher 2013, Espinoza & Uribe 2014, going back to Schottenloher 1995, but repeated by Atiyah & Segal 2004.

The mistake arises because multiplication in B()B(\mathscr{H}) (bounded linear operators) is not generally continuous in the strong operator topology, according to

whence GL()GL(\mathscr{H}) is indeed not a topological group in the strong operator topology, which however does not imply that its subgroup U()GL()U(\mathscr{H}) \subset GL(\mathscr{H}) cannot have that property (cf. Schottenloher 2013 p 2-3).

Proof that U()U(\mathcal{H}) is a topological group in the strong operator topology:

Proof that the weak- and strong- operator topologies agree on U()U(\mathcal{H}):

Proof that weak-, strong- as well as the compact-open topology all agree on U()U(\mathcal{H}):

Previous influential but wrong claim that they do not and that the compact open topology does not even make a topological group (cf. above, concerning Simms 1968 p 10):

Further properties:

Proof of complete metrizability for the strong (and hence weak) operator topology:

Proof of failure of local compactness:

  • Rostislav Grigorchuk, Pierre de la Harpe, Amenability and ergodic properties of topological groups: from Bogolyubov onwards, in: Groups, Graphs and Random Walks, Cambridge University Press 2017 (arXiv:1404.7030, doi:10.1017/9781316576571.011)

Proof of Kazhdan's property (T) for U()U(\mathcal{H}):

Last revised on June 24, 2025 at 08:57:15. See the history of this page for a list of all contributions to it.