I found this formula for the gravitational redshift: [tex](adsbygoogle = window.adsbygoogle || []).push({});

\frac{\lambda_R}{\lambda_E} = \sqrt{\frac{1-2GM/r_R}{1-2GM/r_E}}

[/tex] where the indexes R and E are for receiver and emitter respectively, and the speed of light is normalized to 1.

Most other sources on the net I found only show the limit for ##r_R\to\infty##, so I was happy to see the explicit formula without the limit, because I wanted to see the frequency shift when the photon approaches the Schwarzschild radius of the mass ##M##. This looks like ##\lambda_R## then tends to zero, meaning the photon's frequency and energy go to infinity.

But for which observer does this hold? I take it it is the one near the Schwarzschild radius, not someone looking from the outside?

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# Gravitational red- or rather blue-shift when approaching Schwarzschild radius

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