Robert McNees
Pound and Rebka submitted "The Apparent Weight of Photons" to Physical Review Letters #OTD in 1960.
They described the measurement of minuscule gravitational red shifts and blue shifts as photons moved up or down in the Earth’s gravitational field. Einstein predicted this effect in 1911!
https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.4.337
Robert McNees
Their beautiful 1959 experiment was made possible by Rudolph Mössbauer’s discovery of recoilless nuclear resonance fluorescence the previous year.
The “Mössbauer effect” provides an extremely sensitive mechanism for detecting energy shifts in photons.
Here's a thread about Mössbauer from last month. If you are using @ivory this will hopefully show up as quote post:
Robert McNees
That is precisely what Pound and Rebka needed, as they sent photons up and down a 74 foot tower and tried to measure a tiny fractional decrease and increase, respectively, in their frequency.
Here is Robert Pound seated at the top of the tower, which stretched from the attic to the basement of Harvard's Jefferson laboratory. He is speaking on the phone to Glenn Rebka, stationed at the bottom.
Images: Harvard University News Office
Robert McNees
The Equivalence Principle says that photons with energy E should lose / gain energy as they move up / down in a gravitational field, like an object with mass m = E/c².
For the 14.4 keV gamma rays used by Pound and Rebka, this amounts to ΔE = mgh = Egh/c² = 3.5 x 10⁻¹¹ eV. The difference between ΔE on the way down and on the way up should be about 4.9 parts in 10¹⁵.
Pound and Rebka’s careful experiment yielded (5.13±0.51) parts in 10¹⁵ — a spectacular confirmation of Einstein’s prediction!
Robert McNees
This was the last of Einstein's three "classical tests of general relativity" to be verified. The others were the anomalous precession of the orbit of Mercury, and the deflection of starlight by the gravitational field of the sun.
The paper is open access, so you should have no problem downloading it if you'd like to take a look.
Also, the American Physical Society has a lovely and very accessible discussion of the experiment here:
Chirp! is a social network. It runs on GNU social, version 2.0.1-beta0, available under the GNU Affero General Public License.
All Chirp! content and data are available under the Creative Commons Attribution 3.0 license.