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.