RECENT EVENTS IN RELATIVITY — ROTHMAN 



393 



-0.8 



-0.4 



■•-0.4 



+ 0.8 



RELATIVE VELOCITY (cm. per second) 



Figure S. — Typical resonance curve obtained by the apparatus of figure 4. 



absorber. (This refers to those photons which are moving toward 

 the absorber.) 



In addition, there is another, much smaller, reduction of frequency 

 which always takes place, no matter what the direction of motion. 

 This is an effect of relativity, and has been of the greatest interest in 

 the recent experiments. 



Wlien the source (or the observer) is moving at right angles to the 

 motion of the radiation, only the small effect of relativity is observed. 

 This reduction of frequency — the "transverse Doppler shift" — arises 

 from the slowing down of the atomic clocks in the moving source. This 

 is the relativistic time dilatation, and the amount of slowing down 

 depends only upon the magnitude of the relative velocity between the 

 source of radiation and the observer. 



The transverse Doppler effect was recently observed in an experi- 

 ment performed at Harwell, England, using the Mossbauer effect as a 

 tool (Hay et al., 1960). The radioactive source was placed at the 

 center of a rotating wheel, while the absorber was at the edge. The 

 scintillation counter was at rest outside the periphery of the wheel. 

 In this arrangement the absorber is always moving at right angles to 

 the photons, and so any shift in frequency is a result of the transverse 

 Doppler effect, and is therefore a manifestation of the time dilatation. 



