Comment on "Cutoff dependence of the Casimir force within an inhomogeneous medium"

TL;DR

This paper corrects a previous claim that the inhomogeneous Casimir force diverges with cutoff removal, demonstrating instead that it is cutoff independent when properly calculated using first-order perturbation theory.

Contribution

The authors identify and correct a miscalculation in prior work, providing a first-order perturbative framework for analyzing inhomogeneous Casimir energy and force.

Findings

Casimir pressure is cutoff independent when correctly calculated.

Parallel wave contributions and operator non-commutativity are important for understanding divergences.

First-order perturbation theory suffices for analyzing inhomogeneous Casimir effects.

Abstract

Horsley and Simpson [Phys. Rev. A 88, 013833 (2013)] recently claimed that the inhomogeneous Casimir pressure in a piston model is cutoff dependent, and diverges when the cutoff parameter is removed ({\xi}->0). We demonstrate that, there is a miscalculation in their derivation, and our correction results in a cutoff independent Casimir pressure, based on first-order perturbation theory. We give the general expressions of first-order perturbative inhomogeneous Casimir energy which make it convenient to analyze the divergence problem or to yield the Casimir force. The Casimir pressure contribution of parallel waves (with wave vector parallel to the Casimir plates) together with the non-commutativity of limit and summation operators, are discussed and found to be useful for understanding the inhomogeneous divergence declared in another paper [Phys. Rev. A 87, 043806 (2013)]. We should emphasize that we cannot yet give an exact expression of inhomogeneous Casimir pressure beyond first-order perturbation, which is worth future investigation.