Newton's Material Aether : Problems of Internal Coherence and Rational Reconstruction

ORGANON 25:1989/1993 AUTEURS ET PROBLÈMES

P ra b ir M itra (Poland)

N E W T O N ’S M A T E R IA L A E T H E R : P R O B L E M S O F IN T E R N A L C O H E R E N C E A N D R A T IO N A L R E C O N S T R U C T IO N

1. Prelim inaries

In this paper an exam ination o f N ew ton’s “m aterial” aether pro gram has been undertaken w ith several parallel aim s. First is to exam ine th e internal coherence and unity o f ideas within the “m aterial” aether p rogram (inter­ program coherence, i.e., unity betw een “m aterial” aether, “im m aterial” aether, phenom enological program and actio in distans, w ould not be d is­ cussed in this paper). The search for internal coherence and unity is im p o r­ tant, for m any historians and philosophers o f science believe that a “failed” research program m ight be very m uch w anting in internal coherence. A l­ though “rationality” question cannot be entirely reduced to internal coher­ ence and unity o f ideas, these m ust still be regarded as im portant criteria. B esides N ew ton ’s texts are dense, difficult to interpret and an alternative interpretation is offered here.

Second related aim is to exam ine the question of relationship betw een “rational reconstruction” and the “descriptive” epistem ology o f history o f science, through this reconstruction o f N ew ton’s aether program . M uch d is­ cussion has gone on this topic philosophical literature, with m any d istin ­ guished philosophers - e.g., Agassi, G runbaum , Lakatos etc. - p articipating in it.1 Laudan (1977) has suggested draw ing a line betw een tw o types o f history o f science - called HOS1 and H O S2. T he distinction is useful, even though it m ay be challenged by m any on theoretical grounds, as all such distinctions in the past have been (e.g., distinction betw een theory and ob­ servation etc.). Even if w e accept this distinction betw een H O S 1 and H O S2, the position is not altogether free o f difficulties. An overw helm ingly large part o f the HOS1 m ust be consisting o f “failed” research pro gram and it

will be difficult to explain m any o f them in the “rational reconstruction” m ode. The picture gets further com plicated by the fact that p re-a na lytica l

intuition about scientific rationality (or PI) is seen as im portant elem ent o f

theory acceptance/rejection. H O S 1 contains m any situations w here a “failed” research program w ith very string P i ’s w ere considered along w ith alterna­ tive counter-intuitive program s, with well articulated m odular justification. Such was the case o f N ew ton’s “m aterial” aether program , w hich had to com pete for a valid explanation o f transm ission o f gravitational forces with tw o alternative m odels, nam ely space-body interaction m odel an d the actio

in distans model, w hich appeared to be totally counter-intuitive. H ere we

see a case o f post-facto developm ent o f PI and creation o f a post-facto m od­ ular justification for the alternative program s. The present case study, besides highlighting the com plexity o f relationship betw een “rational reconstruction” and “descriptive” epistem ology, also m akes another point, w hich although not original, is nevertheless o f som e value. Physical explanations, even in absence o f adequate m odular justification, often strive to go beyond logically transparent inferential structure by assum ing a fundam entally d ifferent nature of physical reality. T hat the explanations are counter-intuitive or inconsistent w ith intuitively acceptable logical relationships, m ay not have anything to do with the “truth” or the scientific w orth of the im plications o f these ex ­ planations. A dequate m odular justifications are of course eventually developed to post-facto legitim ize all the successful explanations, but for obvious reasons these justifications have no profound heuristic value. B e­ sides these conclusions, some observations are also added on the dis­ covery/justification dichotom y in the concluding rem arks.

II. Explanation and gravitation in N ew ton’s w ritings

N ew ton ’s p re-P rincipia w ritings on gravitation show that he m ainly used three approaches in order to explain the causes and the m echanism o f opera­ tion o f the phenom enon of gravitation: (a) he used the “m aterial” aether as the “causal agent” - an approach that shared the inter-phenom enal aether with the tradition o f 17th century “m echanical” philosophy, but otherw ise differed substantially w ith this tradition, (b) he introduced the so-called “im ­ m aterial” aether hypothesis, in the paper De gravitatione in 1668, and (c) he used, what we shall call here, the “phenom enological” approach to develop a w orkable dynam ics of m otion o f bodies under gravitation. The last approach to the problem o f gravitation or (d) com prised o f putting for­ w ard and testing the em pirical consequence o f “m odels” or m athem atical constructions that depicted the relationship betw een dynam ic variables of bodies in m otion under gravitation. This approach unlike the oth er two did not attem pt to describe the m echanism by which gravitational forces were

Newton ’s Material Aether ₆₉ transmitted and in the “phenomenological” papers by and large, N ew ton re­ frained from offering any conjecture on the ultimate causes o f gravitation. Such was the approach o f the papers like Laws o f M otion or the paper on calculation o f lunar surface gravity and also the approach taken in the P rincipia?

O bviously the aether conceptions w ere originally introduced into the field of physics in answ er to a question that N ew ton tried to solve for m any years, nam ely: how are the gravitational forces transm itted across the vast expanse of cosm ic void from one body to another? The basic m odel or the m echanism o f transm ission of force in the tw o aether program s are very different from each other. In case o f the “m aterial” aether the transm ission o f forces was caused by either im pact phenom enon or by “push ing ” resulting from the pressure gradient o f the aether. In other w ords, the “m aterial” aether explanation could be reduced to som e sort o f contact action betw een a body and aether (w hich in turn also com prised o f “m aterial” substance). In case o f “im m aterial” aether how ever New ton introduced a very different species o f argum ent - that the absolute space itself in som e way “acted” on bodies to produce their inertia and their observed gravitational m otion. Since ab ­ solute space, unlike the “m aterial” aether, was not “constituted o f m aterial substance” , this action could neither be contrived as contact action no r as a type o f m echanical interaction. W hat w as the m odel o f transm ission of force underlying the “phenom enological” w ritings? W ith the publication o f the P rincipia in 1687, N ew ton was alm ost “universally m isunderstood” to be advocating the phenom enon o f actio in distans. This how ever is an in­ correct supposition. The “phenom enological” m odel was sim ply neutral to m echanism o f transm ission o f forces and N ew ton in a num ber o f docum ents - e.g., the R ules o f Reasoning o f the P rincipia , the B entley Letters o f 1692 etc. - expressed grave doubts about the possibility o f actio in d ista n s.4 N evertheless there are other docum ents am ong N ew ton’s m anuscripts in w hich he did argue on the basis o f som e observed phenom enon about the possible existence of an asym m etrically propagating “double force” acting

at a distance on m icro and m acro objects.5

In considering these three types o f explanations w e im m ediately see that the “m aterial” aether program had an explanatory advantage w hich both the other types o f explanations generally lacked. The contact action and m e­ chanical interaction m odel had an explanatory transparency that m ade it im ­ m ediately intelligible. Both actio in distans and the space-body interaction m odel lacked this transparency and im m ediacy and in general sense these explanations were counter-intuitive. Furtherm ore there w as no m odal ju s ti­ fication for these program es available in the 17th century. G iven the criteria o f acceptability o f scientific theories in the 17th century therefore, the aban­ donm ent of the “m aterial” aether appeared to be the abandonm ent o f the very notion o f m echanical causation o f generation and transm ission o f the gravitational forces. At a more abstract plane the rejection o f “m aterial”

aether caused even deeper and other significant philosophical problem s. If the generation and transm ission o f the gravitational forces w ere not as­ sociated w ith som e sort of “m aterial substance” then w hat w ere they rooted in? This placed the “theoretical” entities like “force” on a very unsound ontological and m ethodological footing. For this reason and others m any thinkers o f 17th century, like H uygens and Leibniz, preferred “m aterial” aether explanations and struggled to develop a “m aterial” aether explanation even after the great success o f N ew ton’s Principia. N ew ton h ow ever choose to part w ays with the “m aterial” aether and explore other possible options. In doing so he confronted a m ost com plex philosophical problem , which M cG uire (1968) has m ost aptly called the “ontological problem o f causation o f force” .

III. N ew ton’s “m aterial” aether writings

There are two distinct types o f “m aterial” aether explanatory “m odels” that N ew ton uses in order to explain gravity. In som e papers he uses the

descending aether collision m odel and in others a variable density gradient o f the aether is held to be responsible for causing the gravitational phenom ­

enon. In an im portant paper Rosenfeld (1969) has term ed the form er the “kinetic” aether and the latter the “static” aether6. W e shall be using here the term s kinetic aether m odel as term inologically equivalent alternative for the descending aether collision m odel and the static aether m od el for the

variable density aether model. The kinetic aether is predom inantly used in

N ew ton’s early prQ-Principia papers, m ost notably in the W aste B ook (the

Questiones, therein) and in the paper H ypothesis. The Q uestiones gives us

the basic form at o f the descending aether explanation. A corpuscular aether radially descending tow ards the center o f earth gets into continuous inelastic collision with bodies. A part o f the aether is absorbed by bodies and another part ascends upw ard in “lesser consistency” . The conceptual problem s arising out of this descent-ascent m echanism and the problem o f continual absorp­ tion o f aether by bodies is elaborately discussed in the Q uestiones. The m ain problem s are what happens to the aether that gets absorbed and w hat m akes the aether ascend upw ards again?

The paper H ypothesis elaborates further on all these problem s and gives us the m ost exhaustive picture o f N ew ton’s early aether “philosophy” . In this paper New ton attem pts to develop an overall theory o f aether to explain all types o f natural phenom enon. T he basic tenets o f this theory o f aether consisted o f (a) a cosm ic circulation o f aether, (b) conversion o f aether and m atter through condensation and evaporation, (c) m echanism o f de­ scending/ascending aether and varying densities .of aether substance sur­ rounding m atter etc., causing gravitation and other natural phenom ena. The

Newton's Mute rial Aether 71 long list o f natural phenom ena that N ew ton attem pted to explain through the operation o f aether m echanism in this paper is revealing. B esides grav­ itation, electricity, m agnetism , chem ical reaction, reflection and refraction o f light etc., he also devised an elaborate explanation o f anim al m otion through condensation and dilation o f aether in the m uscles.7 T he last was undoubtedly an attem pt to furnish a “m echanical” solution to the com plex m ind-body problem , w hich was turning out to be the bane o f the 17th century “m echanical” philosophy.

A ether in this paper was seen as the fundam ental “cosm ic” constituent - a source o f all cosm ic activity - w hich the Sun im bibed copiously to produce the heat, light and planetary m otion, w hich the brain directs through the nervous system in living organism s to produce the m uscular m ovem ents and w hich through its layers o f varying density etc. could produce gravitation and other attractions. As we have already suggested, this paper was definitely conceived within the fram ew ork of C artesian “m echanical” philosophy. God was seen as the first cause, while the “ m echanical” nature proceeded with its ow n autonom y, aided by the aether that like the C artesian “first m atter” constituted the “w hole fram e o f nature” . The “m ain body o f the aether” here is functionally sub-divided into electrical, m agnetic, gravitational types of “aether substance”, “spirits” and “effluvia”, each having its special charac­ teristics and each operating through distinct type o f m echanism . The d is­ tinction - perhaps in anticipation o f N ew ton’s future view s - also extends to the ontological planes. The aether as a “substance”, bears the burden of perform ing the bodily work; condensing, evaporating, dilating, m oving bo­ dies etc. T hereafter New ton goes into its attendant “form al” characteristics: “In the second place it is to be supposed that aether is a vibrating m edium like air, only vibrations far more sw ift and m inute; those o f air m ade by m an ’s ordinary voice, succeeding one another at m ore than h alf a foot or a foot distance; but those of aether at a less distance; but those o f aether at a less distance than the hundred thousandth part o f an inch...” 8. A lthough the “pulses” in the “vibrating m edium ” is also used to explain chem ical reaction and production o f flam es besides the optical phenom enon, the distinction clearly suggests N ew ton’s recognition o f the “form al” or the “m edium ” aspect o f the aether. This distinction as w e shall see w ould becom e prog res­ sively m ore acute in N ew ton’s future w ritings, heralding with it the problem of elasticity of this m edium and the philosophical problem o f “ontology” of all such “theoretical” entities as “force” , to account for w hich the “m aterial” aether was originally invoked.

There are some evidences that New ton w anted to give a m athem atical shape to this descending collision aether hypothesis. Thus in a letter to H ai­ ley w ritten in 1986, ju st before the publication of the P rincipia he referred to “m aterial” aether again and m entioned his attem pts to form ulate a q u an ­ titative version o f the descending aether theory. He suggested that the

de-scending aether could be given a m athem atical treatm ent and m ade to h ar­ m onize w ith the inverse square law, and consequently w ith the K ep ler’s law: ”1 there suppose that the descending spirit acts upon bodies here on the superficies o f the earth with force proportional to superficies o f their parts; which cannot be, unless the demunition o f its velocity in acting upon the first part of any body it meets with, be recompensed by increase o f its density arising from that retardation. Whether this be true is not material. It suffices that was the hy­ pothesis. Now if this spirit descends with accelerated motion, its density would everywhere diminish as much as its velocity increase; and so its force (according to the hypothesis) will be same as before, that is still reciprocally as the square of its distance from the center”.9

N ew ton never really m athem atically elaborated this idea, and the basic idea is not very clear. Prof. Rosenfeld has offered a sim ple m odel o f the idea underlying the passage quoted ab o v e:10 N aether particles descending p er unit tim e tow ards the center o f Earth with radial velocity V at a distance R from the center of earth, w ould have a surface density o f (N/47tR2V). This stream will exert a force directed towards the center o f E arth of m agnitude (NmV/47tR2) on bodies that it encounters on its way, thus m aking the “cen ­ tral force” inversely proportional to the distance from the center as required by N ew ton ’s theory o f gravitation. D espite this possibility o f being able to m athem atically harm onize the “kinetic” aether with the inverse square law, w e find that New ton preferred the “static” aether approach in the p ost-P rm -

cipia period. The only exception to this is the b rief episode in the early

1690s w hen N ew ton is know n to have approved o f F a tio ’s “kinetic” aether explanation of the gravitational phenom enon. The Fatio episode has becom e som ew hat of a puzzle for the history o f science. W hy did N ew ton support F a tio ’s explanation o f gravity when in 1690s he generally harbored strong reservations against the w hole “m aterial” aether program itself? Fatio im ­ agined a very rare aether - o f an extrem ely low density - such that the effect of collision am ong the aether particles could be considered negligible. These aether particle m oved rectilinearly and very sw iftly and through in ­ elastic im pact with bodies they caused the gravitational m otion. O bviously the random collision of aether and m atter w ould introduce secular aberrations in the observed m otion o f the bodies. Fatio thought that by reducing the density o f the aether suitably and sim ultaneously attributing higher velocity to its particles he could m ake the secular effects vanish. F a tio ’s own calcu­ lations show that he did not quite succeed in this and he certainly did not convince any o f his adversaries on this point. But perhaps from N ew ton’s point o f view F atio ’s system had several attractive features. N ew ton had never com pletely accepted the hypothesis o f actio in distans. The Fatio m odel had rectilinearly m oving corpuscles, w hich w as in com pliance with the first law of m otion. It em phasized high degree o f vacuum and a relative paucity o f m atter in the universe, which was very m uch in accordance with N ew to n’s ow n world view. If some o f this aether in inelastic impact with

Newton’s Material Aether ₇₃ the bodies had to be absorbed by the bodies, Newton, unlike Huygens, had no difficulty in accepting this. Indeed, as we have already seen, he had always entertained the possibility o f absorption of aether in the “descending aether m odel”. Besides these the Fatio idea was novel and showed the possi­ bility of developing further. None of these how ever furnish us with a com plete reason and we still do not know conclusively as to why, for a brief period in early 1690s, Newton thought that “kinetic” aether m odel could possibly furnish an explanation of gravity consistent with his Principia.

Only few years after the H ypothesis in a L etter to B o y le, n N ew ton

introduces the “variable density” or the “static” aether hypothesis. The m ain focus o f the Boyle L etter was explanation o f “chem ical” phenom enon and gravitational phenom enon is dealt only in passing in a single paragraph at the very end of the letter. In this paragraph N ew ton introduces an aether of differentiated corpuscular size - com prising o f aether particles o f “finer” and “grosser” size - m ixed in different proportion to each other such that as to result into an aether o f variable density around different bodies. T he interacting density gradients o f different bodies produces an aether pressure and the consequent gravitational m otion (or the force) o f the bodies. The exact m echanism of explanation is som ew hat com plex. The relevant p a ra ­ graph reads as follows:

”1 shall set down one conjecture more, which came into my mind now, as I was writing this letter. It is about the cause o f gravity. For this end I suppose aether to consist o f parts differing from one another in subtlety by indefinite degree; that in the part o f bodies there is less o f the grosser aether, in proportion to the finer, than in the open spaces; and consequently, that in the great body o f earth there is much less o f the grosser aether, in proportion to the finer, than in the regions o f the air: and that yet the grosser aether in the air affects the upper regions o f the earth, and the finer in the earth the lower regions o f the air, in such a manner, that from the top o f the air to the surface o f the earth, and again from the surface o f the earth to the center thereof, the aether is insensibly finer and finer. Imagine now any body suspended in the air or lying on the earth: and the aether being by the hypothesis grosser in the pores which are on the upper parts o f the body than in those which are in the lower parts and that grosser aether being less apt to be lodged in those pores, than the finer aether bellow, it will endeavor to get out and give way to the finer aether below, which cannot be without the bodies descending to make room above for it to go out into.” 12 The B oyle L etter is perhaps the last full-length paper on “m aterial” aether o f the pre-P rincipia period o f w ritings on “m aterial” aether or o f the first phase o f “m aterial” aether writings. It is generally agreed by the N ew ton scholars that betw een 1678 and 1707. N ew ton harbored strong reservations against the “m aterial” aether program . During this period he authored several docum ents and texts sum m arizing his objections against the “m aterial” aether hypothesis. W hen New ton finally returned to the them e o f “m aterial” aether again around 1707, we find that he preferred the “static” aether or the “v ari­ able density hypothesis” to the “kinetic” version. His interests w ere p re ­

sum ably rekindled by Francis H auksbee’s electrical experim ents before the Royal Society around 1706-07. Inspired by this N ew ton once again started considering the possibility o f “short-range forces” betw een particles being o f electrical nature. This led him to postulate the existence o f an electrical “subtle spirit” m uch like he had done thirty years earlier in the H ypothesis. In two draft versions o f Q ueries o f the O pticks, published later in 1717-18 - titled Q uaest 24 and 25 and widely quoted in all im portant studies o f N ew ton’s aether - New ton asks if the “force by w hich small particles o f bodies cohere and act upon one another at small distance” m ay not be of electrical origin? In a tentative answer to this he conjectures if “particles of all bodies m ay abound with electric spirit” w hich when “rarefied” or “agi­ tated by friction” may produce various natural phenom ena. H e sum m arizes the Q uaest 24 thus:

"And if there be such an universal electric spirit in bodies, certainly it must very much influence the motions and actions o f the particles o f bodies amongst one another, so that without considering it, philosophers would never be able to give an account of the Phenomenon arising from those motions and actions. And so far as these phenomena may be performed by the spirit which causes electrical attraction it is unphilosophical to look fo r any other c a u s e ’} '

N ew to n ’s renew ed conviction that no com plete description o f natural “phenom enon” w ithout involving the aether is possible is evident from this passage. This was the spirit in which New ton added the “new ” Queries to the different editions of O pticks betw een 1707 and 1717. M any of the new

Q ueries therefore, e.g., Query J7, 18, 19, 21, 22, 23, 24 etc., dealt with the

“ m aterial” aether, reviving anew alm ost the entire aether problem atic that N ew ton had dealt with in his earlier papers like the H ypothesis. These Q uer­

ies once again postulated the vibration o f the aetherial m edium , differential

densities o f the aetherial m edium , to explain the sam e phenom enon that New ton had attem pted to explain in the H ypothesis som e forty years earlier, namely, optical effects, gravitation, electricity, and propagation o f sensation etc. Radically differing from the position vis a vis the “m aterial” aether in the P rincipia, the Query 22 suggested that the planetary bodies w ould en ­ counter alm ost negligible am ount o f resistance from an aetherial m edium .

The “m aterial” aether o f the new Q ueries how ever differed from the previous one in many ways. The variable aether particle size hypothesis, introduced in the Boyle L etter, was om itted once for all (in fact in the 1690s New ton had discarded the hypothesis on a num ber o f interlinked considera­ tions), instead a hom ogeneous aether with uniform ly varying density around bodies was ascribed the task o f causing the phenom enon o f gravitation. Even this hom ogeneous “static” aether differed rem arkably from the robust “kinetic” aether of the earlier writings. All through the 1690s New ton had em phasized a num ber o f interlinked basic characteristic o f m atter and u n i­ verse, e.g., extrem e paucity o f m atter in the universe and the presence o f a high degree o f dissem inate and interstitial vacuum. At the end o f this period

Newton 's Material Aether 75 w hen N ew ton once again returned to the “m aterial” aether, the aether itself had becom e m uch m ore rare and vacuous entity. The “m ed ium ” aspect of this aether and the conjoint characteristic o f high degree o f “elasticity” of the aether m edium is repeatedly stressed, as opposed to the ontological and kinetic characteristics o f the aether o f the W aste Book.

IV. Elasticity o f A ether

If the aether particles were separated from each other by large stretches o f cosm ic void then how could this aether behave like a continuous elastic m edium ? On w hat basis did this m edium get its attendant characteristics o f “elasticity”? This interesting question has not been given its due attention in the historical literature. R osenfeld (1969) observes that “the origins o f aeth er’s elasticity raised a problem whose solution lay beyond N e w to n ’s conceptual horizon” . 14 O ther im portant studies have by and large ignored this question, supposedly im plying thereby that New ton never gave m uch thought on this problem . I w ould like to differ with this supposition. N ew ton was in the habit o f deliberately and very carefully considering the im plication of any hypothesis that he adopted and it seem s hardly likely that he did not ponder upon this im portant point before w riting the passages in the Q ueries and the Boyle Letter. Besides it seem s that during the “aether-less” period o f 1678-1707, N ew ton m ade several observations on the nature and com ­ position o f m atter which were prim arily aim ed at resolving the question o f elasticity of aetherial m edium . A “m edium ” had to be continuum . B ut as we know New ton from his very early w riting had very strongly rejected the C artesian and other plenum theories. If how ever the aether corpuscles (as well as other elem entary m icro-particles o f m atter) could a ct a t a distance on each other in such a way that the total system could com e to a springy pulsating m edium . The first suggestion tow ards this we find in the paper

De aere et aether in which m icro-corpuscles o f “air” agitated by heat and acting upon each other at distance by a “force o f repulsion” , produce such

a pulsating and springy m ediu m .15 In the follow ing paragraph this springi­ ness o f the m edium is used to explain the transm ission o f the sound w aves also. N ew ton ’s attitude to actio in distans how ever is very com plex and we shall not be in a position to exam ine it in greater detail here. Suffice it to say here that in the 1690s New ton m ade an im aginative attem pt to construct an “elastic” aether out o f tw o tentative properties o f “substances” : (a) the existence o f m icro-structural “double forces” betw een elem entary particles and, (b) the property o f extrem e porosity o f m atter. In another separate paper (yet unpublished) I have argued that the “double force” hypothesis was not entirely an autonom ous line o f argum ent nor a byproduct o f alchem ical

beliefs, but was a com posite part o f gravitational research program , rooted very largely in explanation o f “elasticity” o f “m aterial” aether.

V. Refutation o f the “M aterial” A ether H ypothesis

W hat were the considerations that m otivated New ton to drop the hy­ pothesis o f “m aterial” aether betw een 1678 and 1707 and later after 1717 to generally abandon it altogether? This has been the direct or indirect focus o f inquiry o f a num ber of historical work. W e can begin by exam ining all those basic problem s that New ton had to overcom e in order to develop the “m aterial” aether program .

If the aether substance continuously m oved “dow nw ards” , tow ards the center o f the earth, than what m oved the aether itself in the first place? The 17th century “m echanical” philosophy generally overcam e this paradoxical question in an orthodox C artesian way. A “first m otion” w as im agined to have been im parted on the universe by its creator, w hich w as later continued by the Cartesian “Law o f C onservation of M otion”. This Law , originally form ulated by D escartes, was w idely upheld in the 17th century. It pro­ claim ed that all m otion in the universe was perpetually conserved by a general concourse o f God, such that the total m otion in the universe at all tim e rem ained a constant. It appears from N ew to n’s early w ritings that he did not quite clearly answ er this tantalizing question as to w hat m akes the aether m ove dow nw ards. In the H ypothesis he explained aw ay all the aether functions e.g., descent, ascent, condensation and evaporation etc., by declar­ ing that nature was a perpetual circulatory “w orker” : “for nature is a per­ petual worker, generating fluids out of solid, and solids out o f fluid, fixed things out o f volatile, subtle out o f gross and gross out o f subtle, som e things to ascend and by consequence, others to descend to requital to the form er” 16. In the Boyle Letter how ever he produced an entirely different line of argu­ m ent as we have already seen. H ere the “static” aether m oved “dow nw ards” ow ing to its variable density gradient. But what kept the aether in this state o f variable density? W hy didn’t the aether everyw here get m ixed up into an uniform m ass of average density? In his early papers N ew ton furnished a “m echanical” answ er to this question. Bodies o f all types w ere full o f m inute pores. A ether in these extrem ely narrow pores stood rarer than the aether in the “free spaces” . This, New ton thought, was dem onstrated by various nat­ ural and “experim ental” evidences, e.g. by G rim aldi’s effect, by the fact that w ell-polished pieces o f flat glass got stuck together when pressed strongly, by the cohesion o f bodies, by “filtration” process, and m ost strongly by the phenom enon of capillary action, w here the fluid rose up in the tubes because, as N ew ton writes, “aether may stand rarer, not only in insensible pores o f bodies, but even in the very sensible cavities o f these pores (capillaries)” .17

Newton ’s Material Aether 77 So while the aether density inside a body dropped, the outer aether density gradually increased form ing a halo o f uniform ly varying aeth er density around all bodies. New ton how ever was not very com fortable w ith this m echanism as it is apparent from the hesitant approach o f the B oyle L etter w here this m echanism is discussed at som e length. Prof. W estfall has also persuasively argued that the incom plete paper D e aere - w hich m ost likely w as w ritten around the same tim e as the Boyle L etter - m arked the crucial turning point in N ew ton ’s thinking, w hen he finally parted w ays w ith the “m aterial” aether.

The next objection to the “m aterial” aether, w hich appears to be the m ost sustained o f all the objections, appeared in the Principia. If the m ed i­ ating “m aterial” m edia were to have its own “inertia” (’’inertia” being an “essential” property of all m atter), then it w ould ham per the m otion o f bodies through it in such a way that the laws o f m otion w ould not be valid for them . This argum ent actually stood at the core o f N ew ton ’s disagreem ent with the “m aterial” aether. W e see it for the first tim e being introduced in 1668 in the De g ra v, then repeated in the Principia and later in the 1690s in a num ber o f docum ents connected with the revision o f the P rin cip ia 18. In the Principia Newton estimated, how much the circulation of aether would hinder the motion of the planets in a Cartesian vortex and finds it to be con­ siderable19. The ostensible target o f this calculation in the Principia is the C ar­

tesian “theory o f vortex”, hut it basically applies to the hypothesis o f “m ate­ ria l” aether. In a Draft paper written in connection with the projected second

edition of the Principia written in the 1690s, Newton linked up this supposed non-inertial characteristic of the “m aterial” aether with other aspects of his theory of matter and with his doctrine o f “essential” qualities to form a gener­ alized refutation of the “m aterial” aether hypothesis.

Theory o f m atter and the doctrine of “essential” qualities got hitched up with the critical consideration sim ply because the aether being “m aterial” substance had to conform to these standards. In the D e grav N ew ton differ- entiates betw een “space” and “body” , by using am ong other characteristics such as “resistance to penetration” and “hardness” etc. - that bodies offer som e sort o f resistance, albeit in varying degree depending upon their inter­ nal construction. In the Rule III o f the Rules o f R easoning in P hilosophy o f the second edition o f Principia, N ew ton once again includes “h ardness” and “im penetrability” am ong the five "universal qualities o f the bodies” .22 The “universal qualities” or the “essential qualities” are them selves defined here as those properties o f bodies that are given to “neither intensification nor rem ission o f degree, and which are found to belong to all bodies w ithin the reach o f our experim ents” .23 Here was the “m aterial” aether that penetrated all bodies with ease, seeped into the B oyle’s apparatus when everything else was em ptied out o f it and was not supposed to offer any hindrance to bodies travelling through it. In not having “inertia” and not offering any sort of

resistance to penetration, the “m aterial” aether violated som e o f “essential” traits o f m atter. Besides N ew ton’s m uch m aligned C onversion H ypothesis or the theory that all m atter had a com m on “m aterial” substratum , was fun­ dam entally based on the aether-m atter convertibility hypothesis. If the “m ate­ rial” aether substance was itself devoid o f these “essential” traits o f “m atter” then could the C onversion H ypothesis be justified? Could aether that itself lacked “im penetrability” and inertial characteristic, condense into com m on m atter? It has becom e alm ost a convention to attribute the origins o f N ew ­ to n ’s conversion hypothesis to his b elief in alchem y. Y et we see that the aether-m atter conversion hypothesis - which probably was at the root of the “com m on m aterial substratum ” - b elief - em erging from the dem ands o f the gravitational explanations in his early w ritings in the W aste Book. W hat happened to the great quantity o f aether that was perpetually converging on the bodies? W e see N ew ton’s discom fort in handling this question in the

Q uestiones - the bodies would “ sw ell” up or do they have hidden cavities?24

A nd we see this sam e question loom ing up again when in 1690s Huygens considered F atio ’s “m aterial” aether explanation o f gravity.25 It had to be absorbed into these bodies, at least a large part o f it, if it has to be reflected back in a lesser “consistency” . It is possible that this hypothesis o f continu­ ous absorption o f aether by bodies was w hat caused N ew ton to postulate the hypothesis o f aether-m atter conversion and subsequently led him to the generalized conversion hypothesis. At least from the earliest texts onwards, like the Q uestiones we see that the “m aterial” aether was the “m aterial sub­ stratum ” that condensed into m atter. In the H ypothesis and in the O ldenberg

L etter it becam e the elem ental prop to the fram e on nature, for, as Newton

says, the “fram e o f nature m ay be nothing but various contextures o f some certain aetherial spirits or vapours condensed as it were by p recipitation” .26 It also explains why New ton linked up the conversion hypothesis with the refutation of the “m aterial” aether hypothesis in the 1690s. A D raft Revision o f the Corollary III, Proposition VI o f Book III o f the P rin cipia , that New ton w rote in 1690s in connection with the planned but later abandoned second edition o f the Principia, N ew ton interlinked all the objections against the “ m aterial” aether hypothesis to form an unified critique:

”If anyone should deny these hypothesis and have recourse to a third hypothesis, namely that one admits more matter with no gravity by which gravity o f perceptible matter may be explained: it is necessary for him to assert two kinds o f solid particles which cannot be transmuted into one another: the one (kind) o f denser (particle) which are heavy (have gravity) in proportion to the quantity o f matter, and out o f which all matter with gravity and con­ sequently the whole perceptible world is compounded and other (kind) o f less dense particles which have to be the cause o f the gravity o f the denser one but them selves have no gravity, lest their gravity might have to be explained by a third kind and that (again by fourth and) so on to infinity. But these have to be very much less dense so as by their action shake apart and mutually scatter the dense ones: by which means all bodies composed o f the denser one

Newton’s Material Aether 79 would be quickly dissolved. And since the action o f the less dense upon the denser w ill have been proportional to surface o f the denser, w hile gravity arises from that action and is in proportion to the matter o f which the denser ones consist, it is necessary that the surface o f the denser ones must be in proportion to their solid content, and therefore that all those particles must be equally dense and that they can neither be broken nor worn away nor in any manner de­ stroyed: or else the ratio o f the surface to the solid content, and consequently the ratio o f the gravity to the quantity o f matter would be changed. Therefore one must altogether determine that the denser particles cannot be changed into less dense ones, and thereupon there are two kind o f particles, and these cannot pass into one another”.27

T he hypotheses that N ew ton is alluding to in the first sentence are the w ell-know n H ypothesis III - or the “transm utation” hypothesis o f the first edition o f the P rincipia and another hypothesis o f the D ra ft w hich later becam e the Rule III of the second edition o f the Principia w hich w e have quoted above. The passage has been variously interpreted. W hat N ew ton is pointing out here is that a conceptual disharm ony exists betw een various com ponents of “m aterial” aether explanation, e.g., the “p h y sical” ch arac­ teristics o f the “m aterial” aether, the “essential” qualities doctrine o f the

P rincipia, the conversion hypothesis etc. T hat this leads to an anom alous

epistem ic situation w ithin the netw ork of theories used for explanation of gravitation is obvious. The passage shows that even if the “essential” q u ali­ ties doctrine is abandoned, the variable density aether hypothesis still leads to an infinite regress and com es into conflict w ith the conversion hypothesis on considerations independent o f “essential” qualities doctrine.

T he passage quoted above shows us the com plex variety o f factors that together determ ined the unacceptability o f the “m aterial” aether hypothesis. In w hat sense w as it correct to assum e the aether substance was “m aterial” if it violated m ost o f the know n “essential” characteristic o f “m atter” ? Could this aether that lacked the ability to offer any resistance to bodies be v isu ­ alized to “m echanically” interact with bodies? T hese w ere w eighty co n ­ siderations, going far beyond the operational - m echanistic aspect o f the explanations and, although discussed in terms of “internal consistency” , they actually touched upon larger questions dealing with the basic dom ain of all the gravitational explanations. W hat was to be considered natural and legitimate

mode o f interaction between ‘‘physical entities”, and what was the m eaning o f

“m echanical” action in this context? Newton surely did not satisfy him self entirely on all the points, for as we know he abandoned this “refutation” of the “m aterial” aether and went back to the “universal electrical aether” after 1707. Perhaps, as Prof. Guerlac has suggested, owing to H auksbee’s and D esaguliers’ electrical experiments Newton once again started entertaining the possibility that aether could be considered as an “experim ental” entity. There were certainly some aspects in these experiments which convinced New ton that the “electrical spirits”, however unsubstantial and intangible, m ay be treated ass a “m echanical, experim ental, phenom enon.”

A com plete abandonm ent o f the “m aterial” aether cam e m uch later, som ew here around late 1716, at the end o f the second phase o f aether w rit­ ings, w hen N ew ton penned dow n some O bservations on the electrical ex ­ perim ents as a revision o f the texts intended for inclusion in the 1717-18 edition o f the Opticks. There is also another docum ent w ritten during 1716 and m eant to be published with the Book III o f the third edition o f the

P rincipia, w hich further substantiates the view that N ew ton at this period

not only abandoned the “m aterial” aether, but also considered aether as such outside the realm s o f “phenom enon” . In two very im portant studies o f N ew ­ to n ’s gravitational writings M cG uire (1966 & 1968) has exam ined both these docum ents. Follow ing im portant points em erge from these tw o docum ents: (a) N ew ton once again m akes a strong distinction betw een “body” and “space” (or aether) on the grounds o f essential qualities o f im penetrability, (b) rem arks on the highly speculative character o f the conversion hypothesis and, (c) places the aether outside the lim its o f “phenom enon” . In one part o f the O bservation I New ton writes: “To distinguish this m edium from the bodies w hich flote in it, & from their effluvia & em anetion & from Air, I will henceforth call it A ether & by the word bodies I will understand bodies w hich flote in it, taking this nam e not in the sense o f the m odern m etaphys­ ician, but in the sense of com m on people & leaving it to the m etaphysicians to dispute w hether the aether and bodies can be changed into one another”28. In a sim ilar spirit the other docum ent goes on to define “bo dy ” as everything that can be “m oved”, “touched” and offers “resistance to tangible things” . The gulf betw een “aether” and “m atter” are now unbridgeable and Newton notes in this docum ent: “The subtle m atter in which planets flote, and in w hich bodies m ove w ithout resistance is not a phenom enon.” 29

VI. Concluding Rem arks

The original purpose with w hich we set upon this rather long exam ina­ tion of N ew ton’s text may now be recalled again: (a) to reconstruct the conceptual unity within the “m aterial” aether program , and (b) to explore the relationship betw een “descriptive” epistem ology and “rational recon­ struction” . As a subject of case study N ew ton ’s “m aterial” aether program has som e interesting features. “M aterial” aether program being essentially a degenerate program - in the sense that the entire program was eventually refuted and banished out of physical explanation - poses a special challenge for forw ard-looking character o f philosophical reconstruction. Can we learn som e m ethodological lessons o f general value from this defunct structure? I w ould be taking this up presently. On the other hand, contact action, which lay at the foundation of the m aterial” aether program was the only m echa­ nism that was transparent, im m ediately intelligible and intuitively acceptable

Newton ’s Material Aether ₈₁ am ong all the alternative explanations. Y et we find that the counter-intuitive program s generally flourished. W hile the actio in distans found m uch favor in the 17th & 18th century and a very devious ju stificatio n from K ant and Priestley, and the basic presupposition o f the other program - that space can be considered an active determ inant o f m otion - eventually found inco rp ora­ tion into our contem porary physics. This rather well dem onstrates th e lim it to w hich the intuitive criteria o f logicality and o f logical transp arency /im ­ m ediacy can be considered a factor o f validity o f hypotheses in physical sciences. W e will be elaborating on this point a little later. T hree m ain co n­ clusions that I w ould like to draw from our study, having bearing on the question o f “rational reconstruction” as well as on the problem o f “dis­ covery” are presented below.

(1) One o f the greatest difficulties in philosophical reconstruction o f N ew ton’s texts is to m ake explicit the im plications o f different concepts and to establish conceptual linkages betw een different theoretical entities. In this being totally faithful to N ew tonian texts is not very helpful, as N ew ton uses a num ber o f these ideas in a “tacit” way, not alw ays elaborating on all the im plications. A num ber o f exam ples can be cited to illustrate this difficulty. In N ew to n ’s writings on absolute space, for instance, term s like “fram e of reference” and “inertial system ” do not occur - this term inology was created by further elaboration o f the im plicit aspects o f N ew tonian absolute space. Y et reading betw een the lines o f the Scholium on absolute space in the

Principia, and o f the texts o f De gravitatione, we see a num ber o f im p lica­

tions as w ell as alternative form ulations of w hat w as “disco v ered ” later in the p o st fa c to analysis. A philosophical clarification o f the significance o f the idea o f absolute space in N ew to n’s system therefore calls fo r an im agi­ native conceptual reconstruction o f the m eaning o f the texts and not m ere dogm atic faithfulness to the texts. This, I believe, in a w ay distinguishes the philosophical reconstruction or the “rational reconstruction” from the h is­ torical “descriptive” epistem ology w here it is custom ary to attach great sig ­ nificance to textual com parison.

A nother exam ple o f this distinction is also w ell illustrated by the q u es­ tion o f elasticity o f the “m aterial” aether, w e have dealt w ith in our text briefly. Is it possible that N ew ton, w ho - as it is generally know n - was really fussy about details, did not ponder over the question that if aether particles m erely floated around in void than w herefrom the com posite “m e­ dium ” got its characteristic elasticity? Prof. R osenfeld is p erfectly right in suggesting that the answ er to this question lay beyond the conceptual hori­ zons o f New ton and of the 17th century physics. This how ever did not detract N ew ton from attem pting to solve the problem and considering various p o ssi­ bilities. H ow ever the possibilities N ew ton considered can only be w orked out by carefully considering the im plications o f interconnection o f various hypothesis/ideas N ew ton introduced around 1690s, when he attem pted to

grapple w ith this com plex question. In the text o f this paper I have already outlined a fram ew ork w hich inter-connects the different key ideas that N ew ­ ton em phasized in the 1960s, e.g., the “double force”, the vacuity o f the universe, the connection betw een the internal architecture o f m atter and its physical properties etc. Y et we do not see all the links in the chain very clearly until we refer back to the texts o f De aere et a e th e r, w here New ton uses the concept o f actio in distans to develop a pulsating and springy aerial m edium .

But why w ere these aspects considered im plicitly? H istorians have often suggested that it was custom ary o f N ew ton to treat speculative aspects o f m aterial aether and o f theory o f m atter w ith extrem e caution - exam ple: N ew ton ’s treatm ent o f “double force” hypothesis in the C onclusio and the eventual w ithdraw al o f the Conclusio from publication. T he sam e can be said o f the hypothesis o f “im m aterial” aether - the idea that space could be considered an “active” participant in determ ining m otion o f the bodies was so radical and so m uch against the accepted tradition that N ew ton chose to articulate it only with som e reservations. But his is not all. M any o f the ideas/concepts that N ew ton treated im plicitly w ere pregnant with p ossibili­ ties and the discussion often apprehend future lines o f investigations includ­ ing future discoveries - e.g., treatm ent o f absolute space in the De gravita-

tione, the speculations on nature o f light in the m any versions o f the Q ueries

o f the Opticks. This “latent” or the “tacit” aspect is, I think, a basic feature o f all types o f creative scientific activity. The philosophical “reconstruction” being m uch m ore forw ard-looking enterprise attem pts to capture this dim en­ sion, often w ithout sticking to the texts dogm atically and seldom with any regards for the chronology.

(2) W e have m ade an attem pt to trace the long chain o f argum ents through w hich N ew ton tried to establish and later refute the “m aterial” aether hypothesis. In dealing with this we see that in our contem porary m ethodo­ logical literature an exaggerated am ount o f im portance has been given to the form s o f inference, i.e., if the inference could be reduced to an induction (am ong others R eichenbach’s program ) or if a valid inference it should fit the hypothetic-deductive form . N ew ton’s argum ents are usually elegant. In establishing the generalized characteristics o f the aether, he usually argues from the “know n” and “observed” to a certain general conclusions and then, under the supposition o f “consistency of nature” , applies these general co n­ clusions to develop the m odels and m echanism s o f unobservable m icro-phe­ nom enon (the “invisible realm ” ). In case o f refutation o f the “m aterial” aether we see a different pattern o f argum entation. H ere a w hole ensem ble o f concepts are put to test by assum ing a thesis to be correct and then dem onstrating that (a) it leads to logical absurdity, e.g., infinite regress, (b) that it results in incom patibility with observed results, and (c) it leads to inconsistency am ong supporting hypotheses. Looking at the nature of argu­

Newton ’s Material Aether ₈₃ m entation, one w ould be hard put to say, even for this case o f a “degenerate” research program , that the process o f argum entation was faulty and it does not seem very relevant to ask if it w ould fit the hypothetic-deductive o r the inductive pattern. W e therefore feel that in dealing w ith broader m eth od o­ logical questions and in dealing w ith the issues o f theory assessm ent, the criteria o f “form o f argum entation” should not be given the central position that it occupies today. The em phasis should rather shift on actual ex am in a­ tion o f prem ises o f argum ents, with the im plicit heuristic and w ith the m o d ­ ular articulation o f the “logic o f the situation” .

(3) Lastly, an interconnected issue is that of counter-intuitive hypothesis, which - as we have already mentioned - demonstrates the lim itations o f ap­ plicability o f methodological regulae in physical sciences rather well. Form al methodological regulae mostly tend to take for granted logical transparency and immediacy. If, however, the empirical results tend to support a generally counter-intuitive solution the situation is either judged (a) as anomalous and totally unintelligible (Huygens’ and Leibniz’s response to the possibility of actio

in distans), (b) as an incomplete description, needing further elaboration (N ew ­

ton’s attitude to the “phenomenological” program), and (c) or a situation need­ ing a critical review and a new modular logic (K ant’s exam ination o f New tonian m echanics and Reichenbach’s (Putnam ’s plea for a three valued logic for Q uan­ tum M echanics etc.). The developments in physical sciences have shown that there are no special reasons why an absolute preference m ust be given to trans­ parent and immediately intelligible hypothesis. This is also one strong reason why the “discovery m achine” or the algorithm o f discovery approaches are untenable. General reason is o f course that we cannot have any prior logic for prediction of all possible empirical results', it is even more difficult to imagine that we will ever have prior modular justification of something that we have never encountered. A counter-intuitive hypothesis therefore, is a point o f asym ­ metry in the explanatory complex, which bifurcates the predictive logic, chang­ ing its character fundamentally.

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