I have read this question:
where it says:
As light travels upward in the earth’s gravitational field, it loses energy, and so its frequency goes down. (This means that the length of time between one wave crest and the next goes up.) To someone high up, it would appear that everything down below was making longer to happen.
Now I do understand GR time dilation. I do understand the gravitational potential, and the difference between two places in space (where the gravitational potential is different) will cause time dilation. I understand the Shapiro delay too.
Now what I do not understand is why does a photon's frequency decrease as it travels upwards from Earth? I understand it travels in a changing gravitational field, as it travels upwards, the gravitational potential decreases. But I do not see how the decreasing gravitational field (potential) causes directly the photon's frequency to decrease. Is there a QM explanation to this? Or is there a GR explanation?
Why is a photon's frequency decreasing because it travels upwards in Earth's gravitational field, that is it travels in a decreasing gravitational field (potential)?
Do all photons coming out of the Sun have a decreased frequency compared to when they were emitted nearer the core?