Robert Pound and Glen Rebka

Robert Vivian Pound (1919–2010) was a Canadian-American experimental physicist who joined the Harvard physics department in 1948 after wartime radar work at MIT and a brief stay at Bell Labs. He was a co-discoverer of nuclear magnetic resonance in solids (with Edward Purcell, 1946), and through the 1950s built a reputation as one of the most skilled microwave and radio-frequency experimentalists alive. Glen Anderton Rebka Jr. (1931–2015) arrived at Harvard as a graduate student in 1958, shortly after Rudolf Mössbauer's 1958 discovery of recoilless gamma-ray emission in solid lattices.

The Mössbauer effect made it possible to produce gamma-ray sources whose emission lines were narrow enough to resolve frequency shifts at the parts-per-quadrillion level. Pound and Rebka realized this could be used to test general relativity's prediction that a photon climbing out of a gravitational potential should redshift — that is, lose energy / drop in frequency — by a factor Δν/ν = gh/c² for a tower of height h on Earth's surface. The Jefferson Physical Laboratory tower at Harvard was 22.5 meters tall, predicting Δν/ν = (9.8 m/s²)(22.5 m)/(3 × 10⁸ m/s)² ≈ 2.46 × 10⁻¹⁵ — far below any pre-Mössbauer detection threshold.

Their 1960 paper *Apparent Weight of Photons* (Pound & Rebka, Physical Review Letters 4, 337) reported a measured fractional shift of (2.57 ± 0.26) × 10⁻¹⁵ — the predicted value to better than 10% precision. A 1965 refinement (Pound & Snider) tightened agreement to 1%. The result was the first laboratory confirmation of the equivalence principle's most direct kinematic prediction, derivable from EP alone (no field equations required). The Pound-Rebka experiment remains the canonical undergraduate example of how a precision laboratory instrument can probe the geometry of spacetime. Rebka went on to become professor at the University of Wyoming; Pound continued at Harvard until his retirement in 1989, mentoring two generations of precision experimentalists.