Has the gravitational red shift been proven for electromagnetic waves only or also for a single photon?
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The first I believe was the Pound-Rebka experiment in 1959 in a tower at Harvard.
The best experimental proof we currently have available is the daily successful operation of the Global Positioning System (GPS).
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6$\begingroup$ Here I was writing an answer using the same examples. That makes you a party pooper. Pssssstttthhhhhh.... Note that because they used the Mössbauer effect on gamma rays the Rebka and Pound experiment counted discrete photons and of course the GPS system uses EM wave antennas, so both bases are well covered. $\endgroup$ Commented Feb 23, 2012 at 20:44