252 PHYSICAL SCIENCE 



steel seems to exert no drag on the aether just 

 outside it. 



On the other hand, a classical experiment made 

 in 1887 by the American physicists Michelson and 

 Morley, indicated a contrary result. If the aether 

 is quiescent, as the earth moves through space 

 there is relative motion between it and the aether, 

 and a stream of aether must pass through a 

 laboratory. 



Now a swimmer can pass across a rapid river 

 and back again quicker than he can swim an 

 equal distance up and down stream. Hence a 

 ray of light should take longer to pass along the 

 aether stream and back from a mirror, than if it 

 travelled at right angles to its first path and 

 were reflected back from across the stream. 



When Michelson and Morley carried out this 

 experiment, they found to everyone's surprise that 

 no difference could be detected. 



Of course they could not know beforehand 

 in which direction the aether was moving through 

 their laboratory, but by rotating the apparatus 

 into another position, and trying the experiment 

 at different seasons of the year, they got over 

 this difficulty. But the two rays, at right angles 

 to each other, one going across and one along 

 the stream, always arrived at the observation 

 post together — the race was always a dead-heat, 

 however the apparatus was placed and whatever 

 were the time of year. 



This experiment has been repeated in more 

 recent years to a higher order of accuracy with 

 the same result. It clearly indicates that the 

 measured velocity of light is the same when 

 travelling with and against the motion of the 



