K20and the Equivalence Principle

It is shown that the existence of the long-lived neutral $K$ meson, and the absence of its decay into two pions, establishes that the gravitational masses of the $K^0$ and ${\overline{K}}^0$ are equal to a few parts in ${10}^{-10\mathrm{}}$ of the $K$ inertial mass. This is of interest since the ${\overline{K}}^0$ is the antiparticle of the $K^0$, and is not identical with the $K^0$. The gravitational mass of such a nonidentical antiparticle has never been directly measured.