PoundRebka

Pound–Rebka, often cited as the Pound–Rebka experiment (or Pound–Rebka effect), is a landmark laboratory test of gravitational redshift predicted by general relativity. Conducted in 1959–1960 by Robert Pound and Glen A. Rebka Jr., it used the Mössbauer effect in iron-57 to measure the tiny shift in gamma-ray frequency as photons moved through Earth's gravitational field.

In the experiment, a 14.4 keV gamma ray emitted from an iron-57 source at the top of a tall tower was aimed toward a solid iron-57 absorber at the bottom (approximately 22.5 meters below). Because light loses energy climbing in a gravitational field, the photons experience a redshift. To detect this very small effect, the researchers relied on the recoil-free Mössbauer transition, which provides an extremely narrow spectral line. They adjusted the relative motion of the source and absorber to introduce a Doppler shift that could compensate for the gravitational redshift, allowing the gamma rays to be resonantly absorbed.

The results showed agreement with the gravitational redshift predicted by Einstein’s equivalence principle, within about one percent accuracy. The experiment thus provided a direct, laboratory-scale confirmation of a relativistic effect that governs time dilation in a gravitational field.

Pound–Rebka is regarded as a foundational test of general relativity and has influenced subsequent precision tests of gravitational redshift and time dilation. It also highlighted the utility of the Mössbauer effect for high-resolution tests of fundamental physics. Further refinements and alternative implementations later improved the precision of gravitational redshift measurements, building on the legacy of the original Pound–Rebka work.