thumb|Jefferson laboratory at Harvard University. The experiment occurred in the left "tower". The attic was later extended in 2004.
The Pound–Rebka experiment monitored frequency shifts in gamma rays as they rose and fell in the gravitational field of the Earth. The experiment tested Albert Einstein's 1907 and 1911 predictions, based on the equivalence principle, that photons would gain energy when descending a gravitational potential, and would lose energy when rising through a gravitational potential. and was the last of the classical tests of general relativity to be verified.
In the accelerated system, light emitted from <math>S_2</math> takes (to a first approximation) <math>h/c</math> to arrive at <math>S_1.</math> But in this time, the velocity of <math>S_1</math> will have increased by <math> v = gh/c </math> from its velocity when the light was emitted. The frequency of light arriving at <math>S_1</math> will therefore not be the frequency <math>f_2,</math> but the greater frequency <math>f_1</math> given by
<math display="block">f_1 \approx f_2 \left( 1 + \frac{v}{c}\right) = f_2 \left( 1 + \frac{gh}{c^2}\right) .</math>
According to the equivalence principle, the same relation holds for the non-accelerated system in a gravitational field, where we replace <math>gh</math> by the gravitational potential difference <math> \Phi </math> between <math>S_2</math> and <math>S_1</math> so that Gravitational redshift and two other predictions from his 1916 paper, the anomalous perihelion precession of Mercury's orbit and the gravitational deflection of light by the Sun, have become known as the "classical tests" of general relativity. The anomalous perihelion precession of Mercury had long been recognized as a problem in celestial mechanics since the 1859 calculations of Urbain Le Verrier. The observation of the deflection of light by the Sun in the 1919 Eddington expedition catapulted Einstein to worldwide fame. Gravitational redshift would prove to be by far the most difficult of the three classical tests to demonstrate.
There had been little rush by experimenters to test Einstein's earlier predictions of gravitational time dilation, since the predicted effect was almost immeasurably small. Einstein's predicted displacement for spectral lines of the Sun amounted to only two parts in a million, and would be easily masked by line broadening due to temperature and pressure, and by line asymmetry due to the fact the lines represent the superposition of absorption from many turbulent layers of the solar atmosphere. However, even Adams's measurements have since been brought into question for various reasons.
Mössbauer effect
In atomic spectroscopy, visible and ultraviolet photons resulting from electronic transitions of outer shell electrons, when emitted by gaseous atoms in an excited state, are readily absorbed by unexcited atoms of the same species. However, a corresponding absorbance of photons emitted by the nuclei of γ-emitters had never been observed because recoil of the nuclei resulted in so much loss of energy by the emitted photons that they no longer matched the absorbance spectra of the target nuclei. In 1958, Rudolf Mössbauer, who was analyzing the 129 keV transition of Iridium-191, discovered that by lowering the temperature of the emitter to 90K, he could achieve resonant absorbance. Indeed, the energy resolutions that he achieved were of unheard-of sharpness. He had discovered the phenomenon of recoilless γ-emission.