Experiment #24 · Scientific experiment

The Pound–Rebka Experiment

Gravitational redshift in a Harvard tower

Robert Pound and Glen Rebka · 1959 · General relativity

First published: R. V. Pound & G. A. Rebka Jr., "Apparent Weight of Photons", *Phys. Rev. Lett.* 4 (1960): 337–341.

A gamma ray climbing 22.5 metres in Earth's gravitational field is redshifted by one part in 10¹⁵ — exactly as Einstein predicted.

Using the Mössbauer effect to produce gamma rays with extraordinarily narrow line widths, Pound and Rebka measured the frequency shift of photons travelling up and down the 22.5-metre Jefferson tower at Harvard. General relativity predicts that photons climbing out of a gravitational well lose energy (redshift), even though they have no rest mass; this is the gravitational redshift, distinct from but related to the equivalence principle Einstein had derived in 1907. The observed shift matched the GR prediction to within ~10%, and a 1965 refinement (Pound-Snider) tightened the agreement to ~1%. The experiment is the first laboratory-scale confirmation of GR.

Formulation

⁵⁷Fe γ-ray source at top of tower, absorber at bottom (and vice versa). Predicted fractional redshift Δν/ν = gh/c² ≈ 2.46 × 10⁻¹⁵. Observed: (5.13 ± 0.51) × 10⁻¹⁵ for the round-trip (close to the 2× expected for source-to-absorber and back via reflection). Subsequent precision: ~1% agreement with GR.

Compare this experiment

Dimensions Engaged

⧖

Time

Directly confirms gravitational time dilation: clocks lower in a gravitational well run slower. Time is not a global parameter but is local and metric-dependent.

✦

Space

Bears on Space · Ontological Status: the redshift is a geometric consequence of curved spacetime, not a mechanical interaction between photons and gravity.

Responses — How Schools Engage

Affirms / takes the bait 5

Relationalism

A clean confirmation of the local, relational character of time intervals: there is no privileged global clock; each location has its own rate determined by its gravitational potential.

Naturalism

A laboratory-scale GR confirmation, achievable by two people in a tower with sufficient ingenuity — the kind of result that solidifies a theory beyond astronomical edge cases.

Eternalism

Local proper time varies across the spacetime manifold; the block-universe accommodates this naturally, while presentism must accept that "now" is a foliation choice, not a fact.

Structuralism

Time intervals are structural — defined by their place in the spacetime metric. The experiment is precision structural physics.

Logical Positivism

A model of operational confirmation: a quantitative prediction made by GR, measured to high precision, rules out the relevant alternative theories.