Hawking radiation as perceived by different observers (ERE2011 proceedings)

TL;DR

This paper investigates how different observers perceive Hawking radiation near a black hole, revealing that free-falling observers can detect radiation at the horizon due to Doppler effects, challenging previous assumptions.

Contribution

It introduces an effective-temperature function to analyze observer-dependent perceptions of Hawking radiation in a dynamic vacuum state.

Findings

Free-falling observers perceive Hawking radiation at the horizon.

Diverging Doppler shift explains perception of radiation by free-fall observers.

The vacuum state is modeled as non-stationary to mimic realistic black hole formation.

Abstract

We study the perception of Hawking radiation by different observers outside a black hole. The analysis is done in terms of an effective-temperature function that varies along the trajectory of each observer. The vacuum state of the radiation field is chosen to be non-stationary, so as to mimic the switching-on of Hawking radiation that would appear in a real black hole collapse. We analyse how this vacuum is perceived by observers staying at a fixed radius, by observers coming in free-fall from radial infinity at different times, and by observers in free-fall released from finite radial positions. Results found have a compelling physical interpretation. One main result, at first unexpected, is that in general free-falling observers do perceive particle emission by the black hole when crossing the event horizon. This happens because of a diverging Doppler shift at the event horizon.