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ANNUAL REPORT SMITHSONIAN INSTITUTION, 1964 



frequency of this radiation is decreased. The wavelength is shifted 

 toward the red. 



Astronomers have sought to observe this in the light coming from 

 very heavy stars. If we look at this light by means of a spectroscope, 

 and compare the wavelength of a particular spectral line with the same 

 line from a terrestrial source, the line from the star should be shifted 

 toward the red. This is a very small shift, and is suxDerimposed upon 

 the normal Doppler shift due to the motion of the star away from the 

 earth, so that the measurement is very difficult to make. 



The same sort of shift should be observed if we use a source of light 

 at the surface of the earth, while the observer or detector is stationed 

 at some height above the surface of the earth (fig. 1). In this case, 

 the amount of shift would be very small indeed. 



There are a number of ways of "explaining" how this comes about, 

 all of which are ultimately equivalent in terms of the theory of rela- 

 tivity. One way of understanding the reason for the gravitational 

 red shift is to consider that the light emitted from the source is in 



J 



DETECTOR 



SOURCE OF 

 MONOCHROMATIC LIGHT 



EARTH 



Figure 1. — Light traveling up from the surface of the earth has a longer wavelength than 

 light from the same source traveling parallel to the earth's surface. 



