Massive Unruh particles cannot be directly observed

TL;DR

The paper argues that massive Unruh particles are practically unobservable due to their confinement near the horizon, while massless particles may offer a more feasible observation of the Unruh effect.

Contribution

It demonstrates the physical limitations of detecting massive Unruh particles and highlights the potential for observing the effect with massless particles.

Findings

Massive particles are confined within a Compton wavelength of the horizon.

Detection of massive Unruh particles is beyond current technological reach.

Massless particles show different behavior, making their observation more promising.

Abstract

We show that massive particles created in a relativistically accelerated reference frame, as predicted by the Unruh effect, can only be found in a tiny layer above the event horizon, whose thickness corresponds to a single Compton wavelength. This is beyond the reach of any detector and suggests that the Unruh effect may not ever be directly observed for massive fields. The case of massless particles is also examined, for which qualitatively different behaviour is observed in a low-acceleration regime, suggesting that an observation of the Unruh effect for massless particles is more promising.