Temperature dependence of atom-atom interactions

The interaction potential at finite temperature between two atoms in the asymptotic limit of large separations, $R\gg ħ{c/k}_{B}T,$ is $V\left(R\right)=-{3k}_{B}T{\alpha }^2{\mathrm{}\left(0\right)/R}^6,$ identical to the expression for two classical polarizable particles. This is in contrast to the zero-temperature, large-separation Casimir-Polder retarded potential $V\left(R\right)=-23ħc{\alpha }^2\mathrm{}\left(0\right)/4\mathrm{}\pi R^7.$ The result is demonstrated by two independent methods, so as to facilitate a reinterpretation of retardation effects in interatomic interactions. Here, we argue that the conventional mechanistic interpretation of retardation at large separations in terms of the loss of interatomic correlations due to the finite velocity of light is too simplistic, and relies on nonquantum, time-dependent concepts to explain an equilibrium, quantum effect. We offer an alternative picture in terms of the nature of the thermal excitations of the electromagnetic field, which results in a remarkable manifestation of the correspondence principle at sufficiently large interatomic distances.