Recent Events in Relativity^ 



By Milton A. Rothman 



Research Physicist, Plasma Physics Laboratory, 

 Princeton University 



It may happen in some future time that a man will be able to step 

 into a spaceship and travel to another solar system at a speed ap- 

 proaching that of light. If this ever occurs, certain events predicted 

 by the theory of relativity will take place, events decidedly peculiar 

 by present standards. 



Suppose, for example, that our traveler sets out for a star 10 light- 

 years distant, and is quickly able to attain a velocity of 90 percent 

 that of light. It will take him about 11 years to reach his destination, 

 and if he then turns around and comes back at the same speed, 22 years 

 will have elapsed on earth by the time he makes his landing. 



To the voyager, matters appear somewhat different. Once he 

 reaches a constant velocity, he feels no sense of motion. However, he 

 sees the earth receding and the destination approaching at a speed 90 

 percent that of light. Owing to the contraction of length predicted by 

 relativity, he finds that the distance to be traveled is only 4.35 light- 

 years, rather than 10 light-years. Therefore, he finds it takes him 

 only 4.85 years to go, and an equal time to return, or a total round trip 

 time of 9.70 years. 



As a result, a person who has remained on earth finds himself aged 22 

 years, while the person who went on the trip is aged 9.7 years. Time 

 has been going more slowly on the spaceship than on the earth. This 

 is in agreement with rule 4 of table 2, which gives some of the con- 

 clusions drawn from the Special Theory of Relativity. 



On the other hand, we might raise an objection to this conclusion. 

 Wliile a clock in motion appears to be going more slowly than a clock 

 which is at rest with respect to the observer, the direction of motion 

 does not enter into the equation. If motion is completely relative, 

 then as far as the man in the ship is concerned, it is the earth which is 



1 Reprinted by permission from Foote Prints, vol. 33, No. 1, pp. 15-24, copyright 1963 

 by the Foote Mineral Company, Exton, Pa. 



766-746—65—29 385 



