RECENT EVENTS IN RELATIVITY — ROTHMAN 397 



"spaceship" and "earth." The clock, in this case, is the resonant fre- 

 quency of the nucleus in the act of emitting or absorbing the radiation. 



The effect actually observed is this : The nucleus traveling with the 

 highest average velocity (at the highest temperature) has the lowest 

 resonance frequency. In other words, its clock runs at the slowest rate, 

 as seen by the observer in the laboratory, who considers liimself 

 motionless. 



In terms of our original paradox, the man who goes off in the space- 

 ship will always return to find himself younger than the man who 

 stayed at home. His time has been passing at a lower rate. 



Wliat is it that makes the difference between the clock on the ship 

 and the clock on earth? What is it that makes the situation 

 unsymmetrical ? 



It is simply the fact that in order for the spaceship to go away and 

 come back, it must undergo acceleration at least once during the trip. 

 The clock on earth has been moving at a constant velocity in the mean- 

 time. It is this difference which allows us to put a label on the one who 

 is going to emerge with the slower clock at the end of the voyage. 



The experiment of Pound and Rebka has verified that the magnitude 

 of this effect — the amount by which the clock slows down — depends 

 only upon the mean-square velocity of the moving bodies. It does not 

 depend upon the magnitude of the acceleration, or upon the amount of 

 time between accelerations. In this experiment both the source and 

 absorber nuclei are moving, both clocks are slowed down relative to the 

 laboratory observer, and therefore the difference between the two 

 clock rates depends upon the difference in temperature between source 

 and absorber. 



This experiment is not the first time that the time dilatation effect 

 has been observed in the laboratory. The relativistic Doppler shift 

 was measured by H. E. Ives in 1938 by observing the light emitted by 

 rapidly moving hydrogen atoms. However, this new experiment 

 marks the first time that the effect has been observed using the radia- 

 tion from a source which is moving back and forth, thus duplicating 

 the situation of the clock paradox. 



CONCERNING THE SHAPE OF MOVING OBJECTS 



For many years we have agreed that an object moving at a liigh 

 velocity will appear to be shortened in the direction of motion. This 

 idea originated even prior to Einstein. It is, in fact, called the Lor- 

 entz -Fitzgerald contraction in honor of the prerelativistic scientists 

 who conceived it in order to explain away the observation that light 

 always has the same velocity regardless of the motion of the observer. 



As a result of tliis, writers of science fiction have spoken of long, 

 thin spaceships appearing to be short and squat when in motion, while 

 the passing stars are turned into ellipsoids. 



