Question:
Why do photons blue shift in gravitational fields, and what is the mathematical formula for me to calculate how much a photon will blue shift in a gravitational field?

Replies:
Hi Aaron,

The wavelength appears to shift because of time dilation in the reference frame of the observer which is a consequence of mass-energy equivalence and Integrated Sachs-Wolfe effect.

Wikipedia gives a rigorous explanation of V under "gravitational potential" which could be combined to express frequency(F) or wavelength. Then, for frequency, the shift, z = (Fe - Fob)/Fob ; z < 0 for blue shift (toward observer) and z > 0 for red shift (away). However, the limits for validity of simple formulas for these potentials vary considerably.

Hoping this helps, Peter E. Hughes, Ph.D. Milford, NH

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