Timelapse

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 $\tilde{\tau }=1\mathrm{/}{M}_0$. Consequences for the exponential decay formula and the modifications that $\tilde{\tau }$ introduces into the Breit-Wigner mass formula are described.