Waving Aether vs. Corpuscular Bullets

This is a continued investigation into the historic search for mediators of invisible phenomena like light, gravity, and magnetism.  The blog series starts here.  Last time we talked about color.  Hooke had a pretty loony idea about color but correctly believed it a product of motion.  He was a proponent of a sort of undulatory mechanism of light — vibrations in the aether. 

Newton, in alternative, provided a more coherent explanation of color, suggesting that white light consisted of rays of all colors.  Newton’s success in color theory would have alone guaranteed him a place in history.  He immediately won over the hearts and minds of his contemporaries, so it should come as no surprise that they followed him when he promulgated a less accurate conception of light’s mediator:  the corpuscle.

The corpuscle of light, according to Newton was a physical object like a bullet.  Light is a radial flurry of projectiles.  Newton imagined that color was a product of the size of the corpuscle, much as we consider wavelength the size of light today.  He wrote:

“If corpuscles of various sizes exhibiting the colours of the second & third order be mixed, they should rather constitute an imperfect whitenesse or Grey …”

and

“…for the production of Black the corpuscles must be lesse then any of those which exhibit colours. For at all greater sizes there is too much light reflected to constitute this colour.”

Newton gets a lot of credit for the corpuscle, although a close read suggests he had his doubts about the light-as-a-physical object paradigm.  It seems that popular science writers of the day, such as Desaguliers, ran away with the idea, ascribing a greater certainty to the physicality of the corpuscle than Newton himself.  Newton was always careful in making physical interpretations of the phenomena he so carefully described with mathematics from his experimentation.  In his work on colors addressed to the Royal Society he wrote:

“But to determine more absolutely what light is… is not so easie. . And I shall not mingle conjectures with certaintyes.”

Despite these misgivings about the corpuscle, Newton could not tolerate the undulatory theories of his day.  For him, there were issues that Hooke and Descartes’ aether waves could not rectify.  He based his confidence on three main lines of evidence against the undulatory theory:  

1) Its inability to explain rectilinear propagation

2) Its inability to explain the strange behavior of double-refracting crystals and

3) The mysterious Bologna Stone’s apparent storage of light (phosphorescence).  

The first two complaints remained a subject of heated debate throughout Newton’s lifetime, for which he held the upper hand.  We will elaborate on these momentarily.  The third line was eventually abandoned when Newton and other corpuscularists realized that the light emitted by phosphorous was of a different quality and color than that which it received.

None of this is to say that Newton did not believe in the aether, which was vibrating, but rather that these vibrations could not directly constitute light.  The aether’s vibrations were closer to his conception of heat.  Vibrations simply could not create a rectilinear (straight) path.  

Newton’s corpuscles satisfied not only color, but also refraction and reflection.  He imagined that the diverse angles of reflection and refraction were the result of differing densities of aether through with the different sized corpuscles must pass.  This was a very satisfying explanation to his peers in England.

Another reason that the corpuscular theory held so much traction was that the undulatory theory was somewhat misconceived.  Today we understand that the vibrations of light are transverse — that is, pulsations emanate perpendicularly to the photon as it travels between atoms.  But in Newtons day, undulatory aether waves were thought similar to longitudinal sound waves— that is, sound consists of rarefication and compaction of air exclusively in the direction of the wavefront’s motion.  This misconception of light’s vibratory nature led to comparisons of color to pitch.  

Newton, too, leaned on this misconception: 

“if by any means those [corpuscles] of unequal bignesses be separated from one another, the largest beget a Sensation of a Red colour, the least or shortest of a  deep Violet, and the intermediate ones, of intermediate colours ; much after the manner that bodies, according to their several sizes, shapes, and motions, excite vibrations in the Air of various bignesses, which, according to those bignesses, make several Tones in Sound.”  (1672, phil. Trans. Vii p. 5088)

 Newton’s criticism of the undulatory nature of light, coupled to the near-universal misconception of light as longitudinal waves succeeded in keeping the corpuscular theory central to optics throughout the 17th and 18th centuries.  There were, however, astute detractors.  

In particular, Christiaan Huygens, a dutch polymath corresponding primarily with the Paris Academie des Sciences, had other ideas.  He published his wave theory of light in 1678, which explained diffraction and reflection geometrically, with enough mathematical rigor to challenge Newton’s standing conceptions.

Huygens reprimanded the corpuscular idea of light by noting that beams of light particles do not collide with one another.  For him, this was evidence that light was a motion in something rather than a volley of bullets.  In Treatise on Light, he wrote:

“When one considers the extreme speed with which light spreads on every side, and how, when it comes from different regions, even from those directly opposite, the rays traverse one another without hindrance, one may well understand that when we see a luminous object, it cannot be by any transport of matter coming to us from this object, in the way in which a shot or an arrow traverses the air; for assuredly that would too greatly impugn these two properties of light”

Huygens imagined that like sound, light propagated longitudinally, but that at each point along the path of light a secondary wavefront appears.  This is perhaps the first conception of the modern transverse wave nature of light.

Huygens’ work proved revolutionary in the end.  More than a century later, Augustin-Jean Fresnel would tie up the mathematical loose ends. Today, the Fresnel-Huygens diffraction principles constitute the basis for our modern understanding of optics.  

Huygens spent much of his efforts investigating the double-refraction of Iceland Spar.  At the time is was unknown how one ray could split into two and his repeating wavefronts accomplished this easily.  Also Huygens saw that the two new rays produced through double refraction of a single ray are not as the original.  A ray incident on a second crystal can give rise to one or two depending on the rotation of that second crystal.  Unfortunately, Huygens was never able to provide a physical mechanism for this rotational aspect of the doubly refractive crystal.

Newton, on the other side of the channel, happily seized the opportunity to defend the corpuscular nature of light.  He noted that if we treat the secondary rays of the Iceland Spar themselves as new objects, with different shapes, the rotational behavior is more easily understood.  Since the spar produced the secondary rays, they could perhaps present a different, asymmetric cross-section.  For instance, we can imagine the secondary rays as a rectangular rod whose cross-section presents a top and bottom while the incident ray’s circular cross-section could be infinitely rotated without noticeable difference.  Newton’s clever reasoning gave the corpuscularists even more confidence in their explanation for light.

Recall one of the chief arguments from Newton against the wave theory was rectilinear propagation.  Light, after all, traveled in straight lines, but why?   Maupertuis reconciled that this was not due to light taking the shortest path, but rather by the light taking the least action.  He, Leonhard Euler, and Joseph-Louis Lagrange developed this into a universal principle of least action for all dynamical systems.  They extended their principle to all of nature. The universe is lazy.  It always tends toward the least effort.

The 18th century found thinkers more concerned with electricity than light.  Both the wave and corpuscular theories seemed compatible with this least-action principle and the jury was still out.  Newton was a huge celebrity for his work on color and gravitation, so many thinkers sided with the corpuscular explanations.  Euler defended the wave theory, while most at the royal society preferred Newton’s bullets.  The Marquis de Courtivron and Thomas Melvill attempted to accommodate colored rays with bullets by suggesting differing forward velocities for each color ray.  Astronomers revealed that this should but did not affect the colors of Jupiter as it approached and receded.

Then a young medical doctor named Thomas Young appeared on the scene.  Young was an extraordinary mind working across disciplines.  In addition to optics he made significant contributions to the study of elasticity.  With light, he played on this velocity mishap of his predecessors — for via Roemer in 1675, it was now established that a powerful ray from the sun and a meek ray from a spark traveled with equal speeds.  This seemed a preposterous quality to Young, were light as bullets.  Not so for disturbances in a stiff, composite media. 

Young also pointed out the superiority of the wave theory in explaining bifringegence, coining the term “interference”.  He based his notions, ironically, on Newton’s explanation of the anomalous tides at Batsha in Tonkin.  Young wrote (Works, I, p 202):

“Suppose a number of equal waves of water to move upon the surface of a stagnant lake, with a certain constant velocity, and to enter a narrow channel leading out of the lake ; suppose then another similar cause to have excited another equal series of waves, which arrive at the same channel, with the same velocity, and at the same time with the first. Neither series of waves will destroy the other, but their effects will be combined ; if they enter the channel in such a manner that the elevations of one series coincide with those of the other, they must together produce a series of greater joint elevations ; series are so situated as to correspond to the depressions of the other, they must exactly fill up those depressions, and the surface of the water must remain smooth. Now I maintain that similar effects take place whenever two portions of light are thus mixed; and this I call the general law of the interference of light.”

Young continued

“…whenever two portions of the same light arrive to the eye by different routes, either exactly or very nearly in the same direction, the light becomes most intense when the difference of the routes is any multiple of a certain length, and least intense in the intermediate state of the interfering portions ; and this length is different for light of different colours.“

Young applied this explanation to the appearance of dispersed colors in thin films of soap.  Immediately after presenting his ideas, Young suffered a fierce attack in the Edinbourg Review from Henry Brougham, an established aristocrat and later prominent head of state.  Brougham actually achieved his goal of discrediting Young for several years following the exchange.

Young moved on and applied his interference principle to explain fringes in shadows.  In the Bakerian lecture for 1803 explained that when one places a hair in a cone of rays, the fringes within the shadow result from interference of the rays inflected by the two edges of the object.  Those dark fringes outside of the shadow were the result of interference between direct rays and the rays reflected at the diffracting edge.  He applied his reasoning though a double-slit experiment that proved infamous in future generations of quantum physicists.

Young then set his sights on the largest problem in optics:  the Iceland spar.  He leaned toward Huygens’ model.  Laplace published a dynamical explanation of the crystals in 1808.  Using least-action principles, Laplace showed that the lattice geometry of the crystal could account for the strange refractive qualities.  Young attacked this as unnecessarily abstract and supplied a physical mechanism based on Huygens principles for the double rays, writing:

“a medium more easily compressible in one direction than in any direction perpendicular to it, as if it consisted of an infinite number of parallel plates connected by a substance somewhat less elastic.”  

Later David Brewster confirmed this mechanism when he found that compression in one direction can cause a transparent material to become doubly refracting.  But Young’s brilliance was often lost on his patrons and peers.  After he presented his work on deformation in the field of elasticity, the relevant authorities replied:

“Though science is much respected by their Lordships and your paper is much esteemed, it is too learned ... in short it is not understood.”

Only years later would Young’s ideas on refraction see their full development.  Brougham’s criticism had hit Young hard and he pledged his attention to issues distant from optics, writing:

I have resolved to confine my studies and my pen to medical subjects only. For the talents which God has not given me, I am not responsible, but those which I possess, I have hitherto cultivated and employed as diligently as my opportunities have allowed me to do ; and I shall continue to apply them with assiduity, and in tranquillity, to that profession which has constantly been the ultimate object of all my labours.”

For most of Europe, the issue of the Iceland Spar still required attention.  A competition was issued by the French Academy in 1810 To furnish a mathematical theory of double refraction, and to confirm it by experiment.  Etienne Louis Malus answered the call.  While playing with the crystal, he found light through the window exhibited extraordinary ray properties (rotational orientation).  He deduced that this could be generalized to all transparent solid and liquids.  He wrote:

“for example, light reflected by the surface of water at an angle of 52*45’ has all the characteristics of one of the beams produced by the double refraction of Iceland spar, whose principle section is parallel to the plane which passes through the incident ray and reflected ray.  If we receive this reflected ray on any doubly-refracting crystal, whose principle section is parallel to the plane of reflection, it will not be divided into two beams as a ray of ordinary light would be, but will be refracted according to the ordinary law”

Malus gave this orientation property of light the name “Polarization.”  But Malus’ mathematical treatment polarization only served to forge the resolve of corpuscularists, who strongly identified with Newton’s rod analogy.  An adequate wave interpretation of polarization would not arrive for nearly a decade.

Augustin-Jean Fresnel answered a similar competition issued by the Academy in 1817.  This time the topic was diffraction:  the phenomena of fringes — or the appearance of light within shadows and vice versa.  Fresnel drew on both Huygens and Young’s principles.  He explained that different portions of the same primary wavefront produce secondary waves per Huygens but they are of different phases, which mutually interfere per Young.  

Several notable corpuscularists sat on the judging committee, including Laplace, Poisson, and Biot.  Poisson recognized extensions of Fresnel’s explanation to circular interference problems.  Fresnel collected his prize when Poisson’s circular interference was evaluated empirically by the Academy and found to be in excellent agreement with the young mathematician’s calculation.

By mid 19th century, the tide was turning in favor of the wave nature of light.  Fresnel had packaged the work of Huygens and Young in a sufficiently mathematically palatable manner that thinkers on both sides of the channel were turning on the corpuscular explanation.  There were but two hurdles ahead for the wave theorists.  Principally, wave-like light required a mediator.  The empirical search for this aether ensued frantically during the latter half of the 19th century.  Second, was the abominable aberration of light — that a distant star’s location appeared subject to the motion of the Earth’s orbit — an explanation initially more suitable to a barrage of bullets than a wave. 

In future posts, we’ll examine these final corpuscular complaints with the luxury of detail.  Only then will we be in a position to understand the death of the aether.  And to ask, what have we been left with in its stead?