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ANNUAL REPORT SMITHSONIAN INSTITUTION, 1964 



WEIGHING THE PHOTON 



Perhaps the most extensive and completely worked-out test of rela- 

 tivity performed with the Mossbauer effect has been that of R. V. 

 Pound and G. A. Rebka, at Harvard, begun early in 1960, and still in 

 progress. 



The initial purpose of this experiment was to check the gravitational 

 red shift, and in the course of doing this it almost incidentally cast 

 a great deal of light on the problem of the clock f)aradox. 



Pound's experiment measured the change in the resonance between 

 an iron-57 source and an iron absorber (enriched in iron-57) differing 

 in height by a distance of 74 feet. The apparatus was set up in a 

 tower at Harvard University which, fortunately, had been built many 

 years previously for an entirely different purpose. 



When the source is at the bottom, the frequency of the radiation is 

 shifted to the lower side of the resonance point, because the photons 

 must lose energy in rising against gravity, as described previously. 

 This, in effect, measures the "weight" of the photons. When the source 

 is at the top, on the other hand, the absorber "sees" that the frequency 



VIBRATOR 

 SOURCE 



Figure 6. — Experimental arrangement for measuring the gravitational red shift by means 



of the Mossbauer effect. 



