3D scalar model as a 4D perfect conductor limit: dimensional reduction and variational boundary conditions

TL;DR

This paper explores how boundary conditions affect the dimensional reduction of scalar and electromagnetic fields, revealing that perfect conductor conditions lead to a scalar limit, while PMC and Neumann conditions preserve lower-dimensional theories.

Contribution

It demonstrates that boundary conditions critically determine the outcome of dimensional reduction, with specific conditions like PMC and Neumann preserving lower-dimensional models in any dimension.

Findings

Perfect conductor conditions reduce EM fields to scalar fields in lower dimensions.

Dirichlet boundary conditions lead to zero Casimir pressure upon reduction.

PMC and Neumann boundary conditions preserve lower-dimensional theories across dimensions.

Abstract

Under dimensional reduction, a system in D spacetime dimensions will not necessarily yield its D-1-dimensional analog version. Among other things, this result will depend on the boundary conditions and the dimension D of the system. We investigate this question for scalar and abelian gauge fields under boundary conditions that obey the symmetries of the action. We apply our findings to the Casimir piston, an ideal system for detecting boundary effects. Our investigation is not limited to extra dimensions and we show that the original piston scenario proposed in 2004, a toy model involving a scalar field in 3D (2+1)dimensions, can be obtained via dimensional reduction from a more realistic 4D electromagnetic (EM) system. We show that for perfect conductor conditions, a D-dimensional EM field reduces to a D-1 scalar field and not its lower-dimensional version. For Dirichlet boundary conditions, no theory is recovered under dimensional reduction and the Casimir pressure goes to zero in any dimension. This "zero Dirichlet" result is useful for understanding the EM case. We then identify two special systems where the lower-dimensional version is recovered in any dimension: systems with perfect magnetic conductor (PMC) and Neumann boundary conditions. We show that these two boundary conditions can be obtained from a variational procedure in which the action vanishes outside the bounded region. The fields are free to vary on the surface and have zero modes which survive after dimensional reduction.