# Berlin 2014 – wissenschaftliches Programm

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# AGPhil: Arbeitsgruppe Philosophie der Physik

## AGPhil 3: Symposium Quantum-Classical Divide

### AGPhil 3.2: Hauptvortrag

### Donnerstag, 20. März 2014, 11:00–11:30, Audimax

**From classical instruments to quantum mechanics and back** — •Reinhard F. Werner — Leibniz Universität Hannover

In the early days of quantum mechanics Bohr and Heisenberg often referred to the indispensability of classical concepts for the quantum object. But increasingly this was applied only to the classical description of the measuring devices, emphasizing the rather obvious need for classical language to communicate the result of experiments. This is the starting point of the quantum axiomatics of Günther Ludwig, the operational approach to quantum physics, and, more recently, quantum information theory. It comes with a choice of "fundamental" concepts (states, observables and channels) in terms of which the whole theory is set up. With regard to Bell's theorem(s) I will show how this leads to a theory which automatically respects no-signalling locality, but gives up "classicality".

I will then briefly describe how one employs symmetries and other structures to fix some basic observables of the theory. As an illustration I will describe the salient formulation of the classical limit. A detailed description of the measurement process then requires the application of quantum theory to (parts of) the measuring instruments. I will briefly describe what one can hope to get out of this theory of measuring processes. One aim is a consistency statement, justifying the initial classicality assumptions about instruments, like the possibility of stable records, from quantum mechanics itself. The core of this problem is the emergence of classicality in much the same way as it is targeted by statistical mechanics.