UNITARITY AND THE TIME EVOLUTION OF QUANTUM-MECHANICAL STATES

The basic requirement that, in quantum theory, the time evolution of a ny state is determined by the action of a unitary operator, is shown t o be the underlying cause for certain ''exact'' results that have rece ntly been reported about the time dependence of transition rates in qu antum theory. Departures from exponential decay, including the ''quant um Zeno effect,'' as well as a theorem by Khalfin about the ratio of r eciprocal transition rates, are shown to follow directly from such con siderations. At sufficiently short times, unitarity requires that reci procity must hold, independent of whether T invariance is valid. If T invariance does not hold, unitarity restricts the form of possible tim e dependence of reciprocity ratios.