The dielectric response to an inhomogeneous electric field has been investigated for Ba and ${\text{Ba}}_{2}$ within a molecular beam experiment. The ratio of the polarizabilities per atom of ${\text{Ba}}_{2}$ and Ba is determined to be $1.30\ifmmode\pm\else\textpm\fi{}0.13$. The experimental result is compared to a high level ab initio quantum chemical coupled cluster calculation with an energy-consistent scalar relativistic small-core pseudopotential for Ba. For the barium atom a polarizability of $40.82\text{ }{\text{\AA{}}}^{3}$ is obtained and the isotropic value of the polarizability calculated for ${\text{Ba}}_{2}$ is $97.88\text{ }{\text{\AA{}}}^{3}$, which is in good agreement with the experimental results, demonstrating that a quantitative understanding of the interaction between two closed-shell heavy element metal atoms has been achieved.