THE VALUE OF SCIENCE 345 



in measuring anything, we shall always be free to say that this is 

 not the absolute velocity, and if it is not the velocity in relation 

 to the ether, it might always be the velocity in relation to some new 

 unknown fluid with which we might fill space. 



Indeed, experiment has taken upon itself to ruin this interpretation 

 of the principle of relativity; all attempts to measure the velocity of 

 the earth in relation to the ether have led to negative results. This 

 time experimental physics has been more faithful to the principle than 

 mathematical physics ; the theorists, to put in accord their other general 

 views, would not have spared it; but experiment has been stubborn 

 in confirming it. The means have been varied; finally Michelson 

 pushed precision to its last limits; nothing came of it. It is pre- 

 cisely to explain this obstinacy that the mathematicians are forced 

 to-day to employ all their ingenuity. 



Their task was not easy, and if Lorentz has got through it, it is 

 only by accumulating hypotheses. 



The most ingenious idea was that of local time. Imagine two 

 observers who wish to adjust their timepieces by optical signals; they 

 exchange signals, but as they know that the transmission of light 

 is not instantaneous, they are careful to cross them. When station B 

 perceives the signal from station A, its clock should not mark the same 

 hour as that of station A at the moment of sending the signal, but 

 this hour augmented by a constant representing the duration of the 

 transmission. Suppose, for example, that station A sends its signal 

 when its clock marks the hour 0, and that station B perceives it when 

 its clock marks the hour t. The clocks are adjusted if the slowness 

 equal to t represents the duration of the transmission, and to verify 

 it, station B sends in its turn a signal when its clock marks ; then 

 station A should perceive it when its clock marks t. The timepieces 

 are then adjusted. 



And in fact they mark the same hour at the same physical instant, 

 but on the one condition, that the two stations are fixed. Otherwise 

 the duration of the transmission will not be the same in the two senses, 

 since the station A, for example, moves forward to meet the optical 

 perturbation emanating from B, whereas the station B flees before the 

 perturbation emanating from A. The watches adjusted in that way 

 will not mark, therefore, the true time; they will mark what may be 

 called the local time, so that one of them will gain on the other. It 

 matters little, since we have no means of perceiving it. All the phe- 

 nomena which happen at A, for example, will be late, but all will be 

 equally so, and the observer will not perceive it, since his watch is 

 slow; so, as the principle of relativity would have it, he will have no 

 means of knowing whether he is at rest or in absolute motion. 



Unhappily, that does not suffice, and complementary hypotheses 



