Machinery can make big changes in social behavior--- the automobile completely changed patterns of courtship, even techniques and positions of sexual intercourse, in America. The development of the rail system, and of high speed printing, over the last half of the 19th Century, made possible the weekly magazine, and opened markets for fiction unprecedented in human history. Writers responded nobly to the demand for quantity and novelty. The type of literature we call ``science fiction'' today thus became a recognizable genre of popular fiction more than a century ago. Completely fictional themes and gimmicks developed during this heroic age have so deeply penetrated public consciousness, through use and reuse and retread and re-re-retread during the following ages of film and television, that it is often difficult to realize they have no counterpart whatsoever in reality.

The gimmick of invisibility was not invented by H. G. Wells, but he got there first with the best story. His inventor, Griffin, by swilling a magic potion, made himself totally transparent. (He would also have made himself totally blind, since the backs of his eyes would no longer absorb light, but Wells chose to ignore this complication.) The conceptual problem with this, as Wells was well aware but also chose to ignore, is that transparent objects are not invisible. Water, air and glass are fairly transparent, but quite visible. Another, physical problem is that transparent objects refract light--- change its direction slightly--- and also scatter or diffusely reflect light--- bounce it in all directions from their surfaces.A transparent drop of water flying through the air, or a transparent air bubble in water, are clearly visible, by the light they refract and reflect.

The earth's air is transparent, but the air between you and those distant hills is visible because of the lovely shade of pale blue it scatters. And the air between you and the black of outer space is transparent, but clearly visible because of the lovely shade of deep blue it scatters. Clouds and fogs consist of transparent, tiny water droplets suspended in transparent air, but clouds and fogs are clearly visible by their scattering of all wavelengths of light... they look brilliant white in sunlight, grey in subdued light.

Furthermore, transparency and refractive index are intrinsic, inherent properties of a given material. They cannot be changed, being determined by the internal molecular structure, and the overall spacing of the molecules in the material. In order for substance A to be invisible in substance B, both materials must be transparent and both must have about the same index of refraction. No solid or liquid comes with an index anywhere close to the index of any gas. Thus, if you want to explore this kind of invisibility, you need a solid and a liquid. Try ordinary Wesson Vegetable Oil and laboratory pyrex glass. Fill a small pyrex container with Wesson oil and then submerge it in a large jar of Wesson oil and watch the pyrex container vanish! Wow, golly, Mr. Science!! But so long, Mr. Invisible Man.

Decades after Wells, writers tried other tactics. An object could be made invisible if you could ``bend light around it.'' The conceptual problem here is that, except in rare cases, we don't see objects as silhouettes! We see objects mainly because their surfaces scatter light diffusely. Since the light is scattered in all directions, no matter how it is bent, some gets to our eyes. A physical problem is that, in empty space or any given material, light travels as straight as it can travel. Only something that can change ``the shape of space itself,'' like gravity, can bend light ``around an object.'' For example, distant massive galaxies gravitationally bend the light from still more distant galaxies directly behind them, so that the more distant galaxies are visible as distorted arcs or in multiple images surrounding the central galaxy, in the beautiful ``gravitational lens'' phenomenon first predicted by Einstein. The Hubble Space Telescope has provided many wonderful examples of such lenses. Note that in every case the central galaxy that is bending the light is clearly visible--- indeed, it dominates the image! As it must.

The daffiest ``invisible'' scenario was the ``coexistent ultraviolet (or infrared) world''. What looks to you like the streets of a city is in fact a dense jungle full of exotic alien animals. It's just that the trees and animals reflect only ultraviolet (or maybe infrared, or something) light. So we don't know they are there! The conceptual problem should be obvious here. The fact that we can't see something doesn't keep us from walking into it or tripping over it, painfully! The writers confused invisibility with intangibility. The physical problem should also be obvious. An object that does not reflect or scatter visible light is clearly visible, because it appears black.

With the real external world so inhospitable to invisibility, only the human mind is left. In the exciting world of pulp fiction, in the 1920s and '30s, various justice figures came up with a new way to be unseen. Like radio's Shadow, who could ``cloud men's minds so that they cannot see him,'' perhaps by ``guesturing hypnotically'' like the comic strips' Mandrake the Magician, these justice figures could tumble helpless bad guys around like so many ten-pins. And that's another story, for another time, plunging us into yet another myth, the myth of ``mass hypnosis.''

In recent years the news media have sometimes carried almost completely nonsensical stories about "inventors" or "professors" or "government labs" that have achieved supposed "invisibility." What is referred to in every case that I am familiar with is instead "adaptive camoflage." A typical idea is to dress in a cloak that somehow is made to carry an image of the background; depending on how accurately the background is reproduced, the cloaked individual might (or might not!) be not noticable upon a very casual glance, although hardly invisible. Such systems as have been "shown" to date involve such things as video cameras with operators, and large free-standing video projectors, and are totally impractical for any imaginable purpose.

There is another impractical approach to invisibility, making use of the modern science of “metamaterials.” It is possible to make a delicate, rigid structure out of tiny bits of some material, having the property that nothing placed inside the structure can be seen in specific wavelengths of light. The structure's index of refraction is tuned by arrangement and size of its pieces, so that light entering from any direction is guided through it and exits on the opposite side, with the result that at the particular wavelength chosen, the structure is effectively transparent, and the solid object inside it cannot be seen. Again, because the structure is incredibly delicate and rigid, it does not provide invisibility for a moving, flexible object such as a human being.

Onward to Next Science Page?

Based on material first published by me in a 1995 issue of the Internet magazine Baudeville.

A test tube in Wesson oil... the part of the tube below the oil is invisible since the indices of refraction of pyrex glass and Wesson oil are close to the same.

I like it, I like it, but it's not invisibility!