newton's views on aether and gravitation - l. rosenfeld

7/29/2019 Newton's Views on Aether and Gravitation - L. Rosenfeld

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N ewton ' s V i ews on A eth er and Gr avi tat ionL. ROSENFELD

I n t h e Principia [1] NE W T ON p r e s e n t e d t h e t h e o r y o f g r a v i t a t i o n f r o m t h ep o i n t o f v i e w wh i c h we n o wa d a y s c a l l "phenomenological":h e b a s e d t h e wh o l ed e s c r i p t i o n o f p l a n e t a r y m o t i o n s o n a n a s s u m e d l a w o f d y n a m i c a l i n t e r a c t i o n ,a n d d e l ib e r a te l y r en o u n c e d a n y a t t e m p t a t a c c o u n t i n g f o r t h is p a r t i c u l a r f o r mo f i n t e r ac t i o n b e t w e e n d i s t a n t b o d ie s b y s o m e m e c h a n i s m o f p r o p a g a t i o n t h r o u g ha n i n t e rv e n i n g m e d i u m . T h i s a p p r o a c h w a s s u c h a n i n n o v a t i o n t h a t i t w a s t h o r -o u g h l y m i s u n d e r s to o d e v e n b y t h e g r e a t e s t o f t h e c o n t e m p o r a r y p h y s ic i st s ;HUYGE NS i n p a r t i c u l a r o b j e c t e d t o i t w i t h o u t a p p a r e n t l y r e a l i zi n g t h a t h e wa sh i m s e l f c u r r e n t l y u s i n g t h e s a m e l o g i ca l p ro c e d u r e i n h is m e c h a n i c a l i n v e s t i g a ti o n s ,wh e n h e j u s t i f i e d h i s p o s t u l a t e s b y p o i n t i n g t o t h e i r e m p i r i c a l o r i g i n .

NEWTON was a t g r ea t pa ins to d i spel th e mi s tak en susp ic ion tha t he r egardedg r a v i t a t i o n a s a n i n t ri n s ic q u a l i t y o f m a t t e r . H i s f i rs t o p p o r t u n i t y t o d o so wa sh is cor r espondence wi th BENTLEY ~2], bu t in the f ina l s chol ium of the secondedi t ion of the Principia [3] h e is m o r e e x p l ic i t a b o u t t h e n o v e l t y o f h i s m e t h o d - -a n d m o r e o u t s p o k e n a b o u t h i s f a il u re t o fi n d in t h e k n o w n p r o p e r t i e s o f g r a v i t a t i o nany sure c lue to i t s "cause". He m a k e s q u i t e c l e a r t h e d e e p - l y i n g r e a s o n f o r h i sd i s s a t i sf a c t i o n a b o u t m e c h a n i c a l e x p l a n a t i o n s o f g r a v i t a t i o n : t o b e a c c e p t a b l ea c c o r d i n g t o t h e e x a c t i n g c r i te r i a o f h is e x p e r i m e n t a l p h i l o s o p h y , s u c h a n e x -p l a n a t i o n o u g h t t o b e r e l a t e d i n a unique w a y t o t h e p h e n o m e n a - - o t h e rw i s ei t is an "hypothesis" o f t h e k i n d h e d o e s n o t wa n t t o "feign". T h e l a st p a r a g r a p hof the schol ium, then , in which he a l ludes to h i s concept ion of a un ive r sa l ae the rwh o s e m u l t i p l e f u n c ti o n s a s t h e a g e n c y o f t h e m o s t v a r i e d p h e n o m e n a h e c a r e f u l l ye n u m e r a t e s , c a n o n l y b e i n t e n d e d t o i n t i m a t e t h a t h e r e g a r d s th e m e c h a n i c a lc a u s e s o f t h e s e p h e n o m e n a a s m o r e s e c u re l y fo u n d e d , o r a t l e a s t l es s h y p o t h e t i c a l ,t h a n t h a t o f g r a v i ta t i o n .

I n d e e d , t h e p r o b l e m o f t h e c a u s e of g r a v i t y wa s o f q u i t e a d i f f e r en t o r d e r o fd i f f i cu l ty f ro m the o th e r p rocesses he cons ide red : in the l a t t e r , such as the ac t iono f g r o ss m a t t e r u p o n t h e p r o p e r t i e s o f l ig h t, o n e is co n c e r n e d w i t h i m m e d i a t e ,s h o r t - r a n g e , i n t e r a c t i o n s wh i c h e a s il y f i t i n t o t h e g e n e r a l l y a c c e p t e d c a r t e s i a nf r a m e w o r k o f m e c h a n i c a l e x p l a n a t i o n ; t h e p h e n o m e n a o f g r a v i t at i o n , b y c o n t r a s t,h a d b e e n e x t e n d e d b y NEW TON h i m s e lf t o t h e g r a n d s c al e of t h e u n i v e r s e : t h e r ewa s h e r e n o t o n l y a p e c u l i a r i n t e r a c t i o n b e t we e n a e t h e r a n d m a t t e r a t p l a y ,b u t a d y n a m i c a l p r o c es s w i t h i n t h e a e t h e r i ts e lf h a d t o a c c o u n t f o r t h e t r a n s -miss io n of a fo r ce of un l im i ted r ange . T his s ingula r pos i t ion of the g r a v i ta t ionpro blem i s s t r ik ing ly r e f lec ted in the ev idence we hav e of NEWTON'S ima gin a t iv ea t t e m p t s a t a n a l l - e m b r a c i n g d e s cr i p ti o n o f t h e a e t h e r ' s wo r k i n g s : wh i le i n t h e s es u c c e s s iv e f o r m u l a t i o n s h e h a r d l y v a r i e d i n t h e c o n c e p t i o n o f t h e s h o r t - r a n g e: in te r ac t ions be tween ae the r and mat te r r e spons ib le fo r a l l o the r e f f ec t s , them e c h a n i s m s h e p r o p o s e d f o r e x p l a i n i n g g r a v i t a t i o n e x h i b i t c o n s i d e r a b l e a n ds ign i f ican t d i f f e r ences .


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3 0 L . R O S E N F E L D :

In the extensive memorandum sent to the Royal Society [4] in 1675, heoutlined a remarkable mechani sm of kinetic character, according to which materialbodies set up around them a cyclical circulation of aether which carries towardstheir surface the smaller material bodies it "p er va de s" . In a letter to BOYLE [5 ]of 1679, however, purporting to acquaint the latter with the content of thememorandum, he surprisingly ends up with an entirely new "conjecture" re-specting gravitation, without even alluding to his previous theory. As if carriedaway by his eagerness to unfold the whole vision of the world that is disclosingitself to his mind, he tells his friend that the new idea "came into my mind nowas I was writing this letter". Although such spontaneity is not a usual moodwith NEWTON, it is in har mony with the tone of confident friendship whichmarks his relations with BOYLE,and there is no reason to doubt his word; whatthis ex tra ordinary circumstance reveals seems obvious: NEWTON was SO littleconvinced of the adeq uacy of his circulatory mechanism that when he was aboutto put it down in writing, he started thinking again about the problem andreadily yielded to the alternative possibility that offered itself to his powerfulimagination. In fact, when he later returns to the circulation theory in his cor-respondence with HALLEY [6], he calls it in so man y words "o ne of my guesseswhich I did not rely on " [7]. Nevertheless, this ver y correspondence shows tha the not only did not definitely discard the circulation theory, but on the contrarykept it in mind and even persisted in attaching great importance to it. The newtheory s ummarily formula ted in the letter to BOYLE, while still in the cartesianspirit, differed essentially from the previous one by being static rather thankinetic: gravitation was still conceived as the result of a pressure exerted bythe aether on material bodies and tending to push them towards each other,but this pressure was now thought to result from the inhomogeneity of the aether,assumed to consist of two kinds of particles, finer and coarser ones, interactingdifferently with matter. It thus looks as if NEWTON was contemplatin g as alter-native possibilities the opposite extremes of stationary circulation and staticdensity distribution of aether particles to account for the maintenance of theforce of gravity.

If we now turn to his last pronouncement on the subject, which he only putforward in the form of additional queries at the end of the second English editionof the Opticks [8], as late as t 717*, we see him inclining to the static alternative,which he develops in an apparently simpler form than in the letter to BOYLE:he now operates with an aether of homogeneous composition, but whose densityis assumed to be influenced by the presence of material bodies in such a waythat it is lowest inside and around these bodies and increases continually atlarger distances. This superficial resemblance should not mislead us to any facileconclusion, for the aether of NEWTON'S later years, such as he describes it inthe Queries, is radically different from the medium bearing the same name, towhich, in his youthful speculations, he assigned such a prominent role in theeco nomy of the universe. Originally, NEWTON took for granted the existence of

* Essentially similar ideas occur already in manuscript drafts which can be datedto the period of writing of the Pvincipict(t687), and recur in drafts of the finalscholiumof the second edition (1713) [t0]. A general discussion of NEWTON'S thoughts on theconstitution of matter has been given by A. R. & M. B. HALL [10].


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Newton on Aether and Gravitation 31a medium filling all space, like DESCARTES' "m a t i ~ r e subtile", not for the meta-physical reasons adduced by the French philosopher, but simply as part of thegeneral mechanical view of tile universe, which was then accepted without ques-tion. The constitution of the aether discussed in the Queries, on the other hand,is that of an elastic medium of such extremely low density that its single con-stituent particles must be assumed to be separated by considerable spatial ex-tensions void of every kind of matter: the origin of this aether's elasticity thenraised a problem whose solution lay bey ond NEWTON'S concept ual horizon.

In order to understand why the development of the aether problem took thisdram ati c turn, let us now look more closely at its successive phases. Th e mem oran -dum of 1675 gives us a curious picture of the youn g NEWTON'S fiery imaginatio n,taking at times a poetical turn, as when he evokes his vision of Nature as a great" c i r c u l a t o r y wo rk er " [41 ; but we also observe how he holds his fanta sy in checkby his other l e i t -mo t i v , the "analogy" of Nature: in his boldest speculations healways seeks guidance in comparisons with known phenomena, arguing thatNature proceeds in all her operations according to the same few principles. Themechanism of gravitation he proposes in the memorandum was at first meantto account for the force of gravity experienced on the surface of the earth; asan afterthought E71, he pointed out that it could also be applied to the attract ionof the planets by the sun: at that time he was not at all persuaded that thisatt rac tio n was the only force regulating p lane tary motions [1 t l, and the problemof gravitation as a universal agency had not yet presented itself to his mind.

He imagines that there is in the aether a component of a " s t i c k y " nature,which is continually streaming towards the surface of the earth, where it is partlyabsorbed*, partly re-emitted into the surrounding space, and is thus involvedin a steady circulation. The downward aether stream exerts upon the materialbodies it traverses a force which NEWTON wishes to ident ify with gravi ty. Tothis end, he must specify the wa y in which the aether particles transfer mom ent umto those of gross matter: he assumes [t21 that the material bodies are actedupon by the aether in proportion to the "superficies of their parts", i .e. by thepressure of the aether stream, so that the radial force upon a unit volume ofmatter is given by the corresponding pressure gradient in the aether stream.The latter quantity is equal to the product of the local aether density and theamount of momentum which an aether particle transfers on the average per unittime to t he gross matter . NEWTON takes this averag e force to be pr oport ionalto the radial velocity of the aether particle; in this way - - as he explains toHALLEY It2~ -- since the aether densi ty is inversely proportional to the samevelocity, the pr oduct is independent of the variation th at this velocity undergoes

* The summary account of NEWTON'S circulatory mechanism given in my paperon NEWTON'S theory of gravitat ion [11 ] misses the essential part played by thisabsorption, and is accordingly quite inadequate. This was noticed by Dr. E. J. AITONin the course of all investigation of the controversy between HOOKE and NEWTONabout the priority of the discovery of the inverse square law. I am most thankfulto Dr. AITONfor calling my at tention to this point and for sending me the manuscriptof his forthcoming paper, which contains in particular the correct interpretation ofNEWTON'S argument [1 3J.


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3 2 L . ROSENFELD:

as a result of the interaction of the aether stream with the material bodies. Infact, on NEWTON'S assumption, ever y material b ody at the same distance fromthe centre of the earth experiences the same centripetal acceleration, proportionalto the aether flux, and according ly var ying inversely as the square of the distance.The inverse square law is thus only established, however, in the space outsidethe earth, in which the total aether flux around the earth is not sensibly affectedby the bodies it traverses; but it is not expected to hold inside the earth [7],since in this region the " s t i c k y " aether particles will be gradually absorbed bythe matter and the total aether flux will decrease appreciably, in some unspecifiedway*.

The interest of this hypothesis lies in NEWTON'S endeav our to give it quanti-tative precision, aiming at an understanding of the inverse square law -- albeitthis result could only be achieved at the cost of much arbitrariness. This wasat any ra te NEWTON'S own motiva tion for exhuming it from the register of theRoyal Society ill t686, when he felt compelled to evidence his early knowledgeof the law in question aga inst HOOKE'S priori ty contention. By t ha t time, how-ever, his view of the constitution of the aether had already undergone a radicaltransformation. The exact fulfilment of KEPLER'S laws throughout the wholesolar system, while vindicating the universality of the simple inverse square lawof gravita tion, had the reby forced NEWTON to the unwelcome conclusion thatsince the aether did not oppose any appreciable resistance to the passage of thecelestial bodies, it must be a medium of such extremely low density that itsrole as the agent of gravitation was in jeopardy: thus arose, paradoxically, fromthe very triu mph of the universal law of gravitation, the acute dilemma regardingthe origin of this force, which weighed so heav ily on NEWTON'S mind dur ingthe rest of his life [11 ].

It was at this critical juncture, in the spring of 1687, that a gifted and enter-prising young man, FATIO DE DUILLIER, arrived in Lon don Et4]. He came fromHolland, where he had been ill relation with HUYGENS,and was well acquaintedwith the mechanical explanation of gravity that the latter had developed a longtime before [t 5], but had left unpublished. FATIO mus t have discussed it soonafter his arrival at the Royal Society (whose meetings he attended), for onJune t4 , t687 he reports to HUYGE~IS [16] tha t some of the members told h imhe was too mu ch of a Cartesian, and t ha t the ideas set fo rth in NEWTON'S forth-coming book had changed the whole of physics. Still, the following year, asappears from entries in the journal of the Society [t7] for June 27 and Ju ly 4,1688, FATIO was asked, on his accession to member ship [14], to give a fulleracco unt of I-IuYGENS' theory before the Society. This th eor y was indeed ascartesian in spirit as NEWTON'S circulat ory hypothesis, inasmuch as it tre atedthe aether as a continuous fluid; in fact, it was a modernization of the cartesian

* A total number S of aether particles per unit time steadily streaming towardsthe centre of the earth produce at the distance R from the centre an aether fluxF = S / 4 ~ R 2, and a local aether density F/v , if v is the radial velocity at this distance.With the assumption that the average force exerted by an aether particle on thegross matter is proportional to v, the total force on a unit volume of matter will beindependent of v and proportional to the aether flux F. This again is inversely propor-tional to R 2 so long as the total aether flux S is sensibly constant.


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N e w t o n o n A e t h e r a n d G r a v i t a t i o n 33v o r t e x i d e a. HUYGENS, l i ke NEWTON, h a d r e a l i z e d t h e i n a d e q u a c y o f D E SC A RT ES 'p l a n e v o r t ic e s , a n d t r i e d t o r e p l a ce t h e m b y s p h e r i c a l o ne s. T h u s , h e i n t e r p r e t e dt h e f o rc e o f g r a v i t y a s t h e c e n t r i p e t a l f o rc e w h i c h a m a t e r i a l b o d y i m m e r s e d i ns u c h a n a e t h e r v o r t e x a r o u n d t h e e a r t h w o u l d e x p e r i e n c e ; t h e q u a n t i t a t i v ei d e n t i f i c a t i o n r e q u i r e s a n a n g u l a r v e l o c i t y o f t h e a e t h e r p a r t i c l e s a b o u t t 7 t i m e st h a t o f t h e e a r t h ' s r o t a t i o n , c o r r e s p o n d i n g t o a p e r i o d o f t h 2 5 r a i n ( a s s t a t e di n t h e r e p o r t [ t 7 ] o f FA T IO 'S a c c o u n t ) .

C l e a rl y , H U Y GE N S' p r o p o s a l - - w h i c h h e e v e n t u a l l y p u b l i s h e d i n t 6 9 0 , w i t hc r i t i c a l c o m m e n t s o n t h e Principia [ t 8 t - - w a s a t h i n g o f t h e p a s t , w h i c h c o u l dn o t i m p r e s s N E W T O N'S f o ll o w e rs . F A TIO s e e m s t o h a v e r a p i d l y a s s i m i l a t e d t h eN e w t o n i a n p r in c ip le s a n d n o t h i n g w a s m o r e n a t u r a l f o r h i m t h a n t o a t t e m p t an e w m e c h a n i c a l e x p l a n a t i o n o f g r a v i t a t i o n , i n h a r m o n y w i t h t h e i n f e re n c e s a b o u tt h e a e t h e r ' s p r o p e r t i e s t h a t N EW T ON h a d d r a w n f r o m h i s a n a ly s i s o f t h e p l a n e t a r ym o t i o n s . H i s s t a r t i n g p o i n t is a k i n e t i c c o n c e p t i o n o f t h e a e t h e r ' s c o n s t i t u t i o na n d a c t io n u p o n m a t t e r - - a s r a d i c a l i n d e e d a s t h e c o n d i t i o n o f e x t r e m e l y lo wd e n s i t y c a l ls f o r: h e v i s u a l i z e d t h e a e t h e r a s a m e d i u m c o n s i s t in g o f p a r t i c l e s i ns w i f t r a n d o m m o t i o n , a n d s o r a r e t h a t t h e m u t u a l c o ll is io n s o f t h e s e p a r t i c l e sm a y b e i g no r ed . C o u ld t h e i m p a c t s o f t h e s e a e t h e r p a r t i c le s o n a p a i r o f m a t e r i a lb o d i e s i m m e r s e d i n t h e m g i v e r is e to a n e t m o t i o n o f th e s e b o d i e s t o w a r d s e a c ho t h e r ? If so , th i s a p p a r e n t a t t r a c t i o n w o u l d i n d e e d o b e y t h e i n v e r s e s q u a r e l a w .F A TIO r e a l i z e d t h a t n o s u c h m o t i o n c o u l d b e p r o d u c e d b y p e r f e c t l y e l a s ti c c o l-l is i on s b e t w e e n t h e p a r t i c l e s of a e t h e r a n d m a t t e r . O n t h e o t h e r h a n d , i n e l a s t i c i t yi n t r o d u c e s s e c u l a r a l t e r a t i o n s o f t h e m o t i o n s o f t h e s y s t e m , a n d i t is a m o o tp o i n t w h e t h e r t h e s e c a n b e k e p t d o w n s u f f ic i e n tl y n o t t o c o n f li c t w i t h t h e o b -s e r v e d s t a b i l i t y o f th e p l a n e t a r y m o t i o n s . F A TIO 'S p r o g r e s s w a s h e l d u p b y t h i sd i f f ic u l t y f o r m o r e t h a n t w o y e a r s , u n t il h e t h o u g h t h e c o u l d p r o v e t h a t t h es e c u l a r e f f e c t o f i n e l a s t i c c o l l i s i o n s c o u l d b e r e d u c e d a s m u c h a s d e s i r e d b y d e -c r e a s in g t h e d e n s i t y o f t h e a e t h e r p a r t i c l e s a n d , i f n e e d b e , i n c r e a s i n g t h e i rv e l o c i t y . O n F e b r u a r y 2 4, t 6 8 9 /9 0 , h e a n n o u n c e s b r i e f l y t o N E W T O N [ t 9 ] t h a th is t h e o r y i s n o w " c l e a r o f o b j e c t i o n s " : f r o m w h i ch o n e m a y i n f e r t h a t h e h a dp r e v i o u s l y d i sc u s s e d i t v e r b a l l y w i t h h i m . O n t h e s a m e d a y , h e w r i t e s t o H U YG E NS[20~ a l e t t e r c o n t a i n i n g a d e t a i l e d a c c o u n t o f h i s t h o u g h t s , a n d t w o d a y s l a t e r ,h e r e a d s t h i s l e t t e r a t a m e e t i n g o f t h e R o y a l S o c i e t y [ 2 t ] . M o r e o v e r , h e c o m p o s e sa p r o l i x e s s a y [ t 4 ] a f t e r th e f a s h i o n o f t h e t i m e , i n w h i c h h e s e t s d o w n o n c em o r e t h e w h o l e a r g u m e n t a t g r e a t l e n g t h ; o n M a r c h t 9 , t 6 9 0, t h i s e s s a y is i nt h e h a n d s o f H A L L E Y a n d N E W T O N [2 21 .

H U Y GE N S s a w a t o n c e t h a t F AT IO 'S w a y o f d e a l i n g w i t h t h e c o n s e q u e n c e so f i n e l a s t i c c o l l is i o n s w a s a p a r a l o g i s m , a n d i n h i s r e p l y [ 23 ] o f M a r c h 2 1 , t 6 9 0( n .s .) , h e p o i n t e d o u t t h a t n o t h i n g s h o r t o f a c t u a l a b s o r p t i o n o f a e t h e r p a r t i c l e sb y t h e m a t e r i a l b od i e s c o u ld p r o d u c e th e e x p e c t e d a t t r a c t i o n b e t w e e n t h e m .F AT IO p r e s e n t e d [2 4] a n i n e f f e c t u a l d e f e n s e , b u t n o t r e c e i v i n g a n y f u r t h e r r e p l yfrom HUYGENS, h e f o n d l y i m a g i n e d t h a t t h e l a t t e r h a d y i e l d e d * , a n d h i s o w nb e l ie f i n t h e s o u n d n e s s o f h i s t h e o r y r e m a i n e d u n s h a k e n . I n f a c t , a s a p p e a r s f r o mh i s m a r g i n a l a n n o t a t i o n s t o FAT!O'S t w o l e t t e r s , a s w e l l a s f r o m l a t e r o c c a s i o n a l

* He says so in a le t ter of March 30, t694 wr i t t en to th e B raunsch weig res identin London and intended for LEIBNIZ [25], and also in a later memorandum E22~.3 Arch. Hist. Exact Sci., Vol. 6


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3 4 L . ROSENFELD :

utt eran ces* HUYGENS tho ugh t th at the necessity of assuming a continu al ab-sorption of aether by the material bodies reduced the whole theory to an ab-surdity.

As to NEWTON, we gat her from a manuscr ip t E26], as well as from FATIO'Sreports [25, 22], that he looked more favou rably on the latter's theory. Heseemed to regard it as a possible mechanical explanation of gravity -- indeedhe even says: the only possible one, but not with great conviction, for we knowfrom FATIO'S own testimony that he often seemed to incline, as an alternative,to the opinion "that Gravity had its Foundation only in the arbitraryWill of Go d [22~ ". It is underst anda ble th at NEWTON would not feel as stronglyas I-IUYGENS the inconvenience of secular per turb atio ns in t he solar sys tem; incon tra st to HUYGENS and LEIBNIZ,he never thought that the solar system couldsubsist indefinitely by itself. His own circulatory mechanism was also essentiallydependent on some absorption of aether by matter, and (as we may infer fromFLAMSTEED'S te st imony [30]) as late as t685 he was still c ont emp la ting thepossibility of a slowing down of planetary motions as the result of an increasein mass due to accretion of aetherial matter; he was then in doubt about a sus-pected slowing down of Saturn's motion, which seemed to follow from tablesdrawn up by HALLEY [301 and was too large to be explained as a perturbationdue to the at tra cti on of Jupi ter [31]. Eve n after he had conv inced himself, inthe course of writing the Principia [tt], that gravitation was the only forcegoverning the solar system, he remained persuaded that in any case the per-turbations arising from the mutual attraction of the planets would in the longrun upset its structure E32~.

The FATIO episode is instruct ive inasm uch as it evidences the persistence ofkinet ic conceptions in NEWTON'S mind as offering at least a possible basis for amechanical theory of gravitation. There is no telling when or why he gave upthis prospect and turned again to the static type of mechanism of which he hadsketch ed out an impro vised va ria nt in his letter to BOYLE. It is first in the newEnglish edition of the Opticks [8] (of which two issues were pr int ed in t 717 and17t8) t ha t he resolved to make public, in some additional queries **, his last viewsabout the aether and its possible function as the agent of gravitation; theseviews, however, harmonize with those expressed in the other queries, which datefrom the first latin edition [32] of 1706. We may therefore surmise that NEWTON'Srenewed interest in optics, which led him to the elaboration of his curious theoryof the fits of easy reflexion and easy transmission, can have been instrumental

* In a letter of December 29, 1692 (n.s.) to the Marquis DE L'I-IosPITAL[27] andtwice [28] in reply to enquiries which LxIBNIZ directed to him [29] after receivingFATIo's letter mentioned in the preceding footnote.**The first edition of the oplichs (t704) contained only queries 1--16, whichwere kept (with some additions) in all the following editions. In the latin edition [32]of t 706, a number of new queries appear, there numbered 17--23: they coincide inall essentials with the queries numbered 25- 31 in all later editions, and they containa detailed discussion of the aether's role in optical and chemical phenomena, as wellas (in the last query) physical and theological considerations about the constitutionof the universe. New queries, numbered t 7--24, were finally inserted in the secondEnglish edition of 17t 7- -t 8 and the later ones.


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N e w t o n o n A e t h e r a n d G r a v i t a t i o n 3 5i n o r i e n t in g h i s t h o u g h t s t o w a r d s t h e m a n i f e s t a t i o n s o f t h e e l a s t i c i t y o f t h e a e t h e r ,w h i c h h e w a s n o w c o n s i d e r i n g t o b e o n e o f i t s e s s e n t i a l a t t r i b u t e s .

I n o r d e r t o a c c o u n t f o r t h e p h e n o m e n a o f t h i n p l a te s , N E W TO N h a d i n d e e ds e e n th e n e c e s s i t y o f i n t r o d u c i n g a n e l e m e n t o f p e r i o d i c i ty in t h e p r o p a g a t i o n o fl i g h t : h e i m a g i n e d t h a t t h e l i g h t c o r p u s c l e s c o u l d e x c i t e i n t h e a e t h e r w a v e s o fd e f in i te p e r io d s , w h i c h w o u l d o v e r t a k e t h e m a n d p u t t h e m i n f it s of e a s y r e fl e x io no r e a s y t ra n s m i s s i o n . T h i s m e c h a n i s m f i x es a l o w e r li m i t t o t h e v a l u e o f t h er a t i o o f e l a s t i c i t y t o d e n s i t y o f t h e a e t h e r i a l m e d i u m , w h i c h is e q u a l t o t h es q u a r e o f t h e v e l o c i t y of p r o p a g a t i o n o f w a v e s i n i t; t h i s r a t io m u s t b e a t l e a s to f t h e o r d e r o f t0 12 t i m e s t h e c o r r e s p o n d i n g r a t i o f o r a i r. O n t h e o t h e r h a n d ,t h e d e n s i t y o f t h e a e t h e r m u s t b e s m a l l e n o u g h t o m a k e i t s re s i st a n c e t o t h em o t i o n o f m a t t e r t h r o u g h i t n eg l ig i b le ; i f w e t a k e i t 106 t i m e s r a r e r t h a n a i r ,i t s r e s is t a n c e w i l l b e a b o u t t 09 t i m e s l e ss t h a n t h a t o f w a t e r . I t s e l a s t i c i t y w o u l dt h e n h a v e t o b e a t l e a s t 1 0 s t i m e s l a r g e r t h a n t h a t o f a ir . N E W TO N a p p l i e s h e r ew i t h w o n t e d v i r t u o s i t y hi s f a m i l i a r m e t h o d o f e x t r a p o l a t i n g f r o m k n o w n p r o p -e r ti e s o f m a t t e r t o a p l a u si b le c o n s t r u c ti o n o f a h y p o t h e t i c a l m e d i u m s a t i sf y i n gt h e i m p o s e d r e q u i re m e n t s . I f h e n o w w a n t e d t o f it g r a v i t a t i o n i n t o t h e p i c t u r e ,h e h a d t o i n t ro d u c e s o m e i n h o m o g e n e i t y in t h e d i s t r ib u t i o n o f t h e a e t h e r w i t hr e s p e c t t o t h e m a t e r i a l b o d i e s ; a n d h e s a w t h a t h e w o u l d a c h i e v e h i s p u r p o s eb y a s s u m i n g t h a t t h e p r e s e n c e o f m a t t e r p r o d u c e s in t h e a e t h e r a g r a d i e n t o fd e n s i t y , w h i c h in i ts t u r n m a i n t a i n s t h e p r e s s u r e g r a d i e n t r e q u i r e d t o b r i n g a b o u tt h e p h e n o m e n a o f g r a v i t a t i o n .

I t c o u l d n o t e s c a p e h i m , h o w e v e r , t h a t h i s n e w a e t h e r m o d e l , in s p i t e o f i t sa p p a r e n t s im p l ic i ty , w a s m u c h n e a r e r " t h e a r b i t r a r y w il l o f G o d " t h a n t h o seh e h a d p r e v i o u s l y d e v i se d o n t h e a n a l o g y o f c o n t i n u o u s f lu i ds - - a n a n a l o g y h en o w f i r m l y r e j e c t e d ( q u e r y 2 8) . F o r h e w a s n o w f a c e d w i t h t h e q u e s t i o n o f h o wt o a c c o u n t i n m e c h a n i c a l t e r m s f o r t h e e l a s t i c i t y o f t h e a e t h e r . I n t h e P r i n c i p i a E l 1 ,h e h a d d i sc u s se d - - a s a " m a t h e m a t i c a l " p o s s i b il i ty - - a s t a t ic m o d e l o f f lu i d si n w h i c h a p r e s su r e i s m a i n t a i n e d b y a s h o r t - ra n g e r e p u l s io n b e t w e e n t h e c o n -s t i t u e n t p a r t i c l e s o f t h e f lu i d . T h i s i s p r o p o s i t i o n X X I I I o f b o o k I I , i n w h i c hh e s h o w s, b y a si m p l e a r g u m e n t o f p r o p o r t i o n a l i ty , t h a t a r e p u ls i v e fo r c e b e t w e e nn e a r e s t n e i g h b o u r s v a r y i n g a s t h e - - n - t h p o w e r o f t h e i r d i s t a n c e gi v es r is e t o ap r e s s u re p r o p o r t i o n a l t o t h e p o w e r ( n + 2 ) o f t h e d e n s i ty . H e d o e s s u g g e s t( q u e r y 2 t ) t h a t s u c h a m u t u a l r e p u l s io n b e t w e e n t h e p a r t ic l e s o f t h e a e t h e rm a y b e t h e c a u s e o f i t s e l a s t i c i ty ; b u t t h i s c a n n o t b e a m e c h a n i c a l c a u s e , f o rt h e a n a l o g y w i t h o r d i n a r y f l u id s h e r e b r e a k s d o w n o n a n e s s e n t i a l d i f f i c u l ty : t h en e c e s s i t y o f a s s u m i n g t h a t t h e a e t h e r p a r ti c le s a r e o n t h e a v e r a g e s e p a r a t e d f r o me a c h o t h e r, w i t h o u t a n y p o s s i b il i ty of m a t e r i a l c o m m u n i c a t i o n b e t w e e n t h e m .

I n t h e f a m o u s q u e r y 3 1 ( w h i c h w a s p u b l i s h e d i n t 7 06 , a n t i d a t i n g b y 7 y e a r s t h ep u b l i c a t i o n o f t h e f i n a l s c h o l i u m a d d e d t o t h e P r i n c i p i a ) N E W T O ~ r a i s e s t h ep r o b l e m o f a c t i o n a t a d i s t a n c e i n f u l l g e n e r a l i t y : g r a v i t a t i o n i s o n l y o n e o f t h ei n t e r a c t i o n s b e t w e e n m a t e r i a l p a r t i c l e s w h i c h c a n b e d e s c r i b e d i n t h i s w a y ;e l e c tr i c it y a n d m a g n e t i s m o f f er o th e r e x a m p l e s , a n d c h e m i c a l p ro c e ss e s g i v ee v i d e n c e o f s h o r t e r - r a n g e f o r c es o f a s im i l a r t y p e . L i k e w i s e , o p t i c a l p h e n o m e n ap o i n t t o a n a c t i o n o f m a t t e r u p o n l ig h t c o r p u sc l es w h i c h is n o t o n e o f i m m e d i a t ec o n t a c t . H o w s u c h a c t i o n s a r e t r a n s m i t t e d i s a q u e s t i o n w h i c h N E W T O N p r o f e s s e sn o t t o b e c o nc e r n ed w i t h ; t h e y " m a y b e p e r f o r m ' d b y im p u l se , o r b y s o m e3*


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3,6 L . ROSENFELD :o t h e r m e a n s u n k n o w n t o m e " . J u s t a s i n t h e Princ@ia , h e is a n x i o u s s h a r p l y t od i s s o c i a t e h is p h e n o m e n o l o g i c a l a p p r o a c h f r om t h e v e x e d p r o b l e m o f t h e c a u s e s ,f u l l y c o n s c i o u s a s h e is o f t h e i m m e n s e p r o g r e s s t h is m e t h o d r e p r e s e n t s o v e r t h ec a r t e s i a n p h i l o s o p h y . N e v e r t h e l e s s , h e n e v e r g a v e u p h i s d r e a m o f a u n i v e r s a la e t h e r p r o v i d i n g t h e m e d i u m t h r o u g h w h i c h a n o m n i p r e s e n t G o d w a s c o n t i n u a l lya c t i n g u p o n h i s c r e a t i o n ; b u t h e h a d t o c o nf e ss t h a t t h e o n l y w o r k i n g s o f t h ea e t h e r h e c o u l d u n d e r s t a n d w e r e t h e t r a n s m i s s i o n o f s h o r t - ra n g e i n t e r a c t i o n si n s i d e t h e m a t e r i a l b o d i e s ; t h e s e h e f e l t c o n f i d e n t c o u l d b e a t t r i b u t e d t o " a v e r ys u b t l e s p i r i t p e r v a d i n g t h e g r o s s b o d i e s a n d l y i n g h i d d e n i n t h e m " ( " s p i r i t uq u o d a m s u b t i l i s s i m o c o r p o r a c r a s s a p e r v a d e n t e , & i n i i s d e m l a t e n t e " [ 3 ]) : f o rt h a t p a r t o f t h e a e t h e r w a s a c t in g u p o n s u f f ic i e n t l y c o n d e n s e d m a t t e r t o m a k ei t s b e h a v i o u r c o n c e i v a b l e in m e c h a n i c a l t e r m s , - - i n c o n t r a s t t o t h a t o f t h e r a r em e d i u m o f i n t e r p l a n e t a r y s p a c e . T h e n o b l e s t f u n c t io n o f t h e a e t h e r , t h e r e g u l a t i o no f t h e h e a v e n l y m o t i o n s , e l u d e d h i m t o t h e l as t .

L i t e r a t u r e *1. N EW T ON , I . , P h i l o s o p h i a e n a t u r a l i s p r i n c i p i a m a t h e m a t i c a , f i r s t e d i t io n 16 87 .2 . T h e c o r r e s p o n d e n c e o f I S A A C N E W T O N , ( f u r t h e r q u o t e d a s " C o r r e s p o n d e n c e " ) ,v o l . 3 : l e t t e r 3 9 9 o f J a n u a r y 1 7 , 1 6 9 2 / 9 3 , e s p . p . 2 4 0 .3 . N EW T ON , I . , P h i l o s o p h i a e n a t u r a l i s p r i n c i p i a m a t h e m a t i c a , s e c o n d e d i t i o n 1 7 t 3 :S c h o l i u m g e n e r a l e .4 . C o r r e s p o n d e n c e , v o l . 1 : l e t t e r 1 4 6 t o O L D E N B U R G o f D e c e m b e r 7 , 1 67 5, e s p . p . 3 6 6 .5. C o r r e s p o n d e n c e , v o l . 2 : l e t t e r 2 33 o f F e b r u a r y 2 8 , 1678/79, e s p . p . 2 9 5 .6 . C o r r e s p o n d e n c e , v o l . 2 : l e t t e r 2 8 8 o f J u n e 2 0 , t 6 8 6 a n d l e t t e r 2 9 t o f J u l y 2 7 , 1 6 8 6 .7 . C o r r e s p o n d e n c e , v o l . 2 : l e t t e r 2 8 8 , p . 4 4 0 .8 . N EW T ON , I . , O p t i c k s , s e c o n d e d i t i o n 1 7 1 7 - - 1 8 ; n e w q u e r i e s n u m b e r e d 1 7 - - 2 4 .9 . H A L L, A . R . & M . B . , U n p u b l i s h e d S c i e n t i fi c P a p e r s o f I s a a c N e w t o n ( C a m b r i d g e

U n i v e r s i t y P r e s s , 1 96 2), p . 3 2 1 - - 3 3 3 , 3 4 9 - - 3 5 9 .10 . Ibid., p. 1 8 3 - - 2 1 3 .11. ROSENFELD, L., N e w t o n a n d t h e l a w o f g r a v i t a t i o n ( A r c h iv e fo r H i s t o r y o f E x a c tSc iences 2 , 365 (1965) .1 2. C o r r e s p o n d e n c e , v o l . 2 : l e t t e r 2 9 1 , p . 4 4 7 .t 3 . A IT ON , E . J ., N e w t o n ' s A e t h e r S t r e a m H y p o t h e s i s a n d t h e I n v e r s e S q u a r e L a wo f G r a v i t a t i o n ( A n n a l s o f S c i e n c e ) .1 4. G A GN EB IN , B . , D e l a c a u s e d e l a p e s a n t e u r , m 6 m o i r e d e N i c o l a s F a t i o d e D u i l l ie r( N o t e s a n d R e c o r d s o f t h e R o y a l S o c i e t y o f L o n d o n 6 , 10 5 ( t9 4 9 )) ( f u r t h e rq u o t e d a s " G A G N E B I N " ).t 5 . ( E u v r e s c o m p l e t e s d e C . H U Y GE N S ( f u r t h e r q u o t e d a s "CEuvres") , vo l . 19 , p . 619 ;v o l . 2 t , p . 3 7 9 .1 6. ( E u v r e s , v o l . 9 , P . 1 67 ( a l s o C o r r e s p o n d e n c e , v o l . 2 : l e t t e r 3 0 7 , a n d G A G N E B IN ,p . t 0 9 ) .1 7. G A G N E B I N , p . 1 1 4 - - 1 1 5 ( a ls o C o r r e s p o n d e n c e , v o l . 3 : l e t t e r 3 5 2, n o t e ( t ) , p . 69 ,w h e r e t h e s e c o n d e n t r y , h o w e v e r , i s w r o n g l y i n t e r p r e t e d a s r e fe r r in g t o t h ee x p o s i t i o n o f F A T IO 'S o w n t h e o r y ) .1 8. ( E u v r e s , v o l . 2 1 , p . 4 2 9 .1 9. C o r r e s p o n d e n c e , v o l . 3 : l e t t e r 4 6 3 , p . 3 9 0.2 0 . ( E u v r e s , v o l . 9 , P . 38 1 .

21. GAGNEBIN, p. t t 5- - 1 t 6.22. GAGNEBIN, p. t1 6- -1 17 .2 3 . G A G N E B I N , p . 1 5 4 - - 1 5 8 ( a ls o , b u t i n c o m p l e t e , ( E u v r e s , v o l . 9 , P . 3 91 ).* A l l d a t e s a r e q u o t e d i n th e J u l i a n c a l e n d a r , e x c e p t th o s e m a r k e d " ( n . s . ) "( n e w s t y l e ) .


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N e w t o n o n A e t h e r a n d G r a v i t a t i o n 372 4 . ( E u v r e s , v o l . 9 , P . 4 0 7 ( l e t t e r t o H U Y G E N S o f A p r i l 1 t , 1 6 9 0 ).2 5 / ( E u v r e s , v o l . 10 , p . 60 8 ( a l s o C o r r e s p o n d e n c e , v o l . 3: l e t t e r 4 4 0 ) .26. Loc. cir. i n n o t e 9 , P . 3 t 3 .27 . (Euvres , vo l . 10 , p . 354 .2 8 . ( E u v r e s , v o l . t 0 , p . 6 1 3 ( l e t t e r t o L E IB N IZ o f M a y 2 9 , 1 6 9 4 ( n . s. ) ) a n d p . 6 6 9( l e t t e r t o L E IB N IZ o f A u g u s t 2 4 , t 6 9 4 ( n . s. ) ).2 9 . ( E u v r e s , v o l . 1 0, p . 6 0 3 ( l e t t e r t o H U Y G E N S o f A p r i l 2 6 , t 6 9 4 ( n .s . )) ~ n d p . 6 4 4( l e t t e r t o H U Y G E N S o f J u n e 2 2 , 1 6 9 4 ( n . s .) ) .3 0. C o r r e s p o n d e n c e , v o l . 2 , n o t e ( 4) t o l e t t e r 2 7 5, p . 4 t 2 .3 1. C o r r e s p o n d e n c e , v o l . 2 : l e t t e r 2 76 o f J a n u a r y t 2 , 1 6 8 4/ 85 t o F LA M S TE E D.3 2. N E W TO N , I . , O p t i c e ( t r a n s l a t e d b y S . CLARKE) t 7 0 6 , q u a e s t i o 2 3 ( = q u e r y 3 1 i nl a t e r e d i t i o n s ) ; s e e a l s o R O B II~ E% A . , ( e d .) , C o r r e s p o n d a n c e L e i b n i z - C l a r k e( P r e s s e s U n i v e r s i t a i r e s d e F r a n c e , P a r i s 1 95 7).

N O R D I T AC o p e n h a g e n(Received March 19, 1969)

newton's views on aether and gravitation - l. rosenfeld

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