By resolving the following problem. A light source at the top of a tower
with height h emits light and this light reaches a receiver on the ground
with a speed (as judged by the receiver): A) c ; B) c+v, v>0
One applies the equivalence principle and obtains that, if a rocket with
length h accelerates with acceleration g, light emitted by the front end
will take the time h/c to reach the back end (receiver). Accordingly, at the
moment the light reaches the receiver the latter will have a speed v=gh/c
(relative to the original frame at rest).
Now the receiver calculates the frequency of the received light by using the
formula
f = V/lambda
where V is the receiver's relative speed with respect to the light. If A is
used (V=c), the receiver obtains
f = fo
where fo is the initial frequency of the light. In other words, if the speed
of light is constant, there will be no frequency shift. If B is used
(V=c+v), the receiver obtains
f = fo(1 + gh/c^2)
where (1 + gh/c^2) is the frequency shift factor obtained also
EXPERIMENTALLY. In other words, the hypothesis that the speed of light is
VARIABLE is confirmed experimentally.
Ironically, Einstein has managed to convince the world that the redshift
just provides one of the glorious confirmations of relativity. But otherwise
he would not be one of the greatest jugglers in science (perhaps only
Prigogine is greater).