Leggett-Garg inequalities and decays of unstable systems

We apply the Leggett-Garg inequalities (LGI) to the case of classical and quantum unstable systems. For classical systems the two assumptions of macroscopic realism and non-invasive measurements imply that the three-measurement string $K_3$ is identically equal to one. Also for quantum mechanical systems -- for which the two assumptions are in general not valid -- we find that $K_3=1$ for purely exponential decays ($K_3\leq1$ is the general LGI). On the other hand, the necessary deviations from the exponential decay law at short and long times predicted by quantum mechanics lead to values of $K_3 \neq 1$. Moreover, a strict violations $K_3>1$ occurs typically at short times. Thus, we conclude that experiments in which such deviations from the exponential decay law have been observed should also find in their data violations of the LGIs.