Editing Gravitational redshift (section)

=== Uniform gravitational field or acceleration ===
Einstein's theory of general relativity incorporates the [[equivalence principle]], which can be stated in various different ways. One such statement is that gravitational effects are locally undetectable for a free-falling observer. Therefore, in a laboratory experiment at the surface of the Earth, all gravitational effects should be equivalent to the effects that would have been observed if the laboratory had been accelerating through outer space at ''g''. One consequence is a gravitational [[Doppler effect]]. If a light pulse is emitted at the floor of the laboratory, then a free-falling observer says that by the time it reaches the ceiling, the ceiling has accelerated away from it, and therefore when observed by a detector fixed to the ceiling, it will be observed to have been Doppler shifted toward the red end of the spectrum. This shift, which the free-falling observer considers to be a kinematical Doppler shift, is thought of by the laboratory observer as a gravitational redshift. Such an effect was verified in the 1959 [[Pound–Rebka experiment]]. In a case such as this, where the gravitational field is uniform, the change in wavelength is given by
: <math>z = \frac{\Delta\lambda}{\lambda}\approx \frac{g\Delta y}{c^2},</math>
where <math>\Delta y</math> is the change in height. Since this prediction arises directly from the equivalence principle, it does not require any of the mathematical apparatus of general relativity, and its verification does not specifically support general relativity over any other theory that incorporates the equivalence principle.

On Earth's surface (or in a spaceship accelerating at 1&nbsp;''g''), the gravitational redshift is approximately {{val|1.1|e=−16}}, the equivalent of a {{val|3.3|e=−8|u=m/s}} Doppler shift for every 1&nbsp;m of altitude.