The Thomist 68

success

fail

Feb	MAR	Jun
	05
2011	2012	2017

2 captures

05 Mar 2012 - 27 Jun 2017

About this capture

Proponents of the perennial philosophy tend to be embarrassed by its natural science, and this is, to some extent, understandable. That the progress of the sciences in the past four hundred years coincided with a widespread repudiation of Aristotle's philosophy in general, and his natural philosophy in particular, is not coincidental. As the natural philosophers of the 1600s looked at nature more and more closely, evidence began to accumulate that much of what Aristotle thought was true about nature was not. Perhaps, many suspected, none of it was true.

The most obvious instance of this challenging of Aristotelian natural philosophy came from the Copernican revolution, in which the Earth was elevated from the status of an immobile lump of dross at the center or bottom of the universe to that of "planet," one of the heavenly bodies orbiting the immobile sun.⁽¹⁾ Our promotion seemed to fly in the face of Aristotle's now frequently derided bifurcation of nature into two regions, the celestial and the terrestrial (or more accurately, the supra- and

sublunary), each corresponding to two radically different kinds of matter: aether and the familiar Empedoclean elements. If the Earth is just another one the planets, and the planets and the other stars are the only sensible evidence of Aristotle's aethereal substance, then the case for positing aether weakens; the planets, or "wandering stars,"⁽²⁾ are no more aethereal than is the ordinary kind of matter with which we are intimately familiar.

That was the beginning of the end for Aristotle's incorruptible aether. Although the notion that an aether was still needed as a medium for conveying gravitational and electromagnetic forces would occasionally surface, by the end of the nineteenth century the view prevailing among experimental scientists was that aether was superfluous. Modern-day Thomists and disciples of Aristotle were forced to choose between clinging to doctrines against which the entire scientific community was arrayed, and admitting that their masters were egregiously mistaken in a large part of their philosophy. That many have taken the latter path, trying to ameliorate the situation by claiming that Aristotle's natural philosophy is not foundational for his metaphysics or ethics, or by insisting that St. Thomas Aquinas's philosophy, unlike that of Aristotle, is essentially metaphysical or theological, is well known. This essay, however, will, in a manner of speaking, take the former path, arguing that while experimental science has indeed made a definitive case against certain particularities of Aristotle's aether, the existence of some kind of aether, one not entirely unlike his celestial matter, has not yet been refuted. Indeed, a positive case can be made in favor of it still, a case based upon recent developments within experimental science itself. In short, we will argue that there was a real insight driving the Philosopher's claim that to explain the cosmos more is needed than just the sort of matter that we can touch and grip in our

hands, an insight of which contemporary physicists and philosphers of nature are beginning to catch a glimpse.

In order to appreciate both the degree to which Aristotle's aether has been rejected and the degree to which it has been resuscitated and modified by recent physics, this paper will be divided into three parts. We will first examine the nature and properties of Aristotle's aether, summarizing what arguments lead him to posit its existence, and evaluating the strength of these arguments, both in themselves and according to Aristotle. However, because the Copernican revolution was only an implicit attack on aether, while the most direct and successful challenge to it (via a modern stand-in, the luminiferous aether of electro-magnetism) was the Michelson-Morley experiment of 1887, in the second part we will briefly recount the fate of aether in the hands of early modern natural philosophers, and its rejection as a result of this experiment and Einstein's special theory of relativity. Lastly, we will focus on the twentieth century's gradual recognition of a critical need for aether in explaining both the very small--in the so-called vacuum of quantum electrodynamics--and the very large--in the curved space-time of the general theory of relativity and astrophysics's recent postulation of a cosmological constant.

In the first book of De Caelo, his only extended discussion and defense of aether as such, Aristotle offers what might be characterized as four or five different arguments for aether's existence, concluding with

Thus reasoning from all of these things, we come to believe that there is something besides the bodies nearby and around us, something other than and

separate from them, something having a more honorable nature to the degree that it is distant from the world at hand.⁽³⁾

Rather than expound each argument in detail, we will present an overview of the reasoning as Aristotle's attempt to explain certain observed facts about the heavens, facts that modern man is so habituated to explaining away that he finds it difficult even to notice their peculiarity.

Although Aristotle invokes premises about the perfection, simplicity, and priority of certain kinds of local motion, the principal datum of nature that he wishes to explain with aether can be experienced firsthand by spending the night under the stars and watching their motion as the night hours pass. One finds himself at the center of a perfectly circular pilgrimage of stars traveling from east to west, as though each of the heavenly bodies were embedded on a dark orb revolving around the Earth. This nightly, and a related yearly, uniform circular motion of the stars should provoke a question: Why should this apparently natural motion occur in the sky, indeed in most of the cosmos,⁽⁴⁾ but not here below, where few things seem to move in circles without being coerced? This peculiarity is all the more striking when one notices that these same heavenly bodies and their motions are never seen to change, much less corrupt or cease--perhaps the reason why, Aristotle suggests, the heavenly matter was named aether, from aei thein, "always running."⁽⁵⁾ This appearance of

eternity and incorruptibility is strengthened by the astronomical records Aristotle has at his disposal: "For in all time gone by, according to all records handed on from one [generation] to the next, no change has ever appeared either in the whole of the containing heaven or in any proper part of it."⁽⁶⁾ To assume that a radical difference between natural motions in the terrestrial region and those in the heavenly does not derive from a radical difference in the natures of the bodies in these regions appears foolish.

Besides clarifying the aether doctrine, however, this line of reasoning also suggests its weaknesses, especially for the modern reader. Perhaps, one could suggest to Aristotle, the Philosopher has not observed the heavenly motion long enough to make the judgment that it is moving uniformly and is incorruptible; perhaps such perfect circular motion could be accomplished by ordinary matter by way of various combinations of rectilinear motions; and, most importantly, perhaps it is the Earth itself rather than the heavens that has the daily rotation and yearly orbit.⁽⁷⁾ While Aristotle is often accused of being insufficiently empirical in his

study of nature, perhaps here just the opposite is the case; perhaps he is relying too much on (mere) appearances.⁽⁸⁾

With this in mind it is worth recalling what kind of certitude Aristotle claims to be offering here. Unlike the core arguments and principles in the more foundational Physics, the arguments in De Caelo are consistently and explicitly characterized by Aristotle as tentative and merely probable.⁽⁹⁾ For example, in asking why the heavens rotate from east to west rather than the opposite, he pauses:

Now, it may be objected that to try to explain everything without distinction appears to be a sign either of excessive foolishness or of excessive zeal. But this criticism is not always equally just. Rather, one must see what cause there is for saying something, and further, what sort of belief in it one may have, whether it be [merely] human or something more unassailable. Thus, although if someone ever chances upon more strictly necessary [accounts], one must be grateful to him; nevertheless for now one must state how things appear.⁽¹⁰⁾

Sometimes we must be satisfied with a "consistent account that merely harmonizes with our suspicions," restraining these suspi-cions so that "the appearances are always lorded over by sense."⁽¹¹⁾

Aristotle likewise implies, when he summarizes the conclusions reached about aether's nature in the first book, that the case for the heavenly substance in particular, while assumed throughout most of De Caelo, is also tenuous: "Taking belief from the things said, [we must say] that the entire heaven was not generated nor can it be destroyed (as some say), and that it is one and eternal."⁽¹²⁾ Similar language of belief, or tentative conviction, is present in the passage quoted at the beginning of this section, when Aristotle concludes the arguments for aether by saying that we have reason

"to believe that there is something besides the bodies nearby and around us."⁽¹³⁾ Aristotle refuses to characterize the case for aether as a strict demonstration, leaving open the possibility that his reader has not been convinced and may simply have to assume or hypothesize the existence of aether to understand the rest of the work. He explains this ambiguity in the same discussion, before noting the aforementioned astronomical records, when he says that the immutability of the heavenly substance "follows from the senses, at least sufficiently to speak on behalf of human belief [proV" ajnqrwpivnhn pivstin]."⁽¹⁴⁾

Hence, even with an imperfect foundation, Aristotle encourages us to strive to understand the nature of the heavenly bodies as best we can, given the intrinsic desirability of the subject matter:

It is good to inquire about these things and so to deepen our understanding, although we have little to go on and we are situated at such a great distance from the attributes of these things. Nevertheless, from contemplating such things nothing [we infer] should seem to be unreasonable, holding them now as fraught with difficulties.⁽¹⁵⁾

Here Aristotle points out the reason that we are restricted to mere "human belief" when we try to discern the nature of the heavens: Like detectives without witnesses, we have little to go on. He elucidates this later when he says, "We are far away from the things we are trying to inquire into, far away not only in place but more so in that we have sensation of exceedingly few of their accidents."⁽¹⁶⁾ We sense little to nothing of the heavens; with the exception of the luminous stars and planets, we perceive none of the properly sensible attributes (e.g., color, smell, and sound) in aether, but only some of the common ones (e.g., magnitude and motion), which latter are in fact difficult to detect without the

former.⁽¹⁷⁾ Nor is this limitation simply in the acuity of our sense powers; by definition aether, an invisible, incorruptible body, has little in common with ordinary matter. As will be shown below, it cannot be acted upon by ordinary matter and can act on ordinary matter and our senses only indirectly or in a hidden way. Thus, aether's hypothesized nature must be detectable via argument, not through mere experience.

This leads us to an implicit, and in fact more certain, reason Aristotle thinks that the four elements are insufficient to explain the heavens. In writing De Caelo, Aristotle assumes that the reader understands and accepts his arguments from the fourth book of the Physics,⁽¹⁸⁾ namely, that a void, a region not filled by a material substance, is not physically possible.⁽¹⁹⁾ After a careful considera-tion of what place is and what void would have to be, Aristotle sets out a number of arguments, some merely dialectical and others more decisive,⁽²⁰⁾ to show that it is impossible that there be

a place or quantity without a subject or material in which to inhere. Thus any region that appears empty must not be so. Looking to the heavens, then, we conclude that the vast expanse between the visible heavenly bodies and the world in which we live, the region that the life-giving light of the sun must traverse to allow animals to see and plants to grow, must itself be filled with an invisible or, better, a transparent medium. Not only are the stars and planets made of a different kind of substance, but--given that such perfect transparency is present in something that manifests no signs of ordinary matter's downward or upward tendency, but either is perfectly yielding to the visible circular motion of the stars and planets, or moves with them--so must be the subtle matter surrounding them. Thus, Aristotle applies the name "aether," or more frequently, "the first body,"⁽²¹⁾ to whatever fills the volume of space between the moon and the outermost sphere of the fixed stars. It is itself "the heaven . . . the continuous body in the place after the outermost circumference of the whole,

in which are the moon, and the sun, and some of the stars [i.e., the planets]."⁽²²⁾ Likewise this is the body whose existence Aristotle is trying to make probable in the opening chapters of De Caelo.⁽²³⁾

While an adequate defense of Aristotle's critique of void would take us somewhat afield,⁽²⁴⁾ it is crucial at least to notice that this critique is not merely the scaffolding but more the infrastructure of the general case for aether. The heavens appear to be empty, yet we may offer compelling arguments that such an emptiness is physically impossible. Thus there should be a prima facie case in favor of an essentially insensible substance that pervades apparent voids, an aether. In the modern context this manner of inter-preting De Caelo 1.2 is all the more noteworthy, for since Aristotle's time we have taken a closer look at the heavens, and in the past half century we have literally gotten within arm's reach of the hypothetical aether. Nonetheless, the implicit results are the same now as they were when Aristotle looked up at the night sky: We lack evidence of any tangible matter, besides stars and space dust, filling outer space.

Although Aristotle admits that there is little we can sense about aether, from the arguments for its existence he finds that he can infer certain properties of it that emphasize how different it is from ordinary matter. In order better to assess its likeness with and difference from recent physics's version of aether, then, it will be helpful to sharpen our image of Aristotle's aether by sum-marizing some of its distinctive marks and how Aristotle is driven to them.

Since the heavenly substance is first apparent to us in virtue of its enduring circular motion, Aristotle immediately deduces three properties of the aether, two pertaining to its motion, and one pertaining to its endurance.⁽²⁵⁾ Because we clearly see that the heavens have a perpetual, and therefore probably natural, circular motion, and not an upward or downward rectilinear motion, we may take as corollaries that celestial matter is simple, that is, not a compound of elements, and that it is neither heavy nor light. Given that the circle is the simplest possible shape and that the path of this motion is perfectly circular, it seems that it would be effected by only one internal principle or cause, that is, by only one elementary nature; so, aether must be simple.⁽²⁶⁾ Likewise, defining heaviness and levity as natural inclinations toward or away from the center of the Earth, and seeing signs only of a natural inclination to rotate around the center, it seems "im-possible that the body being borne in a circle has heaviness or levity."⁽²⁷⁾

Aristotle infers a third mark of aether when he says, "Likewise, then, with good reason we may posit concerning [the heavenly

substance] that it is ungenerable and incorruptible, and that it is not capable of growth or alteration."⁽²⁸⁾ Although aether is evidently subject to change in place, it is not subject to changes in substance, quantity, or quality. Whatever undergoes a substantial change, Aristotle points out, does so because it is acted upon by a body with a contrary nature, while aether, whose only positive formal attribute noted so far is its circular motion, has no contrary, since circular motion has no contrary.⁽²⁹⁾ For the same reason, since growth always involves substantial change between opposites, change in the size of aether is not possible; and since alteration is ordered toward substantial change and involves expansion or contraction, "then just as the body with circular [motion] is not capable of growth or diminution, it is quite reasonable also [to say] it is inalterable."⁽³⁰⁾ With a reminder that this depends on whether "one has trust in those things laid down before," Aristotle reinforces the conclusion that the aether is immutable by saying, "the account, it seems, bears witness to the appearances and the appearances to the account."⁽³¹⁾ He notes that all men have thought the heavens divine, and therefore immortal, and have even named the heavens with its eternity in mind, calling it (as we said before) the aei thein, "the always running." Perhaps most compellingly, Aristotle emphasizes in a passage quoted earlier that in the extensive and precise Babylonian, Persian, and Greek records of the positions and risings of the various constellations, no star has been observed to deviate from its

ordinary motion, to slow down or speed up, much less simply to go out or come into existence.⁽³²⁾

Besides going on to argue that the heavens are finite in extension and that the incorruptibility of the aether is part of what makes possible the eternity of the cosmos as a whole,⁽³³⁾ this is all Aristotle concludes about the matter of the heavens in the first book of De Caelo. However, from these few properties of the aether--although they are almost entirely negative (i.e., what aether is not)⁽³⁴⁾--in other contexts Aristotle and his disciples, most notably St. Thomas,⁽³⁵⁾ infer more corollary attributes of this new matter. Insofar as they shed further light on this almost inscrutable substance, we will briefly enumerate and explain them.

If aether is incorruptible two conclusions follow right away, one pertaining to its substantial principles and the other pertaining to its qualities. First, aether's prime matter and substantial form must be so perfectly united that the latter must actualize and thereby exhaust the potency of the former, insofar as an incorruptible body by definition must lack the potential to become anything else; aether must possess a "certain total and universal perfection" that thoroughly fulfills its potency for existence.⁽³⁶⁾ Indeed, if one were not to distinguish fulfilled and

unfulfilled potencies, one might be tempted to say that the heavenly substance has no prime matter. More accurately, however, one should conclude that, unlike sublunary composites, aether's prime matter is always perfectly fulfilled, so it is inseparable from its form, and in this sense is not really distinct from it. Likewise, since its prime matter would not be a principle of aether's coming to be, but only of its being, it would not be the same sort of prime matter that is a principle of mundane substances (which is a principle both of coming to be and of being); it would be called prime matter only analogously.⁽³⁷⁾

Further, anything that cannot be destroyed or even altered qualitatively must somehow be intangible. Aether must lack and not be susceptible to the action of the tangible qualities of temperature and pressure. If aether were cool, for instance, then it could be heated up by the immediate contact of a hot body, and likewise pressure exerted upon it by any contiguous sublunary body over time would incline it toward destruction, or at

minimum would alter it in some measure.⁽³⁸⁾ Both of these results are impossible if the celestial body is wholly immutable, and thus elsewhere Aristotle emphasizes that wherever there is no shared matter there cannot be mutual agency.⁽³⁹⁾ But if aether cannot be pressed upon by ordinary matter, then if some body were to try to press upon it, that body would cut right through the aether unhindered; even more than the ever-present yet barely noticeable medium of air through which we walk and run, the aether would yield and be cleft without any resistance. Paradoxically put, being wholly impervious to alteration entails that aether be perfectly pervious to something trying to press upon it.⁽⁴⁰⁾

These properties remain largely negative. There is a related, more positive avenue for detecting distinctive marks of aether. Besides its circular motion, one other aspect of aether might

almost be called sensible in virtue of the noticeable connection between certain heavenly and terrestrial motions, for example, seasonal changes and lunar tides. Even in Aristotle's time it was evident that these latter were the effects of the relative positions and motions of the sun and the moon, respectively.⁽⁴¹⁾ In addition, the heavenly bodies' constant illumination of things here below shows that the aethereal agency emanates not only from the sun and the moon, but from the stars too, and that this agency must communicate itself across the tremendous expanse of the aether to reach us. Further, Aristotle and St. Thomas would argue, the aethereal substance would produce the seasons by being the cause not only of the changes in the length and temperature of the day, but also in some way of the cycle of life itself, the blooming of vegetation and the seasonal generation of animals. As Aristotle puts it in a well-known but cryptic passage, "Man and the sun generate man";⁽⁴²⁾ the heavenly substance, due to the perfection of its form, appears to be the ultimate physical cause not only of the seasons but also of all terrestrial change.⁽⁴³⁾

Saint Thomas, developing Aristotle's aether doctrine and perhaps suspecting that the fact that aether both illuminates the Earth and simultaneously effects generation is not a coincidence, suggests that it is precisely in virtue of its luminescence that heavenly matter acts upon ordinary matter. Thus he states, "the powers of the heavenly bodies are participated in by the inferior bodies by the mediating of light."⁽⁴⁴⁾ When we recall that Aristotle and St. Thomas understood light to be the "act of the transparent as such," we see that on this account light is present not merely in

the stars but in the entirety of the aether.⁽⁴⁵⁾ Indeed, if we consider that nothing around us is perfectly transparent--one can see only so far even through air--and that the distance between the Earth and the stars is almost inconceivable,⁽⁴⁶⁾ one sees that aether must be the most perfectly transparent substance in the cosmos. Thus St. Thomas will conclude that transparency is not only the active but also the proper quality of the heavens; all other bodies are called transparent only by participation in the nature of aether, the way other things are hot by participation in the nature of fire.⁽⁴⁷⁾ Further still, if transparency, rather than being merely a privation of color, is a positive nature, as Aristotle believes,⁽⁴⁸⁾ we have found our first positive quasi-sensible quality in the aether: its supreme transparency and, in some cases, luminescence.⁽⁴⁹⁾

Further, considering aether's universal agency in conjunction with its immutability, two critical consequences follow. First, we have implicitly granted that aether acts upon ordinary matter and, since it is inalterable, that it is not acted upon in turn by it; aether can "push" on ordinary matter without being "pushed back." Thus Aristotle will conclude that

While usually the thing touching is touched by what it touches--for nearly all the things we come upon move while also being moved, and in these cases it is necessary and apparent that the thing touching is touched by what it touches--still it also occurs (as we sometimes say) that only the mover may touch the moved, while the thing touched does not touch the one touching it. But because things of the same kind are moved [in return] when they move others, it seems to be necessary that [movers] be touched by what they touch. Whence

if something unmoved moves another, although it will touch the thing moved, nothing [will touch] it.⁽⁵⁰⁾

Bodies act upon each other by touching, whence it follows that they are simultaneously acted upon [in return], since what touches is acted upon. But this should be understood [only] when there is mutual contact [mutuus tactus], as happens in those things that share in a common matter, each of which is being acted upon by the other while they are touching each other. The heavenly bodies, however, because they do not share a common matter with inferior [i.e., sublunary] bodies, act upon them such that they are not acted upon by them [in return]; they touch and are not touched.⁽⁵¹⁾

This one-way contact and causality may seem absurd at first glance. While the doctrine is contrary to our Newtonian "equal and opposite reaction" prejudices, Aristotle argues persuasively that it is not impossible, as motion is an actuality not in the mover as such but in the mobile. Thus it is not essential to the notion of one thing's moving another that the latter move the former in response.⁽⁵²⁾ Indeed, the cause of such agency being only one-way is the aether's incorruptibility, and therefore, at least in part, the aforementioned difference between its prime matter and ours.

Second, while aether lacks the qualities of sublunary matter, it must be able to generate them in the latter; that is, aether must be an equivocal cause, possessing the predicates it gives its patients in an equivocal, or more accurately, analogous, and therefore

higher, or intentional, mode of existence.⁽⁵³⁾ For example, the sun induces heat in the bodies around us, but it is not itself hot (for its heat would eventually consume it, and also it would have a natural motion upward), so it must possess heat according to a different ratio, in such a way that heat does not inform the aethereal matter.⁽⁵⁴⁾ Likewise, then, St. Thomas says that aether is so radically unlike sublunary matter that any predicates that the two have in common will be only analogous:

Many things that are not equivocal according to the abstracted consideration of either the logician or the mathematician nevertheless are in a certain mode said equivocally according to the concrete notion of the physicist applying [them] to matter. For such [predicates or forms] are not received according to the same notion in every matter whatsoever, just as it happens that one does not find quantity and the unity that is the principle of number according to the same notion [rationem] in the celestial bodies and in fire, in air, and in water. . . . Some equivocations, however, are proximate, on account of an agreement in genus, just as if "body" be said of the celestial body and of a corruptible body, it is said equivocally, speaking according to the natures of things [naturaliter loquendo], since their matter is not one. Nevertheless, they agree in logical genus [in genere logico], and on account of this agreement of genus appear not to be wholly equivocal. . . . Whence, on account of this proximity of genus or of likeness, they do not appear to be equivocations, while nevertheless they are.⁽⁵⁵⁾

The student of nature, paying attention not merely to the abstracted and most generic consideration of a form--the playing field of the logician or mathematician--will study the being of the specific nature of a physical substance and, if necessary, will

qualify the logician's univocal predication of some terms. Although an investigation of the relationship of the logician and the physicist, and of that between logical and natural genera, is beyond the scope of this article, this much is clear: Celestial matter's heterogeneity with mundane matter requires that it be called a quantity, one, a body, and even matter and substance only by way of analogy.⁽⁵⁶⁾

A provocative example of this ambiguity about how or whether the heavens possess attributes like those of ordinary matter concerns the predication of motion, place, and time. We have seen that Aristotle attributes to aethereal matter both im-mutability, in virtue of which it endures through all ages, and circular local motion, which reflects its perfection and by which it acts upon the sublunary world. While this appears to be almost a contradiction--a perfect lack of three kinds of change but an inalterable possession of the fourth--it is not. For while Aristotle admits that aether moves locally, by granting it circular motion he mitigates its existence as a motion. What Aristotle means by circular motion is not the motions of the stars and planets, which appear to be progressive motions of bodies tracing out circles. Rather, circular motion is the motion proper to a sphere, a revolution about its axis, and the motion of the aether as a whole: a stationary rotation, not an orbit.⁽⁵⁷⁾ On this account circular motion is not strictly a change of place, but a change within a place. Only the parts of the heavenly sphere, and not the whole, can leave one place and enter another.⁽⁵⁸⁾ The most perfect and primary source of natural motion within the physical order is least

of all in motion, both because local motion is the least of all changes, and because circular local motion is least of all a local motion.⁽⁵⁹⁾ Hence there is a sort of equivocation when we predicate local motion of the heavens, and Aristotle may coherently say, as he does at the end of his discussion of place, "the periphery of that which is carried in a circle [i.e., the aether], always bearing itself in the same way, rests . . . [and] in a way moves and in a way not."⁽⁶⁰⁾

For related reasons aether does not simply speaking have a location and is only analogously in place. To the extent that heavenly matter is not univocally said to be in motion we would think it is at rest, and therefore in place, but Aristotle says that we must resist or at least qualify this inference. The root of this restraint lies in his discussion and ultimate definition of place in Physics 4 as the "first immobile limit of the containing body."⁽⁶¹⁾ Because the substance continuously filling the cosmos from the sublunary regions to the periphery obviously has no container of its own--it is the first container of everything else--then it must not have a place.⁽⁶²⁾ Being itself the ultimate source and measure of all other bodies' locations and local motions,⁽⁶³⁾ aether is not itself, properly speaking, located.

Place may, however, be attributed to aether in secondary or extended ways. Because the heavenly substance gives all other things place, place may be predicated of it according to the mode in which an effect is always somehow predicable of its per se cause, just as, for example, "healthy" is predicated of a climate conducive to health. Likewise, one can say aether is in place

because it is in itself, meaning merely that it is not contained by another.⁽⁶⁴⁾ In addition, the celestial matter can be truly said to have place, and even motion, per accidens and secundum rationem, that is, in virtue of its parts having place and motion per se.⁽⁶⁵⁾ In any of these senses, however, there is an evident loosening or extension of the meaning of the word "place"; place is being said of aether only analogously.⁽⁶⁶⁾

The other external measure of natural bodies (besides location) is time, and in this case too we find that there is a degree of equivocation when it is said of the heavens. Again looking back to Physics 4,⁽⁶⁷⁾ we see that the nature of time is that it be the number or measure of motion, and the uniformity and eternity of time come from its being the measure of local motion most of all, since only this motion, specifically circular motion, can be interminable and uniform. Thus, while aether's motion is not the same thing as time, Aristotle suggests that it seems in some way to be the proper subject of time--that is, what is being counted when one distinguishes the before and after in motion.⁽⁶⁸⁾ But if the motion of the heavens underlies time, then time cannot itself underlie and be prior to that motion.⁽⁶⁹⁾ While in an analogous sense aether may be said to be in time, meaning that it is simultaneous with time and not utterly without per se relation to it,⁽⁷⁰⁾ nevertheless its infinite duration precludes its substance being bounded or limited by ordinary temporal predicates. Thus the heavenly substance is

not in time;⁽⁷¹⁾ more properly speaking it transcends time, or is atemporal.⁽⁷²⁾

Although this ambiguous predication of motion, place, and time in aether is at first perplexing, it is a necessary consequence of making the heavens an ultimate cause and measure of place and motion in sublunary matter. The unending rotary motion of the heavens must evidently be motion and not rest--for it moves in a circle--and yet also, not motion but rest in place--for it moves in a circle. Likewise, this uniform and eternal motion is the foundation of time, so it does not exist at or for a certain time. Put another way, while aether is obviously a mobile (indeed, the primum mobile), it is somehow immobile because it is an ultimate mover. While it is obviously somewhere, it is not in place because it somehow is place. While it obviously exists now and always, it does not exist within time because somehow time exists in it.

Obviously there is little in common between the matter with which we have immediate experience and this subtle and obscure celestial matter. As St. Thomas puts it,

The celestial bodies are far away from us not only according to quantity of spatial distance, but even more so in that few of their accidents fall under our senses, while it is nevertheless connatural to us that we proceed from accidents, i.e., sensibles, to cognizing the nature of some thing. . . . But the accidents of the celestial bodies are of a different notion altogether [alterius rationem] and are wholly disproportionate to the accidents of inferior bodies.⁽⁷³⁾

We may say many things negative about aether, and what positive predicates we may apply to it must be extensions of the first impositions of words--they must be analogies. As Aristotle has said, we have little to go on in determining the nature of aether; what we do have, however, is enough to make some elementary deductions about it, even if they finally must entail analogical predication. While not all of these properties have survived the

test of time, the aether itself and many of these distinctive marks have, specifically within the theories of twentieth-century physics. But before we turn to the recent rehabilitation of aether, we must say a few things about its precarious perpetuation, revision, and demise in early modern physics.

II. THE FATE OF AETHER IN CLASSICAL PHYSICS
AND THE SPECIAL THEORY OF RELATIVITY

The heliocentric hypothesis in the late sixteenth and early seventeenth centuries, while it did not overturn the idea of a heavenly aether, induced a skepticism in some about its necessity. If the daily circular motion of the stars is attributable to the Earth itself, the only foothold we have on aether seems to slip. Although for Copernicus the planets and moon must still forever move in perfect circles--thus tempering the temptation to discard the argument of De Caelo 1.2--so must the Earth, while the outer sphere containing the so-called fixed stars must not; thus circular motion is not a peculiar property of aether as such. Aether, it seems, is not to be posited to explain the night sky, since its motion is as mundane as it is celestial.

This does not by itself respond to Aristotle's critique of void, and therefore of the assumed emptiness of the heavens. Granting the real possibility that the luminous part of the heavens (i.e., the stars and planets) may not be essentially different from ordinary matter, most of the heavenly expanse remains unaccounted for. For if nothing is in any manner detectable there, but something must be there, the natural suspicion would be that this physical "something" is a different order of matter, that is, is essentially aethereal.⁽⁷⁴⁾

While this argument may have been in the background during the early modern era, Isaac Newton embraced aether by way of another. Rejecting the idea of action at a distance, calling it "inconceivable," and "so great an absurdity that I believe no man who has in philosophical matters a competent faculty of thinking can fall into it,"⁽⁷⁵⁾ Newton speculated about how else to explain the distant communication of gravitational forces mathematically described in his Principia. Following something akin to the Aristotelian recognition that an aethereal medium is needed as an instrumental agent of the action of the stars on bodies around them, Newton posited aether as the medium through which the sun holds the planets in orbit.⁽⁷⁶⁾ Although unsatisfied with his mechanical models of how aether could exercise its agency, and hesitant to publish his general speculations, Newton does not seem to have doubted that some such principle was necessary to complete his account of gravitational attraction.⁽⁷⁷⁾ Further, and again like Aristotle and his disciples, Newton saw light as additional evidence for an aether, though, unlike his predecessors, he did not see aether as the subject or medium of light. Specifically, Newton posited aether to explain certain diffractive and refractive properties of propagated light and related electrical phenomena.⁽⁷⁸⁾ Thus while aether was needed in the heavens, it

would have to exist here below as well; in line with Copernicus, aether was no longer only celestial.

Many of Newton's contemporaries and successors, however, presented a conception of light even more like Aristotle's actuality of the transparent by arguing that light needs aether merely to exist. While Newton insisted that light is a particle, Thomas Young and others offered compelling arguments that light is a wave,⁽⁷⁹⁾ and, as Oliver Lodge would one day reiterate, "waves we cannot have, unless they be waves in something."⁽⁸⁰⁾ Implicitly invoking an Aristotelian principle at the root of Aristotle's rejection of void, Young and others inferred that if light is not a substance but an accident, it must be an accident of something. Looking to the expanse of apparently empty space across which light radiates, wave theorists saw the presence of a luminiferous aether. This need for a light-bearing aether was reinforced in the nineteenth century by James Maxwell's discovery that light, electricity, and magnetism are different aspects of the same physical phenomenon. The empirical data and mathematical formalism of electromagnetic theory suggest that, as Maxwell put it, "there is an aethereal medium filling space and permeating bodies."⁽⁸¹⁾ The electromagnetic field is not a mere mathematical abstraction, but a description of a modality or stress in the ubiquitous and immobile aether.

Even so, early modern revisions of aether as a medium of gravitational and electromagnetic interactions should be carefully distinguished from Aristotle's heavenly substance. Unlike the latter, this new aether is in a real way not aethereal. Newton, for example, knew that any substance lacking inertial mass, and therefore resistance to pressure--in the language of Aristotle and St. Thomas, intangible and perfectly yielding matter--would violate his laws of motion;⁽⁸²⁾ it would not follow what came to be

called "classical" mechanics. Likewise, because inertial mass is proportional to gravitational attraction, such a body would not gravitate; universal gravitation would be not quite universal. But most importantly, a truly aethereal substance like this would in principle be experimentally undetectable, for it would not resist and thereby affect measuring devices or test-particles. Although a perfectly noninertial medium could make more substantial and intelligible Newton's problematic doctrine of absolute space, the immobile reference frame of true motions and the forces that cause them, nevertheless he was adamant: Such matter would not be "a phenomenon," and would have "no place in experimental philosophy."⁽⁸³⁾ While admitting a need for a pervasive medium-- although not a truly continuous one, since he embraced atomism, and therefore also the reality of voids⁽⁸⁴⁾--Newton granted it a very small, but nevertheless in principle detectable, mass, and therefore also a proportionate resistance and gravitational attraction.⁽⁸⁵⁾ Thus, when he retains the word "aether," he means something that differs from ordinary tangible matter not in kind but only in degree.

Young and Maxwell's medium of light and electromagnetism was likewise assumed to be minimally inertial, massive, and tangible in theory. Despite the occasional discovery of evidence challenging the hypothesis that the luminiferous aether was merely a more tenuous kind of ordinary matter, the hypothesis that all matter must be inertial was not questioned.⁽⁸⁶⁾ The

electromagnetic wave was understood to be a measurable force and transient attribute of the aether not unlike the stresses and strains in an elastic body or set of bodies, and therefore this aether by definition would be subject to Newtonian mechanics.⁽⁸⁷⁾ In addition, the luminiferous aether was posited as the reference frame in which Maxwell's electromagnetic field equations obtained perfectly, and therefore was the coordinate system or stationary container of all local motion. It was assumed to be necessarily immobile.⁽⁸⁸⁾ But this assumes that place and motion, or the lack thereof, are univocally present in the aether--unlike in the traditional aether.

Although Newton and the others did not invoke a full-blooded aether concept, nevertheless with the empirical success of universal gravitation and electromagnetism came the embrace of the reformed (or perhaps to Aristotle, deformed) aether among physicists. This embrace lasted until the dawn of the twentieth century and was so tight that renowned physicists would say, "We know there is an ether," and "The probability of the hypothesis of the existence of this element is extremely close to certainty."⁽⁸⁹⁾ The aether of classical physics had the virtue of undoing what is commonly considered an embarrassingly ad hoc Aristotelian

partitioning of nature into two essentially heterogeneous categories of substance. Nevertheless, this homogeneity and unification of the cosmos came at a price, for it required a dilution of the meaning of the word "aether." This price did not at first seem costly; indeed, it seemed to be an asset because it rendered aether more accessible to experimentation and mathematical conceptualization. But there were hidden expenses that would prove problematic and insupportable. The particular nature of classical physics' aether would make it vulnerable to a kind of refutation that Aristotle would have considered irrelevant: the Michelson-Morley experiment.

On the supposition that the immobile luminiferous aether follows Newton's laws and resists the motion of other bodies, physicists after Maxwell looked for ways to measure its mass and inertial properties. In one such test at the end of the nineteenth century, Albert Michelson and Edward Morley attempted to estimate the speed of the Earth relative to Maxwell's stationary medium. The idea was that a sort of aether "wind," caused by the Earth's motion through the aether, might be detected by shining a light in the direction of the Earth's motion and comparing it with light shining in a direction less affected by the motion. Taking the absolute speed of light to be the same as its speed relative to the aether, Michelson and Morley inferred that the apparent speed of the two light beams should not be a constant; rather, when shining in the direction of the Earth's motion, it should be slower than when shining in a direction perpendicular to that motion. The experiment was done with a simple inter-ferometer, with light rays simultaneously sent in perpendicular directions toward mirrors, which reflected them back to the source, where they would intermingle and betray signs of the anticipated unequal speed in the interference pattern. Such signs, however, did not turn up; regardless of direction, the light rays seemed to travel at the same speed. Apparently there was no

relative motion between the Earth and the aether.⁽⁹⁰⁾ Michelson and most of his contemporaries concluded that aether near the surface of the Earth must not be immobile but dragged along by the Earth's motion.⁽⁹¹⁾ Because of this "atmosphere" of aether, no relative motion occurs in the Earth's immediate vicinity. This notion of a mobile luminiferous aether, however, was soon rejected on the basis of further experimentation.⁽⁹²⁾ How do we explain the null-result of the Michelson-Morley experiment?

H. A. Lorentz and George FitzGerald stepped in to propose an answer. A small but fixed contraction of matter in the direction of the Earth's motion would shorten one arm of the interferometer, thereby giving the light beam less distance to cover and thus allowing it to return to the source at the same time as the light beam directed along the other, uncontracted arm. Although he could not offer a precise mechanism for this phenomenon, Lorentz attributed it to some hitherto unknown influence of the aether.⁽⁹³⁾ Lorentz-FitzGerald contraction saved the appearances and was mathematically sound,⁽⁹⁴⁾ but was criticized as ad hoc, having the virtues neither of elegance nor of making new pre-dictions.⁽⁹⁵⁾ And it was finally replaced by a theory that had both.

In 1905 Albert Einstein published his special theory of relativity and shortly thereafter declared that the luminiferous aether is an "outdated point of view. . . . [A] satisfactory theory is only achieved by renouncing the ether hypothesis."⁽⁹⁶⁾ This revolutionary theory encompasses not just the Michelson-Morley null-result, but also a wide range of physical phenomena. Combining the empirically based principle that the speed of light in a vacuum is a constant in all reference frames⁽⁹⁷⁾ and the philosophical principle that all laws of physics are the same for all coordinate systems regardless of their states of motion changes the entire worldview or natural philosophy of mathematical physics--so much so that the advent of relativity came to mark the transition from so-called classical (i.e., Newtonian) to modern physics. Put simply, Einstein's union of these two principles denies that there is any meaning to Newton's notions of absolute space, motion, and time. Thereby it offers what appears to be the simplest possible explanation of the Michelson-Morley null-result: there is no absolute reference frame for the Earth's motion, and therefore no aether, and therefore no "aether wind," which explains why no difference in the speeds of the two beams of light is detectable in the interferometer.⁽⁹⁸⁾

Reducing measurements of time, space, rest, and motion to mere relative terms, although philosophically problematic to many, then and now,⁽⁹⁹⁾ has its merits. Not only did special

relativity shed the artificiality of Lorentz-FitzGerald contraction, it made new predictions--the most famous of which still is the convertibility of matter and energy, empirically vindicated quite publicly less than forty years later in the nuclear fires of Hiroshima and Nagasaki. And with the removal of the privileged coordinate system, the immobile luminiferous aether lost its relevance for the physicist, for, according to the prevalent attitude of logical positivism, what cannot be measured may just as well not exist.⁽¹⁰⁰⁾ Within a few decades after 1905, aether became a "metaphysical concept in the pejorative sense,"⁽¹⁰¹⁾ as outmoded as Aristotle's geocentric universe,⁽¹⁰²⁾ or his bifurcation of nature into ordinary matter and his full-blooded, though more subtle, aether. Yet the latter was about to make a comeback.

According to most accounts of the history of the aether in physics, that was the end of the story. Aether is dead and the Michelson-Morley experiment and special relativity killed it.⁽¹⁰³⁾

Rumors of its death, however, had been greatly exaggerated, and a closer look at the content and history of the theories that have been accepted in twentieth-century science makes this manifest. Not the least among these theories is Einstein's generalized theory of relativity, just as not the least among those who interpret this theory in terms of aether was Einstein himself.

A decade passed between the publication of the special theory of relativity and the 1915 completion of the general theory, immediately after which Einstein realized that he had overstated his case against aether. General relativity--indeed, even special relativity--was compatible with and, in fact, implied an aether;⁽¹⁰⁴⁾ aether's "story, by no means finished, is continued by the relativity theory."⁽¹⁰⁵⁾ Relativity initiated a development and further aetherealizing of the physicist's understanding of the luminiferous aether that, we will see, bespeaks a conception more reminiscent of the intangible substance proposed by Aristotle.

Because special relativity had seemed to discard the luminiferous aether, we should start with it. In spite of his hyperbole in the decade after 1905, Einstein would later temper his remarks:

careful reflection teaches us, however, that the special theory of relativity does not compel us to deny ether. We may assume the existence of an ether; only we must give up ascribing a definite state of motion to it, i.e., we must by abstraction take away from it the last mechanical characteristic which Lorentz had still left it. . . . The special theory of relativity forbids us to assume the ether to consist of particles observable through time, but the hypothesis of ether in itself is not in conflict with the special theory of relativity.⁽¹⁰⁶⁾

Special relativity is congenial to aether as long as one withholds from it all the properties underlying classical mechanics, such as mass, inertial resistance, atomic composition, and even a determinate state of motion or rest, in order to be consistent with the Michelson-Morley experiment and the equivalence of reference frames. Aether thereby becomes essentially unobservable and "appears at first to be an [empirically] empty hypothesis."⁽¹⁰⁷⁾ This, however, would be to ignore

a weighty argument to be adduced in favor of the ether hypothesis. To deny the ether is ultimately to assume that empty space has no physical qualities whatever. The fundamental facts of mechanics do not harmonize with this view. . . . [B]esides observable objects, another thing, which is not perceptible, must be looked upon as real, to enable acceleration or rotation to be looked upon as something real.⁽¹⁰⁸⁾

Insofar as certain aspects of "empty space" are elements in the equations describing accelerative motions (special relativity focuses on only uniform motions) an aether still seems necessary. The "four dimensional space[-time] of special relativity is to some extent a four-dimensional analogue of H. A. Lorentz's rigid three-dimensional aether."⁽¹⁰⁹⁾

While special relativity is compatible with an aether (when each is properly understood), general relativity positively demands it in its notion of space-time curvature. Although even special relativity employs the notion of space-time--the idea that no local description of a body should be considered without specifying when it is at this location--general relativity goes further, attributing to space-time a certain quality or mutable property, dubbed "curvature" because of the likeness between the way this property and the curvature of a surface affect the speed and

direction of a body's motion on it.⁽¹¹⁰⁾ Space-time curvature, mathematically described by a metric tensor and metric field,⁽¹¹¹⁾ is general relativity's modification and reinterpretation of Newton's gravitational force.⁽¹¹²⁾ According to general relativity, then,

the metrical qualities of the continuum of space-time differ in the environment of different points of space-time. . . . [T]he recognition of the fact that "empty space" in its physical relation is neither homogeneous nor isotropic [i.e., uncurved] . . . has, I think, finally disposed of the view that space is physically empty. But therewith the conception of ether has again acquired an intelligible content. . . . [S]pace is endowed with physical qualities; in this sense, therefore there exists an ether. According to the general theory of relativity space without ether is unthinkable; for in such space there not only would be no propagation of light, but also no possibility of existence for standards of space and time (measuring-rods and clocks), nor therefore any space-time intervals in the physical sense.⁽¹¹³⁾

Thus, aether is essential to general relativity, for it is ultimately what is being described by general relativity--it is what the curvature is a curvature of. True, it is no longer the privileged reference frame of classical physics, for it cannot, properly speak-ing, be said to be at rest (or in any mathematically expressible

state of motion). It is, however, the substrate of the physical properties or quasi-geometrical structure that is "characterized mathematically by the components of the gravitational potential [i.e., the metric tensor], which describes the metric behavior of this part of space, as well as its gravitational field,"⁽¹¹⁴⁾ and therefore also the motion of bodies in this part of space. This aether's "state varies continuously from point to point"⁽¹¹⁵⁾ in space and in time, and thereby it affects the metrical properties (i.e., the measurements of space-time variables) of any body at any point in it.

The function of space-time curvature, and therefore of aether, as determining how bodies move bears emphasis. At the root of the relativistic aether's causality, like that of both the Aristotelian and Newtonian aethers, is its role as an intermediate agent, a medium through which one massive body--whether a planet or an atom--could attract or in any way act upon another. According to Einstein, no theory

involving action-at-a-distance . . . merits serious consideration. . . . [Thus] we will not be able to do without the ether in theoretical physics, i.e., a continuum which is equipped with physical properties; for the general theory of relativity, whose basic points of view physicists surely will always maintain, excludes direct distant action. But every contiguous action theory presumes continuous fields, and therefore also the existence of an "ether."⁽¹¹⁶⁾

Taking the rejection of action at a distance as a philosophical first principle in interpreting the mathematical theory, Einstein maintains that agent causes will act only by contact, specifically by alterations of the space or aether surrounding distant patients, and general relativity explains how this happens. Although Einstein's aether "is itself devoid of all mechanical and kinematical qualities . . . [it] helps to determine mechanical (and electromagnetic)

events."⁽¹¹⁷⁾ Specifically, relativity's aether determines both inertial and gravitational motions, the former by the abstracted special theory which makes it a principle of motions in nearly flat regions of space, and the latter by the general theory which makes it a principle of motions in significantly curved space.⁽¹¹⁸⁾ So-called empty space not only has attributes, it also has causally active attributes, and enters into "the causal nexus of physics, . . . [so] this ether would be a physical reality as good as matter."⁽¹¹⁹⁾

Unlike the aether of Aristotle, however, Einstein's aether enters into the "causal nexus of physics" by also being acted upon by ordinary matter. The degree of curvature in a region of space-time varies with--indeed, Einstein sometimes even says, "is generated and defined"⁽¹²⁰⁾ by--the presence of ordinary matter. A massive body curves ambient space-time, but this curvature, in turn, determines the path and speed of that same body and of bodies near it. While Einstein grants that aether has some sort of priority over ordinary bodies--sometimes he says, perhaps hyperbolically, that inertial matter is nothing more than a state or modification of the aether⁽¹²¹⁾--he nevertheless does not simply speaking attrib-ute to it the sort of one-way causality found in the Aristotelian aether.

Still, this is not to say that the relativistic aether is simply another form of ordinary, Newtonian matter, for the way massive bodies produce this curvature in the aether is not properly explicable in terms of Newtonian mechanics. The not-so-aethereal aether of classical mechanics--which, ironically, Einstein calls "ether in the traditional sense"⁽¹²²⁾--is indeed a dead hypothesis. The relativistic aether, however, is still radically unlike ordinary

matter because it does not exert a force of resistance when massive bodies enter it and begin to generate the gravitational field, and likewise massive bodies do not curve aether by exerting a force or pressure on it. Rather, just as Aristotle's aether would docilely yield to ordinary matter entering it, so Einstein's aether naturally curves with the presence of a massive body. Thus Einstein insists again that this is aether of "a more sublimated form [than the luminiferous aether] . . . [For it] differs from the one of earlier optics by the fact that it is not matter in the sense of mechanics. Not even the concept of motion can be applied to it";⁽¹²³⁾ thus, it "may not be thought of as endowed with the quality characteristic of ponderable [i.e., massive, inertial] media, to consist of parts that may be tracked through time."⁽¹²⁴⁾ Not unlike Aristotle's celestial matter, relativistic aether is deprived of univocal spatio-temporal predicates, is continuous, intangible, and perfectly transparent--for it is also the medium of electro-magnetic energy, light. However, unlike Aristotle's aether (though not perhaps directly opposed to it), this aether possesses specific properties describable only by way of abstract mathematical for-mulae and geometric analogies. This substance is truly aethereal.

We find another ambiguity of predication that Aristotle and St. Thomas noted (although for only partly overlapping reasons)

regarding the celestial substance: Is it right to call this aether a substance or matter at all? Einstein, after all, sometimes says that in general relativity, "instead of speaking of an aether, one could equally speak of physical qualities of space," and that here "the concepts of 'space' and 'ether' merge together."⁽¹²⁵⁾ Has he only tagged the vacuum with the name "aether" without meaning to describe a material being of any sort? Einstein cannot simply mean this, as he just as frequently (and often in the same writings) says that relativity has "finally disposed of the view that space is physically empty," and that "space as opposed to 'what fills space' . . . has no separate existence. . . . Space-time does not claim existence on its own but only as a structural quality of the field,"⁽¹²⁶⁾ that is, of the aether. What is being called "space" is not a void, an expanse of nothingness, for it is not empty and it is positively causal. Whether we call it a material substance, then, seems to depend on what is meant by "matter" and "substance." On the classical assumption that matter is the same thing as massive, atomic, and inertial, or "ponderable," matter, the aether would not properly be a material substance.⁽¹²⁷⁾ However, keeping in mind the Aristotelian insight that matter most properly is that out of which a physical substance is composed, and therefore is the very potency for natural being,⁽¹²⁸⁾ we see that this subtle yet mutable physical entity is essentially a material substance--though these predicates are appropriate to it only in an extended sense.⁽¹²⁹⁾ As Lorentz puts it, given its function, any aether must be "endowed with a certain degree of substantiality, however different

it may be from ordinary matter,"⁽¹³⁰⁾ and according to one Einstein specialist,

In Einstein's concept of the ether there also occurs a gradual materialization of physical space-time. . . . [But this means] ascribing it a specific type of materiality, very different from the materiality of the substances we encounter in physics, to which we refer when we [usually] use the word "matter."⁽¹³¹⁾

Matter and substance, like predicates pertaining to motion, space, and time, can be said of aether only by means of rational equivocation, that is, by analogy. Nevertheless, granting attributes, especially causal attributes, to "empty" space is incoherent, Einstein, Aristotle, and St. Thomas argue; void is no longer a possibility. So an underlying causal physical entity unlike those we experience daily must be posited by relativity. Aether's executioner turns out to be its savior.⁽¹³²⁾

The other pillar (besides relativity) on which modern physics is based is quantum theory. Just as Einstein saw his aether as the medium not only of gravitational fields, but also of electro-magnetic ones,⁽¹³³⁾ so does the second candidate for a modern

aether come from a branch of quantum theory called quantum electrodynamics. As with relativity, we will not try to explain quantum theory in detail,⁽¹³⁴⁾ but will restrict ourselves to salient points pertaining to the idea of aether.

During the decades surrounding the publication of Einstein's theory of relativity, evidence was accumulating and another theory developing that together indicated that all energy is transmitted in discrete units called quanta. One of the implications of this new theory was that the position and momentum of a particle cannot be simultaneously determined with perfect accuracy; the more precise the position measurement, the less precise that of the momentum, and vice versa. Likewise, this inverse relationship was found to apply to the combination of energy and time-interval measurements. Most importantly, however, this "uncertainty principle" was further shown to be not merely a limitation in our knowledge of a particle, but even to imply an indetermination in the particle itself. A precise position measurement means that the particle at that moment in fact has no precise momentum. While some have seen hints of an aether in various parts of quantum theory,⁽¹³⁵⁾ the uncertainty principle argues for it most directly and persuasively. The branch of quantum theory connected with how electromagnetic energy (i.e., a photon) interacts with electrons--namely, quantum electrodynamics, or QED⁽¹³⁶⁾--posits the notion of an effervescent and active "vacuum," which looks suspiciously aethereal.

A region of space in which there is no detectable matter or energy, even at the quantum level, is described by QED as possessing a quantum field that is "inactive" or "unexcited." This is the ground, or lowest possible, energy state of the field, so the quantum system here is said to be physically empty of literally everything detectable, a "quantum vacuum." However (to revise a pun from Aristotle),⁽¹³⁷⁾ this vacuum does not seem so vacuous. In 1925 Werner Heisenberg, who first articulated the uncertainty principle, inferred that it requires that even this vacuum must possess throughout a certain residual and irremovable energy, paradoxically named "zero-point energy."⁽¹³⁸⁾ Because of the irreducible minimal uncertainty we must have about the energy status of every point within a quantum system, even one apparently evacuated of all energy, Heisenberg argued, the vacuum must be allowed occasionally and spontaneously to "fluctuate," that is, to generate real particles with real effects. Hence, where QED says there is nothing, it also says that there will always be the possibility that this nothing will turn into something, even if for only an instant.⁽¹³⁹⁾

This sounds of course like science fiction; the uncertainty principle must be wrong if this is what it implies. Either a quantum system is empty or it isn't, and it is inconceivable that a void would, even occasionally, spit out a body. There is something right about this objection; nonetheless, vacuum fluctuations are not objects of mere theory. Even according to theory they should be measurable--and, disturbingly, they are. Besides nearly a century of unblemished empirical success for quantum theory and its cornerstone uncertainty principle, there are many well-established phenomena apparently intelligible only in terms of

these vacuum fluctuations. While the first experimental evidence of zero-point energy turned up in the same year that Heisenberg predicted its existence, two more recent and more deeply studied examples bear singling out. In the 1940s certain unexplained permutations, dubbed "Lamb-shifts," were first observed in the spectra emitted by excited hydrogen and muonic helium. Insofar as changes in the spectral emission of an atom are directly connected to changes in electron structure or configuration, the only account that was and remains forthcoming is that the vacuum surrounding the electrons of the hydrogen and helium, in virtue of a fluctuation or actualization of some of its zero-point energy, is energizing them.⁽¹⁴⁰⁾

A second well-established witness to the existence of zero-point energy in the vacuum is called the Casimir effect. Predicted by QED in the 1940s, only in the past decade has it been directly and accurately measured.⁽¹⁴¹⁾ The Casimir effect is a delicate but measurable attractive force generated, without the presence of an electromagnetic field, between a pair of parallel metal conducting plates; this force is inversely proportional to their separation.⁽¹⁴²⁾ As with the Lamb-shifts, the measure of this force is exactly that predicted by fluctuations of the vacuum occurring around the plates, those occurring between the plates being overcome by those occurring outside of them. These and other phenomena,⁽¹⁴³⁾ then, indicate that zero-point energy seems to be more than a theoretical entity.

Is it right to call this a vacuum at all? One physicist, reflecting on this ambiguity in QED's use of the word, notes that

The excitation and de-excitation of these modes [of the quantum vacuum field] are interpreted as the "creation" and "annihilation" of "particles." In this context, a new notion of a "vacuum" is introduced. . . . There are several features of these vacuum states that make it hard to conceive of them as "empty."⁽¹⁴⁴⁾

that the concept of the vacuum must be somewhat realigned. It is no longer to be associated with the idea of the void and of nothingness or empty space. Rather, it is merely the emptiest possible state . . . the state from which no further energy can be removed. . . . [The "vacuum"] is what is left when everything is removed from space that can be removed.⁽¹⁴⁵⁾

The so-called vacuum is full--it is filled by the irremovable zero-point energy--so its name will be a contradiction in terms unless it is a vacuum only relatively or loosely speaking, that is, unless it is empty only of a certain genus of things, while it may remain full of something of another genus.⁽¹⁴⁶⁾ This need to say that it is essentially full, a plenum rather than a vacuum, increases when we recall the Presocratic dictum that is one of the first principles of natural philosophy: ex nihilo nihil fit--nothing comes from nothing. The sudden generation of particles or their agency is physically impossible if there is not some matter or physical potency permeating the "vacuum" already. Manifestations of zero-point energy, then, not only keep us from describing it as "empty," or a "vacuum," without further qualification, but they also tell us that it is material, for it is that which is able to become actual particles (of energy or massive particles). It is not, of course, the ordinary matter of common experience.

Is it not right to call it aether, then? Although few physicists are in the habit of calling it anything other than a vacuum--with the constant double-speak that it is not really empty--some think aether is a more meaningful appellation. For example, some insist

that the energy of the ground-state is "a kind of reintroduction of the ether," a "quantum field-ether," although "it is a pale and ghostly shadow of its old self," that is, of the luminiferous aether of classical physics.⁽¹⁴⁷⁾ Indeed, the best-known proponent of reappropriating the name and notion of aether is the founder of QED itself, Paul Dirac. Despite special relativity's much-sung elimination of aether, because of the enduring need to explain not only vacuum fluctuations, but also aspects of general relativity, Dirac argues, we should admit that we are implicitly invoking aether in our general account of the phenomena and the mathematical theories that pretend to be studying a void:

Physical knowledge has advanced much since 1905, notably by the arrival of quantum mechanics, and the situation [about the scientific plausibility of aether] has again changed. If one examines the question in the light of present-day knowledge, one finds that the aether is no longer ruled out by relativity, and good reasons can now be advanced for postulating an aether. . . . We can now see that we may very well have an aether, subject to quantum mechanics and conformable to relativity, provided we are willing to consider a perfect vacuum as an idealized state, not attainable in practice. From the experimental point of view there does not seem to be any objection to this. We must make some profound alterations to the theoretical idea of the vacuum. . . . Thus, with the new theory of electrodynamics we are rather forced to have an aether.⁽¹⁴⁸⁾

We would not be accused of reading too much into these words to say that, at minimum, the notion of the aether is anything but scientifically naive or reactionary. Outside of refusing to try to make sense of QED and phenomena like Lamb-shifts and the Casimir effect, positing a peculiarly unobservable--but inferrable--medium seems inescapable. And what is this but aether? Objecting to the reappropriation of the name in principle, as though motivated by an undue syncretism, is unreasonable. As one physicist soberly points out, in attempts to unify physics in recent decades, "Increasingly, this vacuum is reminiscent of ether . . . [and the parallels are drawn] not out of any great fondness for

the concept of ether, but through the perception of its unifying role in dynamics."⁽¹⁴⁹⁾ This is aether not merely in name, but in function and in essence.⁽¹⁵⁰⁾

No claim has been made that the aether implied in quantum electrodynamics is identical to the aether of relativity, as each explains a different, and apparently irreducible, form of energy

(electromagnetic and gravitational, respectively). Likewise, while QED's zero-point energy is postulated to explain the very small (e.g., spectral variations in an excited atom), relativity's curved space-time is postulated to explain only the very large (e.g., the precession of Mercury's orbit around the sun). Thus, assuming that there is only one aether filling a given space at a given time, it is difficult to see, barring some future unification of these forms of energy--which, admittedly, many scientists, including Einstein, have devoted much effort to finding--how these two modern aether-candidates can be fused. Harmonizing quantum theory and relativity is not my aim here, as their partial incompatibility is well known and has troubled better minds. Nevertheless, there are some signs, courtesy of recent astrophysics, of a partial overlap between the quantum and relativistic aethers.⁽¹⁵¹⁾ In attempting to understand the macro-world (e.g., stellar motions), usually the domain of general relativity, astrophysicists are now feeling a need to posit a macro version of QED's micro-world zero-point energy. Thus, although the jury is still out among the physicists on what to make of the phenomena in question--so any evaluation must remain tentative--still the evidence should be noted.

According to accepted theory, the expansion of the universe should be decelerating due to the gravitational drag of massive bodies, such as planets and stars. However, observations on a number of distant supernovae over the past ten years are suggesting that some hitherto unknown repulsive force from an

unknown energy source is accelerating the expansion.⁽¹⁵²⁾ And worse, this force does not appear to be coming from one region of the universe; rather, it appears to be coming from all directions, or more specifically, from space itself. The comparison with Einstein's original idea of a "cosmological constant," an irremovable repulsive force built into the texture of the universe, has been difficult to avoid, although for half a century it was common opinion that its addition to relativity theory was ad hoc. While little is certain about this accelerative force, one thing seems clear: As one physicist puts it, "the energy density associated with the [new] cosmological constant is not possessed by matter or radiation, but by 'empty' space."⁽¹⁵³⁾

Perhaps, then, it is not coincidental that researchers have tentatively named this mysterious energy source not only "X-matter" (thereby treating it as material in some sense), but even more strikingly, "quintessence,"⁽¹⁵⁴⁾ the name used by Aristotle's medieval disciples for the fifth element, aether. As one physicist specializing in interstellar dark matter puts it,

A decade ago, it seemed to me that dark matter was a sort of modern "fifth essence." But even closer in spirit to Aristotle's heavenly aether or "quintessence" is the currently favored possibility that a nonzero energy exists throughout empty space. . . . [P]erhaps nothing in the history of physics resembles more the quintessence of Aristotle than . . . [this] vacuum energy, comprising 50-70 percent of the energy density of the universe.⁽¹⁵⁵⁾

As in general relativity and QED, this use of the word "aether" does not seem to be pure metaphor. Besides apparently possessing the aethereal character of QED's vacuum, this "quintessence," unlike all other known matter, does not exhibit gravitational attraction, it is not pulled or pushed toward other bodies--indeed, it appears only to repulse or push upon ordinary matter, implying both its uniqueness and its causal agency. Likewise, in virtue of its dominance over ordinary matter and energy, this energy-source seems to be the single most important factor determining the shape of the universe, the arrangement of stars and galaxies within it, of space itself, and the eventual fate of the universe as a whole. Further, this mysterious energy source appears to act on ordinary matter without being acted upon in return. Empirical data suggest and general relativity theory requires both that it be accelerating the expansion of bodies in the cosmos and that it remain unaffected by their motion or presence. In short, this cosmic vacuum energy seems to be immutable, a cosmological constant--all of which are marks of Aristotle's weightless and intangible, but causal, celestial substance.⁽¹⁵⁶⁾ While not identical to Aristotle's conception of aether, this modern "quintessence" does appear to be its intellectual heir.

The supernova evidence of a cosmological constant is hardly definitive. Likewise, general relativity and quantum electro-dynamics are only theories--well-tested theories (especially QED), to be sure, but theories nonetheless, successful attempts at making more intelligible what is observed.⁽¹⁵⁷⁾ Just as Aristotle's geocentric universe was overturned in the sixteenth and

seventeenth centuries and was replaced by Copernicanism and Newtonian universal gravitation, and just as the unthinkable happened when Newtonian physics failed at the beginning of the twentieth century, each proving unable to account for newly detected phenomena, so the same may someday occur to relativity and quantum theory. The experimental method of natural science is tentative by nature, its inductions always being only partial and incomplete.⁽¹⁵⁸⁾ However, just as Newtonian physics is an approximation to the truth, so relativity and quantum theories, if or when challenged, will turn out to have been like the truth. Thus, they should not be dismissed as being entirely off on the wrong track, and their different but similar needs for an aether suggest that there is something right about the aether as a principle of nature. At minimum one will grant that the foregoing excursions into relativistic, quantum, and astrophysical theory and observation amount to a powerful dialectical argument in favor of something like Aristotle's celestial substance.

Nevertheless, it seems silly, one might say, to pretend that the new aether-candidates are what Aristotle was trying to say all along. After all, there are two critical ways in which they are decidedly unlike Aristotle's aether. His first and most straightforward reason for positing a new kind of matter is his belief that he could see the heavens moving in a way that nothing else around him moves. This simple circular motion was rejected long before Einstein and Dirac formulated their revolutionary theories, and their aethers possess no such motion. Likewise, Aristotle's aether was peculiar to the celestial regions; it was not "here below," whereas space-time and vacuum energy are everywhere. Thus (the objection goes) what contemporary science has hit upon bears only a superficial, coincidental resemblance to Aristotle's aether.

There is, of course, something to be said for this. Aristotle's aether is not simply identical to those implied in relativity and

quantum theories, as the most obvious marks of the former are not preserved in the latter. However, a case has been made that, while the nightly circular motion of the heavens is no doubt what first caught Aristotle's attention and made him suspect that there is an unusual kind of matter up there that is not down here, the underlying and in many ways most critical drive to posit aether is the impossibility of void. If there cannot be a void, and yet the senses detect nothing in the heavens besides a few shining lights, then we are left with good reason to believe that whatever fills the heavens is unlike any matter of which we have experience. The circular motion of the stars, then, only adds to this conviction.⁽¹⁵⁹⁾

This situation is identical to the one in which we find ourselves today. Space still appears to be empty. Certainly it is probable that, if there were any subtle sensible attributes permeating the heavenly regions, some sign of an ubiquitous medium of ordinary matter, we would have detected something by now. What we have found, from random stellar dust and gas to a uniform scattering of microwaves, is not enough literally to fill the heavens. Sensation and even the mechanical measuring devices acting as extensions of sensation have not given us reason to believe that there is anything but emptiness. Rather, what we have done is argue to the existence of properties in space, properties that thereby imply a substrate. Einstein has argued to there being a quasi-geometric structure filling space that affects the gravitational motions of bodies in it. Heisenberg has argued that the uncertainty principle entails vacuum fluctuations, and therefore some kind of "vacuum energy." Thus, the consistently vacuous appearance of outer space, when combined with the troublesome philosophical baggage that comes with void, is itself an argument in favor of aether, just as was Aristotle's wonder at the apparent emptiness between himself and the distant stars. Modern science has only strengthened the argument. Likewise, the modern aethers and that of Aristotle have a common core: They are aethers of the same sort. Each posits an

ubiquitous, space-filling, utterly insensible medium whose existence we cannot directly measure or detect but which can be inferred from things we can measure and detect.

Nor are only the basics shared. In the foregoing, we have demonstrated profound similarities between the old and the new aethers that are worth reviewing. Recall Einstein's curved space-time which has no determinate velocity, location, or history; spatio-temporal predicates can be applied to it only analogically, not univocally. Likewise, Aristotle's heavenly matter has no place, no motion or rest, and does not exist in time, without some kind of loosening of the meanings of those words. Einstein's aethereal space-time, moreover, is a principle and cause of the local and temporal properties of ordinary matter and in some way determines the nature of their motions. Aristotle's "first body" is the ultimate principle in virtue of which all other bodies have place and are measured by a common time, and it is the first physical agent cause of natural motions. In both relativity and QED, one finds an ambivalence among the physicists about calling their respective aethers "material" or "immaterial"; likewise, Aristotle and St. Thomas insist that aether can be named "matter" and "substance" only equivocally, even occasionally arguing that it partakes of "immateriality."⁽¹⁶⁰⁾ Aristotle and St. Thomas, on the one hand, argue that aether seems to be immutable and impassive to ordinary matter, that is, it cannot be touched or pushed. Relativity and QED, on the other hand, while admitting that ordinary matter somehow causes curvature of space-time, and that the relative location of conducting plates can indirectly effect a net attracting force in the ambient quantum vacuum, require that aether not be a ponderable or inertial sort of matter--the quasi-agency of ordinary matter on it is not intelligible as common efficient causality, which involves an equal and opposite reaction. And lastly, Einstein, Heisenberg, Dirac, on the one hand, and Aristotle and St. Thomas, on the other, all insist that light and light-related phenomena have this medium as their proper subject. p

These likenesses seem too particular yet profound to be chalked up to coincidence.⁽¹⁶¹⁾

But is not Aristotle's restriction of aether to the heavens significant? Of course, and Aristotle no doubt would have second-guessed his sharply spatial bifurcation of nature if he had known what experimental science tells us now. There is even some vague evidence that he considered the possibility that the aether or some kind of participation in the nature of aether could exist in the sublunary regions.⁽¹⁶²⁾ Regardless, Aristotle's account would have

to be modified in this respect to make it compatible with better data--as he himself insisted, for he said that his account was plausible only given the information he had at present.⁽¹⁶³⁾ Thus, just as there must be some way in which, it now seems, ordinary matter can act upon aether, and that particles of dust can exist in the interstellar sea of aether, so aether seems to exist in closer proximity and interaction with the ordinary matter around us. Hence, we are not arguing that Aristotle's aether can be preserved without refining and updating it; but neither has it been simply chameleon-like, utterly changing its colors for Aristotle, then for Newton, and then again for Einstein and Heisenberg. Indeed, if modern physicists can see themselves as rehabilitating the Newtonian aether in quantum and relativity theories, a fortiori would it be appropriate to say the same about the Aristotelian aether, which is much more like what they are talking about than is the aether of classical physics.

"Nature loves to hide," Heraclitus said, and the evidence for aether is a case in point. Its existence is by no means self-evident, and is only detected by inference--sometimes lengthy and complicated inference, punctuated by many premises that are merely tentative. While the argument for aether was first made by Aristotle, and many of the fundamental insights contained in this argument are still valid, the cause of aether has now been taken up by the most empirically successful theories of contemporary science. As one physicist puts it, with relativity, quantum theory, and astrophysics, "we are going full cycle, back to the aether and quintessence of Aristotle. . . . [This is] a true 'quintessence,' in the

spirit of Aristotle."⁽¹⁶⁴⁾ The allegedly different subjects of natural science and natural philosophy have reached the same conclusion, though by way of somewhat different means, suggesting that perhaps the disjunction between the philosopher and the scientist has been too radical and thorough. The myth that experimental science invariably refutes the perennial natural philosophy, and that the aether in particular is a prime example of the casualties of this conflict, is itself being rethought and repudiated. Phoenix-like, the aether, after having received what appeared to be a mortal wound, is still with us in both philosophy and experimental science, and it bids fair to remain.⁽¹⁶⁵⁾

1. It is often repeated that the Copernican revolution was a demotion for the Earth and for mankind in general, taking him from the center of the universe, a privileged place in contemporary speech, and placing him in a position of subordination and subservience. Now man would realize, the story goes, his own insignificance in the great scheme of things. (I doubt I need to cite evidence of this claim; examples are legion.) Regardless of whether some may have derived an overly anthropocentric world view from the centrality of the Earth, Aristotle did not. He consistently argued that the part of the cosmos beneath the moon was the least both in quantity and quality. See, for example, Meteor., 2.1.353a35-b6.

2. Setting aside the presence or absence of twinkling, to the naked eye a planet looks no different from a star, and hence a planet was thought to be one of the stars, distinguished from the others only because its circular motion had certain irregularities, earning for it the name plants, "wanderer." Likewise, then, if the Earth is one in kind with the planets, it seems likely that it is one in kind with the stars in general.

3. De Caelo, 1.2.269b14-17. (All translations of Aristotle and St. Thomas will be my own.) After some preliminary distinctions in the previous chapter, Aristotle presents the core of his argument for aether in 269a2-269b17. St. Thomas, in his exposition of this chapter, divides these arguments into five (I De Caelo, lect. 4), but notes that they may all be treated as one primary argument complemented by defenses of certain key premises and responses to natural objections; see ibid., nn. 15 and 17.

4. In spite of the caricature one often sees of the ancient and medieval idea of the cosmos, Aristotle and all the ancient astronomers knew that the Earth and its atmosphere must be of an insignificant size relative to the cosmos as a whole. See Aristotle, De Caelo, 2.14.297b24-298a21; St. Thomas, I De Caelo, lect. 28, nn. 3-4; Ptolemy, Almagest 1.6.

5. See Aristotle, De Caelo, 1.3.270b16-24. Aristotle may be deriving this etymology from Plato's Cratylus, 410b, where Socrates tells Hermogenes that "'Aether' [jv] I would interpret as 'the always running' [jvv]; this may be correctly said, because this element is always running in a flux around the air." Plato, however, is implicitly identifying aether with the sphere of fire surrounding that of air; Aristotle was the first to use the word as designating a kind of matter that is not found below the sphere of the moon. Aristotle notes that Anaxagoras and Empedocles applied the word to fire or air (De Caelo, 1.3.270b24-25; 2.13.294a25-27).

6. De Caelo, 1.3.270b12-17. While it is unclear how many centuries of records Aristotle personally knew to show that stars do not come to be or cease to be, at his time the Egyptians and Babylonians were well known to have over a thousand years of accurate astronomical records that indicate no change in the relative positions, speeds, and number of the heavenly bodies. See D. R. Dicks, Early Greek Astronomy to Aristotle, ed. H. H. Scullard (Ithaca, N.Y.: Cornell University Press, 1970), 166-75.

7. Emphasizing the dialectical character of these arguments for aether, St. Thomas notes that although all the evidence suggests that the heavenly bodies and their motions are incorruptible, it remains a possibility that we have not observed them long enough (I De Caelo, lect. 7, nn. 5-6). Aristotle himself does not try to show that the Earth is immobile at the center of the universe until De Caelo, 2.13-14, and St. Thomas points out that until this issue is settled it is possible that the Earth, not the heavens, is moving (II De Caelo, lect. 11, n. 2). While Aristotle could not be accused of circular reasoning here, since the later arguments against the Earth's mobility do not appear to rest on the assumption that the heaven is a different kind of matter, nevertheless these arguments are not demonstrative.

8. As one commentator puts it, "If ever an empirical attempt was made to save the phenomena, it is to be found in the De Caelo. In no other Aristotelian writing is it more true that the 'mistakes' come from this implicit reliance on what 'we see' as beyond question" (John H. Randall, Aristotle [New York: Columbia University Press, 1960], 153).

9. See, for example, De Caelo, 1.7.274a30-34; 1.8.277a9-13; 2.12.291b24-28; 2.13.294b30-295a2; and 3.1.299a1-6.

11. De Caelo, 2.1.284b4-5, and 3.7.306a17, respectively; see also 2.13.293a25-30.

15. Ibid., 2.12.292a14-18. A parallel text and locus classicus is at De Partibus Animalium, 1.5.644b23-645a7.

17. On the distinction between proper and common sensibles, see De Anima, 2.6; on the visibility of the transparent, and therefore of its magnitude, via the colored bodies bounding it, see ibid., 2.7.418b3-15.

18. Physics, 4.1-9; while only chapters 6-9 treat void directly, the previous chapters on place and the Platonic idea of space are crucial for fully appreciating the arguments in the later chapters.

19. While there is some controversy about the chronological order of some of Aristotle's works, there is broad scholarly agreement that the traditional pedagogical order of Physics-before-De Caelo is correct; see Werner Jaeger, Aristotle: Fundamentals of the History of His Development, trans. Richard Robinson (Oxford: Clarendon Press, 1934), 294-306. Not only are there numerous references in De Caelo to matters settled already in Physics, but not vice versa (e.g., 272a30, 274a19-23, 275b22, 299a10); some such references are even to Physics 4 (e.g., 303a22). At any rate, many De Caelo passages indicate that Aristotle is assuming the refutation of void as an underlying principle in the aether doctrine. For example, just before making the case for aether's existence in De Caelo, 1.2, he argues that there are only three dimensions by invoking as a premise that every three-dimensional continuum is a material body (268a1-10), something that can be assumed only if the possibility of void (a nonmaterial three-dimensional magnitude) has been implicitly rejected already. Further, after discussing aether's properties, he denies off-handedly the possibility there is a void outside the cosmos, treating the question as though it had already been settled (279a11); see also ibid., 2.4.287a7-12, and 2.8.290a7, where he explicitly assumes that there are no empty spaces or discontinuities in the cosmos.

20. Making this distinction is of course critical, as many commentators will take as the fundamental argument against void what is really just a preparatory dialectical argument, and will therefore ridicule the weakness of Aristotle's position. The problematic arguments about motion in a void, for example, are frequently taken to be either Aristotle's primary reasons for rejecting void or his only ones; see Randall, Aristotle, 127, and Friedrich Solmsen, Aristotle's System of the World: A Comparison with His Predecessors (Ithaca, N.Y.: Cornell University Press, 1960), 136-41. A careful survey of Physics, 4.8 manifests that some of the arguments pertain to motion in void, and others to void as such (the division occurring at 216a26), as most commentators will agree (e.g., Edward Hussey, in Physics, Books III and IV, trans. Edward Hussey [Oxford: Oxford University Press, 1993], xxxv, 133; David Furley, The Greek Cosmologists, vol. 1, The Formation of the Atomic Theory and Its Early Critics [Cambridge: Cambridge University Press, 1987], 190-91).

21. Other than De Caelo, 1.3.270b22, nowhere else in De Caelo, Physics, or Metaphysics does Aristotle unambiguously use the word jv to describe the heavenly substance, usually preferring the names "first body" (ibid., 1.3.270b2-3), or "divine body" (ibid., 2.3.286a11-12), or generically "the heaven" (ibid., 1.9.278b11-18). Occasionally he does name it from its circular motion (ibid., 1.3.269b29-30), or from its immutability (De Anima, 2.6.418b9). From Aristotle's medieval disciples we received the name quinta essentia, "fifth essence," the root of our modern words "quintessence" and "quintessential" (which will turn up in a surprising context later in this essay). This is of course the origin of the commonly heard claim that Aristotle posits a "fifth element." While he does call it the "first element" on one occasion (Meteor., 1.3.340b12), he avoids calling it an element and never calls it the fifth element, perhaps because "element" designates only what enters into composition with other things, and because counting it as the fifth implies too much homogeneity between it and the traditional other four; calling it "first," however, follows the order of nature, whereas calling it "fifth" follows the order of coming to know. For better or for worse, however, Aristotle's first body is now referred to as aether, and it was under this appellation that it was later rejected and then more recently revised and rejuvenated. We too will follow the convention.

22. De Caelo, 1.9.278b16-18. Aristotle here notes two other uses of the word ouranos, "heaven," one more general and one more restrictive, saying that "heaven" is also used to name the universe as a whole and also the outermost sphere of the fixed stars (278b11-21). The one we have quoted, however, seems Aristotle's preferred use.

23. If it were only the stars and planets in which Aristotle was interested, the argument for their existence would of course be quite simple: Look up. Aristotle, however, argues in a philosophical mode in De Caelo, 1.2, presenting multiple reasons, all based indirectly on the experience of the heavenly motion, but none simply reducible to that experience. At any rate, the nature of the stars and planets is not taken up until 2.7, where he argues from the nature of the heavenly substance in general to that of the stars, for "the most reasonable and fitting account for us [to offer] is that each of the stars is made from that sort of body in which happens its habitual motion, since we have said that there is something naturally apt to moving in a circle" (289a14-16). This implies that in the earlier treatment of aether in 1.2ff. Aristotle does not yet distinguish between stellar/planetary aether and the aether of the entire heavenly region.

24. For a compelling exposition and defense of Aristotle's arguments against the void, see St. Thomas, IV Phys., lects. 1-14. For recent critiques of the notion of void underlying the Newtonian idea of absolute space, see R. Glen Coughlin, "Immobility of Place in Aristotle," Philosophia Perennis 1 (1994): 3-34; idem, "Some Considerations on Aristotelian Place and Newtonian Space," Aquinas Review 1 (1994): 1-48.

26. The simplicity of aether is most explicitly defended in the course of manifesting aether's existence in 1.2, e.g., at 269a2-7, 269a18-28. On the perfection of circles relative to straight lines, see De Caelo, 1.2.269a19-25.

27. De Caelo, 1.3.269b31. At 269b32-270a5 Aristotle bolsters this conclusion by noting that every nature is inclined only toward one place, and therefore toward one kind of motion to that place, and adds that it does not even appear possible that aether could be forced to move upward or downward. The reason for this will become clear shortly in discussing aether's intangibility.

29. Ibid., 270a15-33. Defending the claim that there is no motion contrary to circular motion is Aristotle's sole objective in De Caelo, 1.4; it is easy to see when the simple motions are defined as toward the center or centripetal, away from the center or centrifugal, and around the center or orbiting. For what is the opposite of an orbit? One might think of circular motions in opposite directions--clockwise vs. counter-clockwise--as contraries, but such directions are not really opposed. For what is clockwise when viewed from above is counterclockwise when viewed from below, and since "above" and "below" taken this way have no absolute significance (whether in a geocentric or in a heliocentric cosmos), any opposition or contrariety based on them would be merely subjective.

32. Ibid., 270b5-26. Likewise, although the planets, or "wandering stars," do appear to admit of changes in speed and direction, these phenomena are easily accounted for in terms of combinations of regular circular motions.

33. Ibid., 1.5-7, 10-12. Although Aristotle has already determined in Physics, 3.5-6, that the universe is finite, at that point he had not yet shown that there is any matter besides what is composed of the four elements; thus, because some of the arguments there are based on the nature of the four known elements, Aristotle returns to the question here to see if the exotic nature of aether changes the conclusion. See St. Thomas, III Phys., lect. 8, nn. 5-9.

34. St. Thomas points out that our knowledge of the nature of the heavenly bodies, like our knowledge of God and of anything whose existence we must prove, must be principally if not entirely negative; see STh I, q. 88, a. 2, ad2.

35. While some tend to see in St. Thomas a radical divergence from his teacher on some matters, there is no evidence of this in the case of the celestial substance. Not only is St. Thomas's exposition of Aristotle's arguments for aether and its properties (I De Caelo, lects. 4-8) among the longest lectiones in the work, it is packed with nearly thirty additional objections to the doctrine (many coming from Philoponus) and St. Thomas's rebuttals of them. Clearly the Angelic Doctor is committed to defending, and sometimes even elaborating, the notion of aether.

36. St. Thomas, I De Caelo, lect. 6, n. 6; see also STh I, q. 55, aa. 1 and 2.

37. See Aristotle, Metaphys., 8.1.1042b2-8; 8.4.1044b3-8; 12.2.1069b24-27. Thus St. Thomas will say that when "Averroes denies that the celestial body has matter. . . . if he understands that the celestial body does not have matter insofar as 'matter' is said in the order to motion or change, he speaks truly. . . . But if he understands the celestial body in no mode to have matter or some sort of subject, then he manifestly speaks falsely" (I De Caelo, lect. 6, n. 6, emphasis added). As St. Thomas explains elsewhere, "it must be admitted that the [prime] matter of the celestial body, considered according to itself, is not in potency except to the form that it has. . . . Whence that form so perfects that matter that in no mode does there remain in it a potency toward being [esse], but only toward where [ubi], as Aristotle says. And thus there is not the same matter of the celestial body and of the elements except according to analogy, insofar as they agree in the notion of potency" (STh I, q. 66, a. 2). When he says there is no potency toward being he of course means no unfulfilled or deprived potency; the aethereal form perfects the matter, it does not destroy it. Prime matter is still present as a principle in aether because aether is not itself a subsistent form and it retains a potency or ordering toward its substantial form (see De sub. sep., cap. 8, � 86); however, its potency is exhausted by this form, thereby removing all privation from it. Thus, its matter and its form cannot exist apart from each other, and the physicist, who comes to the notion of prime matter through an analysis of substantial change and the real separation of the principles, would not even be inclined to say that there is a distinction between the principles in aether. However, the metaphysician, whose interest is being as such, would make a resolution of the heavenly substance revealing prime matter and form as really distinct principles even in aether--albeit form and matter in an equivocal sense when compared to those of mundane substances.

38. Aristotle notes elsewhere (e.g., De Gen. et Cor., 1.5.320b18-21) that pressure results in heating, and so again the aether would have a degree of heat in it.

40. Aristotle speaks of the intangibility or subtlety of aether only occasionally and in passing (e.g., De Caelo, 2.4.287b15-22; 4.4.382a7-21; 4.5.382a14-28), for the obvious reason that the thought-experiment of someone pushing on the heavens is imaginable to him only per impossibile. (On the other hand, Aristotle may be entertaining the possibility of sublunary matter somehow violently being pushed over the boundary into the lower part of the aethereal region when he speculates that the region near the orb of the moon may be filled with impure aether; see Meteor., 1.3.340b6-14.) He does, however, present a simple argument that aether is perfectly yielding when he says that if aether resisted terrestrial bodies, their motion against each other would produce a tremendous sound, which is not observed; see De Caelo, 2.9. At any rate, St. Thomas rightly sees that this subtlety and intangibility follows from what Aristotle has said, and so dwells upon it more frequently; see IV Phys., lect. 12, nn. 8, 13; I De Caelo, lect. 8, n. 15; II De Caelo, lect. 10, n. 13; III De Anima, lect. 17, n. 13; STh III, q. 54, a. 2, ad 2. Note also that it is tempting to confuse the nonresisting or yielding character of a substance with an allowance for interpenetration. They are not the same thing. Saint Thomas grants the former in the case of the aether, but rejects the latter (except by way of a miracle; cf. STh suppl., q. 83, aa. 1-4). One must recall that for one body to move another out of its way does not necessarily imply that the latter simultaneously resists being so moved--to be moved does not mean to be moved violently (pace Newton)--and only by equating these things would one be led to think that not resisting means not being moved, and therefore being interpenetrated. As we will show in a few pages, St. Thomas and Aristotle are more explicit in claiming that the aether can touch and move (per se and in all species of motion) sublunary matter without being touched and moved in return; the (in their minds) bizarre situation I am describing of aether being moved (per accidens and merely locally) by sublunary matter would be contrived and more of a thought-experiment for them, but not absolutely impossible.

41. On the sun's motion and relative position as the cause of seasonal changes, see Meteor., 1.9, 2.2-3; De Gen. et Cor., 2.10.336a32-b19; 337a1-32; on the moon's relation to the tides, see St. Thomas, STh I, q. 70, a. 1, ad 5.

42. Aristotle, Phys., 2.2.194b13 (emphasis added). Saint Thomas explicitly proposes that aether is the cause of substantial forms of all bodies here below, expanding on things Aristotle only hints at (II De Caelo, lect. 10, n. 12).

43. De Caelo, 2.1.284a9-12. This is also part of the upshot of the argument in Physics 8 that there is a first mobile whose natural motion is circular; he is referring of course to the heavens.

44. St. Thomas, II Sent., d. 17, q. 3, a. 1; see also II De Caelo, lect. 10, n. 12; STh I, q. 67, a. 3, ad 3.

45. Aristotle, De Anima, 2.7.418b8-9; St. Thomas, II De Anima, lect. 14, nn. 6-7.

47. St. Thomas, II Sent., d. 13, q. 1, a. 4; STh I, q. 67, a. 3; II De Anima, lect. 14, n. 22; De Sensu, lect. 6, nn. 7-9.

48. See De Anima, 2.7.418b4-15. Transparency is in some way more positive than color is, the latter being a deficient participation in lumen, whereas the former is a more perfect one. Aristotle and St. Thomas further argue that since colors can exist in an intentional mode in what is actually transparent, this too suggests that transparency is a higher, and therefore not merely privative, mode of being; see St. Thomas, De Sensu, lect. 6. Note also that modern science also implicitly denies that transparency is merely a privation; put simply, if darkness is the privation of light and color, transparency cannot be.

49. See St. Thomas, II De Caelo, lect. 10, nn. 3 and 12-13; STh I, q. 66, a. 3, ad 4.

50. De Gen. et Cor., 1.6.323a26-32 (emphasis added). In this same passage Aristotle argues that mutual agency and mutual contact require that both bodies have position and place. If, however, place is not univocally said of celestial and terrestrial matter, again they must lack mutual contact and agency; see 322b27-323a13.

51. III Phys., lect. 4, n. 5 (emphasis added); see also II De Caelo, lect. 10, n. 13; ScG II, ch. 56, n. 5.

52. Aristotle, Phys., 3.2.202a3-12. Aristotle's application (and, implicitly, restriction) to sublunary matter of his own version of Newton's third law can be spotted at De Motu Anim., 698b13-18. Obviously if it were a part of the notion of agency that every mover be moved in reaction, any argument from motion to God's existence would be doomed.

53. Although Aristotle does not seem to use the name "equivocal cause," he does insist that the cause is sometimes like the effect by being only "of the same genus. . . . For the hard is not generated by the hard" (De Gen. et Cor., 1.5.320b20-22; cf. also Metaphys. 7.8.1033b30-1034a9). On equivocal causality in general, see St. Thomas, STh I. q. 4, a. 3; VII Metaphys., lect. 7, nn. 16-19; lect. 8, nn. 13-27) and with reference to the heavens, see II Phys., lect. 4, n. 10; lect. 6, n. 3; lect. 11, n. 2; VIII Phys., lect. 10, n. 4; II De Caelo, lect. 1, n. 4; De Pot., q. 3, a. 11, ad 14. This higher mode of possession, while necessary, is difficult to describe; St. Thomas refers to it variously as an "intentional," or "excelling," or "more eminent mode of being," or as a "total," or "less contracted and more universal" form, one that is "more dominating over matter."

54. De Caelo, 2.7.289a11-35; Meteor., 1.3.341a17-19; St. Thomas, II De Caelo, lect. 10, nn. 6-10.

56. Analogy is itself analogous, and I do not mean to say that each of these is predicated according to exactly the same sort of analogy. For example, "body" (and likewise "quantity") is first said of heavenly and terrestrial bodies with the same notion in mind--and hence the logician predicates these terms univocally--although in the determinate natures themselves their corporeity is not the same. "Matter" said of the heavenly and sublunary, however, seems to signify different notions and different realities. To see the basis for this distinction, consider the passages quoted above in conjunction with De Pot., q. 7, a. 10, ad 7; and STh I-II, q. 113, a. 5, ad 1.

57. De Caelo, 2.8, and St. Thomas, II De Caelo, lect. 12, nn. 1-4; lect. 13, n. 3.

58. See Aristotle, Phys., 4.4.211a19-23; 4.5.212b2-13. Even the parts, the stars, do so such that they are always approaching both the end and the beginning, namely, the center, so that here too the motion is somehow static.

59. See Aristotle, Phys., 8.8-9, especially 265a13-b15; St. Thomas, I De Caelo, lect. 6, n. 7; lect. 8, n. 13.

60. Aristotle, Phys., 4.4.212a24, 4.5.212a35. Saint Thomas suggests a number of reasons why the aethereal circular motion cannot be compared with the natural rectilinear motions we experience in tangible matter, all of which suggest that there is some kind of equivocation when we call the former a local motion; see VII Phys., lect. 7, n. 5.

62. See Aristotle, Phys., 4.5.212a32-b21; St. Thomas, IV Phys., lect. 7, n. 2; II Sent., d. 2, q. 2, a. 1, conclusion.

63. See St. Thomas, IV Phys., lect. 3, n. 2; lect. 6, nn. 9, 14-16; lect. 8, n. 7; STh I, q. 66, a. 3, ad 2; I, q. 66, a. 4, ad 5.

65. On aether's per accidens possession of place via its parts, see St. Thomas, IV Phys., lect. 7, nn. 7-9; on its per accidens possession of local motion via its parts, see VIII Phys., lect. 7, n. 2; I De Gen., lect. 11, n. 5.

66. This becomes still more evident when one notes that the parts of aether are only potentially distinct from each other, and so have place and motion only potentially or virtually. See St. Thomas, IV Phys., lect. 7, nn. 7-14.

68. See Phys., 4.14.223b13-34; St. Thomas, IV Phys., lect. 19, n. 4; lect. 23, nn. 11-13.

69. See St. Thomas, II De Caelo, lect. 1, n. 12; De Malo, q. 16, a. 4; St. Thomas adds that therefore aether is measured not by time but by aevum, a sort of imperfect image of the Divine eternity; see also I Sent., d. 19, q. 1, a. 1.

70. See Aristotle, Phys., 4.12.221a9-21; St. Thomas, IV Phys., lect. 20, n. 3.

71. See Aristotle, Phys., 4.12.221a1-24, b25-31; St. Thomas, IV Phys., lect. 20, nn. 2-4; II De Caelo, lect. 1, n. 2.

72. St. Thomas, IV Phys., lect. 20, n. 6; I De Caelo, lect. 6, n. 5; II De Caelo, lect. 1, n. 2; De Pot., q. 5, a. 4, ad 1.

74. Perhaps having heard an argument of this sort, Galileo takes some pains to establish the reality of a void; see Two New Sciences, trans. Stillman Drake, 2d ed. (Toronto: Wall and Thompson, 1989), 20-80. He even addresses the weaker Physica 4 arguments. While his arguments about free-fall correct some of Aristotle's views, his arguments on the whole, turning on a confused and confusing understanding of the infinite and the indivisible as principles of magnitude, are less convincing.

75. Isaac Newton, "Letter to Bentley, Feb. 25, 1692-3," in Isaac Newton's Papers and Letters on Natural Philosophy, ed. I. Bernard Cohen, 2d ed. (Cambridge: Harvard University Press, 1978), 302-3.

76. Isaac Newton, "An Hypothesis Explaining the Properties of Light," in ibid., 180-81; Opticks, query 21, in Great Books of the Western World, vol. 34 (Chicago: Encyclopedia Britannica, 1952). See also Ernan McMullin, Newton on Matter and Activity (Notre Dame, Ind.: University of Notre Dame Press, 1978), 75-109.

77. On Newton's many attempts at formulating a mechanically sound aether theory, see G. N. Cantor and M. J. S. Hodge, "Major Themes in the Development of Ether Theories from the Ancients to 1900," in Conceptions of Ether: Studies in the History of Ether Theories, 1740-1900, ed. G. N. Cantor and M. J. S. Hodge (Cambridge: Cambridge University Press, 1981), 19-24. Newton's awareness of the severe wrinkles in need of ironing out of his aether theory is probably one of the primary reasons he is so noncommittal about the seat and cause of gravitational forces in the Principia. (Note that "ether" is an alternate spelling of "aether"; I use the latter spelling, although when authors quoted use "ether" I will not change it.)

78. It also served in Newton's mind to explain certain chemical processes and the deceleration of the pendulum in an "evacuated" vessel. See Isaac Newton, "Letter to Boyle, Feb. 28, 1678-9," in Cohen, ed., Isaac Newton's Papers and Letters, 250-53.

79. See Thomas Young, "On the Theory of Light and Colours," Philosophical Transactions of the Royal Society 92 (1802): 12-48.

81. Quoted in K. F. Schaffner, Nineteenth Century Aether Theories (New York: Pergamon Press, 1972), 81.

82. Especially his third law, that "To an action there is always a contrary and equal reaction."

83. Quoted by McMullin, Newton on Matter and Activity, 97. McMullin notes, however, that Newton could not consistently hold himself to this bar; Newton knew that natural science does not really offer an understanding of nature until it asks and tries to answer the more philosophical questions. See ibid., 125-27.

84. See, for example, Newton, "Letter to Boyle," 250-53; Opticks, queries 21 and 31.

85. See Newton, Opticks, query 21; McMullin, Newton on Matter and Activity, 96-101.

86. For example, there was great difficulty in identifying the luminiferous aether as a solid, a liquid, or a gas; while its subtlety and minimal mass per volume suggested that it is gaseous, the swiftness with which it transmitted light suggested that it is solid. Likewise there were difficulties with saying that it transmitted light as a longitudinal (i.e., compression) wave or as a transverse wave (i.e., a displacement of the medium, like a water wave); on the one hand, light propagates spherically, suggesting a compression wave, while polarization phenomena suggest transverse displacement. See Henry Margenau, Open Vistas: Philosophical Perspectives on Modern Science (Woodbridge, Conn.: Ox Bow Press, 1983), 108-9. Oliver Heaviside wrote in 1889 that "It often occurs to me that we may be wrong in thinking of the aether as a kind of matter ([an] elastic solid for instance) accounting for its properties by those of the matter in bulk with which we are acquainted" (quoted in Schaffner, Nineteenth Century Aether Theories, 90). Lodge was likewise cautious about making too strong an analogy between ordinary and aethereal matter: "Ether is often called a fluid, or a liquid, and it again has been likened to a jelly because of its rigidity; but none of these names is very much good; all are molecular groupings and therefore not like ether" (Oliver Lodge, "The Ether and its Functions," Nature 27 [1883]: 304). Perhaps more attention should have been paid to these cautionary remarks.

87. Margenau, Open Vistas, 105-11. This dilution of aether until it becomes essentially indistinguishable from ordinary matter is confirmed in the constant tendency (in Newton, Young, Maxwell, and most nineteenth-century physicists) to imagine and treat aether, even in the mathematical formalisms, as atomic; see ibid., 110-11.

88. See ibid., 112-13. The possibility was entertained--and then refuted (see n. 92)--that aether was dragged along by the Earth via a convection current; convection too, however, implies that luminiferous aether acts like ordinary matter.

89. Heinrich Hertz in 1889 and J. Chwolson in 1902, respectively, the former quoted by Schaffner in Nineteenth Century Aether Theories, 101; the latter by Albert Einstein in "The Development of Our Conception of the Nature and Constitution of Radiation," in The World of Physics, vol. 2 (New York: Simon and Schuster, 1987), 295.

90. Michelson and Morley's three papers, originally published in the American Journal of Science in 1881, 1886, and 1887, are reprinted as appendices in Lloyd S. Swenson, Jr., The Ethereal Aether: A History of the Michelson-Morley Aether-Drift Experiments, 1880-1930 (Austin, Tex.: University of Texas Press, 1972), 247-85. Swenson offers a more detailed account of the experiment at ibid., 65ff.

91. See Edmund Whittaker, A History of the Theories of Aether and Electricity, vol. 1, rev. ed. (London: Nelsen, 1951), 390-92.

92. See Swenson, The Ethereal Aether, 190-233; H. A. Lorentz, "Michelson's Interference Experiment," in H. A. Lorentz et al., The Principle of Relativity: A Collection of Original Memoirs on the Special and General Theory of Relativity, trans. W. Perett and G. B. Jeffrey (New York: Dover, 1952), 3-4. The aether "atmosphere" idea implied that light from distant stars should undergo an aberration depending the direction from which it comes, and this aberration is not observed.

94. Lorentz-FitzGerald contraction has been recently defended by Rudoph Peierls ("Relativity," in The World of Physics, 172-73) and John Bell (quoted in The Ghost in the Atom: A Discussion of the Mysteries of Quantum Physics, eds. P. C. W. Davies and J. R. Brown [Cambridge: Cambridge University Press, 1986], 48-49).

95. Max Born, for example, says Lorentz's theory is full of "very artificial assumptions" ("On the Meaning of Physical Theories," in The World of Physics, 63). While there is merit to this criticism, it seems to me hyperbolic, given what little was then (and is still) known about the nature of light. Born, after all, is writing in the wake of the success of relativity theory, which offered the account of the null-result that is still accepted; hindsight is always 20/20.

96. Einstein, "The Development of Our Conception of the Nature and Constitution of Radiation," 295, 299.

97. This assumption of the constancy of the speed of light "in a vacuum" does not implicitly assume that aether does not exist; I am simply following common usage. "In a vacuum," of course, would also mean "in the luminiferous aether," until after aether was rejected.

98. There are a number of more detailed yet simple explanations of how special relativity explains the results of the Michelson-Morley experiment; see, for example, Simon Saunders and Harvey R. Brown, "Reflections on Ether," in The Philosophy of Vacuum, ed. Simon Saunders and Harvey R. Brown (Oxford: Oxford University Press, 1991), 44-45.

99. Bell still insists that a Lorentz-type aether is more plausible an account of nature than relativity, which he believes allows for time-travel and destroys any hope for a realist understanding of nature; see The Ghost in the Atom, 48-50. Those who recognize the great insights in the perennial philosophy of Aristotle and St. Thomas, of course, and therefore start from things better known to all men, will also conclude that the "orthodox" interpretation of relativity cannot be true without qualification. Unfortunately, there are few among us who have attempted alternative interpretations; nor will I propose one here, leaving that for better minds.

100. On the prevalence of positivism among physicists as a partial cause of the acceptance of relativity, see Swenson, The Ethereal Aether, 185-88, 201, 231; and Bell, quoted in The Ghost in the Atom, 49.

102. In histories of science it is often forgotten that, ironically, according to relativity it is as true to say that the sun revolves around the Earth, with Aristotle, as it is to say that the Earth revolves around the sun, with Copernicus. In a real sense Einstein undoes Copernicus, for he says that Copernicus and Aristotle were both right (and wrong).

103. This is true most often of popularized accounts, even by serious physicists; see, for example, Margenau, Open Vistas, 114-15; Louis de Broglie, Matter and Light: The New Physics (New York: Dover, 1939), 266. More rigorous scholarly work tends to be more evenhanded about aether's continued tenure in recent physics; see Swenson, The Ethereal Aether, 187ff.; and especially Ludwik Kostro, Einstein and the Ether (Montreal, Quebec: Apeiron, 2000), passim. This latter work thoroughly explodes the myth that the mature Einstein saw himself as aether's hangman; rather, Kostro shows, after 1916 Einstein consistently understood aether to be essential to relativity theory. (Note also that many of Einstein's writings quoted below are available in English only in Kostro.)

104. In a number of writings, both published and unpublished, Einstein expresses his regret about his overzealous claims to have eliminated aether; see Kostro, Einstein and the Ether, 1-2, 76.

105. Albert Einstein and Leopold Infield, The Evolution of Physics (New York: Simon and Schuster, 1938), 153.

¹⁰⁶ Albert Einstein, Sidelights on Relativity, trans. G. B. Jeffrey and W. Perrett (New York: Dover, 1983), 13, 15. Lodge, a defender of aether even during the period immediately following upon relativity's alleged elimination of it, would argue that "A superstition has recently arisen that the ether is an exploded heresy and is unnecessary; but that is an absurd misunderstanding. The theory of relativity says nothing of the kind. . . . [I]gnoring a thing is not the same as putting it out of existence" (Oliver Lodge, "Speech Through the Ether," Nature 108 [1921]: 88).

110. This appears to be an analogous rather than metaphorical predication, as space-time curvature is mathematically similar to the curvature of certain two-dimensional surfaces. Note that in Einstein's use of the word, the surface of a cylinder is not "curved"; his is both a looser and a more specific (i.e., technical) use of the word than that of common speech.

111. A "metric" is possessed by a slice of space (or, in general relativity, space-time) describable by an equation incorporating infinitesimal differences between its endpoints. If a metric is a "tensor," it allows the components of the system or equation to be transformed from one set of endpoints to another; thus it plays the central role in general relativity of defining the geometry of space-time and giving the prescription for integrals and derivatives. (Where the space is not flat, it is said to be permeated by a "tensor field.") For an in-depth account of general relativity's metric tensor, see P. J. E. Peebles, Principles of Physical Cosmology (Princeton: Princeton University Press, 1993), 227-44.

112. Curved space-time is sometimes described as doing away with Newton's forces and with gravitational agent causality in general; see, for example, Robert Lindsay and Henry Margenau, Foundations of Physics (New York: Dover, 1957), 96, 358-61. This sort of reductionism (ironically) applied to Newtonian physics may be precipitate, as forces seem rather to be the effects of space-time curvature; at any rate, to attempt to explain motions in nature without recourse to agent causes is to mistake physics for mathematics. See Aristotle, Phys., 2.2.193b31-194b15.

114. Einstein, "Dialogue concerning Accusations against Relativity Theory," in Kostro, Einstein and the Ether, 76.

116. Einstein, "On the Ether," in The Philosophy of Vacuum, 15, 20; see also Sidelights on Relativity, 4-6.

118. On the inertial and gravitational causality of the aether, see Kostro, Einstein and the Ether, 166-67, 177-80.

120. Quoted in Kostro, Einstein and the Ether, 112; see also the "Morgan Manuscript," � 13, quoted in ibid., 78.

122. Einstein, "Dialogue concerning Accusations against Relativity Theory," quoted in Kostro, Einstein and the Ether, 76. Einstein never compares his aether to the Aristotelian aether, apparently unaware that there was one.

123. Einstein, "Morgan Manuscript," � 22, in Kostro, Einstein and the Ether, 78. Einstein frequently reiterates that his aether is devoid of all predicates pertaining to motion, place, and time, but he implicitly admits that such predicates may apply to it in an extended sense. What Einstein is saying most precisely is that aether lacks a determinate and mathematically expressible velocity, or state of motion or rest, or a trackable temporal history at any point within it; see, for example, Sidelights on Relativity, 13-15, 19. Relativistic aether is nevertheless ubiquitous and has points and spatio-temporal coordinates within it, each possessing its own distinct curvature, and which change with time--all of which are spatial and temporal predicates that must have some (perhaps nonmathematical) sense to them. Likewise, by surrounding bodies, the relativistic aether locates them or gives them place, and it is the substratum of space-time; therefore, it bears an intrinsic relation to space and time as a principle of them. Thus, distinguishing analogical (i.e., rationally equivocal) from univocal predication, and following Aristotle's account, we may rightly say that even Einstein's aether both has and does not have place, motion, and time.

124. Einstein, Sidelights on Relativity, 23-24. Unlike the classical account of aether and ordinary matter, general relativity's aether is not atomic; it is strictly continuous. See, for example, ibid., 15; "On the Ether," 14.

125. Respectively, "On the Ether," 13; and "Morgan Manuscript," � 22, quoted in Kostro, Einstein and the Ether, 78.

126. Respectively, Einstein, Sidelights on Relativity, 18; and Albert Einstein, Ideas and Opinions (New York: Crown Publishers, 1960), 375-76; on p. 15 of the former he also calls aether an "extended physical object."

128. See Aristotle, Metaphys., 5.12, 9.1, 9.7. On the root notion of matter as "that from which a thing is composed," see Charles DeKoninck, "Abstraction From Matter (part one)," Laval th�ologique et philosophique 13 (1957): 148-62.

129. As we said earlier, the matter of aether is a potency for natural being, but not for natural becoming, except perhaps accidentally, whether that be considered with regard to place via the rotation of the heavenly sphere, or with regard to relativistic curvature.

131. Kostro, Einstein and the Ether, 180; on the materiality of this aether in relation to Einstein's search for a unified field theory, see ibid., 137, 143-44, 147. Some, on the other hand, tend to see in relativity a dematerialization of aether, and of nature as a whole, such that "matter is no longer material" (Margenau, Open Vistas, 113-15, 126-27). Such a conclusion leads one to suspect that perhaps there has been a misunderstanding of what matter really is.

132. Einstein is not alone in his opinion that aether is at the heart of his curved space-time. See, for example, Robert Weingard, "Making Everything out of Nothing," in Philosophy of Vacuum, 202-3; for an enumeration of others, see Kostro, Einstein and the Ether, 88-90, 98-101, 185-88. Some physicists and historians of science who are aware of Einstein's aether-interpretation of relativity tend to dismiss it as no longer tenable or "a purely artificial concept" (Max Born, "On the Meaning of Physical Theories," 64). We should note that Einstein explained relativity in terms of an aether on fewer occasions as years went on not because he changed his mind, but because of Nazi physicists' use of the aether concept and their bitter mockery of "Jewish physics," epitomized, in their eyes, by Einstein; see Kostro, Einstein and the Ether, 137, 147, 150-52.

133. See Einstein, Sidelights on Relativity, 19, 21-23, and Kostro, Einstein and the Ether, 6-7, 96-97, 100, 116ff.

134. Thus we will leave out discussions of Planck's constant, quantum leaps, wave-particle duality, and complementarity. Summaries and interpretations of quantum theory have been presented recently in this journal; see Wolfgang Smith, "From Schr�dinger's Cat to Thomistic Ontology," The Thomist 63 (1999): 49-63. For more in-depth but manageable presentations that concern themselves with the philosophical interpretation of the theory, see the following: David Z. Albert, Quantum Mechanics and Experience (Cambridge: Harvard University Press, 1992); R. I. G. Hughes, The Structure and Interpretation of Quantum Mechanics (Cambridge: Harvard University Press, 1989); Werner Heisenberg, Physics and Philosophy: The Revolution in Modern Science (New York: Harper, 1958); Wolfgang Smith, The Quantum Enigma: Finding the Hidden Key (Peru, Ill.: Sherwood Sugden, 1995).

135. See Henri Poincar�, Mathematics and Science: Last Essays, trans. John W. Boldoc (New York: Dover, 1963), 86ff; Whittaker, A History of the Theories of Aether and Electricity, 2:268ff.

136. QED is a part of quantum field theory, which is quantum theory with a special emphasis on the fields associated with elementary particles.

138. More specifically, he argued that the uncertainty principle implies that the mean square average of the field in its ground state is a nonvanishing value--that is, there is a tiny but irremovable probability amplitude for the presence, or rather generation, of a proportionately tiny amount of electromagnetic energy. See D. W. Sciama, "The Physical Significance of the Vacuum State of a Quantum Field," in The Philosophy of Vacuum, 139-42.

139. Zero-point energy is often interpreted, obscurely, in terms of a continuous sea of "virtual particles" in the quantum field, so named because they are not in principle measurable (although their effect on real particles is measurable).

140. See Sciama, "The Physical Significance of the Vacuum State of a Quantum Field," 144.

141. For more detailed summaries, see C. J. Foot, "Something from Nothing," Nature 362 (1993): 206-7; John D. Barrow, The Book of Nothing: Vacuums, Voids, and the Latest Ideas About the Origins of the Universe (New York: Pantheon, 2000), 204-11.

142. More precisely, the attraction is predicted and observed to be in a subquadruplicate ratio to the separation.

143. For more empirical evidence of fluctuations of the quantum vacuum in fields as diverse as spectroscopy, solid-state physics, and chemical reactions, see Sciama, "The Physical Significance of the Vacuum State of a Quantum Field," 142-50, and Aitchison, "The Vacuum and Unification," 180-85.

144. B. J. Hiley, "Vacuum or Holomovement," in The Philosophy of Vacuum, 222-23.

146. In chemistry, for example, a "vacuum" is not necessarily devoid of any matter, but rather is defined as any volume of gas that exerts less than 1/1000 of an atmosphere of pressure.

147. The first quotation is taken from Sciama, "The Physical Significance of the Vacuum State of a Quantum Field," 137; the second and third from Saunders and Brown, "Reflections on Ether," 29 and 59.

150. Perhaps the reason the name is avoided now is that there is a desire to avoid confusion, for the Michelson-Morley experiment did indeed refute the existence of what is often simply called "aether," and to say that QED and relativity vindicate aether may seem incoherent. Some physicists, however, think that there is more to the common resistance to speaking of aether now than the purely pedagogical concern, noting an instinctive hostility among other physicists to the word "aether" when interpreting zero-point energy:

The reaction against the reintroduction of such an ether or plenum has been so strong that any theory that dared to call on such a notion was for a time [after the advent of special relativity] deemed to be unacceptable and even preposterous. In the 1960s and 1970s I often came across such a reaction when I tried to discuss de Broglie's use of a "sub-quantum medium" as a means of providing a possible explanation of the quantum formalism. The objection was not so much against the attempt to find a more physically intuitive explanation of quantum phenomenon [sic], but rather against the introduction of the "sub-quantum medium." The retort, "Surely Einstein has shown us that the vacuum is 'empty' and the reintroduction of such an outmoded way of thought will not provide a satisfactory understanding of the phenomena," was not uncommon. Yet in relativistic quantum field theory the notion of "vacuum polarization" had already emerged and was being used quite freely, . . . [and] Einstein himself (1924) did not react so strongly against the notion of an ether. (Hiley, "Vacuum or Holomovement," 219)

(Vacuum polarization is another phenomenon manifesting the existence of zero-point energy. The 1924 reference to Einstein is to the essay "On the Ether," cited above.) Is this refusal to countenance aether perhaps motivated by a desire to "save face," not to admit that in a sense physicists were wrong to see the Michelson-Morley experiments and special relativity as the definitive overthrow of aether? Hiley does not pursue the question much further, but he does go on to say that this visceral rejection of aether as an allegedly appropriate heuristic for the quantum vacuum and general relativity's curved space-time has been fading in the past two decades.

151. While special relativity is compatible with quantum theory, general relativity is not. However, quantum field theory, of which QED is a part, is so far the first partial harmonization of quantum theory and general relativity, for it imitates the latter's emphasis on the influence of the geometrical structure of space-time--which field or space-time structure we saw was the segue into each theory's need for an aether. Recall, however, that Einstein saw both gravitational and electromagnetic fields as aspects of one aether; see above, note 119. Note also that Dirac himself thoughtfully argues that the quantum and relativistic aethers are essentially compatible, different sides of the same coin. Pointing out that the indeterminacy of the spatio-temporal predicates that relativity implies for its aether is required by the uncertainty principle, he says that the less massive a body is, the more indeterminate its velocity becomes; thus, if aether is massless, its velocity will be perfectly indeterminate, as special relativity requires; see Dirac, "The Evolution of the Physicist's Picture of Nature," Scientific American 208 (May 1963): 51.

152. See J. P. Ostriker and P. J. Steinhardt, "The Observational Case for a Low Density Universe with a Non-Zero Cosmological Constant," Nature 377 (1995): 600-602; James Glanz, "Astronomers See a Cosmic Antigravity Force at Work," Science 279 (1998): 1298-99; idem, "Exploding Stars Flash New Bulletins from Distant Universe," Science 280 (1998): 1008-9; Barrow, The Book of Nothing, 184-92, 297-301; Lawrence Krauss, Quintessence: The Mystery of Missing Mass in the Universe (New York: Basic Books, 2000), 107-9, 332-36. Although this last work is mainly focused on astrophysics' search for dark matter, it is particularly noteworthy insofar as it enumerates and explains other phenomena besides the supernova data that suggest a need for a cosmic-scale vacuum energy; see Krauss, Quintessence, 222-28.

153. Peter Coles, "The End of the Old Model Universe," Nature 393 (1998): 744.

154. Glanz, "Exploding Stars Flash New Bulletins from Distant Universe," 1008-9; Krauss, Quintessence, 335.

156. On each of these properties of the cosmological constant, see Barrow, The Book of Nothing, 185, 244-45, 290-91, 297-301; Krauss, Quintessence, 223, 334.

157. There remain few physicists who believe that the standard interpretation of either relativity or quantum theory is simply untrue, but many believe each is only a half-truth that will be superseded by later, more unifying fundamental physics. Certainly there are philosophical problems in the way the theories are articulated or made intelligible by their proponents, the solution of which will probably require some revision of standard interpretations.

158. For an illuminating discussion of the threefold distinction among the incomplete induction of the experimental scientist, the complete induction for which he hopes, and the intuitive induction of per se nota propositions that are the basis of natural philosophy and of knowledge in general, see DeKoninck, "Abstraction from Matter," 139-45.

159. Indeed, St. Thomas argues that circular motion is not an inseparable accident of the heavens, arguing that the outermost heavenly sphere beyond the fixed stars, the "empyrean heaven," is thoroughly immobile and the most foundational of the concentric spheres of aether; see STh I, q. 66, a. 3, ad 1 and ad 2; I, q. 66, a. 4, ad 5.

160. On the latter, see St. Thomas, I De Caelo, lect. 18, n. 7; STh I, q. 58, a. 3.

161. One could reasonably respond that nevertheless in the first book of De Caelo Aristotle derives most of these properties from the circular motion of the heavens, which no contemporary scientist would accept, and thus the similarities noted are indeed purely coincidental. However, this would be to delineate too narrowly the sources of Aristotle's doctrine about aether. For example, we see Aristotle laying the groundwork for the idea of aether throughout the Physics, and especially in the fourth and eighth books in their recurring reference to the outermost container of the cosmos as the ultimate source of place and motion. Such is the source of his aether's nonlocalized, nonmoving, yet universally causal properties. Likewise, this would imply aether's one-way causality, immutability, and intangibility, all without reference to its circular motion, and these lines of reasoning are neither opposed to nor even radically unlike those that drive Einstein and the others to posit their aethers. Thus, we suggest, although the line of reasoning in De Caelo is in many ways the most accessible, it is not the sole path of discerning aether's properties. Nevertheless, Aristotle's emphasis on the circularity of the motion as an epistemological principle of aether's properties makes one wonder whether some kind of circularity might still be present in the aether, especially given that Aristotle speaks in De Caelo, 1.3, as though a natural circular motion is somehow necessary for the perfection of the cosmos. General relativity's geodesic light path is a possible candidate for such an aetherial circular motion, since in many other ways it resembles Aristotle's primum mobile, although to consider this in detail would take another paper.

162. Occasionally Aristotle seems to speak of there being some quasi-admixture of aether with sublunary matter; see Meteor., 1.3.340b6-341a37. Much has been made of this "sublunary aether" in recent years; see John Thorp, "The Luminousness of the Quintessence," Phoenix 36 (1982): 104-23; Aristotle's De Motu Animalium, ed. and trans. Martha C. Nussbaum (Princeton: Princeton University Press, 1978), 143-64. Still, it is difficult to see how this notion is consistent with Aristotle's overall claims that aether does not alter or corrupt, so could not enter into the constitution of a complex substance, and that aether moves only circularly, so could never get down here to begin with. As was said earlier, Einstein sometimes (intemperately) claims that ordinary matter is nothing more than a special state of aether; this precipitate conclusion derives from an ambiguity inherent in the mathematical formalism of relativity. For relativity implies that metric tensor fields can be found not only in "empty" space, but in all space; it seems that aether is literally everywhere, even where something else is. Unless one is willing to grant an interpenetration of physical substances, one must say that either the aether is the substance at that point in the field or ponderable matter is, and Einstein takes the former option. However, we know that aether exists at all only because we have started with the self-evident fact that ordinary matter exists and is substantial, and to sacrifice what is evident for the sake of an hypothesis is incongruous. Thus, a more reasonable interpretation of the pervasiveness of the field would be that, just as aether seems to be able to exist in close proximity with ordinary matter, ordinary matter can participate by degrees in the nature of aether, and this participation is what is symbolized by the continuity of the mathematical formalism. Perhaps such a doctrine of aethereal participation can be made along the lines of Aristotle and St. Thomas's doctrine of participation in grades of transparency in all bodies; see Aristotle, De Sensu et Sensato, 3.439a21-25; and St. Thomas, De Sensu, lect. 5. The connection between aether and light (or electromagnetism) is not accidental.

165. Gratitude is owed to Stephen Baldner, Peter Orlowski, Ronald Richard, and my wife Rose, who read and offered many helpful comments on an earlier version of this paper. The remaining mistakes are, of course, my own.