Quantum evolution according to real clocks

We characterize good clocks, which are naturally subject to fluctuations, in statistical terms. We also obtain the master equation that governs the evolution of quantum systems according to these clacks and find its general solution. This master equation is diffusive and produces loss of coherence, Moreover, real clocks can he described in terms of effective interactions that are nonlocal in time. Alternatively, they can be modeled by an effective thermal bath coupled to the system. [S1050-2947(99)04905-7].