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arXiv:0710.3604

On the existence of Lyapounov variables for Schroedinger evolution

Y. Strauss

Published Oct 18, 2007• 0 citations
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Abstract

The theory of (classical and) quantum mechanical microscopic irreversibility developed by B. Misra, I. Prigogine and M. Courbage (MPC) and various other contributors is based on the central notion of a Lyapounov variable - i.e., a dynamical variable whose value varies monotonically as time increases. Incompatibility between certain assumed properties of a Lyapounov variable and semiboundedness of the spectrum of the Hamiltonian generating the quantum dynamics led MPC to formulate their theory in Liouville space. In the present paper it is proved, in a constructive way, that a Lyapounov variable can be found within the standard Hilbert space formulation of quantum mechanics and, hence, the MPC assumptions are more restrictive than necessary for the construction of such a quantity. Moreover, as in the MPC theory, the existence of a Lyapounov variable implies the existence of a transformation mapping the original quantum mechanical problem to an equivalent irreversible representation. In addition, it is proved that in the irreversible representation there exists a natural time observable splitting the Hilbert space at each t>0 into past and future subspaces.

DOI
10.48550/arxiv.0710.3604

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The theory of (classical and) quantum mechanical microscopic irreversibility developed by B. Misra, I. Prigogine and M. Courbage (MPC) and various other contributors is based on the central notion of a Lyapounov variable - i.e., a dynamical variable whose value varies monotonically as time increases. Incompatibility between certain assumed properties of a Lyapounov variable and semiboundedness of the spectrum of the Hamiltonian generating the quantum dynamics led MPC to formulate their theory in Liouville space. In the present paper it is proved, in a constructive way, that a Lyapounov variable can be found within the standard Hilbert space formulation of quantum mechanics and, hence, the MPC assumptions are more restrictive than necessary for the construction of such a quantity. Moreover, as in the MPC theory, the existence of a Lyapounov variable implies the existence of a transformation mapping the original quantum mechanical problem to an equivalent irreversible representation. In addition, it is proved that in the irreversible representation there exists a natural time observable splitting the Hilbert space at each t>0 into past and future subspaces.
In nonrelativistic quantum mechanics time enters as a parameter in the Schrödinger equation. However, there are various situations where the need arises to view time as a dynamical variable. In this paper we consider the dynamical role of time through the construction of a Lyapunov variable—i.e., a self-adjoint quantum observable whose expectation value varies monotonically as time increases. It is shown, in a constructive way, that a certain class of models admits a Lyapunov variable and that the existence of a Lyapunov variable implies the existence of a transformation mapping the original quantum mechanical problem to an equivalent irreversible representation. In addition, it is proven that in the irreversible representation there exists a natural time ordering observable splitting the Hilbert space at each t>0 into past and future subspaces.
We discuss the question of the dynamical meaning of the second law of thermodynamics in the framework of quantum mechanics. Previous discussion of the problem in the framework of classical dynamics has shown that the second law can be given a dynamical meaning in terms of the existence of so-called Lyapounov variables-i.e., dynamical variables varying monotonically in time without becoming contradictory. It has been found that such variables can exist in an extended framework of classical dynamics, provided that the dynamical motion is suitably unstable. In this paper we begin to extend these results to quantum mechanics. It is found that no dynamical variable with the characteristic properties of nonequilibrium entropy can be defined in the standard formulation of quantum mechanics. However, if the Hamiltonian has certain well-defined spectral properties, such variables can be defined but only as a nonfactorizable superoperator. Necessary nonfactorizability of such entropy operators M has the consequence that they cannot preserve the class of pure states. Physically, this means that the distinguishability between pure states and corresponding mixtures must be lost in the case of a quantal system for which the algebra of observables can be extended to include a new dynamical variable representing nonequilibrium entropy. We discuss how this result leads to a solution of the quantum measurement problem. It is also found that the question of existence of entropy of superoperators M is closely linked to the problem of defining an operator of time in quantum mechanics.
Given a Hamiltonian HHH on a Hilbert space H\mathcal HH it is shown that, under the assumption that σ(H)=σac(H)=R+σ(H)=σ_{ac}(H)=R^+σ(H)=σac​(H)=R+, there exist unique positive operators TFT_FTF​ and TBT_BTB​ registering the Schrödinger time evolution generated by HHH in the forward (future) direction and backward (past) direction respectively. These operators may be considered as time observables for the quantum evolution. Moreover, it is shown that the same operators may serve as time observables in the construction of quantum stochastic differential equations and quantum stochastic processes in the framework of the Hudson-Parthasarathy quantum stochastic calculus. The basic mechanism enabling for the definition of the time observables originates from the recently developed semigroup decomposition formalism used in the description of the time evolution of resonances in quantum mechanical scattering problems.
The time irreversible evolution of quantum systems with infinite number of particles (QSINP) was studied within a newly constructed algebraic framework. The QSINP could be described by the quantum infinite lattice field. The *-Algebra R is the set of all dynamic variables of the QSINP, in which a special addition and multiplication operations were defined. The full pure state vector (FPSV) ρ, the pure state vector {ρ_f} and the equivalent relations within them are also well defined. The set of all pure state vectors, which are equivalent to the FPSV ρ, is a Hilbert Space {H_ρ}, if the addition, the inner product and the norm were defined in it. The set of all linear transformation {N_ρ} on {H_ρ} is isomorphic to R, and {{N_ρ},{H_ρ}} is the representation of R, associated with ρ. The *-Algebra R has infinitely many non-equivalent irreducible GNS constructions (representations), associated with different nonequivalent FPSVs respectively. It is proved that, the dynamical motions of the QSINP is totally within the GNS construction associated with the initial pure state vector; However, the time reversal transformation makes the initial FPSV and its corresponding dynamical evolution into another non-equivalent GNS construction. Therefore, within the GNS construction associated with the FPSV, the dynamics of the QSINP is time irreversible. Finally, due to the Liouville operator has real continuous spectrum within the GNS construction, the longtime asymptotic solution of Liouville equation in the GNS construction could be treated by the formal scattering theory. The Master equation with dissipative term was obtained, which is formally irrelevant of the initial state and the corresponding GNS construction. This master equation could be regarded as the evolution equation of the QSINP on R.
An adapted representation of quantum mechanics sheds new light on the relationship between quantum states and classical states. In this approach the space of quantum states splits into a product of the state space of classical mechanics and a Hilbert space, and expectation values of observables decompose into their classical value plus a quantum correction. The splitting is preserved under time evolution of the Schrödinger equation under certain assumptions, and the time evolution of the classical part of a quantum state is governed by Hamilton's equation. The new representation is obtained from the usual Hilbert space representation of quantum mechanics by introducing a gauge degree of freedom in a time-dependent unitary transformation, followed by a non-conventional gauge fixing condition.
Time flow has been embodied in time-dependent Schroedinger equation representing one of the foundations of quantum mechanics. Pauli's criticism (1933) has, however, indicated that the assumptions concerning representation Hilbert space have led to some contradictions. Many authors have tried to solve this discrepancy practically without any actual success. The reason may be seen in that two different problems have been mixed: the problem of Pauli (being more general and more important) and non-unitarity of exponential phase operator of linear harmonic oscillator introduced by Dirac, as demonstrated in 1964. The problem will be discussed in a broad historical context and a solution based on extension of representation Hilbert space will be shown.