It is well known that successive observations of the instantaneous state of a decaying system lead to a modified decay law. In the limit of infinitely frequent observations, the modified lifetime becomes infinite (“Zeno's paradox”). We study here the behavior of decaying systems under continuous rather than successive observations. Such continuous observation is achieved by a permanent coupling of the decaying system to a counter, which is sufficiently sensitive to the presence of the decay products. For two explicitly soluble models (...) we obtain a result very similar to Zeno's paradox: The observation again strongly modifies the decay law, and prevents the decay completely in the limit of a perfect (i.e., infinitely sensitive) counter. Like Zeno's paradox, however, this “watchdog effect” is not very likely to be detected in actual experiments, e.g., with radioactive nuclei. (shrink)

Scattering states in quantum electrodynamics can not be represented in Fock space (i.e., as states with finitely many incoming and outgoing free photons), since most collisions involve the emission of infinitely many soft photons. At present, there exist two alternative proposals for an appropriately modified structure of the asymptotic state space of quantum electrodynamics. According to the “infraparticle” proposal, each charged particle would be accompanied by an appropriate cloud of infinitely many soft photons, whereas according to the “infravacuum” proposal these (...) clouds would be replaced by a common soft photon background. The second (less popular) idea is illustrated and motivated here by simple model calculations. (shrink)

A quantum mechanical model of a counter monitoring the decay of an unstable microsystem is constructed. Detailed investigation of the time evolution of this model shows that the counter behaves essentially classically; i.e., its discharges may be considered as objective, observer-independent events. The possible relevance of this result for the physical interpretation of quantum mechanics is discussed.