Time evolution of unstable quantum states and a resolution of Zeno's paradox

The time evolution of quantum states for unstable particles can be conveniently divided into three domains: the very short time where Zeno's paradox is relevant, the intermediate interval where the exponential decay holds more or less, and the very long time where the decay is governed by a power law. In this work, we reexamine several questions relating to the deviations from the simple exponential decay law. On the basis of general considerations, we demonstrate that deviations from exponential decay near t=0 are inevitable. We formulate general resonance models for the decay. From analytic solutions to specific narrow-width models, we estimate the time parameters T₁ and T₂ separating the three domains. The parameter T₁ is found to be much much less than the lifetime ^-1, while T₂ is much greater than the lifetime. For instance, for the charged pion decay, T₁10^{-14} and T₂190. A resolution of Zeno's paradox provided by the present consideration and its limitaions are discussed.