If the speed of light is variable in a gravitational field, it is variable in gravitation-free space as well:

Michael Fowler, University of Virginia: "What happens if we shine the pulse of light vertically down inside a freely falling elevator, from a laser in the center of the ceiling to a point in the center of the floor? Let us suppose the flash of light leaves the ceiling at the instant the elevator is released into free fall. If the elevator has height h, it takes time h/c to reach the floor. This means the floor is moving downwards at speed gh/c when the light hits. Question: Will an observer on the floor of the elevator see the light as Doppler shifted? The answer has to be no, because inside the elevator, by the Equivalence Principle, conditions are identical to those in an inertial frame with no fields present. There is nothing to change the frequency of the light. This implies, however, that to an outside observer, stationary in the earth's gravitational field, the frequency of the light will change. This is because he will agree with the elevator observer on what was the initial frequency f of the light as it left the laser in the ceiling (the elevator was at rest relative to the earth at that moment) so if the elevator operator maintains the light had the same frequency f as it hit the elevator floor, which is moving at gh/c relative to the earth at that instant, the earth observer will say the light has frequency f(1 + v/c) = f(1+gh/c^2), using the Doppler formula for very low speeds." http://galileo.phys.virginia.edu/cla...elativity.html

Substituting f=c/λ into Fowler's equation gives:

f' = f(1+v/c) = f(1+gh/c^2) = (c+v)/λ = c(1+gh/c^2)/λ = c'/λ

where f' is the frequency measured by both the observer "stationary in the earth's gravitational field" and an equivalent observer who, in gravitation-free space, moves with speed v=gh/c towards the emitter. Accordingly,

c' = c+v = c(1+gh/c^2)

is the speed of light relative to those two observers. Clearly, the frequency shift is due to a shift in the speed of light - the speed of light varies with both the gravitational potential and the speed of the observer, just as predicted by Newton's theory and in violation of Einstein's relativity.

Pentcho Valev