H. Dieter Zeh – författare

When we were preparing the first edition of this book, the concept of de­ coherence was known only to a minority of physicists. In the meantime, a wealth of contributions has appeared in the literature - important ones as well as serious misunderstandings. The phenomenon itself is now experimen­ tally clearly established and theoretically well understood in principle. New fields of application, discussed in the revised book, are chaos theory, informa­ tion theory, quantum computers, neuroscience, primordial cosmology, some aspects of black holes and strings, and others. While the first edition arose from regular discussions between the authors, thus leading to a clear" entanglement" of their otherwise quite different chap­ ters, the latter have thereafter evolved more or less independently. While this may broaden the book's scope as far as applications and methods are con­ cerned, it may also appear confusing to the reader wherever basic assumptions and intentions differ (as they do). For this reason we have rearranged the or­ der of the authors: they now appear in the same order as the chapters, such that those most closely related to the "early" and most ambitious concept of decoherence are listed first. The first three authors (Joos, Zeh, Kiefer) agree with one another that decoherence (in contradistinction to the Copen­ hagen interpretation) allows one to eliminate primary classical concepts, thus neither relying on an axiomatic concept of observables nor on a probability interpretation of the wave function in terms of classical concepts.

The fourth edition contains again various revisions and updates throughout the whole book. There are many new comments, formulations and arguments, several new references, and three minor error corrections (regarding page 22, 112 and 146 of the third edition). This time I am grateful to David Atkinson (for a very useful discussion of radiation damping - Sect. 2. 3), to Larry Schulman (for comments on the problem of simultaneous arrows of time - Sect. 3. 1. 2), and to Paul Sheldon (for a discussion of the compatibility of closed time-like curves with quantum theory - Chap. 1). The most efficient help came from John Free, who carefully edited the whole fourth edition (not only for matters of English language). Heidelberg, April 2001 H. D. Zeh Preface to the Third Edition The third (1999) edition of the Direction of Time offered far more revisions and additions than the second one in 1992. During the seven years in between, several fields of research related to the arrow of time had shown remarkable progress. For example, decoherence proved to be the most ubiquitous man­ ifestation of the quantum arrow, while articles on various interpretations of quantum theory (many of them with inbuilt time-asymmetric dynamical aspects) can and do now regularly appear in reputed physics journals. There­ fore, most parts of Chap. 4 were completely rewritten and some new sections added, while the second part of Chap.

Four previous editions of this book were published in 1989, 1992, 1999, and 2001. They were preceded by a German version (Zeh 1984) that was based on lectures I had given at the University of Heidelberg. My interest in this subject arose originally from the endeavor to better - derstand all aspects of irreversibility that might be relevant for the statistical natureandinterpretationofquantumtheory. Thequantummeasurementp- cess is often claimed to represent an ‘ampli?cation’ of microscopic properties to the macroscopic scale in close analogy to the origin of classical ?uctuations, whichmayleadtothelocalonsetofaphasetransition,forexample. Thisclaim can hardly be upheld under the assumption of universal unitary dynamics, as is well known from the example of Schr¨ odinger’s cat. However, the classical theoryofstatisticalmechanicso?ersmanyproblemsandmisinterpretationsof its own, which are in turn related to the oft-debated retardation of radiation, irreversible black holes with their thermodynamical aspects, and – last but not least – the expansion of the Universe. So the subject o?ered a great and exciting ‘interdisciplinary’ challenge. My interest was also stimulated by Paul Davies’ (1977) book that I used successfully for my early lectures. Quantum gravity, that for consistency has to be taken into account in cosmology, even requires a complete revision of the concept of time, which leads to entirely novel and fundamental questions of interpretation (Sect. 6. 2). Many of these interesting ?elds and applications have seen considerable progress since the last edition came out.

Four previous editions of this book were published in 1989, 1992, 1999, and 2001. They were preceded by a German version (Zeh 1984) that was based on lectures I had given at the University of Heidelberg. My interest in this subject arose originally from the endeavor to better - derstand all aspects of irreversibility that might be relevant for the statistical natureandinterpretationofquantumtheory. Thequantummeasurementp- cess is often claimed to represent an ‘ampli?cation’ of microscopic properties to the macroscopic scale in close analogy to the origin of classical ?uctuations, whichmayleadtothelocalonsetofaphasetransition,forexample. Thisclaim can hardly be upheld under the assumption of universal unitary dynamics, as is well known from the example of Schr¨ odinger’s cat. However, the classical theoryofstatisticalmechanicso?ersmanyproblemsandmisinterpretationsof its own, which are in turn related to the oft-debated retardation of radiation, irreversible black holes with their thermodynamical aspects, and – last but not least – the expansion of the Universe. So the subject o?ered a great and exciting ‘interdisciplinary’ challenge. My interest was also stimulated by Paul Davies’ (1977) book that I used successfully for my early lectures. Quantum gravity, that for consistency has to be taken into account in cosmology, even requires a complete revision of the concept of time, which leads to entirely novel and fundamental questions of interpretation (Sect. 6. 2). Many of these interesting ?elds and applications have seen considerable progress since the last edition came out.

When we were preparing the first edition of this book, the concept of de­ coherence was known only to a minority of physicists. In the meantime, a wealth of contributions has appeared in the literature - important ones as well as serious misunderstandings. The phenomenon itself is now experimen­ tally clearly established and theoretically well understood in principle. New fields of application, discussed in the revised book, are chaos theory, informa­ tion theory, quantum computers, neuroscience, primordial cosmology, some aspects of black holes and strings, and others. While the first edition arose from regular discussions between the authors, thus leading to a clear" entanglement" of their otherwise quite different chap­ ters, the latter have thereafter evolved more or less independently. While this may broaden the book's scope as far as applications and methods are con­ cerned, it may also appear confusing to the reader wherever basic assumptions and intentions differ (as they do). For this reason we have rearranged the or­ der of the authors: they now appear in the same order as the chapters, such that those most closely related to the "early" and most ambitious concept of decoherence are listed first. The first three authors (Joos, Zeh, Kiefer) agree with one another that decoherence (in contradistinction to the Copen­ hagen interpretation) allows one to eliminate primary classical concepts, thus neither relying on an axiomatic concept of observables nor on a probability interpretation of the wave function in terms of classical concepts.

Four previous editions of this book were published in 1989, 1992, 1999, and 2001. They were preceded by a German version (Zeh 1984) that was based on lectures I had given at the University of Heidelberg. My interest in this subject arose originally from the endeavor to better - derstand all aspects of irreversibility that might be relevant for the statistical natureandinterpretationofquantumtheory. Thequantummeasurementp- cess is often claimed to represent an ‘ampli?cation’ of microscopic properties to the macroscopic scale in close analogy to the origin of classical ?uctuations, whichmayleadtothelocalonsetofaphasetransition,forexample. Thisclaim can hardly be upheld under the assumption of universal unitary dynamics, as is well known from the example of Schr¨ odinger’s cat. However, the classical theoryofstatisticalmechanicso?ersmanyproblemsandmisinterpretationsof its own, which are in turn related to the oft-debated retardation of radiation, irreversible black holes with their thermodynamical aspects, and – last but not least – the expansion of the Universe. So the subject o?ered a great and exciting ‘interdisciplinary’ challenge. My interest was also stimulated by Paul Davies’ (1977) book that I used successfully for my early lectures. Quantum gravity, that for consistency has to be taken into account in cosmology, even requires a complete revision of the concept of time, which leads to entirely novel and fundamental questions of interpretation (Sect. 6. 2). Many of these interesting ?elds and applications have seen considerable progress since the last edition came out.

Beschreiben die Begriffe der modernen Physik die „Realität“ oder sind sie nur Hilfsmittel und Rechenwerkzeuge? Wie können reale Objekte aus Atomen aufgebaut sein, wenn diese angeblich gar keine realen Eigenschaften vor deren Messung besitzen? Wie kann Schrödingers Wellenfunktion erfolgreich die Struktur und Stabilität von Atomen, Molekülen und Festkörpern beschreiben, wenn sie nur „menschliches Wissen“ oder abstrakte „Information“ darstellt? Und wie kann die Zukunft unbestimmt sein, wenn Einsteins Raumzeit die Weltgeschichte als ein vierdimensionales „Blockuniversum“ darzustellen erlaubt? Das Buch enthält eine Sammlung von Aufsätzen, in denen sich der Autor  mit diversen „faulen Ausreden“ auseinandersetzt, die Physiker sich ausgedacht haben, um unerwarteten Konsequenzen ihrer eigenen Theorien aus dem Wege zu gehen. Es richtet sich ebenso an Physiker wie an allgemeine Naturwissenschaftler, Philosophen und interessierte Laien mit Grundlagenkenntnissen.

This book arose from a series of lectures which I gave at the University of Heidelberg during the summer terms of 1979,1982 and 1986. They led in 1984 the publication in German of Die Physik der Zeitrichtung, which appeared to as Vol. 200 of the Springer Lecture Notes in Physics. The present English version is not merely a translation of these notes, but has been widely revised and extended. The number of changes and additions roughly increases with chapter number. Chaps. 5 and 6 have been completely rewritten (except for Sect. 5. 1, which is a revised version of the former § 5. 2). The new title is intended to express the somewhat more ambitious program of this book as compared to its German predecessor. My interest in this subject stemmed originally from an attempt to place the quantum mechanical measurement process in its proper relation to other irreversible phenomena. It soon became evident that statistical thermody­ namics is too limited for the search for the common roots of the obviously related arrows of time. It is precisely the interconnectedness of many areas of physics, and not least their relation to some fundamental concepts (or per­ haps prejudices) of epistemology, which sustained my fascination with the subject of this book over many years. Thus it was not my intention to de­ scribe technicalities or mathematical problems, but to point out the essential physical ideas (which are often overlooked).

The asymmetry of natural phenomena under time reversal isstriking. Here Zehinvestigates the most important classesof physical phenomena that characterize the arrow of time,discussing their interrelations as well as striving touncover a cosmological common root of the phenomena, such asthe time-independent wave function of the universe. Thedescription of irreversible phenomena is shown to befundamentally "observer-related". Both physicists andphilosophers of science who reviewed the first editionconsidered this book a magnificent survey, a concise,technically sophisticated, up-to-date discussion of thesubject, showing fine sensivity to some of the crucialphilosophicalsubtleties. This new and expanded edition willbe welcomed by both students and specialists.

For this third edition H. D. Zeh has thoroughly revised his book to include important new results. At the same time it retains the features that make it a classic text on irreversibility, and one which clearly distinguishes the latter from time asymmetry. New findings are presented particularly in the chapters on the arrow of time in quantum mechanics and quantum cosmology. Concepts such as decoherence and timelessness are discussed. The book has gained a reputation for its thorough survey of what is known about irreversibility in physics. It investigates physical phenomena in classical, quantum and cosmological settings. Both physicists and philosophers of science who reviewed earlier editions considered this book a magnificent survey, a concise, technically sophisticated, up-to-date discussion of the subject, showing fine sensitivity to some of the crucial philosophical subtleties.