Earman, John
(2020)
Symmetries for Quantum Theory.
[Preprint]
Abstract
Five conceptually distinct notions of symmetry in quantum theory are studied in the algebraic setting where a quantum system is characterized by a von Neumann algebra of observables and the set of normal states on the algebra. It is shown that all five symmetry notions are closely related and that the glue binding them together is the concept of a Jordan ∗automorphism. For factor algebras a Jordan ∗automorphism reduces either to an ∗automorphism or a ∗antiautomorphism. If the algebra is put in standard form then a ∗automorphism is always unitarily implementable, whereas a ∗antiautomorphism is always antiunitarily implementable. However, there is no guarantee that a general von Neumann algebra admits ∗antiautomorphisms or, if it does, that it admits order two (or involutory) ∗antiautomorphisms). For nonfactor algebras there can be genuine Jordan ∗automorphisms that are neither ∗automorphisms nor ∗antiautomorphisms, and implementation is possible only through partial isometries. These developments enable generalized versions of Wigner's theorem on the implementation of transition probability preserving symmetries for von Neumann algebras. This review is largely an exercise in connecting the dots in existing mathematics and physics literature. But in the service of the philosophy of physics it is an exercise worth doing since the practitioners in this field seem largely unaware of or unappreciative of this literature and how it fits together to yield a multifaceted but unified picture of quantum symmetries. Along the way various interpretations issues worthy of further discussion are flagged.
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