Yet another invisibility story! So many invisibility stories.

So, in the list of stories we’ve looked at so far, we’ve seen invisible people, invisible monsters, invisible buildings, invisible cars, invisible dogs, and invisible spacecraft. But you now what we haven’t seen yet? Invisible robots!

Our invisible robots appear in Harl Vincent’s “Terrors Unseen,” which appeared in the March 1931 issue of Astounding Stories. It is not a particularly astounding story, but it does has invisible robots, so let’s take a look!

The story begins with Eddie Vail, driving on a remote part of the coast on his way to vacation. Out of the corner of his eye, he spots a woman struggling in a strange unnatural position, and he stops to help. When he reaches her, she claims to have been attacked by one of her father’s inventions — a robot which is invisible! The woman is Lina Shelton, and her father is the famed engineer David Shelton. Ever since leaving his job with Universal Electric, he has been performing private experiments in this remote area. And now the robots that he created have gotten out of control!

The trail eventually leads to a disgruntled former employee of Shelton’s, who has seized control of the robots, and a criminal mastermind that has bankrolled the operation for nefarious purposes. Can Vail and Shelton stop the criminals and take back control of the robots?

Like I said, it is not a particularly interesting story, and doesn’t merit much of a synopsis. But what what about the invisibility physics? Let’s look at how Shelton explains it to Vail:

“You can make this one invisible?” Eddie asked incredulously.

“Certainly-from the waist up. This ought to be good.”

“Mind telling me the principle?”

“Not at all-now. I’ve your promise of secrecy, It’-s a simple matter, Vail. really. Just a problem of wave mo­tions-light. Invisible light; the ultra-­violet, you know. My robots are built of specially alloyed metals which permit great freedom of’ molecular vibration. The insulating materials and even the glass of the camera lenses are possessed of the same property. Get it? I merely set up a wave motion in the atoms of the material that is in synchronism with the frequency of ultra-violet light, which is invisible to the human eye. All visible colors are absorbed, or more accurately, none are reflected excepting the ultra-violet. Perfect transparency is obtained since there is neither refraction nor diffraction of the visible colors. And there you are!”

So here we have another explanation that employs the idea that ultraviolet light is invisible. It is mixed with a confusing description of making an alloy that has the right “molecular vibration” to absorb and not reflect or diffract visible light. The explanation doesn’t completely make sense, but it does have some curious connection to so-called “metamaterials,” which have been a huge research area in optics since about 2000!

Natural materials derive their optical properties from the chemistry of the atoms and molecules that they are made of, as well as from the natural structure of the atoms gathered together. A “metamaterial” is a material which has had its structure altered on a scale much smaller than the wavelength of light; an example of this is a so-called “fishnet” structure, constructed from silver. An old illustration of the structure that I made is shown below:

Basically, ordinary silver would be a solid material on this scale; it becomes a “metamaterial” because of the holes and layers in it. This material was designed to produce negative refraction, where the light bends in the opposite direction that it does for normal materials:

John Pendry of Imperial College is one of the pioneers of metamaterial research, which he started in the 1990s. He would later be one of the researchers who introduced the theory of invisibility cloaks in 2006. Relevant to the current invisibility discussion, however, in 1996 he and colleagues at the company GEC-Marconi showed that they could dramatically change the optical properties of a metal and basically move the properties of the metal for visible light into the infrared range instead! Oddly, and very much coincidentally, this seems to be what Harl Vincent is attempting to describe as well.

So again, we have an invisibility story that isn’t terribly interesting as a story, but has curious ideas about invisibility that have now at least partly been done in reality!

PS if you must read this story, here’s the pdf.