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Journal of Physics Conference Series

Dirac's causality leads to time asymmetry

Yoshihiro Sato, Hirofumi Kan

Published Aug 1, 2008• 0 citations
Publisher

Abstract

On the basis of Dirac's causality, we will show that the time evolution is limited to a semigroup. The abstract vector space for states and (yes-or-no) observables are then not the entire Hilbert space but its particular dense subspaces, called Hardy spaces. The Hardy spaces and their functional spaces together make the Hardy rigged Hilbert spaces, which is also called the time-asymmetric boundary condition (TABC). We will illustrate the usage of the TABC with the neutral kaon decay experiment.

DOI
10.1088/1742-6596/128/1/012040

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On the basis of Dirac's causality, we will show that the time evolution is limited to a semigroup. The abstract vector space for states and (yes-or-no) observables are then not the entire Hilbert space but its particular dense subspaces, called Hardy spaces. The Hardy spaces and their functional spaces together make the Hardy rigged Hilbert spaces, which is also called the time-asymmetric boundary condition (TABC). We will illustrate the usage of the TABC with the neutral kaon decay experiment.
The states (Schrödinger picture) and observables (Heisenberg picture) in the standard quantum theory evolve symmetrically in time, given by the unitary group with time extending over − ∞ < t < + ∞ . This time evolution is a mathematical consequence of the Hilbert space boundary condition for the dynamical differential equations. However, this unitary group evolution violates causality. Moreover, it does not solve an old puzzle of Wigner: How does one describe excited states of atoms which decay exponentially, and how is their lifetime τ related to the Lorentzian width Γ? These question can be answered if one replaces the Hilbert space boundary condition by new, Hardy space boundary conditions. These Hardy space boundary conditions allow for a distinction between states (prepared by a preparation apparatus) and observables (detected by a registration apparatus). The new Hardy space quantum theory is time asymmetric, i.e, the time evolution is given by the semigroup with t 0 ≤ t < + ∞ , which predicts a finite "beginning of time" t 0 , where t 0 is the ensemble of time at which each individual system has been prepared. The Hardy space axiom also leads to the new prediction: the width Γ and the lifetime τ are exactly related by τ = ℏ/Γ .
The meaning of time asymmetry in quantum physics is discussed.On the basis of a mathematical theorem, the Stone-von Neumann theorem, the solutions of the dynamical equations, the Schrödinger equation (1) for states or the Heisenberg equation (6a) for observables are given by a unitary group.Dirac kets require the concept of a RHS (rigged Hilbert space) of Schwartz functions; for this kind of RHS a mathematical theorem also leads to time symmetric group evolution.Scattering theory suggests to distinguish mathematically between states (defined by a preparation apparatus) and observables (defined by a registration apparatus (detector)).If one requires that scattering resonances of width Γ and exponentially decaying states of lifetime τ = Γ should be the same physical entities (for which there is sufficient evidence) one is led to a pair of RHS's of Hardy functions and connected with it, to a semigroup time evolution t 0 ≤ t < ∞, with the puzzling result that there is a quantum mechanical beginning of time, just like the big bang time for the universe, when it was a quantum system.The decay of quasi-stable particles is used to illustrate this quantum mechanical time asymmetry.From the analysis of these processes, we show that the properties of rigged Hilbert spaces of Hardy functions are suitable for a formulation of time asymmetry in quantum mechanics.
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.
Experiments on quantum systems are usually divided into preparation of states and the registration of observables. Using the traditional mathematical methods (the Hilbert space and Schwartz space of distribution theory), it is not possible to distinguish mathematically between observables and states. The Hilbert space as well as Schwartz space boundary conditions for the dynamical equations lead by mathematic theorems (Stone-von Neumann) to unitary group with — ∞ < t < ∞ . But in the experimental set-up, one clearly distinguishes between the preparation of a state and the registration of an observable in that state. Furthermore, a state must be prepared first before an observable can be measured in this state (causality). This suggests time asymmetric boundary conditions for the dynamical equations of quantum theory. Such boundary conditions have been provided by Hardy space in the Lax-Phillips theory for electromagnetic and acoustic scattering phenomena. The Paley-Wiener theorem for Hardy space then leads to semi-group and time asymmetry in quantum physics. It introduces a finite "beginning of time" t 0 for a time asymmetric quantum theory, which have been observed as an ensemble of finite times t ( i ) 0 , the onset times of dark periods in the quantum jump experiments on a single ion.
The exact relation τ = ~/Γ between the width Γ of a resonance and the lifetime\nτ for a decay of this resonance could not be obtained in the conventional quantum\ntheory based on the Hilbert space as well as on the Schwartz space in which the spaces\nof the states and of the observables are described by the same space. Furthermore,\nboth dynamical evolution of the states (in Schrodinger picture) and observables (in\nHeisenberg picture) are symmetrically in time, given by an unitary group with time\nextending over −∞ < t < +∞. This time symmetric evolution is a mathematical\nconsequence of Von Neumann theorem for the dynamical differential equations,\nSchrodinger equation for the state or Heisenberg equation for the observable, under\neither the Hilbert space or Schwartz space boundary condition. However, this unitary\ngroup evolution violates causality. In order to get a quantum theory in which the\nexact relation and causality are obtained, one has to replace the Hilbert space or the\nSchwartz space boundary condition by a new boundary condition based on the Hardy\nspace axioms in which the space of the states and the space of the observables are\ndescribed by two distinguishable Hardy spaces. As a consequence of the new Hardy\nspace axiom, one obtains, instead of the time symmetric evolution for the states and\nthe observables, time asymmetrical evolutions for the states and observables which\nare described by two semi-groups. The time asymmetrical evolution predicts an finite\nbeginning of time t0(= 0) for quantum system which can be experimental observed\nby directly measuring the lifetime of the decaying state as the beginning of time of\nthe ensemble in scattering experiments. A resonance obeying the exponential time\nevolution can then be described by a Gamow vector, which is defined as superposition\nof the exact out-plane wave states with exact Breit-Wigner energy distribution.
Abstract A slight modification of one axiom of quantum theory changes a reversible theory into a time asymmetric theory. Whereas the standard Hilbert space axiom does not distinguish mathematically between the space of states (in‐states of scattering theory) and the space of observables (out‐“states” of scattering theory) the new axiom associates states and observables to two different Hardy subspaces which are dense in the same Hilbert space and analytic in the lower and upper complex energy plane, respectively. As a consequence of this new axiom the dynamical equations (Schrödinger or Heisenberg) integrate to a semigroup evolution. Extending this new Hardy space axiom to a relativistic theory provides a relativistic theory of resonance scattering and decay with Born probablilities that fulfill Einstein causality and the exponential decay law.