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.

中文翻译：

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史瓦西黑洞内部的红移/蓝移

我们考虑一个在史瓦西黑洞地平线下移动并吸收光子的观察者。当 (i) 光子来自无限远，(ii) 它是由地平线下的另一个观察者（例如，坍缩恒星的表面）发射的，有两种不同的情况。我们分析了事件视界附近、中间区域和奇点附近吸收的频率变化，并比较了两种情况的结果。在奇点附近，在两种情况下，无限红移（在纯径向情况下）或无限蓝移都是可能的，这取决于光子和观察者的角动量。如果来自恒星表面的光子在地平线附近发射并在中间区域或奇点附近被观察者以非零角动量接收，则两种情况之间的主要区别就表现出来了。