THE STATUS OF TEE ETHER in 



reached by the latter. The object of Lorentz in his papers was to 

 explain the transmission of waves in moving media, beginning with the 

 explanation of astronomical aberration. Singularly enough this was 

 the one phenomenon which was better explained on the emission than 

 on the undulatory theory, and which had proved a stumbling-block 

 for the latter. If the ether is a substance, the question arises whether 

 it is carried along by the earth in its motion, or whether it remains 

 fixed. Lorentz assumed that it remains fixed, and thus satisfactorily 

 explained aberration. But if the earth moved through the ether, the 

 velocity of light between terrestrial points should be affected in the 

 same way that the velocity of sound is affected by the wind. To test 

 this a celebrated experiment was made by Michelson in 1881, repeated 

 by Michelson and Morley in 1887, and several times later, which showed 

 the failure of the earth's motion to influence the velocity of light from 

 a terrestrial source. This classical experiment may prove to be the 

 beginning of the end of the ether. It is evident that if light is propa- 

 gated through the ether in waves which have a velocity peculiar to the 

 ether, and not influenced by the velocity of the source, then light will 

 take longer to reach a point a given distance from it when both are 

 moving in the direction of the line joining them when the second point 

 is ahead than when it is behind, in the ratio of the sum of the velocities 

 of the source and the waves to their difference. The time for the light 

 to go to the forward point and come back is greater than it would be if 

 the system stood still by an amount inversely proportional to 1 — /? 2 where 

 /? is the ratio of the speed of the source to that of light. In the case of 

 the earth this is about one part in one hundred millions, and it was 

 shown by Michelson that no such effect existed. Michelson assumed 

 that this showed that the ether was fixed to the earth. For the contrary 

 explanation, Lorentz adopted an hypothesis already proposed by Fitz- 

 gerald, namely, that all bodies in motion are thereby shortened in the di- 

 rection of their motion, in precisely this ratio. This hypothesis, though 

 startling, has now obtained great weight. In connection with it, 

 Lorentz introduced the idea of local time, which is different for dif- 

 ferent points of the same system moving with a uniform velocity of 

 translation. The modification, by the motion, of both distance and 

 time leads to a most fundamental principle for all our physical notions, 

 called the principle of relativity, which, though brought about by 

 Lorentz, was most clearly expounded by Einstein, who is probably the 

 high priest of the ultra-modern school. The principle of relativity 

 assumes as a postulate that all phenomena are the same if observed 

 with reference to a body moving with constant velocity with respect to 

 the ether as if with respect to a body at rest. If this is so, and no 

 experiments have contradicted it, we have as much right to suppose the 

 ether at rest with respect to one body as another. It seems then unnat- 

 ural to characterize one body as moving relative to a fixed ether. Hence 



