Pound–Rebka experiment

The Pound–Rebka experiment monitored frequency shifts in gamma rays as they rose and fell in the gravitational field of the Earth.

[p 1][p 2] It was proposed by Robert Pound and his graduate student Glen A. Rebka Jr. in 1959,[p 3] and was the last of the classical tests of general relativity to be verified.

In 1916, Einstein used the framework of his newly completed general theory of relativity to update his earlier heuristic arguments predicting gravitational redshift to a more rigorous form.

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.

Adams's 1925 measurement of shifts in the spectral lines of the white dwarf star Sirius B.

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.

[1][note 1] In 1959, several research groups, most notably Robert Pound and Glen Rebka in Harvard and a team led by John Paul Schiffer in Harwell (England), announced plans to exploit this recently discovered effect to perform terrestrial tests of gravitational redshift.

In February 1960, Schiffer and his team were the first to announce success in measuring the gravitational redshift, but with a rather high error of ±47%.

[p 6] It was to be Pound and Rebka's somewhat later contribution in April 1960, which used a stronger radiation source, longer path length, and several refinements to reduce systematic error, which was to be accepted as having provided a definitive measurement of the redshift.

[p 4] After evaluating various γ-emitters for their study, Pound and Rebka chose to use 57Fe because it does not require cryogenic cooling to exhibit recoil-free emission, has a relatively low internal conversion coefficient[note 2] so that it is relatively free of competing X-ray emissions that would have been difficult to distinguish from the 14.4 keV transition,[note 3] and its parent 57Co has a usable half-life of 272 days.

[5] Pound and Rebka found that a large source of systematic error resulted from temperature variations, which they attributed primarily to a second order relativistic Doppler effect due to lattice vibrations.

A mere 1°C difference in temperature between emitter and absorber caused a shift about equal to the predicted effect of gravitational time dilation.

[p 7][6] They also found frequency offsets between the lines of different combinations of source and absorber stemming from the sensitivity of the nuclear transition to an atom's physical and chemical environment.

[note 4] They therefore needed to adopt methodology which would allow them to distinguish these offsets from their measurement of gravitational redshift.

[p 4] The experiment was carried out in a tower at Harvard University's Jefferson laboratory that was, for the most part, vibrationally isolated from the rest of the building.

[5] The vibrating speaker coil imposed a continuously varying Doppler shift on the gamma ray source.

The hydraulic cylinder motion was reversed multiple times during each data run after a constant integral number of transducer vibrations.

[5] Among the other steps used to compensate for possible systematic errors, Pound and Rebka varied the speaker frequency between 10 Hz and 50 Hz and tested different transducers (ferroelectric transducers versus moving coil magnetic speaker coils).

[5] Although the 14.4 keV recoilless emission line of 57Fe had a half-width of 1.13×10−12, the anticipated gravitational frequency shift was only 2.5×10−15.

[note 5] Counts that were received in the quarter cycles of the oscillation period centered around the velocity maxima were accumulated in two separate registers.

Eight runs with the source at the top, after temperature correction gave a weighted average fractional frequency shift of −(15.5±0.8)×10−15.

Taking half the sum of the weighted averages yielded the inherent frequency difference of the source/absorber combination, −(17.6±0.6)×10−15.

Taking half the difference of the weighted averages yielded the net fractional frequency shift due to gravitational time dilation, −(2.1±0.5)×10−15.

[p 4] Over the full ten days of data collection, they calculated a net fractional frequency shift due to gravitational time dilation of −(2.56±0.25)×10−15, which corresponds to the predicted value with an error margin of 10%.

[p 4] In the next several years, the Pound lab published successive refinements of the gravitational redshift measurement, finally reaching the 1% level in 1964.

[p 9][1] In the years subsequent to the series of measurements performed by the Pound lab, various tests using other technologies established the validity of gravitational redshift/time dilation with increasing precision.

A notable example was the 1976 Gravity Probe A experiment, which used a space-borne hydrogen maser to increase the accuracy of the measurement to about 0.01%.

[p 10] From an engineering standpoint, after the launch of the Global Positioning System (which depends on general relativity for its proper functioning[9]) and its integration into everyday life, gravitational redshift/time dilation is no longer considered a theoretical phenomenon requiring testing, but rather is considered a practical engineering concern in various fields requiring precision measurement, along with special relativity.

It is widely recognized that general relativity, despite accounting for all data gathered to date, cannot represent a final theory of nature.