Claus Kiefer (editor) - From Quantum to Classical: Essays in Honour of H.-Dieter Zeh

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H. Dieter Zeh in September 2005. Photo Rolf Kickuth

Why does a world described fundamentally by quantum theory look classical? Where does the observed arrow of time come from? Are these questions related? These are the questions that were at the heart of Dieter Zehs thinking over many decades.

Dieter started his career as a theoretical physicist when working on nuclear physics in the sixties. His interests included the theory of -decay, nucleosynthesis and the dynamics of deformed nuclei. It was his concrete research about the deformed nuclei that eventually led him, in 1968, to the discovery of what was later called decoherence. Dieter was puzzled by the fact that individual nucleons could experience an orientation in spite of the whole nucleus being in a stationary quantum state. In his article on Roots and Fruits of Decoherence, he describes this as follows:

If the nucleons in the deformed nucleus dynamically feel a definite orientation in spite of the global superposition, would an internal observer then not similarly have to become aware of a certain measurement result?

He then envisages a stationary quantum universe (wave function of the universe) in which observers experience definite properties such as orientations in spite of the total quantum state being a superposition of all orientations. These observers are decohered, and this decoherence arises from their quantum correlation (entanglement) with many inaccessible degrees of freedom in the global superposition. Dieter identified this entanglement as the key to an understanding of the quantum-to-classical transition. This was not recognized by the physics community for many years. In his own words:

I am indeed convinced that the importance of decoherence was overlooked for the first 60 years of quantum theory precisely because entanglement was misunderstood as no more than a statistical correlation between local objects.

Dieter always entertained a realistic interpretation of the wave function. Through this, he independently arrived at an Everett-type of interpretation without knowing about Everetts work. He was a proponent of this interpretation for the rest of his life, although he was not sympathetic to calling it a many-worlds interpretation, for the reason that we have only one quantum world (though with many quasi-classical branches). The only alternative to the Everett interpretation that Dieter accepted were theories with a dynamical collapse of the wave function, for the reason that these are realistic theories. But so far no empirical hint for collapse theories exists.

His later interests in the arrow of time and in quantum cosmology grew out of these early insights, because he recognized that the emergence of classical properties in quantum theory cannot be understood without understanding the irreversible nature of our Universe.