392 



SCIENCE 



[N. S. Vol. XXXVIII. No. 977 



relatively to us, light can be affected inside 

 that matter; but when matter is relatively 

 stationary to matter nothing observable 

 takes place, however fast things may be 

 moving, so long as they move together. 



Hence arises the idea that motion with 

 respect to ether is meaningless: and the 

 fact that only relative motion of pieces of 

 matter with respect to each other has so 

 far been observed is the foundation of the 

 principle of relativity. It sounds simple 

 enough as thus stated, but in its develop- 

 ments it is an ingenious and complicated 

 doctrine embodying surprising consequen- 

 ces which have been worked out by Pro- 

 fessor Einstein and his disciples with con- 

 summate ingenuity. 



What have I to urge against it? Well, 

 in the first place, it is only in accordance 

 with common sense that no effect of the 

 first order can be observed without rela- 

 tive motion of matter. An ether-stream 

 through our laboratories is optically and 

 electrically undetectable, at least as re- 

 gards first-order observation ; this is clearly 

 explained for general readers in my book, 

 "The Ether of Space," chapter IV. But 

 the principle of relativity says more than 

 that, it says that no effect of any order of 

 magnitude can ever be observed without 

 the relative motion of matter. 



The truth underlying this doctrine is 

 that absolute motion without reference to 

 anything is unmeaning. But the narrow- 

 ing down of "anything" to mean any 

 piece of matter is illegitimate. The near- 

 est approach to absolute motion that we 

 can physically imagine is motion through 

 or with respect to the ether of space. It is 

 natural to assume that the ether is on the 

 whole stationary and to use it as a stand- 

 ard of rest; in that sense motion with ref- 

 erence to it may be called absolute, but in 

 no other sense. 



The principle of relativity claims that 



we can never ascertain such motion: in 

 other words, it practically or pragmatically 

 denies the existence of the ether. Every 

 one of our scientifically observed motions, 

 it says, are of the same nature as our pop- 

 ularly observed ones, viz., motion of pieces 

 of matter relatively to each other ; and that 

 is all that we can ever know. Everything 

 goes on — says the principle of relativity — 

 as if the ether did not exist. 



Now the facts are that no motion with 

 reference to the ether alone has ever yet 

 been observed: there are always curious 

 compensating effects which just cancel out 

 the movement-terms and destroy or effect- 

 ively mask any phenomenon that might 

 otherwise be expected. When matter 

 moves past matter observation can be 

 made; but, even so, no consequent locomo- 

 tion of ether, outside the actually moving 

 particles, can be detected. 



(It is sometimes urged that rotation is a 

 kind of absolute motion that can be de- 

 tected, even in isolation. It can so be de- 

 tected, as Newton pointed out ; but in cases 

 of rotation matter on one side the axis is 

 moving in the opposite direction to matter 

 on the other side of the axis; hence rota- 

 tion involves relative material motion, and 

 therefore can be observed.) 



To detect motion through ether we must 

 use an etherial process. We may use radi- 

 ation, and try to compare the speeds of 

 light along or across the motion; or we 

 might try to measure the speed, first with 

 the motion and then against it. But how 

 are we to make the comparison? If the 

 time of emission from a distant source is 

 given by a distant clock, that clock must 

 be observed through a telescope, that is, by 

 a beam of light; which is plainly a com- 

 pensating process. Or the light from a 

 neighbox'ing source can be sent back to us 

 by a distant mirror ; when again there will 

 be compensation. Or the starting of light 



