Casimir Theory of the Relativistic Composite String Revisited, and a Formally Related Problem in Scalar QFT

TL;DR

This paper revisits the Casimir energy of a relativistic piecewise uniform string using multiple regularization methods, explores its thermodynamics, and introduces a scalar QFT model analogous to the string's potential structure.

Contribution

It provides an updated analysis of Casimir energies for multi-piece strings and introduces a scalar field theory model that mimics the string's potential profile.

Findings

Casimir energy is negative and increases in magnitude with more string pieces.

The formalism is demonstrated for two-piece and multi-piece strings with a recursive approach.

A scalar QFT model is proposed as an analogy to the string potential.

Abstract

The main part of this paper is to present an updated review of the Casimir energy at zero and finite temperature for the transverse oscillations of a piecewise uniform closed string. We make use of three different regularizations: the cutoff method, the complex contour integration method, and the zeta-function method. The string model is relativistic, in the sense that the velocity of sound is for each string piece set equal to the velocity of light. In this sense the theory is analogous to the electromagnetic theory in a dielectric medium in which the product of permittivity and permeability is equal to unity (an isorefractive medium). We demonstrate how the formalism works for a two-piece string, and for a 2N-piece string, and show how in the latter case a compact recursion relation serves to facilitate the formalism considerably. The Casimir energy turns out to be negative, and the more so the larger the number of pieces in the string. The two-piece string is quantized in D-dimensional spacetime, in the limit when the ratio between the two tensions is very small. We calculate the free energy and other thermodynamic quantities, demonstrate scaling properties, and comment on the meaning of the Hagedorn critical temperature for the two-piece string. Thereafter, as a novel development we present a scalar field theory for a real field in three-dimensional space in a potential rising linearly with a longitudinal coordinate z in the interval 0<z<1, and which is thereafter held constant on a horizontal plateau. The potential is taken as a rough model of the two-piece string potential under simplifying conditions, when the length ratio between the pieces is replaced formally with the mentioned length parameter z.