386 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1964 



moving. Therefore the man in the ship should find that the clock 

 on the earth is going more slowly than his clock. 



The traveler feels that he should be the older one at the end of the 

 trip — not the one who stayed home ! 



This famous "clock paradox" has been well-known for many years, 

 and has been discussed thoroughly by a great many writers. The 

 consensus has been that indeed the traveling man would come back to 

 earth younger than the stay-at-home individual. In spite of this, there 

 has been a vocal minority which has maintained that there would be 

 no difference in age between the two people (Dingle, 1956, 1957) . 



Until very recently there was no experimental evidence bearing 

 upon this paradox one way or the other. After all, it has proven 

 difficult enough to get an observer out in space without getting him up 

 to relativistic velocities — that is, velocities great enough to observe 

 these small effects. It requires a speed of 42,000 kilometers per second 

 to produce a 1-percent change in the length, mass, or time rate of a 

 body. With ordinary laboratory or rocket-type velocities, the effects 

 are exceedingly small. 



However, during the past 2 years a new laboratory tool (the Moss- 

 bauer effect) has made possible experiments of such precision that 

 previously they were not considered feasible. As a result of this, 

 interest in experimental proof of the Principle of Relativity is now 

 at a higher level of activity than ever, despite the fact that a great 

 many facets of the theory have already been proven in the 55 years 

 since Einstein first proposed it. 



Since relativity is the foundation of modern physics, any experi- 

 ments which help establish its validity are considered very fimda- 

 mental and important. The newly invented techniques illuminate 

 certain aspects of the theory which have been inaccessible up to now. 



THE PRINCIPLE OF EQUIVALENCE 



The Special Theory of Relativity is based upon the two assumptions 

 listed in table 1. While the Special Theory (published by Einstein 

 in 1905) deals with observers moving at constant velocity, the later 



Table 1. — Basic Assumptions of the Special Theory of 

 Relatimty 



1. The velocity of light in free space is always a constant, 

 regardless of the motion of source or observer. 



2. The laws of nature are always the same to any observer 

 moving with constant velocity, regardless of this velocity. 



The Principle of Equivalence: A body in a gravitational 

 field behaves exactly the same as it would if it were subjected 

 to an equivalent acceleration, without the presence of the 

 gravitational field. 



