THE STATUS OF THE ETHER Drs 
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—? where 
B 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 
