Redshift/blueshift inside the Schwarzschild black hole

TL;DR

This paper analyzes how photon frequencies are affected inside a Schwarzschild black hole, comparing scenarios where photons come from infinity or are emitted by a collapsing star, revealing differences in redshift and blueshift near the horizon and singularity.

Contribution

It provides a detailed comparison of frequency shifts for photons absorbed inside a Schwarzschild black hole in different emission scenarios, highlighting the role of angular momentum.

Findings

Infinite redshift or blueshift possible near the singularity depending on angular momenta.

Distinct frequency change behaviors between photons from infinity and those emitted by a star.

Differences become prominent when photons are emitted near the horizon and received in the intermediate region or near the singularity.

Abstract

We consider an observer who moves under the horizon of the Schwarzschild black hole and absorbs a photon. There are two different situations when (i) a photon comes from infinity, (ii) it is emitted by another observer under the horizon (say, by a surface of a collapsing star). We analyze the frequency change for absorption near the event horizon, in an intermediate region and near the singularity and compare the results for both scenarios. Near the singularity, in both scenarios infinite redshift (in the pure radial case) or infinite blueshift are possible, depending on angular momenta of a photon and an observer. The main difference between both scenarios manifests itself, if photons from the star surface are emitted near the horizon and received in the intermediate region or near the singularity by the observer with a nonzero angular momentum.