International Journal of Theoretical Physics, Vol. 37, No. 1, 1998 Phase Dynamics at the SQUID and Macro-Realism F. Hofmann 1 and A. Rieckers 1 Received July 4, 1997 Taking into account some philosophical notions on realism a reformulation of ªmacro-realismº according to Leggett-Garg is put forward. A macroscopic phase dynamics based on a microscopic SQUID-model is discussed in this context. 1. INTRODUCTION Macroscopic quantum phenomena (MQP) have been brought to our attention by John Leggett (Leggett and Garg, 1985) in order to remind us of the possibility of their being systems whichÐas he puts itÐare not always in a `definite macrostate.’ He mentions the ring SQUID, which consists of a bulk superconducting ring interrupted by a single Josephson junction. Thus he wants to pose a problem for attempts to solve the measurement problem by appeal to the idea that all macro-observables necessarily lead to superselection rules (breaking of coherent superposition). At the same time he wants to make use of the ring SQUID system in order to refute a certain realistic interpretation of QM, namely, what he calls `macro-realism.’ We would like to take up this latter idea. First, we will translate and discuss it in a different quantum mechanical formalism, namely, in an operator-algebraic frame. By thus providing a more adequate quantum mechanical model, we would like to do more justice to the macroscopic character of the system. Second, we would like to elaborate on the clarity of the relevant general notions involved in the conflict between QM and `macro-realism.’ Most importantly, an improved definition of `macro-realism’ is sought. 1 Institut fuÈr Theoretische Physik, UniversitaÈt TuÈbingen, D-72076 Tu Èbingen, Germany. 537 0020-7748/98/0100-053 7$15.00/0 q 1998 Plenum Publishing Corporation