Several books (e.g. “Raum, Zeit, Materie” by H Weyl) say,“Frequency that an observer receives is the same to the frequency that a light source in a different gravitational field (at a standstill) emits. But it’s not the same to the frequency that the same source at hand emits". By interference of this two lights (one waves pass the other waves ; different from interference of MM experiment), it seems to be clarified whether phenomenon gravitational redshift occurs or not (after enough time ; even by earthly experiment).
If phenomenon gravitational redshift is true, explanation (quantitative) as follows seems to be possible on an elevator cabin stays in a gravitational field. Light speed is slower in a gravitational field. So, speed of light emitted upward becomes faster and wavelength becomes longer. Then, redshift occurs.
-P.S.- In an elevator cabin in a uniform accelerated motion (upward), this change is contrary.
Gravitational redshift (both in a gravitational field and in a uniform accelerated frame) will be invisible. Because, colors are caused by the difference of frequency (not by wavelength).
In an accelerated elevator cabin (upward ; uniformly), behavior of every wave will be the same. So, difference of frequency at the roof and at the floor will be impossible (in an elevator in gravitational field, it will be the same).
There is a space ship (accelerating upward). Books say that "gravity" at roof and floor is different.
Disproof ; There are two probes (accelerating upward). Position of the two is apart vertically (distance is the height of the space ship). "Gravity" at the two will be the same.
Many books explain bending of light with an elevator cabin in free fall. Now, two elevator cabins are falling before an observer's eyes. Falling speed of the two is different because starting points of falling is different. Does the observer observe different bending of light (gravity g is the same) ?
P.S. In many books, light ray is coming from the outside of the elevator cabin. In some books, light source is set on the wall of the cabin. In the former, falling of the cabin will have no effect on light ray (imagine a cabin without wall). In the latter (if the emission theory is true), behavior of light ray will be easy to imagine.
For a 10 seconds, an elevator cabin accelerated upward and stopped. Number of waves of light that appears and vanishes during that time will be the same.
