Based on general relativity (see book Gravity; an introduction to Einstein's General relativity by James B. Hartle), the frequency of light (emitted at R) is modified by the factor SQRT (1 - 2 G M / c**2 R) (see page 191).(adsbygoogle = window.adsbygoogle || []).push({});

On the other hand, based on the equivalence principle, one arrives at the factor (1- G M /c**2 R). It is clear that the former equation reduces to the latter for small values of G M /c**2 R.

What I do not understand however is why the simple straightforward derivation, based on the equivalence principle, apparently gives only an approximation to the real red-shift. So, what could be wrong or missing in the original derivation (same book, page 116-119) ?

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# Correct formula for gravitational redshift

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