  We discuss the existence in an arbitrary frame of a finite time for the
transformation of an initial quantum state into another e.g. in a decay.
  This leads to the introduction of a timelapse $\tilde{\tau}$ in analogy with
the lifetime of a particle. An argument based upon the Heisenberg uncertainty
principle suggests the value of $\tilde{\tau}=1 / M_0$. Consequences for the
exponential decay formula and the modifications that $\tilde{\tau}$ introduces
into the Breit-Wigner mass formula are described.
