Eur. Phys. J. C (2011) 71: 1765
https://doi.org/10.1140/epjc/s10052-011-1765-4

Regular Article - Theoretical Physics

Repulsive Casimir–Polder forces from cosmic strings

Department of Physics, Yerevan State University, 1 Alex Manoogian Street, 0025, Yerevan, Armenia

^* e-mail: saharian@ysu.am

Received: 5 August 2011
Revised: 7 September 2011
Published online: 28 September 2011

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 on 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 with the fifth power of the distance. In the non-retarded regime, corresponding to separations smaller than the relevant transition wavelengths, the force decays with 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 a moment of force acting on the particle.