Conjecture that the crossed-product algebra is no longer type III

From papers

Let M\mathcal{M} be a local von Neumann factor, let σs\sigma_s be its modular action, and let R(M,σs)\mathcal{R}(\mathcal{M},\sigma_s) be the algebra generated by M\mathcal{M} together with the modular evolution exp(isHT)\exp(isH_T). For a typical observable B^\hat{B} of R(M,σs)\mathcal{R}(\mathcal{M},\sigma_s), let its spectral projectors be formed through the coupling of elements of M\mathcal{M} and exp(isHT)\exp(isH_T). Crossed-product type conjecture. Such spectral projectors are not related by partial isometries to projectors in, for example, M1\mathcal{M}\otimes\mathbf{1} or to the identity projector 1\mathbf{1}, because their information contents differ. Hence R(M,σs)\mathcal{R}(\mathcal{M},\sigma_s) is no longer of type IIIIII. The paper explicitly says that this is not proved mathematically and argues for it on physical grounds. It is intended to explain how coupling a local algebra to Tomita evolution can change the type of the resulting algebra.

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Sources & referencesView supporting material

Primary source

Manfred Requardt, “The Crossed Product, Modular (Tomita) Dynamics and its Role in the Transition of Type III to Type II_ v.Neumann Algebras and Connections to Quantum Gravity”, arXiv:2507.01419 (2025).

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