Thermality Breakdown in Null-Shifted Rindler Wedges

TL;DR

This paper studies how mass affects the thermal nature of quantum fields in null-shifted Rindler wedges, revealing a breakdown of thermality and showing that massive fields do not exhibit the usual Unruh effect.

Contribution

It demonstrates that mass breaks the conformal symmetry necessary for thermality in accelerated frames, leading to nonthermal particle spectra in null-shifted Rindler wedges.

Findings

Massive fields do not exhibit exponential frequency mixing.

Thermality depends on conformal symmetry, broken by mass.

Massive fields remain unexcited, showing nonthermal response.

Abstract

We investigate the behaviour of quantum fields in null-shifted Rindler wedges and analyse the particle spectra perceived by accelerated observers associated with these null deformations. Unlike the standard Unruh effect, our analysis compares two accelerated frames connected by a null displacement. We consider both massive scalar and Dirac fields, constructing their corresponding mode solutions in Rindler coordinates. Using normalised field expansions, we compute the Bogoliubov transformations between modes defined in the two null-shifted wedges. Our results demonstrate a fundamental breakdown of thermality: the presence of mass modifies the mode structure, rendering the characteristic exponential mixing of frequencies absent. This suggests that the massive field remains unexcited on this background, leading to a manifestly nonthermal response. These findings highlight that thermality in accelerated frames depends sensitively on the conformal symmetry of the field, which is broken by the introduction of a mass term.