390 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1964 



distance apart — they are at rest with respect to each other. A photon 

 emitted by the source at A travels toward the observer at B. However, 

 by the time this photon reaches the observer, the latter has reached 

 the point D. In addition, the observer is now traveling faster than 

 he was at point B, because he has been under constant acceleration. 



In other words, although the source and observer always remain the 

 same distance apart, the acceleration produces an effect as if the 

 observer were always nmning away from the source. If we calculate 

 the additional velocity acquired by the observer during the time it 

 takes the photon to reach it, we can then calculate the Doppler shift 

 resulting from this velocity. This turns out to be exactly the same as 

 the shift which is calculated on the principle that the photon is rising 

 against the acceleration of gravity. 



The statement is sometimes made that the gravitational red sliift is 

 observed when the source of light is in a stronger gravitational field 

 than is the observer. However, this is incorrect, since the calculation 

 outlined above shows that the shift can be observed even when source 

 and observer are in a uniform field. The only tiling that matters is 

 that the observer must be higher than the source. In other words, it 

 is the difference of potential that enters into the calculation. If the 

 observer is lower than the source, the result will be a blue shift. 



Measurements of the gravitational frequency shift within the con- 

 fines of a laboratory were formerly unheard of, because there was no 

 way of measuring the tiny amount of shift produced by the earth's 

 gravitational field. With the advent of satellites, proposals were made 

 to send up very precise radio-frequency oscillators, whose signals 

 would be compared with those of an identical oscillator down on the 

 ground. Since the amount of shift depends on the difference in alti- 

 tude between transmitter and receiver, it was calculated that a meas- 

 urable effect would be obtained. 



However, before this could be done, a development in a totally 

 unexpected direction made these satellite experiments obsolete before 

 they were even undertaken. This new development came from the 

 field of nuclear physics, and it came from a rather obscure corner of 

 a specialty known as low-energy nuclear spectroscopy. 



THE MdSSBAUER EFFECT 



The story illustrates quite beautifully the strongest argument in 

 favor of basic research : You never know when a piece of research will 

 turn out to have important consequences in an unpredictable appli- 

 cation. 



For several years a number of nuclear physicists have been using the 

 phenomenon of nuclear resonance fluorescence to measure properties 

 of the excited states of various nuclei. This is based upon the idea 



