Many pixels have been devoted to exotic materials that could one day make “invisibility cloaks” a reality, thrilling science fiction fans who dream of, say, their own Tardis equipped with a “chameleon circuit.” (Time travel, alas, is a thornier issue).
The latest research on this comes from the University of Michigan, where engineering professor L. Jay Guo has developed a “perfect black” type of cloth that can render a 3D object “invisible.” Guo’s work is described in a new paper in Applied Physics Letters.
It’s basically a black carpet made out of carbon nanotubes (CNT), capable of absorbing 99.9 percent of the light that hits it. “It’s not cloaking, as the object can still cast a shadow,” Guo cautioned. “But if you put an object on a black background, then with this coating, it could really become invisible.”
And there are implications for astronomy! Coating the inside of telescope tubes with this stuff could pretty much eliminate the problem of internal light scattering in those instruments.
CNTs are part of the family of molecules known as fullerenes, related to “buckyballs” (C60), a carbon molecule shaped like a soccer ball. In contrast, CNTs boast a long, hollow, tube-like structure with walls of one-atom-thick sheets of carbon (graphene).
Many accounts credit NEC scientist Sumio Iijima with the “discovery” of CNTs back in 1991, but evidence of these unique structures can be found as far back as 1952, when a paper appeared in the Soviet Journal of Physical Chemistry, that included images of 50 nanometer diameter carbon tubes. The paper was in Russian, and was not widely distributed thanks to the Cold War, and thus did not attract much attention.
From the mid-1970s on, numerous sightings of CNTs appeared in the scientific literature. So saying Iijima “discovered” CNTs is a thorny issue among scientists. Let’s just say he was instrumental in developing methods for producing both multi-walled and single-walled CNTs, and in raising the profile of these amazing structures within the scientific community.
Depending on their structure, CNTs have unique mechanical and electrical properties, opening up a host of potential applications. They can have terrific tensile strength, for example, as well as being highly conductive. And they are almost perfect absorbers of light.
It’s that last property that Guo’s team is exploiting for their CNT coating. They grew a series of vertical carbon nanotubes on the surface of a silicon wafer etched with a pattern of an itsy-bitsy tank (see top image), forming a “perfect black” carpet.
When placed against a black background, the coated tanks seemed to disappear. Oh yes, they had a control: they carved out a rectangle and omitted the CNT coating. Said rectangle was visible on the chip, but the coated tank was not.
The coating essentially changes the optical properties of the silicon tanks. Objects are visible to the human eye because they reflect or scatter light; how much a given material does this depends on a property called its “refractive index.” So-called metamaterials have a negative refractive index, making them potentially useful for cloaking applications.
Guo’s carbon coating, in contrast, has a refractive index similar to that of air; usually light doesn’t scatter when traveling through air. This is achieved by carefully growing the vertical CNT “trees” in his silicon “forest” in such a way as to leave some space between them.
Combine that with the fact that CNTs are near-perfect absorbers of light — if any light does scatter or reflect, it gets absorbed before it reaches our eyes — and you’ve got a recipe for at least a kind of “invisibility”: making 3D objects appear 2D, and vanishing from view when placed against a black background.
Unlike metamaterials, which usually only work within limited spectrums of light — microwaves, for example — the CNT coating works across a much broader range.
So, that’s cool and all, but Guo also speculates that a similar type of substance might be found in the universe, effectively hiding entire planets or stars from our telescopes’ probing eyes.
“Since deep space itself is a perfect dark background, if a planet or star were surrounded by a thick, sooty atmosphere of light-absorbing carbon nanomaterial gases, it would become invisible due to the same principle,” Guo is quoted as saying in the Michigan press release. “It would become totally dark to our instruments that rely on the detection of electromagnetic waves. Could this explain some of the missing matter in the universe?”
Wait — what? Okay, here’s where Guo and I part ways, since the whole point of dark matter in physics is that it’s not the same thing as ordinary (baryonic) matter. It’s an entirely new kind of particle that interacts so weakly with everything else in the universe, we can only detect it through its gravitational effects and the kinds of experiments one uses to search for neutrinos.
So if something like the CNT coating is effectively “veiling” planets and stars from our telescopes, it’s got nothing to do with the dark matter question. But I still hold out hope for my very own vanishing Tardis someday, and researchers like Guo could help make it happen.