## Isaac Newton… Father of invisibility physics?

My blog has been a good impetus to research a number of interesting scientific topics more deeply than I would otherwise have had the ambition to do.  For instance, since the blog’s inception, I’ve been pushing the origins of “invisibility physics” further and further back in time.

When I wrote my Ph.D. dissertation on the subject, I believed the earliest article connected with invisibility was a 1910 article by Paul Ehrenfest on radiationless accelerations of charged particles.  However, blog-related research eventually turned up a 1902 paper on a curious and crude invisibility device, and that paper in turn was inspired by a comment made by Lord Rayleigh in an encyclopedia article in 1884.

I’ve had a suspicion for some time, though, that the trail must go back further.  The evidence for this, albeit thin, is a weird tale written in 1859 by Irish-born American writer Fitz James O’Brien, titled “What Was It? A Mystery“.  In the story, a group of lodgers take up residence in an abandoned house that is widely reputed to be haunted.  Their initial amusement turns to horror when one of the group, the narrator Harry, is attacked by a creature that is invisible but very much of flesh and blood.  They manage to subdue the monster, alive, and tie it to the bed.  What follows is colleague Hammond’s attempt to explain the seemingly supernatural phenomenon:

We remained silent for some time, listening to the low, irregular breathing of the creature on the bed, and watching the rustle of the bedclothes as it impotently struggled to free itself from confinement. Then Hammond spoke.

“Harry, this is awful.”

“Aye, awful.”

“But not unaccountable.”

“Not unaccountable! What do you mean? Such a thing has never occurred since the birth of the world. I know not what to think, Hammond. God grant that I am not mad, and that this is not an insane fantasy!”

“Let us reason a little, Harry. Here is a solid body which we touch, but which we cannot see. The fact is so unusual that it strikes us with terror. Is there no parallel, though, for such a phenomenon? Take a piece of pure glass. It is tangible and transparent. A certain chemical coarseness is all that prevents its being so entirely transparent as to be totally invisible. It is not theoretically impossible, mind you, to make a glass which shall not reflect a single ray of light—a glass so pure and homogeneous in its atoms that the rays from the sun shall pass through it as they do through the air, refracted but not reflected. We do not see the air, and yet we feel it.”

“That’s all very well, Hammond, but these are inanimate substances. Glass does not breathe, air does not breathe. This thing has a heart that palpitates,—a will that moves it,—lungs that play, and inspire and respire.”

Though invisibility has played a large role in mythology through the ages, O’Brien’s story is evidently the very first story that provides a scientific explanation for the phenomenon.  Though O’Brien was ahead of his time, by the late 1800s/early 1900s a number of fiction authors would propose their own quasi-scientific theories of invisibility, including H.G. Wells in his famous 1897 novel The Invisible Man.

But what inspired these stories?  Clearly O’Brien and, as we will see, H.G. Wells had some specific science in mind when they concocted their tales of invisibility, but what science, and from whom?  In this post, I would like to engage in some speculation and propose that they were inspired, directly or indirectly, by Isaac Newton!

A few caveats before I begin.  First, let me emphasize that the argument presented in this post is a hypothesis.   As I will explain below, no record survives of the inspiration for either Wells’ or O’Brien’s work.  (As far as I can tell, anyway.)

Second, let me be clear that I am not proposing that Isaac Newton himself was studying invisibility physics!  Rather, a series of Newton’s experiments were suggestive of the possibility of invisibility, and Wells and O’Brien seem to have picked up on this.

If you can accept these caveats, then we can continue!  I’ll introduce my hypothesis by explaining how it occurred to me in the first place.

In exploring the history of invisibility physics, I’ve spent a lot of time employing “brute force” methods: namely, searches through journal archives for the word “invisibility”, among others.

You might think this would be a straightforward task; alas, “invisible” is used historically in many different ways unrelated to true optical invisibility.  For instance, ultraviolet and infrared radiation was regularly referred to as “invisible rays”.  Objects viewed in dim light were referred to as “invisible to the human eye”.  Perhaps most significant, objects of microscopic (or smaller) size were considered part of an “invisible world”.  Simple searches on “invisibility” and synonyms result in a lot of false positives.

But, about a month ago, I got an intriguing hit! A 1902 paper in the journal Physical Review with the provocative title “Optical notes”*, begins with the following passage:

Occasionally there appear notices of methods for making objects invisible by selecting combinations of media having about the same index of refraction as the immersed solid.  This subject was first investigated by Christiansen in 1884.

The statement above (emphasis mine) refers to the phenomenon of index matching.  The simplest way to characterize the optical properties of a material is by its refractive index, labeled $n$, which is the fraction by which the speed of light is reduced within the material.  That is, considering the speed of light in vacuum is $c = 3\times 10^8$ meters per second, the speed of light in a material is $v = c/n$.  The index of refraction depends on the material: for visible light, the refractive index of water is $n = 1.33$, and the refractive index of ordinary glass is roughly $n =1.5$.  This means that the speed of light in water is roughly 3/4 the vacuum speed, and the speed of light in glass is 2/3 the vacuum speed.

Light reflects or refracts, or generally is distorted, when it runs into an interface between materials of different refractive index.  The difference of refractive index between air $n=1$ and glass $n =1.5$ is why we can see reflections even in the clearest pane of glass.

It is possible, however, to submerge a transparent object in a liquid that has the same refractive index, in which case (with caveats to be discussed below) it becomes invisible!  Because the refractive index is the same for the object and the liquid, light passes smoothly through the object without distortion; this selection of a liquid that has the same refractive index as the object to be hidden is the process of index matching.  This is illustrated below:

A somewhat cheating example of index matching is shown in the following video:

The spheres in this video are super-absorbent polymers that are saturated with up to 300 times their weight in water — this example is “cheating” because the spheres are almost entirely water themselves!

Index matching is a viable strategy to reduce reflections in a number of imaging systems.  Early CAT scan machines, for example, required brain scan patients to wear a water-filled cap to reduce the refractive index contrast between the patient’s head and the surrounding region.  Index matching oils are also used in microscopy to avoid image-degrading refraction effects.

Trying to use index matching to make an object entirely invisible is much more challenging, however, due to the phenomenon of dispersion.  In short, the atoms in a material respond differently to light of different frequencies (colors), and the refractive index of the material and hence the velocity of light is different at each frequency.  Though an object may be index matched to a liquid and hence invisible at one frequency, it will in general not be matched at other frequencies and hence visible at those.

In 1884, the aforementioned Danish physicist Christian Christiansen came up with a clever application** for such imperfectly matched materials.  He was investigating index matching of powdered glass, and was mixing the powdered glass in a liquid mixture of benzene and carbon disulphide.  He found that the powdered glass became almost invisible in the liquid, but furthermore found that the light transmitted by the mixture was very brightly colored.

The effect is easy to explain: because of dispersion, the glass and the liquid will typically only have the same refractive index at a single frequency (color).  At that frequency, the glass and liquid act as a uniform medium that readily transmits light; at any other frequency, the light is scattered by the densely-packed glass spheres and either absorbed or reflected.  A simple illustration of this idea is presented below:

Because the refractive indices of the liquid and the powder typically only match at a single frequency, such an arrangement can be used as an optical filter, transmitting only a single color and blocking the rest.  By changing the refractive index of the liquid, one can adjust the specific frequency that is transmitted.  Such a filter is now known as a Christiansen filter.

###### 1897 advertisement for The Invisible Man, appearing in The Publisher’s Circle and Bookseller’s Record of British and Foreign Literature.

So glass submerged in an index-matching liquid can be made “invisible”, in a sense; what does this have to do with the invisibility stories of H.G. Wells and Fitz James O’Brien?  The Invisible Man was written in 1897 by H.G. Wells, and is almost certainly the most famous story about invisibility of all time.  In the story, a scientist named Griffin manages to find the secret of invisibility, but soon realizes that such a power is as much a curse as a blessing.  Without being able to make his clothes invisible as well, and unable to wander the countryside naked during winter in the British Isles, he quickly finds himself in desperate straits.  Fortuitously, he stumbles into the home of a former colleague, Dr. Kemp, and ends up relaying his entire story — including the general method by which invisibility is achieved.

“If a sheet of glass is smashed, Kemp and beaten into a powder, it becomes much more visible while it is in the air; it becomes at last an opaque white powder. This is because the powdering multiplies the surfaces of the glass at which refraction and reflection occur. In the sheet of glass there are only two surfaces; in the powder the light is reflected or refracted by each grain it passes through, and very little gets right through the powder. But if the white powdered glass is put into water, it forthwith vanishes. The powdered glass and water have much the same refractive index; that is, the light undergoes very little refraction or reflection in passing from one to the other.

“You make the glass invisible by putting it into a liquid of nearly the same refractive index; a transparent thing becomes invisible if it is put in any medium of almost the same refractive index. And if you will consider only a second, you will see also that the powder of glass might be made to vanish in air, if its refractive index could be made the same as that of air; for then there would be no refraction or reflection as the light passed from glass to air.”

“Yes, yes,” said Kemp. “But a man’s not powdered glass!” “No,” said Griffin. “He’s more transparent!

The experiment being described by Griffin, the invisible man, is exactly the type of experiment undertaken by Christiansen and others.

Did Wells know about Christiansen’s work?  It seems clear that he was aware of either Christiansen’s work, which was the first of that era, or some of the research that followed it.  This is not as surprising as it might first appear!  In 1884, Wells won a scholarship to study biology at The Normal School of Science (now part of Imperial College) under Thomas Henry Huxley, “Darwin’s Bulldog” himself!  In 1890, he graduated with a B.S. in zoology, and became a science teacher.  His first book, in fact, was a textbook on biology and zoology, published in 1893, two years before The Time Machine.

Zoology may not seem like an ideal background to be acquainted with optical research, but H.G. Wells was also intensely interested in science pedagogy in all fields.  In one of his letters in his pre-fiction days, he speaks critically of a textbook on “Sound, light and heat” by Benjamin Loewy, and clearly had some familiarity with the subject matter.

So, assuming that Wells was inspired by Christiansen, one might ask if Christiansen himself was inspired.  Quoting from (a translation of) his paper,

As is known, all transparent bodies form finely divided white powders; ice and water form snow and foam, white wool and silk show up under the microscope completely transparent, most white powders manifest themselves in larger crystals as a transparent bodies, and one is certainly entitled to establish the general principle: All bodies are translucent white. There is therefore nothing new said; Newton explains the white color in the same way and Brücke*** literally says “They (white pigments) are all colorless bodies in a very finely divided state.”

Emphasis mine! The original observations of index matching, and making powdered glass disappear in a liquid can be traced back to Isaac Newton!

###### Isaac Newton (1642-1727), painted in 1689 (image source).

I hopefully shouldn’t have to say too much about Newton!  He was one of the greatest physicists of all time, first describing the law of universal gravitation and the famous three laws of motion.  He was one of the developers of calculus, sharing the credit with Gottfried Leibniz, who formulated his own theory independently.

Newton also published a book on optics, Opticks, that relays his detailed experimental observations on the nature of light and color and the conclusions that he drew from them.  The book was incredibly influential, both due to the thoroughness of the experiments and Newton’s massive reputation.  Among the noteworthy results of the book are the conclusion that light consists of a stream of particles (later shown to be not quite correct), and the realization that white light is in fact a combination of light of all colors.  This latter conclusion, and the method of proving it, was later immortalized in a classic Pink Floyd album cover:

Newton’s experiments also led him to draw conclusions about the nature of matter and the interaction of light with matter.   In Book II, Part III of Opticks, Newton provides a few observations highly relevant to our discussion (apologies in advance for the length of the passages):

Prop. II.

The least parts of almost all natural Bodies are in some measure transparent: And the Opacity of those Bodies ariseth from the multitude of Reflexions caused in their internal Parts.

That this is so has been observed by others, and will easily be granted by them that have been conversant with Microscopes.  And it may be also tried by applying any substance to a hole through which some Light is immitted into a dark Room.  For how opake soever that Substance may seem in the open Air, it will by that means appear very manifestly transparent, if it be of a sufficient thinness.  Only white metalline Bodies must be excepted, which by reason of their excessive density seem to reflect almost all the Light incident on their first Superficies; unless by solution in Menstruums they be reduced into very small Particles, and then they become transparent.

Here Newton is speculating that most matter is, when divided into its smallest pieces, transparent.  Opacity, in Newton’s view, is due to the fact that light gets reflected and refracted by these tiny components, and the cumulative effect of all these deflections and reflections is an obstruction of the light.  An illustration of this would be very similar to the lower illustration of the Christiansen filter.

The key passage for our purposes, however, follows next:

Prop. III.

Between the parts of opake and colour’d Bodies are many Spaces, either empty, or replenish’d with Mediums of other Densities; as Water between the tinging Corpuscles wherewith any Liquor  is impregnated, Air between the aqueous Globules that constitute Clouds or Mists; and for the most part Spaces void of both Air and Water, but yet perhaps not wholly void of all Substance, between the parts of hard Bodies.

The truth  of this is evinced by the two precedent Propositions: For by the second Proposition there are many Reflexions made by the internal parts of Bodies, which, by the first Proposition, would not happen if the parts of those Bodies were continued without any such Interstices between them; because Reflexions are caused only in Superficies, which intercede Mediums of a differing density, by Prop. I.

But farther, that this discontinuity of parts is the principal Cause of the opacity of Bodies, will appear by considering, that opake Substances become transparent by filling their Pores with any Substance of equal or almost equal density with their parts.  Thus Paper dipped in Water or Oil, the Oculus Mundi Stone steep’d in Water, Linnen Cloth oiled or varnish’d, and many other Substances soaked in such Liquors as will intimately pervade their little Pores, become by that means more transparent than otherwise; so, on the contrary, the most transparent Substances, may, by evacuating their Pores, or separating their parts, be render’d sufficiently opake; as Salts or wet Paper, or the Oculus Mundi Stone by being dried, Horn by being scraped, Glass by being reduced to Powder, or otherwise flawed; Turpentine by being stirred about with Water till they mix imperfectly, and Water by being form’d into many small Bubbles, either alone in the form of Froth, or by shaking it together with Oil of Turpentine, or Oil Olive, or with some other convenient Liquor, with which it will not perfectly incorporate.

The emphasis is, again, mine.  Having established that the smallest parts of matter are transparent, it naturally follows that the refractive index of the “stuff” between the parts must be different.  He justifies this in both directions: putting an opaque substance like paper in oil can make it transparent (if you’ve eaten greasy pizza on a paper plate, you’ve experienced this), and taking a transparent substance like glass and grinding it up can make it opaque.

In Newton’s views, opacity is due to the irregular microscopic structure of matter; transparency is conversely present when a substance has a very regular and homogeneous structure.

But this in turn brings us back to Fitz James O’Brien’s description of the physics of invisibility:

Take a piece of pure glass. It is tangible and transparent. A certain chemical coarseness is all that prevents its being so entirely transparent as to be totally invisible. It is not theoretically impossible, mind you, to make a glass which shall not reflect a single ray of light—a glass so pure and homogeneous in its atoms that the rays from the sun shall pass through it as they do through the air, refracted but not reflected.

It certainly sounds like O’Brien is working from Isaac Newton’s views of optics and the structure of matter!  If opacity is due to the irregular microscopic structure of matter, then one would naturally conclude that a very homogeneous material could be completely transparent.

It is less clear how O’Brien would have come into contact with Newton’s Optical ideas.  He was educated at the University of Dublin, and certainly could have learned some optics there.  It is noteworthy, though, that O’Brien’s other famous weird tale, The Diamond Lens, deals heavily with microscopy: it concerns a man who builds a super-microscope, discovers a microscopic world, and falls in love with one of its inhabitants.

If you’ve followed me this far, then hopefully you agree that it is plausible that both Wells and O’Brien were influenced by experiments that originated with and were popularized by Isaac Newton.

Unfortunately, it seems likely that this will remain an intriguing hypothesis!  Though H.G. Wells was a prolific letter-writer, his letters were generally short and he did not talk much about his stories in them.  (I looked through his collected letters from the 1880s through the year 1900, with no luck.)  We might have had better luck with O’Brien, save for a tragic stroke of bad fortune: the man entrusted to publish O’Brien’s collected letters after his death himself died soon afterwards, and the letters were apparently lost.

It is a compelling idea, however.  Wells’ story, and to a lesser extent O’Brien’s, led generations of people to imagine the scientific possibility of invisibility; it is not too big of a stretch to say that modern scientific attempts to make cloaking devices have their origins in these early works of fiction.  What better way to complete the story then to find that these authors, in turn, were inspired by one of the greatest physicists in history?

***********************

* William Coblentz, “Optical notes,” Phys. Rev. 2 (1902), 89-97.

** C. Christiansen, “Untersuchungen über die optischen Eigenschaften von fein verteilten Körpern,” Ann. Phys. Chem. 23 (1884), 298-306.

*** Ernst Wilhelm Ritter von Brücke (1819-1892) was a German physician and physiologist who did work on the physiology of the eye.