What Einstein found as he developed these theories is that gravity changes the wavelength of light. The force of gravity resists escaping light, so the light loses energy and is shifted toward the red (low-energy) end of the spectrum. This prediction was verified in the 1960's at Harvard University. As the speed of light is constant, the increased interval between peaks of the light wave must indicate that time itself is slowed down by gravity. So, time flows faster at the ceiling than on the floor! Note, however, that this is a different form of time dilation than the type that occurs in inertial frames.