H. Dieter Zeh

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.

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.

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.