Spontaneous excitation of an atom in a Kerr spacetime

TL;DR

This paper investigates how atoms interact with quantum fields near rotating black holes, analyzing spontaneous excitation and emission influenced by vacuum states, Hawking radiation, and superradiance effects.

Contribution

It provides a detailed quantitative analysis of atomic radiative processes in Kerr spacetime, highlighting the roles of different vacuum states and physical effects like Hawking radiation and superradiance.

Findings

Spontaneous excitation linked to Unruh-Starobinskii effect in Frolov-Thorne vacuum.

Boulware vacuum exhibits outward particle flux due to superradiance.

Static atoms show different excitation behaviors depending on vacuum state.

Abstract

We consider radiative processes of an atom in a rotating black-hole background. We assume the atom, represented by a hypothetical two-level system, is coupled via a monopole interaction with a massless quantum scalar field prepared in each one of the usual physical vacuum states of interest. We constrain ourselves to two different states of motion for the atom, namely a static situation in which the atom is placed at a fixed radial distance, and also the case in which it has a stationary motion but with zero angular momentum. We study the structure of the rate of variation of the atomic energy. The intention is to clarify in a quantitative way the effect of the distinguished contributions of vacuum fluctuations and radiation reaction on spontaneous excitation and on spontaneous emission of atoms. In particular, we are interested in the comprehension of the combined action of the different physical processes underlying the Hawking effect in the scenario of rotating black holes as well as the Unruh-Starobinskii effect. We demonstrate that, in the case of static atoms, spontaneous excitation is also connected with the Unruh-Starobinskii effect, but only in the case of the quantum field prepared in the Frolov-Thorne vacuum state. In addition, we show that, in the ZAMOs perspective, the Boulware vacuum state contains an outward flux of particles as a consequence of the black-hole superradiance. The possible relevance of the findings in the present work is discussed.