  A quantum mechanical wave of a finite size moves like a classical particle
and shows a unique decay probability. Because the wave function evolves
according to the Schr\"{o}dinger equation, it preserves the total energy but
not the kinetic energy in the intermediate-time region of a decay process where
those of the parent and daughters overlap. The decay rate computed with Fermi's
golden rule requires corrections that vary with the distance between the
initial and final states, and the energy distribution of the daughter is
distorted from that of plane waves. The corrections have universal properties
in relativistically invariant systems and reveal macroscopic quantum phenomena
for light particles. The implications for precision experiments in beta decays
and various radiative transitions are presented.
