Gravitational red shift. An important consequence of Einstein’s general relativity is that gravity must affect a light wave’s frequency and wavelength. As light moves upward from the Earth’s surface, the wavelength of the light increases and the frequency decreases as gravity “drains” the light of some energy. In a famous experiment in 1960, Robert Pound and Glen Rebka of Harvard University successfully tested this effect to within 10% of the predicted value. Later, in 1964, they improved the agreement to within 1%. The experimental procedure involved placing a detector 22.6 m above a radioactive source placed on the ground. Quantized “particles” of light called photons (Chapter 28) are given off by the source with an energy of 14.4 keV. Using a highly sensitive technique called the Mössbauer effect, they were able to measure a small shift in the energies of the photons as they moved upward through the Earth’s gravitational field. Although photons are mass-less, we can treat their energy as rest energy and find a corresponding “effective mass” for the photons. Using this approach, determine the shift in the photons’ energy as they move the 22.6 m up to the detector. This shift is called a gravitational red shift because it results in a shift toward a lower frequency and longer wavelength.