796 



NATURE 



[February 17, 192 1 



electric and magnetic forces. The prominent merit 

 of these equations is that they replace any ap- 

 parent predilection for forces acting at a distance, 

 by explicit recognition of a modified medium (or 

 at least a modified space) in contact with the 

 accelerated particle. 



The beauty and ingenuity of this scheme, and 

 of the reasoning associated with it, are apt to 

 overpower the judgment at times, and to lead to 

 pseudo-philosophic conclusions which are not 

 really justified, and some of which are repugnant 

 to common-sense. It is scarcely wise to seek to 

 interpret physically every link in a chain of mathe- 

 matical reasoning. If the chain is coherent and 

 if the terminal hooks are firm — that is, if the end 

 results are intelligible and verifiable — no more 

 need be expected from any system of equations. 

 The astonishing thing is that statements of such 

 extensive generality can be written in so compact 

 a form, and that so considerable a range of ex- 

 perience can be summarised in terms of a pure 

 hypergeometry, even though that be of a complex 

 character. 



It is undeniable that mathematicians, with a 

 self-denying ordinance about coefficients, can thus 

 attain remarkable criteria, and are able to antici- 

 pate definite results ; but we need not seek to 

 engraft their modes of expression on the real 

 world of physics. We need not consider realities 

 superseded, because a system of pure space and 

 time can be devised which can formulate, and be 

 consistent with, the movements observed by 

 ordinary men and animals. What we observe is 

 not motion and position alone : a wealth of colour, 

 form, and beauty is also within our ken, and will 

 not readily evaporate into a geometrical modifica- 

 tion of empty space. 



The Relativity argument is based on a policy 

 of exclusion. It rejects everything that seems 

 unnecessary ; it dispenses with many of our long- 

 standing conceptions ; and accordingly is hailed 

 as a simplification. The first simplification was 

 the denial of any test for motion through a con- 

 tinuous fundamental medium, and a consequent 

 ignoring of such a medium. The second step 

 was to eliminate gravitational and other forces, 

 with further denial of a power of discrimination 

 between different kinds of acceleration. A third 

 simplification, and further introduction of coeffi- 

 cients, enabled electro-magnetic forces to be simi- 

 larly eluded. And if our conceptions permit of 

 any further simplification, perhaps the additional 

 properties of matter studied bv Chemists and Bio- 

 logists and Artists may be extruded too, and the 

 rich fullness of the universe be impoverished into 

 a mental abstraction. 



To summarise, then :■ — 



In such a system there is no need for Reality ; 

 only Phenomena can be observed or verified : abso- 

 lute fact is inaccessible. We have no criterion 

 for truth ; all appearances are equally valid ; 

 physical explanations are neither forthcoming noi 

 required ; there need be no electrical or any other 

 theory of the constitution of matter. Matter is, 

 indeed, a mentally constructed illusion generated 

 NO. 2677, VOL. 106] 



by local peculiarities of Space. It is unnecessary 

 to contemplate a continuous medium as a universal 

 connector, nor need we try to think of it as suffer- 

 ing modification transmitted from point to point 

 from the neighbourhood of every particle of gravi- 

 tational or electrified matter : a cold abstraction 

 like a space-time-manifold will do all that is 

 wanted, or at least all that the equations compel. 

 And, as a minor detail, which will bring us to the 

 point, it is not necessary to invoke a real Fitz- 

 Gerald contraction in order to explain the result 

 of the Michelson Experiment. 



The Experiment. 



With this prelude, which merely summarises a 

 great deal more that in time I wish to say, let us 

 proceed to the M.M. Experiment and recall atten- 

 tion to what it really does demonstrate rather than 

 to what it is too often imagined to prove. 



Take a horizontal square slab of stone (or wood), 

 float it above an annular trough of mercury so that 

 it can be steadily turned round a vertical axis. 

 Fix mirrors to its surface, so arranged that a beam 

 of light, split into halves, can be sent, one half to 

 and fro lengthways, and the other half to and fro 

 breadthways, each half travelling an equal dis- 

 tance as fixed by the slab. Reunite the two half- 

 beams, observe the interference bands so formed, 

 and see -if they shift in a periodic manner during 

 a leisurely rotation of the stone and observer 

 through all azimuths. 



If light is really a wave and not a projectile 

 motion, and if we are living in a virtual drift of 

 ajther due to our real orbital motion through that 

 medium, and if the stone or other material of the 

 slab preserves and determines unalterably the 

 actual distances travelled by the two half-beams 

 of light in the interval between their split and 

 their reunion — then the bands should undergo a 

 sinuous shift, with amplitude representing a lag 

 of the order lo-^, as the stone revolves. 



But, on the electrical theory of matter, cohesion 

 is a residual chemical affinity or electrostatic at- 

 traction ; and such attraction is known to be modi- 

 fied electrokinetically if the charged atoms are 

 in rapid motion through the dielectric medium ; 

 hence it becomes not only possible but likely that 

 the dimensions of the slab will change if we are 

 moving through the aether; the square being 

 subject to a slight distortion. .And the change 

 metrically to be expected, in accordance with elec- 

 trical theory, is found on examination to be also 

 of the order io~*, and in fact was ultimately 

 shown by Larmor to be of a precisely compen- 

 sating character, ("^ther and Matter," 1900, 



P- 175-) 



The fact is that the lines of force on an isolated 

 charged sphere redistribute themselves, if it be 

 moving through the aether, as if it had become a 

 stationary oblate spheroid with axis in the direc- 

 tion of motion. Such a spheroid becomes the 

 static representation of a moving sphere, and may 

 be held to take its place as the most symmetrical 

 figure ; for the only way in which we can infer the 



