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arXiv:quant-ph/0506184

The Arrow of Time in Rigged Hilbert Space Quantum Mechanics

Robert C. Bishop

Published Jun 22, 2005• 0 citations
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Abstract

Arno Bohm and Ilya Prigogine's Brussels-Austin Group have been working on the quantum mechanical arrow of time and irreversibility in rigged Hilbert space quantum mechanics. A crucial notion in Bohm's approach is the so-called preparation/registration arrow. An analysis of this arrow and its role in Bohm's theory of scattering is given. Similarly, the Brussels-Austin Group uses an excitation/de-excitation arrow for ordering events, which is also analyzed. The relationship between the two approaches is initially discussed focusing on their semi-group operators and time arrows. Finally a possible realist interpretation of the rigged Hilbert space formulation of quantum mechanics is considered.

DOI
10.48550/arxiv.quant-ph/0506184

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•Jan 1, 2015•0 citations
Arno Bohm and Ilya Prigogine's Brussels-Austin Group have been working on the quantum mechanical arrow of time and irreversibility in rigged Hilbert space quantum mechanics. A crucial notion in Bohm's approach is the so-called preparation/registration arrow. An analysis of this arrow and its role in Bohm's theory of scattering is given. Similarly, the Brussels-Austin Group uses an excitation/de-excitation arrow for ordering events, which is also analyzed. The relationship between the two approaches is initially discussed focusing on their semi-group operators and time arrows. Finally a possible realist interpretation of the rigged Hilbert space formulation of quantum mechanics is considered.
Quantum mechanics in the Rigged Hilbert Space formulation describes quasistationary phenomena mathematically rigorously in terms of Gamow vectors. We show that these vectors exhibit microphysical irreversibility, related to an intrinsic quantum mechanical arrow of time, which states that preparation of a state has to precede the registration of an observable in this state. Moreover, the Rigged Hilbert Space formalism allows the derivation of an exact golden rule describing the transition of a pure Gamow state into a mixture of interaction-free decay products.
Despite the fact that the fundamental physical laws are symmetric in time, most observed processes do not show this symmetry. Especially the phenomenon of decay seems to involve a kind of irreversibility that makes the definition of a microscopic arrow of time possible. Such an intrinsic irreversibility is incorporated within the Rigged Hilbert Space quantum mechanics of the Brussels School, contrasting to the statements of standard quantum mechanics. As shown in this paper, the formalism bears significant advantages in the description of decaying systems, however the breaking of time symmetry can be avoided.
A quantum theory that applies to the (closed) universe needs to be time asymmetric because of the cosmological arrow of time. The preparation $\ensuremath{\Rightarrow}$ registration arrow of time (a state must be prepared before an observable can be detected in it) of the quantum mechanics of measured systems provides a phenomenological reason for an asymmetric semigroup time evolution. The standard theory in the Hilbert space (HS) is inadequate for either since the mathematics of the HS allows only reversible unitary group evolution and time symmetric boundary conditions. The mathematical theory that describes time-asymmetric quantum physics in addition to providing the mathematics for the Dirac kets is the rigged Hilbert space (RHS) theory. It uses a pair of RHS's of Hardy class with complementary analyticity property, one for the prepared states (``in states'') and the other for the registered observables (``out states''). The RHS's contain Gamow kets which have all the properties needed to represent decaying states and resonances. Gamow kets have only asymmetric time evolution. The neutral kaon system is used to show that quasistationary microphysical systems can be experimentally isolated if their time of preparation can be accurately identified. The theoretical predictions for a Gamow ket have the same features as the observed decay probabilities, including the time ordering. This time ordering is the same as the time ordering in the probabilities of histories for the quantum universe. The fundamental quantum mechanical arrow of time represented by the semigroup in the RHS is therefore the same as the cosmological arrow of time, assuming that the universe can be considered a closed quantum system.
The fundamental problem on which Ilya Prigogine and the Brussels-Austin Group have focused can be stated briefly as follows. Our observations indicate that there is an arrow of time in our experience of the world (e.g., decay of unstable radioactive atoms like Uranium, or the mixing of cream in coffee). Most of the fundamental equations of physics are time reversible, however, presenting an apparent conflict between our theoretical descriptions and experimental observations. Many have thought that the observed arrow of time was either an artifact of our observations or due to very special initial conditions. An alternative approach, followed by the Brussels-Austin Group, is to consider the observed direction of time to be a basics physical phenomenon and to develop a mathematical formalism that can describe this direction as being due to the dynamics of physical systems. In part I of this essay, I review and assess an attempt to carry out an approach that received much of their attention from the early 1970s to the mid 1980s. In part II, I will discuss their more recent approach using rigged Hilbert spaces.
Traditional Quantum Scattering theory is built upon a Hilbert space formalism. The nature of the Hilbert space has led to a number of ideas about the nature of scattering systems, chief among them being the understanding that the decay of unstable quan- tum systems cannot be purely exponential and that there can only be an approximate relationship between the width Γ of a scattering resonance and the lifetime τ of the resonant state whose formation gives rise to it. That such physical conclusions are de- rived from purely mathematical properties of the underlying topological vector space inform the notion that, in choosing a particular space, one is choosing the mathemat- ical representation of the physical universe for the system. With this notion in mind, purely physical characteristics of quantum systems are used to answer the question: “What is the proper mathematical space for quantum mechanical calculations?” In the particular case of quantum scattering, it is shown that this approach casts the choice of the Hilbert space into doubt and motivates instead the use of rigged Hilbert spaces employing Hardy spaces; in these Hardy rigged Hilbert spaces, exponential decay is permissible and the time evolution of decaying states is governed by semigroups. The original theories concerning deviation from exponential decay in the Hilbert space are revisited to reveal how their conclusions flow from a mathematical property whose physical interpretation is problematic. Finally, an application of a Hardy rigged Hilbert space formalism is presented: Time Asymmetric Quantum Mechanics, as developed by A. Bohm, M. Gadella, and S. Wickramasekara.