Thermal Casimir vs Casimir-Polder forces: Equilibrium and non-equilibrium forces

TL;DR

This paper critically examines the differences between thermal Casimir and Casimir-Polder forces, highlighting the non-equilibrium contributions and the limitations of the Matsubara sum representation for ground-state atoms.

Contribution

It reveals that the thermal Casimir-Polder force involves non-equilibrium effects and virtual-photon absorption, challenging the common assumption of a Matsubara sum representation.

Findings

Virtual-photon absorption contributes to the force in ground-state atoms.

Equilibrium force may be overestimated using ground-state polarizability.

Non-equilibrium dynamics are signatures in the force calculations.

Abstract

We critically discuss the common belief that the thermal Casimir-Polder force can always be represented as a Matsubara sum, as it is true for the Casimir force. On the contrary, an exact treatment of the atom-field coupling reveals that for a ground-state atom, terms associated with virtual-photon absorption contribute to the force which are identified as a signature of non-equilibrium dynamics. Even the equilibrium force on a thermalized atom or molecule may be overestimated when using the ground-state polarizability instead of its thermal counterpart.