Observable Algebras  

In General > s.a. C*-Algebra; W*-Algebra; operator theory; symmetries in quantum theory.
* Idea: Observables form an algebra with addition and operator product as the algebra operations; In the quantum theory, this is usually the Banach algebra \(\cal B\)(\(\cal H\)) of bounded self-adjoint operators on some Hilbert space \(\cal H\), with norm |A|:= sup{|Ax|, |x| = 1}, or some sub-algebra thereof (C*-algebra, W*-algebra).
* Remark: They can be made into a commutative algebra with a · b:= (ab+ba)/2.
* Quantum-classical correspondence: A classical Poisson algebra of observables is promoted to an algebra of quantum operators in a Hilbert space, with the condition that commutators correspond to Poisson brackets in the sense that \([\hat A,\,\hat B] = {\rm i}\hbar\, \widehat{[A,\, B]}\), at least in the \(\hbar\) → 0 limit.
$ Von Neumann algebra: A subalgebra R of \(\cal B\)(\(\cal H\)) which is weak-operator closed, contains I, and is closed under adjoints; It is a C*-algebra, and if Abelian it is isomorphic to some algebra C(X) of real functions on a compact Hausdorff extremely disconnected manifold; > s.a. types of metric spaces.
* Relationships: Von Neumann algebra theory can be considered as a non-commutative generalization of analysis (i.e., of C(X)) and of measure theory (projection-valued measures).
@ General references: Jordan, von Neumann & Wigner AM(34); Murray & von Neumann TAMS(37); in Hermann 66, ch16; Haag in(71); Sunder 87; Li Bing-Ren 92 [intro]; Lledó CM-a0901 [modular theory, introduction]; Zalamea a1612-PhD [observables as Jordan-Lie algebras, etc].
@ Operator algebras: Segal 52; Takesaki 79; Bratteli & Robinson 81, 87; Kadison & Ringrose 83; Kadison in(90); Murphy 90; Schroer mp/01-ln; Guido et al JFA(17)-a1512 [Gromov-Hausdorff distance between von Neumann algebras].
@ Quantum-classical correspondence: Kryvohuz & Cao PRL(05) [commutators, in terms of response theory].

Factors (Murray & von Neumann terminology) > s.a. modified approaches to quantum gravity.
$ Def: A factor is a von Neumann algebra whose center consists of scalar multiples of I. Types:
- I (In, I): The simplest possibility, perfectly adequate for quantum systems with a finite number of degrees of freedom; It has a minimal projection (and is thus isomorphic to some \(\cal B\)(\(\cal H\)), with \(\cal H\) of dimension n; The Hilbert space \(\cal H\) can be split into \(\cal H\)1 ⊗ \(\cal H\)2 such that ...
- II (II1, II): No minimal projection, but it has a non-zero finite projection (I1 if I is finite relative to the factor, I otherwise); In some sense nicer than I, seems to be the right framework for describing a system in a thermal bath; Was claimed to be more generally useful for quantum field theory, but this idea does not seem to work.
- III: All non-zero projections are infinite; A pathological case, essential for quantum field theory and statistical mechanics of infinite systems.
* Decomposable algebras: An algebra \(\cal A\) is said to be decomposable in two factors if there are two commuting subalgebras which together generate \(\cal A\) (this can happen only if there are no superselection rules).
* Coupled factors: The factors are called coupled factors if ...
@ References: Yngvason RPMP(05)mp/04 [type III]; Nobili a0809 [introduction, and extension]; Valente SHPMP(08) [von Neumann's program and difficulties].

Other Topics and Specific Theories > s.a. causality; GNS construction; lattice gauge theory; quantum probability.
* More general situations: Non-associative algebras of observables may appear in certain physical situations, but they cannot be represented as operators on a Hilbert space.
@ General references: Dimock CMP(80) [on a Lorentzian manifold]; Balian & Vénéroni AP(88) [expectation values]; Zafiris IJTP(07)gq/04 [sheaf theoretical, and abstract differential geometry]; Hamhalter IJTP(04) = IJTP(04) [states]; Bojowald et al JHEP(15)-a1411 [non-associative algebras in quantum theory]; Bodendorfer et al JHEP(16)-a1510 [in Gaußian normal spacetime coordinates]; Piparo a1707 [pseudo-observable algebra]; Etesi a1712 [the von Neumann algebra of a smooth 4-manifold, and quantum gravity].
@ Representations: Gotay & Grundling in(99)dg/97 [on a non-compact symplectic manifold].
@ Subalgebras: Halvorson & Clifton IJTP(99) [maximal beable]; Olkiewicz AP(00) [stable with respect to environment interaction].
@ Causal nets: Baumgärtel & Wollenberg 92; Thomas & Wichmann JMP(98); Ruzzi RVMP(05) [net cohomology of posets]; > s.a. causal sets.
@ Types of theories: Rudolph & Schmidt JMP(09)-a0807 [gauge theories].
> Related topics: see approaches to quantum field theory; formulations of quantum mechanics.

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