In the theory ofvon Neumann algebras, a part of the mathematical field of functional analysis, Tomita–Takesaki theory is a method for constructing modular automorphisms of von Neumann algebras from the polar decomposition of a certain involution. It is essential for the theory of type III factors, and has led to a good structure theory for these previously intractable objects. The theory was introduced by, but his work was hard to follow and mostly unpublished, and little notice was taken of it until wrote an account of Tomita's theory.
Modular automorphisms of a state
Suppose that M is a von Neumann algebra acting on a Hilbert spaceH, and Ω is a cyclic and separating vector of H of norm 1. We write for the state of M, so that H is constructed from using the Gelfand–Naimark–Segal construction. We can define an unbounded antilinear operator S0 on H with domain MΩ by setting for all m in M, and similarly we can define an unbounded antilinear operator F0 on H with domain M'Ω by setting for m in M′, where M′ is the commutant of M. These operators are closable, and we denote their closures by S and F = S*. They have polar decompositions where is an antilinear isometry called the modular conjugation and is a positive self-adjoint operator called the modular operator. The main result of Tomita–Takesaki theory states that: for all t and that the commutant of M. There is a 1-parameter family of modular automorphisms of M associated to the state, defined by
The modular automorphism group of a von Neumann algebraM depends on the choice of state φ. Connes discovered that changing the state does not change the image of the modular automorphism in the outer automorphism group of M. More precisely, given two faithful states φ and ψ of M, we can find unitary elements ut of M for all real t such that so that the modular automorphisms differ by inner automorphisms, and moreover ut satisfies the 1-cocycle condition In particular, there is a canonical homomorphism from the additive group of reals to the outer automorphism group of M, that is independent of the choice of faithful state.
KMS states
The term KMS state comes from the Kubo–Martin–Schwinger condition in quantum statistical mechanics. A KMS state φ on a von Neumann algebra M with a given 1-parameter group of automorphisms αt is a state fixed by the automorphisms such that for every pair of elements A, B of M there is a bounded continuous functionF in the strip, holomorphic in the interior, such that Takesaki and Winnink showed that a state φ is a KMS state for the 1-parameter group of modular automorphisms. Moreover, this characterizes the modular automorphisms of φ.
Structure of type III factors
We have seen above that there is a canonical homomorphism δ from the group of reals to the outer automorphism group of a von Neumann algebra, given by modular automorphisms. The kernel of δ is an important invariant of the algebra. For simplicity assume that the von Neumann algebra is a factor. Then the possibilities for the kernel of δ are:
The whole real line. In this case δ is trivial and the factor is type I or II.
A proper dense subgroup of the real line. Then the factor is called a factor of type III0.
A discrete subgroup generated by some x > 0. Then the factor is called a factor of type IIIλ with 0 < λ = exp < 1, or sometimes a Powers factor.
The trivial group 0. Then the factor is called a factor of type III1.
Hilbert algebras
The main results of Tomita–Takesaki theory were proved using left and right Hilbert algebras. A left Hilbert algebra is an algebra with involution x → x♯ and an inner product such that
The subalgebra spanned by all products xy is dense in A.
A right Hilbert algebra is defined similarly with left and right reversed in the conditions above. A Hilbert algebra is a left Hilbert algebra such that in addition ♯ is an isometry, in other words =. Examples:
If M is a von Neumann algebra acting on a Hilbert space H with a cyclic separating vector v, then put A = Mv and define = and ♯ = x*v. Tomita's key discovery was that this makes A into a left Hilbert algebra, so in particular the closure of the operator ♯ has a polar decomposition as above. The vector v is the identity of A, so A is a unital left Hilbert algebra.
