Apparent Weight of Photons
R. V. Pound · G. A. Rebka, Jr.
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This is the experiment that weighed light. General relativity says a photon climbing out of a gravitational field arrives with a slightly lower frequency, and near the earth the effect is so small that nobody could measure it until the Mossbauer effect gave physics a gamma ray with an absurdly sharp energy. Pound and Rebka put a cobalt-57 source at one end of the enclosed tower of the Jefferson Physical Laboratory at Harvard and an iron absorber 74 feet away at the other, flushed the path with helium so the gamma rays were not absorbed by air, and vibrated the source so that a known Doppler shift could be dialled in against the gravitational one. Then they ran it both ways up and down, because the difference between rising and falling gamma rays is the part that gravity causes and the average is the part it does not. The measured shift is five parts in a thousand million million, and it agrees with the prediction to within ten per cent.
Por qué importa aquíChapter 4 teaches that the metric tensor sets the rate of a clock as well as the distance between two points, and this is the first laboratory measurement of that rate changing with height. Everything the site says about gravitational time keeping, including the satellite navigation system that has to correct for it continuously, rests on this being a measured fact rather than an inference from astronomy. It is also the cleanest example on the shelf of an experiment designed around its own systematic errors rather than around its signal.
Qué afirma
01The effect of gravitational potential on the apparent frequency of electromagnetic radiation was measured in a laboratory using the sharply defined energy of recoil-free gamma rays emitted and absorbed in solids, the Mossbauer effect, on the 14.4 kiloelectronvolt level of iron-57, whose line was prepared to a fractional full width at half height of 1.13 parts in a million million.Opening paragraph
Settled physics02The vertical baseline was 74 feet, in the enclosed isolated tower of the Jefferson Physical Laboratory at Harvard, giving a two-way height difference of 148 feet; the path was run through a 16-inch diameter Mylar bag flushed with about 30 litres per hour of flowing helium to stop the air absorbing the gamma rays.Paragraphs on the baseline and the helium bag; Figure 1
Settled physics03The measurement is a difference, not an absolute reading. A frequency offset inherent to any particular source and absorber combination exists and varies from one pairing to another, so the experiment compares gamma rays travelling up against gamma rays travelling down and takes the difference; the average of the two directions measures the offset of other origin, which in this apparatus was about four times larger than the gravitational difference being sought.Paragraphs on sources of systematic error; the discussion of Table I
Settled physics04Temperature control is not a detail but a precondition. The frequency of source or absorber varies with temperature through the second-order Doppler effect, and the paper states that a temperature difference of only 0.6 degrees Celsius would produce a shift as large as the one being measured, so the difference has to be known accurately and corrected for before any change can be interpreted.Paragraphs on the temperature-dependent shift and the correction column of Table I
Settled physics05All data collected after the need for temperature correction was recognised yield a net fractional shift of minus 5.13 plus or minus 0.51 parts in a thousand million million, against a predicted gravitational shift of minus 4.92 parts in a thousand million million for this two-way height difference. Expressed as a ratio the result is 1.05 plus or minus 0.10, with the sign indicating that the frequency increases in falling, as expected. The data were collected in about ten days of operation.Closing paragraphs, the net shift and the ratio of experiment to theory
Settled physics06What to watch, as the authors set it out: they expected to continue counting with improved sensitivity and to cut the statistical uncertainty about fourfold, with a comparable reduction in the total error because they believed the systematic errors could be held at that scale, and they name what would be needed to go substantially beyond that, a higher baseline or a narrower gamma ray.Closing paragraph
What to watch
La puerta de entrada
https://doi.org/10.1103/PhysRevLett.4.337SOURCE READ IN FULL, TEXT NOT REPRODUCED. The paper was read on 2026-09-11 from a scan of the original Physical Review Letters pages published on the public physics-teaching pages of the University of Pavia, at fisica.unipv.it/percorsi/pdf/rebka_1960.pdf. That is a way to read the paper, not a licence to republish it: the pages carry the 1960 Physical Review Letters masthead and no open licence of any kind, so no sentence of the paper is carried here. The summary and the claims are written in this site's own words from the paper read in full, and each locator names the paragraph, table or figure the statement comes from. The measured values are quoted as figures because a measurement is a fact rather than expression. The scan is optical character recognition of 1960 typesetting and it garbles some symbols; every number reproduced here was checked against the surrounding sentence, and where a digit could not be read with confidence it is not stated.
Cómo citarlo
R. V. Pound, G. A. Rebka, Jr. (1960) Apparent Weight of Photons. doi:10.1103/PhysRevLett.4.337
Dónde encaja en el currículo
La métrica, los motores de curvatura y los agujeros de gusano