We saw in Sec. 7.4 that light leaving a region where the gravitational force is large will be shifted towards the red (its wavelength increases; see Figs. 7.21, 7.9); similarly, light falling into a region where the gravitational pull is larger will be shifted towards the blue. This prediction was tested in Harvard by looking at light as it fell from a tower (an experiment requiring enormous precision since the changes in the gravitational force from the top to the bottom of a tower are minute) and the results agree with the predictions from the General Theory of Relativity.
The gravitational red-shift was also tested by looking at the light from a type of stars which are very very well-studied. The observations showed that the light received on Earth was slightly redder than expected and that the reddening is also in agreement with the predictions from the General Theory of Relativity.