RECENT EVENTS IN RELATIVITY — ROTHMAN 



395 



of the descending photons has shifted toward the high side of the 

 resonance frequency. 



The actual percentage shift in frequency measured in this experi- 

 ment is rather small, amounting to about 5 parts in 10^^. If you were 

 performing this experiment with a 100 megacycle radio-frequency 

 oscillator, you would have to detect a change of five cycles out of about 

 4 months' operation of the oscillator to obtain the same sensitivity. 

 The method for measuring this small change by means of the Moss- 

 bauer effect is as follows (fig. 6) : 



The radioactive source is mounted on a vibrator so that it moves 

 rapidly up and down — toward and away from the absorber. The 

 scintillation counter is connected to scalers through electronic switches 

 so that one scaler is counting while the source is moving towards the 

 absorber, and the other scaler is counting while the source is moving 

 away from the absorber. 



The Doppler shift due to this motion changes the frequency of the 

 gamma rays to points above and below the center of the absorption 

 resonance (fig. 7). If both source and absorber were at the same 

 height, the scalers would be counting at points A and B on the curve. 

 However, if the source is higher than the absorber, the photons are 

 shifted to slightly higher frequencies when they reach the absorber. 



RELATIVE VELOCITY OF 

 SOURCE AND ABSORBER 



Figure 7. — Effect of gravitational shift upon resonance curve. 



