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Discrete Dynamics in Nature and Society
Volume 2004, Issue 1, Pages 75-83

Irreversibility in quantum mechanics

1Department of Philosophy, Logic and Scientific Method, London School of Economics, London WC2A 2AE, UK
2Center for Junior Research Fellows, University of Konstanz, P.O. Box M682, Konstanz D-78457, Germany
3Physics Department, The University of Texas at Austin, Austin, TX 78712-0264, USA
4Departamento de Física Teórica, Universidad de Valladolid, Valladolid E-47011, Spain

Received 19 January 2004

Copyright © 2004 Hindawi Publishing Corporation. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.


Time asymmetry and irreversibility are signal features of our world. They are the reason of our aging and the basis for our belief that effects are preceded by causes. These features have many manifestations called arrows of time. In classical physics, some of these arrows are described by the increase of entropy or probability, and others by time-asymmetric boundary conditions of time-symmetric equations (e.g., Maxwell or Einstein). However, there is some controversy over whether probability or boundary conditions are more fundamental. For quantum systems, entropy increase is usually associated with the effects of an environment or measurement apparatus on a quantum system and is described by the von Neumann-Liouville equation. But since the traditional (von Neumann) axioms of quantum mechanics do not allow time-asymmetric boundary conditions for the dynamical differential equations (Schrödinger or Heisenberg), there is no quantum analogue of the radiation arrow of time. In this paper, we review consequences of a modification of a fundamental axiom of quantum mechanics. The new quantum theory is time asymmetric and accommodates an irreversible time evolution of isolated quantum systems.