Repulsive Casimir-Polder forces from cosmic strings

TL;DR

This paper studies the Casimir-Polder force on a microparticle near a cosmic string, revealing conditions under which the force is attractive or repulsive, depending on polarizability anisotropy and separation distance.

Contribution

It derives the Casimir-Polder force in the cosmic string geometry for anisotropic polarizability, showing the force can be repulsive or attractive and depends on orientation and distance.

Findings

Force can be repulsive or attractive depending on polarizability anisotropy.

At large distances, the force decreases as the inverse fifth power of separation.

In the non-retarded regime, the force decays as the inverse fourth power of distance.

Abstract

We investigate the Casimir-Polder force acting on a polarizable microparticle in the geometry of a straight cosmic string. In order to develop this analysis we evaluate the electromagnetic field Green tensor on the imaginary frequency axis. The expression for the Casimir-Polder force is derived in the general case of anisotropic polarizability. In dependence of the eigenvalues for the polarizability tensor and of the orientation of its principal axes, the Casimir-Polder force can be either repulsive or attractive. Moreover, there are situations where the force changes the sign with separation. We show that for an isotropic polarizability tensor the force is always repulsive. At large separations between the microparticle and the string, the force varies inversely as the fifth power of the distance. In the non-retarded regime, corresponding to separations smaller than the relevant transition wavelengths, the force decays as the inverse fourth power of the distance. In the case of anisotropic polarizability, the dependence of the Casimir-Polder potential on the orientation of the polarizability tensor principal axes also leads to the moment of force acting on the particle.