MODERN CONCEPTS IN PHYSICS — LANGMUIR 223 



that which resulted from Einstein's relativity theory, first stated in 

 1905. In 1895, as we have seen, electromagnetic waves and matter 

 were thought to be manifestations of the properties of an all 

 pervading ether. 



As an example of the way that the physicists thought of the ether 

 I will quote from the preface to Lord Kelvin's Baltimore Lectures. 

 This preface was written in 1904, but the lectures were those that 

 were delivered at Johns Hopkins University in 1884. 



I diiise ns subject the " Wavo tlieoi'j' of light" with the intention of 

 acxjentuathig Its failures ; rather than of setting forth the admirable success 

 with which this beautiful theory had explained all that was known of light 

 before the time of B>esnel and Thomas Young, and had produced floods of new 

 knowledge splendidly enriching the whole domain of physical science. My 

 audience was to consist of professorial fellow students in ithysical science. 

 • * • I spoke with absolute freedom and had never the slightest fear of 

 undermining their perfect faith in ether and its lighl-^'iviiig waves by any- 

 thing I could tell them of the imperfection of our mathematics, of the insuffi- 

 ciency or faultiness of our views regarding the dynamical qualities of ether, 

 and of the overwhelmingly great difficulty of finding a field of action for ether 

 among the atoms of ponderable matter. We all felt the difficulties were to be 

 faced and not to be evaded ; were to be taken to heart with the hope of solving 

 them if possible. • * • It is in some measure satisfactory to me and I hope 

 it will be satisfactory to all my Baltimore coefficients still alive in our world 

 of science, when this volume reaches their hands to find in It dynamical 

 explanations of every one of the difficulties ^^ith which we were concerned 

 from the first to the last of our 20 lectures of 1884. All of us will, I am sure, 

 feel sympathetically interested in knowing that two of ourselves, Michelson 

 and Morh'y, have by their great experimental work on the motion of ether 

 relatively to the earth, raised the one and only serious objection against our 

 dynamical explanations. 



This Michelson and Morley experiment of 1887, through the 

 theoretical investigations of Lorentz and others, kept growing in 

 importance until it finally stimulated Einstein to evolve his relativity 

 theory. 



According to this theory space and time can not be considered as 

 existing independently of each other. They can not in any sense be 

 regarded as absolute but are both dependent upon the point of view 

 of the observer. For example, Einstein showed that it has no 

 meaning to say that two events which took place at a great distance 

 apart occurred simultaneously. Some observers knowing of both 

 events would have to say that event A occurred before B, while 

 other observers moving at a different velocity from the first observers 

 would conclude that B occurred before A. 



It is not my plan to try to explain the relativity theory to you even 

 if I knew how to do so, but it is rather to discuss the way in which 

 this theory and others of a somewhat similar nature have gradually 

 brought about profound changes in the viewpoint of the physicistg 



