ABSTRACT OF PROCEEDINGS. XX111. 



Summary of lecture, September 16th, 1920, entitled 



"Einstein's Theory of Space and Time." 



By E. M. Wellish. 



According to the undulatory theory, light consists of vibrations 

 in a medium commonly known as the 'aether of space'; this medium 

 transmits electrical and optical disturbances through it with the 

 velocity of 186,000 miles per second. Bradley's experiments on 

 the aberration of light prove that the aether outside matter is at 

 rest. Michelson and Morley attempted in 1887, by a refined 

 optical experiment to measure the aether drift, i.e. the velocity 

 with which the earth moves through the aether. The result of 

 the experiment was entirely negative, the most careful observations 

 failing to bring out this aether drift. 



Lorentz and Fitzgerald accounted for the negative result by 

 assuming that the dimensions of a moving body contract slightly 

 in the direction of motion, but this explanation was not satisfac- 

 tory, as it led to other difficulties of a serious nature. 



In 1905 Einstein, a Swiss, now Research Professor at Berlin, 

 investigated the logical consequences of the hypothesis that it is 

 utterly impossible to bring out experimentally motion with respect 

 to the aether, the only possible motion being that of matter with 

 respect to matter. In his earlier or restricted theory he confines 

 his considerations to the case when the body (usually the earth) is 

 moving uniformly in a straight line. He shows that all observers 

 must agree in assigning the same velocity to a beam of light, even 

 though the observers may be in very rapid motion with respect to 

 one another. The standards of length and time employed by the 

 observers must therefore alter according to their state of motion. 

 Einstein gave the exact relations according to which these stand- 

 ards would alter with the motion. Among other results he showed 

 that two observers in relative motion could record a definite event 

 as occurring at slightly different positions in space and at slightly 

 different times. 



