In the POUND REBKA experiment, the gravitational spectral shift of gamma radiation in the Earth's gravitational field is detected.
Interpretation 1: Photons gain/lose energy when they move towards a higher/lower gravitational potential (blue/red shift).
Interpretation 2: Photons are indivisible quanta whose energy content does not change. The energy source emitted higher/ lower energy photons when it is in a higher/lower gravitational potential (blue/red shift).
<aside> 📌 Presumtion: Gravitational red/blue shift is based on the location of the source in its gravitational potential relative to the normal in our gravitational potential. ↓16↓
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