Macroscopic aspects of the Unruh effect

TL;DR

This paper clarifies that the Unruh effect, often considered thermal, does not produce macroscopic thermal effects for accelerated observers in the vacuum, emphasizing the distinction between microscopic responses and macroscopic observations.

Contribution

It provides a macroscopic analysis of the Unruh effect showing it is not a thermal phenomenon, contrasting microscopic responses with observable macroscopic effects.

Findings

Accelerated observers do not detect global thermal effects in the vacuum.

Genuine equilibrium states in accelerated frames are spatially inhomogeneous.

The Unruh effect's thermal interpretation is not supported at the macroscopic level.

Abstract

Macroscopic concepts pertaining to the Unruh effect are elaborated and used to clarify its physical manifestations. Based on a description of the motion of accelerated, spatially extended laboratories in Minkowski space in terms of Poincar\'e transformations, it is shown that, from a macroscopic perspective, an accelerated observer will not register with his measuring instruments any global thermal effects of acceleration in the inertial (Minkowskian) vacuum state. As is explained, this result is not in conflict with the well-known fact that microscopic probes respond non-trivially to acceleration if coupled to the vacuum. But this response cannot be interpreted as the effect of some heat bath surrounding the observer. It is also shown that genuine equilibrium states in a uniformly accelerated laboratory cannot be spatially homogeneous. In particular, these states coincide with the homogeneous inertial vacuum at sufficiently large distances from the horizon of the observer and consequently have the same (zero) temperature there. The analysis is carried out in the theory of a free massless scalar field; however the conclusion that the Unruh effect is not of a thermal nature, viz. induced by heat transfer, is generally valid.