March ;, 1918] 



NATURE 



15 



-s in the use of language. For example, the " boil- 

 ^ point of water " as a thermometric fixed point, like 

 . "absolute" scale of temperature, is a loose ex- 

 . ssion, only understood by those who know; and 

 unless some looseness be permitted the measurement 

 of the "specific heat of copper" would have to dis- 

 appear from the elementary course. With the two 

 exceptions mentioned, the adoption of "absolute " units 

 for atmospheric measurements, which was not " made 

 ill Germany," has been received • with profound in- 

 'forence in scientific circles. But the whole question 

 units and their nomenclature is of great importance 

 us at this juncture. Our practice of using one set 

 units in tho laboratory and ^mother set in practical 

 can only be described as stupid. Although the par- 

 ular point raised is not a Crucial one, it is much to 

 desired that Prof. Marvin's note may be the bcgin- 

 li; of the serious consideration of this important 

 iject by the exponents of the physical sciences. 



\AV1TATI01\ AND THE PRINCIPLE OF 

 RELATIVITY.^ 



[HERE were many ditTicuIties to encounter in 



entering the room just now. To begin with, we 



to bear the crushing load of the atmosphere, 



)unting to 14 lb. on every square inch. At each 



forwards it was necessary to tread gingerly on a 



of ground moving at the rate of twenty miles a 



jn^^l on its way round the sun. We were poised 



Scariously on a globe, apparently hanging by our 



head outwards into space. And this acrobatic 



was performed in the face of a tremendous wind 



ither, blowing at I do not know how many miles 



:ond literally through us. We do not claim much 



it for overcoming these ditlficulties — because we 



;r noticed them. But I venture to remind you of 



1, because I am xibout to speak of some other extra- 



inary things that may be happening to us of which 



i^are quite unconscious. 



tot to go too far back in history, the present sub- 

 arises from a famous experiment performed in 

 ;year 1887, known as the Michelson-Morley experi- 

 it. The apparatus was elaborate, but the principle 

 le experiment is not very difficult. If you are in a 

 , which will be the quicker — to swim to a point 

 yards up stream and back again, or to a point 

 yards across stream and back again? Mathe- 

 ticallv the answer is, perhaps, not immediately 



* >us, because the net effect of the current is a delay 

 )th cases. But I think that anyone who has swum 



'a river will have no hesitation about the answer. 



"he up-and-down journey takes longer. Now we are 



in a river — of aether. There is a swift current of aether 



Aving through this room; or, if we happen to be at 



I in the aether at the present moment, six months 



ice the earth's orbital motion will be reversed, and 



n there must be a swift civrrent. Michelson divided 



• am of light into two parts; he sent one half swim- 

 !ig up the stream of aether for a certain distance, 

 I tJK'n bv a mirror back lo the starting point; he 

 .i the other half an equal distance (as he thought) 

 i >ss the stream and back. It was a race; and with 

 - apparatus he could test very accurately which part 



back first. To his surprise, it was a dead-heat. 



.irly the two paths could not really have been equal, 



along-stream path must have been a little shorter 



■ ompensate for the greater hindrance of the current. 



it objection was foreseen, and the apparatus, which 



> mounted on a stone pier floating in mercury, was 

 ited tlfrough a right angle, so that the arm which 



> formerly along the stream was now across the 



' Discourse delivered at th- Royal Institution on Fritlav. February i, by 

 Vof. A. S. Eddington, F.R.S. 



NO. 2523, VOL. ioi] 



stream, and vice versa. Again the two portions of the- 

 beam arrived at the same moment; so this time the 

 other arm had become the shorter— simplv bv altering 

 its position. In fact, these supposedlv rigid arms had 

 contracted when placed in the up-a'nd-down stream 

 position by just the amount necessarv to conceal the 

 effect which was looked for. 



That is the plain meaning of the experiment; but 

 we might well hesitate to accept this straightforward 

 interpretation, and try to evade it in some wav, were 

 it not for some theoretical discoveries made later, li 

 has gradually appeared that matter is of an electrical 

 nature, and the forces of cohesion between the par- 

 ticles, which give a solid its rigiditv, are electrical 

 forces, l^irmor and Lorentz discovered that this pro- 

 perty of contraction in the direction of the aether cur- 

 rent was something actually inherent in the formula- 

 for electrical forces written down by Maxwell man\ 

 years earlier and universally adopted; it onlv waited 

 for some mathematician to recognise it. It would be 

 going too far to say that Maxwell's equations actualh 

 prove that contraction must tal^e place; but thev are, 

 as it were, designed to fall in line with the contraction 

 phenomenon, and certain details left vague by Max- 

 well have since been found to correspond! 



We are then faced with the result that a material 

 body experiences a contraction in the direction of its 

 motion through the aether. According both to theorv 

 and experiment the contraction is the same for ail 

 kinds of matter — a universal propertv. One reserva- 

 tion should be made; the experiment has onlv been 

 tried with solids of laboratory dimensions, which are 

 held together by cohesion. There is at present no 

 experimental evidence that a body such as the earth 

 the form of which is determined' by ^gravitation will 

 suffer the same contraction ; we shallj however, assume 

 that the contraction takes place in this case also. 



I am going to ask you to suppose that we in thi> 

 room are travelling through the aether at the rate of 

 161,000 miles a second, vertically upwards. Let us 

 be bolder and say that that is our velocitv through 

 the aether — because no one will be able to contradict 

 us. No experiment yet tried can detect or disprove 

 that motion ; because all such experiments give a null 

 result, as the Michelson-Morley experiment did. With 

 that speed the contraction is just one-half. This 

 pointer, which I hold horizontally, is 8 ft. long. Now 

 [turning it verticallv] it is 4 ft.' long. But, you may 

 say, it is taller than I am, and I must be approaching 

 6 ft. No, if I lay down on the floor I should be, 

 but as I am standing now I am under 3 ft. The con- 

 traction affects me just as it did the pointer. It is no 

 use bringing a standard yard-measure to measure me, 

 because that also will contract and represent only half 

 a yard. "But we saw that the pointer did not changf 

 length when it turned." How did you tell that? Whai 

 you perceived was an image of the pointer on the 

 retina of your eye, and you thought the image occu- 

 I pied the same space of retina in both positions; but 

 your retina has also contracted in the vertical direction 

 I without your knowing it, so that your estimates of 

 I length in that direction are double what they should 

 j be. And similarly with every test you could apply. If 

 everything undergoes the same change, it is jiist as 

 though there were no change at all. 



We thus get a glimpse of what, from our present 

 point of view, must be called the reai world, strangely 

 different from the world of appearance. In the real 

 world, by changing position you extend yourself like 

 a telescope; and the stoutest individual may regain 

 slimness of figure hv an appropriate oritmtation. It 

 must be something like what we see in a distorting 

 mirror; and you can almost see a living-picture of 

 this real world reflected in a oolished door-knob. 



